Internal damage characterization for composite materials under biaxial loading configuration

Abstract

This thesis contains the results of a composite material database developed for fiber glass laminates using test data from the in-plane loader (IPL). The IPL is a unique multi-axial test machine developed at Montana State University. The research was completed with the aim to improve the reliability of composite materials, namely fiber glass for use in wind turbine blades. An energy method was used to characterize strain-induced damage in fiber glass coupons. The energy dissipated by internal failure mechanisms was employed as a metric of internal damage. Thus, by means of a deconvolution procedure data from the IPL was used to obtain a dissipated energy density function. The dissipated energy density function was utilized to characterize the behavior of fiber glass between the onset of damage through ultimate material failure. Two cases studies were used to evaluate the current capabilities of the dissipated energy density function created from IPL data.

Open-hole compression and bearing tension were selected as representative composite structure. The load-displacement response was predicted for both of the structures. By comparing the predictions with experimental results a determination was made concerning the transportable nature of the dissipated energy density function between composite structures of different geometry and laminate lay-up. Under the current dissipated energy density paradigm a linear finite element model was used to determine the strain field for a post damaged coupon. The validity of this procedure is unclear. As a part of this work, the effects of material nonlinearity on damage induced strain redistribution was explored. A best fit bilinear in-situ material constitutive response was determined based on experimental data obtained from the IPL. The level of strain redistribution was determined by directly comparing the strain field computed from the nonlinear finite element model with the linear counterpart.