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