Experimental Study on the Static Behavior and Recovery Properties of CFRP/SMA Composites

Strengthening reinforced concrete elements with externally bonded prestressed fiber reinforced polymer (FRP) sheets has become a popular reinforcement technology in recent years. However, in practical engineering applications, due to the limitations of construction operation space and the need for specialized design of tensioning and anchoring devices, it is very cumbersome to apply prestressing force to FRP sheets. Therefore, using the recovery effect of shape memory alloys (SMA) to introduce prestressing into FRP sheets can innovate a new approach by combining FRP sheets and SMA wires. In order to study the basic mechanical properties of FRP/SMA composites, carbon fiber reinforced polymer and shape memory alloys were used to make the composite specimens, and uniaxial tensile tests were carried out on them. The mechanical properties such as the stress-strain curve, failure mode, ultimate tensile strength and fracture strain were obtained. The test results show that CFRP sheet exhibits obvious linear elastic behavior in tensile tests. The stress-strain curve of SMA wire can be divided into four stages: the linear elastic stage, yield stage, strengthening stage and failure stage. The fracture strain at failure can reach 7%, which indicates excellent deformation properties. The loading and unloading cycles have little effect on the mechanical properties of SMA wire. With the increase in the loading rate, the ‘stress plateau’ section of the phase transformation section of the SMA wire hysteresis curve gradually transits to an oblique upward curve. Increasing the pre-strain value within a certain range can improve the resilience of SMA wires. SMA wires with a pre-strain value of 8% can provide a maximum resilience of 514 MPa after heating to the austenitic state. A prediction model for the number of temperature cycles and maximum recovery force of SMA was proposed and validated. According to this model, the SMA wires can still provide stable resilience after 30 cycles. Increasing the amount of wire (volume ratio) can improve the maximum fracture strain and ultimate tensile strength of CFRP/SMA composite specimens, and the more wire is added, the greater the residual strength after fracture. The diameter of the fiber can significantly reduce the maximum fracture strain and ultimate tensile strength of the FRP/SMA composite specimen.