X-Ray Diffraction Evaluation of Adhesive Bonds and Damage in Composites

Stresses can be measured by X-ray diffraction, not only in metals but also in polymeric composites containing some crystalline filler particles. Diffraction is found effective in disclosing the distribution of stresses over the surface of adhesively bonded joints in aluminum strips when loads well below the yield point are applied. When two 606l-T6 aluminum strips 1/16" or l/32" thick and 3/4" wide are adhesive ly bonded in a single lap joint and loaded in tension, maps giving the distribution of the X-ray-measured stresses show clear evidence of the way in which stresses are transferred from one adherend to the other. The maps show the limits of the bonded area with an accuracy about equal to the width (1 mm) of the irradiated area along the specimen. Attendant bending stresses resulting from the loading are also registered. Stress values can be obtained from the observed diffraction angles by calibration with tensile tests of a single unbonded strip. Similar results are obtained for graphite/epoxy laminates adhesively bonded to aluminum when diffraction is from the aluminum, but a much lower accuracy was obtained when diffraction was from the filled composite. Another X-ray method was developed that appears to be a viable though less accurate method for measuring applied (not pre-existing residual) stresses, and for mapping their distribution around a joint. A thin layer of epoxy paint containing a diffracting filler is applied to a specimen and cured. Diffraction from this paint yields shifts in diffraction angle approximately proportional to the magnitude of applied stresses. A diffracting paint containing, say, aluminum or silver powder can be used on an object that piffracts poorly or in which a filler has not previously been embedded, but the accuracies attainable in ~tress values are apparently somewhat lower and are beinq investigated further. When this method is used on the aluminum adherend of a single lap joint and a load is applied, the limits of the bonded area are disclosed by the diffracted beam from the filler in the epoxy paint. This method appears, therefore, to be useful for mapping the areas that are properly bonded and for associated bending stresses, and possibly for non-destructive evaluation of bond defects. Details will be published in Advances in X-Ray Analysis, Vol. 24. Detection of damage in polymeric composites is possible by X-ray determination of residual stresses. By locally deforming a graphite/epoxy laminate the diffraction from embedded filler powder discloses effects of the deformation not only where damage has occurred but also throughout the immediate neighborhood of the visible damage. Investigation of this method continues with details to be published elsewhere.