ASTM-E587:2020 pdf free download – Standard Practice for Ultrasonic Angle-Beam Contact Testing.
6.1.3 Couplant—A couplant, usually a liquid or semiliquid. is required between the face of the search unit and the examination surface to permit the transmission of ultrasonic waves from the search unit into the material under examination. Typical couplants include glycerin, water, cellulose gel, oil, water-soluble oils, and grease. Corrosion inhibitors or wetting agents or both may be used. Couplants must he selected that are not detrimental to the product or the process. The couplant used in standardization should be used for the examination. The standardization and examination surface temperatures should be within ±25°F (14°C) to avoid large attenuation and velocity diflérences in the wedge material.
6.1.3.1 The coupling medium should be selected so that its viscosity is appropriate for the surface finish of the material to he examined. The examination of rough surfaces generally requires a high-viscosity couplant. The temperature of the material’s surface can change the couplant’s viscosity. As an example, in the case of oil and greases, see Table 1.
6.1.3.2 At elevated temperatures (above 125 °F (52 °C)), heat-resistant coupling materials such as silicone oils, gels, or greases should be used. Further, intermittent contact of the search unit with the surface or auxiliary cooling of the search unit may be necessary to avoid temperature changes that ailed the ultrasonic wave transmission properties of the wedge material or the characteristics of the transducer. At higher temperatures, certain couplants based on inorganic salts or thermoplastic organic materials, high-temperature wedge materials, and transducers that are not damaged by high temperatures, may be required.
6.1.3.3 Where constant coupling over large areas is needed, as in automated examination, or where severe changes in surface roughness are found, other couplings such as liquid- gap coupling will usually provide a better examination. In this case, the search unit face does not contact the examination surface hut is spaced from it a distance of about 0.02 in. (0.5 mm) by integral rails or a fixture. Liquid flowing through the search unit fills the gap. The flowing liquid provides the coupling path and has the additional advantage of keeping the search unit temperature low if the examination surface is hot.
6.1.3.4 An alternative means of direct contact coupling is provided by the wheel search unit. The transducer is mounted at the required angle to a stationary axle about which rotates a liquid-tilled flexible tire. A minimum amount of couplant provides ultrasonic transmission into the examination surface since the elastic tire material is in rolling contact and conforms closely to the surface.
6.1.4 Reference Reflectors—Reference reflectors of known dimension, artilicial reflectors, or distance-amplitude relationships of known reflector sizes for a particular search unit and material may be used for standardization. The artificial reflectors may be in the form of side-drilled holes, notches, or hat-bottom holes. The reference standard and the production material should have similar velocity. attenuation, curvature, and surface finish.