Research On Delamination Failure Of Composite Laminates Under Hygrothermal Environment Using Acoustic Emission Technique

Carbon fiber reinforced resin matrix composites have been widely used in areas of aerospace, pressure vessels, ships and wind power because of high strengths and stiffnesses and low density. However, composites show complicated failure mechanisms, including fiber breakage, matrix cracking, fiber/matrix debonding and interlaminar delamination, and delamination will decrease the stiffness, strength and structural integrity of composites severely. This presents a large challenge to lightweight design and practical application of composites. Nowaday, such as acoustic emission(AE), ultrasound, X-rays and other non-destructive test technique develop rapidly at home and abroad. Particularly, AE technique is increasingly used in composites because it shows many advantages such as dynamic monitoring of failure properties of composites. Currently, there is already much research on the failure mechanisms of composites by AE technique. However, most of the study is limited to the single physical field. Since composite structures are often used in high temperature, hygrothermal and other multi-physics environments, failure mechanisms become more complicated. Unfortunately, AE technique and analytical method for composites are still immature under multi-physics environments, which urgently needs related fundamental research. In this paper, the three-point bending test, acoustic emission test and delamination analysis of composites under room temperature, high temperature and hygrothermal environment respectively. By analyzing the responses of AE signals such as amplitude and energy, analytical method for AE signals is developed under multiphysical fields. Besides, the effects of some factors such as the loading modes, structure sizes and layup patterns on the delamination failure mechanisms and loading response are also studied. The detailed research work is as follows:First, composite laminates with interlaminar and intralaminar initial defects for two kinds of carbon/epoxy composites T700/8911 (Carbon fiber/bismaleimide resin) and T700/9916 (Carbon fiber/epoxy resin) are designed. Besides, the test method of mixed-mode Ⅰ/Ⅱ delamination of composites is developed under room temperature, high temperature and hygrothermal environment.Second, three-point bending delamination and AE test are carried out forT700/8911 composite specimens under room temperature and 70℃ high-temperature environments. By analyzing the load-displacement curves and the evolvement of AE characteristic parameters such as energy, amplitude and count, the effects of the loading modes, structure sizes and layup patterns on the delamination failure mechanisms and mechanical degradation mechanisms as well as the R-curve are studied.Finally, the three-point bending delamination and AE test are carried out for T700/9916 composite specimens under room temperature and hygrothermal environment. By analyzing the load-displacement curves and the characteristic parameters of AE signals, the evolvement behaviors of AE signals corresponding to different failure mechanisms and the R-curve are obtained by combining with the scanning electron microscope (SEM), Fast Fourier Transformation (FFT) and wavelet energy spectrum analysis. Besides, the analytical method using AE signals for composites under multiphysical fields is summarized. The present research provides support for improving the structural design and application under multiphysical field environments.