Strain-field analysis acquired through correlation of X-ray radiographs of a fiber-reinforced composite laminate

Abstract

The strain fields can be determined on the surface of fiber-reinforced composites by correlation analysis of X-ray radiographs of the specimen. One radiograph is taken of the specimen in an unloaded state and another radiograph of the same specimen is taken in a deformed state. Each radiograph is then imaged by a video camera and digitized using a Matrox digitization board. The displacement map is obtained from the two radiographs by dividing one image into subsets of 20 pixels by 20 pixels and using a correlation algorithm to identify the corresponding subsets in the second image. The correlation is completed between patterns of the grey-level intensities for each subset. The appropriate discrete derivatives can then be evaluated by first difference taken to form the in-plane strains.

The X-ray radiographs correlation analysis strain-field method outlined above was applied to a uniaxially loaded [90, 0] _s Gl-Ep coupon. Simultaneously, an extensometer measured the average longitudinal strain over a 2.54-cm (1-in.) gage length. The error between the two methods was less than three percent of the applied strain of 1.3 percent. The same specimen was impacted and re-examined. A radical shift in the strain field was observed when the specimen was reloaded. Further investigation showed the method reliable down to 0.5-percent strain.