Temperature effects on mechanical property performance.
In today’s ever more stringent economic climate, reliable material performance is becoming even more important, as premature failures are not readily tolerated, especially when they can be anticipated and avoided. Sometimes such failures are related to the temperature behaviour of materials, and this month’s Technical Tip refers to two examples that illustrate such temperature related failures.
In one case, an offshore operator utilised components that were fabricated from low carbon ‘mild’ steel, but at very low stresses (mostly tensile but some bending) so that corrosion concerns were not too much of an issue within the operational lifetime - in the vicinity of six months to a year. In an attempt to improve this performance, the owner specified a 0.4% C steel (as opposed to the existing 0.02% C component), but now failure occurred (by complete component fracture) within days, rather than months. This was because there was some fatigue crack growth, which had never previously been a concern, as the fracture toughness of the 0.4% C steel (at 13⁰C) was now five times lower and the critical flaw size 25 times lower, with this higher strength steel. In this case the direct cause of failure was the ‘effective reduction’ in fracture toughness at the low temperature (due to increased carbon content), which in turn led to a significant, order of magnitude, reduction in critical flaw size, and ultimately – premature failure!
In the second case, a water pipeline system employed PVC piping, which is widely used for cold water reticulation worldwide, with great success. The pipeline failure in this case occurred in the Middle East, in conveying desalinated water (incoming at 55⁰C) in desert environments, where the ambient temperature in summer is in the order of 45⁰C. At these temperatures the strength of the PVC material is reduced by about 45%, to about 55% of its value at ambient temperature. Consequently the safety factor in civil engineering design (typically approximately two) is now virtually nullified by the property degradation change due to the high operating temperature. Perhaps more importantly fatigue cracks were initiating during testing in 4 000 cycles rather than in 45 000 cycles at lower temperatures – a full order of magnitude lower. The net effect was that the (normal) conventional fatigue loading led to the development of fatigue cracks and complete pipe fracture within a year, well short of the anticipated 25 to 30 year design life, that would have been expected if the operating temperature had indeed been 20 – 25⁰C.
These examples highlight the importance of ensuring the material properties are suited to the actual operating temperature, and assessing the structural integrity and service performance in the light of this information.
Published in Technical Tips by Origen Engineering Solutions on 1 February 2017