Impact of Foreign Object Damage to Leading Edge on Fatigue Capability of CMC Turbine Vanes

Ceramic matrix composites (CMCs) are expected to make turbine components lighter because they have lower density than nickel-based super alloys which have been used for turbine components. Also they are expected to improve the efficiency of gas turbine engines by realizing higher turbine inlet temperature (TIT), because they have higher temperature capability. One of the technical issues of CMCs is that they have relatively low impact resistance comparing with nickel-based alloy. In the previous work, it was estimated that the foreign object damage (FOD) on CMC turbine vanes under the actual engine condition is less than 0.36-mm-depth. The concern of such damaged CMC turbine vanes is a decrease in a fatigue capability due to the dent particular to the leading edge (L/E). The fatigue strength reduction can be caused by the stress concentration due to the dented shape and the oxidation due to the coating spallation. There are various works about the impact resistance or the fatigue capability of CMCs, but there are little works which assess the fatigue capability of CMCs which was damaged by foreign object. The objective of this work was to clarify the impact of FOD to L/E on the fatigue capability of SiC/SiC CMC turbine vane. The tension-tension low cycle fatigue (LCF) tests and high cycle fatigue (HCF) tests were conducted using four types of test pieces at elevated temperature in the steam environment. The first one was smooth test piece with anti-oxidation coating, the second one was the test piece which had notch with coating, the third one was the test piece which had notch without coating at notched area and the fourth one was the test piece which was damaged by dropped weight test simulating FOD. The result showed that impact of stress concentration due to notch shape on fatigue capability is small. It was also cleared that uncoated condition did not have impact on the LCF capability but had impact on the HCF capability. The HCF strength reduction of TP without coating to the TP with coating was about 35% at 107 cycles. More fiber breakage was observed at the fracture surface of the TP without coating tested in HCF condition, on the other hand more fiber pullout was observed at that of other TPs. The results suggest that the HCF strength reduction was caused by oxidation when the bare CMC was exposed by FOD. From this work, it was concluded that the loss of coating due to FOD impacted on HCF strength of CMC turbine vane, but it can be accepted because turbine vane is normally designed so that it can endure the stress concentration at L/E due to FOD (typically kt = 3).

Some Aspects of Operated Blades HCF Analysis: Rejuvenation and Life Estimation

The lifecycle of modern industrial gas turbines can reach hundred thousand hours and usually the turbine blades need to be replaced. The use of super alloys and application of advanced coatings makes the cost of turbine lifecycle rather high. The methods for blade rejuvenation and life extension are based on the analysis of the main defects which can considerably reduce blade strength. The effect of long operation and typical defects in turbine blades has been studied in correlation with HCF. The decrease of blades HCF under the effect of operation has been considered as the result of influence of mechanical and thermal factors. The influence of FOD on the blade HCF strength is studied. Some random defects in turbine blades which resulted in HCF decreasing and blade failure are considered. The rejuvenation heat treatment for the blades of ZhS6K and EI893 and its positive effect on metal properties is demonstrated. The ultrasonic shot peening for operated blades have been considered. It is demonstrated that HCF strength of blades after shot peening is about 25–30% higher. Relaxation of compressing stresses in operation is shown as not essential. The remaining life of operated blades can be estimated using the correlation of endurance limit and run time.

Mechanical Surface Treatment Technologies for Improving HCF Strength and Surface Roughness of Blisk-Rotors

The manufacturing of electron beam welded blade integrated disk (blisk) rotors sets new demands on mechanical surface treatment technologies. High durability and high efficiency of the blades are in general strict requirements for the component and have ideally to be increased by a mechanical surface treatment. The high complex 3D shape of the blades and the need to treat the blades on the rotor caused by a post weld heat treatment are additional challenges to solve. Limited clearance between the blades and low space between the individual blisk’s increase as well the requirements on the technology. Conventional technologies reach their limits and have to be improved. This paper gives an insight into the capabilities of different mechanical surface treatments regarding the treatment of blades on blisk-rotors. Compared with the benchmark shot peening, surface roughness, residual stress depth distribution and high cycle fatigue (HCF) are investigated on deep rolled and vibropeened specimen. Assets and drawbacks are shown and discussed.

Probabilistic HCF-Investigation of Compressor Blades

During the design process of compressor blades predominantly deterministic models are used for High Cycle Fatigue (HCF) strength investigations. The scatter of HCF that results e.g. through abrasion of the production machines [1] or inhomogeneities of the blade material, is accounted by safety factors and conservative assumptions. A more realistic approach to consider these uncertainties is the application of probabilistic methods. Therefore, further information about HCF and eigenfrequency scatter of the really produced blades can be used for a robust design during the design process. Within a measurement campaign 400 blades of a Rolls-Royce High Pressure Compressor were randomly selected and scanned using an automated process that applies the optical measurement technique of strip projection. The measurement data of the airfoil were subdivided into constant spanwise profile slices. Geometric airfoil parameters were determined on each of the profile slices [2]. Due to the large number of scanned blades each geometric airfoil parameter can be described as a distribution function with corresponding parameters. These distribution functions are the input parameters for the probabilistic investigation — the Monte-Carlo-Simulation (MCS). Within the MCS an automatically transfer process varies at first the profile slices of a CAD-airfoil and in a second step morphs an existent 3D finite element mesh applying the meshmorphing tool of the FE preprocessor Hypermesh. The HCF and eigenfrequency scatter of all blades were calculated with the interpretation of the MCS results and parameters were detected with the largest influence on HCF-strength and eigenfrequencies. A detailed interpretation of the HCF-strength at one example shows the power of the probabilistic investigation. The interpretation helps the engineer to understand the entire system and to design a robust blade.

Influence of Post-ECAP TMT on Mechanical Properties of the Wrought Magnesium Alloy AZ80

It is well-known that the high-cycle fatigue (HCF) performance of severe plastically deformed wrought magnesium alloys is not as good as one might expect from the significant grain size refinement. Although enhanced HCF strength after ECAP as compared to as-cast material was observed its value was significantly lower than after conventional extruding. The present investigation was undertaken to determine whether the relatively poor HCF strength of the ECAP processed wrought magnesium alloy AZ80 is associated with the ECAP-induced unfavorable crystallographic textures. Post-ECAP thermo-mechanical treatment (TMT) was found to result in favorable texture modifications as well as in markedly improved HCF performance. The proposed novel technique consists of a not yet used combination of severe plastic deformation via ECAP followed by a 1-step swaging process. It is shown that the resulting combination of both ultrafinegrain sized material and beneficial crystallographic texture results in superior HCF performance not achievable by ECAP-processing alone.

Simulated Foreign Object Damage on Blade Aerofoils: Real Damage Investigation

During operating service, gas turbine aero-engines can ingest small hard particles which typically produce damage to the aerofoils. If the damage found is a tear or a perforation at the leading edge, it is known as a Foreign Object Damage or FOD and this leads to a reduction of the subsequent High-Cycle-Fatigue (HCF) strength. The objective of research work in this area is to assess the effect of FOD on the residual fatigue strength of compressor blades and to provide predictive tools for engineering judgment. The methodology followed is normally to carry out experimental simulation of FOD, followed by fatigue tests to assess subsequent performance. To date, research related to fatigue following FOD events has concentrated on HCF loading and the impact geometry is frequently that of a sphere against a flat surface or the edge of a blade-like specimen. Both of these aspects do not correspond to the worst cases of real FOD. Here it is intended to investigate the effect of a V-notch geometry, which is more representative of severe FOD found in service. Alongside this, numerical models can be used to simulate the damage and to evaluate the residual stress field. In addition analytical model are used to predict the residual fatigue strength. The current work explains the development of a new rig impact test and discusses the improvements necessary to obtain a sufficient repeatability of the impacts. From the experience gained with a gas gun, an alternative method using a pistol and a barrel, capable of achieving the necessary velocity of simulated FOD, was developed. The applied velocity was in the range of 250m/s to 300m/s and a technique to describe the impact is here discussed. Furthermore the introduction of a high speed camera has allowed to have a complete description of the impact scene and to better understand the impact. The impacted blades were measured and HCF tested. As a result, this has produced a large scatter in the residual fatigue strength. The current method to describe a notch using a 2D approach, which was applied to several geometries of notches, is here critically reviewed. The proposed method would incorporate a more sophisticated method, which reconstruct the real geometry using optical measurement. This latter measurement can fully describe the 3D geometry, showing particularly zones inside the notch where compressive residual might appears. Tears and shear of the material can also be described by applying this technique. The findings are compared with the residual HCF strength and the results are compared to special cases of HCF to justify the results out of theoretical prediction.