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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

account the compliance

account the compliance of the test system. The support distances were 20 mm and 40 mm in accordance with EN 843-1 (167) . The probability of failure, Pf , of the single fracture stresses σ was assumed to follow the two parameter Weibull distribution as: Pf m ⎛ ⎞ ⎜ ⎛ σ ⎞ = 1 − exp − ⎟ ⎜ ⎜ ⎟ Equation 45 ⎟ ⎝ ⎝σ o ⎠ ⎠ where σ0 is the characteristic stress at which 1/exp of specimens survived, and m the Weibull modulus. The latter is a measure of the distribution of flaws. It is a dimensionless number corresponding to the variability in measured strength and reflects the distribution of flaws in the material. A high Weibull modulus indicates little variation from sample to sample. Both m and σ0 were calculated using the maximum likelihood method in accordance with EN 843־5 (168) . The fracture toughness was evaluated using single edge V-notched bars (SEVNB) as described by Kübler et al. (169) . The mechanical tests were conducted by the Fraunhofer Institut für Werkstoffmechanik in Freiburg, Germany. 3.5.6. Tribological test The model system investigated for tribological wear consisted of sliding a hardened steel cylinder (rc=6 mm, lc=22 mm) on a polished MMC sample with a diameter of 24 mm and a thickness of 7.9 mm. Standard motor oil (SAE 15-W40) was chosen as the tribological medium between the two materials. The translational-reverse movement of the cylinder was conducted on a SRV III-Optimol tribometer. A schematic of the test set-up is shown in Figure 3.14 b). F E N P H = 0.418 Equation 46 rc lc The normal force FN was kept constant at 100 N resulting in an initial Hertzian pressure PH of 100 MPa in accordance to Equation 46 (170) . The calculations were based on the elastic 89

modulus Edyn of the unreinforced alloy IS, which was reported to be 73 GPa (1) . The amplitude of the 30 minute test was 2 mm at a frequency of 20 Hz and a test sample temperature of 150°C. Friction coefficient Sliding distance / m Figure 3.14 a) Typical graph of friction coefficient as a function of sliding distance. b) Schematic of friction piaring and wear path width on MMC sample. The friction coefficient was calculated as the ratio between the normal force FN and the friction force FR. It was recorded continuously as shown in the graph in Figure 3.14 a). The characteristic friction coefficient, f50m , was calculated as the friction coefficient of linear regression at 50m sliding distance. As a single parameter of wear, the mean width of the wear path, ww, was measured with an optical microscope. The wear path microstructures were investigated using a LEO Gemini 1525 SEM. 90

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