Elsevier

Scripta Materialia

Volume 48, Issue 12, June 2003, Pages 1637-1642
Scripta Materialia

Dual fatigue failure modes in Ti–6Al–2Sn–4Zr–6Mo and consequences on probabilistic life prediction

https://doi.org/10.1016/S1359-6462(03)00132-5Get rights and content

Abstract

The variability in fatigue life of the Ti–6Al–2Sn–4Zr–6Mo (Ti-6-2-4-6) alloy was investigated. Cumulative life distribution plots were found to be composed of two failure mechanisms. The data could be closely represented by a cumulative distribution function (CDF) resulting from the superposition of the CDFs of the individual mechanisms. An approach for life prediction based on the data due to the worst-case mechanism is suggested.

Introduction

The current trend in the aircraft industry towards extending lives of in-service components beyond their initial design life [1], [2] has generated considerable interest in a more accurate assessment of life and its variability. One approach in life assessment of fracture-critical turbine engine components is based on life for “initiation” of a crack in one out of thousand components. Thus many components with considerable residual life may be prematurely discarded. Conventionally, this type of approach involves empirical extrapolation of data. The source of variability remains unclear in such approach. However, considerable potential for life extension appears to exist through a more physically based assessment of variability.

Fatigue life variability is most commonly described by the lognormal probability density function (PDF) [1], [3], [4], [5], [6], [7]. There have been attempts to track the variability in life through the size distribution of the relevant microstructural feature [1], [3]. In these cases [1], [3], the variability in life was thought to be controlled by the crack growth life. The life calculations were, therefore, based on crack growth from a size equivalent to the relevant microstructural feature until fracture [1], [3]. In all these studies [1], [3], [4], [5], [6], [7], however, the variability in life was controlled by a single failure mechanism, and the probability-of-failure (POF) plot could be represented by a single cumulative distribution function (CDF).

This paper discusses results from our research on the variability in the fatigue life of Ti-6-2-4-6, a high strength α + β titanium alloy. The POF (or the CDF) plots were found to have a step-like shape at certain stress levels, and the data could not be described by a single CDF. The shape of the CDF was shown to result from a superposition of two types of failure mechanisms. Such behavior of a CDF has been previously reported [8], [9], although these studies were not related to fatigue. Nevertheless, the existence of two mechanisms as causing the step-like behavior of the CDF was suggested [8], [9]. The objective of this paper is not to discuss the microstructural source of life variability. It is, however, intended as a step towards a mechanism-based assessment of variability. It is demonstrated that a mechanism-based evaluation of the CDF has a significant potential for life extension.

Section snippets

Material and experimental procedure

The microstructure of the Ti-6-2-4-6 alloy used in this study is presented in Fig. 1. As shown in the figure, the microstructure had a duplex nature and consisted of equiaxed primary-α particles and α platelets in a transformed β matrix. Testing was performed on round-bar stress-life (S–N) fatigue specimens with a uniform gage section of length 12 mm and diameter of 4 mm. All fatigue specimens were machined from the same parent disk forging, which exhibited a highly uniform microstructure. The

Variability in total life

The fatigue life behavior of the Ti-6-2-4-6 alloy is shown in Fig. 2(a). Only the mean lives are plotted in this figure. Multiple tests were conducted at selected stress levels as indicated by numbers in parentheses (Fig. 2(a)). The variability in life is shown in Fig. 2(b). As shown in the figure, decreasing the stress level, σmax, produced an increase in the average fatigue life, but this was accompanied by a substantial increase in the variability in life. While most failures occurred by

Dual failure mechanisms

For clarity, the POF plots at the lower stress levels (σmax=900, 860 and 820 MPa respectively) are plotted separately and presented in Fig. 4(a–c). In each case, a step-like shape of the CDF was observed. At the higher stress level (σmax=1040 MPa), however, the data were well represented by a single line (Fig. 3). The step-like behavior of CDF at lower stress levels indicated a grouping of data into two failure populations and pointed towards the possibility of two distinct failure mechanisms.

Conclusions

Based on the current research on the fatigue life variability of Ti-6-2-4-6, the following conclusions can be drawn:

  • (i)

    The variability in fatigue lives of Ti-6-2-4-6 increased with decreasing stress amplitude.

  • (ii)

    Two types of failure mechanisms were identified based on the step-like morphology of experimental data on the CDF plot. The mean lives of the two types of failures differed by two orders of magnitude. Further, the likelihood of one type of failure vs. the other shifted with stress level.

  • (iii)

    A

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