Tsukuba, Japan, Feb 6, 2020 - (ACN Newswire) - For the past 50 years, researchers at the National Institute for
Materials Science (NIMS) have been conducting detailed short- and long-term testing of a wide variety of structural
materials manufactured in Japan to ensure they can withstand long-term stresses. Now, NIMS scientists have reviewed this
data, in the journal Science and Technology of Advanced Materials, summarizing the institute's major findings.
In 1966, NIMS's predecessor, the National Research Institute for Metals, launched its 'creep data sheet project'. The
aim of this project was to determine the stress required to rupture heat-resistant steels and alloys in 100,000 hours
(about 11.4 years) at high temperatures. This 'creep rupture strength' data was initially needed to determine the
allowable stresses metals could be exposed to in power plants. But more recently, this data has been used to assess how
much longer power plant parts have before they begin to wear and tear.
Just over a decade later, in 1978, NIMS also began assembling what has become a huge database of fatigue properties of
structural materials used in numerous industries, including automobiles and aircrafts. Fatigue describes how cracks
propagate in a metal over time. Fatigue tests involve placing a metal sample under repetitive loads, called cycles, to
see how long it takes for a crack to develop and propagate. These tests are conducted at room temperature and high
temperatures. Samples are exposed to a relatively small number of cycles (in the range of 10 million cycles) or up to 10
billion cycles, lasting for several years.
NIMS data has revealed that the long-term creep strength of materials varies, and that scientists need to choose the
type of analysis method for creep rupture data according to the type of material. How creep happens in materials during
testing not only depends on the amount of stress applied, but also on the temperature conditions. The researchers have
found that materials react differently to varying temperature depending on their chemical composition, the amounts of
minor elements in them, and the crystal grain size. Ferritic heat-resistant steels, which are commonly used in thermal
power plants, were found to have very long-term, inherent creep strength. But this creep strength is dependent on the
amount of minor solutes present in the steel.
Fatigue limits, on the other hand, are affected by a metal's tensile strength and hardness. Interestingly, NIMS
scientists have found that some metals can last for an incredibly long time without forming cracks as long as they are
constantly exposed to room temperatures. These same metals, however, would eventually form cracks if exposed to the same
stress but at high temperature.
Up until now, the creep and fatigue data sheets developed at NIMS have been used mainly for industrial purposes. The
institution is now aiming to improve accessibility so the data can also be used by academics.