Failure Investigation of Graphite/Polyimide Fabric Composites at Room and Elevated Temperatures Using the Biaxial Iosipescu Test

Abstract

The biaxial and modified biaxial Iosipescu shear test methods were applied to determine the shear dominated, biaxial mechanical response of graphite/PMR-15 and graphite/Avimid-R woven fabric composites at room and elevated temperatures. Three different composite architectures were examined: T650-35 warp-aligned, 8-harness satin (8HS) fabric in a PMR-15 matrix, T650-35 warp-aligned, 8HS fabric in an Avimid-R matrix and T650-35 0°/90°, 8HS fabric in an Avimid-R matrix. Several biaxial Iosipescu tests were performed at room temperature under shear, shear-tension, and shear-compression loading conditions to characterize damage and obtain biaxial, shear dominated failure properties. Shear tests were also conducted at elevated temperatures approaching 316 °C to determine the effects of temperature on the shear strengths of the composites investigated. A nonlinear finite element analysis is briefly introduced to evaluate the effects of specimen sliding and geometric nonlinearities on the stress and strain distributions in the biaxial Iosipescu specimens. In addition, the effects of different loading block geometries on the stress distribution in the Iosipescu specimens subjected to biaxial loads were also investigated. Within this investigation, it was found that graphite/Avimid-R was more resistant to biaxial, shear dominated failure at room temperature in comparison to graphite/PMR-15. However, the graphite/PMR-15 composite system exhibited better shear strength properties at elevated temperatures above 232 °C. It was also found that the effect of compression along the notch root axis generated by the loading blocks did not affect the loads at failure.

The Effect of Eccentric Loads on the Macroscopic Strain and Stress Distributions in Woven Fabric Composite Iosipescu Specimens

“…Similar to the stress gradients, the long block configuration also weakens the effect of eccentric loads on the strain gradients. It can be observed from the data presented in Figures 4, 5, 6(b) and 7(b) that the eccentric loads are responsible for the difference in shear strains measured for a variety of Iosipescu specimens subjected either to shear, shear-tension or shear-compression [9,[11][12][13][14][15]. Since the eccentric loads also affect the stress distributions through the specimen thickness, it can be expected that they should affect the failure process.…”

“…From the previous 2-D studies of the Iosipescu test [15,[18][19][20], the coefficient of friction between the blocks and specimen contacting surfaces was chosen as 0.3.…”

“…Previous experiments [15,21] provided the basis for the composite material properties of quasi-isotropic graphite-PMR15 assumed for this study. These properties are listed in Table 2.…”

“…Two 3-D numerical models were proposed for this study and are shown in Figure 2. These models were adopted from previous experimental and 2-D numerical studies [15,16] on the mechanical behavior of composite Iosipescu specimens using the biaxial and modified biaxial fixtures. The geometry of each specimen and its dimensions are illustrated in Figure 3 and Table 1, respectively.…”

“…To this end, a great deal of time has been spent by many to understand these problems and interpret which behavior is truly representative of the test. Likewise, time has been invested in attempts to optimize the design of the specimen, fixture and testing apparatus with a fair amount of documented success [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”