Steel foil reinforced composites: Study of strength, plasticity and ply size effects

The objective was to study the bearing properties of steel foil and carbon fibre reinforced polymer composite (St-CFRP) laminates. Extremely thin carbon fibre plies offer an improved onset of damage and less accumulation of damage prior to ultimate failure; ply level hybridisation can enhance the bearing properties. Both effects were studied to better understand the progression of damage and effect of ply thickness. Both CFRP and Hybrid laminates of ply thicknesses 0.3 and 0.03 mm were manufactured and tested. The thin CFRP laminate has 31% higher tensile strength and a reduction in open hole strength of 28 % due to lack of delamination, critical load reached across the 0° plies more or less instantaneously. The lower delamination tendency of the thin ply composites was reflected in the open hole tensile measurements, where, the suppression of delamination resulted in higher notch sensitivity (2.26 versus 1.25). Hybridisation reduced the effects whilst increasing the open hole strength of the composite laminate further. When the bearing stress was studied, the thin CFRP laminate was observed to increase the bearing stress by 28 %. The hybrid laminates show small differences in the achieved maximum bearing stresses (circa 1,100 MPa - double that of the thick CFRP laminate), although the observed failure modes remain very different: delamination and buckling versus brittle failure. Optical analysis showed extensive plastic deformation in the joint, with evidence of good adhesion between the steel foils and thin CFRP plies. Plastic failure of the bearing joint was maintained due to the steel foils, producing a joint that may fail safely. A user subroutine was used to degrade the damaged material properties of the 3D solid model according to the Hashin failure criterion. The developed finite element models were able to recreate the observed experimental failure modes.