Multidirectional Damage Theory Of Composite Laminates With Experimental Verification

As composite has been used in engineering more and more widely,conservative-leaning application in the aerospace industry will not satisfy the requirement of market and society development.This raises higher demand toward the analysis method on composite mechanical properties,with studying damage mechanism deeply and systematically.Based on the framework of composite damage mechanics,this paper built the relationship between composite mechanical properties and load outside with the ―bridge‖ – damage.This paper researched on the change of multidirectional stiffness properties of composite with damage and the change of damage with loading history respectively.Therefore,the change of stiffness properties with load outside was obtained.Three models were established,including continuum multidirectional damage mechanics(CMDM)for stiffness properties of composite laminates,multidirectional damage collaborative evolution(MDCE)model for matrix cracking in composite laminates under quasi-static loading and fatigue loading.Firstly,this paper reviewed damage mechanics of composite,by introducing the research progress the effect of damage on stiffness properties and on the damage evolution.From the perspective of System Theory of Failure Study,this study elaborated the key factors in the study of damage mechanics.According to the characteristics of composite damage,synergistic multi-scale modeling was employed.Multidirectional damage theory was proposed with studying multidirectional influences of damage at different scales.A series of experiments for E-glass/YPX-3300 glass fiber reinforced composite were tested,including experiments for the basic mechanical properties,damage evolution experiments of plate specimens with composite laminates under quasi-static loading and under fatigue loading,verification experiments for multidirectional damage with tubular specimens.For stiffness properties of composite laminates,continuum multidirectional damage mechanics is established at multi-scales.At micro-scale,the model defined a damage tensor,and obtained multidirectional micro-responses with the finite element method and the parametric modeling.At macro-scale,the stiffness properties of single ply and laminates were deduced as functions of the damage tensor.This model has been applied to characterize the influences of the diffused damage modes on the stiffness properties.Adopting the experiment data both from Varna’s work and this paper,the capacity of the present model to predict multidirectional stiffness properties was verified.Multidirectional damage collaborative evolution model under quasi-static loading employs an energy-based method.At macro-scale,the expression of the energy released rate for matrix crack damage mode was derived in terms of the micro-responses of damage.And the matrix cracking process was simulated by the energy criterion.This model provided the simulation flow,considering several key factors,such as damage interaction,residual stress,the nonlinearity of matrix,initial damage distribution and the inhomogeneity of damage evolution.The simulation results agreed well with the experiment results.And it is shown that this model is capable to predict composite damage evolution under quasi-static loading.At last,―in-situ‖ effect in laminates was discussed.Under fatigue loading,multidirectional damage collaborative evolution model adopted the life curve for initial matrix cracking to characterize the fatigue performance of the cracking ply in composite.Damage accumulation theory was based on the classical Miner fatigue damage theory.As same as the quasi-static model,those key factors should also be taken into consideration.Among the factors,initial damage distribution influences the fatigue performance of the cracking ply.After bringing in the distribution of initial damage,the life curve for any matrix cracking in the potential place would be modified.In addition,the influence of the fatigue stress level was also taken into consideration.According to the simulation flow,the experiments for the damage evolution in cross-ply laminates under constant fatigue loading were analyzed to verify this model.The test data of damage evolution mostly located between the simulation boundary lines.And the simulation curves of the residual stiffness fitted well with the test results at both early and middle stages.Iso-damage curves for the laminates were obtained by summarizing the simulation results.And these curves can be utilized to analyze damage evolution subjected to any loading level conveniently...