Start date and planned duration: January 2019, 36 months
- Obtain high resolution (micro-scale) three-dimensional information on slow crack growth (SCG) initiation and propagation in PE using µCT.
- Comparison of the crack tip micromechanisms associated with different SCG tests (creep rupture, cracked round bar, and strain hardening).
- Use interrupted in situ tests and µCT to follow the crack propagation during the tests (time-lapse study).
- Start to formulate an ECA methodology framework for PE materials.
In order to determine the effect of flaws on the long-term integrity of PE pipes and welded joints it is currently not possible to use the ECA methodology that is used for metals. The fracture mechanics methods used in BS 7910 are not applicable to PE due to its ductility, which causes crack blunting, resulting in a slip-stick crack growth mechanism, called Slow Crack Growth (SCG). The plastic zone in front of the crack tip consists of microvoids that grow in size and join up to produce a network of fibrils, usually referred to as a craze. Eventually these fibrils fail, the crack propagates and the process is repeated. Therefore, a new methodology is needed, which will require the modelling of SCG and crack initiation.
The long duration of, for example, creep rupture tests, causes testing to evaluate SCG to be unpractical or uneconomic. Faster alternative tests have been proposed, but correlations between SCG measurements from constant load creep rupture test and alternative methods have been provided mainly as macroscopic correlations only. This project will identify an improved strategy for characterising creep rupture behaviour in PE using relatively short-term tests. The use of accelerated tests that maintain the creep rupture micromechanisms are fundamental if a correlation between long-term and short-term tests can be found by using or modifying defined procedures, or by creating new ones.
X-ray microtomography (µCT) will be used to obtain physical-based observations on SCG, such as crack location, crack morphology, and crack growth during the different mechanical tests in order to gather insights into the material behaviour for different loading conditions and sample geometries. The results will be used to measure and develop an understanding of SCG initiation and propagation that can be used to develop models for this process, which can then be incorporated into an ECA approach for PE material.
Benefits to Industry
The benefits of ECA for polymers are similar to those that apply for metals, including appropriate time between safe inspection intervals, improvements in the certainty of component life, and life extensions. The procedures and information generated in this project will promote the use of PE pipes for safety-critical applications, reducing costs and improving reliability for end users, and benefiting the PE pipe supply chain.
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