Integrity evaluation of glass-fiber composites with varied fiber/matrix interfacial strength using acoustic emission

Abstract

An advanced acoustic emission analysis method was used to study the dynamics and sequence of microfractures in uni-directional glass-fiber reinforced plastics (UD-GFRPs). UD-GFRPs (60 wt% fiber) with different interfacial qualities were prepared by adding a small amount (4 or 8 wt%) of paraffin wax into vinylester matrix without affecting much the mechanical properties of the matrix. The orientation dependence of both the P-wave velocity and attenuation were accurately measured by the laser ultrasonic method, and incorporated into the acoustic emission analysis of visco-elastic media. The analysis iteratively compares the acoustic emission signals (measured as out-of-plane displacements) with theoretical waveforms calculated assuming the time history and type of an acoustic emission source. The waveform simulation allows us to estimate the fracture dynamics and the sequence of the Mode-I fiber fracture (Type 1), Mode-I debonding (Type 2) and Mode-II disbonding (Type 3). The fiber fracture in UD-GFRPs without wax started at 0.1% strain. The initiation strain increased to 0.25 and 0.3% in the specimens with wax. The specimens with lower interfacial strength showed rapid succession of microfractures at higher stresses (above 100–200 MPa). Fracture dynamics showed distinct differences by wax addition. Using the present analysis method, we can determine quantitatively effects of interfacial quality on the microfracture processes and assess the integrity of UD-GFRPs.