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The ultimate strength (measured in psi or MPa of stress) of the material is the highest strength a material will see before breaking in tension. This value is derived from plotting a stress strain curve using specially machined coupons and a tensometer. Structural designers generally don’t use this number in design because it is in the plastic deformation zone (meaning the geometric shape of the test piece is changing in an unpredictable fashion) and doesn’t recover after being unloaded.