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1. Questions About Tensile Force-Elongation Curve And Stress-Strain Curve

Metal Stamping Material Stress-strain curve of low carbon steel

a. Deformation during stretching:

Elastic deformation, yield deformation, work hardening (uniform plastic deformation), uneven concentrated plastic deformation.

b. Related formulas:

Engineering stress σ=F/A0; engineering strain ε=ΔL/L0; proportional limit σP; elastic limit σε;

Yield point σS; tensile strength σb; breaking 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 and the stress-strain curve are basically the same; the plastic deformation stage is significantly different.

a. Related concepts

• Elasticity: characterize the ability of a material to deform elastically
• Rigidity: characterizes the resistance to elastic deformation of a material
• Elastic modulus: A constant reflecting 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.
• The specific energy of elasticity: It is called the specific energy of elasticity or the specific energy of strain. It is the ability of a material to absorb the work of deformation in the process of elastic deformation, and it is used to evaluate the elasticity of the material.
• Bauschinger effect: A small amount of plastic deformation is produced by pre-loading of metal materials, and then loaded in the same direction, the residual elongation stress is increased; reverse loading, the phenomenon that the residual elongation stress is reduced.
• Elastic hysteresis loop: In the case of non-ideal elasticity, because the stress and strain are not synchronized, the loading line and the unloading line do not overlap and a closed loop is formed.
• The ability of metal materials to absorb irreversible deformation work under the action of alternating loads is called the cyclic toughness of metals, also called internal friction. b. Related theory: elastic deformations are all reversible.
• The ideal elastic deformation is single-valued, reversible, and instantaneous. However, since the actual metal is polycrystalline and has various defects, it is not complete when deformed elastically.
• The essence of elastic deformation is the reflection of the reversible deformation of the self-equilibrium position of the atoms or ions or molecules that constitute the material.
• The elastic modulus of single crystal and polycrystalline metals mainly depends on the nature of the metal atom and the crystal type.
• Bauschinger effect; anelasticity; pseudo-elasticity; viscoelasticity.
• Bauschinger effect elimination method: large plastic deformation in advance, annealing at the temperature of recovery or recrystallization.
• Cycle toughness indicates the shock absorption ability of the material.

3. Questions About Shaping And Deformation

a. Related concepts

• Slip: the more the slip system, the better the plasticity; the slip system is not the only factor (factors such as lattice resistance); the slip surface-affected by temperature, composition and deformation; the direction of slip-comparison Stable twinning: fcc, bcc, and hcp can all produce plastic deformation by twinning; it usually occurs under low temperature and high speed conditions; the amount of deformation is small, adjust the direction of the slip surface
• Yield phenomenon: Annealed, normalized, quenched and tempered medium and low-carbon steels and low-alloy steels are more common, divided into discontinuous yielding and continuous yielding;
• Yield point: the corresponding stress value when the material yields in tension, σs;
• Upper yield point: the maximum stress value before the sample yields and the force drops for the first time, σsu;
• Lower yield point: the minimum stress in the yield stage of the specimen, σsl;
• Yield platform (yield tooth): the horizontal line segment or zigzag line segment corresponding to the yield elongation;
• Lüders belt: uneven deformation; for stamping parts, it is not allowed to appear to prevent wrinkles.
• Yield strength: characterize the resistance of a material to small amounts of plastic deformation

Yield strength of continuous yield curve: Use specified microplastic elongation stress to characterize the resistance of material to microplastic deformation

• (1) Provision of non-proportional elongation stress σp:
• (2) Specified residual elongation stress σr: After the tensile force of the sample is removed, the residual elongation of the gauge length reaches the specified percentage of the original gauge length; when the percentage of residual elongation is 0.2% , Denoted as σr0.2
• (3) Specified total elongation stress σt: the stress when the total elongation (elastic elongation plus plastic elongation) of the gauge length of the specimen reaches the specified percentage of the original gauge length.

Lattice resistance (Penna force); resistance to dislocation interaction

Hollomon formula: S=Ken, S is true stress, e is true strain; n—hardening index 0.1~0.5, n=1, perfectly ideal elastomer, n=0, no hardening ability; K——hardening coefficient

Necking is a special phenomenon in which the deformation of ductile metal materials 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 the ductile metal sample. Represents the maximum tensile stress that a metal material can withstand, and characterizes the resistance of the metal material to the maximum uniform plastic deformation. It is related to strain hardening index and strain hardening coefficient. Equal to the original cross-sectional area of ​​the maximum tensile stress ratio.

Plasticity refers to the ability of metal materials to undergo irreversible permanent (plastic) deformation before fracture

b. Related theories

Common plastic deformation methods: slip, twinning, sliding of grain boundaries, diffusive creep.

Characteristics of plastic deformation: the different time and unevenness 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, more System slip; Von Mises has at least 5 independent slip systems).

Determination of hardening index:

• ①test method;
• ②graphic method lgS=lgK+nlge

Influencing factors of hardening index: related to stacking fault energy, stacking fault energy decreases, hardening index increases; it is also very sensitive to the cold and hot deformation of metal materials; it is not equal to the strain hardening rate.

The criterion of necking (critical condition of instability) The criterion of stretching 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, formed as a necking

ψ=δ or ψ<δ, no necking is formed

4. Regarding The Toughness And Fracture Of Metals

a. Related concepts

• Toughness: Ability to absorb plastic deformation work and fracture work before fracture
• Toughness: the work absorbed per unit volume of material before it breaks
• Ductility fracture: energy is consumed in the process of slow crack propagation; fracture first occurs in the fiber area, then rapidly expands to form radiation and finally fractures to form a shear lip. The radiation zone is formed during the rapid crack propagation process, and the convergence direction of the radiation zone generally points Crack source.
• Brittle fracture: Basically no plastic deformation occurs, which is very harmful. Low stress brittle fracture, working stress is very low, generally lower than the yield limit; brittle fracture cracks always start from the internal macroscopic defects; the temperature decreases, the strain rate increases, and the brittle fracture tendency increases.
• Through crystal fracture: The crack penetrates the crystal, which can be ductile fracture or brittle fracture, and the fracture is bright.
• Fracture along the grain: The cracks propagate along the grain boundary, all of which are brittle fractures, caused by the second degree of brittleness at the grain boundary, and the fracture is relatively dark. Transgranular fracture and intergranular fracture can occur together. At high temperatures, the transition from transgranular fracture to intergranular ductile fracture.
• The fracture fracture along the crystal: the fracture is rock sugar-like; if the crystal grains are small, the fracture is crystal-like.
• Shear fracture: the fracture caused by the material sliding and separating along the sliding surface under the action of shear stress. (Sliding fracture, microporous aggregation type fracture)
• Cleavage fracture: The brittle transgranular fracture that occurs along a specific crystal plane due to the destruction of the original intermolecular bond under the action of normal stress. The strength of a metal refers to 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, which results in a large bonding force between the atoms and a high fracture strength. The essence of fracture is the process of separating the material along a certain atomic plane under the action of external force.
• Griffith Theory: From a thermodynamic point of view, all processes that reduce energy will proceed spontaneously, and all processes that increase energy must stop unless energy is provided by the outside world. Griffth pointed out that due to the existence of cracks, the elastic energy of the system decreases, which is balanced with the increased surface energy due to the existence of cracks. If the elastic energy is reduced enough to satisfy the increase in surface energy, the cracks will grow instability and cause brittle failure.

b. Related theories

Fracture has three main failure modes: wear, corrosion, and fracture. The fracture of most metals includes two stages: the formation and propagation of cracks.

According to the fracture behavior: ductile fracture and brittle fracture; according to the crack propagation path: transgranular fracture and intergranular fracture; according to the fracture mechanism: cleavage fracture and shear fracture ductile fracture and brittle fracture: according to the material before fracture The size of the macroscopic plastic deformation is determined. Pass

Normally brittle fracture will also cause a small amount of plastic deformation. Generally, it is stipulated that the reduction of area is less than 5%, which is brittle fracture. Conversely, more than 5% is ductile fracture. The brittle fracture is flat and bright, perpendicular to the normal stress, and the fracture often presents a herringbone pattern or radial pattern. Cleavage fracture is a brittle transgranular fracture that occurs along a specific crystal plane, and is usually separated along a certain crystal plane. Cleavage fracture is always brittle fracture, but brittle fracture is not necessarily cleavage fracture. Common crack formation theory:

• ①The theory of dislocation blockage
• ②Dislocation reaction theory

Cleavage and quasi-cleavage

Common points: cross-crystal fracture; small cleavage facets; steps and river patterns

The difference:

• ①The quasi-cleavage facet is not a crystallographic cleavage face
• ②Cleavage cracks often originate from grain boundaries, and quasi-cleavage cracks often originate from hard points in the grain. Quasi-cleavage is not an independent fracture mechanism, but a variant of cleavage fracture.

Graffith theory is a necessary condition for fracture to occur based on the principles of thermodynamics, but it does not mean that it must actually be fractured. The sufficient condition for the automatic crack propagation is that the tip stress is equal to or greater than the theoretical fracture strength.

a. Hardness concept

Hardness is a performance index that measures the hardness of metal materials.

b. Hardness test method:

• Scratch method-characterize metal cutting strength
• Rebound method-characterize the metal elastic deformation work
• Press-in method-characterize plastic deformation resistance and strain hardening ability

Brinell hardness

• Indenter: hardened steel ball (HBS), cemented carbide ball (HBW)
• Load: 3000Kg hard alloy, 500Kg soft material
• Warranty time: 10-15s for ferrous metals, 30s for non-ferrous metals
• The principle of indentation is similar: only one standard load and steel ball diameter is used, and it cannot be adapted to hard or soft materials at the same time. In order to ensure that the hardness values ​​measured by different loads and diameters are comparable, 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 retention time

Pros and cons of Brinell hardness test:

Advantages: Larger indenter diameter→larger indentation area→hardness value can reflect the average performance of each component phase of the metal in a larger range, and is not affected by individual component phases and small inhomogeneities.

Disadvantages: The diameter of the indenter and the test force need to be changed 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

• Rockwell hardness: The hardness value of the material is expressed by measuring the depth of indentation. 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 to operate, the hardness can be read directly; the indentation is small, and it can be tested on the workpiece; different scales can be used to determine the hardness and thickness of the test Sample. Disadvantages: small indentation and poor representativeness; if the material has defects such as segregation and uneven structure, the test value has poor repeatability and large dispersion; the hardness values ​​measured with different scales are not related and cannot be directly compared.
• Vickers hardness: The principle is the same as the Brinell hardness test, and the hardness value is calculated according to the test force borne by the unit area. The difference is that the Vickers hardness indenter is a diamond quadrangular pyramid with an angle α of 136° between two opposing faces.
• Knoop hardness: 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
• Shore hardness: A dynamic load test method. The principle is to drop a weight with a diamond round head or a steel ball on the surface of a metal sample from a certain height, and determine the height of the rebound of the weight. Characterizes the value of metal hardness, also known as rebound hardness. Expressed by HS.
• Leeb hardness: The dynamic load test method uses an impact body of specified quality 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 of the metal. Expressed by HL.

6. Regarding The Mechanical Properties Of Metals 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. The impact absorption work AK of commonly used standard specimens is expressed.

Impact measurement parameters: measure the impact absorption energy (AkU or AKV) after impact brittle fracture. The impact absorption energy does not truly reflect the toughness and brittleness of the material (impact absorption energy is not entirely used for specimen deformation and failure)</p >

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 tough state to a brittle state, the impact absorption energy is significantly reduced, and fracture The mechanism causes the micropores to aggregate into transcrystalline cleavage, and the fracture characteristics change from fibrous to crystalline. tk or temperature range is called ductile brittle transition temperature, also known as cold brittle transition temperature.

b. Related theories

Evaluation method of toughness and brittleness: notched impact bending test of materials, impact toughness of materials, influencing factors of toughness and brittleness: temperature (low temperature brittleness); stress state (three-way 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 the material as the temperature decreases. The yield strength σs increases with decreasing temperature, while the breaking strength σc changes little with temperature. t>tk,σc >σs, yield first and then fracture; t<tk,σc <σs, brittle fracture toughness-to-brittle transition temperature is the toughness index of metal materials, which reflects the influence of temperature on toughness and brittleness.

Metallurgical factors affecting ductile-brittle transition temperature:

• Crystal structure: body-centered cubic metals and their alloys have low-temperature brittleness. The matrix of ordinary medium and low-strength steel is body-centered cubic lattice ferrite, so this type of steel has obvious low-temperature brittleness.
• Chemical composition: Interstitial solute elements dissolve into the ferrite matrix and concentrate near the dislocation line, hindering the movement of the dislocation, resulting in an increase in σs and an increase in the ductile-brittle transition temperature of steel.
• Microstructure: The size of the grains. Refining the grains increases the toughness of the material; reducing the size of the subcrystalline and cellular structure can also improve the toughness.
• Refinement of grains improves toughness: the grain boundary is the resistance to crack propagation; the number of dislocations in front of the grain boundary is reduced, which is conducive to reducing stress concentration; the total area of ​​the grain boundary increases, which reduces the impurity concentration on the grain boundary , To avoid brittle fracture along the crystal.

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 the process of gradual change and accumulation of damage and cracking of the structure in the weak area of ​​the material under the action of variable stress. When the crack has grown to a certain extent, a sudden fracture occurs. It is a damage that starts from a local area. Accumulation, the process of ultimately causing total destruction.

Waveforms of cyclic stress: sine wave, rectangular wave, triangle wave, etc.

The parameters that characterize stress cycle characteristics 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, torsion fatigue, tension and compression fatigue, contact fatigue and compound fatigue;

Fatigue is based on environment and contact conditions: atmospheric fatigue, corrosion fatigue, high temperature fatigue, thermal fatigue and contact fatigue, etc.

Fatigue is divided into high and low stress levels and fracture life: high-cycle fatigue and low-cycle fatigue.

b. Metal fatigue characteristics

Characteristics of fatigue: This failure is a hidden sudden failure. Materials that show toughness or brittle failure under static load will not undergo obvious plastic deformation before fatigue failure, and are 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 notches or cracks 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. develop.

c. Macroscopic fracture of metal fatigue

Characteristics of fatigue macro-fracture: fatigue fracture has experienced crack initiation and propagation process. Due to the low stress level, there are obvious crack initiation and steady-state propagation stages, and the corresponding fractures also show 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 mostly appears on the surface of the machine part, and is often connected with defects such as nicks, cracks, knife marks, and pits. However, if there are serious metallurgical defects in the material (inclusions, shrinkage, pores, white spots, etc.), it will also cause fatigue sources inside the parts due to local material strength reduction.
• Features: Due to the repeated extrusion of the crack surface in the fatigue source area, there are many frictions, the fatigue source area is brighter, and the surface hardness of this area will increase due to work hardening.
• Quantity: There can be one or more sources of fatigue damage of the machine, which is related to the stress state and the degree of overload of the machine. If one-way bending fatigue only produces one source area, two-way repeated bending can cause two sources of fatigue. The higher the degree of overload and the greater the nominal stress, the greater the number of fatigue sources.
• Sequence of generation: If there are several fatigue sources in the fracture at the same time, the order of each fatigue source can be determined according to the size of each fatigue area and the brightness of the source area. The brighter the source area, the larger the connected fatigue area. The earlier it occurs; on the contrary, the later it occurs. The fatigue zone is the area formed by the metastable propagation of fatigue cracks.
• Macroscopic features: The fracture is smooth and has shell patterns (or beach patterns), and sometimes there are crack propagation steps. Smooth fracture is the continuation of the fatigue source area, and its degree gradually weakens as the crack propagates forward, reflecting the difference in the degree of crack propagation and extrusion friction.
• The shell line-the most typical feature of the fatigue zone: Cause: It is generally believed to be caused by load changes, because the machine often starts, stops, and accidentally overloads when the machine is running, and all have to leave an arc at the front line of the crack propagation Traces of shell lines.
• Characteristics: Each group of shell lines in the fatigue zone is like a cluster of parallel arcs centered on the fatigue source, with the concave side pointing to the fatigue source and the convex side pointing to the direction of crack propagation. The fringe lines near the fatigue source area are finer, indicating that the cracks grow slowly; the fringe lines far away from the fatigue source area are sparse and rough, indicating that the cracks grow faster in this section.

Influencing factors: The total metal stamping material range of the shell pattern area is related to the degree of overload and the nature of the material. If the nominal stress of the machine part is high or the material toughness is poor, the fatigue zone will be smaller and the shell lines will not be obvious; on the contrary, the low nominal stress or high toughness material will have a larger fatigue zone and the shell lines will be thick and obvious. The shape of the shell line is determined by the propagation speed of each point of the crack front line, the type of load, the degree of overload, and the stress concentration. The transient zone is the area formed by the crack instability and propagation. In the subcritical fatigue growth stage, with the increase of stress cycles, the cracks continue to grow. When it increases to the critical size ac, the stress field intensity factor KI at the crack tip reaches the material fracture toughness KIc(Kc). The cracks quickly propagate instability and cause instantaneous fracture of the machine parts. The fracture in the transient zone is rougher than the fatigue zone, and the macroscopic characteristics are like static loads, which vary with the properties of the material. The fracture of brittle material is crystalline; the fracture of ductile material is radial or herringbone in the plane strain zone of the core, and there is a shear lip zone in the edge plane stress zone. Location: The transient area should generally be on the opposite side of the fatigue source. But for rotational bending, when the nominal stress is low, the position of the instantaneous breaking zone is deflected by an angle against the direction of rotation; when the nominal stress is high, multiple fatigue sources expand inward from the surface at the same time, moving the instantaneous zone to the center position.

Size: The size of the instantaneous breaking zone is related to the nominal stress of the machine parts and the material properties. For high nominal stress or low toughness materials, the instantaneous breaking zone is large; vice versa. The transient area is smaller.

d. Fatigue curve and basic fatigue mechanical properties

Fatigue curve: the relationship curve between fatigue stress and fatigue life, namely S-N curve.

Purpose: It is the basis for determining the fatigue limit and establishing the fatigue stress criterion. There are horizontal sections (carbon steel, alloy structural steel, ductile iron, etc.): fatigue fracture does not occur after infinite stress cycles, and the corresponding stress is called the fatigue limit, which is recorded as σ-1 (symmetrical cycle) without horizontal section (aluminum Alloy, stainless steel, high-strength steel, etc.): It’s just that the cycle times continue to increase as the stress decreases. At this time, the stress that does not break 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

e. Fatigue process and mechanism

Fatigue process: three processes of crack initiation, metastable propagation, and instability propagation.

Fatigue life Nf = initiation period N0 + metastable growth period Np The fatigue process of metallic materials is also the process of crack initiation phase propagation. Crack initiation is often completed in the weak area or high stress area of ​​the material through uneven slippage, formation and growth of micro-cracks. Fatigue microcracks are often caused by uneven slippage and microcracking. The main methods are: cracking of the surface slip zone; cracking of the second phase, the interface between the inclusion and the matrix or the inclusion itself; cracking at the grain boundary or sub-grain boundary.

f. How to improve fatigue strength

How to improve the fatigue strength-the angle of the crack caused by the slip zone cracking From the fatigue crack formation mechanism of slip cracking, as long as the material’s slip resistance (solid solution strengthening, fine-grain strengthening, etc.) can be improved, fatigue cracks can be prevented Initiation, improve fatigue strength.

How to improve the fatigue strength-the crack angle caused by the phase interface cracking; the mechanism of the second phase or inclusions that can cause fatigue cracks, as long as the second phase or inclusion brittleness is reduced, the phase interface strength is increased, and the second phase is controlled Or the number, shape, size and distribution of inclusions to make them “less, 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-cracks are caused by cracks in grain boundaries; from the point of view of cracks initiating at grain boundaries, all factors that weaken grain boundaries and coarsen grains, such as low melting point inclusions and other harmful elements and component segregation in grain boundaries , Tempering brittleness, hydrogen evolution at the grain boundary, and grain coarsening, etc., are easy to produce grain boundary cracks and reduce fatigue strength; on the contrary, all factors that strengthen, purify and refine the grains can inhibit the formation of grain boundary cracks. , Improve fatigue strength.

g. Main factors affecting fatigue strength

The influence of the surface condition: stress concentration-due to stress concentration, the surface gap of the machine part is often the source of fatigue, causing fatigue fracture. Kf and qf can be used to characterize the impact of the 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 becomes more significant as the strength of the material increases.

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 strength of the material, the more significant the influence of surface roughness on the fatigue limit. The influence of residual stress and surface strengthening: Residual compressive stress improves 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 smaller. 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 size, depth, distribution of residual compressive stress and whether relaxation occurs will affect the fatigue strength.

The effect of surface strengthening-surface strengthening can generate residual compressive stress on the surface of the machine part, while increasing the strength and hardness. Both effects will increase fatigue strength. (Method: 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 → carburizing nitrogen.

The influence of material composition and organization: fatigue strength is a mechanical property that is sensitive to the structure of the material. Alloy composition, microstructure, non-metallic inclusions and metallurgical defects

h, low cycle fatigue

Low cycle fatigue: The fatigue life of metal is 102 to 105 times of fatigue fracture under cyclic loading. The phenomenon of cyclic hardening and cyclic softening is related to the cyclic movement of dislocations. In some soft annealed metals, under constant strain amplitude cyclic loading, due to the reciprocating motion and interaction of dislocations, resistance to the continued movement of dislocations is generated, resulting in cyclic hardening.

The cold-worked metal is full of dislocation entanglements and obstacles, which are destroyed during cyclic loading; or in some alloys that are unstable by precipitation strengthening. As the precipitation structure is damaged in the cyclic loading, it can cause the cycle to soften.

Thermal fatigue: the fatigue of mechanical parts under the action of cyclic thermal stress and thermal strain generated when the temperature is cyclically changed.

Thermomechanical fatigue: Fatigue caused by the superposition of temperature cycles and mechanical stress cycles.

Two conditions for thermal stress:

• ①temperature change
• ②mechanical restraint

Impact fatigue: When the number of impacts N>105 times, there will be a typical fatigue fracture after failure, that is, impact fatigue.