High Temperature Shear Strength of T650-35/HFPE-II-52 Polyimide Matrix Unidirectional Composite
The shear strength of a 4-ply unidirectional composite laminate consisting of carbon fibers (T650-35) and a polyimide matrix (HFPE-II-52) was measured over a temperature range of 25 to 315°C. The tests were performed using a Iosipescu test sample, modified to provide a more uniform shear stress distribution across the gauge section and loaded with an Arcan type test fixture. The test specimen design is based on the results of an extensive finite element study. Shear strength tests were performed on dry, 50% RH and 100% RH moisture saturated samples. Results of the experiment show that shear strength decreases from approximately 120 MPa at 25°C to 60 MPa at 315°C and that moisture saturated samples have a 5–10 MPa lower shear strength than dry samples.
Mechanical response of a woven graphite/polyimide composite to in-plane shear dominated loads at room and elevated temperatures
Abstract In this work, the effect of two types of in-plane shear dominated loads on the initiation of intralaminar damage has been evaluated in an 8-harness satin (8HS) T650-35/PMR-15 (graphite/polyimide) composite tested at room and elevated temperatures. The composite was subjected to either pure in-plane shear or combined biaxial tension and shear, which represented the stress conditions in the gage sections of the Iosipescu shear and ±45° tensile tests, respectively. The stress distributions in the composite were determined as a function of load and temperature by performing non-linear stress computations on the micro-, meso- and macro-levels. Both the concept of representative micro- and meso-unit-cells and the viscoelastic Eshelby/Mori–Tanaka approach were used in the analyses. The macro-response of the composite to the in-plane shear and biaxial loads was predicted at both temperatures and then validated by comparing it with the available experimental data. The micro-stress calculations have shown that on the micro-scale noticeably higher maximum principal stresses in the polyimide matrix in the vicinity of the graphite fibers were determined for the biaxial load case than in pure shear, both at room and high temperatures. It was shown that by using the numerical stress predictions, the experimentally observed differences can be explained in the initiation of tow micro-cracking in the woven composite subjected to the Iosipescu and ±45° tests at room and elevated temperatures.
Residual stresses in unidirectional graphite fiber/polyimide composites as a function of aging
Abstract In this research, the magnitude and distribution of residual aging stresses in unidirectional graphite fiber/polyimide composites were investigated. First, a viscoelastic aging material model was developed by characterizing the isothermal aging strain and stress relaxation of neat PMR-15 polyimide. Then, the aging material model was applied to numerically evaluate the effect of matrix aging and cooling on the residual stress state of a unidirectional graphite fiber (T-650)/polyimide (PMR-15) composite. It has been shown in this work that residual stresses in the composite are significantly affected by high temperature aging. Most importantly, the magnitude of the residual stress caused by aging in N2 at 316 °C for one month was found to be comparable to the cooling stress in the composite.