Micro-strain Measurement And Statistical Characteristics Of Acoustic Emission For Delaminated Composites

Realizing the accurate characterization for delamination damage and evolution of composite laminates is great challenges.In view of the problem that it is easy to have a delamination of the composite laminates structure in the process of service vulnerability.Here,we carried out testing simultaneously for dynamic monitoring and acoustic emission(AE)statistical analysis;AE monitoring and strain measurement;both AE and digital image correlation(DIC)technology towards fiber composite specimens under delamination damage.The load curve,AE relative energy,amplitude distribution are obtained respectively and combining AE statistical analysis and micro-deformation measurement methods to investigate the mechanical properties,failure characteristics of interface and AE responses of the composite laminates.Through investigations contrasts,the following conclusions are obtained:(1)AE dynamic monitoring and AE statistical analysis were used to investigate fiber composites under mode-II delamination damage.Results show that micro-damage accumulation around the crack tip region has a great effect on the evolutionary process of delamination.The physical mechanisms of delamination damage such as matrix cracking,debonding and fiber breakage originating from the hierarchical microstructure evolution.AE characteristics and amplitude spectrum represent the damage and the physical mechanism originating form the hierarchical microstructure.Our finding provides a novel and feasible strategy to evaluating the dynamic response and micro-damage mechanism for the fiber composites simultaneously.(2)AE technology strain measurement were used to monitor the damage evolution and failure process of composite specimens embedded with circular delamination which diameters were 5mm and 10 mm respectively.Results show that the delamination growth and the failure mechanism of two kinds of specimens with different delamination were similar under compressive loading.The area of delamination has great influence on the bearing capacity of specimens.The larger area of delamination,the lower carrying capacity and the more serious damage of the specimen are obtained.The load-longitudinal strain curve changes from liner to approximate liner to nonlinear.The change process was coincided with the analysis results of AE signals.(3)AE and DIC technology were used to simultaneously monitor the buckling process of multi-delaminated composites under compression.The results show that the compressive failure process of the multi-delaminated composites could be divided into three stages: delamination opening,local buckling and global buckling.The buckling load is significantly influenced by the thickness of sub layer,the size and position of multi-delamination defects.AE parameters such as amplitude,durations and relative energy are correlated with the damage initiation and propagation.DIC method is capable of investigating the micro-displacement fields and strain fields for composite specimens,and the remarkable deformation and strain around the delamination regions are obtained accurately.The complementary NDT combining AE with DIC are beneficial for in situ damage monitoring of the composites.