Smart Fasteners and Washers for Preload and Loosening Detection: A Systematic Review of Sensing Technologies

Abstract

This systematic review investigates the state of the art of smart fasteners and smart washers. The collected documents were systematically classified, focusing on fasteners and washers equipped with embedded sensors capable of detecting flaws, corrosion, cracks, or monitoring features like structural response, applied load, and preload. The review emphasizes sensor technologies rather than devices made from smart materials. A total of 148 documents were found through electronic databases, of which 32 were thoroughly analyzed and categorized into 5 categories depending on the sensing capabilities, actuating capabilities, ability to transmit data, and their combinations. The analysis showed that most devices incorporate piezoelectric materials as sensing or actuation elements, primarily aimed at detecting fastener loosening or monitoring preload and load in fastened joints. Despite progress, several challenges limit industrial adoption, including sensor integration affecting bolt integrity, durability and calibration issues, and high costs. Few studies address scalability or measurement performance, highlighting the need for reliable, low-cost, and industrially scalable solutions.

1. Introduction

Bolts, nuts, cap screws and rivets are used for joining parts into a structural assembly; their characteristics must be chosen to ensure integrity under service conditions that may induce loosening, such as vibration and fluctuating loads [1]. Since its introduction, bolted joints have been extensively adopted in both civil and mechanical engineering applications to create non-permanent joints, thanks to their high strength and ease of use. Given that such joints are often subjected to complex, multi-axial loading, their design requires careful consideration. Particularly when considering that the connection is influenced by cyclic loading and various working conditions [2,3,4,5,6].

Since accidents may occur because of loosened bolted joints, periodic manual and visual inspection is adopted for structures such as bridges, skyscrapers, power plants, and aerospace applications. However, manual and visual inspection is time-consuming and subject to human actions [7,8]. Several techniques can be used to inspect bolted joints, such as visual inspection for surface condition monitoring, Non-Destructive Inspection (NDI) for corrosion detection, or acoustic emission to detect material loss gaps that could cause structural integrity loss. Still, these techniques do not provide continuous monitoring of the joint. In this contest, Structural Health Monitoring (SHM) plays a fundamental role, as SHM is the process of continuously assessing the integrity and performance of structures through embedded or attached sensors and data interpretation methods that convert measurements into meaningful information on the structure status [9]. Recent work by Zhaoet al. [10] demonstrated the effectiveness of machine learning models for assessing structural information based on measurement data. Moreover, as highlighted in the in [11], advances in smart sensing technologies have significantly expanded the applicability of SHM, enabling real-time detection of corrosion, fatigue, and cracks in complex structures. The use of smart monitoring systems that do not need human involvement brings several advantages: they are effective, reliable, cost-effective, repeatable, accurate, and, often, they do not require heavy structural changes. Beyond force monitoring, innovative sensors are integrated into bolted joints to support structural health monitoring and facilitate predictive maintenance strategies for timely fault identification [4,12,13,14]. Within this framework, smart fasteners and washers represent a specific class of localized SHM nodes, capable of capturing load, strain, or damage information directly at the joint level. The sensing element can be either the bolt or the washer. Both of them are considered in this systematic review.

To the best of our knowledge to date, there are no studies focused on the systematic review summarizing the available technologies for smart fasteners and washers, both for structures and production systems.

The paper is structured as follows: the search strategy, eligibility criteria, and data analysis procedures are described in Section 2. Section 3 presents the results of the systematic review on sensing techniques applied to bolts and washers. Section 4 summarizes the results and critically analyzes the different techniques. Paper conclusions are drawn in Section 5.

2. Materials and Methods

2.1. Search Strategy

Our research focused on bolts and washers with a generic sensing function. Keywords were derived from the synonyms of the word smart (intelligent, innovative) and the term fastener (screw, bolt, washer).

A systematic literature search was conducted in December 2023 on Scopus, Google Patents, Patentscope, and the ASME Digital Collection databases. Customized queries, including the previously identified keywords and Boolean logic with AND/OR operators, were entered in the search engines, in a form like the following example: TITLE (smart OR intelligent OR innovative) AND TITLE (fastener OR screw OR bolt OR washer) AND ABS (detect OR find OR measure OR determine). The research was also limited by including only documents with the type “paper” or “patent” with a date limit set to 1990.

2.2. Eligibility Criteria and Study Selection

The collected documents were classified as Important, Relevant, or Irrelevant. The classification was performed by first defining three core concepts (i.e., Aim, Context, and Tools) from the objective statement defined before. The Aim was defined as “fastener or washer which has functions to detect any kind of flaws, corrosion, cracks, or other useful features like structural response, applied load, preload, etc.”; the Context as “structural members, machine/assembly components, industrial processes, and vehicles”; the Tools as “any kind of sensor”.

The documents were classified according to the following systematic procedure:

• if the document is a duplicate, it is automatically excluded;
• if the document cannot be retrieved or if is not in English language, is excluded;
• if the document does not match the Aim, it is labeled as Irrelevant;
• if the document matches Aim, Context, and Tools, it is labeled as Important;
• if the document matches Aim and Context or Tools, it is labeled as Relevant;
• else it is labelled as Irrelevant.

2.3. Data Synthesis and Analysis

All the documents labeled as Important or Relevant were formalized into Article Cards, a document that summarizes the article’s fundamental information allowing easier consultation, comparison, and reference. The Article Card structure was inspired by Cicero’s “Rhetoricorum, seu De inventione rhetorica” and it contains for each document the most relevant information: filing number, citation number, title, relevance index (as given by the document classification process), a summary, plus the answer to the questions:

• Quis? (Authors);
• Quando? (Year);
• Ubi? (Context or type of structure);
• Cur? (Aim of the invention/study);
• Quid? (Methods/Tools);
• Quomodo? (Tests and results).

Table 1 shows an Article Card example, while the full document can be provided. Three independent reviewers performed the screening and the analysis of each record.

Table 1. An example of Article Card of the document [15].

[15]	A new Design Concept for Multifunctional Fasteners Using Smart Materials.
Relevance:	IMPORTANT
Authors:	Hwan-Sik Yoon
Year:	2009
Context:	Structural Members
Summary:	These smart fasteners, which integrate a piezoelectric element and control unit, function like conventional fasteners while monitoring structural response and actively generating counterforces to improve dynamic performance and suppress vibrations.
Aim:	A new design concept for piezoceramic actuators is presented to simplify the installation and removal of patch-type actuators for active vibration control of flexible structures. Integrated with a dedicated control algorithm, these multifunctional actuators can fasten structural members and generate control forces to enhance structural dynamics.
Methods and tools:	Two smart fastener configurations have been developed. In the first, a ring-shaped piezoceramic actuator is integrated beneath the bolt head to induce axial strain in the fastener and perpendicular deformation in the clamped component. In the second, a compact piezoceramic element is positioned between the bolt head and a proof mass. When driven at high frequency, the proof mass produces an inertial force on the jointed structure, while a sensing transducer embedded in the nut detects the resulting compressive or inertial loads. In both configurations, the measured response is processed by a control unit embedded in the bolt head, which generates the corresponding actuation signal. A Lyapunov-based control algorithm ensures robust suppression of structural vibrations in flexible members. Beyond vibration mitigation, the smart fastener concept also enables structural health monitoring through an impedance-based technique. By applying electrical signals of varying frequencies to the piezoceramic element and measuring the resulting current, the system computes the electromechanical impedance spectrum. Deviations of this impedance from its nominal baseline over time serve as indicators of local damage or degradation near the fastener or within the connected structure.
Tests and results:	A simulation study was carried out on a clamped–clamped beam to evaluate the effectiveness of smart fasteners in active vibration suppression. An impact applied at the beam’s midpoint was modeled as a disturbance input to excite structural vibrations. In the closed-loop configuration, the vibration amplitude decreased to below 4% of its initial value within 0.8 s, whereas in the open-loop case, oscillations persisted for more than 5 s. These results demonstrate that, despite the limited actuation displacement, Active Boundary Control using smart fasteners can effectively attenuate structural vibrations.

3. Results

3.1. Literature Search Results

A total of 155 documents were retrieved from the electronic databases. 45 of them were excluded as duplicates, and 21 documents were not available or not in English. In total, 39 documents were included; 26 were labeled as Important and 13 as Relevant. Document screening led to the exclusion of 50 papers that failed to meet the inclusion criteria. Of the included ones, 24 are research papers, while 14 are patents, and 1 is a book chapter. The overall selection process of the reviewed documents is summarized in Figure 1, following a PRISMA-like flow diagram to illustrate the identification, screening, eligibility, and inclusion steps. PRISMA checklist has been uploaded as Supplementary Materials.

Figure 1. Flow diagram illustrating the selection process of the studies included in the review.

3.2. Description of the Included Documents

Results of the review have been divided into two main categories: screws or bolts (16 papers, 8 patents, and 1 book chapter) and washers (8 papers and 6 patents). Smart fasteners can be grouped into three main categories:

• Smart Material Fasteners, which use materials that adapt or respond to structural or environmental changes [16,17,18,19,20].
• Sensorized Fasteners, that incorporate different transducer families to monitor tension, temperature, or other physical parameters [4,13,15,21,22,23,24,25,26,27,28,29,30,31,32,33].
• Radio Frequency Identification (RFID) Fasteners use radio frequency to track and identify fasteners wirelessly [14].

Other fasteners combine these technologies, such as those that incorporate a transducer and RFID capabilities [1,3]. Similarly, smart washers were grouped into:

• Sensorized Washers, in which a transducer provides real-time data of preload or other physical characteristics [5,7,8,12,34,35,36,37,38,39].
• Sensorized + RFID Washers, that sense information from the environment and also benefit from RFID for wireless communications [40,41,42].
• Sensorized + Smart Material Washers, which adapt their material characteristics in response to environmental factors and transmit data via RFID [43].

Figure 2 presents the classification scheme of the reviewed technologies, outlining the main categories and subcategories of smart fasteners and smart washers. A summary of the documents’ category and relevance is presented in Table 2, in the following sections, only articles classified as important or relevant will be discussed. Table 2 is organized as follows: the important works are listed first, followed by the relevant works, and finally the non-relevant works. Within each category, studies on fasteners are presented first, followed by those on washers.

Figure 2. Conceptual classification of smart fastener and washer technologies according to sensing principles and functionality.

Table 2. Document list specifying subdivision (cluster) and relevance index.

#	Document	Category	Type	Cluster	Relevance Index
1	[3]	Fastener	Patent	Sensorized + RFID	Important
2	[4]	Fastener	Paper	Sensorized	Important
3	[13]	Fastener	Patent	Sensorized	Important
4	[15]	Fastener	Paper	Sensorized	Important
5	[21]	Fastener	Paper	Sensorized	Important
6	[22]	Fastener	Patent	Sensorized	Important
7	[23]	Fastener	Paper	Sensorized	Important
8	[24]	Fastener	Paper	Sensorized	Important
9	[25]	Fastener	Paper	Sensorized	Important
10	[26]	Fastener	Paper	Sensorized	Important
11	[27]	Fastener	Patent	Sensorized	Important
12	[29]	Fastener	Paper	Sensorized	Important
13	[31]	Fastener	Paper	Sensorized	Important
14	[32]	Fastener	Paper	Sensorized	Important
15	[33]	Fastener	Paper	Sensorized	Important
16	[7]	Washer	Paper	Sensorized	Important
17	[8]	Washer	Patent	Sensorized	Important
18	[12]	Washer	Patent	Sensorized	Important
19	[34]	Washer	Paper	Sensorized	Important
20	[36]	Washer	Paper	Sensorized	Important
21	[37]	Washer	Paper	Sensorized	Important
22	[38]	Washer	Paper	Sensorized	Important
23	[39]	Washer	Paper	Sensorized	Important
24	[40]	Washer	Patent	Sensorized + RFID	Important
25	[41]	Washer	Patent	Sensorized + RFID	Important
26	[43]	Washer	Paper	Sensorized + Smart Material	Important
27	[16]	Fastener	Patent	Smart Material	Relevant
28	[17]	Fastener	Paper	Smart Material	Relevant
29	[18]	Fastener	Paper	Smart Material	Relevant
30	[19]	Fastener	Book	Smart Material	Relevant
31	[1]	Fastener	Patent	Sensorized + RFID	Relevant
32	[14]	Fastener	Patent	RFID	Relevant
33	[20]	Fastener	Paper	Smart Material	Relevant
34	[28]	Fastener	Paper	Sensorized	Relevant
35	[30]	Fastener	Patent	Sensorized	Relevant
36	[44]	Fastener	Paper	Sensorized	Relevant
37	[5]	Washer	Paper	Sensorized	Relevant
38	[35]	Washer	Patent	Sensorized	Relevant
39	[42]	Washer	Patent	Sensorized + RFID	Relevant
40	[2]	Washer	Paper	Sensorized	Irrelevant
41	[6]	Fastener	Paper	Visual Indicator	Irrelevant
42	[45]	Fastener	Paper	Sensorized + RFID	Irrelevant
43	[46]	Fastener	Paper	Smart Material	Irrelevant
44	[47]	Fastener	Patent	Smart Material	Irrelevant
45	[48]	Fastener	Patent	Smart Material	Irrelevant
46	[49]	Fastener	Paper	Smart Material	Irrelevant
47	[50]	Fastener	Paper	Sensorized	Irrelevant
48	[51]	Fastener	Paper	Smart Material	Irrelevant
49	[52]	Fastener	Patent	Data Transfer + Storage + Computation	Irrelevant
50	[53]	Fastener	Patent	Surgical	Irrelevant
51	[54]	Fastener	Patent	Surgical	Irrelevant
52	[55]	Fastener	Paper	Probabilistic Fastener	Irrelevant
53	[56]	Fastener	Patent	Sensorized + RFID	Irrelevant
54	[57]	Fastener	Paper	Sensorized	Irrelevant
55	[58]	Fastener	Patent	Smart Material	Irrelevant
56	[59]	Fastener	Patent	Logistics	Irrelevant
57	[60]	Fastener	Paper	External sensor	Irrelevant
58	[61]	Fastener	Patent	Logistics	Irrelevant
59	[62]	Fastener	Paper	Surgical	Irrelevant
60	[63]	Fastener	Paper	Structural	Irrelevant
61	[64]	Fastener	Paper	Inspection	Irrelevant
62	[65]	Fastener	Paper	External sensor	Irrelevant
63	[66]	Fastener	Paper	Structural	Irrelevant
64	[67]	Fastener	Paper	Material	Irrelevant
65	[68]	Washer	Patent	Sensorized	Irrelevant
66	[69]	Washer	Patent	Sensorized	Irrelevant
67	[70]	Washer	Patent	Sensorized	Irrelevant
68	[71]	Washer	Patent	RFID	Irrelevant
69	[72]	Washer	Patent	Sensorized	Irrelevant
70	[73]	Washer	Patent	Sensorized	Irrelevant
71	[74]	Washer	Paper	Sensorized	Irrelevant
72	[75]	-	Paper	Screwdriver	Irrelevant
73	[76]	-	Patent	Screwdriver	Irrelevant
74	[77]	-	Patent	Out of scope	Irrelevant
75	[78]	-	Paper	Out of scope	Irrelevant
76	[79]	-	Patent	Out of scope	Irrelevant
77	[80]	-	Patent	Out of scope	Irrelevant
78	[81]	-	Patent	Screwdriver	Irrelevant
79	[82]	-	Paper	Out of scope	Irrelevant
80	[83]	-	Paper	Out of scope	Irrelevant
81	[84]	-	Paper	Out of scope	Irrelevant
82	[85]	-	Paper	Out of scope	Irrelevant
83	[86]	-	Paper	Out of scope	Irrelevant
84	[87]	-	Paper	Out of scope	Irrelevant
85	[88]	-	Paper	Out of scope	Irrelevant
86	[89]	-	Paper	Out of scope	Irrelevant
87	[90]	-	Paper	Out of scope	Irrelevant
88	[91]	-	Paper	Out of scope	Irrelevant
89	[92]	-	Paper	Out of scope	Irrelevant

3.3. Summary of Evidence

3.3.1. Smart Material Fasteners

In industrial and critical applications, Shape Memory Alloys (SMA) elements are engineered to utilize their unique thermo-mechanical properties to create high-integrity connections. This is commonly achieved by using a pre-deformed SMA component, which, upon thermal activation, attempts to recover its memorized shape, generating a uniform clamping force against the surrounding joint members [16]. The use of SMA components in shrink-fit joints enables an easier and more reliable assembly process that is less sensitive to surface quality and tooling errors, resulting in superior axial fixation and high friction forces [17]. Furthermore, this technology extends to maintaining structural health; for example, SMA elements can function as self-repairing actuators in bolted joints, automatically restoring lost torque and preload when a joint loosens, significantly increasing the long-term reliability of the assembly [18]. In civil engineering, SMA fasteners are also employed as key components in self-centering systems for seismic applications, providing both energy dissipation during an event and the necessary restoring force to bring the structure back to its original position [19].

An interesting approach in the field of smart material fasteners is the one presented in [20] where Transformation Induced Plasticity (TRIP) steels are used. These alloys can show a phase transition under stress from a non-magnetic austenitic phase to a ferromagnetic martensitic phase. The state of the bolt is assured by monitoring the change in the alloy’s magnetic properties.

3.3.2. Sensorized Fasteners

Different sensors can be integrated into the fastener body, as analyzed in fourteen relevant documents. The proposed solutions use different transducing elements positioned at distinct locations within the fastener. We identified the following categories based on transduction principles:

• strain gauges to measure axial force and bending torque independently [4] or to measure torsional strain and associated preload [24].

In [4], the sensorized fastener is manufactured through metal forming, with strain-sensitive elements directly integrated during fabrication. The authors demonstrated that this configuration provides multidirectional load monitoring, good mechanical–electrical coupling, and thermal compensation; however, challenges remain in terms of calibration complexity, long-term fatigue resistance, and industrial scalability.

In [24] a 3D-printed smart screw was fabricated and tested, demonstrating an additive manufacturing approach where the sensing functionality is co-fabricated with the polymeric fastener itself. This method offers design freedom and cost-effective production for lightweight or prototyping applications, yet its mechanical performance, signal stability, and anisotropic material behavior limit its use in high-load environments.

• piezoelectric or ultrasonic transducers to measure load or apply strain in a longitudinal direction [15,22,23,26].

Early developments, such as those described in [22], rely on external ultrasonic transducers to measure bolt elongation through time-of-flight (ToF) variations induced by the acoustoelastic effect, enabling non-invasive and real-time preload control during tightening. More advanced concepts integrate piezoelectric elements directly within the fastener head, as demonstrated by the embedded PZT-based smart bolt [26] (where PZT, or lead-zirconate-titanate, is a piezoelectric ceramic material capable of converting mechanical stress into electrical signals), which eliminates the need for coupling agents and allows continuous, in situ monitoring of axial stress with improved accuracy. Complementary research extends the functionality of piezoelectric fasteners beyond preload sensing: multifunctional designs embed coupled sensor–actuator systems to enable active vibration control and impedance-based SHM, effectively merging actuation and diagnostics in a single compact device [15]. Additionally, ultrasonic Lamb wave–based implementations [23] exploit the fastener as both transmitter and receiver, forming distributed SHM networks capable of detecting cracks or delamination over wide structural areas.

• piezoelectric cable embedded in a bolt to monitor structural health by measuring relative displacements of joints and structural vibrations [29]. The key strengths of the presented approach are ease of installation and low cost, making it suitable for retrofitting existing structures. However, its limited sensitivity to static loads, potential calibration drift, and reliance on wired data acquisition restrict its use to short-term or localized monitoring.
• multielement electrochemical sensing array to detect corrosion [13,21]. The system presented in [21] integrates an electrochemical smart sensor directly into an aircraft fastener to enable in situ detection of early-stage corrosion. It autonomously monitors key environmental parameters. The sensor array is embedded within the fastener head and connected to a Common Electronics Unit for data storage and later retrieval via a portable computer.
• broadband acoustic emission transducer to assess structural integrity by identifying signs of metal fatigue and cracking [13];
• optical unit (emitter/receiver) combined with temperature or moisture sensors to measure strain [30]. This patent presents an optical smart bolt with a reflective sensor embedded in an axial cavity to track elongation and preload continuously. The system offers high precision and immunity to electromagnetic interference, but it may weaken the bolt and require temperature compensation. Its complexity and cost restrict its use to critical, high-value applications.
• fiber optic and Fiber Bragg Gratings (FBG) sensors to measure axial forces, shear forces, preload or to monitor creep [25,27,28,31,32,33,44].

FBG sensors are primarily embedded within the fastener’s shaft to measure axial elongation, thereby enabling accurate determination of preload and continuous long-term monitoring for creep and loosening [27,32]. Beyond axial measurement, FBG configurations have been specially adapted to measure shear forces (transverse strain), providing critical insights into joint slippage and rigidity loss [25]. A major technical challenge for reliable deployment remains the sensor’s cross-sensitivity to temperature, which necessitates complex compensation; this is being addressed by innovative self-compensating designs that aim to isolate strain from thermal effects [25].

In many cases, the fastener geometry must be changed to allow the positioning of the transducing element. The bolt may present a hollow structure with longitudinal [4,13,29,32] or radial [25,27,28,44] holes [33]. Alternatively, transducers can be mounted in slots on the fastener wall [21] or on the top/bottom of the fastener head in a permanent or removable manner [15,22,23,26,30]. The recent advances in additive manufacturing have allowed building the body of the fastener around the sensor system [24]. In many cases, the necessary electronics are located inside the fastener’s body [13,15,21,23].

Figure 3 illustrates the eight categories used to classify the sensorized fasteners.

Figure 3. Classification of sensorized fasteners into eight categories based on their sensing principles and functional characteristics.

3.3.3. RFID Fasteners

These devices are equipped with RFID antennas communicatively coupled with an integrated circuit including a memory. These devices can be used to easily store and share logistic data, such as time and date of installation, grip length of the fastener, type of fastener, identity of installer, location of the fastener on the item being fabricated, maintenance and revision information, fastener equipment used for installation, and any other relevant information [14].

In [3], a fastener is presented that utilizes an integrated RFID chip coupled to a two-state microswitch. This microswitch functions as a binary torque value sensor, designed to signal whether the fastener torque is within the specification value or not within the specification value. Crucially, the RFID tag memory stores the prescribed torque specification for each fastener, ensuring correct installation. Moreover, the system allows rapid verification of multiple fasteners through a scanning inspection method, significantly reducing inspection time by eliminating manual torque checks. The microswitch is installed beneath the head of the fastener, where it remains in contact with the clamped surface under a defined pressure. The same minimum torque detection could be achieved by putting a ground pin inside the fastener body and in contact with the RFID antenna, such that wireless communication cannot be established. When a torque is applied to the fastener, its body elongates, disconnecting the RFID antenna from the ground pin, allowing the RFID system to communicate the data stored in the memory, such as the fastener identification number. In this way, it is possible to check the correct torque state of the fastener [1].

3.3.4. Smart Washers

Smart washers are mainly used for load or preload measurement and fastener loosening detection. They can be divided into two categories: passive washers [12,34,35,39,40,41,42,43] and active washers [5,7,8,36,37,38]. Passive washers measure the force applied to themselves from the outside, while active washers generate ultrasonic excitation and measure the corresponding mechanical response.

Seven documents present smart washers based on piezoelectric transducers [5,7,8,36,37,38,42] while four documents present smart washers based on strain gauges [12,35,40,41]. Two documents introduce a smart washer based on fiber optics with FBG sensors [34,39] and one document presents a smart washer based on smart material in combination with Hall effect sensor [43]. Smart sensorized washers can also integrate RFID tags for wireless communication [40,41,42].

In [5], the authors present a smart washer composed of two annular disks in which the contact surface is reduced to a few spherical points by machining some sections of the disks into convex and concave, respectively. The convex and concave sections are realized with different radii (i.e., convex smaller than concave) to produce a single point of contact with a small surface for each machined section. Each annular disk has a piezoelectric patch bonded to its non-contact surface: one acts as an actuator emitting ultrasonic waves through the interface, while the other functions as a receiver detecting the transmitted signal. The transmitted energy correlates with the actual contact area between the disks, which depends on the bolt preload Indeed, increased load presses together the convex and concave sections, increasing the contact area between the two of them. Similarly, in [37], two smart washers were developed by integrating piezoceramic patches into pre-machined metal rings. The two washers are mounted on opposite sides of a bolt; one acts as an actuator generating stress waves, while the other functions as a sensor detecting their transmission across the bolted interface. A time-reversal approach is applied, where the signal acquired by the sensing washer is temporally inverted and retransmitted. The reversed signal is then received back from the emitter and the temporal and spatial focusing properties are analyzed to monitor the degradation of the bolted connection in time. Further, a smart washer can be realized by coupling a cantilevered plate with a piezoelectric actuator, enabling integrated sensing and actuation for dynamic characterization [7,36,38]. Through this system, it is possible to detect bolt loosening by measuring the variation in the dynamic characteristics of the smart washer system due to the variation in the tightening axial tension. The loosening of the bolted joint is quantitatively assessed through variations in the natural frequency of the smart washer system.

Other possible variations in an active washer are presented in [8], in which a washer can include a transducer manufactured as a segmented series, such as quadrants of piezoelectric crystals recessed into four cavities in the washer body. Alternatively, a ring-shaped transducer or multiple concentric ring-shaped transducers may be recessed, respectively, into single or multiple ring-shaped cavities. Different non-destructive inspection methods can be applied through these smart washers. For instance, a single active washer can operate in pulse-echo mode for one-sided inspection, while a pair positioned on opposite sides of a bolted joint enables through-transmission testing. The resulting data can then support defect detection, structural health assessment, and condition-based maintenance strategies.

FBG sensors also represent a valuable option for smart fastening systems. In [34] the FBG sensor is embedded within a circumferential groove to monitor preload by directly measuring the resulting circumferential strain. This approach effectively converts axial clamping forces into measurable radial strain. Other designs exploit intentionally induced non-uniform strain fields across the FBG to monitor bolt torque, producing a measurable spectral broadening (FWHM) that correlates linearly with the applied torque, thus enabling dual-purpose sensing [39]. Figure 4 illustrates the categories used to classify the sensorized washers.

Figure 4. Classification of sensorized washer into categories based on their sensing principles and functional characteristics.

4. Summary and Discussion

4.1. Smart Fasteners

4.1.1. What to Measure

Standards such as ISO 16047 [93] and ASTM F606 [94] specify methods to verify the torque of bolted joints to ensure structural integrity, prevent loosening, and maintain safety. These standards represent a barrier to the adoption of technologies that monitor quantities other than torque. Since they focus primarily on torque as the key parameter for ensuring structural integrity and safety, alternative approaches—such as monitoring preload, bolt elongation, or strain—face challenges in gaining widespread acceptance. While many solutions based on strain gauges or FBGs enable the measurement of bolt preload, deriving the corresponding torque from axial strain is not a straightforward process. Therefore, in the future, it will be necessary to adapt the current standards or safety procedures to replace the torque verification with some of the techniques currently used for bolt state monitoring.

4.1.2. Strain Sensors

A common method to embed sensors into sensorized fasteners is by using a hollow structure in which the sensor is inserted. However, fasteners with a hole to carry the sensor present reduced load-carrying capacity when compared to whole structures. The reduction in load capacity can be offset by employing higher-strength materials. In [4], a rectangular cross-section sensor body is built, and three strain gauges are applied on three different faces of the sensor body to identify axial forces and bending torques independently. The whole sensor body is inserted in a hollow load-carrying structure in the form of a bolt. Similarly, FBG sensors are integrated into the fastener body through holes drilled in the bolt. Different designs are proposed: in [27,28,44] where an optical fiber containing one or more FBGs is inserted through a centrally drilled fastener and secured to the inner walls using adhesive; alternatively, four FBG sensors were embedded into the fastener body through multiple drilled holes [25] to measure the stress distribution in the fastener body. One FBG is placed centrally and used to measure the axial stress, while the other three are equally spaced at 120° from each other and surrounding the central FBG to measure the shear and bending loads. In [31], an optical fiber with a single FBG, half of which is metalized, is inserted into the cavity of a 3D-printed bolt. This smart fastener monitors the bolt’s load conditions and offers self-compensation for temperature variations, ensuring accurate measurements under varying conditions. In all these cases, a reduction in bolt load capacity is expected. In [33], a reusable bolt tension monitor is introduced, where an FBG sensor is temporarily embedded inside the bolt shaft using fusible paraffin wax or thermoplastic adhesive. This configuration allows precise measurement of the clamping force while maintaining the bolt’s structural integrity and enabling quick sensor installation, removal, and reuse. The thermoplastic-embedded sensor provides stable and linear strain transfer, accurately tracking preload changes and relaxation effects in both steel–steel and steel–composite joints. More recently, Yang et al. [32] embedded FBG sensors directly into stainless-steel M20 bolts to evaluate how the embedding depth and radial position influence sensitivity. The optimal configuration, with the sensor positioned 7 mm from the bolt center in a fully penetrated bolt, achieved the highest sensitivity (0.00296 nm/N·m, R² = 0.9984). Light is also used in [30], where the smart fastener has a control unit and an optical unit in conjunction with additional sensors, such as temperature or moisture sensors. The optical unit includes a light emitter and a detector to send light pulses into a cavity obtained in the fastener body, and to receive back the reflected light. The control unit measures the Time of Flight (ToF) of the reflected light signal and computes the physical deformation of the faster body to obtain the strain. Alternatively, for fiber optic and light emission, a piezoelectric sensor in the form of a cable, covered with a cylinder of urethane resin, was inserted into a hollow bolt structure in [29]. When the bolt deforms due to an external action, the piezoelectric cable also deforms, generating a voltage signal correlated to the applied strain.

4.1.3. Corrosion Sensing

Other than a hollow structure, a dangerous reduction in load-carrying capacity of the fastener can also be caused by external factors, such as corrosion. Conventional Non-Destructive Inspection (NDI) methods, such as infrared or visible light imaging, eddy current, acoustic emission (AE), and X-ray techniques, are effective for detecting surface buildup or material loss but cannot reliably identify hidden or inaccessible corrosion. To address this limitation, ref. [13] proposed a smart aircraft fastener capable of in situ electrochemical corrosion detection. The fastener integrates a corrosion-sensing element and embedded electronics within a standard HI-LOK fastener body (HI-LOK is a lightweight, two-piece aerospace fastener system, comprising a threaded pin and a self-locking breakoff-collar, it is designed to provide precise, uniform preload and high tensile strength in extreme environments). A multifunctional applicability is achieved with the incorporation of a broadband acoustic emission transducer to detect structural integrity related to metal fatigue and cracking. To detect corrosion, a multielement electrochemical sensor is located in a corrosion-sensing environmental chamber inside the fastener. Corrosive electrolytes seep into the environmental chamber through multiple radial capillary tubes in the fastener wall. Consequently, the electrochemical sensor measures the electrolyte’s properties and stores the data in analog memory for later analysis. Similarly, the Smart Aircraft Fastener Evaluation (SAFE) system [21] is being developed to enable early detection of hidden corrosion in aircraft structures. It is based on a smart multielement sensor array composed of three working electrodes and a platinum reference electrode. Each is designed to measure pH, chloride ion concentration, and the free potential of 2024 aluminum, respectively. The sensor outputs a voltage proportional to ion concentration, which is processed and stored by the SAFE electronics.

4.1.4. Smart Materials and Active Fasteners

Hundreds of works focused on the development of fasteners that use Shape Memory Alloys to modify the bolt characteristics in the presence of a phase transition and are not the objective of this review. In [20], the transition from a non-magnetic material to a ferromagnetic material when subjected to strain was detected by the variation in a coil inductance. The contactless technique provides a signal proportional to the damaged state of the bolt; optimal performance can be achieved by machining a stress-concentration groove beneath the bolt head and placing the detection coil either in the groove or within a washer.

Structural characteristics can be modified by Piezoelectric materials [15,23] with different arrangements. For instance, attaching a ring-shaped piezoceramic element beneath the bolt head generates a strain along the bolt’s longitudinal axis.

4.1.5. Piezoelectric Active and Passive Sensors

Piezoelectric elements can be used as passive sensors, i.e., by measuring the charge generated by the load applied on the element, or as vibration generators. In this second case, small piezoceramic elements can be sandwiched between two elements; when the piezoelectric element vibrates, the proof mass exerts an inertial force on the joined structure. In other solutions [48], the piezoceramic element is bonded to the center of the bolt head using epoxy glue and an external pulse generator/receiver, as in ultrasonic non-destructive testing; in this case, the bolt preload is estimated starting from the linear relationship between the change in ToF and the bolt preload.

In general, these systems work at ultrasonic frequencies, and the amount of energy is sufficient to detect structural modifications of the bolt and the neighboring elements.

4.2. Smart Washers

4.2.1. Advantages

The main advantage of smart washers is that they do not require a reduction in the bolt section; on the other hand, their total volume is usually smaller than that of the fastener, and therefore, the miniaturization of the sensing elements may lead to a reduced sensitivity.

4.2.2. Strain Measurements

Strain gauge-based solutions are nowadays commercially available and allow measuring the preload between the bolt head and the joint surface. Miniaturized strain gauges can be positioned in different configurations to measure bending, axial, and radial load, and strain gauges are commonly connected in a Wheatstone bridge configuration to reduce the interfering effects of temperature and to increase the sensitivity.

Strain can be measured by wrapping an FBG sensor around the washer [34]. The axial force exerted on the washer induces strain at the outer diameter, which is directly proportional to the axial strain, as governed by Poisson’s ratio. Typically, this strain ranges from 30% to 40% of the axial strain, resulting in inherently noisier measurements. A different approach based on FBG sensors is presented in [39], where a method for torque monitoring in composite connections is described. The FBG was embedded in the laminate beneath a specially designed washer that imposed a controlled nonuniform strain field on the grating. The spectral broadening of the reflected signal, measured through the full width at half maximum (FWHM), exhibited a monotonic and nearly linear relationship with the applied torque, allowing the sensor to simultaneously function as a conventional strain gauge.

4.2.3. Smart Materials

Similarly to what has been previously shown for fasteners, also in the case of washers, there are several solutions based on shape memory alloys. In other applications, magnetostrictive alloys (Galfenol) combined with bias magnets and a Hall-effect sensor have been employed. The washer enables real-time bolt-tightness monitoring and supports wireless data transmission. When the bolt is tightened, compressive stress on the washer alters Galfenol’s magnetic properties. The Hall-effect sensor detects this change, and with proper magnetic biasing of the washer, bolt load is measured by deriving the relationship between the applied stress and the magnetic variation.

4.3. Data Transmission

The transmission of data is a key point of smart systems. To date, many existing solutions are cabled or embedded with the electronic conditioning system, usually composed of an A/D converter, a microcontroller, and a system for data transmission. Other solutions include RFID technology [1,3,14], in which the RFID tag stores fastener-specific information such as a unique ID, specified torque value, installation date and location, as well as the most recent inspection. Additionally, in the referred documents, a two-state detection is established employing a microswitch or a ground pin technology (as explained in Section 3.3.3) to provide a binary torque condition signal. Also, in this case, the lack of standardization among structural monitoring solutions represents a limit to the diffusion of smart elements.

4.4. Summary

Most proposed approaches are based on piezoceramic (PZT) transducers, with 11 studies employing PZT or ultrasonic techniques—4 applied to fasteners and 7 to washers. FBG-based solutions account for 9 works (7 for fasteners and 2 for washers), while strain-gauge-based approaches are represented by 6 studies (2 on fasteners and 4 on washers). These results can be explained by the fact that both PZT and FBG sensors enable miniaturized and easily embeddable solutions. PZT sensors often allow for a more comprehensive evaluation, monitoring not only the joint condition but also the structural response, whereas FBGs are attractive due to their multiplexing capability and immunity to electromagnetic interference (EMI). Among the important categories, the most cited works are [25,26,34], and [37]. Study [37], with 119 citations, presents a bolt looseness monitoring approach based on the Time Reversal Energy Transfer Matrix technique, which employs piezoceramic transducers embedded in a washer for active sensing. This method is effective, easy to implement on existing bolted joints, and suitable for in situ applications. In [26], a low-cost bolt preload monitoring method using a PZT transducer embedded in the bolt head and fixed with epoxy is proposed, eliminating the need for couplants required in traditional ultrasonic probes. This approach achieves high accuracy and stability, with a relative error of about 1%, making it a promising technique for precise preload measurement. Works [34] (84 citations) and [25] (80 citations) propose FBG-based solutions, implemented, respectively, on a washer [34] and on a bolt [25].

Within the relevant category, the most cited studies are [17,19], which discuss the application of shape memory alloys (SMA) in structural health monitoring (SHM) and industrial contexts, along with [1], a patent describing a load-sensing system with RFID-tagged fasteners, and [5], which presents a smart washer sensor with convex–concave contact surfaces and dual piezoelectric patches that uses the time reversal method to overcome saturation and accurately monitor bolt preload across the full range.

Table 3 reports an analysis of the works classified as important in terms of the following aspects:

Table 3. Summary of the works marked as “important” in Table 1.

Ref.	Support Type	Technology/Sensor	Measurement Type	Measurement Evaluation		Integration Level	Cost/Scalability		Notes
[3]	Screw or BScolt	RFID tag and two-state Microswitch	Torque state	NA (Provides binary compliance check)		Sensor body, RFID tag, and antenna are inserted in the fastener head	Low cost and high scalability		Easy installation	Easy torque check but Limited to status check
[4]	Fastener (screw or bolt)	Strain gauges and temperature sensor	Pretension forces and operating loads. (Capable of identifying axial forces and bending torques independently. measures combined force and torque signals in individual spatial directions)	NA (the achieved prestressing force after forming was calculated with a range of 1900 N to 2900 N ± 250 N)		Integrated into the metallic fastener structure during manufacture	Possibility of cost-effective production.	High scalability	The system measures multi-axis loads, providing reliable insights into the manufacturing process and the condition of machines and tools, as well as enabling compliance evaluation. The load-bearing capacity of the structure is reduced by about 20% due to the required central hole.
[13]	Fastener (screw or bolt)	Corrosion sensing array based on a reference electrode and at least one sensing electrode	Corrosion	NA		Integrated within said fastener body. The sensors are exposed to an electrolyte that enters through a capillary tube in the bolt.	NA		The system includes an analogue storage array and a clock for recording corrosion signals and their respective times, allowing access to data externally of said fastener.
[15]	Fastener	Piezoelectric element and control unit	Structural response based on vibrational control	NA		Embedded	NA
[21]	Bolt	Electrochemical microsensor array and a time-of-wetness sensor	Corrosion-related symptoms: pH, temperature, chloride, free potential, time-of-wetness	NA		Integrated in the bolt	NA		Early warning detection of corrosion-related symptoms in hidden and inaccessible locations. Provides a condition-based corrosion detection system.
[22]	Fastener	Ultrasonic load measurement transducer.	Load in the thread-forming fastener	NA		Coupled externally to the fastener head	NA		Used to measure and control the load during tightening and to inspect the load after assembly
[23]	Fastener	Piezoelectric element and control unit	Structural response detection	NA		Embedded into the bolt	NA
[24]	ISO M6 screw	Resistive strain gauges	Torsional strain in the screw head to compute force and torsional moment	NA		Integrated: sensor foil is interposed/embedded between the printed layers of the screw head/spacer disk	Low cost (if 3D printer available)		Enables the direct integration of sensing elements into additively manufactured parts, allowing material characterization and determination of the screw’s breaking limit. However, for some materials such as ABS Flex, poor adhesion between the material and the sensor foil can reduce mechanical durability, sometimes causing failure above the foil. Manual misalignment of the sensor foil is also a noticeable issue.
[25]	Bolt	FBG sensor	Axial and shear forces	axial force monitoring with a measurement error ranging from 2.1% to 3.6%. The shear force monitoring error ranges from 2.4% to 4.4%		Embedded. The four packaged FBG sensors are longitudinally embedded in pre-machined through holes of the smart bolt using epoxy	NA		Can simultaneously monitor both axial preload and shear loads. FBG sensors offer high accuracy and immunity to electromagnetic interference.
[26]	Bolt	Piezoceramic transducer	Blot preload force	Relative error = 1%		The PZT Transducer is embedded in the bolt head at and fixed with epoxy.	Low cost		Eliminates the need for couplants used in traditional ultrasonic probes, resulting in more precise measurements. It offers high accuracy and stability in preload detection, providing a promising method for measuring bolt preload.
[27]	Fastener	FBG sensor	Strain and temperature	NA		Embedded. The optical fiber is secured within the central opening of the fastener and bonded to with adhesive. It is	NA		Enables periodic or continuous monitoring, overcoming previous limitations by delivering consistent, hysteresis-free data when unloaded. It also allows for remote monitoring.
[29]	Bolt	PZT sensor with piezoceramic transducer	Structural vibration response	NA		Embedded	Low cost		It is convenient to install or remove compared to conventional patch actuators that require epoxy.
[31]	Bolt	FBG sensor	Preload force	NA		Embedded	NA		Temperature self-compensation, which significantly improves force measurement accuracy. Enables in situ health monitoring of fasteners in harsh conditions. Provides high sensitivity and EMI
[32]	Bolt	FBG sensor	Torque	Maximum sensitivity: 0.00298 nm/N-m		Embedded	NA		A linear relationship exists between the FBG wavelength and applied torque, enabling precise measurement and long-term monitoring and control of bolt preload.
[33]	Bolt shaft	FBG sensor	Preload and retained clamping force	NA		Embedded in the bolt shaft using temporary adhesive	No high costs of permanently embedded FBG through reusability and shows strong potential for easy integration into conventional manufacturing processes.		It allows quick assembly, disassembly, and reassembly of the sensor, providing precise monitoring without affecting the bolt’s intrinsic properties thanks to its compact size. The system is also immune to electromagnetic interference.
[7]	Washer	Piezoelectric patches	Bolt loosening monitoring and bolt preload	NA		Bonded on the non-contact surface	NA		Active sensing method: one patch serves as an actuator generating ultrasonic waves, while the other functions as a sensor detecting the response. The saturation issue is resolved using convex and concave contact surfaces.
[8]	Washer	Piezoelectric transducer	Flaw length progression measurement	NA		Applied on the surface or embedded in a cavity	NA		Enable structural inspection in the area immediately surrounding the bolted joint. Configurations with segmented or concentric elements allow imaging and measurement of flaw length progression.
[12]	Washer	Strain gauges and temperature sensor for compensation	Preload estimation based on compression deformation	NA		Embedded into one or more holes.	NA		High Output Signal Sensitivity (thanks to the series connection of strain gauges and optimal positioning). Detects compression deformation, which is more uniform than surface deformation, improving reliability. Provides real-time preload measurement during tightening.
[34]	Washer	FBG sensors	Bolt preload force	Max: sensitivity = 0.0151 nm/kN.	Embedded	NA	The method measures preload through changes in the washer’s outer diameter and circumference, offering high sensitivity and EMI immunity. As a load-path element, the washer enables simple and reliable long-term monitoring.		The method measures preload through changes in the washer’s outer diameter and circumference, offering high sensitivity and EMI immunity. As a load-path element, the washer enables simple and reliable long-term monitoring.
[36]	Cantilever washer	Bolt loosening detection based on natural frequency shift	NA	NA		Bonded to the washer	NA		The system is designed for remote, automated monitoring of bolt loosening. Using an n Adaptive Observer with time-varying gain enables identification of the natural frequency across the full axial tension range.
[37]	Washer	Piezoceramic transducer used in active sensing method	Bolt looseness monitoring base on rime reversal and energy transfer matrix	NA		Embedded into the washer	NA		Easy to apply to existing bolted joints. The Energy Transfer Matrix technique based on Time Reversal is effective and suitable for in situ application for monitoring bolted connections.
[38]	Washer	Piezoelectric sensor	Bolt loosening detection vase on natural frequency variation due to bolt tightenind and axial tension	NA		Bonded to the washer	NA		Further investigation is needed to improve repeatability and data quality under varying ambient temperatures.
[39]	Washer	FBG sensor	Preload torque and uniaxial strain	NA		Embedded into the washer	NA		Create a dual-purpose sensor capable of measuring both preload torque (via bandwidth) and uniaxial deformation (via center wavelength).
[40]	Washer	Sensor coupled with wireless data retrieval module	Strain, stress, temperature, vibration, loads, torque	NA		Embedded into the washer	NA		Enables wireless retrieval of data from integrated sensors. Allows magnetic or radio frequency coupling for contactless energy and data transfer, facilitating measurement and monitoring.
[41]	Washer	Variable resistance strain gauges and RFID tag	Preload	NA		Carried by the washer	NA		Enables wireless access to washer-specific data (e.g., ID, installation, and inspection dates) and can control the torque applied by tools linked to the RFID reader.
[43]	Washer	Magnetostrictive Galfenol (iron and gallium alloy), used with biasing magnets and Hall Effect sensors.	Bolt loads and stresses in bolts	NA		Integrated. The washers are made of Galfenol and are integrated into the joint. The entire system uses the Galfenol washer as a passive sensor, which interacts with external magnets and Hall sensors.	Cost efficient		Accurate, sensitive, and cost-efficient alternative. Enables real-time monitoring of stresses with minimal signal conditioning. Eliminates the need for routine bolt retightening by providing current loads instead.

• Support type
• Technology/sensor
• Measurement type
• Measurement evaluation
• Integration level
• Cost/scalability

It is interesting to note that most studies propose embedded solutions, a choice that undoubtedly enhances system robustness against external factors. However, cost and scalability evaluations are discussed in only a few works, highlighting a gap between academic research and the practical implementation of the proposed solutions. Similarly, the assessment of measurement accuracy and output evaluation is often poorly reported, suggesting the need for more comprehensive validation in future studies.

The review includes both conference or peer-reviewed journal articles and patents describing comparable technological solutions. The consistency of information across independent academic and industrial sources supports a moderate-to-high overall confidence in the synthesized evidence.

5. Conclusions

In this review, 155 documents were retrieved from electronic databases, both papers and patents. After the selection procedure, 39 documents were analyzed and classified to provide a detailed overview of the existing technologies related to smart fasteners and smart washers. The first classification was performed concerning the type of technology characterizing each fastener and washer. The identified categories for smart fasteners are smart material (5 documents), sensorized (17 documents), RFID (1 document), and sensorized plus RFID (2 documents); whereas smart washers belong to sensorized (10 documents), sensorized plus RFID (3 documents), and sensorized plus smart material (1 document) categories.

Results of the analysis showed that smart fasteners and washers are mainly based on piezoelectric elements and FBG sensors, even if some specific variant exists that include an electrochemical multielement array to detect corrosion (in the case of smart fasteners) or strain gauges.

Both devices can take advantage of RFID to enable wireless communication capabilities. Finally, the main objective of the smart elements resulted to be the identification of preload and loads acting on the joints and can be used for structural health monitoring to improve safety in critical applications.

Despite significant progress, this review highlights several technical challenges that currently limit the industrial adoption of smart fasteners. A major issue remains with the need to modify the bolt geometry to embed sensors, which can compromise load capacity and fatigue resistance. FBG sensors require complex temperature compensation, embedded strain gauges face calibration and long-term durability challenges, and solutions based on piezoelectric cables or 3D-printed polymers often suffer from reduced sensitivity or signal instability under high loads. Approaches relying on optical units or complex electronics are further constrained by high costs and integration difficulties. Moreover, few works discuss the scalability, cost, and measurement performance of the proposed approach. Despite many sensing solutions having proven feasible, further efforts are needed to develop methods that ensure industrial scalability at low cost and provide reliable long-term monitoring.

The search strategy was limited to selected academic databases and patent repositories, and relevant industrial documents not publicly indexed may have been missed.