Delamination assessment of a composite laminate interleaved with electrospun TPU fibers under Mode-I and Mode-II loading

The critical strain energy release rate (GC) and the traction-separation law of a composite laminate interleaved with electrospun thermoplastic polyurethane (TPU) fibers were investigated under mode I and mode II loading. Pristine unidirectional prepreg carbon-fiber/epoxy as well as interleaved with TPU membrane in the midplane were manufactured and tested by conducting the Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) tests. The Compliance-based beam method (CBBM), which is based on the equivalent crack length concept, was used as a data reduction scheme to determine Gc through the test. The load-displacement curve in terms of the maximum load was compared between the specimens with and without interleaved membrane. The improvement in delamination resistance and the development of the bridging zones at the crack tip were evaluated between specimens under mode I and mode II. The traction-separation law was only determined for the specimens with interleaved membrane under mode I and mode II loading. The resulting maximum strength and Gc for the interlayer-modified specimens were used in the finite element models (FEMs). Finite element analyses were performed in terms of load-displacement for mode I and mode II loading, and compared with experimental results of the laminate with TPU interleaved membrane embedded in epoxy. The delamination resistance was improved in composite laminates interleaved with the TPU fibers membrane in both modes I and II. In mode II, the improvement in delamination resistance is associated with the plastic deformations at the TPU fibers interlayer. An effective bridging zone was observed only in mode I for both the pristine and interleaved laminates. Good agreement between the FE curves and the experiments in mode I and mode II was observed. The use of electrospun TPU fibers is advantageous compared with other thermoplastic fibers since its melting point is higher than most composite curing temperature, which originates the bridging zone that was observed in the experiments, and the electrospun membranes does not require a dispersion on the resin as the particle toughening method.