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Metal Fatigue
Manufacturing Faults
Bicycle Components
Composite Materials

Tools of the trade, some ways to investigate problems;
Dye penetrant testing

of materials engineering terms

Polymer Failures

Stress corrosion cracking of nylon 6,6 - Page 3

Click on pictures below for full size image.
Click on green text for explanation of term or see the glossary.

Broken Y piece

Image 1

Image 2

Image 3

The parts of the pipe removed from the vehicle showed that the pipe itself had not failed, but rather a plastic Y-connection linking two pipes together (image 1).FTIR and DSC analysis showed that the connection had been injection moulded from nylon 6,6 while the pipe itself was a nylon 12 extrudate.

The failed surface showed several features which gave vital clues as the way the junction had failed in service. (Select image 2 for detailed description of this surface)

But what had initiated the sequence of events? The junction was just below the battery of the vehicle, and a possible cause of the erosion may have been attack by spilled battery acid.

Nylon 6,6 is sensitive to degradation by acids, especially sulphuric acid, a fact confimed directly on a new connector. The nylon 12 material of the pipe itself was unaffected by the same fluid. It thus seemed feasible that battery acid had dripped onto the Y-connection and degraded the material allowing a crack to grow from the inner corner of the junction edge. Further intermittent growth then occurred in a series of steps until the final fracture.

Further details of the fracture surface came with ESEM inspection (image 3). Much finer striations were seen between the large crack arrest lines. They showed that crack growth had occurred more slowly than expected, with the crack jumping intermittently under slight loads. EDAX analysis showed traces of sulphur on the eroded part of the surface, supporting the acid attack hypothesis.

Forensic conclusions >

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ed: PRL 21/6/02

© 2005 Materials Engineering - Page last modified 18-Dec-2007