Effect of stress regime‐dependent creep behaviour on measurement of creep strain rate based on small specimen techniques

Abstract

Small specimen creep testing technique has become a hot topic of research as bulk materials are not available in many occasions. The stress distributions in the small specimens such as small punch and 3‐point bending specimens are essentially nonuniform. As it is known, the creep deformation/damage accumulation mechanisms exhibited at a high stress regime are not the same as the ones at a lower stress regime for many engineering alloys. The potential measurement errors because of stress regime‐dependent creep behaviour, however, has not been considered in the determination of the creep parameters based on small specimen testing in the previous studies. In this paper, 2 kinds of materials that show different Norton's parameters at the corresponding stress regimes are examined. A simple case of 2‐bar structure is firstly adopted to illustrate the measurement error of creep strain rate because of stress regime‐dependent creep behaviour. Furthermore, clamped beam bending testing and small punch testing are investigated to demonstrate the significance of measurement error using the same materials. It is shown that an error of more than 8 times may occur near the transition point of creep deformation mechanism depending on specimen types and materials. Attention should thus be paid to the selection of stress level in the small specimen testing to avoid significant measurement errors.

Influence of constitutive equation and friction on measurement of creep strain rate based on small specimen testing technique

“…This is in good agreement with the previous numerical results on P91 steel and A7N01 aluminum alloy. 32 On the other hand, in the case of the Sinh model, the _ ε eq =_ ε u generally decreases with increasing the stress level. As a consequence, the higher the stress level, the higher the measurement error.…”

“…[29][30][31] Contrary to the uniaxial creep tension specimens, small specimens under creep conditions may exhibit complex mechanical responses, for example, SPT, thereby leading to nonuniform stress and strain fields in the specimen. 17,32,33 Accordingly, even though a constant external force is applied, different regions of the small specimen are likely to sustain simultaneously different creep deformation mechanisms due to the stress regimedependent creep behavior. In view of this, the stress regime-dependent creep behavior of materials may greatly influence the evaluation of creep properties based on SSTT.…”

Numerical Prediction of Creep Rupture Life of Ex-Service and As-Received Grade 91 Steel at 873 K

“…Considering the two-creep mechanism when simulating the crack growth rate gives a better prediction. In another study, Tu et al [11] investigated the effect of different creep mechanisms on creep behaviour and strain rate using a small specimen technique and found that the error may occur at the transition between creep mechanisms. Therefore, more attention should be given when simulating this area.…”

Relationship between the microstructure and the heat treatment and creep behavior of Fe–33Ni–19Cr alloy

“…Numerous reports [12][13][14][15][16][17][18][19][20][21][22][23] prove that by increasing the grain size, the creep rate will decrease or increase based on the experiment conditions, material properties, and role of the grain boundaries. For stainless steel, Lee et al 12 prove that as the grain size increases, the steadystate creep rate decreases to a minimum value and then increases from the intermediate grain size.…”

“…[10][11][12] Thus, several studies have been conducted regarding the influence of grain size on the creep properties, and mechanical properties had been conducted on many metals such as stainless steel, copper, and Ni-based superalloys. [12][13][14][15][16][17][18][19][20][21][22][23] Surprisingly, not many researchers investigated in detail the correlation between microstructures and properties of these alloys. Therefore, this paper aims to discuss the relationship between change in the microstructure, specifically on the grain size after heat treatment, on the high-temperature creep performance of Fe-Ni-Cr alloy.…”