Recent developments in dynamic testing of materials

Three new testing configurations that have been developed since the last DYMAT conference in 2009 are presented. The first is high strain rate testing of Kevlar cloth and Kevlar yarn in a tensile Split Hopkinson Bar (SHB) apparatus. The Kevlar cloth/yarn is attached to the bars by specially designed adaptors that keep the impedance constant. In addition to determining the specimen’s stress and strain from the recorded waves in the bars the deformations are also measured with Digital Image Correlation (DIC). The second testing configuration is a high strain rate shear test for sheet metal. The experiment is done by using a flat notched specimen in a tensile SHB apparatus. The shear strain is measured using DIC within the notch and on the boundary. The third development is a compression apparatus for testing at intermediate strain rates ranging from 20 s−1 to 200 s−1. The apparatus is a combination of a hydraulic actuator and a compression SHB. The stress in the specimen is determined from the stress wave in a very long transmitter bar and the strain and strain rate is determined by using DIC. The results show clean stress strain curves (no ringing). 1 Background The Split Hopkinson Bar (SHB), introduced by Kolsky [1] in 1949, is the most commonly used technique for characterizing the deformation and failure of materials at strain rates ranging from about 400 s−1 to 5, 000 s−1. In this technique, a material specimen that is placed between two bars is loaded by a stress wave that is generated in one of the bars (incident bar). The force applied to the specimen and the average strain in the specimen can be determined from the waves in the bars that remain within the elastic limit during the experiment. The technique was introduced with compression loading and was later modified for tension and torsion loadings. Over many years the SHB technique was mostly used for testing specimens that were subjected to a uniform state of stress and deformation. In these tests the force (or torque) in the specimen is determined from the wave in the transmitter bar and the average strain is calculated from the difference in the motion at the ends of the specimen’s gage section that are determined from the waves in the bars. Recently, the Digital Image Correlation (DIC) method for strain measurement has been incorporated into the SHB test, Gilat et al [2]. DIC measures the full field deformation directly on the specimen and thus provides means for examining whether the deformation in the specimen is uniform and means for conducting tests where in deformation between the ends of the bars is intentionally not uniform. Three dynamic tests associated to the SHB technique with DIC are presented in this paper. In the first application, Kevlar yarn and cloth is tested in a tensile SHB apparatus. Kevlar is sometimes used in applications where dynamic loadings are applied, and the tests were done in order to check whether the mechanical properties of Kevlar are sensitive to the strain rate. The second test is a high strain rate shear of sheet metal. The test is done by using a flat notched specimen in a tensile SHB. Dynamic shear tests are usually done with a torsional SHB using spoolshaped specimens which cannot be machined from sheet metal. The third experiments deals with testing materials at intermediate strain rates between 20 s−1 and 200 s−1. Testing at this range is difficult because the strain rates are too low for the standard split Hopkinson bar technique and are too high for standard hydraulic machines. Deforming a specimen to an appreciable strain takes milliseconds and in a typical SHB apparatus the loading lasts well less than one millisecond. On the other end the actuator of a typical hydraulic machine can move fast enough to deform the specimen at the required strain rate, but results from such tests are noisy with large oscillations (referred to as ringing). This happens because the whole testing machine is not in static equilibrium during the test. To overcome these difficulties some researchers have modified the standard hydraulic machine and/or the method by which the data is analyzed, and others have modified the SHB technique. Song et al. [3] have used a compression SHB with a long bars for testing soft materials. Zhao and Gary [4], and Othman and Gary [5] have used a hydraulic actuator for long loading pulse and separation of waves technique for analyzing the data. The new apparatus for compression testing at intermediate strain rates that is presented here is made up from a hydraulic actuator that can apply dynamic loads for the required duration and a long transmitter bar that can measure the force in the specimen without the effects of wave reflections. 2 Tensile testing of Kevlar in SHB Testing Kevlar cloth and yarn requires special attention to the mounting, or gripping, of the specimen. At low (quasistatic) strain rates there are no limits on the specimen and mounting fixtures geometries. Testing these materials with the tensile SHB apparatus requires special attention since the specimen has to be small, should be attached between the bars while keeping the impedance constant, and the connection has to be strong enough such that the specimen will fracture in the gage section between the bars. The tensile SHB in our lab is made of Aluminum This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20122601002 EPJ Web of Conferences Fig. 1. Kevlar cloth specimen following a tensile SHB test. Fig. 2. Kevlar yarn specimen following a tensile SHB test. -200 0 200 400 600 800 Time (μs) -400 0 400 80


Introduction
Numerical simulation of the response of materials to loads has reached a level of maturity at which it can be used with confidence for design. Numerical codes like LS-DYNA and Abaqus include many material models for plastic deformation and failure (constitutive relations) that can be selected for specific applications. The various models require input parameters that are specific to the material that is being simulated. Accurate experimental results from tests under well-controlled conditions are needed for the determinations of the parameters in the material models as well as for the validation.
The current paper presents two new testing configurations that have been developed recently for the purpose of providing fundamental data for determining the parameters in material models for dynamic plastic deformation and failure. In addition, results from testing Inconel 718 over a wide range of strain rates and temperatures are presented.

Dynamic punch experiment
A dynamic punch test in which three-dimensional Digital Image Correlation (DIC) is used to measure the deformation of the rear surface of a specimen as it being penetrated is carried out by using a large diameter (50 mm) compression Split Hopkinson Bar (SHB) apparatus, Fig. 1. Flat, round Ti-6Al-4V disk specimens are attached to the transmitter bar of a compression SHB apparatus and a tungsten carbide punch is attached to the incident bar and positioned such that it is in contact with the disk, Fig. 2. During a test, a compression wave is introduced into the incident bar which causes the punch to penetrate into the specimen. The specimen is mounted on a die fixture that is slotted on both sides such that the rear surface of the disk specimen is visible to two high speed cameras. This a Corresponding author: gilat.1@osu.edu provides a stereographic view of the specimen that is used to measure full-field displacement directly on the specimen using three-dimensional DIC. The contact force between the punch and the disk is determined from the wave in the transmitter bar.
Two Photron SA1.1 cameras running at 100,000 frames per second (10 µs interval) at 192 pixel by 192 pixel resolution record the rear surface deformation of the disk specimen. The images are processed by commercial Digital Image Correlation (DIC) software (VIC-3D).
Typical wave profiles recorded in a test are shown in Fig. 3. The amplitude of the incident wave in this experiment is 400 kN. Figure 4 shows the history of the strains at two points (the centre point and the point that failure first occurs) at the back surface of the specimen. The last DIC image recorded before failure is shown in Fig. 5.
Results from tests with punches of various geometries show that the punch geometry greatly influences the punching force and the failure mode. The data is used to construct and validate deformation and failure models.   to 5.200 s −1 . A hydraulic frame is used for the quasistatic tests and the SHB technique (with DIC used to measure strains directly on the specimen) is used for the dynamics tests. Results from tensile and compression tests with specimens initially at room temperature are shown in Figs. 6 and 7, respectively.

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The results show a significant effect of the strain rate on the stress. There is also a noticeable difference in

Full-field strain and temperature measurements during dynamic tensile test
Under development is a dynamic tensile experiment that features a full-field strain measurement, and a full-field temperature measurement at the same time (on opposite sides of the specimen). The strains are measured with DIC and the temperature is measured with a high speed IR camera. Pilot experiments have been conducted at strain rate of 150 s −1 and 500 s −1 . Figures 8 and 9 present results from tensile testing of specimen made of 304 stainless steel at strain rate of approximately 150 s −1 . Figure 8 shows the last frame before failure that was recorded with an IR camera (Telops Fast MWIR 1500, resolution 128 × 64, frame rate 10.000 fps). The maximum temperature observed is about 270 • C. The full-field strain measured with DIC at the same time is shown in Fig. 9 (Photron SA1.1, resolution 448 × 592, 20.000 fps). The maximum strain the is measured in the neck is about 0.8.