# Thermoplastic Composites and Their Promising Applications in Joining and Repair Composites Structures: A Review

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## Abstract

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## 1. Introduction

## 2. Strength Testing in FRP Joints

#### 2.1. Strength Testing

#### 2.2. Toughness Testing

#### 2.2.1. Pure-Mode Loading

- Mode I (Figure 3a): The load is applied normally to the bonded joint interface and the rate at which the joint opens can be monitored and measured in the double cantilever beam tests (DCB) (Figure 4). Specimens contain a pre-crack at one of its extremities and the cracked faces are pulled apart with the aid of either piano hinges or loading blocks attached to the specimen on the cracked end. A thin non-stick film is placed between the central plies during curing to introduce the pre-crack, and the sides of the specimen are marked with a millimeter scale in order to quantitatively track the crack growth during testing. The specimen is then loaded and the load-displacement data is recorded and used for computation of the critical strain energy release rate G
_{Ic}. This test can be performed under quasi-static and cyclic loading conditions, and is the most widely used method for measuring Mode I fracture toughness of unidirectional composites. Furthermore, fabrication and testing of DCB specimens is straightforward and relatively inexpensive, which can be tested by using standard mechanical test frames [108]; - Mode II (Figure 3b): The load creates a sliding shear mode in a direction perpendicular to the leading edge of the crack and the joint will exhibit the highest resistance to fracture. The most suitable method to evaluate this failure mode is the End Notched Flexure specimen (ENF) (Figure 5), which consists in a three-point bending test in a pre-cracked specimen. The resulting load creates an almost pure shear stress state at the crack tip, provided that the specimen is designed so that the adherends deform elastically, which provides shear characterization. The simplicity of this specimen is one of the main reasons to be widely used in mode II fracture characterization [109];

#### 2.2.2. Mixed-Mode I + II Loading

## 3. Comparison of TPC and TSC Fracture Toughness

#### 3.1. Pure Modes

_{Ic}and G

_{IIc}, respectively, for several composite systems picked from the literature review with thermoset and thermoplastic matrices. In order to discuss the relationship between the fracture toughness and the materials’ strength, the elastic modulus in longitudinal direction is plotted with the G

_{Ic}and G

_{IIc}values. It can be observed that most of the TPC taken from the literature reveal much higher fracture energies in mode I and mode II when compared to the TSC counterparts, which constitutes a remarkable advantage of the former ones. This is an expected result, since interlaminar toughness of the TPC is known to be higher than the thermoset ones. Although the distribution of the longitudinal modulus reveals lower strength when E-glass fibers are used as reinforcement material, they should not be disregarded for structural applications—their lower cost make them very attractive, and the fracture toughness plays an important role in many FRP applications when impact loading is susceptible to occur. In fact, it is important to note that the modulus and strength of TPC are mainly controlled by the fiber properties, fiber weight fraction and fiber orientation when continuous fiber reinforcement lay-up is used, while the fracture toughness is primarily governed by the matrix properties [25].

#### 3.2. Mixed Modes

_{I}–G

_{II}space. According with the collected data, the thermoplastic based composites presents higher toughness when compared with the thermoset ones, mainly when mode I loading is predominant in the mixed mode ratio. Since the mode I fracture toughness is generally lower than corresponding mode II in most of the thermoset based composite materials, this aspect presents a notable advantage in this context. Therefore, thermoplastic polymers matrices reveals to be promising candidates to replace thermoset based ones in many applications. For example, structural components that are eventually exposed to low velocity impact loading will improve their interlaminar resistance if TPC is employed instead of thermoset based ones. The linear failure criteria can satisfy most of the existing cases displayed in Figure 7. However, some cases reveal an increase of the mode I component with a certain amount of G

_{II}, which decreases after to zero when the applied G

_{II}equals G

_{IIc}. This behavior is known as “overshoot” phenomenon, and has not been observed for mixed-mode fracture for isotropic materials. Only a few of failure criteria can capture this behavior [131].

## 4. TPC Joining and Repair Techniques

#### 4.1. Adhesive Bonded Joints

#### Numerical Modelling of Adhesive Joints

#### 4.2. Fusion Bonded Joints

_{g}for amorphous polymers, and the melting temperature T

_{m}for semi-crystalline ones, while keeping the maximum temperature below the degradation point of the polymer. Semi-crystalline thermoplastic polymers need higher heating energy to flow because they have orderly molecular arrangements. The heat affected zone strongly affects the quality of the welded joint. In general, a plastic material should not be welded at a temperature above than 75% of its glass transition point T

_{g}for amorphous polymers, and 75% of its melting point T

_{m}for semi-crystalline ones [89]. Therefore, the study of temperature distribution is of significant importance to optimize any fusion bonding process. Thermocouples placed between the welding interface may be used to monitor the temperature changes during the experimental trials.

^{®}thermoplastic composites was recently investigated by [146], revealing an increase of fatigue life by 10–12% when compared to adhesively bonded joints. Fatigue performance on lap shear unidirectional carbon fiber reinforced PEI and PEKK, and weave glass fiber reinforced PEI resistance welded joints was performed by Dubé et al. [80]. Linear S-N curves were observed for all three materials, but infinite life occurred for different percentages of static lap shear strengths for the different TPC materials. Therefore, according with O’Shaughnessey et al. [48], the selection of a fusion bonding process for a particular application should be determined by factors such as the material type, weld size and geometry. However, Davies et al. [66] found that resistance heating provides significant stronger repairs comparatively with ultrasonic heating when using a PEI heating element. Therefore, this aspect is not clear.

#### 4.2.1. Ultrasonic Heating

#### 4.2.2. Induction Heating

#### 4.2.3. Resistance Heating

_{g}), respectively. The moisture did not affect the lap shear strength, thanks to the low moisture uptake of TPC materials.

_{Ic}and G

_{IIc}) which measure the fracture toughness of the material. In addition, mixed-mode fracture tests must also be performed in order to evaluate the energetic fracture criterion characterizing the connection. This is an important quality indicator of any adhesive or fusion bonded joint as it quantifies its damage resistance, which is a very important property of FRPs applied in composite structures.

_{Ic}), which is associated with adhesive failure due to the low energy surface observed in these polymers. On the other hand, fusion bonding techniques revealed much higher values, even exceeding the critical energy release rate of the material in bulk state when resistance heating is used to perform the weld. Contrary with the previous comparison for lap shear strength results (Figure 9), fracture toughness measured by DCB tests points the resistance welding method as the most reliable fusion bonding technique for PEEK based TPC. This result suggests that lap shear test may not give the full picture of the strength of fusion bonded joints.

#### 4.2.4. Non-Destructive Evaluation Techniques

#### 4.2.5. Numerical Modelling of Fusion Bonded Joints

_{p}is the specific heat, and k

_{i}and $\frac{\partial T}{\partial i}$ (i = x,y,z) are the thermal conductivities and the temperature gradients along the three cartesian directions, which represents the balance of flux of energy through the control volume. The right-hand side of the equation represents the increase of the internal energy per unit of time. The material parameters can be estimated by applying the rule of mixtures be assumed constant within a layer [61]. However, some studies have reported the dependency of electric and thermal materials properties with the temperature, such as electrical conductivity and heat capacity, to have a significant importance in the prediction of the temperature field along the welding interface welding processes [48]. Suitable boundary conditions and initial conditions must be defined for each specific case. The volumetric heat generation term $\dot{Q}$, defined according each fusion bonding technique to account for the rate of heat generation or absorption (Table 3), is defined according with three main mechanism, as follows [61]:

^{®,}have been successfully used to perform this task [25,89]. Comsol Multiphysics

^{®}finite element software is well known for its multiphysics capabilities and have been successfully used to model RW, IW, and UW processes [48]. Since these models are time dependent, they are particularly useful to determine the time to melt the polymer and the time to cause thermal degradation. A micro-mechanics model to investigate the thermal de-consolidation and re-consolidation phenomena was established by Ye et al. [29], allowing one to determine the applied critical pressure at which none of these phenomena takes place.

_{ic}at any time during the pressure application P, as follows:

_{c}is an empirical roughness parameter (see [38] for more details), t

_{p}is the duration of pressure application, and µ is the temperature dependent fiber matrix viscosity given by an Arrhenius type correlation:

_{h}, which depends on the instantaneous temperature T and the reptation time t

_{r}, which is the necessary time for the polymer chains to diffuse across the interface (i.e., time to get the maximum bond strength). Accordingly, the degree of healing D

_{h}at a time instant t can be defined as follows [34,36,165]:

_{r}described by an Arrhenius type law:

_{r}and B

_{r}are experimentally determined parameters (see [38] for more details). Since the chains have lower mobility with longer chain lengths, the reptation time t

_{r}increases with the molecular weight. This way, a degree of bonding D

_{b}can be defined as function of the degree of intimate contact and the degree of healing, as follows at a given time t [36]:

_{r}and the development of the instantaneous mechanical strength σ can be described as follows:

_{c}at a given time as follows [165]:

## 5. Conclusions

- It is not clear if different fusion bonding techniques provide different weld strengths for the same substrate. Some authors observed similar weld strength for a TPC laminate using different fusion bonding techniques (RW, IW, and UW), claiming that the selection of a fusion bonding process for a particular application should be determined by other factors such as the material type, weld size and geometry. However, some published works reported RW providing stronger repairs than UW;
- It is not clear if the conductive implant remaining inside the part affects negatively the mechanical performance of RW and IW welded joints. This is an importance aspect since the presence of the conductive implant may be the useful for further reprocessing operations; and
- Is it not clear which one of the three heating mechanisms in IW is the dominant one: Joule heating by eddy currents traveling along the conductive fibers, Joule heating by contacting fibers at the junctions (i.e., where fibers from adjacent plies overlap), or heating by dielectric hysteresis when the fibers are separated by a small gap of dielectric polymer matrix. A deeper insight on IW modelling may be required to clarify this aspect.

_{ic}for RW welded joints. Moreover, the influence of environmental conditions on static and fatigue behavior should also be addressed by submitting specimens to adverse and representative temperature and moisture conditions, to which TPC can be submitted in structural applications. However, it should be noted that not one joining technology can be applicable to all cases. In fact, all joining methods present advantages and drawbacks, and they may be more or less suitable to a particular application depending on its specific requirements.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Fusion bonding techniques categorized by the technology used to generate heat. The main fusion bonding techniques are highlighted in yellow.

**Figure 2.**(

**a**)—Main thermoplastic composite materials used in fusion bonding research techniques: Graphite fiber/PEEK [58,59,60,61,62,63,64]; Carbon fiber/PEEK [23,47,60,65,66,67,68,69,70,71,72,73,74,75,76,77] Carbon fiber/PEI [69,78,79,80,81,82]; Glass fiber/PP [34,40,83,84,85,86]; Carbon fiber/PPS [29,32,33,48,87]; Carbon fiber/PP [36,70,88]; PE/HDPE [42,89,90]; Glass/PEI [31,80]; Glass fiber/PA [83,85]; Carbon fiber/PA [36,91]; Glass fiber/PPS [92,93]; PU + Fe particles [94]; Polymethylmethacrylate (PMMA) [95]. Keywords of the search scope: “Fusion bonding”, “Resistance welding”; “induction welding”; “ultrasonic welding”. (

**b**)—Distribution of manufacture techniques to perform bonded and welded joints on the thermoplastic composite materials presented in Figure 2a.

**Figure 3.**The three loading modes that promote the crack propagation: (

**a**)—Mode I (opening), (

**b**)—Mode II (in-plane shear), and (

**c**)—Mode III (out-of-plane shear).

**Figure 4.**Schematic representation of the double cantilever beam (DCB) test. P is the imposed load, δ is the crack opening displacement, and a is the crack length.

**Figure 5.**Schematic representation of the End Notched Flexure (ENF) test. P is the imposed load, δ is the specimen’s deflection, L is the length of the specimen, and a is the crack length.

**Figure 6.**Comparison of critical energy release rates G

_{Ic}—(

**a**) and G

_{IIc}—(

**b**), and elastic modulus E

_{1}of unidirectional thermoset and thermoplastic based composites. Filled and hollow marks stands for thermoplastic and thermoset systems, respectively [7,16,121,122,123,124,125,126,127,128,129,130].

**Figure 9.**Experimental mean lap shear strength values of several of adhesive bonded and welded joints using thermoplastic based composites substrates.

**Figure 12.**Experimental critical fracture energy release rates (G

_{Ic}) obtained in pure mode I loading fracture tests for several thermoplastic based composites. Legend (joining method, surface treatment): i—Bulk state [128]; ii—Resistance heating [61]; iii—Bulk state [124]; iv—Epoxy adhesive Dexter, abrasion [96]; v—Epoxy adhesive Cyanamid, abrasion [96]; vi—Adhesive Araldite AY103 [66]; vii—Adhesive Araldite AV118(M) [66]; viii—Hot melt adhesive [96]; ix—Ultrasonic heating [66]; x—Resistance heating [66]; xi—Resistance heating [66].

**Figure 13.**Intimate contact and autohesion stages of the consolidation phenomena: (

**a**) Two distinct interfaces; (

**b**) achievement of intimate contact; (

**c**) collapse of the interface (autohesion).

**Table 1.**Chronological representation of TPC materials applied in Resistance welding (RW), Induction welding (IW), Ultrasonic welding (UW), and Adhesive Bonding (AB) procedures. Gr—Graphite fiber; Gl—Glass fiber, CF—Carbon fiber. Highlighted references represents works including numerical modelling.

Year | RW | IW | UW | AB |
---|---|---|---|---|

1988 | Gr/PEEK [58,59] | Gr/PEEK [58] | Gr/PEEK [58] | CF/PEEK [96] |

1989 | Gr/PEEK [63]; PE [42] | Gr/PEEK [63] | Gr/PEEK [63] | |

1990 | Gr/PEEK [61,62,64]; CF/PEEK [65] | Gr/PEEK [62]; CF/PEEK [65] | CF/PEEK [65] | CF/PEEK [65] |

1991 | CF/PEEK [66] | CF/PEEK [66] | PEEK [97]; CF/PEEK [66]; Gr/PEEK [98] | |

1992 | CF/PEEK [67]; CF/PP [88]; Gl/PP [84] | |||

1993 | Gr/PSU [99] | CF/PEEK [100]; Gl/PP [100] | ||

1996 | CF/PEEK [68] | |||

1997 | CF/PEEK [47] | |||

1998 | CF/PEEK [69,70]; CF/PEI [69,70] | |||

1999 | CF/PEI [78] | |||

2000 | CF/PEI [79]; Gl/PEI [79] | |||

2006 | Gl/PPS [92] | |||

2007 | HDPE [89]; ABS [89] | |||

2008 | CF/PEI [80]; CF/PEKK [72,80]; Gl/PEI [80] | |||

2011 | CF/PEEK [73] | |||

2012 | Gl/PP [86] | HDPE [90]; PA6 [90] | PMMA [95] | PP [101] |

2013 | CF/PPS [33]; Gl/PEI [31] | CF/PPS [33] | CF/PPS [33]; CF/PEI [81] | |

2015 | Pu + Fe particles [94] | |||

2016 | CF/PPS [48] | CF/PPS [48] | CF/PPS [48] | |

2017 | CF/PPS [87]; CF/PEI [82] | |||

2018 | Gl/PPS [93] | CF/PEEK [75] | ||

2019 | CF/PEEK [76]; Gl/Ellium^{®} [43] | Gl/Ellium^{®} [43]; CF/PEEK [74] | CF/PA6 [91]; CF/PEEK [23] | |

2020 | Gl/PP [40]; PEEK [102]; CF/PPS [32] |

**Table 2.**Process parameters and its typical values of resistance welding (RW), induction welding (IW), and ultrasonic welding (UW).

Process | Heating Time [s] | Process Parameters | Typical Values | Influence |
---|---|---|---|---|

RW | 30–300 | Power input (kW/m^{2}) | 30–160 | Determines the energy input into the weld |

Welding pressure (MPa) | 0.4–1.4 | Provide intimate contact and prevent delamination of the heated affected zones | ||

Clamping pressure (MPa) | 4–20 | Promotes the lowest resistance on the electrical contact | ||

Resistance of the heating element (Ω) | $R=\gamma \frac{L}{W}$ | Influences the heat generation L—length of the heating element; W—width of the heating element; γ—specific resistance of the material | ||

IW | 10–360 | Power input (kW/m^{2}) | Determines the energy input into the weld | |

Welding pressure (MPa) | 0.8 | Provide intimate contact and prevent delamination of the heated affected zones | ||

Frequency (Hz) | 60–100 | Affects quadratically the heating generation | ||

UW | 3–4 | Power input (kW/m^{2}) | 80 | Determines the energy input into the weld |

Welding pressure (MPa) | 2.2 | Affects the heating generation | ||

Frequency (Hz) | 20–50 | Affects quadratically the heating generation | ||

Vibration amplitude (µm) | 50–85 | Affects the heating generation |

Process | Reference | Heat Generation Rate (W/m^{3}) | Heat Absorption Rate (W/m^{3}) | Parameters |
---|---|---|---|---|

RW | [61,67,73,159] | ${\dot{Q}}_{gen}=\frac{{I}^{2}R}{V}$ ${\dot{Q}}_{crys}={X}_{mr}{H}_{f}\rho \frac{d{X}_{vc}}{dt}$ | ${\dot{Q}}_{melt}={X}_{mr}{H}_{f}\rho {X}_{vc}\frac{d{X}_{f}}{dt}$ where $\frac{d{X}_{f}}{dt}={K}_{0}exp\left(\frac{-{E}_{a}}{GT}\right){\left(1-{X}_{f}\right)}^{n}$ | I—applied current (A) V—volume of the heating element (m ^{3})R—resistance of the heating element (Ω) f—frequency of the coil (Hz) µ—magnetic permeability of the composite H—magnetic field intensity (Wb) A—cross sectional area of the conductive loop (m ^{2})R _{f}—electrical resistance of the conductive fibers (Ω)H _{f}—enthalpy of fusion (J/kg)X _{mr}—mass fraction of matrixX _{vci}—initial crystallinity of the compositeX _{j}—degree of meltingE _{a}—activation energy (J/mol)K _{0}—pre-exponential factorn—kinetic order G = gas constant (8.314 J/mol.K) |

IW | [24,74,161] | ${\dot{Q}}_{gen}=\frac{4{\pi}^{2}{f}^{2}{\mu}^{2}{H}^{2}{A}^{2}}{{R}_{f}}$ ${\dot{Q}}_{crys}={X}_{mr}{H}_{f}\rho \frac{d{X}_{vc}}{dt}$ | ||

UW | [89,162] | ${\dot{Q}}_{gen}=\frac{E\u2019\u2019\omega}{2}{\epsilon}^{2}$ | E’’—loss modulus (MPa) ε—amplitude strain (mm) ω—vibration frequency of the sonotrode (Hz) α _{h}—empirical hammering correction factor (0 < α_{h} < 1) | |

[150] | ${\dot{Q}}_{gen}={\alpha}_{h}^{2}\frac{E\u2019\u2019\omega}{2}{\epsilon}^{2}$ |

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**MDPI and ACS Style**

Reis, J.P.; de Moura, M.; Samborski, S.
Thermoplastic Composites and Their Promising Applications in Joining and Repair Composites Structures: A Review. *Materials* **2020**, *13*, 5832.
https://doi.org/10.3390/ma13245832

**AMA Style**

Reis JP, de Moura M, Samborski S.
Thermoplastic Composites and Their Promising Applications in Joining and Repair Composites Structures: A Review. *Materials*. 2020; 13(24):5832.
https://doi.org/10.3390/ma13245832

**Chicago/Turabian Style**

Reis, João Pedro, Marcelo de Moura, and Sylwester Samborski.
2020. "Thermoplastic Composites and Their Promising Applications in Joining and Repair Composites Structures: A Review" *Materials* 13, no. 24: 5832.
https://doi.org/10.3390/ma13245832