Investigating fracture behavior of polymer and polymeric composite materials using spiral notch torsion test

Wind turbine blades are usually fabricated from fiber reinforced polymeric materials, which are subject to complex loading conditions during service. The reliability of the blades thus depends on the mechanical behaviors of these composites under various loading conditions. Specifically, the composite fracture behavior is of great importance to both the scientific research community and the wind industry.In the current study, a new testing technique is proposed based on the spiral notch torsion test to study the fracture behavior of composite structures under Mode I or mixed mode loading conditions, particularly under combined Mode I (flexural or normal tensile stress) and Mode III (torsional shear stress) loading. For this test method, round-rod specimens with spiral V-groove are subjected to pure torsion. Depending on the pitch angle of the spiral lines, pure Mode I, pure Mode III, or mixed Mode I/Mode III loading conditions can be simulated. A three dimensional finite element analysis was used to estimate the fracture toughness.In the current study, both epoxy and fiberglass reinforced epoxy materials are investigated using spiral notch torsion test. This paper will discuss the fracture behaviors of Mode I and mixed loading conditions with and without fatigue pre-crack. In addition, results from fractographic study and finite element analysis will be presented and discussed in detail.

► The spiral notch torsion test (SNTT) provides intrinsic fracture toughness data. ► The SNTT system overcomes size dependent limitation inherent in traditional techniques. ► SNTT was applied to wind turbine blade polymeric composite reliability study. ► SNTT can test tensile normal stress and torsional shear stress simultaneously. ► The SNTT technique offers new opportunities for use in interfacial toughness study.