Size Estimation of Fatigue Crack Appearing at Bolt Joints of Aluminum Alloy Plates by Synchronized SAW Measurement

Abstract

The synchronized ultrasonic measurement with the surface acoustic wave (SAW) was carried out during the fatigue testing of bolted specimens of the aluminum alloy in synchronization with a fatigue cycle. The stationary SAW measurement was also carried out in the fatigue testing under pausing the fatigue tester at the mean load. In both measurements, the intensity of the scattered wave gradually increased with the number of fatigue cycles in the fretted region ahead of a bolt hole due to surface roughness, and the steep increment occurred following appearance and propagation of the fatigue crack. The steep increment was detected earlier in the synchronized measurement than that in the in-process measurement. In the synchronized SAW measurement, the intensity of the reflection from the fatigue crack increased with increasing stress level in the fatigue cycle. Then, the crack length was estimated with the intensity change of the reflection from the crack in the fatigue cycle following the Elber's model modified by Suresh on the crack opening and closure following the stress intensity factor. The method developed in the present work shows the potential of the synchronized surface acoustic wave measurement as a tool for the early detectability of the fatigue crack as well as for the evaluation of crack opening/closure behavior in the bolted joint.

Depth Evaluation of Fretting Fatigue Crack Appearing at Bolt Joints of Aluminum Alloy Plates by Synchronized Surface Acoustic Wave Measurement

“…In the previous work, 13) the SAW intensity distribution changed following the change in the stress level in the fatigue cycle. Figure 3 shows the SAW distributions measured at an early stage of the fatigue process, in which measurement, the number of fatigue cycles proceeded from 380,000 to 388,000 cycles.…”

“…8,13) A transducer generating a longitudinal wave of 20 MHz in frequency with a focal distance of 25.4 mm in water was used in the measurement. The longitudinal wave was irradiated to the specimen surface at an angle of 30°from the specimen normal to generate the SAW.…”

“…The position (Bh) of the echo intensity reflected from the bolt hole edge sinusoidally changed with the delay angle due to movement of the specimen following loading and unloading cycle, as was observed in the previous report. 13) From the fitted curve to the position of Bh, the change of the nominal stress (the stress averaged through the specimen cross section) in a fatigue cycle was evaluated, and showed a maximum at a delay angle of about 180 deg. Figure 4 shows the change in the SAW distributions obtained at the delay angle of 180 deg (i.e.…”

“…To overcome this difficulty, Min and Kato 11) and Rokhlin et al 9,10) carried out the real-time ultrasonic measurement during the fatigue testing of the specimen to obtain the change in the ultrasonic signals at any loading level in a fatigue cycle, but still remained limitations. The authors 12,13) presented the other way of the ultrasonic monitoring technique, called as the synchronized ultrasonic measurement, in which the ultrasonic pulse signal is generated in synchronization with the loading cycle in the fatigue testing to obtain the data at any stress levels in a fatigue cycle, and they evaluated a crack length from the change in the SAW intensity reflected from the fatigue crack with the stress level in the fatigue cycle. 13) Ultrasonic evaluations of the depth of the surface crack also have been carried out to understand the crack growth by different researchers.…”

Survey on techniques to detect micro-cracks in steel after bending

“…Out of the time difference, the distance to the discontinuity can be calculated. According to Wagle and Kato [1], this technique can detect cracks underneath other components, such as washers in nut-bolt connections. The technique is suited for larger areas.…”