7/30/2019 Angle Probe

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Angle beam probe :Angle probes are used for detecting flaws oriented at an angle to

the material surface. They will miss flat reflectors, parallel orperpendicular to the scanning surface.The angled beam must hit thereflecting plane of the discontinuity at 900 for maximum reflection.Angle beam reflects well from corners but may undergo wave modeconversion.Usually shear wave probes are used in angle beam testing, because

the refracted longitudinal wave probe will have a shear wavecomponent of weaker intensity in the test material. Longitudinal waveangle probes are specially useful for testing austenitic stainless steelwelds and inside surface of highly curved hollow parts and welds.Shear wave probes are used for testing welds, material defects inpipes, tubes, plates and sheets and in irregular shapes where fullcontact testing area is not available.Angle beam probes use the principle of refraction and mode

conversion to produce refracted longitudinal and shear waves in thetest part. In an angle beam probe, longitudinal wave is converted torefracted longitudinal or shear wave by means of an angled plasticwedge. The wedge is cut at an angle to provide an incident beamangle to produce refracted longitudinal or shear wave at the desiredangle in Steel, because steel is tested in most applications. Fordesigning small crystal low frequency probes, incident angle calculationbased on snells law fails because of wider beam spread associated

with these probes.The refracted angles will change in other materialsbecause of velocity difference.A probe of 450 in steel will equal 430in aluminum.The wedges have serrations cut on the front surfaceand fitted with absorbing medium to prevent internal reflections fromproducing unwanted echoes but some spurious echoes are oftendetectable just after the initial pulse. The wedge can be an integralpart or can be detachable. In detachable system, a single transducercan be configured to different angles by changing the wedge and iseconomical. The detachable wedges are bigger and absorbs moreenergy. The detachable type requires couplant between the transducerand the wedge to permit transmission of ultrasound into the wedge. Standard angles are 35, 45, 60, 70 and 800 in steel. 800 and 350

probes have limited applications due to prevalence of surface wavesfrom 800 and presence of longitudinal waves from 350 probes. Crystal

sizes around 10 mm for miniature and 20 mm for bigger probes aremost popular. Probes with 12.5 mm crystal size is normallyrecommended for standard test applications. Rectangular crystals areoften used. Bigger probes have higher sensitivity and sharply focusedsound beam and permit flaw detection at higher depth but cannot beplaced very close to the weld edge. 4 MHz, 8 X 9 / 10 mm probes arevery popular for their small contact area, high resolution and sharpechoes and are used up to a test distance of 200 mm in steel. Largerprobes are mostly used for thick welds and for applications wheresufficient probe contact area is available and a large area is to bescanned. A 20 X 22 mm, 2 MHz probe can detect a 2 mm reflector upto a distance of 700 mm.Twin crystal angle probes, longitudinal and shear, are available for

direct scanning on ground weld surfaces and low thickness materials.These probes are more suitable for testing stainless steel and soundabsorbing materials.With use, the beam exit point and the refracted beam angle will

change with acrylic wear and should be checked before each use.The beam angle change should be controlled within + / - 20. Newacrylic soles can be pasted with araldite to repair the wear.The frontcorner of the sole should be shaped to reduce spurious echoes.Performance :A good angle probe with a standard flaw detector, should produce aminimum 75% signal from the 100 mm radius of a standardIIW - V1 block with a minimum of 40 dB gain reserve.The probe should resolve at least three of the five holes [ clearly

detectable peaks ] in the IOW beam profile block.Noise from internal reflections should not exceed 5% screen height,

at all the working test sensitivity levels.

Beam axis abnormality should be checked by maximizing a signaland moving the probe forward and backward and rotating left andright, while monitoring the falling signal. The signal should fallcontinuously without any sudden rise in signal height.

Compiled by K. Chatterjee, 75643 Center for NDT P 00 Rv 05 Self study material.