Effect of Surface Roughness on Determination of Creep Parameters using Impression Creep Technique
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1224-GG05-12
Effect of Surface Roughness on Determination of Creep Parameters using Impression Creep Technique Wuzhu Yan, B. Zhao and Zhufeng Yue Department of Engineering Mechanics, Northwestern Polytechnical University, Xi’an, 710129, PR China ABSTRACT The indentation creep test, especially the impression creep, exhibits a magic appealing in the determination of creep properties of small structures in industry for its simplicity, efficiency and non-destruction merits. Most of previous researches of indentation or impression creep neglect the effect of surface roughness of materials, which plays a crucial role in extracting creep properties of materials. The FE results showed that the surface roughness has no effect on the determination of creep exponent when the punching stress is larger than 150MPa. However, under a smaller punching stress the stress exponent is decreased due to the “Tuner” effect of asperities. The conclusions drawn in the present study provide an important guidance on experiment results amendment for impression creep technique. INTRODUCTION One of the crucial aims of the impression creep test is to bridge it and uni-axial tensile/compression creep test in the determination of creep parameters of materials [1-2]. The impression creep law of steady stage which is analogous with Norton law in format was exploited by Yu and Li [3] and is n' h& = C 'σ m
(1)
where h& is the steady state punch velocity, n’ is the stress exponent of impression creep, σm is the applied punching stress, and C’ is the temperature and material dependent pre-exponential factor. However, most of investigations on impression creep including tests and numerical simulations neglected influence of surface roughness. In the present study, the impression creep performed on a solid with a rough surface is investigated by numerical simulations to explore how the surface roughness works on determination of creep parameters. FINITE ELEMENT MODEL
A solid pressed by a flat cylindrical punch with radius r can be simplified into a two-dimensional axial symmetric for the finite element calculation of impression creep. A uniform punching stress σm is acted on the tested material via the punch. The cosine surface profile is used to exhibits fractal behavior of surface in the simulation. In this paper, the surface roughness was quantified by:
l
Ra =
1 y ( x) dx l ∫0
(2)
Where y(x) is the height of the surface profile based on the centerline, l is the concerned length of the sample. Keeping the wave length fixed, the amplitude of the cosine curve was varied to generate Ra=0, 6.4, 12.7 and 25.5 µm respectively [4]. The finite element model with partially refined (at peaks and valleys) in the zone beneath the punch is designed as shown in Fig. 1.
Fig. 1. Schematic of cosine profile surface and mesh design used in the numerical simulation of impression creep.
The typical structure steel mechanical properties is selected as a representation with Young’s Modulus E=138GPa, Poisson’s ratio µ =0.3. The Katchanov-Robotnov (K-R) model is emp
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