![]() ![]() have proposed some modifications to the CWA model to deal with these limitations. Also, it has been indicated that for specific materials the k sp factor is not a constant, but instead needs to be optimized against uniaxial creep date. showing that k sp correlations for inherently brittle superalloys used for high-temperature applications are not as useful as in the case of Titanium aluminides. Also, the accuracy of the k sp method has been shown to be limited to ductile materials-with the work done by Lancaster et al. In some studies on P91 this method has indicated a difference in low and high load conditions, indicating a load dependence. ![]() ![]() This model, also referred to as the Classic CEN Workshop Agreement model (CWA), has already been applied to a range of materials, where, for example, the work done by Jeffs on CMSX-4 and the work done by Milička and Dobeš on P91 have met with some success. ![]() In this model, Ψ is a function of the test geometry of the small punch creep test and the k sp correction factor. The k sp method is based on a constant force-to-stress ratio ( ψ = F/ σ) and is based on the Chakrabarty membrane stretching equations. Therefore, data obtained from SPC tests cannot be used to find or to compare to the values of conventional creep parameters (such as stress)-hence the need for a form of correlation to bridge the gap between the two test types. This is because uniaxial test specimens are only subjected to a single tensile stress state, while in the SPC test the specimen experiences a multiaxial stress state. Results from small punch creep (SPC) tests are typically shown through time/displacement curves, which while appearing comparable to that of conventional uniaxial creep curves, in reality are quite different due to the differing creep mechanisms present in each of these test techniques. Reliability and repeatability of testing has been a strong focus within the research community, leading to the establishment of a European Code of Practice. When it comes to the application of the small punch creep test, there have been many reservations surrounding the repeatability and application of the small punch data itself. In the small punch creep test, the disc specimens typically have dimensions of just 8 to 9.5 mm for the diameter and 500 µm for the thickness. However, small punch testing allows for a mini-invasive assessment of the component requiring very little material and therefore less damage is incurred to the component in use. The traditional uniaxial creep test used to obtain mechanical data is often insufficient due to the large volumes of material that are required to produce test specimens, as these large volumes will in time undermine the integrity of these components in use. However, further potential was seen in small scale testing to analyze the residual life of key “in-service” components approaching the end of their life. The small punch creep method was originally developed in the 1980s with the idea of estimating the properties of irradiated materials in the power generation industry. Further, the random error associated with these conversions were dramatically increased. It was found that this latter approach enabled the accurate conversion of SPC minimum displacement rates to equivalent uniaxial minimum creep rates which, when combined with the Wilshire equations, enabled SPC test loads to be converted into equivalent uniaxial stresses (and visa versa) with levels of accuracy that were significantly reduced when compared to using the k sp method. This paper shows this to be the case for Waspaloy and proposes some alternative methods of correlation based on combining the Monkman–Grant relation and the Wilshire equations for both uniaxial and small punch creep. Recently, and for low chrome steels, this has been achieved through use of the k sp method, but there are good reasons for believing this technique will not work so well for Nickel-based super alloys. As such, the focus of this paper is to correlate the creep behavior of Waspaloy obtained through conventional uniaxial testing to that obtained via small punch creep testing. SPC testing can be a valuable tool for validating models of creep deformation, but the key to unlocking its full capability is through the accurate correlation of the creep material properties measured through both techniques. Small punch testing offers the potential for understanding creep behavior using much less material than conventional uniaxial testing but in contrast to uniaxial creep tests, the stress in small punch creep (SPC) tests is multiaxial. Within the aerospace sector, the understanding and prediction of creep strains for materials used in high-temperature applications, such as Nickel-based super alloys, is imperative. ![]()
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