The physical fundamentals of the method for nondestructive SIM are closely related to new science, i. e. mesomechanics of media exhibiting structural inhomogeneity. Within its context, plastic deformation is viewed as a process having a multilevel organization, with its different events ranging widely in scope, depending on specimen size, the dimensions of material's structure elements and deformation conditions. As to the most hazardous event, i. e. fracture this is the final stage of the evolution of the material under loading to which there corresponds a transition to the macroscopic scale - that of the specimen under loading. It is therefore concluded that speckle interferometry can be capable of measuring deformation parameter in a region comparable in size to the specimen tested or to that portion of the specimen which is the most critical and liable to fracture.

A laboratory version of the automated laser measuring complex [1] is provided by us. This features the following components:

• laser for specimen illumination;
• photographic recording unit for obtaining specimen image with speckles superimposed on its surface;

Use of the above technique to study the plastic flow allowed to reveal that in the early stages of deformation, virtually from the very onset, an inhomogeneous flow behaviour is observed and a localization of strains. In the different stages of deformation, the localization of strains might take different forms; however, as the stage of fracture is approached, the distribution pattern of the components of the plastic distortion tensor begin to reveal certain trends, which can serve as fracture symptoms of significance for pinpointing the onset of fracture and consequently, for predicting the exact moment in the material at which a transition to a critical state. Upon incipient of a seat of fracture the _xx (x) curve shows a pronounced maximum indicating at which particular point fracture is bound to occur. The site at which fracture is to occur can be determined to an accuracy of about 1 mm.

Thus, in steels subjected to deformation at a rate of =10^-4s^-1. This can be predicted when the attainment of the total amount of deformation just sufficient cause cracking is a matter of some 5- 6%, that is 500 - 600 s before the onset of failure.

REFERENCE

1. Method and Apparatus for Nondestructive Testing of Mechanical Behaviour of Solid State Objects under Loading, V. E. Panin, L. B. Zuev, V. I. Danilov, Ruspatent Moscow, Pat. No. 93002731, 1993.