Dynamic intralaminar fracture toughness of carbon/epoxy composites at intermediate strain rates using size effect

Abstract

The design of composite transportation structures with improved crashworthiness behaviour requires a reliable and accurate dynamic material characterisation database covering various loading rates expected in crash events. For vehicle crash scenarios experimental results at strain rates up to 200 s-1 are particularly important. However, conducting dynamic testing at those strain rates with a high speed servo-hydraulic testing machine remains a challenge due to intrinsic aspects associated with inertia effect and stress wave propagation. The inertial effect known as system ringing is one of the major obstacles to obtain the true dynamic properties at intermediate strain rates, which results in unacceptable oscillations in load signals. Conducting compact tension (CT) tests is a typically first choice when measuring the intralaminar fracture toughness for fibre tensile failure. However, the CT specimens have several limitations when those used under dynamic loading rates. For example, inertial effects cause unsymmetrical opening of the CT specimens, which induce mixed mode fracture in the specimen. To overcome this limitation, G. Catalanotti proposed a new procedure using size effect law to measure the intralaminar fracture toughness for fibre tensile failure using double-edge notched tension (DENT) specimens. The tests using DENT specimens were successfully conducted under quasi-static loading and at loading strain rate of 60-1 using the split Hopkinson Tension Bar (SHTB) technique. This paper reports an experimental investigation on dynamic fracture toughness of IM7/8552 carbon/epoxy composites at intermediate strain rates. Dynamic tensile testing with three different sizes of DENT specimens is performed at the strain rates of 60 and 100 s-1 using a high-speed hydraulic testing machine. To that aim an improved load introduction device is used to handle the inertial effect during the testing. Obtained results from current work are then compared to previous results gained with SHTB technique. These results provide the competence of the proposed testing method to measure dynamic fracture toughness. This study is a first step towards the establishment of a reliable test method for measuring dynamic fracture toughness at intermediate strain rates.