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Metal Fatigue in Engineering
Fatigue is the failure mechanism which is a kind of cyclic or fluctuating stress occured by the cracking of materials and structural components. It includes the stress like tensile, compressive or torsional, crack initiation and propagation are occured due to the tensile component. The fatigue is developed because of cracking fatigues that is initiated and propagated at stresses well below the yield strength of the material of construction and these stresses are usually related to elastic deformation, not plastic deformation.The fatigue failure involves three stages i.e. Crack initiation , Crack propagation and Failure.
METAL FATIGUE IN ENGINEERING
The weakening of material caused by repeatedly applied loads is called as fatigue in engineering. It occurs in a situation when a material is subjected to repeatedly loading and unloading. Maximum stress value causes such damage and is less than the strength of material. Various characteristics of fatigue are as follows:
- Applied stress is inversely proportional to the life of material i.e. greater the stress shorter the life
- Fatigue is due to tensile strength but fatigue cracks are due to compressive loads
- High degree of randomness
- Cumulative damage
- High cycle fatigue strength is due to stress based parameters
- Low cycle fatigue is due to local behavior of plastic
Microscopic cracks begin to form at the area where stress concentration is more when load is applied above its threshold value. Square holes and sharp corners lead to a situation where these fatigue cracks starts to initiate. Whereas round holes and smooth transitions increases the strength of fatigue.
Topics we study under this section are given below:
- Design methods of fatigue
- Cyclic Deformation
- Statistical aspects of fatigue
- Fatigue crack growth
- Fatigue tests
The subject not only deals with the above mentioned topics but also include macro/micro aspects of metal fatigue, stress-life approach, strain-life approach, fundamentals of LEFM, fatigue from variable amplitude loading, environmental effects, fatigue due to weldments, notches and its effects, effects of microstructure on fracture in metals, polymers, thin film of metals, composites, biological materials, creep fractures, toughening mechanism, interface fracture mechanisms etc.
Fracture and fatigue introduce the failure modes of materials such as:
- Physical basis of fracture
- Crack tip stress and stress intensity factor
- Plasticity considerations
- Small scale yielding
- Plane stress vs. plane strain
- Resistance curves
- Test methods (polymers, ceramics, fracture in metals, glasses)
These are just a few major cause of fatigue and fracture failures there are more such as low tolerance, crack initiation due to lifetime approach, stress-life vs. strain-life, crack growth, fatigue mechanism etc.
The fatigue mechanism involves cyclic softening and hardening of materials, effect of stress, neuber rule, design procedures, fatigue in welds and joints, optical technique, SEM technique, Notch effect, concept of fatigue limit, cyclic load spectra and cycle counting, damage nucleation etc.
Types of fatigue loading include zero-to-max-to-zero and varying load superimposed on constant load. They can be explained as:
- Zero-to-max-to-zero is a kind of fatigue loading where a part carrying no load is then subjected to load and later the load is removed and hence again the part goes to a no load condition.
- Varying load superimposed on constant load is another kind of fatigue loading this can be explained with the help of an example i.e. wires having constant static tensile load from the weight of bridge and some extra tensile load when a train is on the bridge.
Factors which affect the life of any material or the fatigue life are given as:
- Surface Condition (Ka): Surface acts as an important feature on the life of fatigue such as: polished, grounded, corroded, machined etc.
- Size (Kb): It is responsible for the changes which occur when there is change in the actual size or the cross-sectional area of the test specimen
- Load (Kc): It is responsible for loading differences between the actual part and the test specimen
- Temperature (Kd): With change in temperature with respect to the room temperature there is reduction in fatigue life
- Reliability (Ke): It is responsible for handling all the scattered data. Example: 8% standard deviation in test data requires Ke value of 0.868 for 95% reliability
- Miscellaneous (Kf): It handles the reduction of other effects such as residual stress, corrosion, plating, metal spraying, fretting etc.
Fracture toughness is that property of science which determines the ability of material containing a crack to resist fracture and is used by many design applications. Fracture toughness is a quantitative method of expressing material’s resistance to brittle fracture when a crack is present. The materials having high fracture toughness are known as ductile fractures and those having low fracture toughness are known as brittle toughness.
Fracture mechanics which leads to fracture toughness is based on the study of behavior of cracks in brittle materials. The stress intensity factor determines the linear-elastic fracture toughness which grows with a thin crack in the material. The S.I. unit of plastic-elastic fracture toughness is J/cm2.
Multiaxial stress is another major concept in this in which fatigue occurs on the surface of zero principle stress which results in biaxial nature of multiaxial fatigue. It is analyzed that stress normal to free surface is always zero. Various amplitude multiaxial stress analysis defines a series of steps:
- Applied stress
- Material properties
- Computing fatigue damage
- Searching failure planes
While, Multiaxial fatigue counts the cycle and sums damage for all cycles. Calculators are used for constant amplitude loading and largest cycle handles the fatigue damage.
Linear Elastic Fracture Mechanics (LEFM): Let us assume an isotropic material having linear elasticity. The crack occurs when stress near the crack tip exceeds the fracture toughness of that material. LEFM is used for condition that is linear elastic during fatigue process. Mostly all formulas are derived from either plane stress or plane strain and the three most important modes in which loading on any cracked body done are: opening, sliding, tearing. LEFM can be applied only when inelastic deformation is very small in comparison with the crack. In case large of large zone of plastic deformation we use Elastic Plastic Fracture Mechanics (EPFM).
Stress corrosion crack (SCC) is a kind of cracking influenced by combining tensile stress and corrosive environment. It lies between dry cracking and fatigue threshold of that material. The tensile stress required are either in form of residual stress or directly applied stress.
- Chloride SCC: It is one of a kind which occurs in austenitic stainless steel under tensile stress in presence of oxygen, chlorine ions and high temperature. This form of corrosion can be controlled by the using low chloride ions and low carbon steels.
- Caustic SCC: A number of corrosion problems arise with inconel tubing. It include problems such as wastage, tube denting, pitting and other inter granular attacks.
Griffith fracture theory is one of the major advanced concepts that show a relationship between applied nominal stress and crack length at fracture. It is concerned with the energy changes associated with crack extension. It contributes the energy changes in the new fracture surface and change in potential energy in the body.
Various advanced topics included in metal fatigue engineering are:
- Toughening mechanisms
- Stress concentration
- Stress-strain curves
- Crack propagation
- Uniaxial and biaxial fatigue analysis
- Plasticity and crack-tip plasticity
- Signal processing
- Wang-Brown and Dang Van criterion
- Weibull and Gaussian distribution
- Fatigue design: strategy, criteria, inspection, reliability, Aspects of fatigue: macro , micro, Fatigue tests and stress life approach:, testing,S-N curves, factors, Cyclic deformation and strain life approach: cyclic σ-ε curves,, ε-N approach.
- Linear Elastic Fracture Mechanics, Fatigue Crack Growth, Notch analysis, Residual stress, Fatigue from variable loading:, spectrum loading,, cumulative damage, load interaction, life calculation., Multiaxial Fatigue., Environmental fatigue:, corrosion fatigue, fretting fatigue, low-temperature and high temperature fatigue, fatigue, cyclic stress-strain behavior, fatigue analysis: stress based approach.
- constant amplitude loading, mean stress effects, stress concentration effects, variable amplitude loading, cumulative damage, rainflow cycle counting, multaxial stresses, fatigue analysis: strain based approach, constant amplitude loading.
- mean stress effects, multaxial stresses & strains, fatigue analysis, local stress strain approach, fatigue crack propagation, fracture mechanics, constant amplitude loading, crack closure, variable amplitude loading, fatigue crack growth threshold.
- fracture, fatigue life based on small crack behavior, constant aplitude loading, variable amplitude loading, increasing fatigue resistance, role of stress concentration and surface finish, beneficial residual stresses, statistical aspects of fatigue, bolted joints, welded joints