TABLE OF CONTENTS There are a number of variables that can influence an ultrasonic examination. Because of the many different materials tested, the component configurations, and the anomalies that can occur during an ultrasonic inspection, it is impossible to list or predict all of these variables. The following subsections deal with some of the more common variables that may have an affect on the results of a test.
Temperature is an important factor when considering ultrasonic examination. Temperature affects the velocity of sound in most materials to a greater or lesser degree depending on the material. Water, for instance, undergoes velocity changes with temperature. The velocity of water at 68 F (20 C) is approximately 1480 meters per second, whereas at 86 F (30 C) the velocity is approximately 1570 meters per second. At the other end of the scale, water at approximately 34 F (1 C) is approximately 1414 meters per second. Velocity changes in metals are not as dramatic as with water, but changes do occur. When conducting examinations, consideration should be given to temperature variations between the calibration standard and the component. Some specifications require that the temperature difference of the calibration standard and the component be within 20 F ( ?7 C). Of greater influence than the component is the temperature in the Plexiglas wedge or delay line. Temperature variations can cause beam angle changes and/or alter the apparent delay on the time base. When conducting examinations at elevated temperatures, special high-temperature transducers and couplants are usually necessary. Calibration blocks need...
