Donald F. Adams

Current Status of the Iosipescu Shear Test Method

During the past several years, considerable study of the Iosipescu shear test method has been conducted by a number of investigators, including the present authors. Several dif ferent test fixture designs have been developed, and variations of the specimen size and notch configuration have been considered. Materials with properties ranging from iso tropic to highly orthotropic have been tested. The response of such materials has also been predicted using finite element techniques, and attempts have been made to correlate these predictions with available experimental data.

Hygrothermal Microstresses in a Unidirectional Composite Exhibiting Inelastic Material Behavior

A finite element numerical analysis is presented which models the influ ences of temperature variations and dilatations due to moisture absorption on the local stress state in a unidirectionally-reinforced composite material. These influences are admissible in both fiber and matrix, and all material properties are assumed to be temperature- and moisture-dependent. The fiber is assumed to be elastic and transversely isotropic, the matrix material inelastic and isotropic. A generalized plane strain condition is formulated, which permits the analysis of applied normal loadings in three directions (i.e., longitudinal and biaxial transverse) combined with arbitrary tempera ture and moisture content changes. Numerical results are presented for a typical graphite/epoxy composite, indicating the residual microstresses induced during cooldown from the curing temperature, and how they can be altered by a subsequent moisture absorption at room temperature. Results are also given for predicted microstress states and failure initiation in a graphite/epoxy composite modeled both with and without curing stresses and moisture dilatation, for a transverse normal applied loading continued beyond the elastic limit, to first failure.

Residual Strength of a Carbon/Epoxy Composite Material Subjected to Repeated Impact:

The residual tensile and compressive strengths of specimens cut from composite plates subjected to repeated impact at various energy levels were measured. The 16-ply quasi- isotropic composite plates were 2 mm thick, fabricated from Hercules AS4/3501-6 car bon/epoxy prepreg. Impact energy level and number of impacts were found to be major factors influencing strength degradation. However, strength degradation was limited to the region near the impact point.

Finite Element Micromechanical Analysis of a Unidirectional Composite Including Longitudinal Shear Loading.

Abstract A two-dimensional finite element analysis has been extended to include longitudinal shear loadings in the third direction, thus permitting a more complete micromechanical analysis of a unidirectional composite material subjected to combined loading states. The numerical analysis includes temperature and moisture loadings as well as mechanical loadings, and models temperature—and moisture—dependent material properties, into the inelastic range to first failure. Correlations of the predictions of composite material response with available experimental data indicate good agreement.

Instrumented drop weight impact testing of cross-ply and fabric composites

Abstract A state-of-the-art instrumented drop weight impact test system developed at the University of Wyoming was used to investigate the impact performance of thin, simply-supported composite laminates. System calibration, data acquisition and data reduction techniques developed for this impact test system, which makes use of a piezoelectric force transducer, are presented, along with insights into system resonance characteristics. Composite material plates were tested to identify performance differences between cross-ply and fabric material forms. The six composite material systems investigated included cross-ply and fabric laminates of Hercules AS4 graphite, DuPont Kevlar 49 and Owens-Corning E-glass fibres impregnated with a Hercules 3501-6 epoxy resin. Test results are presented along with results of a literature review in this area.

An experimental investigation of the biaxial strength of IM6/3501-6 carbon/epoxy cross-ply laminates using cruciform specimens

Abstract Several variations of a thickness-tapered cruciform specimen have previously been used to experimentally determine the biaxial strength of an AS4/3501-6 carbon/epoxy cross-ply laminate. The present work represents a follow-up study of the original specimen design, and incorporates numerous specimen improvements made in an attempt to generate more accurate biaxial results. A total of 52 tests were performed at numerous biaxial stress ratios, utilizing six different specimen configurations. The experimental data generated in the present study for all specimen geometries, as well as a complete biaxial failure envelope in σ1–σ2 stress space for this laminate configuration, are presented. A desirable failure mode in the gage section of the specimen was achieved for all specimens tested in the present study, indicating that accurate biaxial stress states were being generated at ultimate specimen failure. The ability of the thickness-tapered cruciform specimen to determine the biaxial strength of composite materials at any stress ratio has been demonstrated.

Micromechanics Prediction of the Shear Strength of Carbon Fiber/Epoxy Matrix Composites: The Influence of the Matrix and Interface Strengths

Presently there is great interest in understanding and improving the bond ing between the fibers and matrix in high performance composite materials. Indeed, the fiber-matrix adhesion in many recently-developed systems is poor. To improve bonding, various fiber surface treatments have been developed. These treatments are often evaluated by measuring their effect on a composite property sensitive to the interfacial bond strength (e.g., the composite shear strength). Such methods are, however, inferential, and do not provide a direct measure of the strength of the interfacial bond. A method is presented to estimate the influence of the matrix and the interfacial bond strength itself on the composite shear strength for a given fiber/matrix composite. Analyti cal prediction of these effects has been achieved using a finite element micromechanics model. Numerical results generated using this model have been shown to compare well with experimental data, which suggests that the micromechanics approach to predicting constituent and interface effects on the composite shear strength is a potentially valuable tool.

A plasticity model for unidirectional composite materials and its applications in modeling composites testing

A simple three-dimensional plasticity model has been developed to describe the plastic response of unidirectional composite materials. The orthotropic parameters in the model were determined from the stress/strain responses of composites subjected to various experimental loading conditions. The plasticity models for several composite materials were obtained, the stress/strain responses under various loadings being generated by a micromechanics analysis instead of real experiments. A three-dimensional finite element analysis employing the plasticity model was used to model compression and short-beam shear tests of composite materials to demonstrate the application of the model.

A Micromechanics Analysis of the Influence of the Interface on the Performance of Polymer-Matrix Composites:

A specially developed two-dimensional finite element micromechanics analysis was used to predict the transverse tensile response of three different carbon fiber-reinforced, polymer matrix unidirectional composites. Experimental data were available for four dif ferent fiber sizings. The composites were tested both dry and moisture-conditioned, at room and elevated temperatures. Analytical/experimental correlations are presented and discussed.

Combined loading micromechanical analysis of a unidirectional composite

A microscopic region of a unidirectional composite is modelled by a finite element micromechanical analysis using a generalized plane strain formulation, but including longitudinal shear loading. The analysis is capable of treating elastic, transversely isotropic fibre materials, as well as isotropic, elastoplastic matrix materials. Matrix material properties are considered to be temperature- and/or moisture-dependent. The longitudinal shear loading capability permits the analysis of the shear response of unidirectional composites in the fibre direction. In conjunction with a laminated plate point stress analysis, the present micromechanical analysis has been used to predict the stress/strain response into the inelastic range of a graphite/epoxy [±45]4s laminate. Available experimental data for various environmental conditions indicate excellent agreement with the analytical predictions.