Abstract:
A specimen for measuring a material under multiple strains and strain rates. The specimen including a body having first and second ends and a gage region disposed between the first and second ends, wherein the body has a central, longitudinal axis passing through the first and second ends. The gage region includes a first gage section and a second gage section, wherein the first gage section defines a first cross-sectional area that is defined by a first plane that extends through the first gage section and is perpendicular to the central, longitudinal axis. The second gage section defines a second cross-sectional area that is defined by a second plane that extends through the second gage section and is perpendicular to the central, longitudinal axis and wherein the first cross-sectional area is different in size than the second cross-sectional area.

Description:
[0001]    Applicants claim, under 35 U.S.C. §119(e), the benefit of priority of the filing date of May 12, 2011 of U.S. Provisional Patent Application Ser. No. 61/485,163, filed on the aforementioned date, the entire contents of which are incorporated herein by reference. 
     
    
       [0002]    This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    1. Field of the Invention 
         [0004]    The present invention pertains to systems and methods for characterizing mechanical properties when subject to multiple strains and strain rates. 
         [0005]    2. Discussion of Related Art 
         [0006]    With the rising cost of fuel, automobile manufacturers are looking for ways to improve fuel mileage. One way to do this is to use lightweight materials, such as alloys of Magnesium (Mg), when manufacturing an automobile. In the case of using Magnesium alloys, automobile manufacturers desire to use them because of their high strength-to-weight ratios. The application of magnesium alloys in the automobile industries will satisfy the goal of vehicle weight reduction and fuel efficiency improvement. 
         [0007]    Before a material is used in an automobile, a number of information is needed. For example, mechanical properties under impact, damage and failure characterization, material and failure models and Finite Element Models (FEM) technology need to be determined. In addition, methods for material characterization under impact need to be developed. In the case of Magnesium alloys, little information regarding crashworthiness of the material is available. With that said, there are differences between the mechanical properties of conventional materials, such as High-Strength Low-Alloy Steel (HSLA), and Magnesium alloys. For example, as shown in  FIGS. 1(   a )-( f ) the impact energy dissipation in HSLA during a crashworthiness test occurs by a different process than in a Magnesium alloy tube as shown in  FIGS. 2(   a )-( f ). Faced with the difference in mechanical properties, current automobile designs that employ lightweight materials, such as Magnesium alloys, result in overdesigned components in order to compensate for the uncertainties in the deformation and failure mechanisms. Such overdesigning can be avoided by understanding the initiation and evolution of internal state and failure processes in the lightweight materials as functions of loading type and loading rates. Reducing uncertainties in component design would greatly improve the overall vehicle system reliability and enable weight reduction of the automobile. 
         [0008]    Another property to test for a material is their response at different strain rates. Testing at low (quasi static) rates is performed in order to observe the situation when the system is in equilibrium at all times. At the other end of the spectrum, fast rate tests have been performed wherein a single impact pulse travels through the system. A more problematic area of interest is so-called intermediate rate tests performed in the range of between 1/s and 1000/s with multiple wave reflections in the system. This area is important because maximum strain rates are in the interval of 10-1000/s for automotive crashes. For intermediate rates, it is difficult to establish dynamic equilibrium in the sample and the system and it is a fact that intermediate strain rate tests have not been established. 
         [0009]    With the above said regarding strain rates measurements, it is important to apply certain principles to the measurement of strain rates for lightweight materials. Such principles include: reducing mass in the system; developing lightweight load cells and sensors; understand and control oscillations in the system; and combine multiple measurement techniques for the same data. 
         [0010]    One way to understand the structure of materials, such as lightweight materials, is to study the response and microstructure changes in the design of the materials. Material mechanical response and microstructure changes, such as microstructure defect evolution, are often dependent on the levels of imparted strains and strain rates. The changes in microstructure can provide understanding about processes of material mechanical degradation that can lead to structural failure. Many materials exhibit different mechanical response when loaded by different deformation rates. This property is called strain rate sensitivity and is conventionally examined by using multiple specimens and tensile tests. 
         [0011]    For tensile test configurations, standard dog-bone specimens (ASTM E8) are used. For the sample geometry of standard dog-bone specimens, uniform deformation is achieved within the gage section and the measured strains and strain rates are related to the displacements in this region. In order to characterize the evolution of an internal state of a material at different rates of strain, it is required to instantly stop (interrupt) the deformation from a current loading speed. This interruption of the deformation is possible at very low loading speeds, but at velocities necessary to generate strain rates of 1/s and higher, the inertia of the loading equipment and control system makes this task impractical using conventional test methods. Performing strain-interrupted tests using dog-bone specimens becomes exceedingly difficult, if not impossible, at high rate testing speeds. Complicated testing fixtures have been proposed and have shown to be impractical at high rates of strain. These fixtures add extra mass in the loading train of the testing equipment and consequently introduce additional oscillations that reduce the quality of the measurements. Low (quasi-static) rate tests—entire system is in equilibrium at all times. 
         [0012]    Note that conventional methods of calculating displacement or strains for materials in general from stroke (i.e., the actuator motion) are not accurate. For example, at low strain rate (1/s), strain calculated from stroke tends to overestimate the average strain of the gage section. At high strain rate (500/s), strain calculated from stroke tends to underestimate the average gage section strain. Such conventional methods give inaccurate measurements that need to be filtered and cannot provide data for small strains and for high strain rates. Furthermore, at high strain rates conventional methods results in the sample being difficult to control and an increase in measurement problems as the speed is increased. 
         [0013]    Thus, there is a need for new testing methodologies and material information for the strain rates of interest in vehicle design when lightweight materials are employed. In particular, systems and methods need to be developed that can measure a wide range of strain rates from low to intermediate to high rates. It is envisioned that such systems and methods would employ multiple types of sensors, wherein one type of sensor would be configured to measure one range of rates and other types of sensors would be configured to measure other ranges of rates. In such a system, there would be a transition going from one type of sensor to another. Other parameters to be measured by new methods and systems would be: 1) strain-interrupted tests at high rates, 2) methods for characterization of material property degradation (damage) evolution under high rates, 3) methods for failure characterization at high rates, 4) constitutive models for FEM simulations, and 5) investigating formation and growth of voids using microscopy for strains and strain rates of interest. 
       SUMMARY OF THE INVENTION 
       [0014]    One aspect of the present invention regards a specimen for measuring behavior of a material under multiple strain rates with only a single strain test. The specimen including a body having a first end, a second end and a gage region disposed between the first end and the second end, wherein the body has a central, longitudinal axis passing through the first end and the second end. The gage region includes a first gage section and a second gage section, wherein the first gage section defines a first cross-sectional area that is defined by a first plane that extends through the first gage section and is perpendicular to the central, longitudinal axis. The second gage section defines a second cross-sectional area that is defined by a second plane that extends through the second gage section and is perpendicular to the central, longitudinal axis and wherein the first cross-sectional area is different in size than the second cross-sectional area. 
         [0015]    A second aspect of the present invention regards a system for characterizing material behavior under multiple strain rates using a single specimen in one test, the system including a device for applying a load, the device comprising a first jaw and a second jaw, the device capable of applying strain loads at greater than 500 inches per second. A specimen having a body with a first end, a second end and a gage region disposed between the first end and the second end, wherein the first end is engaged by the first jaw and the second end is engaged by the second jaw, wherein the body has a central, longitudinal axis passing through the first end and the second end, wherein the gage region comprises a first gage section and a second gage section, wherein the first gage section defines a first cross-sectional area that is defined by a first plane that extends through the first gage section and is perpendicular to the central, longitudinal axis and the second gage section defines a second cross-sectional area that is defined by a second plane that extends through the second gage section and is perpendicular to the central, longitudinal axis and wherein the first cross-sectional area is different in size than the second cross-sectional area. The system further includes a camera focused at the specimen and generates an image of the specimen and a control and data acquisition unit that receives signals from the camera representative of the image and calculates a strain experienced by the specimen based on the signals. 
         [0016]    A third aspect of the present invention regards a method for characterizing material behavior under multiple strain rates using a single specimen in one test. The method including applying a load to a specimen having a body with a first end, a second end and a gage region disposed between the first end and the second end, wherein the first end moves wherein the body has a central, longitudinal axis passing through the first end and the second end, wherein the gage region comprises a first gage section and a second gage section, wherein the first gage section defines a first cross-sectional area that is defined by a first plane that extends through the first gage section and is perpendicular to the central, longitudinal axis and the second gage section defines a second cross-sectional area that is defined by a second plane that extends through the second gage section and is perpendicular to the central, longitudinal axis and wherein the first cross-sectional area is different in size than the second cross-sectional area. The method further including generating an image of the specimen when a first deformation is experienced by the specimen and generating a second image of the specimen when a second deformation is experienced by the specimen. The method including calculating strains experienced by the specimen at the first and second deformations. 
         [0017]    One or more aspects of the present invention provide the advantage of reducing the number of samples needed for material measurements. 
         [0018]    One or more aspects of the present invention provide the advantage of measuring multiple strain rates in an efficient manner. 
         [0019]    One or more aspects of the present invention provide the advantage of testing the structural characteristics of lightweight materials to be sued for automobiles in an efficient manner. 
         [0020]    One or more aspects of the present invention provide the advantage of reducing the number of required tests for characterization of material strain rate sensitivity. 
         [0021]    One or more aspects of the present invention provide a new capability of imparting prescribed strains at high strain rates and thereby enables characterization of material internal state evolution in this loading regime. 
         [0022]    Further characteristics and advantages of the present invention will become apparent in the course of the following description of an exemplary embodiment by the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIGS. 1(   a )-( t ) show impact energy dissipation in HSLA during a crashworthiness test; 
           [0024]      FIGS. 2(   a )-( f ) show impact energy dissipation in a Magnesium alloy during the same crashworthiness test as in  FIGS. 1(   a )-( f ); 
           [0025]      FIG. 3  schematically shows an embodiment of a measuring system in accordance with the present invention; 
           [0026]      FIG. 4 , schematically shows an embodiment of a hydraulic tensile machine to be used with the measuring system of  FIG. 3  in accordance with the present invention; 
           [0027]      FIG. 5A  shows a first embodiment of a test fixture to be used with the hydraulic tensile machine of  FIG. 4  in accordance with the present invention; 
           [0028]      FIG. 5B  shows a second embodiment of a test fixture to be used with the hydraulic tensile machine of  FIG. 4  in accordance with the present invention; 
           [0029]      FIG. 6  shows a first embodiment of a specimen to be used with the measuring system of  FIG. 3  in accordance with the present invention; 
           [0030]      FIG. 7  shows a second embodiment of a specimen to be used with the measuring system of  FIG. 3  in accordance with the present invention; 
           [0031]      FIG. 8  shows a possible plot of strain versus distance from fracture location for the specimen of  FIG. 7  using the measuring system of  FIG. 3 ; 
           [0032]      FIG. 9  shows a possible plot of strain versus time for the specimen of  FIG. 7  using the measuring system of  FIG. 3 ; 
           [0033]      FIGS. 10A and 10B  show the difference between the conventional method of calculating from stroke versus the use of digital image correlation (VIC) in the measuring system of  FIG. 3 ; 
           [0034]      FIGS. 11A and 11B  show the difference between the conventional method of testing strain rate using a LVDT/load washer versus the use of the samples of  FIGS. 6 and 7  in the measuring system of  FIG. 3 ; 
           [0035]      FIG. 12A , shows a non-uniform distribution of the pores/microstructure needs to be considered when locating an area of interest; 
           [0036]      FIGS. 12B-D  show how void statistics/data are generated by image processing; 
           [0037]      FIGS. 13A-I  show void profile examples; 
           [0038]      FIG. 14  shows the measurement of porosity versus strain and strain rate at various strain rates; 
           [0039]      FIGS. 15A-B  show an example of the evolution of the shear strains during a high-rate shear test run with the system of  FIG. 3 ; and 
           [0040]      FIG. 16  shows the measurement by the system of  FIG. 3  of the shear stress versus the shear strain. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    As schematically shown in  FIG. 3 , a measuring system  100  that performs various measurements regarding strain and strain rates is provided. The hydraulic test system includes a test frame  106  and a hydraulic actuator  131  that pulls down on a grip  120 . Such pulling motion results in deformation of the specimen  116 ,  116 ′ (specimen  116 ′ of  FIG. 7  is shown, for example). The operation mechanism of the hydraulic system is described by Y. Wang et al. entitled “Characterization of High Strain Rate Mechanical Behavior of Az31 Magnesium Alloy using 3D Digital Image Correlation.” The deformation is measured via a 3D Digital imaging correlation system  104  that can generate a full field displacement map. The system  104  includes two high speed digital cameras  130  and a control and data acquisition unit  105 . An example of a suitable digital camera  130  is the Photron FASTCAM SA5 which provides 7,500 frames per second (fps) at resolution of 1024×1024 pixels and reduced resolution operation to a maximum of 1,000,000 frames per second. The digital cameras  130  are staged so that the area of interest on the sample is visible to both cameras. Note that control and synchronization of multiple data sources is desired for accuracy at high speeds. 
         [0042]    Prior to tensile testing, a standard calibration procedure is performed to determine the camera parameters, such as focal length, radial distortion coefficients, center position of the lens, skew of the sensor grid and the relationship between the two cameras  130 . A rigid calibration grid with known spacing is used to perform the calibration. Within the depth of field, images are taken simultaneously while the calibration grid is placed to cover the image field and positioned to have tilt/rotation and translation along all three axes. An acceptable calibration for this study is when the standard deviation of residuals for a minimum 20 views of the calibration grid at various positions is less than 0.05 pixels. 
         [0043]    During the deformation process, the cameras  130  receive images of an area of interest of the specimen. In the area of interest, a speckle pattern is present. The signals representative of the images of the speckle pattern present in the area of interest are then sent to control and data acquisition unit  105 . The signals from each camera are synchronized and combined so that a three-dimensional full field deformation map is generated for a number of instances of the deformation process. Preferably, consecutive instances of time are separated from one another by an equal amount of time, with separations that can range from 10 s to 1×10 −6  s being possible. The three-dimensional full field deformation maps are formed in a well-known manner using software available under the trade name of VIC-3D 2010 Digital Image Correlation made by Correlated Solutions, Inc. 
         [0044]    With the above described deformation maps, the control and data acquisition unit  105  using the above described VIC-3D 2010 Digital Image Correlation software is able to determine for each speckle in the area of interest how its position changes from one map to the next. In particular, the deformation strains are resolved by post-processing the sequential speckle images of the tested specimen  116  based on a pattern-matching algorithm, such as available from the commercial software VIC-3D 2010 (Correlated Solutions, Inc.). The standard deviation for strain measurements is about 60 microstrain. An example of the above mentioned measurement of strain using high speed cameras is described in the article by Y. Wang et al. entitled “Characterization of High Strain Rate Mechanical Behavior of Az31 Magnesium Alloy using 3D Digital Image Correlation,” Advanced Engineering Materials, Vol. 13, No. 10, 2011, pp. 943-948. 
         [0045]    As shown in more detail in  FIG. 4 , the pulling actuator  131  can be thought of as being part of a tensile machine  102  that includes a frame  106  to which a load cell  108  is attached. The load cell  108  and load washer  110  is in communication with control and data acquisition unit  105 . Top grip  112  engages a top portion or tab  114  of the lightweight material sample  116 . A bottom portion or tab  118  of the sample  116  engages a bottom grip  120 . In  FIG. 5A , fixture  124  engages the top  114  of the specimen  116  and allows the bottom  118  of the specimen  116  to pass through to engage the bottom grip  120 . The mechanical stopper  125  is engaged to the bottom  118  of the specimen  116 . 
         [0046]    An alternative test fixture  124 ′ is shown in  FIG. 5B . The amount of specimen deformation is defined by the spacing between the mechanical stopper  125  and the stationary part of the fixture  126  while the specimen is tested under tension. The specimen will break at the notch section after the mechanical stopper engages the stationary part of the fixture  126 . The stationary part of the fixture  126  in  FIG. 5A  is a solid metallic frame with a through slot at the bottom to allow the specimen to pass through. The stationary part of the fixture  126  in  FIG. 5B  is a solid metallic tube. 
         [0047]    As shown in  FIG. 4 , the specimen  116  includes strain gages  126  located at the top portion  114 , central portion or gage section  128 , and bottom portion  118 . Each strain gage  126  is an electrical bridge circuit that indirectly measure a load applied to the strain gage in a well-known manner. Signals  111   a, b  from the strain gages  126 , signals  113  from the load washer  110  and signals  115  from the load cell  108  are sent to control and data acquisition unit  105 . These signals are synchronized with the images from cameras  130  during the tensile test. The control and data acquisition unit  105  sends command signals  111   c  to the actuator  131  to control its motion. Note that the control and data acquisition unit  105  sends command signals  111   c  to the actuator for testing of the specimen and collects the data present in signals  111   a, b ,  113  and  115 . Such collection of data is synchronized with the acquisition of images by system  104 . In one embodiment, synchronization can be achieved by generating a 50 millisecond pulse on an M-series PXI-6259 multifunction IO board. This pulse is then simultaneously detected by the data acquisition and waveform generation tasks on the same PXI-6259 IO board that were programmatically configured pre-test. Similarly, a second board, such as the M-series PXI-6250 board, is also triggered off the same pulse along with the two high speed cameras in the imaging system. All triggers are synchronized through the PXI chassis in which the M-series cards are installed. The chassis backplane runs a 10 MHz system clock yielding 100 ns trigger synchronization. 
         [0048]    In operation, the machine  102  may have the following properties: 
         [0049]    Max Velocity=700 in/s (18.5 m/sec) over approx. 4 in (100 mm) Range; 
         [0050]    Load Capacity: 9000 lbs (40 kN) static, 5500 lbs (25 kN) dynamic; 
         [0051]    Total Stroke: 15.5 in (400 mm); 
         [0052]    Working Stroke: approx. 7.0 in (175 mm) with slack adapter in the load train; 
         [0053]    Control: MTS 407 servo-hydraulic controllers, with external command signal (drive file); and 
         [0054]    Synchronization and DAQ systems. 
         [0055]    With the above discussion of the measuring system  100  in mind, some general principles of measurement and operation should be kept in mind. For example, elimination of noise (the mechanical vibration present in the dynamic test) in the system  100  is the most challenging task along with having the load cell fight the effect of inertia of the devices in order to keep accuracy at high rates. In the system  100 , the strain gages  126  on the specimen  116  are used for strain and stress measurements in the central portion  128 . In addition, optical strain measurements from the high speed cameras  130  are used for strain and stress measurements across the specimen  116 . Measurements from 1) different sensors  126  located in the tab and gage sections, 2) the load washer  110  and  3 ) the load cell  108  are compared with optical measurements performed by the cameras  130  for different strain rates in order to establish correlations and estimate errors. 
         [0056]      FIG. 6  shows an embodiment of a specimen  116  to be tested by the measuring system  100  of  FIG. 3 . Possible materials for the specimen  116  are AZ31 sheet metal, AM60B cast (top hat), AM60B unprocessed and Advanced High Strength Steels (AHSS). The goal of the specimen design is to enable application of a desired strain and strain rate distribution across the specimen gage length. The possibility of changing mechanical properties of a test specimen through mechanical work and/or thermal process in order to create desired property distribution and thereby tailor the strain and strain rate distribution is also envisioned. Specimen design enables application of desired strain under specific strain rate by ensuring specimen rupture at a prescribed location so that it is not necessary to insert additional fixtures (stoppers, displacement limiters) into the loading train. 
         [0057]    In the specimen design of  FIG. 6 , a standard dog-bone specimen geometry is modified to have multiple gages sections (i.e., gage l 1 , l 2 , . . . l n ) rather than a single gage. So, in contrast with the standard dog, bone specimen, the specimen of  FIG. 6  has a multiple gage sections with changing cross-section areas. Each gage length can be the same or different than each other. The total specimen length must be practically possible for the test instrument and the area of interest should fit into the digital imaging windows. Each gage has different width with d 1 &gt;d 2 &gt; . . . &gt;d n  and the idea is to achieve yielding in l 1  after the minimum width gage reaches ultimate tensile strength. The initial specimen width dimensions can be estimated through the ratios between the yield strength and ultimate tensile strength, but the accurate strain and strain rate in each gage should be analyzed through 3D Digital Imaging Correlation via system  104 . 
         [0058]      FIG. 7  shows a specimen  116 ′ that can achieve a continuous and infinite distribution of strains and strain rates. Possible materials for the specimen  116 ′ are AZ31 sheet metal, AM60B cast (top hat), AM60B unprocessed and Advanced High Strength Steels (AHSS). Instead of a standard dog-bone gage section, this design has a gage with a curvature of R. The initial estimation of R is based on the ratio of the material ultimate tensile strength, σ UTS , and yield strength, σ ys  under static test rates. In other words, the specimen should yield at +−L 0  when the middle location fractures. For a sheet specimen with width W 0  and the desirable length for continuously varying the strains of 2L 0 , the curvature R, is given by equation (1) below. For accurate specimen design and analysis, Finite Element Modeling technique is suggested. 
         [0000]    
       
         
           
             
               
                 
                   R 
                   - 
                   
                     
                       
                         L 
                         0 
                         2 
                       
                       + 
                       
                         
                           
                             W 
                             0 
                             2 
                           
                           4 
                         
                          
                         
                           
                             ( 
                             
                               
                                 
                                   σ 
                                   UTS 
                                 
                                 
                                   σ 
                                   ys 
                                 
                               
                               - 
                               1 
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0059]    The dimension of T and W are designed to fit in to the specimen grips. Other designs of this end tab (such as a pinhole design) can be employed. The only requirement is that the tab region is long and wide enough so that its deformation during the test does not affect the area of interest region of 2L 0 . 
         [0060]    Note that other shapes for the specimen  116  are possible without departing from the spirit of the invention. The shape is determined by Finite Element Modeling. 
         [0061]    The speed of the tensile test performed by system  100  should be chosen as the maximum speed of interest, and images of the sample should be taken during the test with appropriate frame rates to calculate the full-field displacement map within the area of interest using 3D digital image correlations. 
         [0062]    Using the specimen geometry shown in  FIG. 7 , the plastic strain will decrease continuously from the middle of the specimen to +−L 0 , which can be shown from the full-field displacement map. Because of the difference in the strain levels, within the same amount of test duration time, continuous strain rates are also achieved at locations further away from the specimen center. Because the deformation of the specimen is recorded as a function of time, the strain and strain rate at each location (l, w) are resolved from the full field displacement map.  FIGS. 8 and 9  show an example of one application of this specimen geometry to magnesium alloy AZ31. The specimen  116 ′ has a curvature of 20 inches and is tested at a speed of 500 in/s. The strains and strain rates at each location of the interested area are resolved through the previously mentioned 3D digital image correlation. The specimen  116 ′ experiences continuous deformation of 0-˜23% within a one inch region that is centered about midpoint M of the specimen  116 ′. Results for three selected locations are shown in  FIGS. 8-9  to demonstrate strain variation from 6% to 20% with strain rates from 192/s to 580/s. In particular,  FIGS. 8-9  show the results at various distances (5.5 mm, 12 mm and 18 mm) from a fracture formed in the specimen  116 ′. The new method generates smoothly increasing strain path to the final strain and strain rate distribution. 
         [0063]    In summary, with this test design, to achieve desired strains with various strain rates using a single specimen is emphasized. The present invention reduces the number of required tests for characterization of material strain rate sensitivity. Since there is no added mass, it results in a simpler and cheaper measurement process. It also provides a new capability of imparting prescribed strains at high strain rates and thereby enables characterization of material internal state evolution in this loading regime. Evidence of the advantages of the measuring system  100  of  FIG. 3  and the samples of  FIGS. 6 and 7  are discussed below. 
         [0064]    For example,  FIGS. 10A and 10B  show the difference between the conventional method of calculating from stroke versus the use of digital image correlation (VIC) in the measuring system  100 . Indeed, digital image correlation enables measurement of strains well beyond the range of fast-response bondable foil gages. 
         [0065]      FIGS. 11A and 11B  show the difference between the conventional method of testing strain rate using a LVDT/load washer versus the use of the samples of  FIGS. 6 and 7  in the measuring system  100 . Indeed, significant improvement in measurements of stresses, strains and strain rates in the intermediate strain rate regime is accomplished with the present invention. 
         [0066]      FIGS. 12-16  show various measurements performed by system  100  on the Magnesium alloy AM60. For example,  FIG. 12A , shows a non-uniform distribution of the pores/microstructure needs to be considered when locating an area of interest.  FIGS. 12B-D  show how void statistics/data are generated by image processing. Such data can be used for calibration of micromechanics-based material and failure models for FEM simulations.  FIGS. 13A-I  show void profile examples and  FIG. 14  shows the measurement of porosity versus strain. Measurements show that void nucleation and growth (damage) intensifies with strain rates for AM60.  FIGS. 15A-B  show shear normal stresses surrounding a slant notch and  FIG. 16  shows the measurement by system  100  of the shear stress versus the shear strain. The system  100  generates reasonable strain distributions in the test specimen and test data correlates well with tension tests. 
         [0067]    In summary, the new method specimen design and testing procedure can produce continuously varying levels of plastic strain achieved at various locations in the specimen and at different strain rates. The new specimen design also reduces the size of conventional test matrices, overall testing time and permits more focus on test analysis and modeling. 
         [0068]    The foregoing description is provided to illustrate the invention, and is not to be construed as a limitation. Numerous additions, substitutions and other changes can be made to the invention without departing from its scope as set forth in the appended claims.