5 | Discussion
Surface condition such as different processing methods35 or surface roughness36,37 has a
critical effect on fatigue strength and fatigue life. It should be
pointed out that in most cases it is the micro scratch that finally
affects fatigue behavior among different processing methods or surface
roughness. Micro scratch is one of the most concerned problems in
structure service.\(\ \sqrt{\text{area}}\) brought Murakami theory
into micro dimension level, which aimed at actual micro scratch in order
to provide an effective parameter for fatigue damage estimation.
To examine fatigue strength of component with micro scratches, the depth
and width of detectable scratches should be collected. Fatigue damage
parameter \(\sqrt{\text{area}}\) can be calculated using Equation 1.
Combined with Vickers hardness of material, conditional fatigue strength
can be achieved by Equation 4. The accuracy of \(\sqrt{\text{area}}\)in conditional fatigue strength prediction was checked by FV520B-I,
Ti-6Al-4V and EA4T steel using Equation 4.16,18Prediction error is lower than 10%.
Classic fatigue-life curve method can provide essential parameters of
material fatigue properties for infinite fatigue life design. Obviously,
the presence of micro scratch can reduce fatigue properties including
fatigue strength and fatigue life. \(\sqrt{\text{area}}\) can be
applied to describe fatigue damage caused by micro scratch
quantitatively. The relationship between fatigue life and stress
amplitude affected by micro scratch can be revealed by modify fatigue
strength using \(\sqrt{\text{area}}\). According to Fig. 5,
acceptable result was also achieved combining with three-parameter model
and \(\sqrt{\text{area}}\).
Crack growth analysis in fracture mechanics is mainly adopted for damage
tolerance design. Fatigue life of component with initial defect
generated by manufacturing or assembly process can be assessed using
fatigue crack growth rate \(\frac{\text{da}}{\text{dN}}\). A problem in
the fracture mechanics-based life prediction is to determine the
equivalent initial defect size \(a_{0}\) for crack growth analysis.
Because of the presence of micro scratch, the initiation and propagation
life of small cracks can be ignored. In such a situation,\(\sqrt{\text{area}}\) provide another method to estimate equivalent
initial defect size \(a_{0}\) for component with micro scratch.
Reasonable fatigue life result was examined its ability in the
combination with the fracture mechanics and crack growth analysis of
TC17 titanium alloy18.
Since a new fatigue damage parameter for micro scratch is obtained, more
attempts can be made to combine \(\sqrt{\text{area}}\) with the
existing fatigue analysis theories to study the fatigue behavior
affected by micro scratch. These results support the validity of\(\sqrt{\text{area}}\) in fatigue analysis, even though it is evident
that more work has to be done in this area to more deeply investigate
its accuracy and reliability.
Facing with the characteristics of random length and direction of micro
scratch, \(\sqrt{\text{area}}\) highlights the effect of depth and
width on fatigue damage estimation. This is proved to be useful and
convenient to service safety concerned with surface condition. To a
certain extent, it can facilitate the engineering application in dealing
with fatigue problem affected by micro scratch.
Due to the small size at micron level, there may be some obstacles in
measuring the actual dept and width of micro scratch. To a certain
extent, optical profiler instrument can reflect the actual geometric
parameters of micro scratch. As a traditional and widely method, stylus
method can present an alternative method to determine the approximate
size scratch. Other measuring methods such as 3D mapping
technology38,39 and powder filling
method11 are also recommend for micro scratch to
guarantee prediction accuracy. Multiple scratch on the surface of
component is general situation in engineering practice, the principle of
measuring the maximum scratch and cross-section should be followed