System and method for testing true stress and true strain

A method for testing true stress of a specimen, the method comprising the steps of:

• positioning the specimen in a force input machine;
• positioning a first light source such that the first light source faces the specimen;
• aligning a first camera with the first light source on an opposite side of the specimen;
• projecting a first light beam from the first light source towards the specimen in a first direction so that at least some of the first light beam reaches the first camera;
• receiving at least some of the first light beam via the first camera so as to create a first shadow image of the specimen;
• applying a force to the specimen via the force input machine;
• determining a minimum diameter of the specimen perpendicular to the first direction at a point in time over a plurality of points along the specimen such that the processor 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;
• and
• determining a true stress of the specimen according to the force applied to the specimen at the point in time and the minimum diameter of the specimen perpendicular to the first direction at the point in time.

A system for testing true stress and true strain of a specimen, the system comprising:

• a force input machine for imparting a force on the specimen along a force axis, the force input machine including an adjustable alignment head for aligning the test specimen;
• an imaging system comprising:
• a first light source positioned perpendicular to the force axis and configured to project a first light beam at the specimen in a first direction;
• a first camera configured to be aligned with the first light source on an opposite side of the specimen such that the first camera receives a first shadow image of the specimen;
• a second light source positioned perpendicular to the force axis and angled ninety degrees from the first light source, the second light source being configured to project a second light beam at the specimen in a second direction;
• and
• a second camera aligned with the second light source on an opposite side of the specimen such that the second camera receives a second shadow image of the specimen;
• a plurality of clips configured to be connected to the specimen, each clip being configured to cast a clip shadow on the first camera and/or the second camera;
• a computer having a processor configured to:
• determine a minimum diameter of the specimen perpendicular to the first direction for a plurality of points in time while the force is being applied to the specimen;
• determine a minimum diameter of the specimen perpendicular to the second direction for the plurality of points in time;
• determine a true stress of the specimen according to the force applied to the specimen at the plurality of points in time and the minimum diameters of the specimen at the plurality of points in time;
• and
• determine a true strain of the specimen based on a change in spacing between the clip shadows for the plurality of points in time.