Editing Compressive strength (section)

=== Frictionless contact ===
With a compressive load on a test specimen it will become shorter and spread laterally so its cross sectional area increases and the true compressive stress is<math display="block">\acute{\sigma} =F/A</math>and the engineering stress is<math display="block">{\sigma_e} =F/A_o</math>The cross sectional area (<math display="inline">A</math>) and consequently the stress ( <math display="inline">\acute\sigma</math>) are uniform along the length of the specimen because there are no external lateral constraints. This condition represents an ideal test condition. For all practical purposes the volume of a high [[bulk modulus]] material (e.g. solid metals) is not changed by uniaxial compression.<ref name=":0" /> So<math display="block">A l=A_o l_o</math>Using the strain equation from above<ref name=":0" /><math display="block">A=A_o/(1+\epsilon_e)</math>and<math display="block">\acute{\sigma} = \sigma_e(1+\epsilon_e)</math>Note that compressive strain is negative, so the true stress (<math>\acute\sigma</math> ) is less than the engineering stress (<math display="inline">\sigma_e</math>). The true strain (<math>\acute \epsilon</math>) can be used in these formulas instead of engineering strain (<math display="inline">\epsilon_e</math>) when the deformation is large.{{anchor|Contact with friction}}