DYNAMIC LOCAL STRAIN-MEASUREMENT FROM THE DIGITAL IMAGE-PROCESSING OFTHE GRATING INTERROGATION

The direct measurement of local strains during a dynamic loading is pr oposed from the spectral analysis of a crossed,orating using optical d iffraction under oblique incidence This procedure is experimentally ac hieved with acousto-optic modulators which allow to record 24 strain s tates at a maximum frequency near to 1MHz. The diffraction figures can be recorded on a photographic film or directly on a C.C.D. camera. Th e automatic localisation of the diffracted spots with the help of data image processing gives the components of the strain tenser. The strai n measurement method is performed for compression impact rests using a n Hopkinson bar loading.