J.N. Yang

A Stiffness Degradation Model for Graphite/Epoxy Laminates

Stiffness reduction in composite laminates is an important measure of fa tigue damage. The determination of fatigue damage and the prediction of fatigue life can be made through the development of a stiffness degradation model. This paper proposes a stiffness degradation model that can be used to predict the statistical distribution of the residual stiffness of composite laminates subjected to fatigue cycling. Based on the pro posed model, two analytical methods are presented, which are capable of predicting the stiffness degradation of a particular composite specimen under cyclic loading. One method is based on the linear regression analysis and the other on the Bayesian approach. Experi ments have been performed on graphite/epoxy [90, +45, —45,0], laminates to generate sta tistically significant data for evaluating the proposed analytical models and for verifying the predicted results. It is shown that theoretical predictions for the stiffness degradation of an individual specimen and for the statistical distribution of the entire population corre late reasonably well with experimental results.

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.

Modulus reduction and fatigue damage of matrix dominated composite laminates

Abstract Through the monitoring of fatigue modulus degradation, the cumulative damage of matrix dominated composite laminates due to fatigue can be evaluated. A fatigue modulus degradation model is presented in this paper to predict statistical distributions of fatigue modulus reduction for matrix dominated composites. Regression analysis and Bayesian statistical approach are used with the aid of the model to forecast the fatigue modulus reduction for an individual specimen. Experimental tests were performed on the [±45°]2s graphite/epoxy laminate to generate statistically meaningful data for verifying the model. It is shown that the predictions using the fatigue modulus degradation model correlate well with the experimental results.

Prediction of fatigue damage and life for composite laminates under service loading spectra

Abstract A residual stiffness degradation model, which has been proved to be capable of estimating the cumulative damage in composite laminates under constant amplitude loading, is used to investigate the fatigue damage of composite laminates subjected to service loading spectra. Baseline stiffness reduction data under spectrum loadings are incorporated in this model to predict the statistical distributions of the residual stiffness as well as the residual stiffness of an individual specimen. An empirical failure criterion is proposed and fatigue life distributions for composite laminates subjected to different levels (or severities) of spectrum loadings are theoretically derived. Experimental tests were conducted on graphite/epoxy laminates to generate statistically meaningful data for the verification of the theoretical approaches presented. Experimental test results demonstrate that the predictive capabilities of the proposed theoretical methods are quite satisfactory.

A Stiffness-Based Statistical Model for Predicting the Fatigue Life of Graphite/Epoxy Laminates

A statistical model for predicting the fatigue life distribution of graphite/epoxy laminates under constant-amplitude tensile cyclic loadings has been proposed based on a stiffness degradation model and a failure stiffness criterion. The prediction of fatigue life requires base-line experimental data for the static ultimate strength and stiffness degradation. An experimental program has been conducted using graphite/epoxy [90, +45, −45,0] s laminates to generate statistically meaningful data for the verification of the proposed model

Prediction of fatigue life for matrix-dominated composite laminates

Abstract Fatigue life for matrix-dominated composite laminates under constant amplitude tensile fatigue loading can be theoretically predicted on the basis of a fatigue modulus degradation model along with an appropriate failure criterion. The fatigue modulus degradation model used in this study takes account of the effects of a non-linear stress/strain relationship and plastic deformation which are significant for matrix-dominated composites under fatigue loading. The failure criterion used herein states that a laminated specimen fails when its fatigue modulus falls by a certain amount. From the fatigue modulus degradation model and the failure criterion, an analytical model for the statistical distribution of fatigue lives for matrix-dominated composites is derived. Experiments were also conducted on [±45] 2s graphite/epoxy laminates to verify the theoretical model. Correlations between the analytical predictions and experimental results are shown to be reasonable.

Load sequence effects on the fatigue of unnotched composite materials

A more comprehensive version of an earlier fatigue and residual strength degradation model is proposed to predict the effect of load sequence on the statistical fatigue behavior of composite laminates. The model, which reduces to various fatigue models proposed in the literature by means of approximations, is verified by a survey of experiments on glass/epoxy laminates. It is shown that the correlation between the model and the test results under dual stress levels is reasonable, and that a simplified version of the model is verified by experiments on graphite/epoxy laminates in which the correlation between theoretical predictions and results under dual stress levels is satisfactory. The model is also shown capable of predicting the effect of proof loads on the fatigue behavior of composite materials.

Durability design requirements and analysis for metallic airframes

Air Force durability design requirements are reviewed and durability analysis methodology capable of satisfying these requirements are presented. The proposed methodology includes (1) durability critical parts, (2) guidelines for economic life criteria, and (3) analytical procedures for quantifying overall structural damage due to fatigue cracking. Two analytical formats for quantifying economic life are proposed: probability of crack exceedance and ratio of repair cost/replacement cost. Although the analytical methodology has been developed and verified using a fastener hole crack as the prototype, the basic concepts and methodology apply to cracks in other structural details as well. The durability analysis methodology is based on a fracture mechanics philosophy, using a probabilistic format but with deterministic crack growth. The methodology accounts for initial quality, crack growth accumulation in a population of details, load spectra, and structural properties. Essential elements of the methodology are presented, including an evaluation and illustration of the analytical methodology using test results for the F-16 full-scale durability test article.

Effect of High Load on Statistical Fatigue of Unnotched Graphite/Epoxy Laminates*

The purpose of this paper is to verify experimentally the theoretical model for predicting the effect of the high load (or proof load) on the fatigue behavior of unnotched composite laminates. It is confirmed by the present test program that a specimen is guaranteed to survive a certain number of load cycles as predicted by the theoretical model, after the specimen has survived a high load or proof load prior to fatigue loading. The correlation between the test results and the theoretical distributions of the fatigue life and the residual strength due to the effect of high load is satisfactory.

Proof Test and Fatigue of Unnotched Composite Laminates

A comprehansive Fatigue and residual strength degradation model has been used to predict the effect of proof loads (or high load) on the statisti cal fatigue behavior of composite laminates. The validity of the theoretical model is confirmed by the experimental test results. The correlation be tween the test results and the theoretical distributions of the fatigue life and the residual strength for composite specimens with or without the effect of proof loads is shown to be very good.