The ACPD technology has been used for monitoring crack extension in laboratory tests and also for surface crack shape evolution studies.
TSC equipment is used in test laboratories to provide data on fatigue resistance of a material under given conditions (e.g. environment, load, geometry etc.). In these tests, the components can be standard defined geometries (such as compact tension or tensile test specimens) or small pieces representative of a larger structure (such as T-butt welds or tubular intersections).
Crack monitoring can also be required on-site to give early warning of crack initiation in a critical area, for example, or to monitor a known defect up to the point where a failure may be imminent; allowing repair to be scheduled at a convenient time (i.e. at a planned shut-down rather than an emergency one).
The most common crack sizing applications include:
- Welded connections, butt welds, tee butts, tubulars, pressure vessels (refinery, nuclear, offshore topside and underwater)
- Threaded connections, nuts, bolts, drillstring threads, large diameter threads
- Automated inspection using tracking systems, robots
- Monitoring of cracks to give crack growth data
- Monitoring of critical regions for crack detection
- Turbine blades, blade roots
- Castings, forgings, ground repairs.
Fatigue Testing Crack Growth Rates
The National Physical Laboratory undertake a variety of fatigue tests to determine properties of metals under various conditions. One program involved measurement of crack growth rates under corrosion conditions, initiated from a small corrosion pit.
The samples were standard cylindrical tensile specimens enclosed in a corrosion cell.
This involved measurements of shorter and shallower defects than had been attempted before. The solution was to use TSC’s ACPD instrument, with special wiring onto spot-welded pins covered in a gel to withstand the corrosion. Depth measurements were compared with those obtained after the test from staining and beach-marking, with good results.
UCL Lab Testing
Studies at University College London, on electrical field perturbation, have produced theoretical interpretations of the ACPD measurements eliminating the need for calibration. TSC can advise on application of correction factors based on these results.
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ACPD MK IV - Desktop Crack Microgauge. Long term single or multi-site monitoring of up to 128 channels, with threshold alarms.
Amigo - Portable, rugged and versatile ACFM instrument. Options for dual frequency and array probes support for general inspection.
Bespoke Product Solutions
TSC has worked on many collaborative projects with the British National Physics Laboratory and several UK Universities on research projects.
An example of a Lab Test project:
University of Waterloo in Canada
The Department of Civil and Environmental Engineering at the University of Waterloo in Canada had a requirement to carry out fatigue tests of welded steel specimens under simulated in-service loading typical of highway bridges. The aim was to compare results with a theoretical model, and to measure the benefits of needle peening to increase fatigue life.
The samples were cruciform-shaped, with four separate welds, all of which needed to be monitored. The solution was to use TSC’s Mark IV ACPD instrument, with four separate current outputs, and magnetically-attached array probes.
Repeatable results were obtained of crack growth curves from initiation to failure, in good agreement with the theoretical model. This allowed the model to be used for further predictions, without the expense of large numbers of fatigue tests.