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Crack arrest toughness KIa is considered to represent the minimum fracture toughness of a material. The crack arrest toughness standard ASTM E1221 defines a crack arrest toughness reference temperature, TKIa. TKIa is the temperature corresponding to a median crack arrest toughness value of 100 MPa√m. Previously, the TKIa temperature has been successfully correlated to the 4 kN crack arrest load transition temperature (TFa4kN) from the instrumented Charpy-V impact test for low to high strength steels. In this work, the correlation is expanded to cover also sub-sized specimens and ultra high strength steels.
Crack arrest toughness usually indicates the material toughness available to arrest a running brittle crack. The estimation of crack arrest toughness is made complicated by the fact that the crack driving force for a dynamically running crack is not necessarily equivalent to the static crack driving force for the same crack size. The dynamic stress intensity factor for a running crack is usually denoted KID and the value of KID at crack arrest is called KIA. This dynamic crack arrest toughness is considered to represent a true material parameter. Unfortunately the dynamic crack driving force is not as simple to estimate in a test specimen. Therefore, the crack arrest toughness is commonly based on a static analysis and is then identified as KIa. For many specimen geometries KIa and KIA are sufficiently close so that KIa can be used to estimate KIA. Usually KIa is smaller than KIA, thus providing a conservative estimate of crack arrest toughness. However, there are geometries where this is not the case, and for those a dynamic analysis is required. One such geometry is the single edge cracked tension specimen (Link, 2006), where the static analysis appears to overestimate the dynamic value. The standard definition of crack arrest toughness relies on the static estimate KIa (ASTM E1221).
Crack arrest toughness KIa is considered to represent the minimum fracture toughness of a material. If the fracture mechanical driving force KI is less than KIa, fracture is not possible. It thus represents a true lower bound fracture toughness. Interestingly, the crack arrest toughness for structural steels follows a similar temperature dependence as brittle fracture initiation toughness. An example of this is shown in Fig. 1.