AE2030 FATIGUE AND FRACTURE
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INTRODUCTION
Fracture mechanics is the study of mechanical behavior of
cracked material subjected to an applied load
Fatigue is the weakening of a material caused by repeatedly
applied loads.
The process of progressive localized permanent structural changes occurring in a material subjected to conditions that produce fluctuating stresses at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations.
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WHY STRUCTURES FAIL? The cause of most structural failures generally falls into one of the following categories:
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Negligence during design, construction, or operation of the structure.
Application of a new design or material, which which produces an unexpected (and undesirable) result.
Common causes of Failure are:
Yielding – critical loading point
Deflection beyond a certain stage
Buckling
Fatigue
Fracture
Creep
Environmental Degradation
Resonance
Impact
Wear
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Fracture mechanisms Ductile fracture
Accompanied by significant plastic deformation
Brittle fracture
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Little or no plastic deformation
Catastrophic
Usually strain is < 5%.
Brittle vs Ductile Materials
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FRACTURE MECHANICS
Study of crack propagation in bodies
Methodology used to aid in selecting materials and designing components to minimize the possibility of fracture.
It begins with the assumption that all real materials contain cracks of some size—even if only submicroscopically
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Based on three types of displacement modes
3 Modes of Crack Propagation
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FRACTURE TOUGHNESS
Fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for many design applications. The linear-elastic fracture toughness of a material is determined from the stress intensity factor (K) at which a thin crack in the material begins to grow.
In fracture mechanics, one does not attempt to evaluate an effective stress concentration, rather a stress intensity factor K
After obtaining K, it is compared with a limiting value of K that is necessary for crack propagation in that material, called K c
The limiting value K c is characteristic of the material and is called fracture 9 toughness
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Fatigue Failure- Mechanism
A fatigue failure begins with a small crack; the initial crack may be so minute and can not be detected. The crack usually develops at a point of localized stress concentration like discontinuity in the material, such as a change in cross section, a keyway or a hole.
Once a crack is initiated, the stress concentration effect become greater and the crack propagates.
Consequently the stressed area decreases in size, the stress increase in magnitude and the crack propagates more rapidly.
Until finally, the remaining area is unable to sustain the load and the component fails suddenly. Thus fatigue loading results in sudden, unwarned failure. 12
Four Different Stages of Fatigue Failure
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Crack initiation at points of stress concentration
Crack growth
Crack propagation
Final rupture
Factors Influencing Fatigue
Loading
Geometry
Material
Manufacturing
Environment
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Stress Concentration Factor
Stress concentration factor (Kt), is a dimensionless factor which is used to quantify how concentrated the stress is in a material. It is defined as the ratio of the highest stress in the element to the nominal stress (reference stress )
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Characteristics of stress-concentration factors:
Function of the geometry or shape of the part, but not its size or material
Function of the type of loading applied to the part (axial, bending or torsional)
Function of the specific geometric stress raiser in the part (such as fillet radius, notch, or hole)
Always defined with respect to a particular nominal stress
Typically assumes a linear elastic, homogeneous, isotropic material
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Stress Concentration Factor
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Fatigue Stress Concentration
The existence of irregularities or discontinuities, such as holes, grooves, or notches, in a part increase the magnitude of stresses significantly in the immediate vicinity of the discontinuity .
Fatigue failure mostly originates from such places. Hence its effect must be accounted and normally a fatigue stressconcentration factor Kf is applied when designing against
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Fatigue Stress Concentration Factor (K f )
Miscellaneous-effect factor (Ke)
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Notch Sensitivity (q) A measure of the reduction in strength of a metal caused by the presence of a notch
The value of ‘q’ usually lies between 0 and 1. If q=0, Kf =1 and this indicates no notch sensitivity. If however q=1, then Kf = Kt and this indicates full notch sensitivity.
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STRESS-LIFE DIAGRAM(Wohler S-N Curves)
Steel, Ti. etc
Al, Cu alloy, Mg, etc.,
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Endurance Limit / Fatigue Limit
The fatigue life reduces with respect to increase in stress range and at a limiting value of stress, the curve flattens off. The point at which the S-N curve flattens off is called the ‘endurance limit’.
Certain materials have a fatigue limit or endurance limit which represents a stress level below which the material does not fail and can be cycled infinitely.
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Unlike steels, most nonferrous metals and alloys (Al, Mg, Cu alloy, etc.,) do not have a fatigue limit i.e. S-N curve continues to fall steadily with decreasing stress, though at a decreasing rate.
Thus, fatigue will ultimately occur regardless of the magnitude of the applied stress. Fatigue response of these materials is specified for a number of stress cycles, normally 10 7 , and is known as fatigue strength.
An effective endurance limit for these materials is sometimes defined as the stress that causes failure at 1x10 8 to 5x10 8
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Metal fatigue is a significant engineering problem because it can occur due to repeated or cyclic stresses below the static yield strength, unexpected and catastrophic failure of a vital structural part may occur and rack initiation may start at discontinuities in highly stressed regions of the component.
Fatigue failure may be due to discontinuities such as inadequate design, improper maintenance and so forth.
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Fatigue failure can be prevented by
Avoiding sharp surfaces caused by punching, stamping, shearing.
Preventing the development of surface discontinuities during processing.
Reducing or eliminating tensile residual stresses caused by manufacturing.
Avoiding
assembling
errors,
improper
maintenance,
manufacturing defects, design errors
Using proper material and heat treatment procedures
Environmental Effects. 26
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