Size and Deformation Limits to Maintain Constraint in Kic and Jc of Bend Specimens

摘要

The ASTM Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials (E399-83)restricts test specimen dimensions to insure the measurement of highly constrained fracture toughnessvalues (Kic). These requirements insure small-scale yielding conditions at fracture, and thereby the validityof linear elastic fracture mechanics. When these conditions are satisfied, the diameter of the plastic zoneis nearly twenty five times smaller than all specimen dimensions. The need for this degree 'of plastic zoneconfinement, set by the factor 2.5 in a, b, B > 2.5(Kq/oys)2 was based on Kic data for many different alloys.These data show that all specimens satisfying the size requirements of E399 produce highly constrainedfracture toughness values. However, the required multiplier ranges from 1.0 for certain steel alloys to 2.5for titanium alloy 6-6-2 in the aged condition. To maintain a standard test method applicable to all materials,the more restrictive 2.5 value has been retained by ASTM Committee E08. Recently, Dodds and Andersonhave proposed a less restrictive size requirement for cleavage fracture toughness measured in termsof the J-integral (Jc ). Dodds and Anderson performed finite element analyses to calculate the ratio of J inthe finite-sized specimen (JSE(B)) to J under small-scale yielding (JSSY) needed to produce equivalentstresses ahead of both crack tips, and thereby equivalent conditions for cleavage fracture. The proposedsize requirement specifies the deformation level at which JSE(B) / JSSY deviates from unity for deeplycracked bend specimens, and is given by a, b, B > 200 Jc/oo . The size requirement proposed by Doddsand Anderson increases the utility of fracture toughness experiments by expanding the range of conditionsover which such data can be reliably measured. This investigation compares the proposed size requirementwith that of AS Trvi Standard Test Method E399 and, by comparison with published experimental data, providesvalidation of the new requirements.