Published on 28 January 2013
Failures in healthcare products take on a seriousness and urgency of their own, often not attributed to more benign engineering structures.
So when TWI was asked to investigate a medical service weld failure it rose to the challenge by immediately mobilising specialists in the fields of both resistance welding and structural integrity.
The product in question.a teeth brace. Or, as it is known in the world of specialist orthodentic products, a molar band-buccal arrangement.
The investigation formed part of a product and process review for Joining Forces, Yorkshire and Humber and was followed by a feasibility study.
The molar band-buccal structure is fabricated in medical standard stainless steel and involves spot welding a bracket some 5mm long to a narrow band which embraces each molar tooth.
And it was the integrity of these tiny welds which was thrown into doubt when a patient returned their broken brace.
A programme of work was scheduled to tackle the problem.
The objective...simply to establish possible causes of failure of the assembled orthodontic band.
First, a dozen static tensile tests were carried out on specimens provided by the client, to determine failure load. The load was applied at 90 degrees to the welds at a rate of two millimetres per minute. The failure loads varied widely from 254 to 441N. The service loads endured by the structure inside the patient's mouth were not known.
Second, the assembled bands were examined metallographically. Two specimens were sectioned horizontally and ground and polished to a one micron diamond finish.
And third, for reasons of comparison, the bands which had failed in service were also examined metallographically.
Examination of the bands using twenty five times magnification under a binocular microscope revealed four to five electrode indentations on each side of the buccal. In several cases small deposits of electrode material were found at the edge of the indent, suggesting that damage to the electrode tip had occurred.
Examination of the fracture surfaces also revealed that failure had involved pulling a plug or button from the parent band material.
There also seemed to be a difference in appearance between the tested failures and the service failure offered by the client.
A closer look was required, using a scanning electron microscope.
This revealed microvoid coalescence in the tested weld, a certain indicator of ductile failure. However the service failures showed crack arrests across the fracture surface indicating a progressive failure. This was borne out by the presence of contamination found at the start of the failure.
It is likely that final failure of the band occurred following overload of a partly broken joint.
TWI was able to conclude that the service failures of the bands could be attributed to cyclic loading and a progressive failure culminating in overload. The joint strength of the bands appears to be limited by the endemic fatigue performance of spot welds in general. Their geometry inherently includes a notch, or crack initiation point.
Increasing the size of the weld may not make any difference.
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