A comparison of fracture toughness parameters for titanium alloys

Abstract

High strength titanium alloys in thick sections are of interest for use in complex structures such as deep-diving vehicles, hydrofoils, and aircraft. Successful application of these materials in such structures requires a knowledge of the notch fracture toughness of these alloys to preclude the possibility of catastrophic fracturing. Experience has shown that no single test method presently exists that can provide reliable fracture toughness information for the whole spectrum of titanium alloys. A previously established relationship between drop-weight tear test energy and explosion tear test performance provides reliable fracture toughness information on those alloys characterized by a toughness level requiring gross plastic deformation to propagate fractures. This analysis has not been extended to the ultra high strength alloys in which fractures can propagate catastrophically at elastic stress levels. For these alloys, analytical methods of linear elastic fracture mechanics provide the required elastic stress level and flaw size relations for fracture. This paper deals with a 'marriage' of the two approaches — the engineering and the analytical — by correlative techniques. A direct correlation has been found for titanium alloys between the drop-weight tear test (DWTT) energy for fracture and the corresponding stress intensity factor, K IC . The relationship appears to be valid up to a DWTT value of 1200ft-lb and may likewise be expressed in terms ofβ IC-DWTT or ℷ IC-DWTT. There is also a correspondence betweenβ IC and a fracture appearance as determined by the percentage of shear lip on the single-edge notch specimen. On the other hand, the Charpy-V notch test proved to be relatively insensitive to change in fracture toughness and could not be correlated with K IC . The practical implication of this study is that reasonable estimates of plane strain fracture stress-intensity K IC , should be possible from results obtained with other reliable engineering methods for measuring fracture toughness.