Understanding the cause of a medical device's failure is crucial to a design engineer, so that the mistake won't be repeated in the future. When the device is made of plastic, there are a number of indi-
cators that can be of help, an expert told conference attendees at January's MD&M West show in Anaheim, CA.
Causes, said Eric J. Moskala, PhD, of Eastman Chemical Co. (Kingsport, TN), include improper material selection, poor design, degradation of plastic materials, residual stresses, and environmental conditions such as elevated temperatures.
The first step, he said, is to perform a molecular-weight analysis on the broken device, using a test such as melt-flow rate. This analysis will determine whether the cause involved anything that would result in a loss of molecular weight.
Next, engineers should perform thermal and stress analyses. They should then identify all the materials present in the broken device via spectroscopy or chromatography. This analysis could indicate whether a foreign substance was involved in the failure or whether a chemical reaction caused the failure.
Failures of plastic materials fall into two main categories, Moskala said. Brittle failure occurs when a complete break or significant crack in the material occurs. In these cases, the stress-strain curve will be triangular, because little or no energy will have been absorbed after the break. There will also be minimal visible plastic degradation of the intact portions of the device. Ductile failure involves significant energy being absorbed after the crack occurs, and gross deformation of the plastic will be visible. The broken pieces won't be able to fit together in any recognizable pattern, he said.
The patterns made in the material after a brittle failure will point to the origin of the crack, he said. A mirror region, which is completely featureless, will surround the point of origin. Adjacent to that is the mist region, which does have some microscopic textures. Beyond that are markings called hackles, Wallner lines, striations, and rib markings. All of them have curves, and they curve away from the origin of the crack. Once the origin of the crack is pinpointed, it is much easier to spot a foreign material that led to its cause. If the cause cannot be identified, then at least the focal point of the stress and thermal tests to be conducted has been found.
Ductile failure, which occurs as a result of fatigue, displays a different pattern. Closest to the point of fatigue are discontinuous growth bonds, then farther away are striations, then farther away are Wallner lines and other markings associated with brittle failure.
To fully understand the cause of a plastic failure, an engineer must be able to pinpoint the ductile-brittle transition, Moskala said. That transition is an abrupt change in a material's toughness associated with a failure. Several causes, including temperature and physical aging, can lead to this transition.
The transition point is determined by the intersection of two curves, both plotted on a stress-versus-temperature axis. The first is the yield curve, which is highly dependent on temperature. The second is the craze or brittle curve, which is highly dependent on stress. Understanding the device's conditions before the failure occurred, then, can indicate whether the failure was caused by temperature or stress.
He noted that the ductile-brittle transition figure should be used in accelerated-aging tests to indicate how much stress the plastic material can withstand at a given temperature.
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