Abstract
In this study the effects of surface finish on the fatigue life of 7449-T7651 aluminium alloy
were evaluated as a result of different surface roughness conditions polished in the longitudinal
direction of the specimens. The efficiency of the boundary parameters of surface roughness as
well as the surface roughness techniques was also investigated. The surface roughness and
residual stress were characterized using contact profilometry and x-ray diffraction techniques
respectively. The 7449-T7651 aluminium alloy round specimens were subjected to uniaxial
constant amplitude cyclic loading using a R-ratio of 0.1. Load controlled high cycle fatigue
testing was carried out at both low and high amplitudes covering four surface roughness
conditions.
The decrease in peak alternating stress from 360 MPa to 320 MPa showed a significant
increase in fatigue life, however at 360 MPa the effect of surface roughness and residual stress
on fatigue life was unfounded mainly due to the effect of adequate plastic strain amplitude
which reduces the crack initiation process and increase the crack growth process in relation to
total fatigue life.
Test results of the specimens, tested at 320 MPa indicated that a decrease of surface roughness
resulted in an increase in the fatigue life. The test results further showed that a surface
roughness more than 0.2 μm Ra has a detrimental effect on fatigue life and below 0.2 μm Ra the
roughness effect on fatigue life is less evident mainly because of lower external surface and
residual stress concentrations. The effectiveness of the arithmetic mean roughness (Ra) and the
mean peak-to-valley height (Rz) was found to be questionable in order to accurately detect the
maximum valley depth on the surface layer where microcrack nucleation is most likely to
initiate. The roughness measurements covering a small localized area of the total surface in the
longitudinal direction of the specimen was found to be inadequate and not representative
enough for the use of semi-empirical fatigue life assessment methods in order to calculate the
surface stress concentration factor Kt, which require the asperity root radius ρ of the surface
topography. Traditional stylus-based methods were not sensitive enough and it is advisable to
use optical methods like white light interferometry or perhaps tomography for surface
topography measurement which is significantly more accurate at 0.1nm resolution. For the
purpose of determining the effect of the surface layer on the fatigue crack initiation process it is
recommended that residual stress measurement to be conducted after final polishing and should
represent the full circumference of the gauge section of round specimens.
M.Tech. (Mechanical Engineering)