Practice for Detection and Evaluation of Discontinuities by the Immersed Pulse-Echo Ultrasonic Method Using Longitudinal Waves

An experimental study by the ultrasonic method of the phase velocity of longitudinal waves transmitted in liquids contained in tubes. Greatly augmented as well as largely decreased velocities may be obtained in any liquid by suitably adjusting the frequency of the wave or the diameter of the containing tube. This phenomenon, described here at length for the first time, is found to be caused by the selective absorption of energy of the longitudinal wave at certain frequencies, resulting in a velocity-frequency curve analogous to the "anomalous dispersion" curve of optics. In the experiments there is strong indication that the absorbing frequency depends inversely on the diameter of the tube.The fact that the absorption frequency does not depend on the material or length of the tube, or for thin walls on the wall thickness, indicates that it is neither longitudinal nor flexural (lateral) vibrations in the tube walls which causes the phenomenon; and the fact that for any liquid the critical frequency shifts with change of diameter indicates that it is in the column of liquid itself that the energy absorption or transference takes place.These experiments show that it is only at frequencies far removed from absorption i.e., on the regular and flat portions of the velocity-frequency curve some distance from the discontinuity, that the usual theories of sound transmission may safely be applied.

Download Full-text

Correction of B-scan distortion for optimum ultrasonic imaging of backwalls with complex geometries

Insight - Non-Destructive Testing and Condition Monitoring ◽

10.1784/insi.2020.62.4.184 ◽

2020 ◽

Vol 62 (4) ◽

pp. 184-191

Author(s):

S Davì ◽

C Mineo ◽

C MacLeod ◽

S G Pierce ◽

A Gachagan ◽

...

Keyword(s):

Ultrasonic Method ◽

Maximum Amplitude ◽

Constant Thickness ◽

Cross Sectional ◽

Industrial Sectors ◽

Aerospace Manufacturing ◽

Lower Amplitude ◽

Complex Shapes ◽

Ultrasonic Phased Array ◽

Pulse Echo

Ultrasound undergoes refraction and reflection at interfaces between media of different acoustic refractive indices. The most common ultrasonic method (pulse-echo) monitors the reflected energy to infer the presence of flaws, whereas the lower amplitude of refracted signals is ignored. When the reflector is orientated normally with respect to the ultrasonic beam, the received echo signal shows the maximum amplitude. The pulse-echo method also relies on monitoring the amplitude of the backwall echo to identify or confirm the presence of defects. This works well for parts with constant thickness and with planar backwalls. Unfortunately, parts with complex backwalls are common to many industrial sectors. For example, applications such as aerospace structures often require parts with complex shapes. Assessing such parts reliably is not trivial and can cause severe downtime in the aerospace manufacturing processes or during in-service inspections. This work aims to improve the ultrasonic inspectability of parts with complex backwalls, through sending ultrasonic beams from the frontwall side. Ultrasonic phased array probes and state-of-the-art instrumentation allow ultrasonic energy to be sent into a part at wide ranges of focusing depths and steering angles. This allows for tracking of the backwall profile, thus hitting it normally and maximising the amplitude of the reflected echo at any point. However, this work has shown that a cross-sectional scan resulting from multiple ultrasonic beams, which are sent at variable incidence angles, can present significant geometrical distortion and cannot be of much use for accurate defect visualisation and sizing. This paper introduces a generalised algorithm developed to remove geometric distortions and the effect that variable refraction coefficients have on the transmitted and received amplitudes. The algorithm was validated through CIVA simulations for two example parts with complex backwalls, considering isotropic materials.

Download Full-text

Detection of contact damage in ceramics by an ultrasonic method

Journal of Materials Research ◽

10.1557/jmr.1998.0269 ◽

1998 ◽

Vol 13 (7) ◽

pp. 1899-1904 ◽

Cited By ~ 1

Author(s):

Hyo Sok Ahn ◽

Said Jahanmir ◽

John A. Slotwinski ◽

Gerald V. Blessing

Keyword(s):

Silicon Nitride ◽

Glass Ceramic ◽

Ultrasonic Method ◽

Circular Ring ◽

Ultrasonic Technique ◽

Surface Cracks ◽

Compressional Waves ◽

Different Depths ◽

Pulse Echo ◽

Hertzian Cone

A pulse-echo ultrasonic technique consisting of focused normal-incident compressional waves was used for the detection and evaluation of surface and subsurface damage in micaceous glass-ceramic and silicon nitride samples. The damage was produced by indentation with a tungsten carbide ball. The nature of the damage was found to be material-dependent and was classified into two types: Hertzian cone cracks in the silicon nitride, and distributed subsurface microcracks in the glass-ceramic. While the cone cracks were visible on the surface as circular ring cracks, the distributed subsurface microcracks were not associated with any visible surface cracks. Both the cone cracks and the distributed subsurface microcracks were easily detected by the ultrasonic technique. In addition, the ultrasonic beam was focused to different depths below the surface of the glass-ceramic sample to probe the subsurface region containing the microfracture damage.

Download Full-text