Significant thermal stresses are introduced into the adhesive layers of a metal–composite bonded joint due to the large differences in coefficients of thermal expansion between the metal and composite adherends. In this study, theoretical analysis of shear and peel stresses in the adhesive layers of a double-lap metal–composite bonded joint was performed to evaluate the effects of thermal and mechanical loads on the stress distribution in the adhesive layer. The results obtained by the theoretical analysis and the finite element method were compared to validate the derived theoretical analysis. The effects of temperature change and adhesive thickness on the shear and peel stresses in the adhesive layer of the bonded joint, with and without external force, were examined theoretically. The results calculated for the conditions involving mechanical load application to the bonded joint and decreasing temperature indicate that the absolute value of shear and peel stresses peak at both ends of the adhesive layer and that the absolute value of the peak stresses increases with a thinner adhesive layer. When mechanical and thermal loads are simultaneously applied to a double-lap joint, shear and peel stresses synergistically increase at one end of the adhesive layer and decrease with offset at the other end.
Impact damage characteristics and compression after impact (CAI) strength of curved CFRP laminates were experimentally investigated. Various curved and flat CFRP laminates were prepared with special impact fixtures and compression loading fixtures. Impact tests with a range of energy levels were executed on the prepared laminates. Damage areas and their shapes after impact on the curved CFRP laminates were measured by an ultrasonic C-scanning machine. It was found that the CFRP curvature affected the impact delamination shapes (aspect ratio: AR). The delamination propagation direction followed the specimen curvature which was increased with increasing AR. After C-scope observations of delamination, compression strength tests were performed. CAI strength degradation was detected on several curvatures of 0.004 and 0.01. To further investigate the relation between CAI strength and delamination shapes, we calculated a shape factor (S) that was considered for a rectangular plate in the buckling equation. Using the actual delamination sizes as the plate size parameters in the rectangular plate buckling equation, S was calculated. The impact energy and curvature as functions of S showed very similar trends when correlated to their CAI strength test results. The results presented herein suggest that the CAI strength of curved specimens is closely related to their delamination sizes and shapes that are determined by the curvatures in the specimens.
The elastic properties of thermoplastic composites reinforced with randomly oriented chopped strands are measured and statistically evaluated. Chopped fiber composites exhibit a probabilistic nature due to the randomness in their fiber orientations, and packing arrangement; hence, a probabilistic estimation is essential to provide an accurate representation of their performance. An analytical approach is then proposed, and explicit expressions for the probabilistic values of the elastic properties are approximately given in terms of basic data such as dimensions of the gage area, and dimensions and elastic properties of the randomly oriented strands. The estimated elastic properties of the composites agree well with the experimental results. Present formulae that can suitably express the effects of the sizes of the gage areas on the mean values and standard deviations of the tested results help to determine some rules on the testing standards and the method of data acquisition.
A longitudinal compression test for a single polyacrylonitrile-based carbon fiber (T300) was performed using a scanning electron microscope. The compressive stress/strain behavior was initially linear, but subsequently became nonlinear. The longitudinal tangent modulus decreased with increasing compressive strain. A cyclic compression test revealed that the T300 carbon fiber deformed elastically up to ∼90% compressive strength. The variability in the compressive strength was evaluated using Weibull analysis. The representative compressive strength of the T300 carbon fiber was nearly the same as the tensile strength. The compressive strength of the T300 carbon fiber was almost same as that of the high-tensile-strength T800S carbon fiber. Finite element analysis was performed to investigate the validity of the test method. The results showed that the longitudinal compressive stress on the carbon fiber varied during longitudinal compressive loading. The maximum longitudinal compressive stress in the carbon fiber was slightly higher than the average compressive strength applied at the end. However, the variability in the measured compressive strength was much higher than that in the longitudinal compressive stress on the carbon fiber, which does not affect the former.