TOFD Solution

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TOFD Solution to Detection of SC (Strain-cracks)
in the PMP (Press-mounted Parts) of Axle Shaft

Oil-pressing technique is usually employed in the PM (press-mounting) process. The PM quality of the wheel-set is then to be evaluated according to its F-L curve. Currently SW (shear wave) probe with bigger IBA (incident beam angle), LW (longitudinal wave) probe of smaller IBA or a combo of the kind is adopted for manual inspection on the PMP of axle shaft. LW probe of small IBA ( 7°~9°) is often applied to the PMP of the brackets respectively at both ends of the axle. Another example for the application of SW probe with bigger IBA, is to move the probe (45°~63.43°) from the neck of axle towards outer bracket, otherwise from the body of axle towards inner bracket. LW probe with smaller IBA and SW probe with bigger IBA can collectively work out the crack in the PMP arisen from fatigue. Anatomy of the axle with SC indicates that such flaws are generally born in the outer fatigue-stricken zone 300mm away from the axle end and extended inward to some length and width, parallel to the axle. But the veinings of a SC, scaly, should be at some angle between 20°~50°to the normal of the axle. Usually, the echo of a SC is so weak as to escape detection, which becomes a challenge to the inspectors.

TOFD Solution to Detection of SCs in the PMP of Axle Shaft is recommended to be followed with two probes and an encoder for probe-tracking on the axle. The pair of probes, one as initial and the other as terminal, are placed respectively on both sides of the PMP. The “initial”, “terminal” and the encoder are respectively connected to corresponding ports on the TOFD instrument. The process goes just as the axle turning, with the “initial” and the “terminal” kept still. Meanwhile, the TOFD instrument acquires the signals of the echoes and the corresponding positions, and visualizes them. The inspector will then diagnose and quantize the condition of SCs in the PMP of the axle shaft.

Interpretation of the image is key to the TOFD technique. You should have great experience and comprehensive understanding over the object you’re working on. Analyzing software is another important contributor. The accuracy of TOFD technique applied to SCs in the PMP of axle shaft relies on mathematic model of fitted curve worked out from simulation, verification by test block and reasoning with data. The mathematic model of the fitted curve for a SC in the PMP of axle shaft is described by the below formula:

d is the function to L, meaning the “depth” to the curve under changeable value of L.
d0 is the max “depth” to the curve
L is the path by which the axle turns, an arc length. Be the L as 0 when the “depth” is as d0, it shall be: .

Picture 1 shows you how such inspection goes with the CTS- 10 08plus , an efficient TOFD instrument made by Goworld. It is inset with curve-fitting feat , and supplied with a specialized scanner. Fig 1 shows the result of curve -fitting. The curve belonging to the flaw is fitted right at the outline.

Pic 1 Inspection           Fig 1 Fitted Curve           Pic 2 The Axle with SCs

Inspection on axles respectively with and without SCs. The results are shown in Fig 2 & 3.

Fig 2 Axle with SCs                    Fig 3 Axle without SCs