B. Benedikt

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.

Visco-elastic stress distributions and elastic properties in unidirectional composites with large volume fractions of fibers

In this work, the visco-elastic stress distributions and elastic properties of unidirectional graphite/polyimide composites have been examined as a function of the volume fraction of fibers. The stress distributions were determined by employing two different methods, namely the finite element method (FEM) assuming either hexagonal or square fiber arrangements, and the Eshelby method modified by Mori and Tanaka to account for the presence of multiple fibers. It has been presented in this research that the Eshelby/Mori-Tanaka approach can be used for the calculations of the stresses inside and outside graphite fibers provided the volume fraction of the fibers does not substantially exceed 35% in the case of the square fiber array and 50% for the hexagonal fiber distribution. It has also been shown that the elastic properties of unidirectional graphite/polyimide composites can be accurately determined using the analytical Eshelby/Mori-Tanaka method even for large volume fractions of fibers.

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.

The use of X-ray diffraction measurements to determine the effect of bending loads on internal stresses in aluminum inclusions embedded in a unidirectional graphite-fibre/PMR-15 composite

A testing methodology for the determination of residual thermal stresses in the polymer-matrix of unidirectional polymer-matrix composites has been proposed in Ref. [1] [Benedikt B, Kumosa M, Predecki PK, Kumosa L, Castelli MG, Sutter JK. An analysis of residual thermal stresses in a unidirectional graphite/PMR-15 composite based on the X-ray diffraction measurements. Composites Science and Technology (in press)]. The methodology is based X-ray diffraction (XRD) measurements of residual strains in embedded metallic particles. The residual stresses in the polymer matrix can be extracted from the X-ray strains in the particles using the visco-elastic Eshelby method for multiple inclusions. The purpose of this work has been to show that the newly developed experimental/analytical methodology can also be applied to composites subjected to external loads, in this case: spherical aluminum particles embedded in a unidirectional graphite/PMR-15 composite subjected to four-point bending loads. The total stresses and strains in the aluminum particles caused by residual thermal stresses in the composite and the applied stresses generated by four-point bending have been determined by XRD measurements under low bending displacements. Subsequently, the total strains and stresses in the aluminum particles have been numerically predicted by applying elastic and visco-elastic laminate theories and the Eshelby method. It has been shown in this research that not only the residual thermal strains and stresses in the aluminum particles and the matrix can be determined by using the proposed technique but also the effect of external loads on the stresses and strains in the particles can be monitored. This research has provided another verification of the newly proposed methodology presented in Ref. [1].

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.

Stress Corrosion Cracking in a Unidirectional E-Glass/Polyester Composite Subjected to Static and Cyclic Loading Conditions

This work evaluates the effects of static and cyclic loading conditions on the stress corrosion process in unidirectional glass/modified polyester composite in the presence of a nitric acid solution. The composites were investigated under static, cyclic, and static with overload conditions using a rectangular specimen with centrally positioned grooves. The stress corrosion process was monitored using acoustic emission (AE). It was found that the time-to-failure of the composite specimens were only slightly affected by loading conditions. However, the large scatter in the time-to-failure of the specimens did not allow for the proper evaluation of the effect of the cyclic loads on the stress corrosion fracture process. It seemed that the time-to-failure were more influenced by the specimen variations than by the type of loading. By using an AE digital set-up based on four transducers, not only can the fiber fracture by stress corrosion be very accurately monitored, but also the location of AE sources can be precisely determined.