Enhancing Fatigue Strength of Adhesively Bonded Composite Joints

Enhancing Fatigue Strength of Adhesively Bonded Composite Joints: Comparison

Please note this is a comparison between Version 1 by Hossein malekinejadbahabadi and Version 3 by Catherine Yang.

Adhesive bonding is widely seen as the most optimal method for joining composite materials, bringing significant benefits over mechanical joining, such as lower weight and reduced stress concentrations. Adhesively bonded composite joints find extensive applications where cyclic fatigue loading takes place, but this might ultimately lead to crack damage and safety issues. Consequently, it has become essential to study how these structures behave under fatigue loads and identify the remaining gaps in knowledge to give insight into new possibilities. The fatigue life of adhesively bonded composite joints is influenced by various parameters, including joint configuration and material properties of adherends and adhesive. Numerous studies with varying outcomes have been documented in the literature. However, due to the multitude of influential factors, deriving conclusive insights from these studies for practical design purposes has proven to be challenging. Hence, this resviearchw aims to address this challenge by discussing different methods to enhance the fatigue performance of adhesively bonded composite joints. Additionally, it provides a comprehensive overview of the existing literature on adhesively bonded composite joints under cyclic fatigue loading, focusing on three main aspects: adherends modification, adhesive modification, and joints configurations. Since the effect of modifying the adhesive, adherends, and joint configurations on fatigue performance has not been comprehensively studied in the literature, this review aims to fill this gap by compiling and comparing the relevant experimental data. Furthermore, this review discusses the challenges and limitations associated with the methods that can be used to monitor the initiation and propagation of fatigue cracks.

adhesively bonded joints
composite materials
fatigue
fatigue life

1. Introduction

The majority of applications that utilize adhesively bonded joints are exposed to high cyclic fatigue loading over their lifespan, and fatigue failure is well known to be one of the primary causes of catastrophic failure in many cases (up to 50-90%) ^[1][35]. This fact highlights the importance of studying fatigue behavior in adhesively bonded joints. This is particularly crucial, especially when designing under safe life and damage tolerance philosophies ^[2][3][17][36]. The fatigue life of a composite adhesive joint is impacted by the distribution of stresses along the adhesive and adherends, as well as their interface, which are a function of various parameters, including joint configurations, material properties of adherends and adhesive ^[4][27], service temperature ^[5][6][20][28], humidity level ^[7][29], manufacturing process, and associated surface treatment ^[8][30]. The fatigue life can be controlled by modifying these parameters, such as hybridizing the adherends and adhesive ^[9][10][31][32], incorporating additive particles ^[11][33], and adjusting the configurations ^[12][34]. This resviearchw aims to provide a comprehensive overview of the existing literature on adhesive bonded composite joints under fatigue loading, focusing on three main aspects: 1. adherend modification, 2. adhesive modification, and 3. joint configurations.

2. Joint configuration and geometry

This section aims to investigate the impact of joint configuration and geometry parameters, such as overlap length (l), overlap geometry (interface configuration), corner geometry, adhesive thickness (ta), adherends thickness (T) on the fatigue behavior of composite adhesive joints (see Figure 1Fig. 3).

Figure. 13.

Design parameters of conventional SLJ.

The study findings indicated that the fatigue strength of double-lap joints was significantly superior to that of SLJs. Overall, the findings indicate that decreasing the scarf angle results in an improvement in the fatigue strength of scarf joints, driven by an increase in the bonded area. This is consistent with the conclusions drawn in other studies ^[13][14][15][53][54][55]. Scarf joints exhibited a higher fatigue life than butt joints of equal CFRP adherends thickness, regardless of the scarf angle. As the thickness of the CFRP adherends in the butt joint increased, the fatigue strength is reduced due to the volume effect of CFRP, and the dominant failure mode shift from primarily delamination to cohesive failure. Figure 2Fig. 5 illustrates the increase in fatigue strength, specifically the load threshold at which the specimen has withstood over 10 million cycles, known as the endurance limit, in comparison to the SLJ.To ensure comparability, the fatigue load is normalized by dividing it by the overlap area for each configuration. This adjustment takes into account the size of the adherends and the adhesive layer used.

Figure. 25. Fatigue strength (corresponding to 1e6 cycle) improvement for different configurations in comparison to the SLJ. (Adapted from ^[12][16]).

Fatigue strength (corresponding to 1e6 cycle) improvement for different configurations in comparison to the SLJ. (Adapted from [34][45]).

The test results showed that the wavy lap joint had a considerably longer fatigue life and higher strength than the conventional lap joint (see Figure 3Fig. 8). The analysis of the failure surface in a wavy joint under fatigue loading revealed that cohesive failure was the primary mode of failure. However, in the case of a conventional SLJ, unstable shear breakage was observed specifically in the middle section of the overlap length.

Figure. 38. Effect of wavy interface on the fatigue and static strength of joints (Adapted from ^[17]).

Effect of wavy interface on the fatigue and static strength of joints (Adapted from [46]).

The resulting improvement in fatigue strength for different configurations is depicted in Figure 4Fig. 9. To allow for direct comparisons in the fatigue performance, the fatigue strength is normalized by dividing it by the overlap area. This normalization takes into account the use of a similar adhesive for the various configurations presented. In conclusion, the wavy joint configuration can lead to improved joint strength compared to conventional SLJs, particularly under fatigue loading conditions. The reduction of peel stresses and stress concentrations at the joint ends are key factors contributing to these improvements.

The final aspect to consider is that utilizing non-flat overlap regions can enhance the fatigue strength and behavior of adhesively bonded joints and the significance of locally modifying adherend geometry becomes even more crucial when joint configurations are limited by their application. However, it is essential to address the complexity of manufacturing and the lack of precise control over the geometry of composite adherends, as these factors can lead to variations and increased cost.

Figure. 49. Fatigue strength improvement for different configurations compared to conventional SLJs (Adapted from ^[17][18]).

Fatigue strength improvement for different configurations compared to conventional SLJs (Adapted from [46][56]).

2.1. Corner Ggeometry and Ooverlap Llength

Although the length of the overlap in adhesively bonded joints has a significant impact on both the initiation and propagation of fatigue crack growth, more so than factors like corner fillet ^[19][83], as shown in Figure 5Fig. 12, the strength of adhesively bonded composite joints (the highest tensile stress that can be applied to the bonded joint without causing failure until 1e 6 cycles) has been enhanced by incorporating a spew fillet at the overlap ends. This can be explained by the reduction of stress concentration within both the adhesive and the adherends ^[20][21][87][88].

Figure. 512. Effect of overlap length vs. corner fillet on fatigue strength ^[19].

Effect of overlap length vs. corner fillet on fatigue strength [83].

2.2 Aadhesive Tthickness

It is recognized by the classic theory that when the thickness of the adhesive increases, the peel stress also increases, leading to a decrease in the fatigue life ^[22][92]. While adhesive thickness is increased, the length of the "moment arm" over which the applied force acts is effectively extended and makes higher moment arm, subsequently leading to increased peel stresses at the edges of the adhesive bond. To support this considering that peel stress is influenced by multiple factors, it becomes necessary to conduct a numerical investigation into the impact of adhesive thickness on peel stress. In ^[23][24][50][98], the finite element analysis revealed that the peel stress increased with greater adhesive thickness.

The thickness of the adhesive also affects the failure mode so that failure occurred withing the adhesive layer, and with an increase in thickness, the failure mode changes and delamination is raised due to an increase in peel stresses ^[24][98]. As previously discussed, it has been observed that peel stress rises in response to an increase in adhesive thickness. Given that peel stress significantly impacts the composite adherends at the ends of the overlap (where peel stress is most pronounced), this leads to a heightened risk of deamination, as depicted in Figure 6Figure 18.

Figure. 617. How the adhesive thickness affects the highest load that can be applied to the bonded joint without causing failure in 1e6 cycles (Adapted from ^[25]).

How the adhesive thickness affects the highest load that can be applied to the bonded joint without causing failure in 1e6 cycles (Adapted from [81]).

3. Adherend modification

3.1. Stacking Ssequence and Iinterface Ply Oply orientation of Aadherends

As shown in Figure 7Fig. 26, as long as the interface plies are placed at 0^º, debonding occurs within the adhesive region in a cohesive manner and final failure occurred in the 0^º adjacent layer. However, in the case of 45° interface plies, debonding happens through a combination of cyclic debonding in the adhesive and delamination in the ±45^° plies of the laminate adherend. In more detail, the crack continues to grow until it reaches the layer with 0^º orientation and then final failure occurs. For specimens with 90^° plies, damage initiates with transverse cracking in the 90^° layers of the strap adherend, and then progresses through delamination failure until it reaches the first 0^° ply in the strap adherend (see Figure 7Fig. 26).

Figure. 726. Changing crack path due to adhesive adjacent composite adherends layer orientation (0/+45/-45/90), (45/-45/0/90), (90/-45/+45/0) (Adapted from ^[26][27][28]).

Changing crack path due to adhesive adjacent composite adherends layer orientation (0/+45/-45/90), (45/-45/0/90), (90/-45/+45/0) (Adapted from[161][162][163]).

In general, based on existing literature, it’s obvious that the stacking sequence and the orientation of adhesive adjacent layer have an impact on fatigue cracks path. Consequently, this affects both fatigue strength and fatigue life. Nonetheless, the impact of these factors on fatigue strength varies significantly based on the joint configuration. As previously discussed, using unidirectional fibers aligned with the load direction results in greater fatigue strength. The least favorable stacking sequence involves a perpendicular orientation, particularly evident in SLJs. However, in lap strap joints, these effects tend to diminish.

3.2. Surface Ttreatment

A surface with an optimized amount of roughness can result in the highest fatigue life due to several reasons, including an increase in the bonded area, enhancement of mechanical locking, and retardation of crack growth. Conversely, a surface that is too rough can lead to the formation of voids and stress concentration at the adhesive/adherend interface ^[29][184]. Surface treatments applied to adherends can impact the behavior of composite bonded joints during fatigue and static loading. However, this influence could depend on factors like the joint configuration and surface treatment parameters (such as surface roughness). As a result, these treatments might either enhance or degrade the strength of the joints.

4. Adhesive modification

4.1. Nano-Reinforced Adhesive Layers

To enhance the performance of adhesively bonded joints, one potential approach involves incorporating nanoparticles into the adhesive, even in small amounts (1–1.5%) . The strength of a joint reinforced with nanomaterials is influenced by various factors that affect the adhesive’s behavior and its ability to transmit stress to the composite adherends. One of the key factors is the type of adhesive, which can be rigid, flexible, or toughened. Another significant factor is the ratio and type of the added nanostructure, including materials such as carbon nanotubes and graphene. Furthermore, the characteristics of the nanostructures such as their size, shape, and aspect ratio, can impact the dispersion and alignment of these structures within the adhesive . Apart from these aspects, other factors such as the loading conditions, preparation of the bonding surface, and the curing process can also play a role in the strength of the joint reinforced with nanomaterials. Hence, considering all these variables is crucial in the design of nano-reinforced adhesive joints to achieve optimal performance.

Using nanofillers to modify adhesives has been shown in many studies to enhance the mechanical properties such as strength, stiffness, toughness, and fracture energy of adhesive joints, making them suitable for bonding composites by improving stress transfer between the adhesive and the composite adherends . This improvement can be attributed to the enhanced wettability of the adherends by the adhesive and also the greater resistance to crack initiation and propagation due to the more complex crack path (see Figure 28), achieved through the utilization of nanoparticles. Moreover, due to their size, they can penetrate the voids within the composite adherends, increasing the mechanical interlock and consequently enhancing the joints’ strength. These changes in behavior change the failure mode from adhesive failure at the interface to cohesive failure within the adhesive, ultimately leading to a more effective transfer of load to the adherends. In contrast, some studies have indicated that the addition of nanofillers can lead to a decrease in the strength of the joints. This decrease may be attributed to an increase in the brittleness of the adhesive and its glass transition temperature (T_g) . For example, Kang et al. indicated that in the case of a composite/aluminum adhesive joint without carbon nanotubes under tensile loading, there was an occurrence of interlaminar failure within the composite adherend. This type of failure occurred because the adhesive was strong enough to transfer the load effectively between the adherends. Conversely, adding carbon nanotubes to the adhesive resulted in adhesive failure at the interface between the adhesive and the composite adherend, as observed. This can be attributed to the degradation of interfacial bonding between the carbon nanotubes and the adhesive, which reduced the strength of the adhesive and its capacity to transfer stress to the composite adherend. Furthermore, the analysis of fracture surfaces did not reveal any significant difference in the behavior of the joints under fatigue compared to static loading conditions. The effect of adding nanoparticles to the adhesive layer on the static and fatigue strength of SLJs is shown in Figure 29. The addition of 2 wt% carbon nanoparticles resulted in a minor enhancement of around 18% of the tensile–tensile fatigue strength, but with a decrease of approximately 51% in static strength (see Figure 29) . The conclusion drawn is that the impact of incorporating nanoparticles on the static and fatigue strength of adhesively bonded composite joints is not necessarily uniform; it varies depending on multiple factors including the type of adhesive and nanoparticles as well as the effect of nanoparticles on the ductility of the adhesive .

Figure 28.

Crack path affected by nanoparticle (greater resistance to crack initiation and propagation due to more complex crack path).

Figure 29. Effect of adding nanoparticles to the adhesive layer on the static and fatigue strength of SLJs (adapted from ^[30]).

Effect of adding nanoparticles to the adhesive layer on the static and fatigue strength of SLJs (adapted from ).

Considering all of the explanations presented above, the process of reinforcing materials with nanoparticles is seen as a process that involves several variables that influence the properties of the resulting composite, including the dispersion of nanoparticles, their structural features, and their length and diameter. Although numerous studies [198] ^[31] have suggested that adding nanoparticles to adhesives can increase fatigue strength and lifespan, the type and content of nanoparticles used should be carefully considered to achieve optimal results ^[32][197].

4.2. Mixed Adhesive Layers

Previous studies have proposed the use of mixed modulus adhesive joints, which consist of two different types of adhesives (one ductile and one brittle), to improve the stress distribution and fatigue strength of adhesively bonded joints . In this approach, the stiff and brittle adhesive is placed at the center of the overlap, while a low-modulus adhesive is applied at the edges where stress concentrations are more likely to occur. Figure 310a shows the schematic of mixed adhesive joints and the combination of two different adhesives . Utilizing a mixed adhesive is a viable option for applications that experience harsh environmental conditions and involve dissimilar adherends with notable variations in properties . Flexible adhesives are preferred for their resistance to peeling, fatigue, crack propagation, and impact loads . However, they typically have lower cohesive strength. Brittle adhesives, on the other hand, have a higher modulus and lower toughness. In , the bi-adhesive technique to enhance the performance of SLJs made of composite and metal materials under impact fatigue loading conditions was proposed. An experimental analysis was conducted to evaluate the impact fatigue life of composite-metal joints using a mixed-adhesives layer. The findings demonstrated that the impact fatigue life of bonded joints could be significantly enhanced simply by employing two different adhesives in the overlap region, following the mixed adhesive layer technique. Furthermore, it was observed that the optimal length ratio of the adhesives, i.e., the fraction of the overlap size occupied by the ductile adhesive, depended on the joint’s stiffness (adherend material) and was more pronounced for bonded joints with lower stiffness. Figure 310b illustrates how varying impact levels affect the fatigue life of mixed adhesive joints, taking into account the length ratio of the mixed adhesive used. Akhavan-Safar et al. investigated the mixed adhesive joints under cyclic low-energy impacts and reported the impact energy vs. impact fatigue life. Similarly to the other study , it was observed that the impact fatigue life of bi-adhesive joints is highly influenced by the length ratio. The results demonstrated that the impact fatigue life of the joints improved significantly by employing the mixed adhesive technique. The experimental results indicated that even small changes in the length ratio of the two adhesives in the joint had a significant effect on its endurance to impact fatigue.

Figure 310. (a) Schematic of mixed adhesive joints. (b) Impact fatigue life assessment of mixed adhesive SLJ under varying impact energies: Exploring the effects of different impact levels on fatigue life of mixed adhesive joints .

As far as the authors are aware, no previous studies have specifically focused on investigating the cyclic fatigue behavior of mixed adhesive joints. This research gap presents an interesting opportunity to explore the effects of employing this method in adhesive bonding. The study may involve comparing the fatigue performance of mixed adhesive joints with those of conventional adhesive joints and other bonding methods. This comparative examination offers valuable insights into the relative benefits and limitations of mixed adhesive joints, while it also highlights their potential utility in diverse industries that encounter high-cycle fatigue scenarios.