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Vol.6 No.1previous AA74-SP11(75-76)-Errata NT 63 -64
Academic Articles
Vol.6, No.1(2014) p.14 - p.32

Special Issue 11

High Cycle Thermal Fatigue Induced by Fluid Temperature Fluctuation

 
Relevant Field
Fluid-structure interaction, Fatigue issue
 
Keywords
Thermal Fatigue, Thermal Hydraulics, Thermal Stress, Knowledge-base, Numerical simulation, Nuclear Piping
 
Preface
Thermal fatigue failure sometimes occurs in nuclear plants even though well recognized failure mode. One of main reasons is their complex failure mechanism among thermal hydraulic loading, thermal stress response and fatigue crack behavior. Therefore, the sets of thermal load and fatigue evaluation methods are necessary to prevent failures. However, there exit many kinds of thermal loads for fatigue failure. They can be classified into several patterns. Among them, two kinds of thermal fatigue failure modes are well known and were codified as the JSME guideline.
(A) Fluid temperature fluctuates at an incomplete mixing area of high and low temperature fluids in nuclear components. It induces random variations of local temperature gradients in structural walls, which lead to cyclic thermal stresses.
(B) Temperature stratified layer tends to appear at cooled stagnant branch pipe. When it locates at the elbow, thermal hydraulic instability oscillates the layer and leads to thermal fatigue.
The JSME guideline provides the conservative evaluation procedure, however, its margin is pointed out to be inhomogeneous. Furthermore, best estimate plus uncertainty evaluation is required for system safety enhancement after the Fukushima-daiichi nuclear plant accident.
From above needs, R&Ds have been promoted to clarify thermal fatigue mechanism and to develop precious evaluation methods. For considering uncertainties, probabilistic approach is also studied.




Guest Editor,
Naoto Kasahara
EJAMAA_SI11_editor_N.Kasahara
 

Naoto KASAHARA, Takamoto ITOH, Masakazu OKAZAKI, Yukihiko OKUDA, Masayuki KAMAYA, Akira NAKAMURA, Hitoshi NAKAMURA, Hideo MACHIDA, Masaaki MATSUMOTO

Nuclear piping has various kinds of thermal fatigue failure modes. Main causes of thermal loads are structural responses to fluid temperature changes during plant operation. These phenomena have complex mechanisms and many patterns, so that their problems still occur in spite of well-known issues. The guideline of the JSME (Japan Society of Mechanical Engineering) for estimation of thermal fatigue failures in piping system is employed as Japanese regulation. To improve this guideline, generation mechanisms of thermal load and fatigue failure have been investigated and summarized into the knowledgebase. And numerical simulation methods to replace experimental based methods were studied.
Furthermore, probabilistic failure analysis approach with main influence parameters was investigated to be applied for the plant system safety. Thus, based on the knowledge, estimation methods revised from the JSME guideline were proposed.

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Masakazu OKAZAKI, Milton MUZVIDZIWA, Akira IWASAKI, Naoto KASAHARA

High cycle thermal fatigue failure of pipes induced by fluid temperature change is one of the interdisciplinary issues to be concerned for long term structural reliability of high temperature components in energy systems. In order to explore advanced life assessment methods to prevent the failure, fatigue crack propagation tests were carried out in a low alloy steel and an austenitic stainless steel under typical thermal and thermo-mechanical histories. Special attention was paid to both the effect of thermo-mechanical loading history on the fatigue crack threshold, as well as to the applicability of continuum fracture mechanics treatment to small or short cracks. It was shown experimentally that the crack-based remaining fatigue life evaluation provided more reasonable assessment than the traditional method based on the semi-empirical law in terms of "usage factor" for high cycle thermal fatigue failure that is employed in JSME Standard, S017. The crack propagation analysis based on continuum fracture mechanics was almost successfully applied to the small fatigue cracks of which size was comparable to a few times of material grain size. It was also shown the thermo-mechanical histories introduced unique effects to the prior fatigue crack wake, resulting in occasional change in the fatigue crack threshold.

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EJAM Vol.6 p.14-32 Academic Articles Special Issue on "High cycle thermal fatigue induced by fluid temperature fluctuation"