Effect of ao/W Ratio on Fracture Toughness Parameter of Extra Deep Drawn Steel Sheets: Experimental and Finite Element Studies

Fracture toughness parameter is significantly affected by specimen dimensions i.e. specimen thickness (B), width (W) and unbroken ligament length (W-ao) in elastic-plastic region. Present study is about the third dimension of test specimen (W-ao). In order to investigate effect of ao/W ratio on fracture toughness parameter, fracture test and finite-element - cohesive zone model (CZM) simulation tool are used. Fracture tests are carried out on extra deep drawn (EDD) steel sheets using compact tension (CT) type specimens with different ao/W ratio (0.5, 0.525, 0.55 and 0.575). After successive experimental attempts, load drop technique is used as a fracture criterion. Critical CTOD is used as a fracture toughness parameter. An alternative constant traction separation law is used to account for maximum load and large load line displacements. Experimental findings as well as finite element studies show that the critical CTOD decreases with ao/W ratio. It has been observed that as ao/W ratio increases, the location of plastic hinge shifts towards the crack tip (i.e. size of tensile plastic zone reduces), which reduces fracture toughness. That is, the material is less resistant to crack growth for deeper cracks.