Characterization of failure modes in GFRP laminates and analysis of composite joints by acoustic emission monitoring

Abstract

Composite materials are among the strongest, lightest and stiffest corrosion-resistant materials widely used in aerospace, marine, civil and automotive industries. Damage in composite materials can be localized to one of the constituents or affect the structure as a whole. There are several methods of characterizing internal damage within composite materials. One of the most widely used nondestructive methods is Acoustic Emission (AE), which is defined as the release of transient elastic waves in solids as a result of rapid localized redistributions of stresses which accompany the operation of damage mechanisms . AE can evaluate damage by detecting the emitting strain energy when elastic waves are generated by the generation and growth of a crack, plastic deformation, fiber breakage, matrix cleavage, or delamination. The structural complexity of composites and the presence of friction-related emissions, produced during mutual rubbing of evolving damage surfaces and fretting of broken fibers with matrix, which may conceal emissions from actual damage. The overall goal of this thesis is to characterize the damages in composite laminate and joint, predicting residual strength using AE. This aim can be reached using either of the experimental approaches described below or a combination of them. The first approach involves the measurement of a number of parameters from the AE wave, divided into signals using appropriate time settings disposed on the AE system (multi parameter approach). The second approach is based on the extraction of frequency content from AE wave with appropriate algorithms, the simplest of these being Fast Fourier Transform (FFT). Laminates with different stacking sequences such as (00, 900, angleply (+450/-450) and cross ply (00/900) are used to trigger different failure mechanisms. From AE data of specimens subjected to tensile load, three different frequency content are identified which are related to matrix cracking (90-100kHz), delamination (130-200kHz)