Fatigue life prediction of TC17 titanium alloy based on micro scratch
Introduction
Compressor blade is a crucial part in modern aircraft engines that subjects to high centrifugal and aerodynamic force at a very high rotating speed. Even though high surface quality of blade is carefully controlled in engineering manufacturing, the random mechanical scratches in micron scale will exist on the part surface. This scratch has large dimensions along the surface, and a depth of a few micrometers with a length of potentially several millimeters. These micro mechanical scratches may be generated by manufacturing or assembly process due to unintentional or wrong operation generally or by in-service damage. Operation at extreme conditions, micro scratches are sufficient to induce early initiation and propagation of fatigue cracks, leading to premature fatigue fracture of blades [1], [2].
The location and the growth of fatigue cracks in the early stages are controlled by the presence and the geometry of grinding scratches [3]. Cracks initiated from scratches were typical short cracks, growing faster than conventional long cracks. Scratches reduced aluminium fatigue life under tensile and bending load up to 97.8% due to multiple crack nucleation [4]. Fatigue life decreased sharply as the scratch depth increased [5]. The fatigue life of scratched structures can be predicted using dynamic analysis of scratch generation combined with the continuum damage mechanics based fatigue damage model [6]. A new method to calculate the fatigue life and defect tolerance for a30CrMnSiA steel specimen with artificial scratches was proposed [7].
Geometric characteristics of scratch such as open angle, tip radius and depth have significant impact on the fatigue life [8]. Among these factors, scratch depth can be the most critical one [5]. Early in 1957, Siebel [9] found that fatigue strength reduced linearly with the increasing of groove depth, but only happened above a critical depth. Fatigue life decreased sharply as the scratch depth increased. Elastic stress concentration Kt and fatigue notch factor Kf is predominantly influenced by root radius according to Neuber equation [10] and Peterson equation [11]. Surface scratch can be treated as a form of micro notch and its Kt can approach to at least 15 [12]. For specimen with circumferential scratches, polishing can extend fatigue life [13]. However, this would not happen to longitudinal scratches, even with larger size. In order to reflect the actual geometric characteristics of surface defect such as pores [14], micro-stiffener topography [15], machining defects [16], surface roughness [17], [18], finite element analysis was widely used to study fatigue performance.
Unfortunately, exact measurements such as open angle and tip radius may not be achieved conveniently from the actual scratch due to the limited resolution of the profilometer and the irregularity of scratch section. Thus, artificial scratches with kinds of designed angle or tip radius are widely used to study its effect on fatigue performance.
In the study of fatigue performance affected by surface defect, Murakami theory has been widely recognized. Murakami et al. provided a quantitative estimation of fatigue damage from surface defect using [19], which is defined as the square root of the area obtained by projecting a surface defect onto a plane perpendicular to the maximum principal stress, as shown in Fig. 1a [20]. In Fig. 1b, artificial defects with a regular section shape such as V-notches, cuboid or semi-circular slit were introduced in Murakami‘s study [21], which can evaluate the area conveniently beforehand. is a parameter of pure projected geometric area of surface defect [22]. It can be conducted that it is the projected area that determines fatigue damage for surface defects with various geometric shape. Takahashi [23] studied the fatigue strength of artificial surface roughness simulated by micro circumferential notch with 30 μm depth, and provided a semi-empirical formula of . Xu [24] estimated the fatigue limit curve for high-speed railway axles with surface scratch using Murakami theory as well.
It can be summarized that artificial scratch was widely adopted in experiments to control the necessary geometric parameters such as depth, width, scratch angle and scratch root radius of the scratches beforehand [25], [26], [27]. However, little work has been performed on the study of fatigue behavior from the perspective of natural micro scratches. There are few methods have been reported to express fatigue damage caused by natural micro scratch.
This study aims to establish an improved analytical model of fatigue life by redefining a fatigue damage parameter for micro scratch. First, the geometric characteristics and necessary geometric parameters of micro scratch, such as the width and depth, is studied and measured by ZYGO 3D optical profiler instrument. To figure out the effect of scratches on fatigue life of TC17, the scratched specimens and smooth ones were prepared for fatigue experiment. The fracture characteristics were observed by SEM (Scanning Electron Microscope). Fatigue damage caused by micro scratch is quantitatively expressed by a new parameter based on Murakami theory, marked as , which is defined as the square root of triangle area of micro scratch. Combined with Paris formula and the maximum stress intensity factor modified by , fatigue life model for TC17 is proposed. The model is verified being satisfactory to the fatigue life prediction of TC17 in the lab experiment.
Section snippets
Material and experiment
TC17 titanium alloy has a high strength to weight ratio, high toughness, and good corrosion and creep resistance, which explains as a potential candidate of materials to manufacture the dual-property blisk [28], [29], [30]. The microstructure consists of primary α grains and acicular α + β colonies. The chemical composition and the mechanical properties of TC17 are given in Table 1, Table 2.
The specimen used in the experiment is the hourglass type with a minimum diameter of 3 mm, a radius
Geometric characteristics of micro scratch
The scratch can be considered as micro notch. The stress concentration phenomenon usually occurs in these discontinuous geometric features. It can be deduced that these scratches would be a crucial factor that induces fatigue failure in this experiment. Thus, in order to estimate fatigue damage caused by micro scratch, geometric characteristics should be clarified first.
ZYGO 3D optical profiler instrument owns numerous advantages such as high precise, non-contact surface measurement. What is
Experiment results
The distribution of stress amplitude and fatigue life is portrayed in Fig. 6, including the data of the scratched and smooth specimens. “Run out” represents to stop loading if fatigue life is over 109.
For the smooth specimens without obvious scratches, the distribution of fatigue life can range from high cycle fatigue regime to very high cycle fatigue regime. However, fatigue life mainly concentrates in the HCF regime () for the scratched specimens. An approximation line showing
Fatigue life prediction considering micro scratch
As discussed above, micro scratches have a preponderant influence on fatigue life of TC17. Therefore, establishing a fatigue damage parameter for micro scratch and having the analytical relationship between micro scratch and HCF life of TC17 is crucial.
Conclusions
The effect of micro mechanical scratch on fatigue life of TC17 is investigated in this study. Fatigue experiments on the scratched and smooth specimens were carried out to analyze the fatigue performance of TC17. A new parameter expressing fatigue damage caused by micro mechanical scratch quantitatively is first proposed based on Murakami theory. The following conclusions are drawn:
- (1)
The fatigue failure mode for TC17 was that surface failure without facets in this experiment. The fatigue life of
Declaration of Competing Interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Fatigue life prediction of TC17 titanium alloy considering micro scratch” (ID: IJFATIGUE-D-20-00264)”.
Acknowledgements
The authors gratefully acknowledge The National Natural Science Foundation of China (No. 51875082). Also gratefully acknowledge the Fundamental Research Funds for the Central Universities (DUT17ZD230) and Key Research and Development Projects of Ningxia Hui Autonomous Region (2018BDE02045) of China.
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