Ultrasonic inspection of tilted defects using the corner effect modelling and experimental validation

• Published in: Insight (Northampton) Vol. 50; no. 2; pp. 66 - 69
• Main Authors: Chapman, R K, Burch, S F, Collett, N J, Fradkin, L J
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Northampton British Institute of Non-Destructive Testing 01.02.2008
• Subjects: Acoustical measurements and instrumentation; Acoustics; Exact sciences and technology; Fracture mechanics (crack, fatigue, damage...); Fundamental areas of phenomenology (including applications); Measurement and testing methods; Physics; Solid mechanics; Structural and continuum mechanics; Vibration, mechanical wave, dynamic stability (aeroelasticity, vibration control...); Tapered waveguide; Acoustic measurement; Pulse echo method; Deformation modulus; Elastic wave; Wedge; Modeling; Tilt angle; Ultrasonic control; Specular reflection; Plane wave; Ferritic steel; Crack tip; Elastodynamics; Non destructive test; Crack
• ISSN: 1354-2575; 1754-4904
• DOI: 10.1784/insi.2008.50.2.66

In ultrasonic NDT, the corner trap mechanism is a common means of detecting planar cracks which break the back-wall surface of the component. The mechanism is most efficient when the crack is normal or near-normal to the back wall, so that a specular reflection is obtained. However, a response is still obtained when the crack is more tilted, due to diffracted edge wave signals from the crack tip and from the wedge formed between the crack and the back wall.In order to model the wedge mechanism using the Geometrical Theory of Diffraction, it is necessary to know the diffraction coefficients from the elastic wedge. For ferritic steel, these wedge diffraction coefficients are given by solving the canonical problem of the scattering of a plane elastic wave by a wedge vertex of arbitrary angle in a homogeneous isotropic elastic medium. We have obtained a theoretical solution to this very challenging mathematical problem. The underlying mathematical theory has been implemented in software, which is used to generate lookup tables of diffraction coefficients for use in our ultrasonic NDT models.The theory has recently been validated against experiment, using two sector-shaped steel test specimens having different vertex angles. Pulse-echo measurements of both amplitude and phase of the diffraction coefficients were made, for both compression and shear waves, and compared with theory. Good agreement between theory and experiment was obtained.