Damage Behavior of Composite Structures and Joints at Room and Elevated Temperatures

In this project the damage behavior of S2 glass/toughened epoxy composites of various lay-up configurations was studied under tensile and bending loads at room and elevated temperatures. First miniature dog-bone shaped specimens were tested in situ in a scanning electron microscope (SEM) at room temperature and also at 75 and 125 deg C under tensile loading. For each specimen, using the load and displacement data a stress-strain curve was constructed. It was observed that the curve, which is linear in the beginning, becomes non-linear at higher loads due to damage in the specimen. This damage consisted mainly of debonding along the fiber/matrix interface and fiber breaking and was recorded through microphotographs. The results indicate that the lay-up configuration and temperature have significant effect on the ultimate strength, stiffness and failure strain of the composite. During the next phase of the project the same composite was also tested under 3-point bending loads again at room and elevated temperatures. Mid-span stress versus normalized displacement curves were constructed for each test. The results show that temperature and lay-up architecture affect significantly the stiffness, deformation and ultimate strength of the composite under bending loads. One of the objectives of the project was to measure strains at high temperature using the laser interferometry technique. During this period the experimental set-up was perfected and the technique was successfully tested on aluminum specimens subjected to bending loads. The tests on composite specimens will be carried out during the second phase of the project. Finally, a finite element model based on micromechanics was developed. The damage observed during testing, that is to say, fiber breaking and interfacial debonding was incorporated into the model. The numerical calculations indicate that the observed behavior can be simulated extremely well with the proper choice of interface frictional stress.