Photo Bradley Forged Takumi (18 inches)
Forged wheel "Bloodley Forged Takumi" adds 300 inches for Land Cruiser 18
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In addition, the design boasts high accuracy without leaving any distortion due to residual stress by carving out all the surfaces with a machining center and various original tools.
4x4 engineering service is available on the forged aluminum wheel "Bloodley Forged Takumi" for Crocan, and the land ... → Continue reading
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Residual stress (Zanryuoryoku, residual stress) exists in an object even after the external force is removed.stressIs that.Hooke's lawCorresponds to residual stressStrain,Residual strainIt is called (Zanryu strain, residual strain).The distribution of residual stress varies, but of the objectbalancePositive and negative residual stresses are balanced throughout the object to satisfy the condition..
The generation of residual stress may or may not be desirable.Typically,compressionResidual stress improves strength,引張Residual stress reduces strength..For example, laser peening imparts beneficial compressive residual stress to metal parts such as turbine engine fan blades.again,Smartphone OfdisplayUsed inTempered glassIt is also applied to, large and thin, andCrack・ We have realized something that is resistant to scratches...However, the generation of unintended residual stress may cause premature fracture of the structure.
Residual stress is generated by various mechanisms.for example,Plastic deformationAnd temperature gradient, materialPhase transitionand so on.WeldingThe heat generated from time to time causes local expansion of the material.During welding, the parts being welded move and the molten metal absorbs the expansion, but when the weld is complete, some parts cool faster than others, resulting in residual stress.
Unintentional residual stress generation is often undesirable, but some designs take advantage of residual stress.especiallyBrittlenessIn the material, tempered glass andPrestressed concreteAs shown above, toughness can be strengthened by applying compressive residual stress in advance.The dominant fracture mechanism of brittle materials isBrittle fractureIt is called and begins with the formation of early cracks.When tensile stress is applied by external force, it is applied to the tip of the crack.Stress concentrationIs generated and the stress at the tip of the crack becomes very large.As a result, the initial crack grows rapidly and eventually leads to destruction.Materials with compressive residual stresses are reinforced against such brittle fractures.This is because even if an initial crack occurs, the crack is compressed by the residual stress.In order for brittle fracture to occur and cracks to grow, the tensile stress applied from the outside must exceed the compressive stress due to the residual stress.
SteelMetal likePlasticityFor materials, by hitting a small steel ball on the surface, work hardening by plastic deformation, homogenization of surface stress, and compressive residual stress are applied to the surface to improve strength.Shot peeningThere is a technique called.fatigueApplying compressive residual stress is also effective for fracture due toFatigue limitContributes to the improvement of fatigue crack growth rate.
swordIn the manufacture ofMartensiteUsing the generation gradient ofHardnessIs producing.especiallyKnifeThe production of is famous.The difference in residual stress between the hard cutting edge and the relatively soft back surface gives the curved shape characteristic of some sword types.
In tempered glass, compressive stress remains on the glass surface, and tensile stress is generated inside in an equilibrium.Tempered glass has resistance to cracks due to the compressive residual stress on the surface, but when the surface breaks, it has the property of shattering due to the internal tensile residual stress.This effect is created by dropping molten glass into cold water.Dutch tearsIt will be demonstrated in an easy-to-understand manner in an experiment called.In actual tempered glass, tempered glass that blows air onto the glass surface to quench it and give residual stress.Or chemicallyIon exchangeChemically tempered glass that gives residual stress usingand so on.
There are several techniques for measuring residual stress, which can be broadly divided into destructive and non-destructive methods...One of the fracture methods uses the stress or strain released when an object is cut, drilled, or cut.On the other hand, the method using nonlinear elasticity by ultrasonic waves or magnetism requires a standard sample.
X-ray diffraction Is one of the non-destructive methods and can measure local residual stress with a resolution of about 1 micrometers.   Neutron diffraction method Is also a method that enables local residual stress measurement.Which method is best depends on the design of the parts.
Removal of residual stress
When unnecessary residual stress is generated in the process of manufacturing metal parts, the residual stress is removed or reduced by several methods.These methods can be broadly divided into heat treatment methods and non-thermal methods..
As a method by heat treatment, stress relief annealing is used..AnnealingAmong them, it is especially done at low temperature and is also called low temperature annealing...For steel materials, above the recrystallization temperature, A1Stress-relieving annealing is performed by heating and holding below the transformation point temperature and slowly cooling after a certain period of time..
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- Energy-saving technology and computerized equipment for vibration stabilization of residual stress (the only CIS 35-year-old Scientific School, who presented in 1988, the world's first installation of computer diagnostics) are intended to stabilize the residual stresses in welds, castings and other non-rigid dynamic metal
- Residual stress at TWI (The Welding Institute)
- Comprehensive resources on Residual stresses at Cambridge University
- Residual stress measurement based on hole-drilling and ESPI (stresstech group)