Micro mechanical scratches, though preventable, are common occurrences on material surfaces. They are a result of manufacturing or assembly processes due to wrong or unintentional operations. For operations at extreme conditions such as that for compressor blades used in aircraft engines, these scratches may lead to early fatigue cracks that are detrimental to such operations. Micro scratches significantly reduce the fatigue life of materials under tensile and bending loadings due to multiple crack nucleation in the affected areas. This presents a great obstacle to practical applications of different materials in extreme conditions and, therefore, an urgent solution is necessary.
Extensive research has been conducted to study the location and early-stage growth of fatigue cracks. There are numerous methods for determining the fatigue life of structures, including a combination of continuum damage mechanics and a dynamic analysis of scratch generation. The early-stage growth of fatigue cracks is mainly controlled by the geometry of the grinding structures. Among the geometric characteristics of scratch, scratch depth has been identified as the most critical and exhibits a non-linear relationship with fatigue life. Additionally, finite element analysis has been incorporated in the study of fatigue performance to obtain actual and accurate geometric characteristics of surface defects like surface roughness. Unfortunately, obtaining exact measurements of the actual scratches has remained a challenge due to the scratch sections’ irregularity and limited resolutions of the measurement methods.
Artificial scratches with specially designed angles or tip radius have been developed to study the effects of surface defects on fatigue performance. Nevertheless, despite the significant progress in using artificial scratch to control different geometric parameters like scratch angle, width and depth, the study on the fatigue behaviors from the natural scratch perspective are limited. Additionally, very few methods for modeling the effects of micro scratch-induced fatigue damage in titanium alloys are available. To address these challenges, Mingchao Ding (PhD candidate) and Professor Yuanliang Zhang from the Dalian University of Technology, in collaboration with Professor Huitian Lu from the South Dakota State University, presented a feasible method for predicting the fatigue life of TC17 titanium alloys from micro scratch perspective. The work is currently published in the International Journal of Fatigue.
In their approach, the authors developed an improved analytical model of fatigue life from a refined fatigue damage parameter for the micro scratch. First, the geometric characteristics and vital parameters of the micro scratch were studied and measured using a three-dimensional (3D) optical profiler instrument. Both scratched and smoothed specimens were used for the fatigue experiment to determine the effects of scratches. The fatigue damage is expressed using a new parameter based on the Murakami theory. Through a combination of the Murakami theory, maximum stress intensity factor and Paris formula, the fatigue life model for TC17 titanium alloy was proposed and verified.
The authors found that the micro scratches significantly reduced fatigue life from very high cycle fatigue (VHCF) to high cycle fatigue (HCF) regime. This indicated that the fatigue life of TC17 alloys was highly sensitive to the surface conditions. Based on the geometrical characteristics of the micro scratch, the two principles established on Murakami theory exhibited no obvious effects on the fatigue life and scratch direction. The newly proposed parameter for describing the fatigue damage caused by micro scratches proved accurate, robust and reliable.
In summary, the study investigated the effect of micromechanical scratch on the fatigue life of TC17 titanium alloys. The proposed HCF life model of the TC17 titanium alloys, considering the micro scratch effects, enabled the researchers to gain more insights into the different factors affecting the fatigue performance, such as surface conditions. Moreover, the predicted results agreed well with the experimental data. In a statement to Advances in Engineering, the authors explained their study enables compressive fatigue life prediction of different materials, thereby increasing their application in extreme conditions.
Ding, M., Zhang, Y., & Lu, H. (2020). Fatigue life prediction of TC17 titanium alloy based on micro scratch. International Journal of Fatigue, 139, 105793.