Compression Shear Properties of Adhesively Bonded Single-Lap Joints of C/C Composite Materials at High Temperatures

Abstract

The performance of joint structure is an important aspect of composite material design. In this study, we examined the compression shear bearing capacity of the adhesively bonded single-lap joint structure of high-temperature-resistant composite materials (C/C composite materials). The test pieces were produced in accordance with the appropriate ASTM C1292 standard, which were used for the compression shear test. The failure morphology of the layer was observed by a digital microscopic system and scanning electron microscope. The experimental result shows that the load on the test piece increased nonlinearly until the failure occurred, and most of the adhesive layer exhibited cohesive failures at three temperature points (400, 600, and 800 • C), while the interface failures occurred in a small part of the adhesive layer. A numerical analysis model was established using ABAQUS finite element software. The simulation results were compared with the test results to verify the correctness of the model. On the basis of correctness of the model verified by comparing the simulation results and the test results, the influences of temperature and overlapped length on the joint compression shear performance were studied through the validated simulation method. Numerical results showed that the ultimate load of the joint decreased with increases in temperature and that the distribution trends of the shear stresses in the overlapped length direction were substantially the same for joints of different overlapped lengths.Adhesively bonded joints nonetheless have some disadvantages due to their processing and associated overlapping. The bearing capacity of adhesively bonded joints is considerably affected by the manufacturing process, application environment, geometric dimensions, and the gap between adhesive layers. In particular, for bonded composite structures used in extreme environments such as high temperature and high humidity, joint strength presents an obvious dispersion. The strength of the adhesive changes with the temperature, resulting in changes in the bearing capacity of the joint. Although an adhesive layer has a strong shear capacity, its peeling resistance is poor. A reasonable structure should be designed based on the direction of the maximum load so that the joint can transfer the load as much as possible in the form of shearing. For example, in an adhesively bonded single-lap structure, the peeling stress at both ends of the adhesive layer increases due to secondary bending [6], resulting in failure of the adhesive layer. To reduce the torque generated by an eccentric load, the thin plate is usually selected as the adherend of an adhesively bonded single-lap structure. However, for materials with high brittleness such as C/C composite materials and C/SiC materials, the composite material undergoes brittle fracture if the adhesive strength is large when a thin plate is used for the single-lap joint. Therefore, in practical applications, a scarf joint [7] is used, or alternately the single-lap structure of bri...

Compression Shear Properties of Bonded–Bolted Hybrid Single-Lap Joints of C/C Composites at High Temperature

“…Due to the high cost of C/C composites, three sets of test pieces were provided. In order to compare the bonded-bolted hybrid joint test results with the bonded joint test results at 800 °C in [30], in this study, the adhesive type, the adhesive curing method, the surface treatment method of test pieces, the testing equipment, and the fixture and loading method ( Figure 3) were all completely consistent with that study. This study differed in that holes needed to be made in the center of the upper and lower cover plates to connect the adherend with the fasteners to control the adhesive thickness of the hybrid joint.…”

“…The material of the adherend was C/C piercing woven composites used in [30]. The material of the bolt is C/C orthogonal three-directional woven composites.…”

“…In addition, it can reduce the hourglass problem and is not sensitive to element distortion [31]. For the adhesive layer, the same cohesive element as in [30] was adopted for modeling, and the element type was COH3D8R. As shown in Figure 5b, meshes of the adhesive layer, hole, and bolt rod were refined, respectively.…”

“…In order to facilitate the comparison of the calculation results of the hybrid joint and the adhesive joint, the same bilinear constitutive relation as in [30] is used to simulate the failure process of the adhesive layer. For damage to the bolt, the three-dimensional Hashin failure criterion is used, and it can be expressed as follows:…”

“…Moreover, to improve the mechanical properties of the structure, it is essential to reduce the bending effect of single-lap joints due to eccentric loading and improve the shear bearing capacity of the adhesive, and there are few reports [29] on this aspect. Based on the research of [30], in this study, the failure modes and force transfer mechanism of C/C composites single-lap joints at 800 °C were obtained by means of tests and numerical simulation. The experimental results were compared with the calculated results to verify the correctness of the numerical model.…”