Young's Modulus: This is the slope of the linear portion of the stress-strain curve, it is usually specific to each material; a constant, known value. Yield Strength: This is the value of stress at the yield point, calculated by plotting young's modulus at a specified percent of offset (usually offset = 0.2%). Ultimate Tensile Strength: This is the highest value of stress on the stress-strain curve. Percent Elongation: This is the change in gauge length divided by the original gauge length. Load - The force applied to a material during testing.
Strain gage or Extensometer - A device used for measuring change in length (strain). Engineering stress - The applied load, or force, divided by the original cross-sectional area of the material. Engineering strain - The amount that a material deforms per unit length in a tensile test. Plastic Deformation
From an atomic perspective, plastic deformation corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors. After removal of the stress, the large number of atoms that have relocated, do not return to original position. Yield strength is a measure of resistance to plastic deformation. Permanent Deformation
Permanent deformation for metals is accomplished by means of a process called slip, which involves the motion of dislocations. Most structures are designed to ensure that only elastic deformation results when stress is applied. A structure that has plastically deformed, or experienced a permanent change in shape, may not be capable of functioning as intended. Tensile Strength, TS
After yielding, the stress necessary to continue plastic deformation in metals increases to a maximum point (M) and then decreases to the eventual fracture point (F). All deformation up to the maximum stress is uniform throughout the tensile sample. However, at max stress, a small constriction or neck begins to form. Subsequent deformation will be confined to this neck area....
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