Strengthening of mild steel struts using CFRP sheets subjected to uniform axial compression

The behaviour of Carbon Fibre Reinforced Polymers (CFRP) reinforced mild steel isolated plate (strut) under uniform axial compression is not well understood because there is a lack of experimental research. This paper presents experimental results for a series of struts reinforced with CFRP sheets subjected to uniform axial compression. The main parameters examined were the thickness of the steel plates, and the location and the number of the CFRP sheets. Three different thicknesses of steel plates were examined, namely, 2, 3 and 4 mm. The testing program included 21 struts with three different width-to-thickness ratios of 50, 66.7, and 100. The CFRP sheets were adhesively bonded where the fibres were aligned in the longitudinal direction parallel to the direction of the applied force. The results showed that the debonding failure strain in the compression face occurred in the range of 0.3-0.6% for all the composite specimens. It was found that the amount of permanent damage in the composite strut identified by the residual displacement is significantly less than that of the bare strut. Experimental results showed a global increase in strength, energy absorption and ductility for all test specimens where the greatest percentage increases were 452%, 259% and 107%, respectively. The 2 and 3 mm composite specimens showed greater percentage increases than the 4 mm composite specimens due to the relative thickness of the carbon fibre sheets verses steel. All experimental specimens failed by overall buckling, achieving maximum capacity at the end of their elastic range showing that the specimens were successfully bonded and acting as composites within such range. An existing rigid plastic theory considers initial imperfections (defects) was modified and used to predict the response of the struts. Experimental and theoretical curves showed consistency where initial defects controlled the elastic-inelastic behaviour and the load bearing capacity of the strut. In general, specimens with initial defects produced a smooth elastic-plastic curve, while specimens with little to no initial defects showed a sharp peak at the buckling load. The rigid plastic theory well predicted the response of the bare specimens whereas it reasonably predicted the plastic collapse curve of the composite struts.