Influence of tab debonding on measured stiffness evolution in Compression-Compression and Tension-Compression fatigue testing of short gauge length coupons

Abstract Short gauge length coupons of plain woven Carbon/Epoxy with layup [ # ( 0 / 90 ) ] 24 and [ # ( 45 / − 45 ) ] 24 are subjected to Compression-Compression and Tension-Compression fatigue loading at several stress levels. Inspection of the gauge length with Digital Image Correlation (DIC) allows measurement of the strain field. The hysteresis loops, axial and shear stiffness as a function of the number of cycles are obtained from the strain field and measured load. For most of the tests a general degradation of the specimen stiffness is observed. For the coupons subjected to uniaxial Compression-Compression with layup [ # ( 45 / − 45 ) ] 24 , however, a 17% stiffness increase is seen during the first 10 4 cycles. Post-mortem inspection revealed that the aluminium tabs of the coupons, to facilitate load introduction from the clamps, had partially debonded during testing. A 3D finite element analysis of the coupons including tabs is carried out to investigate the phenomena. The degradation of the tab-coupon bond is modelled using contact with debond capability. This confirmed that tab debonding is responsible for significantly altering the stress state in the (short) gauge section which results in an apparent stiffening of the sample during fatigue loading. Hereby, the authors offer a new explanation for stiffness increase of composite coupons in fatigue tests. For future tests it is shown that quantification of tab debonding is necessary to provide a decisive answer about the experimental boundary conditions in the gauge section of the coupon.