Lec 6-8 Creep

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CREEP AND FATIGUE : 

CREEP AND FATIGUE Wg Cdr Shakeel Safdar

Operating Conditions in a Gas Turbine Engine : 

Operating Conditions in a Gas Turbine Engine

Engine Materials : 

Engine Materials

Material Strength VS Temperature : 

Material Strength VS Temperature

Load representation for a turbine blade : 

5 Load representation for a turbine blade DOVE TAIL BLADE TIP L.E T.E

Temperature and Plastic Strain distribution in a Turbine Blade : 

Temperature and Plastic Strain distribution in a Turbine Blade

Shrouded Blade Configurations : 

Shrouded Blade Configurations Part span fixed shroud Part span slipping shroud Tip Shroud

Equivalent Stress distribution in a turbine blade : 

8 Equivalent Stress distribution in a turbine blade

Comparison of Stress distribution for Shrouded and Un-shrouded turbine blade : 

Comparison of Stress distribution for Shrouded and Un-shrouded turbine blade

Vibration modes of turbine blade : 

Vibration modes of turbine blade

Affected areas of Turbine disc : 

11 Affected areas of Turbine disc

Turbine blade in an Aero-Engine Experience : 

12 Turbine blade in an Aero-Engine Experience Mechanical Forces Creep Thermo-Mechanical Fatigue Thermal Fatigue High Temperature Environment Oxidation Hot corrosion Sulfidation

Turbine Blade Microstructure : 

13 Turbine Blade Microstructure

Creep Facts : 

Creep Facts Creep is a process of continued plastic deformation (change in component dimensions) which occurs at high temperatures as a result of the stresses developed in a component during operation and sustained over a period of time. For a constant stress, the operational time for a particular change in dimension is decreased with increasing component temperature. For a constant temperature, the time for a particular change in dimension is decreased with increased sustained stress. If the process of creep is allowed to continue unchecked, deformation becomes localized and the component ruptures (stress rupture). Turbine blade life is limited by creep deformation to a specified increase in blade length. It is not based on creep deformation to rupture. Creep failure (stress rupture) occurs at a blade location that coincides with maximum temperature and increasing mechanical stress – typically around mid span. Failure is preceded by localized plastic deformation (necking) and surface micro-cracking. Extensive micro-cracking is visible on the surface in this region. Microstructural examination would reveal voids adjacent to fracture. Some indicators of creep in the turbine blades are the loss of material at the trailing edge of blade tips, cracks in the trailing edge, necking at the trailing edge, and blade stretch

Dynamic/Fatigue Loading types : 

Dynamic/Fatigue Loading types

Mean Stress level : 

Mean Stress level Important factors are: – mean stress level – geometrical design – surface condition – metallurgical structure – environment

FATIGUE CATEGORIES : 

FATIGUE CATEGORIES MECHANICAL FATIGUE GENERALLY FALLS INTO TWO MAIN CATEGORIES Low Cycle Fatigue (LCF) High Cycle Fatigue (HCF) LOW CYCLE FATIGUE Alternating stresses developed in rotating components through the change in rotational speed. LCF is caused by the nature of flight operation. One Flight Cycle (e.g. start up, taxi, take-off, climb, cruise, descent, approach taxi, shut down) produces One Stress Cycle. The number of cycles to failure is usually in the range of 103-105. Easy to manage by keeping track of the number of cycles Low cycle fatigue cracks initiate in regions where the stresses developed through rotation are highest, eg the rims of rotors and turbine discs, and blade to disc connections. Crack propagation occurs on a plane perpendicular to the direction of maximum stress. HIGH CYCLE FATIGUE Mainly caused by vibration through variation in gas impulse loads, eventually resulting in resonance. Each vibration cycle constitutes a stress cycle. The number of cycles to failure is usually in the range of 107-109. The general location for high cycle fatigue in turbine blades with no tip shrouds is at the base of the blade. Though the number of cycles to failure appear high, but these can be reached very quickly e.g. If the stress level was high enough to cause failure in 107 cycles, then a vibration mode at 2500Hz would take only 4000 seconds ( approx 1 hr). The effect of the creation of structural resonance is to create bending stresses of high magnitude. The excitation of resonance in compressor turbine blades results in a mode 1 bending response. High cycle fatigue cracking develops at the location of the flexural node.

Strain VS Temperatureduring a typical flight cycle : 

Strain VS Temperatureduring a typical flight cycle

Thermal Fatigue : 

Thermal Fatigue Alternating stresses may be created when changes in gas temperature result in regions of a component (turbine blade/vane or combustion chamber) being heated or cooled at different rates to other regions of the component. In turbine terminology this mode of fatigue cracking is known as thermal fatigue. For a turbine blade, a rapid reduction in gas temperature results in the thinner trailing-edge of the blade cooling more rapidly than the thicker middle and leading-edge sections. The difference in cooling results in a difference in the extent of thermal contraction between the trailing edge and the remainder of the blade and creation of tensile stresses in the trailing edge. In the case of thermal fatigue, stress is no longer a function of predictable mechanical loading, but is a function of thermal gradients and local distortions in blade shape. Thermal fatigue generally start in regions of thinnest cross-section in regions where the greatest thermal strains occur. Thermal fatigue cracking is likely to follow irregular crack paths. Thermal fatigue failure is prevented by retiring a component after its exposure to a specified number of thermal cycles (engine start/stop cycles) and limiting the severity of thermal strains through operational techniques, such as, slow power changes.

High Cycle Fatigue (HCF) FACTS : 

High Cycle Fatigue (HCF) FACTS Fatigue failure is usually brittle in nature. Brittle fracture is catastrophic as it occurs without prior plastic deformation. The final fracture occurs at very high speed which is in the range of 300 m/sec. High cycle fatigue is very undesirable as it occurs very suddenly and is very difficult to monitor.

Operational and Financial Implications of HCF : 

Operational and Financial Implications of HCF U.S Military have reported HCF to be major issue affecting operational readiness and considers HCF to be responsible for as much as 50% of all Engine Failures. USAF AND US NAVY estimate that HCF related problems cost approximately $400 million/year. U.S Military have reported that 87% OF THE MAINTENANCE EFFORT ON THE U.S MILITARY AIRCRAFT FLEET IS AS A DIRECT RESULT OF HCF PROBLEM OR INSPECTIONS TO GUARD AGAINST HCF.

Fatigue Crack : 

Fatigue Crack Three steps: Initiation Fatigue crack propagation final failure (when area decreases sufficiently) Fatigue life: Nf = Ni + Np Ni is the number of cycles to initiate fracture Np is the number of cycles to propagate to failure high cycle fatigue ( Low stress levels): most of the life is spent in crack initiation and Ni is high low cycle fatigue (High Stress levels): propagation step predominates and Np>Ni

Fatigue Crack (Contd) : 

Fatigue Crack (Contd) Cracks that cause fatigue failure almost always initiate/nucleate at component surface at some stress concentration: scratches, dents, fillets, keyways, threads, weld beads/spatter….. On very smooth surfaces, SLIP steps can act as stress raisers Fracture Mechanisms stage I propagation crack tends to grow initially along crystallographic planes of high shear stress: high stresses and notches tend to shorten this stage. It may only propagate over a few grains. Length of stage I is controlled by presence of stress raisers stage II - crack growth rate increases (perpendicular to tensile stress direction)

Slide 24: 

Beachmarks are macroscopic evidence of fatigue and can be observed with the naked eye classical fatigue fracture surface (clam-shell markings) always concentric with the fracture origin caused by interrupted loading, e.g. machine being switched on and off during stage II propagation These marks DO NOT indicate the crack growth per stress cycle.

Fractography of Fatigue : 

Fractography of Fatigue Fatigue striations are microscopic and require a scanning electron microscope (SEM) to observe them each beachmark is composed of thousands of striations results from incremental advance of the fatigue crack during stage II propagation region of crack is usually relatively smooth and often discoloured in relation to the final fracture Each of these microscopic striations is usually caused by one stress cycle. If the stress increases, the spacing usually increases. Can count striations/mm to get ESTIMATE of crack growth rate.

Slide 26: 

Beachmarks and striations will not appear on that region over which the rapid failure occurs. Rather, the rapid failure may be either ductile or brittle; evidence of plastic deformation will be present for ductile failure and absent for brittle failure.

Fatigue fracture surface : 

Fatigue fracture surface At low magnifications, The beach mark pattern indicates fatigue as the fracture mechanism. The arrows show the direction of growth of the crack front, whose origin is at the bottom of the photograph. At very high magnifications spaced striations formed during fatigue are observed (x 1000).

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