Dynamic Mechanics of Materials Laboratory: Lab Equipment
Laboratory Equipment
The Dynamic Mechanics of Materials Laboratory is equipped with a compression split Hopkinson bar (SHB) apparatus for high strain rate compression experiments. The apparatus, see Figure 1, is comprised of three 0.5 in. diameter Ti-6Al-4V bars. The compression loading wave is generated by firing a striker bar into the end of the incident bar using a gas gun. The incident and transmitter bars are 6 ft. long and comprised of Ti-6Al-4V. The waves during a test are measured at the center of both the incident (Gage A) and transmitter (Gage B) bars, see Figure 1. At both locations, four 1000 Ohm strain gages are connected in a full Wheatstone bridge circuit configured to measure axial load. The compression SHB apparatus at the Dynamic Mechanics of Materials Laboratory is shown in Figure 2.
Specimens, typically small cylinders, are placed between the bars. The contact surfaces between the specimen and the bars are lubricated to minimize friction and "barreling" deformation. Typical strain rates of 400 to 8000 s-1 are achievable on this apparatus.
Click here for sample compression SHB data from the apparatus shown in Figure 2.
Some materials require large sample sizes to maintain their aggregate mechanical characteristics. These sample sizes may be too large for the apparatus in Figure 2 to accomodate. Because of this, the Dynamic Mechanics of Materials Laboratory has designed and constructed a large diameter compression SHB.
More detail on this experimental technique can be found in the following reference:
Gray, G.T., “Classic Split-Hopkinson Pressure Bar Testing”, Mechanical Testing, Vol. 8, ASM Handbook, ASM International, 2000.
The Dynamic Mechanics of Materials Laboratory uses a torsion split-Hopkinson bar apparatus that is used to investigate the high strain rate behavior of materials subjected to shear stress-states. The apparatus is comprised of two, 0.875 in diameter 7075-T6 aluminum bars. The incident and transmitter bars are 12 ft. and 7 ft. long, respectively. The torsion loading wave is generated by the release of a torque that is initially clamped in the incident bar, see Figure 1. The wave test data are measured at two locations on the incident bar (Gages A and B), and at one location on the transmitter bar (Gage C), see Figure 1. At each location four 1000 Ohm strain gages are configured in a full Wheatstone bridge circuit for torque measurment. The torsion SHB apparatus at the Dynamic Mechanics of Materials Laboratory is shown on the right side of Figure 2.
The specimens are flanged thin-walled tubes (shaped like spools) which are fastened to the bars using a high-strength, two-part epoxy. Typical shear strain rates of 400 to 5000 s-1 are achievable using this apparatus. The torsion SHB has two advantages over compression and tension SHBs. First, shear waves are not susceptible to dispersion (the wave changes shape as it travels down the bar). Second, radial inertia effects in the specimen, for example necking in a tension specimen and "barreling" deformation in a compression specimen, do not effect the experimental data.
Click here to view sample experimental data from the torsion SHB apparatus shown in Figure 2.
More detail concerning this experimental technique can be found in the following reference:
Gilat, A. “Torsional Kolsky Bar Testing”, Mechanical Testing, Vol. 8, ASM Handbook, ASM International, 2000.
The Dynamic Mechanics of Materials Laboratory uses an Instron 1321 axial-torsional load frame, see Figure 1, to conduct experiments at strain rates less than 2.0 s-1. The axial and torsional degrees of freedom allow the user to conduct tension, compression, shear (torsion) as well as combined tension-shear and compression-shear experiments. The load frame is controlled with an MTS FlexTest SE controller and is equipped with the Multi-Purpose Testware software package. The machine has MTS 647.02 axial-torsional hydraulic wedge grips for specimen attachment. Two load cells can be used with this machine. For experiments requiring large loads and torques, a Lebow 193 axial torsional Load cell is available. This cell has an axial load capacity of +/- 20,000 lbf and a torque capacity of +/-10,000 in-lbf. For experiments requiring lesser loads, an Interface 1216CEW-2K load cell is available. This cell has an axial load capacity of +/- 2,000 lbf and a torque capacity of +/-1,000 in-lbf.
Some materials require large sample sizes to maintain their aggregate mechanical characteristics. Examples of these materials include woven composites and concrete. These sample sizes may be too large for the small diameter SHB apparatus to accomodate. Because of this, the Dynamic Mechanics of Materials Laboratory has designed and constructed a large diameter compression SHB. The apparatus, see Figure 1, is comprised of 2 inch diameter Ti-6Al-4V bars. Operation of this device is identical to that of the small diameter compression SHB.
Click here to view sample data from this apparatus.
Most servohydraulic load frames have a functional upper strain rate limit of 1.0 to 5.0 s-1. Servohydraulic data above these strain rates are commonly plagued by oscillations that are superimposed on the load cell response. This is commonly referred to as "ringing" and can lead to questionable stress-strain data. These oscillations arise from the inertial response of the load frame. In other words, the specimen is not in equilibrium for the entirety of the experiment. At the other end of the spectrum, the typical split-Hopkinson bar apparatus has a lower strain rate limit of 400 to 500 s-1. Therefore, there is a strain rate range spanning roughly two orders of magnitude (5.0 to 400 s-1) where it is difficult to obtain high-quality data. Many common engineering applications, including automotive crash and low-velocity impact, lead to strain rates in this range. Because of this, the Dynamic Mechanics of Materials Laboratory designed and constructed an intermediate strain rate apparatus to bridge the gap between the serovohydraulic load frame and the split-Hopkinson bar.
The apparatus, see Figure 1 and Figure 2, consists of a servohydraulic actuator and a 130 ft long transmitter bar. The specimen is placed such that one end is adjacent to the transmitter bar and the other end is free. The specimen is loaded by the actuator as it impacts the specimen’s free end directly. Once loaded, the specimen deforms between the actuator and the transmitter bar. As the specimen is loaded and deformed, a compression wave propagates into the transmitter bar. The amplitude of this wave is measured with four 1000 Ohm strain gages in a full Wheatstone bridge circuit configured to measure axial load. The gages are placed on the transmitter bar at a distance roughly five bar diameters from the specimen. The wave in the transmitter bar propagates to the end of the bar and then reflects back toward the specimen. The experiment can continue until the reflected wave reaches the strain gages (roughly 16 milliseconds). "Ringing" in the measured load pulse is eliminated because the experiment is over before reflections reach the strain gages. Strain is measured using a high speed three-dimensional digital image correlation system. Strain can be measured directly on the surface of the specimen or by measuring the motion of both the actuator and transmitter bar. The apparatus can also be used for tensile testing by fixing the specimen to both the transmitter bar and actuator and reversing the direction of actuator motion.
Click here to view sample data from this apparatus.
The Dynamic Mechanics of Materials Laboratory uses VIC-3D, a commercial three-dimensional digital image correlation (3D DIC) software package that is maintained by Correlated Solutions. Static and dynamic camera systems are available for use. The static system consists of two Point Grey GRAS-20S4M-C cameras. These cameras have 1624 pixel by 1224 pixel resolution and can capture images at a maximum frame rate of 19 fps which is fast enough for most experiments conducted on a servohydraulic load frame. The dynamic system employs two Photron Fastcam SA1.1 high speed cameras. These cameras can achieve 125,000 fps at 256 pixel by 128 pixel resolution. A table that describes the frame rate and resolution capabilities of these cameras is found here. The dynamic system is used for split-Hopkinson bar and intermediate strain rate experiments.
The Dynamic Mechanics of Materials Laboratory uses this technique in experiments regularly. Some examples are highlighted below.
Uniaxial tension experiment of an axisymmetric 2024-T351 aluminum specimen:
In this experiment, a round tension specimen is tested in uniaxial tension. One might think that this would be a relatively simple test and yield rather uninteresting results. The 3D DIC data, however shows an interesting deformation phenomenon that takes place in the specimen (click on the image to play the movie). As the specimen is loaded, the deformation initiates in a region near the left end of the specimen and propagates to the right. This repeats itself several times until the necking localization forms and the specimen fails shortly afterward.
Cyclic loading of a resin mandible specimen:
In this experiment, a cyclic compressive load (that mimics typical chewing forces) is applied to two non-splinted dental prostheses that are retained with screws to dental implants embedded in a resin mandible specimen. 3D DIC is used to monitor the maximum principal strain distribution in the mandible specimen (click the image to play the movie). The data clearly shows a strain concentration underneath the anterior (left) prosthesis. These data help dental researchers evaluate their implants, prostheses and installation techniques.
Comparison of 3D DIC data to numerical simulations:
3D DIC data of a thick, notched tension specimen are compared to data from an LS-DYNA simulation below. Maximum and minimum principal strains measured with DIC are shown in (a) and (c), respectively. Simulated maximum and minimum principal strains are respectively presented in (b) and (d) below. In this case, the experimental strains are similar to simulated strains. This example illustrates that 3D DIC can be used to make detailed critiques of the constitutive models used in numerical simulations.
3D DIC is an optical measurement technique that is rapidly enhancing the experimental mechanics discipline. The technique can determine the three dimensional contour of an object's surface and track the surface displacement field of the object in a series of images. 3D DIC uses digital images from two cameras and the principles of optics to stereo-triangulate the surface contour of the object. An algorithm defines a field of "subsets" on the object's surface using the digital images. These subsets are N by N pixel boxes that contain an array of pixel gray-scale values. An advanced tracking algorithm can determine the translation, rotation and deformation of these subsets in loaded images with respect to a reference frame. The result is a time history of the specimen's surface displacement field. The displacement field is used with continuum mechanics definitions of strain to calculate the strain field on the specimen's surface.
3D DIC is an extremely useful tool for experimental mechanics. It allows the experimentalist to examine, in detail, complex behavior that exist even in relatively simple mechanical tests, such as the necking localization in tension and shear band formation in torsion of a thin-walled tube. The full-field displacements and strains measured using the technique give the user access to significantly more detailed information than previously available with strain gage measurements. These additional data can be used to make detailed comparisons and critiques of numerical simulations.
More detail on 3D DIC can be found in the following reference:
Sutton, M.A., Orteu, J.-J., Schreier, H.W., Image Correlation for Shape, Motion and Deformation Measurmeants, Springer, New York, NY, 2009.
High Strain Rate Tension Tests
The Dynamic Mechanics of Materials Laboratory employs a direct tension split Hopkinson bar (SHB) for high strain rate tension tests. The apparatus is made of two 7075-T6 aluminum bars with diameter of 0.5 in. The incident and transmitter bars are 12 ft. and 5.5 ft. long, respectively. The loading wave is generated by the release of a static tensile force that is initially clamped in the incident bar, see Figure 1. The waves during a test are measured at two locations on the incident bar (Gages A and B) and in one location on the transmitter bar (Gage C), see Figure1. At each location four 1000 Ohm strain gages are connected in a full Wheatstone bridge circuit configured to measure axial load. The tension SHB apparatus at the Dynamic Mechanics of Materials Laboratory is shown on the left side of Figure 2.
A wide array of specimen geometries can be tested with this apparatus. Typical specimen geometries include flat or axisymmetric specimens with a short, constant cross-section gage. Notched flat and axisymmetric samples can also be tested to introduce multi-axial stress states. The specimens are typically fastened to the bars using a high-strength, two-part epoxy. Custom-designed mechanical fixtures can also be used for efficiency. Typical strain rates of 400 to 2000 s-1 are achievable with this apparatus.
Click here for sample data from the tension SHB apparatus.
More detail on this experimental technique can be found in the following reference:
Staab, G. H., Gilat, A., “A Direct-Tension Split Hopkinson Bar for High Strain-Rate Testing”, Experimental Mechanics, Vol. 31, 1991, pp 232-235.
Accordions
