J.K. Sutter

An analysis of residual thermal stresses in a unidirectional graphite/PMR-15 composite based on X-ray diffraction measurements

The purpose of this research is to determine residual thermal stresses in a unidirectional graphite-fiberT/PMR-15 polyimide composite by using crystalline inclusions. X-ray diffraction (XRD) measurements have been made to determine residual stresses in embedded aluminum and silver inclusions placed between the first and second plies of six-ply unidirectional graphite/PMR-15 composite specimens. In the modeling part of this research, residual thermal stresses in unidirectional graphite/polyimide composite plates and in the embedded aluminum and silver inclusions with interlaminar and intralaminar particle distributions have been modeled by using elastic and visco-elastic laminate theories and the Eshelby method. The numerically determined residual stresses in the particles have been subsequently compared to the residual stresses determined from the XRD analysis. It has been shown in this research that the residual stresses in the unidirectional graphite/polyimide composite can be obtained with reasonable accuracy by using the X-ray diffraction technique in conjunction with the application of the visco-elastic Eshelby method of multiple inclusions. The modeling has also shown that the distribution of the aluminum and silver particles and their geometries have a strong effect on the XRD data and the thermal stress analysis based on the concept of embedded crystalline inclusions.

Determination of Interlaminar Residual Thermal Stresses in a Woven 8HS Graphite/PMR-15 Composite Using X-Ray Diffraction Measurements

This work is a continuation of the research recently presented in [1] and [2] on the determination of residual thermal stresses in graphite/polyimide composites with and without externally applied bending loads. In the previous work [1, 2] a combined experimental and numerical methodology for the determination of the residual stresses in unidirectional graphite/PMR-15 composites based on X-ray diffraction (XRD) measurements of residual strains in embedded aluminum (Al) and silver (Ag) inclusions has been presented. In this research, the previously developed approach has been applied to evaluate the residual thermal interlaminar stresses in an 8 harness satin (8HS) woven graphite/PMR-15 composite. First, residual thermal stresses have been measured by XRD in aluminum inclusions embedded between the first and second plies of a four-ply 8HS woven graphite/PMR-15 composite. The measurements have been conducted with the composite specimens subjected to four-point bending deformations. Second, viscoelastic computations of interlaminar residual stresses in the composite have been performed using classical laminated plate theory (CLPT) following the manufacturing procedure. Third, the residual strains and stresses in the inclusions have been numerically predicted using the viscoelastic Eshelby model for multiple spherical inclusions. Finally, the interlaminar residual stresses in the composite have been extracted from the XRD strains in the Al inclusions, again using the viscoelastic Eshelby model, and subsequently compared with the residual stresses from the CLPT. It has been shown in this study that the residual interlaminar thermal stresses can be accurately determined not only in unidirectional graphite/polyimide systems as presented in [1] and [2], but also in woven graphite polymer matrix composites.

Comparison of the ±45° Tensile and Iosipescu Shear Tests for Woven Fabric Composite Materials

The mechanical response of a woven eight-harness satin graphite/polyimide composite has been investigated by performing ±45° tensile and Iosipescu shear tests at room temperature. Nonlinear finite element simulations of the tests have been conducted to determine internal stress distributions in the ±45° tensile and Iosipescu fabric specimens as a function of load. In the experimental part of this study, a series of tensile and Iosipescu shear tests have been performed. Acoustic emission techniques have been employed to monitor damage initiation and progression in the composite. The finite element computations have shown that the internal stress distributions in the Iosipescu and tensile fabric specimens are significantly different. In the gage sections of Iosipescu specimens, the state of stress is essentially pure shear, whereas the tensile tests generate biaxial stress conditions. It has been shown in this research that the shear strength of the composite determined from the maximum loads obtained from the Iosipescu shear tests is significantly higher than the shear strength obtained from the ±45° tensile tests. Moreover, the initiation of intralaminar damage in the tensile specimens occurs at much lower loads than in the Iosipescu specimens. It appears that the ±45° tensile test significantly underestimates the shear strength of the composite evaluated from the onset of intralaminar damage and the maximum loads.

Analysis of Stresses in Aluminum Particles Embedded Inside Unidirectional and Woven Graphite/Polyimide Composites Subjected to Large Bending Loads

ABSTRACT The effect of large bending loads on strains and stresses inside aluminum particles embedded in unidirectional and woven eight harness satin (8HS) graphite/PMR-15 composites has been examined. The stresses and strains in the particles were determined by performing X-ray diffraction (XRD) measurements. It has been shown in this work that when the composites are subjected to large four-point bending loads, above certain critical loads, the normal stresses and strains in the particles no longer respond to an increase in the bending moments. The stabilization of the normal X-ray stresses and strains under large bending conditions was attributed to the onset of plastic deformation of the particles. It has also been shown in this study that the aluminum particles do not noticeably affect the mechanical behavior of the composites under four-point bending conditions.

Mechanical Behavior of a Woven Graphite/PMR-15 Composite at Room and Elevated Temperatures Determined from the ±45° Tensile and Iosipescu Shear Tests

Failure and stiffness properties of a woven 8 harness satin (8HS) graphite/PMR-15 composite have been investigated at room temperature and at 315°C by performing the ±45° tensile and Iosipescu tests. Acoustic emission has been monitored during testing. The critical loads for the initiation of damage in the composite have been determined. In particular, the specimen width effect has been investigated in the case of the ±45° specimens by testing the specimens with their width ranging from 12.7–50.8 mm. The results from the high temperature tests have been compared with the room temperature data presented in Refs. 1 and 2. Similar to the room temperature ±45° tests, the shear stresses at the onset of intralaminar damage in the specimens and the shear stresses at the maximum loads at 315°C are significantly affected by the specimen width effect. The trends in the damage initiation stresses and the maximum stresses as a function of specimen width at 315°C have been found to be very similar to the room temperature data with the stresses increasing almost linearly with the specimen width. It has also been shown in this project that the shear stresses at the onset of intralaminar damage and at the maximum load at 315°C depend very strongly on the specimen type. The shear stresses determined at the onset of damage and maximum loads from the Iosipescu tests at 315°C are noticeably higher than the stresses from the ±45° tests. The ±45° tensile test significantly underestimates the room and elevated temperature shear strength properties of the 8HS graphite/PMR-15 composite in comparison with the Iosipescu shear test.

Failure Analysis of ±45° Off-Axis Woven Fabric Composite Specimens

The purpose of this research is to evaluate the mechanical response of eight harness satin woven graphite/polyimide composite specimens using the ±45° test. A series of tensile tests have been conducted at room temperature to evaluate the effect of specimen width as well as monotonic, progressive, and multiple loadings on the response of the composite specimens. Acoustic emission techniques have been employed to monitor damage initiation and progression in the specimens at different stages of loading. In addition, nonlinear finite element computations have been conducted to determine stress distributions in the ±45° fabric specimens. It has been shown that the specimen width effect is strong and should not be ignored in a failure analysis of the fabric composite using the ±45° test. The shear strength of the composite determined from the maximum loads increases substantially with the width of the specimen. It has also been shown in this research that the initiation of intralaminar damage in the graphite/polyimide composite can be monitored using acoustic emission. The initiation of interlaminar damage can be determined either from the characteristic knees on the load/displacement diagrams or from the acoustic emission data.