Statistical Investigation of Parameters Influence on Fracture Toughness of the Glass Fiber Reinforced Composites
The objective of the work was to investigate statistical analysis of fracture toughness of the glass fiber reinforced composites at different stacking sequences, strain rate and crack length. The glass reinforced vinylester composites prepared by hand layup technique with stacking sequences of 0/90, 45/-45 and chopped strand orientations. 3- point bending for fracture toughness test for various notch depth ratio's as per linear elastic fracture mechanics concept was conducted. Further the initial notch depth method was adopted to find the critical stress intensity factor KIC for the given notch depth ratio. Design of experiment using Taguchi L9 array was formulated to understand the influence of parameters on fracture toughness. The result of surface methodology showed the stacking sequence of (45/- 45/0/90/Chopped)3s, with strain rate of 0.5 mm/min and without notch having the highest fracture toughness of 81.36 Mpa
Fatigue Model for Composites Based on the Cycle-by-cycle Probability of Failure: Implications and Applications
Further implications of a fatigue model based on the cycle-by-cycle probability of failure are presented. A model for the residual strength is developed based on a linear relation between the residual strength at any cycle level and its derivative. This model is shown to be in excellent agreement with published test results. The model is also shown to lead to a constant cycle-by-cycle probability of failure under constant amplitude loading thus verifying the assumption of constant probability of failure made previously. The model is then used to construct constant life (Goodman) diagrams for composite structures and its predictions are compared to published test results. The model follows the test data well but needs further improvement for negative R values. The approach is then extended to the determination of the truncation level required for a structure to meet a certain fatigue life. The truncation level is a function of R ratio and the amount of statistical scatter for static tension and compression tests. Using representative values for the scatter, the predicted truncation level of 38% is shown to cover most applications. Possible reasons for discrepancies and areas where more work is needed are identified.
An Exploratory Study Into the Fatigue of Composites Under Spectrum Loading
A fatigue and residual strength degradation model for constant amplitude cyclic loading has been modified and applied to fatigue of composites under service loading spectra. An exploratory study is made to describe the fatigue behavior of composites subject to spectrum loading using some base-line constant amplitude data. Available test data for both composite laminates and bonded joints under service loading spectra are used to correlate with the theoretical results.