Fatigue Strength and Endurance of Adhesive-Bonded Joints

Abstract

Fiber reinforced plastics (FRP) are increasingly being used in many areas as large-size construction or connecting structures. Adhesive constructions are applied for architectural parts, various kinds of water tanks, sewage purifiers, etc. But the reasons and methods of using the adhesive construction and the estimations of the life of these constructions have not been clarified yet.
This paper concerns with the durability of adhesive joints of FRP by means of an accelerating exposure testing machine, and investigates the fatigue properties of adhesive joints. All of the adhesive joint specimens were treated with popular construction of glass roving and mat structure, and were bonded with epoxy-polyamides adhesives. Accelerating exposure hours for the periods of 0, 1500, 2000, 3000 and 4000 hours (4000 hours are about 6 month) were chosen, and properties of joint structures were examined for the static and the fatigue strengths. The static tests investigated were the average shear strength of adhesive joints and the bending strength of glass fiber reinforced plastics (GFRP) materials exposed to the accelerating exposure testing machine. Fatigue tests were carried out under the stress constant amplitude with tension-shear loads, and the fatigue strength as well as the stress-strain hysteresis loop were examined. According to the test of accelerating exposure, damages of adhesive joints were not detected for 3000 hours in either the static or the fatigue strength, but, with the exposure time over 3000 hours, the static and the fatigue strengths abruptly went down. Therefore, the accelerating exposure time leading to damages is believed to exist between 3000 to 4000 hours.
It is revealed that adhesive structures are strengthened in the beginning of accelerating exposure and break down abruptly with the lapse of exposure time. Accordingly, the design of FRP adhesive joints must be generally taken into consideration of the durable hours of these structures and the fatigue behavior of abrupt fracture under the tension-shear loads.