Test Methods for Determining Temperature Dependent Material Properties and Residual Stress Modeling for Laminated Composites

摘要

Utilizing composites for lightweight cryogenic storage tank applications is challenging because of the potential for residual stress induced micro-cracking. Yet if designed properly, with an appropriate stacking sequence, it has been shown that residual stresses can be kept below that which leads to micro-cracking. The result is a tank that is far lighter than traditional metallic tanks. The technique called the Cure Reference Method (CRM) was developed to determine the strains associated with the cure induced residual stresses through the temperature range from cure to cryogenic. The method is capable of individually isolating contributions due to chemical shrinkage of the matrix and the difference in thermal expansion between the fibers and the matrix. In order to determine the stresses associated with the strains measured by CRM, there arose a need to accurately quantify some of the material properties as a function of temperature. Disregarding the temperature dependent material properties can lead to a meaningful error. From our analysis, it became clear that the three properties that were the most temperature dependent, E_2 transverse modulus, G_12 shear modulus, and α_2 the transverse coefficient of thermal expansion contributed the majority of that error. Other properties such as the fiber dominated modulus and coefficient of thermal expansion could be modeled as temperature independent in the analysis. Our search through the literature yielded very scant information regarding temperature dependent properties, especially for the material system tested (IM7/977-2, X-33 material system). As a result, we initiated a program to measure these quantities. Our procedure for E_2 follows the ASTM suggestions with only a few modifications. In the void of test methods to acquire accurate temperature dependent G_12, we developed a novel test that utilizes the same experimental setup as the E_2 test, except with a unique notched tension specimen. The coefficient of thermal expansion as a function of temperature is automatically acquired during the CRM procedure. This paper will introduce the cure reference method, and describe in details the experimental program to determine the temperature dependent material properties.Then we will demonstrate how to incorporate them into the CRM in order to accurately determine residual stresses at the ply scale for a variety of laminate configurations including the one used on the X-33 as well as an optimized angle ply.