1. Questions about tensile force-elongation curves and stress-strain curves
Stress-strain curve of mild steel
a. Deformation during stretching:
Elastic deformation, yield deformation, work hardening (uniform plastic deformation), uneven concentrated plastic deformation.
b. Relevant formula:
Engineering stress σ=F/A0; engineering strain ε=ΔL/L0; proportional limit σP; elastic limit σε; yield point σS; tensile strength σb; fracture strength σk.
True strain e=ln(L/L0)=ln(1+ε); true stress s=σ(1+ε)= σ*eε The exponent e is the true strain.
c. Related theories:
The true strain is always less than the engineering strain, and the greater the deformation, the greater the gap between the two; the true stress is greater than the engineering stress.
In the elastic deformation stage, the true stress-true strain curve is basically consistent with the stress-strain curve; in the plastic deformation stage, there are significant differences between the two curves.
2. Questions about elastic deformation
a. Related concepts
Elasticity: Characterizes the ability of a material to deform elastically
Stiffness: Characterizes the resistance of a material to elastic deformation
Elastic modulus: a constant that reflects the relationship between elastic deformation stress and strain, E=σ/ε; also called stiffness in engineering, which characterizes the resistance of a material to elastic deformation.
Elastic specific work: called elastic specific energy or strain-specific energy, it is the ability of a material to absorb deformation work in the process of elastic deformation and evaluate the elasticity of the material.
Bauschinger effect: A small amount of plastic deformation occurs when a metal material is preloaded, and then loaded in the same direction, the specified residual elongation stress increases; in reverse loading, the specified residual elongation stress decreases.
Hysteretic elasticity: (Elastic after-effect) refers to the performance of the additional elastic strain generated by the extension of time after the material is rapidly loaded or unloaded.
Elastic hysteresis loop: In the case of non-ideal elasticity, due to the asynchronous stress and strain, the loading line and the unloading line do not coincide to form a closed loop.
The ability of a metal material to absorb irreversible deformation work under alternating loads is called the cyclic toughness of the metal, also called internal friction
b. Relevant theories:
Elastic deformation is reversible.
The ideal elastic deformation is single-valued, reversible, and instantaneous. However, due to the fact that the actual metal is polycrystalline and has various defects, it is not complete when elastically deformed.
The essence of elastic deformation is the reflection of the reversible deformation of the atoms or ions or molecules constituting the material from the self-equilibrium position
The elastic moduli of single crystal and polycrystalline metals mainly depend on the atomic nature and crystal type of the metal.
Bauschinger effect; hysteretic elasticity; pseudoelasticity; viscoelasticity.
Elimination of the Bauschinger effect: pre-large plastic deformation, recovery or annealing at recrystallization temperature.
Cyclic toughness represents the shock-absorbing ability of a material.
3. Questions about plastic deformation
a. Related concepts
Slip: The more slip systems, the better the plasticity; the slip system is not the only factor (factors such as lattice resistance); slip surfaces – affected by temperature, composition and deformation; slip direction – relatively stable
Twining: fcc, bcc, hcp can be twinned plastic deformation; generally occurs at low temperature and high-speed conditions; small deformation, adjust the direction of the slip surface
Yield phenomenon: annealed, normalized, quenched and tempered medium and low carbon steel and low alloy steel are more common, divided into discontinuous yielding and continuous yielding;
Yield point: the stress value corresponding to the material yielding in tension, σs;
Upper yield point: the maximum stress value before the sample yields and the force decreases for the first time, σsu;
Lower yield point: the minimum stress in the yield stage of the specimen, σsl;
Yield plateau (yield tooth): horizontal line segment or zigzag line segment corresponding to yield elongation;
Lüders belt: uneven deformation; for stamping parts, it is not allowed to appear to prevent wrinkles.
Yield Strength: Characterizes the resistance of a material to small amounts of plastic deformation
Yield strength of continuous yield curve: The resistance of a material to microplastic deformation is characterized by a specified microplastic elongation stress
(1) Specify the non-proportional elongation stress σp:
(2) Specified residual elongation stress σr: after the tensile force is removed from the specimen, the residual elongation of the gauge length part reaches the specified original gauge length percentage; when the percentage of residual elongation is 0.2%, it is recorded as σr0.2
(3) The specified total elongation stress σt: the stress when the total elongation (elastic elongation plus plastic elongation) of the gauge length part of the specimen reaches the specified percentage of the original gauge length.
Lattice resistance (Pine force); dislocation interaction resistance
Hollomon formula: S=Ken, S is true stress, e is true strain; n—hardening index 0.1~0.5, n=1, perfect elastic body, n=0, no hardening ability; K—hardening coefficient
Necking is a special phenomenon in which the deformation of a ductile metal material is concentrated in a local area during a tensile test.
Tensile strength: The stress corresponding to the maximum test force during the tensile fracture of a ductile metal sample. It represents the maximum tensile stress that the metal material can bear, and characterizes the resistance of the metal material to the maximum uniform plastic deformation. It is related to the strain hardening exponent and the strain hardening coefficient. is equal to the ratio of the maximum tensile stress to the original cross-sectional area.
Plasticity refers to the ability of a metallic material to undergo irreversible permanent (plastic) deformation before fracture.
b. Relevant theories
Common plastic deformation modes: slip, twinning, slip at grain boundaries, diffusive creep.
The characteristics of plastic deformation: the different timing and inhomogeneity of the deformation of each grain (different orientation; the difference in the mechanical properties of each grain); the mutual coordination of the deformation of each grain (the metal is a continuous whole, multi-system slip ; Von Mises at least 5 independent slip systems).
Determination of hardening index: ① test method; ② drawing method lgS=lgK+nlge
The influencing factors of the hardening index: related to the stacking fault energy, the stacking fault energy decreases, and the hardening index increases; it is also very sensitive to the hot and cold deformation of the metal material; it is not equal to the strain hardening rate.
The criterion for necking (instability critical condition) The criterion for tensile instability or necking should be dF=0
Two plastic indexes: elongation after fracture δ=(L1-L0)/LO*100%;
Shrinkage rate after breaking: ψ=(A0-A1)/A0*100%
ψ>δ, forming a necking
ψ=δ or ψ<δ, no necking is formed
4. About the toughness and fracture of metals
a. Related concepts
Toughness: the ability to absorb plastic deformation work and fracture work before fracture
Toughness: The work absorbed per unit volume of material before it breaks
Ductile fracture: energy is consumed during the slow propagation of the crack; the fracture first occurs in the fiber region, then rapidly expands to form radiation, and finally the shear lip is formed.
Brittle fracture: basically no plastic deformation occurs, and it is very harmful. Low-stress brittle fracture, the working stress is very low, generally lower than the yield limit; brittle fracture cracks always start from internal macroscopic defects; the temperature decreases, the strain rate increases, and the brittle fracture tendency increases.
Transgranular fracture: The crack passes through the crystal, which can be a ductile fracture or a brittle fracture, and the fracture is bright.
Intergranular fracture: The crack propagates along the grain boundary, all of which are brittle fractures, which are caused by the second phase of brittleness at the grain boundary, and the fracture is relatively dark. Transgranular fracture and intergranular fracture can occur in combination. At high temperatures, the fracture is mostly transformed from transgranular fracture to intergranular ductile fracture.
Intergranular fracture: the fracture is rock sugar-like; if the grains are small, the fracture is grain-like.
Shear fracture: The fracture caused by the slippage separation of materials along the slip plane under the action of shear stress. (slip fracture, microporous aggregate type fracture)
Cleavage fracture: The brittle transgranular fracture along a specific crystal plane is caused by the destruction of the original bond between the materials under the action of normal stress.
The strength of a metal refers to the size of the bonding force between the atoms of the metal material. Generally speaking, the metal has a high melting point, a large elastic modulus, and a small thermal expansion coefficient. The essence of fracture is the process of separating materials along an atomic plane under the action of external force.
Griffith’s theory: From a thermodynamic point of view, any process that reduces energy will proceed spontaneously, and any process that increases energy will cease unless external energy is provided. Griffth pointed out that the reduction in elastic energy of the system due to the presence of cracks is balanced with the increased surface energy due to the presence of cracks. If the elastic energy is reduced enough to satisfy the increase in the surface energy, the crack will grow erratically, causing brittle failure.
b. Relevant theories
Fracture Three main failure modes: wear, corrosion, fracture
The fracture of most metals includes two stages of crack formation and propagation.
By fracture state: ductile fracture and brittle fracture; by crack propagation path: transgranular fracture and intergranular fracture; by fracture mechanism: cleavage fracture and shear fracture
Ductile fracture and brittle fracture: Determined according to the amount of macroscopic plastic deformation generated before the material fractures. Usually, a small amount of plastic deformation occurs in brittle fracture, and it is generally stipulated that the reduction of the section is less than 5% to be a brittle fracture. On the contrary, more than 5% is a ductile fracture.
The brittle fracture is flush and bright, perpendicular to the normal stress, and the fracture is often herringbone or radial pattern.
Cleavage fracture is a brittle transgranular fracture that occurs along a specific crystal plane and usually separates along a certain crystal plane.
Cleavage fractures are always brittle fractures, but brittle fractures are not necessarily cleavage fractures.
Common crack formation theories: ① dislocation packing theory ② dislocation reaction theory
Cleavage and Quasi-cleavage
Common points: transgranular fracture; small cleavage facets; steps and river patterns
Differences: ① quasi-cleavage facets are not crystallographic cleavage planes; ② cleavage cracks often originate from grain boundaries, and quasi-cleavage cracks often originate from intragranular hard points. Quasi-cleavage is not an independent fracture mechanism, but a variant of cleavage fracture.
Griffith’s theory is a necessary condition for the fracture to occur according to the principles of thermodynamics, but it does not mean that fracture is in fact necessary. A sufficient condition for automatic crack propagation is that the tip stress is equal to or greater than the theoretical fracture strength.
5. Questions about the hardness
a, hardness concept
Hardness is a performance index to measure the softness and hardness of metal materials.
b. Hardness test method:
Scratch Method – Characterization of Metal Severance Strength
The rebound method – characterizing the elastic deformation work of metals
Indentation Method – Characterization of Plastic Deformation Resistance and Strain Hardening Ability
Indenter: Hardened Steel Ball (HBS), Carbide Ball (HBW)
Load: 3000Kg carbide, 500Kg soft material
Warranty time: 10-15s for ferrous metals, 30s for non-ferrous metals
Indentation Similarity Principle
With only one standard load and ball diameter, it cannot accommodate hard or soft materials at the same time. To ensure comparability between hardness values measured at different loads and diameters, the indentation must satisfy geometric similarity.
Brinell hardness representation method: 600HBW1/30/20
①degree value, ②symbol HBW, ③ball diameter, ④test force (1kgf=9.80665N), ⑤test force holding time
Advantages and disadvantages of Brinell hardness test:
Advantages: larger diameter of indenter → larger indentation area → hardness value can reflect the average performance of each constituent phase of the metal in a wide range, and is not affected by individual constituent phases and minor inhomogeneities.
Disadvantages: It is necessary to change the diameter of the indenter and change the test force for different materials, the indentation measurement is troublesome, and the automatic detection is limited; when the indentation is large, it is not suitable to test on the finished product
The material hardness value is expressed as the measured indentation depth.
There are two types of indenters: a diamond cone with α=120°, and a quenched steel ball with a certain diameter.
Advantages and disadvantages of Rockwell hardness test:
Advantages: easy and fast operation, hardness can be directly read; small indentation can be tested on the workpiece; different scales can be used to measure samples with different hardness and thickness.
Disadvantages: The indentation is small and the representativeness is poor; if the material has defects such as segregation and uneven structure, the test value has poor repeatability and large dispersion; the hardness values measured by different scales are not related and cannot be directly compared.
The principle is the same as that of the Brinell hardness test, and the hardness value is calculated according to the test force per unit area. The difference is that the indenter of Vickers hardness is a diamond quadrangular pyramid with the included angle α of two opposite faces being 136°.
The difference from Vickers hardness 1) The shape of the indenter is different; 2) The hardness value is not the test force divided by the indentation surface area, but divided by the indentation projected area
A dynamic load test method, the principle is to drop a weight with a diamond round head or a steel ball of a certain quality on the surface of the metal sample from a certain height, and characterize the hardness value of the metal according to the height of the rebound of the weight, and also Called rebound hardness. Indicated by HS.
In the dynamic load test method, an impact body of a specified mass is used to impact the surface of the sample at a certain speed under the action of elastic force, and the rebound speed of the punch is used to characterize the hardness value of the metal. As indicated by HL.
6. About the mechanical properties of metal under impact load
a. Related concepts
Impact toughness: refers to the ability of a material to absorb plastic deformation work and fracture work under impact load, which is commonly expressed by the impact absorption energy AK of a standard sample.
Impact measurement parameters: measure the impact absorption energy (AkU or AKV) after impact brittle fracture, the impact absorption energy can not really reflect the toughness and brittleness of the material (the impact absorption energy is not completely used for the deformation and failure of the sample)
Low-temperature brittleness: body-centered cubic or some close-packed hexagonal crystal metals and alloys, when the test temperature is lower than a certain temperature tk or temperature range, the material changes from a ductile state to a brittle state, the impact absorption energy decreases significantly, and the fracture mechanism is small. The pores aggregated into transgranular cleavage, and the fracture characteristics changed from fibrous to crystalline. The tk or temperature range is called the ductile-brittle transition temperature, also known as the cold-brittle transition temperature.
b. Relevant theories
Evaluation methods for toughness and brittleness: notched impact bending test of materials, impact toughness of materials
Influence factors of ductility and brittleness: temperature (low-temperature brittleness); stress state (three-dimensional tensile stress state); influence of deformation speed (impact brittle fracture)
The nature of low-temperature brittleness: Low-temperature brittleness is the result of a sharp increase in the yield strength of a material with decreasing temperature. The yield strength σs increases with decreasing temperature, while the breaking strength σc varies little with temperature.
t>tk ,σc>σs , first yield and then fracture; t<tk ,σc<σs , brittle fracture
The ductile-brittle transition temperature is a toughness index of metal materials, which reflects the effect of temperature on ductile-brittleness.
Metallurgical factors affecting the ductile-brittle transition temperature:
Crystal structure: Body-centered cubic metals and their alloys are brittle at low temperatures. The matrix of ordinary medium and low strength steel is a ferrite of body-centered cubic lattice, so this kind of steel has obvious low-temperature brittleness.
Chemical composition: Interstitial solute elements dissolve into the ferrite matrix, segregate near the dislocation line, hinder the movement of dislocations, cause σs to increase, and the ductile-brittle transition temperature of steel increases.
Microstructure: Grain size, refining the grains increases the toughness of the material; reducing the size of the subgrain and cellular structures can also improve the toughness.
Reasons for refining grains to improve toughness: grain boundaries are the resistance to crack propagation; the number of dislocations plugged before the grain boundaries is reduced, which is conducive to reducing stress concentration; the total area of grain boundaries increases, which reduces the impurity concentration on the grain boundaries and avoids occurrence Intergranular brittle fracture.
7. Questions about metal fatigue
a, metal fatigue phenomenon
Fatigue: The fracture phenomenon of metal parts caused by accumulated damage under the long-term action of variable stress and strain.
The failure process of fatigue is
Under the action of variable stress, the tissue gradually changes, accumulates damage, and cracks. When the crack expands to a certain extent, a sudden fracture occurs. It is a process of accumulation of damage starting from a local area and eventually causing overall damage.
Waveforms of cyclic stress: sine wave, square wave and triangle wave, etc.
The parameters that characterize the stress cycle are:
Maximum cyclic stress σmax, minimum cyclic stress σmin; Average stress: σm=(σmax+σmin)/2; Stress amplitude or stress range: σa=(σmax-σmin)/2; Stress ratio: r=σmin/σmax
Fatigue is divided into stress states: bending fatigue, torsional fatigue, tension and compression fatigue, contact fatigue and compound fatigue;
Fatigue is classified according to the environment and contact conditions: atmospheric fatigue, corrosion fatigue, high-temperature fatigue, thermal fatigue and contact fatigue.
Fatigue is classified according to stress level and fracture life: high cycle fatigue and low cycle fatigue.
b. Metal fatigue characteristics
Fatigue characteristics: This failure is a latent sudden failure. Materials showing ductile or brittle failure under static load will not have obvious plastic deformation before fatigue failure and show a brittle fracture.
Fatigue is very sensitive to defects such as notches, cracks and structures, that is, it has a high degree of selectivity to defects. Because the notch or crack will cause stress concentration and increase the damage to the material; organizational defects (inclusions, looseness, white spots, decarburization, etc.) will reduce the local strength of the material, and the combination of the two will accelerate the onset of fatigue failure and develop.
c. Metal fatigue macro fracture
Fatigue macro-fracture characteristics: Fatigue fracture undergoes the process of crack initiation and propagation. Due to the low-stress level, it has obvious crack initiation and steady-state propagation stages, and the corresponding fracture also shows the characteristics of fatigue source, fatigue crack propagation zone and instantaneous fracture zone.
Fatigue source: It is the source of fatigue crack initiation.
Location: It usually appears on the surface of the machine, and is often connected to defects such as nicks, cracks, knife marks, and pits. However, if there are serious metallurgical defects in the material (inclusions, shrinkage cavities, bollards, white spots, etc.), the fatigue source will also be caused inside the parts due to the reduction of local material strength.
Features: Due to the repeated extrusion of the crack surface in the fatigue source area, the number of frictions is large, the fatigue source area is relatively bright, and the surface hardness of the area will be improved due to work hardening.
Quantity: The fatigue source of the fatigue failure of the machine part can be one or more, which is related to the stress state and the overload degree of the machine part. If one-way bending fatigue produces only one source region, two-way repeated bending can produce two sources of fatigue. The higher the overload, the higher the nominal stress and the higher the number of fatigue sources.
Generation sequence: If there are several fatigue sources in the fracture at the same time, the order of generation of each fatigue source can be determined according to the size of each fatigue zone and the brightness of the source zone. ; On the contrary, it will be late.
The fatigue zone is the region formed by the metastable propagation of fatigue cracks.
Macroscopic features: The fracture is relatively smooth and distributed with shell lines (or beach patterns), and sometimes there are crack propagation steps.
Fracture smoothness is the continuation of the fatigue source area, and its degree gradually weakens as the crack grows forward, reflecting the difference in the degree of fast crack expansion and extrusion friction.
Shell line – the most typical feature of the fatigue zone: Cause: It is generally considered to be caused by load changes, because the machine is often started, stopped, accidentally overloaded, etc., and arc-shaped shell lines should be left on the front line of crack propagation. trace.
Morphological characteristics: Each group of seam lines in the fatigue zone is like a cluster of parallel arcs with the fatigue source as the center, the concave side points to the fatigue source, and the convex side points to the crack propagation direction. The shell lines near the fatigue source are denser, indicating that the crack propagation is slow; the shell lines far away from the fatigue source are sparse and rough, indicating that the cracks grow faster in this section.
Influencing factors: The total extent of the seaweed area is related to the degree of overloading and the properties of the material. If the nominal stress of the machine is high or the material toughness is poor, the fatigue zone will be small and the scallop lines will not be obvious; on the contrary, if the nominal stress or high toughness material is low, the fatigue zone will be larger and the scallop lines will be thick and obvious. The shape of the shell line is determined by the propagation speed, load type, overload degree and stress concentration of each point on the crack front line.
The transient region is the region formed by the unstable propagation of the crack. In the fatigue subcritical growth stage, with the increase of stress cycle, the crack grows continuously. When it increases to the critical dimension ac, the stress field intensity factor KI at the crack tip reaches the material fracture toughness KIc(Kc). The crack will be unstable and expand rapidly, resulting in instantaneous fracture of the parts.
The fracture in the transient zone is rougher than that in the fatigue zone, and the macroscopic characteristics are like static loads, which vary with the material properties.
The fracture of brittle material is crystalline;
The fracture of ductile material is radial or herringbone-shaped in the plane strain area of the heart, and there is a shear lip area in the plane stress area of the edge.
Location: The transient area should generally be on the opposite side of the fatigue source. However, for rotational bending, when the nominal stress is low, the position of the instantaneous break area is deflected by an angle against the rotation direction; when the nominal stress is high, multiple fatigue sources simultaneously expand inward from the surface, causing the instantaneous break area to move to the center position.
Size: The size of the instantaneous breaking area is related to the nominal stress and material properties of the parts. For materials with high nominal stress or low toughness, the instantaneous breaking area is large; otherwise. The instantaneous interruption area is small.
d. Fatigue curve and basic fatigue mechanical properties
Fatigue curve: the relationship between fatigue stress and fatigue life, that is, the S-N curve.
Purpose: It is the basis for determining fatigue limit and establishing fatigue stress criterion.
There are horizontal sections (carbon steel, alloy structural steel, ductile iron, etc.): no fatigue fracture occurs after infinite stress cycles, and the corresponding stress is called the fatigue limit, denoted as σ-1 (symmetric cycle)
No horizontal section (aluminum alloy, stainless steel, high-strength steel, etc.): only the cycle times increase as the stress decreases. At this time, the stress at which no fracture occurs under a certain cycle is specified as the conditional fatigue limit according to the use requirements of the material.
Determination of Fatigue Curve – Determination of Fatigue Limit by Lifting Method
d. Fatigue process and mechanism
Fatigue process: three processes of crack initiation, metastable propagation, and buckling propagation.
Fatigue life Nf = initiation period N0 + metastable extension period Np
The fatigue process of metal materials is also the process of crack initiation phase propagation.
Crack initiation often occurs in weak or high-stress regions of the material through uneven slippage, microcrack formation and growth.
Fatigue microcracks are often caused by uneven slippage and microcracking. The main ways are: surface slip zone cracking; the second phase, inclusion and matrix interface or inclusion itself cracking; grain boundary or subgrain boundary cracking.
e. How to improve fatigue strength
How to Improve Fatigue Strength – Slip Band Cracking Generates Crack Angle
From the perspective of the formation mechanism of fatigue cracks caused by slip cracking, as long as the sliding resistance of the material (solid solution strengthening, grain refinement strengthening, etc.) can be improved, the initiation of fatigue cracks can be prevented and the fatigue strength can be improved.
How to Improve Fatigue Strength – Phase Interface Cracking Generates Crack Angle
From the perspective of the mechanism that the second phase or inclusions can cause fatigue cracks, as long as the brittleness of the second phase or inclusions can be reduced, the strength of the phase interface can be improved, and the number, shape, size and distribution of the second phase or inclusions can be controlled to make “Few, round, small and uniform” can inhibit or delay the initiation of fatigue cracks near the second phase or inclusions and improve fatigue strength.
How to Improve Fatigue Strength – Grain Boundary Cracking Generates Cracks
From the point of view of crack initiation at the grain boundary, all the factors that weaken the grain boundary and coarsen the grain, such as the segregation of harmful elements and components such as low-melting inclusions in the grain boundary, temper brittleness, hydrogen evolution at the grain boundary and grain coarsening, etc. Both are prone to grain boundary cracks and reduce fatigue strength; on the contrary, all factors that strengthen, purify and refine grain boundaries can inhibit the formation of grain boundary cracks and improve fatigue strength.
f. Main factors affecting fatigue strength
Influence of surface state: stress concentration – the surface notch of the machine is often the source of fatigue due to stress concentration, causing fatigue fracture. Kf and qf can be used to characterize the effect of notch stress concentration on the fatigue strength of the material. The larger the Kf and qf, the lower the fatigue strength of the material. And this effect is more significant with the increase of material strength.
Surface Roughness – The lower the surface roughness, the higher the fatigue limit of the material; the higher the surface roughness, the lower the fatigue limit. The higher the material strength, the more pronounced the effect of surface roughness on the fatigue limit.
Effect of residual stress and surface strengthening: residual compressive stress increases fatigue strength; residual tensile stress reduces fatigue strength. The influence of residual compressive stress is related to the stress state of the applied stress. Different stress states have different stress gradients on the surface layer of the machine. When bending fatigue, the effect is greater than torsional fatigue; when tension and compression fatigue, the effect is small.
The residual compressive stress significantly improves the fatigue strength of the notched parts, and the residual stress can be concentrated at the notch, which can effectively reduce the peak tensile stress at the root of the notch. The magnitude, depth, distribution of residual compressive stress, and whether relaxation occurs will affect fatigue strength.
Effect of Surface Strengthening – Surface strengthening can generate residual compressive stress on the surface of the part, while increasing strength and hardness. Both effects will increase the fatigue strength. (Methods: shot peening, rolling, surface quenching, surface chemical heat treatment) The order of hardness from high to low: nitriding → carburizing → induction heating quenching; the order of depth of strengthening layer from high to low: surface quenching → carburizing → infiltration nitrogen.
Influence of material composition and structure: Fatigue strength is a mechanical property sensitive to the structure of materials. Alloy composition, microstructure, non-metallic inclusions and metallurgical defects
g. Low cycle fatigue
Low cycle fatigue: The fatigue life of metal under cyclic load is 102 to 105 times of fatigue fracture.
Cyclic hardening and cyclic softening phenomena are related to the cyclic motion of dislocations.
In some annealed soft metals, cyclic hardening occurs under cyclic loading of constant strain amplitude due to dislocation reciprocation and interaction, which creates resistance to continued dislocation motion.
In cold-worked metals, full of dislocation entanglements and barriers that are destroyed during cyclic loading; or in some precipitation-strengthening unstable alloys. Cyclic softening can be caused by the failure of the precipitation structure during cyclic loading.
Thermal fatigue: The fatigue of a machine part under the action of cyclic thermal stress and thermal strain generated by temperature cyclic changes.
Thermo-Mechanical Fatigue: Fatigue is caused by the superposition of thermal cycling and mechanical stress cycling.
Two conditions for thermal stress: ① temperature change ② mechanical constraint
Impact fatigue: When the number of impacts N>10⁵ times, there is a typical fatigue fracture after failure, which is impact fatigue.