Lucjan Witek

Crack Growth Simulation in the Compressor Blade Subjected to Vibration Using Boundary Element Method

This paper presents results of numerical crack propagation analysis of the compressor blades subjected to transverse vibrations. For stress intensity factor calculation in the half-elliptical crack, a dual boundary element method was used. In this analysis the automated remeshing procedure was used for creation of numerical models with a different crack size. Obtained results of numerical calculations were compared to results of experimental investigations performed for PZL-10W engine compressor blades tested in resonance conditions.

Fatigue Investigations of the Compressor Blades with Mechanical Defects

This paper presents results of experimental fatigue analysis of the compressor blades subjected to high cycle fatigue. The blades used in investigations were preliminary defected to simulate the foreign object damage. The blades during experiment were entered into transverse vibration. In results of investigations, the number of cycles to crack initiation and also the crack growth rate were obtained for the blade subjected to transverse vibrations. Moreover, observations of the beach marks on the blade fractures revealed two main shemes of crack propagation process. In this work, the finite element stress analysis of the blade was also performed. Obtrained numerical results shoved that stress level in the notch vicinity is 3 times higher than the stress in the blade without mechanical defects.

Stress Intensity Factor Calculations for the Compressor Blade with Half-Elliptical Surface Crack Using Raju-Newman Solution

Stress Intensity Factor Calculations for the Compressor Blade with Half-Elliptical Surface Crack Using Raju-Newman Solution This paper presents results of the stress intensity factor calculations for the compressor blade including a half-elliptical crack, subjected to vibration. In this analysis, the Raju-Newman empirical solution for stress intensity factor calculations in the rectangular plate with a half-elliptical flaw was used. The bending stress used in the Raju-Newman solution was computed for the real blade using the finite element method. The K-factor values were calculated only at one point of the crack front, where the crack tip contacts the free surface, because the crack length during experimental investigations was measured just in this direction. In order to determine the stress intensity factors for different crack sizes, ten diverse flaws in the blade were defined. Results of the experimental fatigue tests performed for the blade without preliminary defects showed that the cracks developed from the convex blade surface. On the blade fracture, the beach marks typical of the fatigue damage were visible. The dimensions of cracks in the rectangular plate were defined based on the beach marks shape. In the next part of the work, the stress intensity factor values were used as an input data into the Paris-Erdogan equation. As a result of this calculation, the crack growth rate for the compressor blade vibrating at constant amplitude was estimated. The results obtained were finally compared with the results of the experimental crack growth analysis performed for 1st stage compressor blades of the helicopter turbo-engine.

Numerical Simulation of Fatigue Fracture of the Turbine Disc

Abstract This paper presents the results of the crack propagation analysis of an aircraft engine turbine disc. In the first part of the work the finite element method was used for calculation of the stress state and the stress intensity factor (SIF, KI, K-factor) in the turbine disc with an embedded quarter-elliptical corner crack, subjected to low-cycle thermo-mechanical fatigue. To refine the K-factor calculation, specially degenerated finite elements were used. These elements provide stress singularity suitable for the linear-elastic material of the disc. The performed calculations yielded the stress intensity factor KI for different crack sizes. Subsequently, ΔK parameter was determined as a difference of the KI values calculated for the turbine’s speeds equal to 6373 and 14200 RPM. Based on the Paris-Erdogan equation and the obtained ΔK values, the fatigue crack growth plot for the turbine disc subjected to complex thermo-mechanical loads was determined.

Experimental Fatigue Analysis of Compressor Blades with Preliminary Defects

This work presents results of the experimental fatigue analysis of the compressor blades. In the investigations the blade with the V-notch (which simulates the foreign object damage) was considered. The notch was created by machining. The blades during the fatigue test were entered into transverse vibration. The crack propagation process was conducted in resonance conditions. During investigations both the amplitude of the blade tip displacement and also the crack length were monitored. As the results of presented investigations both the number of load cycles to crack initiation and also the crack growth dynamics in the compressor blade subjected to resonant vibrations were determined. In the work the influence of crack size on the resonant frequency was also investigated.

Fracture Problem of the Valve of Piston Engine

In this paper the fracture problem of exhaust valve of the piston engine was investigated. Visual inspection showed that on the fractured surface of the valve the beach marks, typical for fatigue failure were observed. The crack origin was not covered by corrosion products or material defects. In order to explain the reasons of damage of the valve, the non-linear finite element method was utilized. The numerical model composed of the poppet valve, the guide and the seat face was defined. In the analysis both the mechanical load resulting from the valve spring and also the thermal load arising from a non-uniform temperature field were defined. The loads were at first defined separately in order to check which load component has a dominant influence on the stress level. In third load case (which represents the operational thermo-mechanical engine conditions) the mentioned loads were defined simultaneously. The results of performed computations showed that the operational dynamic stress (in the critical zone of the valve where the crack appeared) is more than 12 times lower than the yield stress of the material. It means that the premature fatigue fracture of the valve was probably caused by any phenomenon concerned with the increase of the operational stress in the valve. The additional observation of the second (non-damaged) valve from the same engine head showed that the carbon deposit was located on the valve face. The results of the stress analysis of the valve with additional carbon particle showed, that in the valve stem a high bending stress was observed.

Influence of engine rotational speed on the natural frequencies of the turbine blade

In this work, the influence of the engine rotational speed on the natural frequencies of the turbine blade was investigated. In first part of the work the experimental modal analysis of the blade were made using the vibration system. The investigated blade was fixed to the movable head of the shaker. The blade amplitude was measured using the piezoelectric accelerometers. As results of experimental modal analysis, the resonant frequencies for first three modes of vibration were determined. In the next step of the work, the numerical models of the blades with different size of the finite elements were performed in order to check the convergence of the numerical solution. For each models the natural frequencies were determined for the non-rotated blade. The numerical results were next compared to the results of experimental modal analysis. After this comparison, one model only was selected for the further computations. In the last part of the work, the complex multi-steps analysis was performed in order to investigate the influence of turbine rotational speed on the modes and natural frequencies of the blade. Obtained results are important from both the research and also the practical point of view and have an influence on reliability of the engine.

Fatigue Analysis of the Compressor Blades with V- Notches

This paper presents results of experimental and numerical fatigue analysis of damaged compressor blades, subjected to vibration. The blades used in experimental investigations were preliminary defected to simulate the foreign object damage. The crack propagation process was conducted in resonance condition. During the fatigue investigations, the crack length and amplitude of the blade tip displacement were monitored. The main result of experimental test is the crack growth rate obtained for the blade including V-notch. In the second part of work, the results of the complex stress and fatigue analysis for the helicopter turbo-engine compressor blades were presented. A nonlinear finite element method was utilized to determine the stress state of the blade during the first mode of transverse vibration. In this analysis, the numerical models without defects and also with V-notches were defined. Obtained results were next used as an input data into crack initiation (e–N) analyses performed for the load time history equivalent to one cycle of the transverse vibration. As a result of e–N analysis, the number of load cycles to the first fatigue crack appearing in the compressor blade was obtained. Moreover, the influence of the blade vibration amplitude on the number of cycles to the crack initiation was analyzed. The numerical results were compared with the results of an experimental high-cycle fatigue (HCF) tests performed for the first stage compressor blades.

Experimental and Numerical Crack Initiation Analysis of the Compressor Blades Working in Resonance Conditions

Experimental and Numerical Crack Initiation Analysis of the Compressor Blades Working in Resonance Conditions This paper presents the results of a complex experimental and numerical crack initiation analysis of the helicopter turbo-engine compressor blades subjected to vibrations. A nonlinear finite element method was utilized to determine the stress state of the blade during the first mode of transverse vibration. In this analysis, the numerical models without defects as well as those with V-notches were defined. The quality of the numerical solution was checked by the convergence analysis. The obtained results were next used as an input data into crack initiation (ε-N) analyses performed for the load time history equivalent to one cycle of the transverse vibration. In the fatigue analysis, the different methods such as: Neuber elastic-plastic strain correction, linear damage summation and Palmgreen-Miner rule were utilized. As a result of ε-N analysis, the number of load cycles to the first fatigue crack appearing in the compressor blades was obtained. Moreover, the influence of the blade vibration amplitude on the number of cycles to the crack initiation was analyzed. Values of the fatigue properties of the blade material were calculated using the Baumel-Seeger and Muralidharan methods. The influence of both the notch radius and values of the UTS of the blade material on the fatigue behavior of the structure was also considered. In the last part of the work, the finite element results were compared with the results of experimental vibration HCF tests performed for the compressor blades.

Thermal Fatigue Problems of Turbine Casing

Thermal Fatigue Problems of Turbine Casing This paper presents numerical stress analysis of the turbine casing of an aero-engine. To solve the problem, the geometrically complicated numerical model was created. The finite element method was used in computations. In results of nonlinear static analyses performed for both mechanical and thermal load occurred under operating condition of engine, the stress and deformation contours were generated. High thermal stress gradients were found at the region of casing where fatigue cracks were detected during engine operation.