J. Byrne

Influence of LCF overloads on combined HCF/LCF crack growth

Abstract Fatigue crack growth rates have been studied in forged Ti–6Al–4V aero-engine disc material under the conjoint action of LCF and HCF cycles at room temperature. An overload has been introduced into the LCF cycle component of the test sequence, which has been based on a ratio of HCF to LCF cycles of 1000:1. Systematic increases in the overload, applied prior to the commencement of the HCF cycles, clearly demonstrated a diminution in the contribution of the HCF cycles to crack growth rates. Accompanying the reduction in crack growth rates is an increase in the stress intensity range at which the HCF cycles begin to contribute to the crack growth rate. For three HCF cycle stress ratios (R=0.7, 0.8 and 0.9), the severity of the overload required to negate the effect of the HCF cycles has been determined. Additionally, threshold values for the same stress ratios have been found and used to predict the onset of HCF cycle contribution to crack growth, and the effect of multiple vs. single overloads has been investigated. Modelling to predict crack growth rates and the onset of minor cycle damage has been undertaken.

Characterization of Powder Metallurgy (PM) Nickel Base Superalloys for Aeronautical Applications

During the last decade, some major improvements have been achieved concerning the evaluation of new types of materials suitable for aeronautical components exposed to severe operational conditions, such as turbine disks. Due to their outstanding mechanical properties, nickel base superalloys assumed a preferential position when compared with other conventional metallic alloys, benefiting from both their superior fatigue strength and high temperature behaviour. However, these alloys evince a high sensibility concerning possible defects that can arise due to certain types of loading, such as porosities and cavities associated with creep-fatigue at high temperatures. The present paper compiles some experimental results obtained for two types of recent nickel base superalloys. Some fatigue tests were performed using two configurations of these materials: a set of Udimet 720Li specimens (CT geometry) and a set of RR1000 specimens (CN geometry). A maximum temperature of 650°C was considered in both types of materials. The mechanical properties of these alloys were inferred via typical FCGR parameters, such as da/dN vs K curves, complemented with detailed analyses of the cracking mechanisms based on SEM observations. Finally, some metallographic characterization tests were carried out in order to determine the average grain size of these PM alloys and to confirm the presence of important microstructural constituents that can influence the overall fatigue performance of these materials.

The Ability of Current Models for Fatigue Life Prediction of Shot Peened Components

It is well known that shot peening has a marked benefit on fatigue life for the majority of applications. This effect is attributed mainly due to the compressive residual stress state at the component’s surface due to shot peening. The present paper evaluates the ability of several fatigue life prediction models, commonly used for general analyses, to predict the behaviour of components with compressive residual stress due to shot peening. Advanced elastic-plastic finite element analyses were carried out in order to obtain stress, strain, strain energy and fracture mechanics parameters for cracks within a compressive residual stress field. With these results several total fatigue life prediction models (including critical distance methods) and fracture mechanics based models were applied in order to predict fatigue life. Fatigue life predictions were compared with several experimental fatigue tests carried out on specimens, representative of a critical region of a compressor disc in a gas turbine aero engine. The results obtained showed that total fatigue life methods, even if combined with critical distance methods, give conservative results when shot peening is considered. Fatigue life was successfully predicted using the method proposed by Cameron and Smith, by adding initiation life to crack propagation life. This last method was also successfully applied for the prediction of non-propagating cracks that were observed during the experimental tests.

Fatigue Behaviour of Scratch Damaged Shot Peened Specimens at Elevated Temperature

It is clearly shown that surface condition has a strong effect on fatigue life. 80–90% of total high cycle fatigue life is taken up by crack nucleation/initiation at the surface. Shot peening is a quite successful surface treatment process for extending the service life of a large number of components. The benefit is created by the compressive stress field at the surface and a limited effect of cold work, which has advantages in reducing the likelihood of crack formation, Webster et al. [1]. However shot peening only introduces a very thin layer of compressive residual stress, in the order of hundreds of micrometres. Below the compressive layer near the surface is an elastic region in a tension state to achieve equilibrium and not cold worked, which may have a detrimental effect on fatigue life if a scratch created in the treated surface is bigger than the compressive layer, Burgess et al. [2]. Little information is available about the influence of small scratches on a shot peened surface and how it will affect the fatigue life of a component.

Residual Stress Analysis Around Foreign Object Damage Using Synchrotron Diffraction

The current study compares the residual strain around foreign object damage (FOD), measured using synchrotron diffraction, to the strain predicted by a plastic model with power-law dependence. It is shown that the measured strains are significantly lower than those predicted by the model. This may be explained in part, by the inability of the model to account for damage mechanisms such as micro-cracking and shear band formation.

Fatigue life prediction of scratch damaged shot peened components

Purpose – This paper aims to present a methodology, based on traditional approaches, to predict the fatigue life and non‐propagating cracks of shot peened components and the damaging effect of a scratch created over the treated surface.Design/methodology/approach – The finite element method is used to determine the actual strain at surface and fracture mechanics parameters calculated from cracks at the surface. The model considers residual stress (in order to introduce the effect of shot peening) and the scratch geometry. The total fatigue life is obtained by adding initiation life, to early and long crack propagation life using appropriate criteria.Findings – Numerical predictions were compared with previous experimental tests, showing that this method is quite reliable for predicting both fatigue life and non‐propagating cracks of shot peened components, including the effect of damage due to a scratch.Research limitations/implications – The proposed method provides good results and a clear understanding...