Search Results (18)

In advanced structural applications—aerospace and automotive—fiber-laminated composite (FRP) materials are increasingly used for their superior strength-to-weight ratios, making the reliability of their mechanical joints a critical concern. Mechanically fastened joints play a major role in ensuring the structural stability of FRP Composite structures; however, accurately predicting their failure behavior remains a major challenge due to the anisotropic and heterogeneous nature of composite materials. This paper presents a progressive damage modeling approach to investigate the failure modes and joint strength of mechanically fastened carbon fiber-laminated (CFRP) composite joints. A 3D constitutive model based on continuum damage mechanics was developed and implemented within a three-dimensional finite element framework. The joint model comprises a composite plate, a steel plate, a steel washer, and steel bolts, capturing realistic assembly behavior. Both single- and double-lap joint configurations, featuring single and double bolts, were analyzed under tensile loading. The influence of clamping force on joint strength was also investigated. Model predictions were validated against existing experimental results, showing a good correlation. It was observed that double-lap joints exhibit nearly twice the strength of single-lap joints and can retain up to 85% of the strength of a plate with a hole. Furthermore, double-lap configurations support higher clamping forces, enhancing frictional resistance at the interface and load transfer efficiency. However, the clamping force must be optimized, as excessive values can induce premature damage in the composite before external loading. The stiffness of double-bolt double-lap (3DD) joints was found to be approximately three times that of single-bolt single-lap (3DS) joints, primarily due to reduced rotational flexibility. These findings provide useful insights into the design and optimization of composite bolted joints under tensile loading. Full article

This study investigates the influence of drilling-induced delamination damage on the compressive mechanical behavior of open-hole carbon fiber-reinforced composite laminates and explores the failure mechanisms under dual-defect coupling effects. Specimens with circular delamination defects of varying sizes were fabricated by embedding polytetrafluoroethylene (PTFE) films during the layup process. Ultrasonic C-scan and digital image correlation (DIC) techniques were used to monitor delamination propagation and deformation behavior. A cohesive zone-based numerical model was developed and validated against experimental results to reveal the three-stage failure process in single-defect cases. The validated model was then used to analyze the coupling effects of dual defects (same side and opposite side). The results show that dual delamination defects significantly reduce the compressive load-bearing capacity of open-hole composite laminates. Specifically, same-side defects exhibit a failure mode similar to single-defect structures, while opposite-side defects display a unique failure behavior characterized by dual-crack propagation, further reducing the compressive load-bearing capacity. Full article

This study aimed to improve the luminous characteristics of laminated OLEDs using the electron injection layer as an adhesive layer by introducing several materials to overcome the limitations of previous studies. We improved the adhesiveness and electron injection ability at the same time by mixing polyethylene glycol (PEG) and D-sorbitol with polyethyleneimine (PEI). In addition, we introduced TPBi and PBD, which are electron-transport and hole-blocking (ET/HB) materials, to improve the electron-transport capability of laminated OLEDs. We fabricated a single-carrier device to analyze the balance of movement of holes and electrons, and a single-substrate OLED device to evaluate the luminance performance by comparing ET/HB materials. Finally, the OLED devices laminating the two substrates were manufactured by applying the optimal conditions found in the above experiments. The results showed that there was a serious imbalance between the electron current and the hole current, which was improved by introducing TPBi and PBD as ET/HB materials. In the case of the laminated OLED device, the device applying D-sorbitol to the adhesive layer showed more stable performance than the device using PEG. As a result, the laminated OLED device applying D-sorbitol showed performance similar to that of the single-substrate OLED, and operated stably even at a high voltage. Full article

Friction-type bolted joints are widely used in both the civil and aerospace industries. Uncontrolled excessive bolt clamping force can cause damage to the laminated fiber-reinforced polymeric (FRP) composite through the thickness and damage the joint before applying the service loads. The effect of the friction coefficient (between 0 and 0.3), bolt clearance, joint type, and other parameters on failure modes and the maximum bolt clamping force of the carbon FRP lapped joint is studied. A three-dimensional finite element (FE) model consisting of a bolt, a washer, a laminate FRP composite plate, and steel plates was developed for the simulation of the double- (3DD) and single (3DS)-lapped bolted joint. The FE model was validated by using experimental results and was able to predict the experimental results by a difference of between 2.2 and 6.7%. The joint capacity of the clamping force was found to be greatly increased by adopting the double lap technique, which involves placing an FRP composite plate between two steel plates. Also, it was recommended to use an internal washer diameter less than or equal to the FRP composite plate hole diameter since a larger washer clearance can produce higher contact pressure and reduce the resistance by 22%. In addition, reducing the bolt head diameter can lead to a 65% reduction in the 3DS joint clamping strength. Full article

A finite element analysis (FEA) was conducted to examine the behaviour of single-lap quasi-isotropic (QI) and cross-ply (CP) hybrid bolted/bonded (HBB) configurations subjected to tensile shear loading. Several critical design factors influencing the composite joint strength, failure conditions, and load-sharing mechanisms that would optimise the joining performance were assessed. The study of the stress concentration around the holes and along the adhesive layer highlights the fact that the HBB joints benefit from significantly lower stresses compared to only bolted joints, especially for CP configurations. The simulation results confirmed the redundancy of the middle bolt in a three-bolt HBB joint. The stiffness and plastic behaviour of the adhesive were found to be important factors that define the transition of the behaviour of the joint from a bolted type, where load sharing is predominant, to a bonded joint. The load-sharing potential, known as an indicator of the joint’s performance, is improved by reducing the overlap length, using a low-stiffness, high-plasticity adhesive, and using thicker laminates in the QI layup configuration. Enhancing both the ratio of the edge distance to the hole diameter and washer size proves advantageous in reducing stresses within the adhesive layer, thereby improving the joint strength. Full article

The elongation of the bolt hole is an important parameter for assessing the failure of bolted joints. However, direct experimental measurement using strain gauges and extensometers is difficult. This article shows that digital image correlation (DIC) can overcome the difficulties and provide important indications of the failure mechanisms of bolted joints. Hole elongation was measured using DIC in the following carbon/epoxy composite configurations: standard open-hole tensile (OHT) and filled-hole tensile (FHT), single-lap shear only-bolted (OB), and single-lap shear hybrid-bolted/bonded (HBB) joints. For each configuration, the hole-elongation changes were tracked for cross-ply (CP) and quasi-isotropic (QI) stacking sequences with two thicknesses. In the tensile load direction for OHT and FHT cases, CP showed a greater hole elongation than QI. However, the opposite trend was observed in the transverse direction. In OB joints, bypass loads contributed more to the hole elongation than bearing action. In HBB joints, it has been observed that the adhesive significantly reduces hole elongation, particularly for CP configurations. Moreover, it was found that in HBB joints, hole elongation was independent of laminate lay-up, while it was very determinative in OB joints. Full article

This study examines the behavior of hybrid bolted/bonded (HBB) joints loaded in tensile shear comprising plain weave carbon/epoxy laminates in quasi-isotropic (QI) and cross-ply (CP) layups. It proposes a combined approach of 3D digital image correlation and finite element analysis (FEA) to assess their behavior. To apply the FEA simulation accurately, a single layer of plain fabric was replaced with ${[0/90]}_s$ lamination. Experimental standard open-hole tension test results, as well as only bolted (OB) and HBB, along with FEA predictions, confirmed the accuracy of the substitution method. The FEA, calibrated by experimental results, provides insight into the distinctive characteristics of HBB joints in comparison with bonded and bolted joints. Critical considerations include material properties, damage modeling, adhesive characteristics, and mass scaling. The FEA results underscored the pivotal role of adhesives in HBB joints, rendering them akin solely to bonded configurations. HBB joints retain their geometry better than OB joints with considerably less out-of-plane displacement, following a sinusoidal trend. Moreover, the overall behavior of the two layups demonstrates that CP benefits from having higher strength than QI, especially at the critical hole located closer to the grip side. Full article

The growth of the Urban Air Mobility (UAM) industry emphasizes the need for considerable study into assembly procedures and dependability to guarantee its effective integration into air transport networks. In this context, this study seeks to evaluate the mechanical characteristics of bolted joint Carbon Fiber Reinforced Plastic (CFRP), with a particular emphasis on bearing strength. By altering the w/D (specimen width to hole diameter) and e/D (distance between hole center and specimen end to hole diameter) ratios, the study investigates how edge and end distances affect material performance. The study discovered a shift from tension to bearing failure at w/D ratios of 4.0, with maximum bearing strength decreases of 90.50% and 69.96% compared to full bearing failure. Similarly, for e/D ratios of 1.5, 2.0, and 3.0, transitioning from shear to bearing failure at 2.0 resulted in maximum bearing strength losses of 94.90% and 75.96%, respectively. Maintaining a w/D ratio of at least 6.0 and an e/D ratio of at least 3.0 is critical for maintaining maximum performance and stability in CFRP structure design. Full article

Fiber Reinforced Polymer (FRP) laminates have been widely used in engineering applications in recent decades. This is mainly due to their superior mechanical properties compared to single-phase materials. High strength-to-weight ratio, high stiffness, and excellent corrosion and fatigue resistance are some of the attractive properties of these materials. In large structures, drilling composite panels is a typical operation to assemble different parts with mechanical fasteners. This operation severely threatens the quality of the holes and, therefore, the joint strength. This study aims to study the wear evolution of two drill bits manufactured with different coatings processes (chemical vapor deposition and physical vapor deposition) and their influence on the quality of the holes. A carbon/glass fiber sandwich structure was selected as the workpiece, and a high-speed machine center was used to drill 1403 holes per tool in the laminates. The wear analysis of the tool was characterized in terms of flank wear and crater wear. For the delamination analysis caused by drilling, two types of delamination are identified (type I and II), and their values were quantified through the equivalent delamination factor (F_ed). The results showed that, in general, the process used to apply the coating to the tool influences the wear mode and the delamination damage. The first tool, diamond coated with Chemical Vapor Deposition (CVD), showed more severe crater wear in the flank face and coating loss at the end of the cutting edges. However, with a Physical Vapor Deposition (PVD) coating process, the second tool presented flank wear more controlled but a more severe coating loss and edge rounding near the tip, producing further delamination. Using a supporting plate showed a reduction of delamination type I but not for delamination type II, which is related to edge rounding. Full article

Due to the impacts on the ecological environment, production safety and the economic benefits of large open-pit water deposits, there is no longer a single drainage method within the pit that can be used to meet the need for further deep mining. Therefore, curtain grouting technology is proposed for use in the prevention and control of mine water. The flow control equation of slurry in vertical fissures under the influence of self-weight stress was derived, and a three-dimensional visualized laminated jointed rock grouting model was developed independently to study the slurry transport mechanism, diffusion pattern, and sealing mechanism, which verified the correctness of the control equation. Field trials of industrial curtain grouting were also carried out at the mines, and the effectiveness of curtain water blockage was tested using visual analysis, inspection hole detection, and geophysical exploration methods. The results showed that after grouting the curtain, water-conducting cracks in the formation can be filled well and sealed, and the curtain has a good water-blocking effect. A complete set of diversified value-taking grouting systems is obtained, which can ensure the development of a green, safe and sustainable mine in the future. Full article

In this paper, a novel wideband transition from a laminated waveguide (LWG) to an air-filled rectangular waveguide (RWG) is proposed for millimeter-wave integration solutions based on multilayer low-temperature co-fired ceramic (LTCC) technology. The integrated transition cavity is divided into several resonators by introducing five grounded via holes. Due to the magnetic wall existing in the symmetry plane, the equivalent circuit of the proposed transition can be simplified as a three-pole filter model to explain the working mechanism with wideband performance. A W-band integrated LWG-to-RWG transition is designed as an example using LTCC technology. Two back-to-back prototypes with different lengths are fabricated and measured. A measured 25.7% bandwidth from 76 GHz to 101 GHz can be achieved for return loss better than 14 dB. The average insertion loss of a single transition is about 0.5 dB. The compact structure and wideband performance give it potential in high-density millimeter-wave and terahertz packaging. Full article

Thin-ply composite failure modes also significantly differ from conventional ply composite failure modes, with the final failure mechanism switching from irregular progressive failure to direct fracture characterized by a uniform fracture with the reduction of the ply thickness. When open holes and bolt joints are involved, thin-ply-laminated composites exhibit more complex stress states, damage evolution, and failure modes. Compared to the experimental study of thin-ply-laminated composite-bolted joints, there are few reports about numerical analysis. In order to understand the damage evolution and failure mechanism of thin-ply-laminated composites jointed by single-lap bolt, a progressive damage model based on three-dimensional (3D) LaRC failure criterion combined with cohesive element is constructed. Through an energy-based damage evolution method, this model can capture some significant mechanical characteristics in thin-ply-laminated structures, such as the in situ effect, delamination inhibition, and fiber compressive kinking failure. The comparisons between the numerical predictions and experimental observations are made to verify the accuracy of the proposed model. It is found that the predicted stress-displacement curves, failure modes, damage morphologies, etc., are consistent with the experimental results, indicating that the presented progressive damage analysis method displays excellent accuracy. The predicted stress at the onset of delamination is 50% higher than that of the conventional thick materials, which is also consistent with experimental results. Moreover, the numerical model provides evidence that the microstructure of thin-ply-laminated composite performs better in uniformity, which is more conducive to inhibiting the intra-layer damage and the expansion of delamination damage between layers. This study on the damage inhibition mechanism of thin-ply provides a potential analytical tool for evaluating damage tolerance and bearing capabilities in thin-ply-laminated composite-bolted joints. Full article

Based on the principle of underwater transducers, an ultrasonic four-laminated transducer with a frequency of 1 MHz was proposed to solve the problem of large energy attenuation when ultrasonic waves propagate in viscoelastic media. First, this study targeted solid rocket propellant as the research object, and the energy attenuation characteristics of ultrasonic waves propagating in viscoelastic media were analyzed through the derivation of the wave equation. Second, the structure of a four-laminated transducer with a frequency of 1 MHz was designed, and the resonance frequency was obtained by a graphical method. The sound field simulation and experimental results showed that the gain of the four-laminated transducer was 15 dB higher than that of the single-wafer transducer. An ultrasonic feature scanning system was built to complete the qualitative and quantitative detection of the smallest artificial hole ($\varphi$2 mm × 10 mm). Finally, two different natural defects were scanned, and the results were compared with those obtained using an industrial computed tomography detection system. The results showed that the ultrasonic method was more accurate in characterizing two natural defects. The primary cause was that the industrial CT was not sensitive to defects parallel to the incident direction of the ray. Therefore, this study not only achieved the qualitative and quantitative nondestructive testing of solid rocket propellants, but also provides an important reference for other viscoelastic components. Full article

The diffusion bonding titanium alloy laminates with preset unbonded area (DBTALPUA) compared with other titanium alloy structural forms has good damage tolerance performance and designability. It is important to fast get the damage estimation of the DBTALPUA with crack. The stress intensity factor (SIF) of the crack is an effective indicator to give the damage estimation. In order to get the SIF fast, this paper proposed an analytical model to calculate SIF for single hole-edge crack in DBTALPUA with hole under tension loading. Comparison of the results obtained through this analytical model and numerical simulation illustrated that the analytical model can rapidly predict the SIF with fine precision. Full article

The aim of the present paper is to study the mechanical performance evolution of pinned hybrid glass-flax composite laminates under environment aging conditions. Hybrid glass-flax fibers/epoxy pinned laminates were exposed to salt-spray fog environmental conditions up to 60 days. With the purpose of assessing the relationship between mechanical performances and failure mechanisms at increasing aging time, single lap joints at varying joint geometry (i.e., hole diameter D and hole distance E from free edge) were characterized after 0 days (i.e., unaged samples), 30 days, and 60 days of salt-fog exposition. Based on this approach, the property–structure relationship of the composite laminates was assessed on these critical environmental conditions. In particular, a reduction of failure strength for long-aging-time-aged samples was observed in the range 20–30% compared to unaged one. Due to the natural fiber degradation in a salt-fog environment, premature catastrophic fractures mode due to shear-out and net-tension were found, related to reduced joint fracture strength. This behavior identifies that this type of joint requires a careful design in order to guarantee an effective mechanical stability of the composite hybrid joint under long-term operating conditions in an aggressive environment. Full article