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The form of stress-strain curve of a plastic material is such that at small stresses, the strain is proportional to the stress but when the stress becomes equal to the strength, the strain continues to increase with no increase of stress. If the stress is reduced to values less than the strength, the reduction in strain is proportional to the reduction in stress. The proportionality factor is often very close to that obtained when the stresses were small. Many materials act substantially in this way and are called elastic-plastic materials. Dry wood acts substantially in this way when subjected to compression parallel to grain (12). It acts differently in tension parallel to the grain. At small stresses, the strain is proportional to the stress and remains so until the stress becomes equal to the tensile strength when catastrophic failure occurs.
Norris, C. B.
Engineer, Forest Products Laboratory, Madison, Wis