Gary D. Roberts

Experimental study of strain-rate-dependent behavior of carbon/epoxy composite

The strain rate dependent behavior of IM7/977-2 carbon/epoxy matrix composite in tension is studied by testing the resin and various laminate configurations at different strain rates. Tensile tests have been conducted with a hydraulic machine at quasi-static strain rates of approximately 10 � 5 s � 1 and intermediate strain rates of about 1 s � 1 . Tensile high strain rate tests have been conducted with the tensile split Hopkinson bar technique at strain rates of approximately 400–600 s � 1 . Specimens with identical geometry are used in all the tests. The standard split Hopkinson bar technique is modified to measure strain directly on the specimen. The results show that strain rate has a significant effect on the material response. # 2002 Elsevier Science Ltd. All rights reserved.

Incorporation of Mean Stress Effects into the Micromechanical Analysis of the High Strain Rate Response of Polymer Matrix Composites

The results presented here are part of an ongoing research program, to develop strain rate dependent deformation and failure models for the analysis of polymer matrix composites subject to high strain rate impact loads. A micromechanics approach is employed in this work, in which state variable constitutive equations originally developed for metals have been modified to model the deformation of the polymer matrix, and a strength of materials based micromechanics method is used to predict the effective response of the composite. In the analysis of the inelastic deformation of the polymer matrix, the definitions of the effective stress and effective inelastic strain have been modified in order to account for the effect of hydrostatic stresses, which are significant in polymers. Two representative polymers, a toughened epoxy and a brittle epoxy, are characterized through the use of data from tensile and shear tests across a variety of strain rates. Results computed by using the developed constitutive equations correlate well with data generated via experiments. The procedure used to incorporate the constitutive equations within a micromechanics method is presented, and sample calculations of the deformation response of a composite for various fiber orientations and strain rates are discussed.

Effects of hygrothermal cycling on the chemical, thermal, and mechanical properties of 862/W epoxy resin

The hygrothermal aging characteristics of an epoxy resin were characterized over a one-year period, which included 908 temperature and humidity cycles. The epoxy resin quickly displayed evidence of aging through color change and increased brittleness. The influence of aging on the material’s glass transition temperature (T g) was evaluated by Differential Scanning Calorimetry and Dynamic Mechanical Analysis. The T g remained relatively constant throughout the year-long cyclic aging profile. Chemical composition was monitored by Fourier Transform Infrared spectroscopy, where evidence of chemical aging and advancement of cure was noted. The tensile strength of the resin was tested as it aged and this property was severely affected by the aging process in the form of reduced ductility and embrittlement. Detailed chemical evaluation suggests many aging mechanisms are taking place during exposure to hygrothermal conditions.

Full-field Strain Methods for Investigating Failure Mechanisms in Triaxial Braided Composites

Composite materials made with triaxial braid architecture and large tow size carbon fibers are beginning to be used in many applications, including composite aircraft and engine structures. Recent advancements in braiding technology have led to commercially viable manufacturing approaches for making large structures with complex shape. Although the large unit cell size of these materials is an advantage for manufacturing efficiency, the fiber architecture presents some challenges for materials characterization, design, and analysis. In some cases, the static load capability of structures made using these materials has been higher than expected based on material strength properties measured using standard coupon tests. A potential problem with using standard tests methods for these materials is that the unit cell size can be an unacceptably large fraction of the specimen dimensions. More detailed investigation of deformation and failure processes in large unit cell size triaxial braid composites is needed to evaluate the applicability of standard test methods for these materials and to develop alternative testing approaches. In recent years, commercial equipment has become available that enables digital image correlation to be used on a more routine basis for investigation of full field 3D deformation in materials and structures. In this paper, some new techniques that have been developed to investigate local deformation and failure using digital image correlation techniques are presented. The methods were used to measure both local and global strains during standard straight-sided coupon tensile tests on composite materials made with 12 and 24 k yarns and a 0/+60/-60 triaxial braid architecture. Local deformation and failure within fiber bundles was observed, and this local failure had a significant effect on global stiffness and strength. The matrix material had a large effect on local damage initiation for the two matrix materials used in this investigation. Premature failure in regions of the unit cell near the edge of the straight-sided specimens was observed for transverse tensile tests in which the braid axial fibers were perpendicular to the specimen axis and the bias fibers terminated on the cut edges in the specimen gage section. This edge effect is one factor that could contribute to a measured strength that is lower than the actual material strength in a structure without edge effects.

Influence of material parameters on strain energy release rates for cross-ply laminates with a pre-existing transverse crack

The formation of intraply cracks in continuous fiber reinforced composite structures can be tolerated if critical mechanical properties are not reduced below allowable design limits. However, the presence of intraply cracks can lead to the formation of other, more harmful, types of damage. The onset of this late stage damage determines the useful service life of the composite structure. In this paper, the damage progression in the vicinity of a pre-existing transverse crack is investigated for [0/903)s and [O2/902]s cross-ply laminates. The types of damage considered are secondary crack formation, 0/90 interface delamination, split cracks in 0° plies, and delamination induced by split and transverse cracks. Strain energy release rates (SERR) associated with the onset and propagation of these types of damage are calculated using three dimensional finite element analysis (FEA) of a representative volume containing a pre-existing transverse crack. The implications of this analysis are considered for several polymer matrix and metal matrix composites.

Characterization and Analysis of Triaxially Braided Polymer Composites under Static and Impact Loads

In order to design impact resistant aerospace components made of triaxially braided polymer matrix composite materials, a need exists to have reliable impact simulation methods and a detailed understanding of the material behavior. Traditional test methods and specimen designs have yielded unrealistic material property data due to features such as edge damage. To overcome these deficiencies, various alternative testing geometries such as notched flat coupons have been examined to alleviate difficulties observed with standard test methods. The results from the coupon level tests have been used to characterize and validate a macro level finite element based model which can be used to simulate the mechanical and impact response of the braided composites. In the analytical model, the triaxial braid architecture is simulated by using four parallel shell elements, each of which is modeled as a laminated composite. Currently, each shell element is considered to be a smeared homogeneous material. Simplified micromechanics techniques and lamination theory are used to determine the equivalent stiffness properties of each shell element, and results from the coupon level tests on the braided composite are used to back out the strength properties of each shell element. Recent improvements to the model the incorporation of strain rate effects into the model. Simulations of ballistic impact tests have been carried out to investigate and verify the analysis approach.

Ballistic impact response of carbon/epoxy tubes with variable nanosilica content:

Laminated fiber reinforced polymer composites are known for high specific strength and stiffness in the plane of lamination, yet relatively low out-of-plane impact damage tolerance due to matrix dominated interlaminar mechanical properties. A number of factors including the toughness of the matrix can influence the response of composites to impact. The objective of the current investigation is to evaluate the ballistic impact response of carbon/epoxy tubes with variable amounts of nanosilica particles added to the matrix as a toughening agent. Mass density, elastic modulus, glass transition temperature and Mode I fracture toughness of the matrix materials were measured. Tubes manufactured with these matrix materials were ballistically impacted using a round steel projectile aimed at normal incidence across the major diameter. After impact, the tubes were nondestructively inspected and subjected to mechanical tests to determine the residual shear strength in torsion. Increasing concentrations of nanosilica monotonically increased the modulus and fracture toughness of the matrix materials. Tubes with nanosilica had smaller impact damage area, higher residual shear strength, and higher energy absorbed per unit damage area versus control materials with no nanosilica. Overall, the addition of nanosilica improved the impact damage resistance and tolerance of carbon/epoxy tubes loaded in torsion, with minimal adverse effects on mass density and glass transition temperature.

Analytical Modeling of the High Strain Rate Deformation of Polymer Matrix Composites

The results presented here are part of an ongoing research program to develop strain rate dependent deformation and failure models for the analysis of polymer matrix composites subject to high strain rate impact loads. State variable constitutive equations originally developed for metals have been modified in order to model the nonlinear, strain rate dependent deformation of polymeric matrix materials. To account for the effects of hydrostatic stresses, which are significant in polymers, the classical 5 plasticity theory definitions of effective stress and effective plastic strain are modified by applying variations of the Drucker-Prager yield criterion. To verify the revised formulation, the shear and tensile deformation of a representative toughened epoxy is analyzed across a wide range of strain rates (from quasi-static to high strain rates) and the results are compared to experimentally obtained values. For the analyzed polymers, both the tensile and shear stress-strain curves computed using the analytical model correlate well with values obtained through experimental tests. The polymer constitutive equations are implemented within a strength of materials based micromechanics method to predict the nonlinear, strain rate dependent deformation of polymer matrix composites. In the micromechanics, the unit cell is divided up into a number of independently analyzed slices, and laminate theory is then applied to obtain the effective deformation of the unit cell. The composite mechanics are verified by analyzing the deformation of a representative polymer matrix composite (composed using the representative polymer analyzed for the correlation of the polymer constitutive equations) for several fiber orientation angles across a variety of strain rates. The computed values compare favorably to experimentally obtained results.

Impact Testing of Composites for Aircraft Engine Fan Cases

Gary D. Roberts and Duane M. RevilockGlenn Research Center, Cleveland, OhioWieslaw K. Binienda and Walter Z. NieUniversity of Akron, Akron, OhioS. Ben Mackenzie and Kevin B. ToddSaint-Gobain Performance Plastics, Ravenna, OhioPrepared for the42nd Structures, Structural Dynamics, and Materials Conference and Exhibitcosponsored by the AIAA, ASME, ASCE, AHS, and ACSSeattle, Washington, April 16-19, 2001National Aeronautics andSpace AdministrationGlenn Research Center

High-speed infrared thermal imaging during ballistic impact of triaxially braided composites

Ballistic impact experiments were performed on triaxially braided polymer matrix composites to study the heat generated in the material due to projectile velocity and penetration damage. Triaxially braided (0/+60/−60) composite panels were manufactured with T700S standard modulus carbon fiber and two epoxy resins. The PR520 (toughened) and 3502 (untoughened) resin systems were used to make different panels to study the effects of resin properties on temperature rise. The ballistic impact tests were conducted using a single stage gas gun, and different projectile velocities were applied to study the effect on the temperature results. Temperature contours were obtained from the back surface of the panel during the test through a high speed, infrared thermal imaging system. The contours show that high temperatures were locally generated and more pronounced along the axial tows for the T700S/PR520 composite panels; whereas, tests performed on T700S/3502 composite panels, using similar impact velocities, demonstrated a widespread area of lower temperature rises. Nondestructive, ultrasonic C-scan analyses were performed to observe the failure patterns in the impacted composite panels and correlate the C-scan results with the temperature contours. Overall, the impact experimentation showed temperatures exceeding 252℃ (485°F) in both composites which is well above the respective glass transition temperatures for the polymer constituents. This expresses the need for further high strain rate testing with measurement of the temperature and deformation fields in order to fully understand the complex behavior and failure of the material and to improve the confidence in designing aerospace components with these materials.