Inconel 718 alloy was tested. A new type of specimens of variable cross-sectional area measuring part was used for the tests. This provided a continuous distribution of plastic strain in that part of the sample. The proposed method enables to replace a series of specimens by one specimen. The degradation of the material was obtained by static tensile test and the creep test. The permanent deformation that varies along the specimen axis allows for an analysis of damage induced by a plastic deformation. The degradation of the alloy corresponds with the changes of acoustics properties of the material - attenuation of ultrasonic waves. It allows to detennine the degree of damage to the material using a non-invasive - ultrasonic method. Using the damage parameter proposed by Johnson allows to obtain correlation between the non-destructive results and a damage degree of the material. The presented testing method delivers information about changes in the material structure caused by permanent deformation.