When Sasol Technology of Secunda in South Africa experienced premature fatigue failures in pressurised aluminium pipework, initial investigations revealed that much of the pipework had longitudinal dents or kinks in it, believed to be manufacturing defects. These kinks were creating local stress concentrations contributing to the premature failure.
A finite element analysis of the effect of the kinks was commissioned, but was not wholly conclusive. At this stage TWI was approached for advice, and recommended model testing as the most direct way of determining the stress concentration due to the kink.
The original pipework was 914mm OD with a wall thickness of 12mm. A one-third scale model was made, and kinks of various depths were pressed into the surface. A string of strain gauges, spaced at 2mm intervals, was banded across the kink on both inner and outer surfaces, and strain measurements recorded as the model was pressurised. The relationship between stress concentration factor and kink depth/profile could be determined from this experimental procedure. Coupled with a comprehensive fatigue analysis of the real system, TWI was able to make positive recommendations concerning the type and extent of repairs needed to ensure satisfactory operation of the remainder of the pressurised pipework.
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