Experimental investigation of fatigue behavior of adhesively-bonded glass fiber joints
The fatigue behavior of adhesively-bonded laminates made out of glass fiber reinforced plastics was experimentally investigated and modeled under constant amplitude loading. Double strap lap joints were examined under seven various stress-ratios (R) denoting the ratio of minimum to maximum cyclic stress. The ratios selected represent tension-tension (T-T), tension-compression (T-C), compression-tension (C-T) and compression-compression (C-C) fatigue-loading conditions. The R-ratios selected for these fatigue-loading conditions were R = 0.5 and R = 0.1 for T-T, R = -1 and R = -0.5 for T-C, R = -2 for C-T, and R = 10 and R = 2 for C-C. The S-N curves under different R-ratios showed both mean and amplitude strength of the joints to be decreasing with an increasing number of applied numbers of cycles. The S-N curves also showed that amplitude strength of the joints was low under tension-tension and compression-compression fatigue loading. Amplitude strength was high under both compression-tension and tension-compression fatigue loading. The mean strength of joints was high under both tension-tension and compression-compression fatigue loading. Mean strength was low under compression-tension and tension-compression loading. The relationship between mean and amplitude stresses was investigated using a constant life diagram (CLD). The diagram demonstrated that joints examined under R = -1 have the highest stress amplitude when plotted against mean stress. For all constant lives, the amplitude stress changed linearly with the mean stress under different loading conditions. Changing of failure type from compression to tension was observed on this diagram as mean stress increases from negative to positive values. The relationship between mean and amplitude stress at any constant life was then modeled using different mathematical formulae. Linear, second-order polynomial, third-order and fourth-order polynomial expressions were used to model this relationship for the entire range of values of mean stress defined between ultimate compression strength and ultimate tensile strength. The predictive accuracy of each of these models was evaluated by calculating the statistical distance (r2-value) between experimentally-derived amplitude values and the amplitude values predicted by the model at randomly chosen number of cycles. Four more models commonly used for composite materials were then applied, to compare with the established formulas. Piecewise-linear modeling was established as a preferable method for modeling mean and amplitude stress.
- Engineering