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Ultrasonic hardness measurements

Hardness measurements are commonly used in industry to assess the quality of materials because a direct correlation exists between their hardness and their strength. The majority of hardness testing methods is of the static type in which an indenter is slowly pressed into the material by either a constant load or one that is being gradually increased up to a required level (Brinnell, Rockwell, and Vickers). A dynamic (Poldi) method is also known in which the indenter is impacted. Results of hardness measurements made on welded joints enable an assessment to be made of the size of the heat affected zone, and, also, approximately, of the impact strength of the joint. One of the limitations of the above listed methods is the fact that they involve the use of stationery equipment (except for the comparative Poldi method which gives approximate results of hardness measurements) which, in turn, limits the size of the investigated part. If the dimensions of the element in question make hardness measurement impossible, then the only solution lies in either machining out a smaller testpiece on which the measurement can be made, or producing a specimen that has been manufactured in the same way as the large part. In the former case, the fabricate is being destroyed, and in the latter, there is no certainty that the hardness of the specimen will be the same as that of the original product. It is rather difficult to imagine hardness testing being carried out by a traditional method on welded joints of a ship's hull without actually destroying it. Hardness measurements made by means of any of the traditional methods carry also with them the errors associated with factors such as the deformation of the indenter, the elastic straining of the elements of the equipment supporting the specimen, and the elastic straining of the testpiece itself.