Does Young’s modulus really exist?
10th August 2016
By: Ruud Selker and Ping Liu
Every young civil or mechanical engineer is taught that the Young’s modulus (E) of steel equals 207 or 210 GPa at room temperature. This value is commonly used as a physical CONSTANT.
However, we recently learned that even such a well-known constant must not be taken for granted. Assessment of measured linepipe material data from the on-going South/Turk Stream Project showed that the elastic modulus in circumferential pipe direction was significantly higher than the expected value of 210 GPa. Accepting this expected value as the truth without confirmation testing can have severe consequences. Firstly, underestimation of the elastic modulus has the potential to result in non-conservatism. Particularly in case of displacement or strain-controlled behavior, acting load or stress will be undervalued. Secondly, the elastic modulus is a parameter that is used in quality control. Incorrect assumption of its baseline can lead to unnecessary rejections or false acceptations during production.
In order to establish a statistical correlation between elastic modulus in longitudinal and circumferential direction, a dedicated test program was conducted. The Young’s modulus in circumferential direction is found to be approximately 15 GPa higher compared to that in longitudinal direction, which does show a typical value of 210 MPa. The distribution of results is presented in Fig. 1. The observed elastic anisotropy is attributed to the plate rolling and manufacturing process (TMCP). Results are consistent for all assessed materials that were produced by various plate manufacturers and pipe mills.
Fig. 1 – Distribution of elastic Modulus (Measured by Compression Testing)
It is not all bad; accounting for a raised elastic stiffness in circumferential direction leads to increased collapse resistance for deepwater pipelines. This can beneficially reduce wall thickness requirement. For load controlled situations, structural resistance is likely to be conservatively underestimated.
We argue that the Young’s modulus as we know it does not exist. It is a parameter that needs to be measured prior to using it in design or quality control. For more information and background, please read Selker et al. (2016)[*].
[*]Selker, R., Liu, P., Chaudhuri, J., Jurdik, E. and Fick, A., “Elastic Anisotropy in Linepipe Materials”. Proceedings of the 26th International Ocean and Polar Engineering Conference, Rhodes, Greece, Vol. 2, pp. 459-465, June 2016