Study on new creep equation using discrete cosine transform for high temperature materials

Abstract

For an equation representing creep strain-time relationship curves of high temperature materials, the equations which represent from the primary creep region to the tertiary creep region using exponential functions such as the modified θ projection [3] have already been proposed. However, the estimated values by the modified θ projection don't necessarily coincide with the measured values, and deviations occur between the estimated values and the measured values [8]. Then in this paper, we propose a new creep equation using the Discrete Cosine Transform (DCT). According to this equation, all the estimated values coincide with the measured values and each interpolated value between the measured points gives a reasonable value. Therefore it is possible to express a creep curve quite well from the primary creep region to the tertiary creep region by using the DCT.

New Numerical Analysis Method for Creep Lifetime of Gas Turbine Materials by Using the Discrete Cosine Transform

“…On the other hand, with use of the Discrete Cosine Transform we can now fit a 40-year long term creep curve. Recent literature provides such data for 0.3C steel [21], [22] and shows that it fits anomalous high temperature creep curves of Ni-Cr-W superalloy very well [23]. These results have never been obtained before.…”

“…For this research, Nickel based 16Cr-8.5Co-3.5Al-3.5Ti-2.6W-1.8Mo-0.9Nb superalloy from the database (No.49A) of National Institute for Materials Science (NIMS) was selected, because this superalloy has been used in the author's previous papers [11], [12] and is used for materials at elevated temperatures and high pressure such as gas turbine blade or so.…”

“…These results have never been obtained before. Because the Discrete Cosine Transform has the ability to be able to pass through all the measured points smoothly [11], [12]. In the 40-year long term creep data analysis, the Discrete Cosine Transform was able to fit the 40-year creep curve from the primary creep region to the tertiary creep region very well using 41 discrete signals [22].…”

“…Because the Discrete Cosine Transform has the ability to be able to pass through all the measured points smoothly [11], [12]. In the 40-year long term creep data analysis, the Discrete Cosine Transform was able to fit the 40-year creep curve from the primary creep region to the tertiary creep region very well using 41 discrete signals [22]. A discrete signal x[n] means a creep strain value at ordinal number n obtained by dividing measurement time by equal intervals.…”

“…A discrete signal x[n] means a creep strain value at ordinal number n obtained by dividing measurement time by equal intervals. According to the previous paper [22], the Discrete Cosine Transform can be applied using 6 discrete signals to 41 discrete signals. The accuracy using 41 discrete signals is much higher than that using 6 discrete signals.…”