huangyhg

tower structures - connection inspections at elevated levels
i need to establish the existing conditions of certain connections on some rather high (400 ft+) towers. has anyone had any experience similar to this and can recommend what inspections techniques they have used? e.g. just visual or perhaps some ut, pt, mt or others? i was wondering if other than visual was even practical at elevated locations. for these towers, i was mainly concerned with detecting cracks in the welds possibily caused by cyclic loadings.
as you are aware, vt will certainly not be sufficient to evaluate subsurface cracking and some very tight surface cracks; other ndt methods like ut should be use to supplement the vt findings.
mt or pt will work well with surface cracks but the former does not work with austenitic stainless steels.
to check the "guts" of the welds in these connections ut is the way to go but the ut inspector has got to be really good, otherwise you may get erroneous results. check the sac report by robert e. shaw "sac/bd-00/06, round-robin testing of ultrasonic testing technicians"
on of the nde techniques that shows good promise for this type of inspection is acfm.
ac field measurement (acfm)
an inspection probe (or array of probes) induces an alternating current in the surface of the component under test. this produces a magnetic field that is distorted by the presence of a crack. components of the magnetic field parallel and perpendicular to the surface are detected and the equipment displays the length and depth of the defect calculated by comparison with a theoretical model of the electromagnetic fields. the technique is relatively insensitive to lift-off and so can be used to test through non-magnetic coatings. an advantage for this method is that the data is recorded (on a laptop computer) as the probe is scanned over the component. meaning:
· the scanning can be conducted by relatively inexperienced staff, with data analysis by more skilled staff.
· data is stored and available for further analysis and checking.
· inspection can be carried out in conditions of poor access, so long as it is possible to manipulate the probe over the test surface.
· stored data can be compared to newer data during future inspections.

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