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FRACTURE BEHAVIOR OF TYPICAL STRUCTURAL ADHESIVE JOINTS UNDER QUASISTATIC AND CYCLIC LOADINGS ABSTRACTStructural adhesive joints are expected to retain integrity in their entire service-life that normally involves cyclic loading concurrent with environmental exposure. Under such a severe working condition, effective determination of fatigue life at different temperatures is crucial for reliable joint design. The main goal of this work was thus defined as evaluation of fatigue performance of adhesive joints at their extreme working temperatures in order to be compared with their fracture properties under static loading. A series of standard double-cantilever-beam (DCB) specimens have been bonded by three structural 3M epoxy adhesives selected from different applications. The specimens were tested under monotonic and cyclic opening loads (mode-I) in order to evaluate the quasi-static and fatigue performances of selected adhesives at room temperature, 80°C and -40°C. The test results revealed that the fatigue damage occurred at relatively low load levels when compared to quasi-static fracture forces. At room temperature, the maximum cyclic fatigue forces varied between 25% and 40% of corresponding quasi-static fracture loads of selected adhesives. More significant reductions in adhesive mechanical performances were observed at 80°C. At cryogenic temperature, the adhesives had their own characterizations; mainly increasing the fatigue resistance but very sensitive to testing parameters such as loading rate or crosshead speed. In conclusion, the experimental observations showed a significant influence of fatigue loading on adhesive joints mechanical performances that should be considered in joint design, particularly at non-ambient temperatures.

Adhesive joints are normally subjected to different working conditions in their service-life. This may involve both static and cyclic loadings. In many instances, a combination of various loading conditions occurs that can be further provoked by exposure to hostile environments. This, in turn, leads to the need to characterize the joint behavior under different combinations of working conditions. Extensive experimental tests are needed in order to evaluate the joint performance under such variable working conditions. This implies the development of low cost and efficient testing techniques that reduces the extra needs to operator time and sophisticated test procedures.Taking this objective into account, a novel technique in mechanical evaluation of adhesive joints was developed in the present work. Alternative monotonic and variable-amplitude cyclic loads were applied on the same double-cantilever-beam (DCB) specimens under cleavage mode. DCB specimens were made from aluminum bars joined together by a two-part toughened structural adhesive. On one face, a series of crack detection sensors were bonded to control the testing machine for switching between monotonic and cyclic loadings. The testing machine had two aligned hydraulic actuators applying bending forces on the upper and lower arms of the DCB * Corresponding author:Tel.: +1 (418) 545-5247; Fax.: +1 (418) 545-5345; email: mojtaba.eskandarian@cnrc-nrc.gc.ca specimen. The effects of testing frequency and applied load history were also investigated within a range of 4 to 20 Hz for a nominal adhesive thickness of 0.5 mm. The fatigue performance of each configuration was represented by a power-law relationship and was compared for different testing conditions. The test results revealed that the fatigue damage occurred at relatively lower load levels (35%) when compared with monotonic fracture load. The power-law constants for the tested adhesive were influenced by testing frequency but not sensitive to loading order.