An investigation of the stitching effect on single lap shear joints in laminated composites

2.1 Test specimens

Fig. 1 presents a sketch of the single lap joint tested in this study. The lapping plates studied here were made of the composite laminate G0827/BA9912. The layup information is listed in Table 1, and the mechanics properties are detailed in Table 2. Paste adhesive EC-3448 was used between the two lap panels. Both the stitched and unstitched composite single lap joints tested in this paper were made by vacuum assisted resin infusion molding technology (denoted as VARI). The modified lock stitch was used (Fig. 2) with a stitching parameter of 5 × 5 mm.

Figure 1

Geometry of the single lap joints

Figure 2

A modified lock stitch

2.2 Test equipment and procedure

Stitched and unstitched single lap joints were tested under different environments. The experiments were grouped into three types: RTD (room temperature/dry), CTD (cold temperature/dry) and ETW (elevated temperature/wet). The cold temperature was 55^∘C below zero, while the elevated temperature was 70^∘C. Six specimens were tested in each group, for a total of 36 specimens. The load was applied at a crosshead speed of 1 mm/min based on the HB7070 specifications. The single lap joints were subjected to static tensile tests using the INSTRON test machine with a load cell of 3T. The tests in the CTD and ETW environments were conducted in a chamber with the temperature controlled from −70^∘C to 300^∘C. The wet specimens were immersed in deionized water at 90^∘C for several days until the moisture reached equilibrium and then placed at the elevated temperature condition of 70^∘C. An extensometer with a maximum measurement range of 25 mm was used to measure the elongation of single lap joint specimens. The experimental setup for the shear test of a single lap joint is shown in Fig. 3.

Figure 3

Experimental setup of the shear test

3.1 Load-displacement curves

A large amount of experimental data was obtained from these tests, but only a few representative results were chosen to present in the paper. Fig. 4 shows the load- displacement curves of stitched and unstitched single lap joints. The experimental data of single lap joints is presented in Table 3. The dispersion in the experimental data for stitched lap joints is considerably less than that for unstitched lap joints under all three environmental conditions (RTD, ETD or CTD). The main reason for the lower dispersion is that the strength of stitch line plays a key role in the bearing capacity of the stitched co-curing lap joints, and the strength dispersion of the stitch line is relatively small. In contrast, the bearing capacity of unstitched co- curing lap joints mainly depends on the adhesive strength of the matrix, which has a relatively large dispersion. To a large degree, the adhesive strength is related to the bonding process, adhesive thickness, surface quality of lapping plates [16, 17, 18].

Figure 4

Plots of load versus displacement

Interfacial damage includes different models, such as model I, model II and model III. Model I is normal mode, model II is first shear mode and model III is second shear model. To describe interfacial damage initiation and evaluation, strain energy release rate is the key parameter. The strain energy release rate of model II is obtained from the stress-strain curves of the samples under shear loading, with the strain energy release rate approximately equal to the integral area of the stress-strain curve. The results can be used in the cumulative damage analysis of a single shear joint.

According to the J integral proposed by Rice [19], the boundary method can be written as:

where G_C is the critical strain energy release rate, and δ_fail is the maximum displacement before the sample fails. G_II and G_III are the critical strain energy release rates in mode II and mode III, respectively. The strain energy release rate of model II is similar to that of model III, denoted as G_II = G_III. The calculation results are shown in Table 4.

From Table 4, some results can be seen as follows. For the case of stitched lap joint, the shear strength values are close together, but for the strain energy release rate, the value of the CTD environment is minimum. For the case of unstitched co-curing, the shear strength values of the RTD environment is minimum, however, the strain energy release rate of the CTD environment is minimum. Shear strength is closely related to interfacial binding force, and stitched lap joints are mainly dependent on the properties of stitch, which has little change in CTD environment and ETW environment. So the stitched single lap joints were almost equal to each other in three environments. But the shear strength of unstitched single lap joints is almost dependent on the properties of adhesive. The adhesive become densification in cold temperature, but occur re-curing in high temperature. Therefore, the shear strength is minimum in RTD temperature. The strain energy release is related not only to shear strength, but also to fracture toughness. Because the adhesive and stitch become brittle in cold temperature, the strain energy release of both stitched and unstitched were minimum.

G_equivC is the critical equivalent strain energy release rate calculated based on the user-specified model-mix criterion and the bond strength of the interface, and G_equiv is the equivalent strain energy release rate at a node. Abaqus/standard provides three common mode-mix formulas for the fracture criterion: the Benzeggagh-Kenane (B-K) law, power law, and Reeder law models. The choice of model is not always clear in any given analysis, and an appropriate model is best selected empirically. The B-K law is always used in aviation epoxy adhesives. G_equivC of the B-K law is shown in equation (2) [20]. If G_equivC is greater than 1.0, the adhesive fails.

3.2 Failure mode

The specimens were clamped in the special fixture, and the environmental chamber provided an accurate testing environment. In the early stage, the load-displacement curves were linear, and the specimens appeared undamaged. In the later stage, the specimen made a slight sound, and a large fluctuation occurred on the load-displacement curve. The sound was predicted to come from the rupture of fibers in the lap transition zone because of stress concentration. When failure occurred, the load-displacement curve dropped rapidly, and the load was not able to keep up with the load in displacement control. The main reason for this failure is that at the later loading stage, the lapinterface had been damaged and that the effective lap area was reduced. Fig. 5 presents fracture photos of the stitched and unstitched single lap joints under a shear load. The location of damage was the interface between the lapping plates. For the unstitched single lap joints, a small amount of fiber is pulled out near the fracture surface. For stitched specimens, the fracture of the suture is flat, occurring at a highly uniform location. As for unstitched single lap joints, a small amount of fiber was also pulled out. The fiber always pulled out on the thinner lapping plate for stitched and unstitched lap joints.

Figure 5

Damage photos

3.3 Comparison and analysis

The main results concerning the joint strength and failure modes of stitched and unstitched single lap joints are summarized as follows.

1. Stitching can greatly improve the properties of single lap joints. In the RTD, CTD and ETW environments, the shear strength and fracture energy release of stitched single lap joints were considerably greater than those of unstitched joints, except in the ETW environment. Tests showed that the shear strength of stitched lap joints was up to 2.5 times that of conventional co-cured specimens and that the fracture energy release rate was up to 4 times greater. The main reason for this difference is that stitching technology improves the bond strength of shear joints.

2. In the ETW environment, the bond strengths of the stitched and unstitched single lap joints were almost the same. The main reason for this result is that the EC-3448 paste adhesive re-cured at 85 degrees, resulting in a considerable increase in bond strength after re-curing. The EC-3448 paste adhesive exhibits excellent heat resistance and humidity resistance below 90 degrees Celsius.

3. Stitching technology enhances the interfacial properties, including the fracture energy release rate. In designing lap joints and stiffened composite panels, stitching technology can be carried out with appropriate parameters.

4.1 Finite element modeling

The method used to model the connection between the stitch line and the lapping plates is the key technology in modeling stitched composite structures.

After comparison with various modeling methods, a new, accurate, and simple method is proposed. The B2 element is used for the stitch lines, and the C3D8 element is used for the lapping plates. The stitch lines and lapping plates were modeled, and the stitch line is connected with the lapping plates by embedded interactions. The geometric relationship is shown in Fig. 7. The finite element model and boundary conditions are shown in Fig. 8 and Fig. 9. The displacement force was applied on one end, and the shear strength was calculated by the reaction force of fixed end. The finite element model of the stitch line is in good agreement with the actual structure, which is also simple to model. The material parameters are defined according to the experimental results.

Figure 7

Geometric relationship between the stitch line and lapping plates

Figure 8

Finite element mode

Figure 9

Load and boundary condition

4.2 Finite element model sensitivity studies

To study the effect of the stitch density on the shear strength, three finite element models with different stitch densities were established. The stitch densities were 8×8 mm², 5×5 mm² and 3.5×3.5 mm², and the unstitched single lap joint was also studied for comparison. The load-displacement curves are shown in Fig. 10.

Figure 10

Load-displacement curves of different stitching parameters

The error between the numerical result and the experimental result is less than 5%. The experimental shear load of the stitched lap joint with a stitching parameter of 5×5 mm² is 23.7 kN, and the simulated result is 25.2 kN. The experimental shear load of the unstitched lap joint is 10.4 kN, and the simulated result is 10.1 kN. The model established in this paper can be used to simulate the failure of single lap joints, including stitched and unstitched lap joints.

The shear strength was considerably improved by stitching, and the bond strength increased rapidly with increasing stitch density. The stitched lap joint with a stitching parameter of 3.5×3.5 mm² had the greatest shear strength, almost 4 times greater than that of the unstitched lap joint.

Fig. 11 shows the damage generation and extension process for the stitched lap joint with the parameter of 5× mm². Fig. 12 shows the change in the stress distribution of the interface with displacement load. The cracks in the stitched shear specimens were initially generated at the edges of the adhesive layer and then spread to the stitch line. When the stress was greater than the maximum stress of the suture, the suture fractured. The first row of the stitch lines ruptured first. After that, the adhesive layer expanded a little further, and then, the second row of sutures fractured. The sutures and the adhesive layer failed one row at a time until all rows of the sutures fractured. The stress contour indicates that the position of the stitch line is the point of local stress concentration on the lap board. As the rigidity of the stitch line is much greater than that of the adhesive layer, the stitch line takes on most of the shear load. Alternatively, the stitch line can be said to block the extension of damage, and the rupture of the stitch line leads the crack to grow. As the fracture load increases, the shear strength of the interface is considerably improved.

Figure 11

Curves of the interface damage generation and expansion of the stitched specimens

Figure 12

Stress distribution diagram of the interface

When the displacement increased from 0 mm to 0.8 mm and the shear load increased from 0 kN to 25.8 kN, the stitch line began to break. After that point, the stitch line gradually fractured, and the shear load decreased rapidly. This process is basically consistent with the experimental results.Therefore, the proposed simulation method can be used to predict the shear load of single lap joints of stitching composite materials.