Damage development in 336 steel and 40 HNMA steel due to creep under uniaxial tension at elevated temperatures is assessed using destructive and non-destructive methods. In the destructive method the specimens after different amounts of creep prestraining were stretched up to failure and variations of the selected tension parameters were taken into account in damage identification. The ultrasonic investigations were selected as the non-destructive method for damage development evaluation. In the case of this method the elasto-acoustic coefficient and acoustic birefringence coefficient were used to identify damage development in the tested material. In order to get more thorough understanding of the phenomena associated with an influence of deformation history on mechanical properties of materials the paper considers two types of deformation processes: deformation due to creep and deformation due to plastic flow caused by the monotonically increasing load.
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