State-of-Art Review on Fatigue and Tension Behavior of Scarf Adhesive Joints

Scarf adhesive joints (SAJs) are increasingly used in composite structures due to their enhanced load transfer capabilities and structural efficiency. This literature review provides a comprehensive overview of SAJs, covering their fundamental geometry and function, and explores key mechanical properties under different loading conditions. The fatigue behavior of SAJs is discussed in terms of endurance limits and failure mechanisms. Tensile properties are reviewed with a focus on the influence of scarf angle and adhesive thickness. This review serves as a foundation for future research and optimization of SAJs in structural applications.

Introduction

Recently, scarf adhesive joints (SAJ’s) have become quite popular among different types of adhesive-bonded joints. They are overlap joints where the angle between the axis of the adhesive layer and the axis of the adherends (of equal width and thickness) is greater than 0° and less than 90° (butt joint) [1]. It is an alternative joint configuration which has the advantage of not requiring the component to modify its original shape. It also a prevalent procedure to repair defective structures [2].When compared to overlapping joints, the smoother taper surface of SAJs reduce stress concentrations caused by eccentricity. Because of this, SAJs was the first method of showing a more even distribution of stress throughout the adhesive layer and absorbing a significant portion of it in shear [3]–[5]. Additionally, over long bonding length, for scarf joints, the smallest ratio of adherend extensional stiffnesses represents a constant proportion towards which the maximum bond stress has an asymptotes value [6], [7]. It also has the benefit of offering a larger adhesive surface area, which increases the joints' strength among other configurations [8]. In terms of fatigue cycle capacity, SAJ provides superior performance in comparison to lap and step joints [9], [10]. Fig. 1 illustrates a schematic of the scarf joint’s parameters.

Fig. 1. Scarf adhesive joint.

The strength of the bonded scarf joint is influenced by techniques of production for parent and repair materials, surface treatments, milling process, overlaminate, scarf angle or scarf ratio, ply stacking sequence, adhesive kinds and their modified forms, and geometry/configurations [11]. The scarf joint segment is often weaker than other sections because there is a break in the reinforcing fibers from one part to the other. However, the average bond stress primarily distinguishes scarf joints approaches a certain constant proportion of the maximum bond stress over extensive bonding length (smaller scarf angel). This proportional fraction is equivalent to the smaller ratio of the adherend’s extensional stiffnesses [6]. Therefore, evaluating the joint section's structural integrity, that is its joint strength, is essential. This leads many researchers to investigate more on this subject. The following sections discuss the behavior of the joint based on the major types of loads mentioned in the literature, namely, fatigue and tensile tests.

Fatigue Properties of SAJ

In practical applications, materials are frequently exposed to complex alternating stresses. The fatigue property, which is represented by a S–N curve that pits cyclic stress (S) against cycles to failure (N), is a crucial indication of material reliability. Fatigue life in adhesive joints is influenced by several parameters which are illustrated earlier in Fig.1. The following sections review how changing the adhesive type, joint geometries and tests’ temperatures on the fatigue properties.

The Effect of Joint Modification

Several research conducted by Khashabah et al. [12]–[19] have reported the fatigue performance of the scarf joints. A resin/epoxy was used in their studies as an adhesive part to join woven CFRE adherends. The research stared by studying the improvement of the scarf joints by incorporating the optimum weight of multi-walled carbon nano tubes (MWCNTs), SiC and Al2O₃ nanofillers, that were investigated earlier [20], [21], into epoxy adhesive [12]. Then, the joints were tested using tension-tension fatigue test. The fatigue lives (Nf) were calculated at fatigue limits of (36 MPa), (42 MPa) and (57 MPa). When comparing the modified adhesives to the neat adhesive specimens, the result of the tests reveals that the fatigue lives of the SAJs were improved for MWCNTs, SiC and Al2O3 SAJs by 42%, 133% and 160%, respectively, at the higher stress level (57 MPa). The author attributed this to the higher improvement in the interfacial bonding and accordingly the interfacial shear resistance of the modified SAJs. However, this improvement was gradually decreased with decreasing the applied stress level (i.e., longer fatigue life). Furthermore, As the number of cycles increased, the stiffness of the SAJs increased up to about N/Nf = 0.01. This outcome was explained by the creation of microcracks in the adhesive phase following a few cycles, which relieve the high concentration of localised stress caused by the exothermic reaction temperature during the curing process.

The Effect of Joint Geometry

Generally, Moreira et al. proved that fatigue life can be improved by decreasing the scarf angle [22]. The modified joint with MWCNT’s of Khashbah et al. [13] was further investigated to study the effect of different scarf angle and bond line thickness. The impact of bondline thicknesses (0.17 mm and 0.25 mm) and scarf angles (5° and 10°) was evaluated. The fatigue strength/life of the SAJs was enhanced by reducing the scarf angle from 10° to 5°, resulting in an enlarged bondline area, and by decreasing the bondline thickness from 0.25 mm to 0.17 mm. The research also shows that the interfacial shear cracks, which began at the joint overlap edges (tip ends) between the adhesive layer and the stiffer adherends, are the primary cause of the fracture of SAJs. These cracks proceeded concurrently to the centre of overlap and towards the bondline centre. Either the interfacial shear jumped from one contact to the other or it propagated between the two surfaces without fracturing the adhesive layer. Moreover, incorporating MWCNTs into the adhesive layer of scarf joints can enhance mechanical properties, stiffness, and overall performance under tensile and fatigue loads.

A continuation to the previous work, the effect of the reduction of the bondline thickness on the failure stress and fatigue lives at room temperature is discussed [18]. It was found that compared to the NE-SAJs, the SiC-SAJs assessed fatigue live at various stress levels dramatically improved by around 12–39 times which accompanied with an improvement in fatigue limit with minimizing the bondline thickness, whereas the tests at 50°C result in a sharp decrease in the fatigue strength of the SiC-SAJ with bondline thickness of 0.17 mm higher than the joints with a bondline thickness of 0.25 mm.

Lastly, the impact of the adhesive materials (neat epoxy (NE), SiC, and Al₂O₃-nanocomposite), as well as bond thickness, on the dynamic characteristics of the scarf adhesive joints (SAJs), was thoroughly examined in relation to the adhesive fatigue lives [19]. Potential energy (Up) was used for monitoring and predicting the damage in the adhesive joints. Comparing the potential energy of the Al₂O₃-SAJs to both the NE and SiC-SAJs, at lower stress levels (σ_max = 59.4 MPa), the Al₂O₃-SAJs showed a growing rate throughout stage II. Compared to both NE and Al₂O₃-SAJs, the fatigue characteristics of SiC-SAJs with a 0.17 mm bond thickness have significantly increased. Fig. 2 shows the improving in fatigue strength at scaf angle of 5° after reducing the adhesive thickness from 0.25 mm to 0.17 mm at room temperature.

Fig. 2. The improvement in fatigue strength after reducing the adhesive thickness from 0.25 mm to 0.17 mm.

The Effect of the Test Temperature

In a following study of the modified joint with MWCNT, the effect of the temperature on the fatigue properties of the bolted/bonded adhesive joint is investigated [14]. The analysis of the scatter in fatigue lives was performed based on Weibull probability. The results showed that, at RT, the mean fatigue lives of the CNT-SAJs are improved compared with those corresponding to the NE-SAJs. At +50°C, the losses in the mean fatigue lives are about 96% and 98% for respectively the NE-SAJs and CNT-SAJs compared with those of the corresponding RT-tests.

In addition, a novel, straightforward method for determining bolt-hole elongation was created. By using their load-displacement relationships, the proposed method effectively distinguishes between the loads and failure mechanisms of both bolted joints and SAJs. It was found that the most common failure mechanisms in SAJs are interfacial shear failure, cohesive failure, and CNT pull-out; for bolted joints, the most common failure modes are bearing, shear-out, and longitudinal splitting.

A dynamic analysis used to find a more useful parameter that may be utilised during fatigue loading at various temperatures (25°C, 50°C and −60°C) to monitor the start and spread of damage [15]–[19]. It was discovered that, at 50°C, the dynamic stiffness was roughly 29.6% higher than the static one. The dissipated energy increased when the temperature was adjusted from room temperature to 50°C to −60°C. This occurs because of the constituent materials of SAJs having differing rates of thermal expansion and contraction. This can result in increased stress concentration and, in turn, the beginning of microcracks even before the application of fatigue loads. Furthermore, fatigue lifetimes at −60°C were longer than those at RT and 50°C at the same stress levels. At 50°C, when there was a lot of creep deformation, the fewest cycles to failure were observed. On the other hand, the fatigue lives of the NE and Al₂O₃-SAJs at 50°C were, respectively, 5 and 1.6 times shorter than those at RT at σ_max of 50.2 MPa. Additionally, in order to monitor joint integrity and prevent catastrophic failure, the potential energy (U_p) can be modelled because its variation with fatigue life is more noticeable than that of the other dynamic parameters [16].

In the later research by Khashaba and Najjar [17], the enhancing of the SAJ with SiC compare adhesive joints with 0.25 mm thickness that was studied earlier in Khashaba et al. [12] and the unmodified joint of 0.17 mm thickness in Khashaba [15]. The researchers also found that the majority of SAJs with and without SiC nanoparticles showed mixed cohesive/adhesive/light-fiber-tear failure modes at lower fatigue load levels (i.e., longer lifetimes) and RT, whereas at high stress levels (i.e., shorter lifetimes), interfacial failure along the bondline without cohesive fracture predominates in the collapse of the joint. At 50°C and lower fatigue load level, the SAJs exhibited interfacial failure. The improvement in fatigue properties of introducing the nanoparticles into the adhesive joint is summarized in Table I.

Table I. The Effect Decreasing the Bondline Thickness with Increasing the Temperature of the Enhanced Joints on Fatigue Properties

CNT	RT	Improvement of 712.1%–97.8% in the mean fatigue life.
	50°C	Losses in the mean fatigue lives are −10.8% to −13.6%.
Al2O3	RT	The fatigue limit improved by 5%.
	50°C	At σmax = 50.2 MPa, the fatigue life reduced by 1.6 times.
SiC	RT	The fatigue limit improved by 55%.	The fatigue lives at stress levels of 55.5 MPa, 59.4 MPa, and 66.7 MPa are in the range of 12 to 39 times higher than those of the NE-SAJs.
	50°C	The fatigue limit is decreased by 17.9% compared with that at RT.

Regardless of the presented effects, an experiment conducted by Feng et al. [23] on a repaired scarf join revealed that a significant portion of the fatigue life is occupied by the crack initiation stage, as shown by the crack observation and stiffness degradation. The fatigue crack begins at the scarf edge in the adhesive layer and spreads across the scarf area. The result of the fatigue life in four different stress levels showed that with the decrease of stress level, the fatigue life increases which also agreed with the result of the fatigue life previous research. Generally, based on the experiment, the dominated failure mode is cohesive failure of adhesive.

Tensile Properties of SAJ

The material’s tensile characteristics show how it will respond to tension-applied forces. A tensile test is a basic mechanical test in which a well-prepared specimen is loaded under extremely strict control to measure the applied force and the specimen's elongation over a certain distance. The modulus of elasticity, elastic limit, tensile strength, and other tensile characteristics of a material are all determined via tensile testing [24].

Test Temperature Effect

The tensile properties of the scarf joint were studied and described by Khashabah et al. in their research that mentioned in the last part, where the effect of modifying the scarf joint by nanoparticles (CNT, Al₂O₃ and SiC) was described broadly. Fig. 3 indicates the effect of infusing the adhesive joints with nanoparticles on the tensile properties at 0.17 mm thickness that was observed from the experiments in [14], [16], [18].

Fig. 3. The improvement of the tensile properties of the modified joints compared to the neat adhesive at different temperatures.

In a variety of heat settings, Hyeon-Seok et al. [25] examined the changes in the composite bonded scarf joint’s tensile strength and failure mechanism. They also investigated the impact of covering the joint region with an external ply. Six distinct ambient temperatures, spanning from 25°C to 150°C, were selected and subjected to testing. It was found that up to 75°C, delamination and intralaminar failures modes; at higher temperatures (beyond 100°C), cohesive failure took front stage. Furthermore, when it comes to adhesive-bonded connections failing at high temperatures and causing the loss of structural function, the adhesive material used for joining should perform as well as or better than the resin used in the original composite structures at that temperature.

Roy et al. [26] assessed the impact of the temperature difference on scarf joints’ tensile strength. The experimental result showed that about 86% of the results obtained at 25°C for joint tensile strength were reached at 75°C. The simulated tensile failure strength varies by up to 15% for quasi-isotropic laminates with a given ply angle count but a different ply sequence. As it was mentioned that the proper joint thickness should be between 0.15 mm and 0.25 mm for a satisfactory bond [27]–[29], therefore comparison of the effect of the temperature and the bondline thickness on the tensile strength of the joint is shown in Table II.

Table II. The Effect of the Adhesive Joints on Tensile Strength with Different the Bondline Thickness at Different Temperatures

Reference	Thickness (mm)	Test temperature (°C)	Tensile strength (MPa)	%
Roy et al. [26]	0.12	25	1092.7	−16.3
		75	939.2
Khashabah [13]	0.17	−70	119	−2.8
		25	122.3	–
		50	101.9	−20.0
Hyeon-Seok et al. [25]	0.2	25	475	−1.3
		50	469
Khashabah [13]	0.25	−70	98.1	−4.1
		25	102.1	–
		50	97.2	−5.0

Hygrothermal and Aging Effect

Scarf joint was widely utilized in the construction or repair of airplanes, automobiles and ships which are constantly subjected to moisture from atmospheric humidity or the direct explosion to water. Thus, different studies have considered the influences of hygrothermal on SAJ. Ageing has also a vital effect on the performance of the adhesive joints.

Gacoin et al. [30] found that the load threshold of the initial start of the microcracks and the final breakdown of the SAJ rised as an influence of ageing. Feng et al. [31] employed a quasi-static tensile test on both unaged and hygrothermal aged specimens with multiple plies sequences. It was observed that hygrothermal ageing affected both elastic modulus and tensile strength negatively as they reduced by 14.7% and 30.7%, respectively. The strength also had a proportional effect with the moisture percentage which is influenced by the laminates’ thickness. At the same environmental conditions, the experiment of Harder et al. [32]. revealed that the strength of the bonded joint diminished consistently over all the configurations used by 5%–20%. Zhang et al. [33] examined the effect of single-face and double-face vacuum bag curing condition on the bonding quality of scarf-repaired composite laminates. The study concluded that single face curing leads to more voids, dislocation and elevated moisture absorption rate. Kashabah et al. [20], [21] studied the effect of moisture absorption of SAJs with modified adhesives. It was pointed out that the immersion in water for 526 hours reduces the UTS for the modified SAJs with MWCNT, Al₂O₃ and SiC by 2.5%, 0.6% and 0.1 %, respectively. Generally, studies showed that moisture absorption increased as the scarf angle and the bondline thickness decrease.

The Effect of Dissimilar Adherend

The presence of some limitations of some fibric materials, e.g., CFRP, such as their sensitivity to the environmental conditions have driven some researchers to study the influence of involving different materials in the structures. However, they have higher joint strength than other plastic materials [34]. On the other hand, hybrid joints have extended fatigue life and superior static failure loads [35]. In a comparative study of joining hybrid and equivalent adherends, Alves et al. [36]–[38] reported that the effect of hybridization is limited with consistently advantageous of the equivalent adherends. Additionally, Comparative studies of dissimilar adherends have been conducted of different scarf angles and different adhesives in [36], [37], [39]–[41]. The adhesives used in this project have ductility ranging from low to high. The result shows that the adhesive with higher ductility had more extensive damage at peak load. Furthermore, unlike bonding identical adherends, joining between adherends with varied stiffness generates a natural asymmetry in the stress plots.

Zhang et al. [42] also studied the effect scarf joining multiple types of metal to a composite adherend. In comparison to steel Q235, the joints made of steel 30CrMnSiNi2A, titanium TC4, and aluminium 7075-T6 have higher strength and less plastic stage and the final tensile loads and failure sites of the composite-to-metal SAJs with various metal plates vary.

Tserpes and Moutsompegka [43] investigated the effect of using a thermally treated CFRP which contaminated with different percentages of a de-icer coating. As a result, only joining the thermally adherend to 220°C with the original carbon fiber have a higher failure load than the reference CFRP which was attributed to the carbonyl on the surface that have improved the matrix properties. Contrarily, a reduction of 38% in the failure load was experienced by the other contaminated samples.

The Effect of Geometry

Most of the research agreed that the smaller scarf angle and bonding thickness improve the joint strength. In the first decade of the 21^st century, Gunnion and Herszberg [44] parametric study revealed that increasing the joint thickness leads to boost the peak peel and shear stresses, whereas they decrease with higher ply thickness and scarf angle with no obvious influence on the average stresses. In terms of failure, Kumar et al. [45], [46] concluded that the primary failure modes for SAJ are the failure that characterized by fiber fracture and pull-out which takes place when the scarf angle is smaller than 2°, whereas the cohesive shear failure happens in case of scarf angle beyond 2° under the tensile load, and 3° accompanied by fiber microbuckling in the case of compression. After that, Khashabah et al. [13] mentioned in their studies that 5°-SAJs have a higher tensile strength, and strains compared to 10°-SAJs while the 5°-SAJs have lower local stiffness. At the same time, the SAJs with thinner bondline (0.17 mm) have higher UTSs and lower strain values compared with those of the corresponding thicker one (0.25 mm) (Table II). Different research also highlighted that the larger strain gradient effect in the confined adhesive layer is responsible for the higher interfacial strength. Moreover, the stress singularity is the most prominent at the interfacial region especially for higher scarf angles [47]–[49]. However, the analysis of the interfacial stress distribution for a range of scarf angles from 45°–90° by He et al. showed that the scarf angle of 60° enhanced the joint strength, while reducing the adhesive thickness or rising its Young’s modulus can decrease the stress singularity [50].

Sonat and Özerinç [51] considered three different scarf angles 1.9°, 2.8°, and 5.7° in their experiments based on the argument that 1.9° and 2.8° are the most common repair scarf angles in the aerospace industry, while 5.7° is used when space restrictions in the repair geometry do not allow smaller angles. However, the optimum scarf angle that is ideal for tensile testing settings where both the adherend and the adhesive have the same load-carrying capability can be obtained as:

$$\begin{array}{r}{\alpha }_{opt}=\frac{1}{2}{\sin }^{-1}\frac{2{\tau }_y}{K_s{\sigma }_f}\end{array}$$

where ${\sigma }_f$ is the tensile strength, ${\tau }_y$ shear strength of the adhesive and $K_s$ is the stress concentration factor.

The outcome demonstrated that at scarf angles of 1.9° and 5.7°, the laminate structure collapses without significant damage to the bond region, but cohesive failure occurs within the glue. A mixture of cohesive failure and laminate failure was seen at the scarf angle of 2.8°. Furthermore, the adhesive-adherent stiffness mismatch is greatest in the 0°-plies, which is the most crucial portion in the bond area. An examination of the bondline flaws revealed that a fault density of around 10% results in a nearly 20% decrease in tensile strength, highlighting the significance of repair quality in attaining appropriate mechanical performance. On the other hand, Marques et al. [52] observed, in a study included 4 different scarf angles from 1.09°–2.42°, that the mechanical tensile characteristics of scarf-repaired laminates were affected by the overlap lengths of the patch plies as the shorter overlap length exhibiting a 37% decrease in failure load relative to the parent laminate.

Liao et al. [53] conducted a research on the influence of various adhesive types and geometries on the performance of SAJ. It is found that the smaller adhesive thickness results in higher ultimate tensile load. The variation in thickness causes diverse failure energy in brittle and ductile adhesives. In addition, ductile adhesive maintained partial load capacity, whereas brittle adhesive loses its ability to support load. Further, SAJs with smaller scarf angle withstood higher tensile load for both adhesives, however, higher load was observed when the brittle adhesive was used followed by sharp decline after reaching the peak as it exhibited less plastic deformation.

Silva et al. [54], [55] investigated the behavior of the tubular scarf joint (TSJ) which was validated with a previous experiment of tubular lap joint. The research was performed to prove the argument that TSJ, which can be created by chamfering the cross-section of the tubes, increases the joint strength in the axial direction as the entire bonding area is increased. The stress analysis reveals that whereas σ_y stresses are insignificant for smaller scarf angle, they approach τ_xy stresses by increasing the angle. Despite low peak stresses for small SAJ angle, τ_xy stresses are almost uniform throughout the bondline, which is a clear benefit over TLJ. When comparing the three adhesive types, it was discovered that, in contrast to what occurs in SLJ, adhesive strength predominates over ductility. It is thus advised to use stringy but brittle adhesives rather than ductile but less robust adhesives.

In the study conducted by Yanen and Solmaz [56], it revealed that, when it comes to joint damage, adhesive and cohesive damages are the main damage processes. Cohesive damage happened in the centre of the joint. In general, the bond strength diminishes as sample widths decrease. The experiment showed opposite of this circumstance in the samples made with scarf joints at 45° and 60° angles.

Sun et al. [57] examined the tensile performance of CFRP SAJs with varying scarf angles beside the failure mechanisms for a ductile adhesive and load-displacement response. It was concluded that the adherend’s layup sequence and the scarf angle determine the non-uniform stress distribution inside the adhesive which influences a maximum peel stress and shear stress. The adhesive failure typically happens when the scarf angle is rather big although cohesive failure generally takes the dominating. Additionally, as the scarf ange decreases, the matrix fracture and delamination are more likely to occur. It is also more influenced by cohesive strength than by fracture toughness, exhibits an exponential rise as θ decreases.

A comparison of the effect of changing the scarf angle on the failure load is shown in Fig. 4a for ductile adhesives and Fig. 4b for brittle adhesives. The lower stiffness of the adhesive used in Yanen and Solmaz’s [56] experiment results in a different behavior of the curve. Moreover, different the bonding thicknesses and adherend materials play an important role resulted load.

Fig. 4. The effect of different SAJ angle on the failure load using a) ductile and b) brittle adhesives.

The Effect of SAJ Design

In a study of the effect of the singularity on damage progression that induced by scarf geometry of single and double scarf joints was conducted by Gacoin et al. [58]. By comparing different scarf angles, the results showed that double scarf joint had a superior endurance to damage when the scarf angle was greater than 18°. This was because the crack initiation began from the inner end in this case as result of the overstress occurred.

A further comparative study for a scarf and curved joints was undergone by Citil [59] to inspect their impact the joint strength. It has been noted that the failure stress borne by the curved lap joints reduced as their radius of curvature decreased, increasing the surface area on which the adhesive was applied. Furthermore, it was noted that the overlap angle increased as the radius of curvature decreased which resulted in a decrease in the load tangential to the surface and an increase in the load applied normally on the surface in the joint region where the adhesive was applied, breaking the joint sample at lower failure loads. The study revealed that the adhesively bonded joint failed in the mixed mode (adhesive and cohesive modes) and showed brittle behaviour.

Similarly, Pitanga et al. [60] compared the performance of the straight scarf joint to an optimized ply-wise wavering angles joint aiming to minimize the repair size. The optimized joints were developed with scarf ratio 1:30 for 0° plies and 1:2 for the 45° and 90° plies (Optimized scarf 1:30/1:2) and with scarf ratio 1:20 for 0° plies and 1:2 for the 45° and 90° plies (Optimized scarf 1:20/1:2). The experiment showed that the optimized joint with a ratio of 1:30/1:2 can attain 64% of the tensile strength of the straight scarf joint while occupying just 40% of the size. This was enhanced to 75% by attaching two 0° plies in order to secure the scarf tip.

Pierce and Falzon [61] proposed a unique fiber-oriented scarf design in which the scarf bonding direction changes simultaneously with changing the scarf angle of any ply aiming to diminish the joint length. Consequently, the scarf angle increases in each ply from the optimal minimum fiber direction. Their parametric analysis perceived notable advantages in area reduction were observed in composites that have higher plies number, thicker plies, or smaller scarf angle.

The Effect of Plies Orientation

Matta and Rajmi [62] studied the mechanical behavior of both adhesively bonded single tapered scarf joint and double tapered scarf joint made of CFRP adherend taking into account a unidirectional [0°]₁₆ and quasi-isotropic [+45/−45/0/90]_2S CFRP adherends.

In the case of UD layup specimens, it is discovered that the largest strain values occur closer to the free surface and the cohesive failure mode took place, while the specimen fails in three stages in the quasi-layup: the 900 plies experience transverse intralaminar failure (also known as matrix failure), which is followed by adherend interlaminar delamination between the 900 plies and neighbouring plies, and lastly, the adhesive layer experiences a final cohesive failure.

To determine the governing differential equations of equilibrium for the adhesively bonded single scarf joint, Sonwani et al. [63] created an analytical model. The analytical model is numerically solved using the finite difference technique and validated by the experiment performed in [62]. In this study, CFRP laminates that are unidirectional or quasi-isotropic are considered adherends. When a laminate is unidirectional, cohesive failure is seen. Nevertheless, matrix failure and interlaminar delamination are also noted in the case of quasi-isotropic laminates. It was also discovered that the single scarf UD laminates’ ends are where the largest axial and shear strains occur. The joints’ ends experience the highest shear stresses also because of the taper in the adhesive layer. This explains why adhesive joins tear under heavy loads.

In order to better design and use scarf joints for wind turbine blade shell repairs, Ghafafian et al. [64] aimed to identify the critical areas and the mechanism of damage in scarf joints to uncover the significance of the fibre orientation mismatch. For this purpose, two orientation, [−45°/+45°]_2S and [0°/90°]_2S were tested. The reason for the failure mechanism of the [±45°] repaired specimens was the same as that of the control specimens because intralaminar strength is lower than interlaminar strength. As a result, the failure route is determined by intralaminar propagation across the thickness of the specimen, and the scarf joint has no bearing on it. The 0°/90° specimens’ failure mechanism altered when a scarf joint is present, failing along the junction between the source material and the repair. This is because, in this case, when there are zero fibres in the direction of load, the processes at work are fibre strength against interlaminar strength. Since interlaminar strength is the weaker of the two, the propagation path mostly follows the scarf joint.

To study the effect of the surface treatment, AlYousef et al. [11] conducted research to investigate the effect of the CO₂ laser on the tensile strength of scarf joint with associated failure modes of unidirectional (UD) and quasi-isotropic (QI) CFRP laminates. The cohesive failure and fibre tear that were encouraged by 90, +45, and −45 plies resulted in an increase in joint strength of around 30% and 8%, respectively, which is why the laser therapy techniques employed were recommended. In contrast, the strength of UD scarf joints is not significantly impacted by the uniform laser treatment. The influence of the plies orientation is concluded in Table III.

Table III. The Effect of Layup Sequence on SAJ

Reference	Orientation	Conclusion
Ghafafian et al. [64]	[0°/90°]	• Stress concentration at the joint interface which leads to delamination with a reduction in the joint strength due to interlaminar failure.
	[±45°]	• Intralaminar failure through the thickness of the specimen leading to fiber-matrix debonding and matrix cracking with minimal impact on the joint.
Sun et al. [57]	[0°]24	• As the proportion of the [0°] increases, the strongest the joint becomes.
	[0°/90°]S6
	[±45°]S6
	[0°/45°/−45°/90°]3S	• The joint’s strength decreases when the 0°, ±45° and 90° plies in same proportion and different stacking sequences.
	[45°/0°/−45°/90°]3S
	[90°/−45°/0°/45°]3S
Matta and Rajmi [62]	[0°]16	• The strain distribution is more uniform along the adhesive bond line.
		• The strain distribution is more uniform along the adhesive bond line.
		• Higher stiffness and strength.
	[45°/−45°/0°/90°]	• The varied fiber orientations distribute the load more evenly but result in lower overall stiffness and strength.
		• The strain distribution is more complex due to the varying stiffness of the different plies.
		• Multiple interacting failure modes.

Joint Imperfection

Several studies have conducted to evaluate the effect of the joint imperfection on its strength either experimentally, numerically or analytically. The analysis of Haraga et al. [65] on SAJ showed that the stress intensity factor of a small interface crack is an effective method for assessing the interface’s bonding strength. Su et al. [66] investigated the effect flow size and flow position on scarf joint of composite material and a variety of observational methods were combined to determine the failure modes. On a stacking sequence of [45/0/−45/90], the study revealed that the adhesive layer failure is particularly more common in 0° and ±45° plies, while interlaminar delamination also occurs inside the composite adherends. Complete peeling of the sharp corners in 90° plies and marginal peeling in ±45° plies on the fracture surfaces affect adhesive layer performance. The scarf joint with a centre imperfection exhibits better strength than the joint with a tip flaw, and the flaw size grows as the joint strength diminishes. Hayes-Griss et al. [67] found that the joint failure had gradually propagated with increasing the load as it started by matrix damage at the crack and scarf tips, delamination in the adjacent damaged plies and finally by adhesive failure. The comparison of the flow size and location conducted by Wu et al. [68] showed that the flow length growth was twice for the central flow, of the mid-thickness plane, in comparison to edge flow. Additionally, setting up the half flow reduces the damage tolerance performance as the strain concentration at the flow tip intensifies as the full- width flow minimized to a half-width which decrease further when the half-width flow was situated on the edge. In contrast, Guo et al. [69] stated that the joint ability of resist the load decrease as the flaw location situated far from the center of SAJ.

Conclusion

It can be concluded from the former research that the scarf angle and the bondline thickness play an important role on the fatigue properties of the adhesive joint. The modification of the joint by incorporating CNT, SiC or Al2O3 has improved the fatigue lives of the bonded joint. Furthermore, the tensile tests verified that failure is determined by the strength of the parent material and that high joint strengths may be attained if the scarf angle is sufficiently small. Despite this, additional angle decreases only little affect the total strength. In contrast, the adhesive handles the majority of the tensile load at large scarf angles, which causes the joint to fail prematurely. At the same time, there is a strong correlation between the material characteristics of the adherend and the damages, which likely affect the bondline stress of the scarf joints. The damage occurred in the most cases in the SAJ tip (adhesive damage) or in the center of the SAJ damages (cohesive damage), which are highly correlated to the fiber orientation.