One of the most sensitive predictive technologies for detecting mechanical problems is ultrasonic monitoring. Frequencies ranging from .5 to 50 MHz are used on metal objects to detect flaws such as cracks or to evaluate the thickness of an object to determine whether there is corrosion. Not only can ultrasonic testing (UT) detect extremely small flaws and at very great depths within machinery, but one only needs one accessible surface to identify these flaws. Also the ultrasonic frequencies do not affect the machines adversely. A transducer is placed on the metal material and the screen on the diagnostic machine shows the amplitude and the time it takes for the wave to return to the transducer. From this information, the inspector can determine where there might be cracks, how long they are, and at what depth. (Wikipedia, 2007, September 2).
For example, tie rods are examined with UT straight and angle beam techniques. The examinations are applied from both ends, and where necessary, the center spans of each tie rod. The examinations are expanded to cover the complete length of each tie rod. The rod ends have to be prepared to an adequate surface condition. Because there are several mechanisms that affect the conductance of an ultrasonic beam within a tie rod, UT straight beams applied from the ends of rods can result in false indications. Ultrasonic instruments with special functions must be used to discriminate between false indications and true cracks. Tie rods are susceptible to cracking in their threaded areas. Cracks usually initiate in the last threads behind the end nuts, but can also be found in the inner threaded sections. Any surface anomalies, such as welds or lifting lugs should be examined with Magnetic testing.
The image above shows an ultrasonic straight beam indication of a crack in the threads of a tie rod. The size of the crack is marked up on the end of the tie rod. The crack is in the third thread from the end where the nut bears on the rear platen.