Patent Publication Number: US-10768082-B2

Title: System and method for testing true stress and true strain

Description:
RELATED APPLICATIONS 
     This patent application is a continuation application, and claims priority benefit with regard to all common subject matter, of earlier-filed U.S. patent application Ser. No. 15/147,055, filed on May 5, 2016, and entitled “SYSTEM AND METHOD FOR TESTING TRUE STRESS AND TRUE STRAIN”. The identified earlier-filed non-provisional patent application is hereby incorporated by reference in its entirety into the present application. 
    
    
     STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No.: DE-NA0000622 awarded by the United States Department of Energy/National Nuclear Security Administration. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Conventional engineering stress tests are often used for determining stress, strain, yield strength, tensile strength, fracture point, ductility, and other mechanical properties of parts and materials. These standard tests (per ASTM E8 and A370) do not take into account the “in test” reduction in cross sectional area as the specimen is stretched (a phenomenon called “necking”) and only uses the original gage area for generation of the engineering stress-strain curve. “In test” measurement of the actual cross sectional area is of key importance in the calculation of the “true stress−true strain” curve where measurement of this reduced area (i.e. necked region) is difficult to consistently and accurately measure in real time all the way to the point of specimen failure. 
     Conventional stress and strain testing systems also only measure the area of the specimens using stationary tags and/or digital markers usually in a single direction or single measuring position. Hence, these systems fail to correctly adjust to the correct area as these tags cannot move and are not usually placed at the location of minimum area throughout the test. This results in significant errors in the calculation of the true stress−true strain curve for the specimen being tested. 
     SUMMARY 
     Embodiments of the present invention solve the above-mentioned problems and provide a distinct advance in the art of stress testing. More particularly, the present invention provides a testing system and method for determining true stress in which a minimum diameter of a specimen is determined over a plurality of points along the specimen to account for changes in location of maximum necking of the specimen. 
     An embodiment of the present invention is a true stress testing system for determining true stress of a specimen being subjected to an applied force. The system broadly comprising a force input machine, an imaging system, and a computer. The force input machine imparts a force on the specimen (e.g., a tensile force along a force axis). The imaging system includes a light source for projecting a light beam in a first direction at the specimen and a camera positioned on an opposite side of the specimen for receiving portions of the light beam not blocked by the specimen. A shadow image of the specimen is thus created via the camera. The computer may determine a minimum diameter of the specimen perpendicular to the first direction at a point in time over a plurality of points along the specimen via the shadow image. The computer can then account for changes in position of the minimum diameter along the specimen as the specimen is subjected to the force over time. A true stress of the specimen may then be determined according to the force applied to the specimen and the minimum diameter of the specimen perpendicular to the first direction at the point in time. Accounting for changes in the position of the minimum diameter along the specimen provides more accurate stress analysis when compared to theoretical stress, which does not take into account movement of the minimum diameter along the specimen. 
     Another embodiment of the present invention is a method of determining true stress of a specimen. The method includes positioning a specimen in a force input machine; aiming a light source at the specimen; aligning a camera with the light source on an opposite side of the specimen; projecting a light beam from the light source in a first direction so that at least some of the first light beam reaches the camera, receiving at least some of the light beam via the camera so as to create a shadow image of the specimen; and applying a force via the force input machine. A minimum diameter of the specimen perpendicular to the first direction at a point in time over a plurality of points along the specimen may be determined so that the computer accounts for changes in position of the minimum diameter along the specimen as the specimen is subjected to the force over a period of time. A true stress of the specimen may then be determined according to the force applied to the specimen and the minimum diameter of the specimen perpendicular to the first direction at the point in time. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other aspects and advantages of the present invention will be apparent from the following detailed description of the embodiments and the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a perspective view of a true stress and true strain testing system constructed in accordance with an embodiment of the present invention; 
         FIG. 2  is a perspective view of a force input machine and imaging system of the true stress and true strain testing system of  FIG. 1 ; 
         FIG. 3  is a block diagram showing certain components of a computer of the true stress and true strain testing system of  FIG. 1 ; 
         FIG. 4  shows shadow images generated via cameras of the true stress and true strain testing system; 
         FIG. 5  is a stress plot comparing true stress determined via the true stress and true strain testing system with engineering stress and theoretical stress determined via conventional systems; and 
         FIG. 6  is a flow diagram of steps in a method of determining true stress and true strain of a specimen. 
     
    
    
     The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following detailed description of the invention references the accompanying drawings that illustrate specific embodiments in which the invention can be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the current technology can include a variety of combinations and/or integrations of the embodiments described herein. 
     Turning to the drawing figures, a true stress and true strain testing system  10  constructed in accordance with an embodiment of the present invention is illustrated. The true stress and true strain system  10  determines true stress of a specimen  100  being subjected to an applied force. The specimen  100  may be a standard test piece for testing material properties, a part formed via conventional machining or molding or via additive manufacturing for testing part geometry or forming techniques, or any other test piece. The specimen  100  may have a round, rectangular, oval, or any other suitable cross section. As best shown in  FIG. 1 , the true stress and true strain testing system  10  broadly comprises a force input machine  12 , an imaging system  14 , and a computer  16 . 
     The force input machine  12  applies a force to the specimen  100  and broadly includes a load frame  18  and a load cell  20 , as shown in  FIG. 2 . The load frame  18  may include hydraulics, pneumatics, or other mechanical systems for applying the force to the specimen  100 . The load cell  20  senses the amount of force exerted by the load frame  18 . The force input machine  12  may also include an alignment head for allowing the specimen  100  to be manually or automatically aligned with components of the imaging system  14  and may include additional hydraulics, pneumatics, and spacers for effecting the alignment. The force may be a tensile or compression force applied along a force axis, a torsional force applied about the force axis, a bending force, or a combination of the above forces, as described in more detail below. 
     The imaging system  14  captures images of the specimen  100  and broadly includes a first light source  22  and a first camera  24 . The imaging system  14  may also include a second light source  26 , a second camera  28 , and/or additional sensors as described below. The imaging system  14  may be mounted on an adjustable plate or may otherwise by adjustable for aligning the imaging system  14  with the specimen  100 . 
     The first light source  22  produces a first imaging light beam and may be positioned facing the specimen  100  so that the first imaging light beam emitted from the first light source  22  is directed towards the specimen  100  in a first direction. Optionally, the first light source  22  may be positioned perpendicular to the force axis so that the first imaging light beam reaches the specimen  100  “head on”. The first light source  22  may be a laser head (e.g., Keyence brand laser head) or any other light source. The first light source  22  may produce a laser beam or other focused light beam in a single electromagnetic wavelength or over a range of electromagnetic wavelengths. 
     The first camera  24  receives a portion of the first imaging light beam not blocked by the specimen  100  and may be aligned with the first light source  22  on an opposite side of the force axis. The first camera  24  may be a Keyence brand imaging camera or any other camera or sensor configured to detect directional light. 
     The second light source  26  produces a second imaging light beam and may be positioned facing the specimen  100  so that the second imaging light beam emitted from the second light source  26  is directed towards the specimen  100  in a second direction. That is, the second light source  26  may be angled from the first light source  22  so that the first imaging light beam and the second imaging light beam reach the specimen  100  in different directions. Optionally, the second light source  26  may be positioned perpendicular to the force axis so that the second imaging light beam reaches the specimen  100  “head on”. In one embodiment, the second light source  26  is angled ninety degrees from the first light source  22  so that the first imaging light beam and the second imaging light beam reach the specimen  100  at ninety degree angles from each other. The second light source  26  may be a laser head (e.g., Keyence brand laser head) or any other light source. The second light source  26  may produce a laser beam or other focused light beam in a single electromagnetic wavelength or over a range of electromagnetic wavelengths. 
     The second camera  28  receives a portion of the second imaging light beam not blocked by the specimen  100  and may be aligned with the second light source  26  on an opposite side of the force axis. The second camera may be a Keyence brand imaging camera or any other camera or sensor configured to detect directional light. 
     The computer  16  determines true stress and true strain of the specimen  100  and broadly includes a processor  30 , a memory  32 , a transceiver  34 , a plurality of inputs  36 , and a display  38 , as shown in  FIG. 3 . The computer  16  may be a desktop computer, laptop computer, tablet, smartphone, computer network, or any other computing device, system, or systems. 
     The processor  30  receives shadow images  102 ,  104  of the specimen  100  from the first camera  24  and the second camera  28  and determines true stress and true strain of the specimen based on the shadow images  102 ,  104  as described below. The processor  30  may include one or more circuit boards, memories, displays, inputs, and/or other electronic components. 
     The processor  30  may implement aspects of the present invention with one or more computer programs stored in or on computer-readable medium residing on or accessible by the processor. Each computer program preferably comprises an ordered listing of executable instructions for implementing logical functions in the processor  30 . Each computer program can be embodied in any non-transitory computer-readable medium, such as the memory  32  (described below), for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device, and execute the instructions. 
     The memory  32  may be any computer-readable non-transitory medium that can store the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-readable medium can be, for example, but not limited to, an electronic, magnetic, optical, electro-magnetic, infrared, or semi-conductor system, apparatus, or device. More specific, although not inclusive, examples of the computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a random access memory (RAM), a read-only memory (ROM), an erasable, programmable, read-only memory (EPROM or Flash memory), an optical fiber, and a portable compact disk read-only memory (CDROM). 
     The transceiver  34  may transmit data and instructions between the force input machine  12 , the imaging system  14 , the computer  16 , and other computers in the network. Alternatively, a wired or integrated setup may be used between these components. 
     The inputs  36  allow a user to activate and control the force input machine  12  and the imaging system  14  and manipulate data via the computer  16 , and other computers in the network. The inputs  36  may comprise a keyboard, mouse, trackball, touchscreen, buttons, dials, virtual inputs, and/or a virtual reality simulator. 
     The display  38  may present the shadow images  102 ,  104  of the specimen  100  (see  FIG. 4 ), input force data, minimum diameter data (i.e., minimum widths or thicknesses), elongation data (i.e., elongation), metadata, computer options, stress plots (see  FIG. 5 ), strain plots, and other information via a graphical user interface (GUI). The display  38  may also include image windows, data spreadsheets, virtual setup windows, and virtual inputs. 
     Use of the true stress and true strain testing system  10  will now be described in more detail. First, the specimen  100  may be positioned in or on the force input machine  12 , as shown in block  200  of  FIG. 6 . The specimen  100  may be positioned such that a longitudinal axis of the specimen  100  is aligned with a force axis of the force input machine  12 . 
     The first light source  22  may be positioned so that it faces the specimen  100 , as shown in block  202 . That is, the first light source  22  should be able to shine a first light beam towards the specimen  100  in a first direction. 
     The first camera  24  may then be aligned with the first light source  22  on an opposite side of the force axis, as shown in block  204 . The first camera  24  should be positioned to receive portions of the first light beam from the first light source  22  that are not blocked by the specimen  100 . 
     The second light source  26  may be positioned so that it faces the specimen  100  at an angle from the first light source  22 , as shown in block  206 . That is, the second light source  26  should be able to shine a second light beam towards the specimen  100  at an angle from the first light source  22  (i.e., in a different direction than the first light beam). In one embodiment, the second light source  26  may be positioned at a ninety degree angle from the first light source  22 . 
     The second camera  28  may then be aligned with the second light source  26  on an opposite side of the force axis, as shown in block  208 . The second camera  28  should be positioned to receive portions of the second light beam from the second light source  28  that are not blocked by the specimen  100 . 
     Once the force input machine  12  and the imaging system  14  are set up, the specimen  100  may then be aligned or adjusted between the first light source  22  and the first camera  24  and between the second light source  26  and the second camera  28  via the adjustable alignment head of the force input machine  12 , as shown in block  210 . This will allow the first camera  24  and the second camera  28  to capture optimally framed shadow images  102 ,  104 , as described below. 
     Optionally, one or more clips or other markers may be connected or attached to the specimen  100  near opposite ends of the specimen  100 , as shown in block  212 . The clips will provide reference points in the shadow images  102 ,  104  for elongation or other deformations of the specimen  100 , as described below. 
     A first light beam may be projected from the first light source  22  towards the specimen  100  in a first direction (i.e, a first cross axis) during the period of time that the force is applied to the specimen  100 , as shown in block  214 . The first light beam may be continuous or may be pulsed according to a predetermined pulse rate (e.g., every twenty milliseconds or fewer). 
     Portions of the first light beam should be blocked by the specimen  100  while other portions of the first light beam should be received by the first camera  24 , as shown in block  216 . A first shadow image  102  of the specimen  100  may thus be created via the first camera  24 . The first shadow image  102  may include clip shadows  106  of the clips connected to the specimen  100 , which may be used for determining true strain, as described below. 
     A second light beam may also be projected from the second light source  26  towards the specimen  100  in a second direction (i.e., a second cross axis) during the period of time that the force is applied to the specimen  100 , as shown in block  218 . The second light beam may be continuous or may be pulsed according to a predetermined pulse rate (e.g., every twenty milliseconds or fewer). 
     Portions of the second light beam should be blocked by the specimen  100  while other portions of the second light beam should be received by the second camera  28 , as shown in block  220 . A second shadow image  104  of the specimen  100  may thus be created via the second camera  28 . The second shadow image  104  may include clip shadows  106  of the clips connected to the specimen  100 , which may be used for determining true strain, as described below. 
     The force input machine  12  may then be activated to apply a force to the specimen  100 , as shown in block  222 . The force may be a tensile force or compression force applied along a force axis, a torsional force applied about the force axis, a bending force, or a combination of one or more of the above forces. The force may be applied and increased in a linear or stepped pattern or at an increasing rate or a decreasing rate. The force may also be held at a steady amount for a predetermined period of time, cycled, ramped up and down, switched or gradually cycled between counter forces (e.g., switched or cycled between tensile and compression forces), such as for studying cyclic fatigue and/or creep testing, or any other force pattern. The force may be applied over a predetermined period of time or until the specimen  100  begins necking, begins fracturing, or fractures completely. 
     The specimen  100  will begin “necking” (i.e., narrowing in diameter along a portion of the specimen  100 ) from elongation due to tensile forces or otherwise deforming due to other types of forces applied to the specimen. The maximum stress on the specimen  100  will be felt where necking is greatest (i.e., where the specimen  100  has the smallest cross sectional area). However, the smallest cross sectional area may change positions along the specimen as viewed from the cameras  24 ,  28 . For this reason, the minimum diameters  108 ,  110  of the specimen  100  may be determined for a range of points along the specimen, as described in more detail below. 
     Imaging masks may be used on the first and second shadow images  102 ,  104  for assisting the processor  30  in determining minimum diameters  108 ,  110  or minimum dimensions (explained in more detail below) and for tracking positions of and changes in the test specimen  100 , as shown in block  224 . Other imaging techniques and data manipulation techniques may also be used for simplifying the image and data analysis. 
     A minimum diameter  108  of the specimen perpendicular to the first direction may be determined at one or more points in time while the force is being applied to the specimen  100 , as shown in block  226 . This may be determined by measuring thicknesses of the first shadow image  102  at a number of points along the specimen  100  and identifying the smallest measured thickness. Note that for each minimum diameter determination, the smallest measured thickness may be located at different points along the specimen  100 . 
     A minimum diameter  110  of the specimen perpendicular to the second direction may be determined at one or more points in time while the force is being applied to the specimen  100 , as shown in block  228 . This may be determined by measuring thicknesses of the second shadow image  104  at a number of points along the specimen  100  and identifying the smallest measured thickness. Note that for each minimum diameter determination, the smallest measured thickness may be located at different points along the specimen  100 . 
     The minimum diameters  108 ,  110  described above may be determined at any point or points in time while the force is being applied to the specimen  100 . In one embodiment, the minimum diameters  108 ,  110  may be determined continuously or every twenty milliseconds or fewer. It will be understood that the shadow images  102 ,  104  may be captured for these points in time while the minimum diameters  108 ,  110  are determined after the fact in a data analysis session. 
     A true stress of the specimen  100  at a point in time may then be determined according to the amount of force applied to the specimen  100  at the point in time and a minimum cross sectional area of the specimen  100  as calculated from the minimum diameter  108  of the specimen  100  perpendicular to the first direction at the point in time and the minimum diameter  110  of the specimen  100  perpendicular to the second direction at the point in time (and/or the known cross sectional shape of the specimen  100 , as shown in block  230 . The calculated minimum cross sectional area of the specimen  100  may be an average of an area measurement based on the minimum diameter  108  in the first direction and an area measurement based on the minimum diameter  110  in the second direction. Alternatively, the calculated minimum cross sectional area may be a direct function of the minimum diameter  108  in the first direction and the minimum diameter  110  in the second direction such as for calculating an oval or rectangular shape. A true stress of the specimen  100  for a plurality of points in time or a period of time may similarly be determined by repeating the above steps for different points in time. The true stress may be plotted against time and/or against force to show a complete behavior of the specimen  100 , as shown in  FIG. 5 . It will be understood that the true stress may be calculated as instantaneous load divided by instantaneous cross sectional area of the specimen  100 . True strain may be calculated as the natural log of the instantaneous cross sectional area divided by the original cross sectional area or the natural log of the instantaneous length divided by the original length of the specimen  100 . 
     Note that the minimum cross sectional area of the specimen  100  may not necessarily correspond to a point on the specimen  100  along the force axis at which both of the minimum diameters  108 ,  110  are located. For example, the first minimum diameter  108  may be so small at one point that the specimen  100  may have significant thickness as seen from the second direction yet still have a minimum cross sectional area at that point. That is, the minimum diameters  108 ,  110  may be located at different points along the specimen  100  for a single point in time. To account for this, the processor  30  may need to calculate cross sectional areas for a number of points along the force axis at a single point in time to determine the actual minimum cross sectional area at that point in time. 
     A distance between the clip shadows  106  in one or both of the shadow images  102 ,  104  at a plurality of points in time as the force is applied to the specimen  100  may be measured, as shown in block  232  of  FIG. 6 . The distance measured may be parallel to a force axis or force direction but may also be measured in other directions. An initial distance between the shadow clips  106  at an initial point in time may be used in determining true strain, as described below. 
     Changes in the distance between the clip shadows  106  from one point in time and another point in time may then be determined, as shown in block  234 . The points in time may be subsequent points in time or may be start and end points. 
     A true strain may then be determined as a change in the distance between the clip shadows  106  divided by an initial distance, as shown in block  236 . Alternatively, the true strain may be determined as a change in a minimum lateral dimension (diameter, width, or thickness) of the specimen  100  divided by an initial lateral dimension. The true strain may be plotted to show true strain over the period of time that the force is applied to the specimen  100 . 
     The above-described true stress and true strain testing system  10  and method provide several advantages over conventional systems. For example, the true stress and true strain testing system  100  scans the entire gage length of the specimen and measures the actual minimum diameters  108 ,  110  of the specimen throughout the test. This provides improved stress analysis over theoretical stress because theoretical stress does not take into account changes in location of the minimum diameters  108 ,  110  of the specimen  100  (see  FIG. 5 ). In contrast, conventional testing systems use reflective laser tags placed on the specimen. Unfortunately, these tags may not be placed on the region where the minimum dimension is located. These tags also cannot move as the location of the minimum dimension moves. This leads to much less accuracy and incorrect stress and strain curves. The true stress and true strain testing system  10  also provides more accurate results because minimum diameters  108 ,  110  as seen from two or more different directions are determined and incorporated into the true stress analysis. This allows for specimens having different cross sections (circular, oval, rectangular, etc.) to be more accurately analyzed. Indeed, the true stress and true strain testing system  10  allows for enhanced analysis of non-linear plastic behavior of additive manufactured materials, organic materials, metals, and other materials. Parts formed via additive manufacturing having elaborate and complicated geometries in particular benefit from the enhanced analysis provided by the true stress and true strain testing system  10 . Note also that the true stress and true strain testing system  10  captures more data points while the specimen is subjected to a test force than can be performed by hand. 
     Although the invention has been described with reference to the embodiments illustrated in the attached drawing figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.