Fatigue of hybrid (adhesive/bolted) joints in SRIM composites

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Abstract

This paper presents a study on the static and fatigue performance of adhesive/bolted (hybrid) joints in a structural reaction injection molded composite. It is shown that the hybrid joints have a higher static failure load and longer fatigue life than the adhesive joints. However, the performance of hybrid joints depends on the washer design. Washers that provide full lateral clamping pressure over the entire overlap area have a better performance than those that provide partial lateral clamping pressure. Finite element analysis of adhesive joints shows that the presence of lateral clamping can significantly reduce the maximum peel stress at the adhesive–substrate interface and thus help in achieving improved joint performance.

Introduction

Joint design in composite materials has been a concern and also, the focus of numerous studies in the past. Most of the publications in this area have dealt with either adhesive or bolted joints in laminated continuous fiber composites [1], [2]. Studies on hybrid joints combining adhesive with bolts are very few. Sawa and his coworkers [3], [4] have considered hybrid butt joints in which two hollow cylinders are fastened by a bolt and a nut with an initial clamping force after being joined by an adhesive. The external load was either tensile normal to the adhesive layer [3] or torsional, and, in both cases, they observed an increase in joint strength relative to the adhesive joints alone. Recently, with increasing interest in random fiber composites for automotive applications, attention is being paid to joint design with sheet molding compound (SMC) composites and structural reaction injection molded (SRIM) composites. Mallick and his co-authors have published several papers on bolted lap joints [5], [6] as well as adhesive lap joints [7], [8], [9] in SMC and SRIM composites. In an attempt to improve the joint strength of such composites, a hybrid of adhesive and bolted joints has also been explored. In Ref. [10], Satish Kumar and Mallick have reported the experimental work on the static behavior of hybrid adhesive/bolted joints (Fig. 1) in polyurethane–SRIM composites. Static tests were conducted with 25.4-mm wide/3.53-mm thick SRIM specimens, 6.35-mm diameter steel bolts and 19-mm diameter round SAE Grade 8 steel washers. An epoxy adhesive was used for these joints and the adhesive thickness was 0.762 mm. The bolt was centrally located in the 25.4-mm×25.4-mm overlap area. In these tests, hybrid joints failed at a higher load than the bolted joints and with the proper clamping torque (between 3.39 and 5.08 N m) reached the same failure load as the adhesive joints. Furthermore, unlike the adhesive joints, hybrid joints failed in two steps, first by initiation of fiber tear (akin to delamination in laminated continuous fiber composites) at one of the lap ends and then by tensile failure across the bolt hole. This led to a slightly higher overall elongation at failure for specimens with the hybrid joints. Later, static tests with 50-mm wide specimens have shown similar, but perhaps less dramatic results. In fatigue tests with 50-mm wide specimens, the hybrid joints with 19-mm diameter round SAE Grade 8 steel washers, on an average, survived longer fatigue cycles than adhesive joints at high-load levels, but they did not show any improved results at lower loads (Fig. 2). Failure in fatigue also started by fiber tear and when the fiber tear progressed to the bolted area, a combination of half-net-tension failure and splitting (cleavage failure) occurred. In both static as well fatigue tests, failure was initiated by fiber tear and the round washers with their edges located slightly away from the lap ends were not effective in preventing fiber tear.

In this study, static and fatigue experiments are conducted with hard round washers as well as both thin and thick square washers. The purpose of this study is to examine the effects of washer type as well as washer geometry on the performance of hybrid joints. The presence of high peel stresses at the lap ends of adhesive and hybrid joints is considered to be the principal reason for failure initiation by fiber tear [11]. Thus, it is felt that a square washer will be more effective in either preventing or delaying the failure by fiber tear than a round washer, which does not provide lateral clamping in the outside corners of the lap ends. In addition to the experimental work, finite element analysis is performed to examine to the effect of washer type on the peel stresses in adhesive joints with lateral clamping pressure.

Section snippets

Experimental

The structural reaction injection molded (SRIM) material used in this study was an E-glass mat reinforced polyurethane. The mat contained randomly oriented continuous E-glass fibers and the fiber content in the composite was 40% by weight. The nominal thickness of the molded plates was 3 mm. Adhesive as well as hybrid joints were prepared with an epoxy adhesive. The lap length in all three types of joints was 25.4-mm long and the specimen width was either 25.4 or 50 mm. The bolted as well as

Static test results

Static tests were conducted on adhesive, bolted as well as hybrid joints at a crosshead speed of 1.27 mm/min until the specimen failed. The static test results for the adhesive and bolted joints are summarized in Table 1 and the static test results for the hybrid joints are summarized in Table 2. The load–displacement diagrams of various joints are compared in Fig. 5.

The maximum load at which the specimens with an adhesive joint failed varied between 4.24 and 5.38 kN, depending on the adhesive

Numerical analysis on single-lap adhesive joints with lateral pressure

Two-dimensional linear finite element analysis was carried out using ABAQUS finite element software on the 24.5-mm wide adhesive as well as hybrid joints to examine the effect of lateral clamping pressure on the stresses at the adhesive–substrate interface. In the analysis, both the SRIM composite and the adhesive were considered to be isotropic and linear elastic materials. Fig. 16 shows the model geometry as well as the boundary conditions for the analysis. Four-noded, plane strain elements

Conclusions

Hybrid joints give better static as well as fatigue performance than adhesive joints in SRIM composites when fiber tear, the primary failure mode in adhesive joints, is either prevented or delayed by the presence of clamping. The finite element analyses proved that the presence of the lateral clamping pressure can significantly decrease the maximum peel stress at the interface, which helps in achieving improved joint performance. The 25.4-mm square washer in the experiment, representing full

Acknowledgements

The authors would like to acknowledge the support provided by the Automotive Composites Consortium and the Michigan Materials and Processing Institute to conduct this project.

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