Abstract
The subsurface nature of Crack Tip Opening Displacement (CTOD) makes its direct measurement very difficult, if not impossible. During fracture toughness testing, CTOD is commonly calculated by applying a plastic hinge model using externally applied clip gauges. However, clip gauge CTOD calculations merely provide information relating to the center of the defect (which is typically the most critical point, but not always). For the case of a finite-length surface defect, CTOD will be variable over the defect front. Exact knowledge of CTOD over the entire front is useful for detailed calculations, such as crack profile evolution due to ductile tearing or calculations involving interacting defects. To experimentally measure the CTOD at locations other than the center of the crack, the authors propose a technique based on full field three-dimensional profile measurement of the notched surface by means of stereoscopic Digital Image Correlation (3D-DIC). The method is based on the plastic hinge model assuming that the crack flanks rotate in a rigid manner around a plastic hinge point in the un-cracked ligament. Having measured full-field out-of-plane displacement at the surface of the specimen around the crack using the 3D-DIC method, CTOD can be inferred over the entire crack front. Results show that, due to the acceptable agreement between the DIC based calculation and CTOD measured from cast replicas, the proposed technique has a sufficient accuracy to measure CTOD on the entire crack front in plastically deforming specimens.
Highlights
Determining criteria for the failure due to pre-existing structural defects is a fundamental driver for structural integrity analysis
Crack Tip Opening Displacement (CTOD), known as one of the most important crack driving force parameters used in fracture mechanics, is defined as the opening of a defect upon loading a test specimen or structure
Exact knowledge of CTOD over the entire front is useful for detailed calculations, such as crack profile evolution due to ductile tearing or integrity assessments involving multiple interacting defects
Summary
Determining criteria for the failure due to pre-existing structural defects is a fundamental driver for structural integrity analysis. Accurate experimental measurement of fracture toughness is required for the application of fracture mechanics methods. Crack Tip Opening Displacement (CTOD), known as one of the most important crack driving force parameters used in fracture mechanics, is defined as the opening of a defect upon loading a test specimen or structure. CTOD toughness is the magnitude of the opening of an initial crack tip in a standard fracture toughness test specimen at the point of stable or unstable crack extension. The CTOD measure first was proposed by Wells [1] in 1963 at the British Welding Institute. The subsurface nature of CTOD makes its direct measurement very difficult, if not impossible
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