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Transcript of Fatigue process in materials - Aalborg Universitethomes.civil.aau.dk/lda/Advanced Structural...
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Fatigue process in materials
Lars Damkilde Section for Structural MechanicsDepartment of Civil Engineering, Aalborg University
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Fatigue and fracture mechanics
• The fatigue process can be considered as a sequence of small fracture processes.
• The crack growth depends on the stress intensity factor.
• The crack growth also depends on some material parameters.
• The final breakdown is a fracture problem. The structure will break when the fatigue crack become unstable.
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Fatigue process
• Crack initiation
• Crack growth (Paris law)
• Final breakdown
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Fatigue process
• The number of cycles in the crack initiation phase will in general be hard to predict.
• Depend on imperfections in material.
• Formations of micro-cracks.
• The number of cycles in the crack growth phase can be predicted quite good.
• Growth of a macro crack.
• The number of cycles in the final stage is hard to predict.
• Very few
• The maximum crack size can be predicted quite well.
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Fatigue process
• Minimum stress required to initiate fatigue cracks (K_threshold)
• Constant growth rate of fatigue cracks (A)
• Constant exponential growth rate depending on stress level (n)
• All the parameters are material values depending also on temperature.
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Test results
• Test are typical done with sinusoidal stress variations.
• Som test on stochastic variations of stresses.
• Stress amplitude and average stress.
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Experimental results
• Large variations (crack initiation phase)
• Tests not finished
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Fatigue process – stress dependency
• Stress variations are the central element.
• Principal stresses, tension or shearing components.
• For some materials the average stress has very little influence.
• Pretensioned steel bolts
• Welded steel
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Fatigue problems
• High cycle-fatigue.
• Large number of cycles (> 10^5)
• Typical in windturbine towers and (most) other components.
• Low cycle-fatigue
• Relative small number of components
• Typical in components used in a very limited period.
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Corrosion and fatigue
• Corrosion may considerably speed up the fatigue growth process.
• Threshold values for minimum stress values for fatigue crack growth are very small (or 0).
• Standards may require increased inspection and lower values of the Palmgren-Miner sum.
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Time-dependent materials
• Time-dependent materials are timber and other polymeric based materials.
• Visco-elasticity – continued deformation under time.
• Time under load is of major importance.
• Interesting for windturbine blades.
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Wind-turbine foundations - reinforced concrete
• Concrete is in general believed to be very resistant to fatigue.
• True?
• Concrete is mostly in compression
• Shear gives both compression and tension.
• Grouted connections.
• Transition piece.
• Crack-formations.
• Soil fatigue
• Monopiles and cyclic deformations.
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Stress or strain driven
• Normally the fatigue process is dictated by load-variations which are transferred into stress variations.
• Bolts in pretensioned connections.
• In other situations the variations are induced by strain variations.
• Structure is deformed between two outer limits. (e.g. component in a device).
• Less severe than stress driven as the load decreases.
• In statically indeterminate structures fatigue will have a component of strain driven.
• Fatigue cracks in circumferential welded connections?
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