Amos Gilat

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.

A direct-tension split Hopkinson bar for high strain-rate testing

A direct-tension split-Hopkinson-bar apparatus is introduced. In this apparatus the specimen is loaded by a tensile wave that is generated by the release of a stored load in a section of the input bar. The system can be used for experiments with test durations of up to 500 μs. The effect of specimen geometry (length to diameter ratio) is investigated. Consistent results are obtained when the ratio is larger than about 1.60. Results from tests with 6061-T651 aluminum are in agreement with published data.

High Strain Rate Response of Angle-Ply Glass/Epoxy Laminates:

The effects of strain rate on the mechanical behavior of Scotchply Type 1002 glass/epoxy angle-ply laminates is investigated. High strain rate tests (approximately 103 sec−1) using a direct tension split Hopkinson bar apparatus and quasi-static tests (strain rate of approximately 10−4 sec−1) using a servo-hydraulic testing machine have been conducted. Results indicate that the maximum normal stress experienced by glass/epoxy laminates is higher for dynamic than for quasi-static loading conditions. Although both fibers and matrix are sensitive to the strain rate, the fibers influence laminate rate sensitivity more than the matrix.

Plastic deformation of 1020 steel over a wide range of strain rates and temperatures

Abstract The response of hot-rolled 1020 steel over a wide range of temperatures and strain rates is investigated. Pure shear tests have been conducted at strain rates of 5 × 10−4, 2 and 1000s−1, and temperatures of 25, 200, 400, and 600°C. Results show that temperature and strain rate greatly affect material response. In general, the stress decreases with increasing temperature and decreasing strain rate. At temperatures of 200–400°C, however, effects of dynamic strain aging are very significant. At this temperature range, negative strain rate sensitivity is observed as well as a region of increasing stress with increasing temperature. Constitutive equations are developed for modeling the material response. The relations are based on a model of thermally activated motion of dislocations that include the effect of dynamic strain aging. The parameters in the constitutive relations are determined, and there is good agreement between the model and the tests.

Modeling torsional split Hopkinson bar tests at strain rates above 10,000 s−1

Finite element analysis is used to study experiments with the torsional split Hopkinson bar technique in which strain rates on the order of 104 s−1 are reached by using specimens with a very short gage length. Time-dependent analysis with a viscoplastic constitutive model for the specimen material is used to analyze the whole apparatus (elastic bars and specimen). Results from modeling tests with 1100-O aluminum show that the waves in the elastic bars can be modeled accurately when a significant increase is the flow stress at strain rates of 104 s−1 is assumed in the constitutive model of the material. The calculated stresses, strain rates, and strains in the specimens gage section show that the state is not exactly of pure and homogeneous shear as assumed when the experimental stress strain curve is determined from the measured waves on the elastic bars, and that the plastic zone extends beyond the gage length. Even with these discrepancies the results show that the experimental curve provides a good estimation to the true material response. A time-independent analysis with rate-dependent material model is also used to analyze only the specimen and a short section of the adjacent flanges. The results show that this analysis can also be used to determine the validity of the assumed constitutive model of the material.

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.

Torsional split Hopkinson bar tests at strain rates above 104s−1

The torsional split Hopkinson bar is used for testing materials at strain rates above 104s−1. This strain rate, which is an order of magnitude higher than is typical with this technique, is obtained by using very short specimens. Strain rates of 6.4×104s−1 have been achieved with specimens having a gage length of 0.1524 mm. Results from tests on 1100 aluminum show an increase in rate sensitivity as the strain rate increases.

Elevated temperature testing with the torsional split hopkinson bar

The torsional split-Hopkinson-bar technique is modified for high-strain-rate testing at elevated temperatures by heating the specimen rapidly and keeping the rest of the apparatus at room temperature. Tests have been conducted with specimens made of several materials (Haynes-188, 1020 steel, and 1151 steel) at temperatures ranging from 650°C to 1060°C and strain rates on the order of 1000 s−1.

High-rate decremental-strain-rate test

A modified torsional split-Hopkinson bar is intoduced and used to study material response associated with a sudden reduction of stain rate during high-rate plastic deformation. In tests on 1100-0 aluminum iniial deformation at a strain rate of approximately 2400 s−1 is reduced by a factor of 15 after 200 μs of high-rate deformation. After the reduction, the deformation continues at the low rate for additional 550 μs. The change in the strain rate is obtained by using a stepped input bar. The results for 1100-0 aluminum show a decrease in the flow stress following the reduction in the strain rate. A short delay exists between the beginning of the strain-rate reduction and the response of the stress. The magnitude of the drop in the stress agrees with the difference in flow stress expected in constant-strain-rate tests in the corresponding high- and low-strain rates. Following the stress reduction. The stress remains essentially constant with no hardening during the subsequent deformation at the low rate.

Strain Measurement at Temperatures Up to 800°C Utilizing Digital Image Correlation

An experimental technique is introduced to measure full field strains using three dimensional digital image correlation at temperatures up to 800°C. Challenges include: thermal air gradients, speckle pattern adhesion, image distortion due to viewing window deformation, camera calibration, and infrared light pollution of the camera sensor. Elements of the test setup are designed to address all of these challenges. The technique is used to measure full-field strains on Ti-6Al-4V specimens as they are loaded to failure in tension. The technique provides substantially more data than traditional elevated temperature strain measurement methods.