Fatigue life prediction of TC17 titanium alloy based on micro scratch

Significance 

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.

Fatigue life prediction of TC17 titanium alloy based on micro scratch - Advances in Engineering

About the author

Mr. Mingchao Ding is currently a Ph.D. candidate in the School of Mechanical Engineering, Dalian University of Technology, China. He received his Bachelor’s degree from Dalian Jiaotong University and Master’s degree in from Shandong JiaoTong University. His research is mainly focused on fatigue performance analysis and remanufacturability evaluation technology of mechanical parts.

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About the author

Dr. Yuanliang Zhang is a Professor of the School of Mechanical Engineering, Dalian University of Technology, China. He received the PhD degree from the Dalian University of Technology, in 1998. Zhang’s scientific research areas include intelligent control, single chip microcomputer application and measurement and control technology, fatigue life prediction and remanufacturability evaluation technology of mechanical parts, nonlinear ultrasonic flaw detection, etc. Dr. Zhang has published 100+ articles and undertakes multiple national scientific research projects.

About the author

Dr. Huitian Lu is a Professor with the Department of Construction and Operations Management, South Dakota State University. He received his MS and Ph.D. degrees from Texas Tech University (TTU) in 1992 and 1998 respectively. He has more than 20 years of university work and research experience in system modeling, dynamics, statistical quality control, reliability modeling, decision theory, etc.

His current research interests include systems remaining useful life (RUL) modeling (theory and method), online assessment, and applications in solid-state battery and biomedical complex systems, and management science. Dr. Lu has published numerous academic articles in the field.

Reference

Ding, M., Zhang, Y., & Lu, H. (2020). Fatigue life prediction of TC17 titanium alloy based on micro scratchInternational Journal of Fatigue, 139, 105793.

Go To International Journal of Fatigue

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