Yield (engineering)

This article is about the science of material yield. For predicting yield, seeMaterial failure theory.

Stress–strain curveshowing typicalyieldbehavior fornonferrous alloys(stress,$\sigma$,shown as a function ofstrain,$\epsilon$):

1. True elastic limit
2. Proportionality limit
3. Elastic limit
4. Offsetyield strength

Inmaterials scienceandengineering,theyield pointis the point on astress–strain curvethat indicates the limit ofelasticbehavior and the beginning ofplasticbehavior. Below the yield point, a material willdeform elasticallyand will return to its original shape when the appliedstressis removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible and is known asplastic deformation.

Theyield strengthoryield stressis amaterial propertyand is the stress corresponding to the yield point at which the material begins to deform plastically. The yield strength is often used to determine the maximum allowableloadin a mechanical component, since it represents the upper limit to forces that can be applied without producing permanent deformation. For most metals, such asaluminiumandcold-workedsteel,there is a gradual onset of non-linear behavior, and no precise yield point. In such a case, the offset yield point (orproof stress) is taken as the stress at which 0.2% plastic deformation occurs. Yielding is a gradualfailure modewhich is normally notcatastrophic,unlikeultimate failure.

Forductilematerials, the yield strength is typically distinct from theultimate tensile strength,which is the load-bearing capacity for a given material. The ratio of yield strength to ultimate tensile strength is an important parameter for applications such steel forpipelines,and has been found to be proportional to thestrain hardening exponent.

Insolid mechanics,the yield point can be specified in terms of the three-dimensional principal stresses ($\sigma _{1},\sigma _{2},\sigma _{3}$) with ayield surfaceor ayield criterion.A variety of yield criteria have been developed for different materials.

Definitions

Material	Yield strength (MPa)	Ultimate strength (MPa)
ASTMA36 steel	250	400
Steel, API 5L X65	448	531
Steel, high strength alloy ASTMA514	690	760
Steel, prestressing strands	1650	1860
Piano wire		1740–3300
Carbon fiber(CF, CFK)		5650
High-density polyethylene(HDPE)	26–33	37
Polypropylene	12–43	19.7–80
Stainless steelAISI 302 – cold-rolled	520	860
Cast iron4.5% C, ASTM A-48	172
Titanium alloy(6% Al, 4% V)	830	900
Aluminium alloy2014-T6	400	455
Copper 99.9% Cu	70	220
Cupronickel10% Ni, 1.6% Fe, 1% Mn, balance Cu	130	350
Brass	200+ ~	550
Spider silk	1150 (??)	1400
Silkwormsilk	500
Aramid(KevlarorTwaron)	3620	3757
UHMWPE	20	35
Bone(limb)	104–121	130
Nylon, type 6/6	45	75
Aluminium (annealed)	15–20	40–50
Copper (annealed)	33	210
Iron (annealed)	80–100	350
Nickel(annealed)	14–35	140–195
Silicon(annealed)	5000–9000
Tantalum(annealed)	180	200
Tin(annealed)	9–14	15–200
Titanium(annealed)	100–225	240–370
Tungsten(annealed)	550	550–620

It is often difficult to precisely define yielding due to the wide variety ofstress–strain curvesexhibited by real materials. In addition, there are several possible ways to define yielding:

True elastic limit

The lowest stress at whichdislocationsmove. This definition is rarely used since dislocations move at very low stresses, and detecting such movement is very difficult.

Proportionality limit

Up to this amount of stress, stress is proportional to strain (Hooke's law), so the stress-strain graph is a straight line, and the gradient will be equal to theelastic modulusof the material.

Elastic limit (yield strength)

Beyond the elastic limit, permanent deformation will occur. The elastic limit is, therefore, the lowest stress point at which permanent deformation can be measured. This requires a manual load-unload procedure, and the accuracy is critically dependent on the equipment used and operator skill. Forelastomers,such as rubber, the elastic limit is much larger than the proportionality limit. Also, precise strain measurements have shown that plastic strain begins at very low stresses.

Yield point

The point in the stress-strain curve at which the curve levels off and plastic deformation begins to occur.

Offset yield point (proof stress)

When a yield point is not easily defined on the basis of the shape of the stress-strain curve anoffset yield pointis arbitrarily defined. The value for this is commonly set at 0.1% or 0.2% plastic strain.The offset value is given as a subscript, e.g.,$R_{\text{p0.1}}=310$MPa or$R_{\text{p0.2}}=350$MPa.For most practical engineering uses,$R_{\text{p0.2}}$is multiplied by a factor of safety to obtain a lower value of the offset yield point. High strength steel and aluminum alloys do not exhibit a yield point, so this offset yield point is used on these materials.

Upper and lower yield points

Some metals, such asmild steel,reach an upper yield point before dropping rapidly to a lower yield point. The material response is linear up until the upper yield point, but the lower yield point is used in structural engineering as a conservative value. If a metal is only stressed to the upper yield point, and beyond,Lüders bandscan develop.

Usage in structural engineering

Yielded structures have a lower stiffness, leading to increased deflections and decreased buckling strength. The structure will be permanently deformed when the load is removed, and may have residual stresses. Engineering metals display strain hardening, which implies that the yield stress is increased after unloading from a yield state.

Testing

Yield strength testing involves taking a small sample with a fixed cross-section area and then pulling it with a controlled, gradually increasing force until the sample changes shape or breaks. This is called atensile test.Longitudinal and/or transverse strain is recorded using mechanical or optical extensometers.

Indentation hardnesscorrelates roughly linearly with tensile strength for most steels, but measurements on one material cannot be used as a scale to measure strengths on another.Hardness testing can therefore be an economical substitute for tensile testing, as well as providing local variations in yield strength due to, e.g., welding or forming operations. For critical situations, tension testing is often done to eliminate ambiguity. However, it is possible to obtain stress-strain curves from indentation-based procedures, provided certain conditions are met. These procedures are grouped under the termIndentation plastometry.

Strengthening mechanisms

There are several ways in which crystalline materials can be engineered to increase their yield strength. By altering dislocation density, impurity levels, grain size (in crystalline materials), the yield strength of the material can be fine-tuned. This occurs typically by introducing defects such as impurities dislocations in the material. To move this defect (plastically deforming or yielding the material), a larger stress must be applied. This thus causes a higher yield stress in the material. While many material properties depend only on the composition of the bulk material, yield strength is extremely sensitive to the materials processing as well.

These mechanisms for crystalline materials include

• Work hardening
• Solid solution strengthening
• Precipitation strengthening
• Grain boundary strengthening

Work hardening

Where deforming the material will introducedislocations,which increases their density in the material. This increases the yield strength of the material since now more stress must be applied to move these dislocations through a crystal lattice. Dislocations can also interact with each other, becoming entangled.

The governing formula for this mechanism is:

$\Delta \sigma _{y}=Gb{\sqrt {\rho }}$

where$\sigma _{y}$is the yield stress, G is the shear elastic modulus, b is the magnitude of theBurgers vector,and$\rho$is the dislocation density.

Solid solution strengthening

Byalloyingthe material, impurity atoms in low concentrations will occupy a lattice position directly below a dislocation, such as directly below an extra half plane defect. This relieves a tensile strain directly below the dislocation by filling that empty lattice space with the impurity atom.

The relationship of this mechanism goes as:

$\Delta \tau =Gb{\sqrt {C_{s}}}\epsilon ^{\frac {3}{2}}$

where$\tau$is theshear stress,related to the yield stress,$G$and$b$are the same as in the above example,$C_{s}$is the concentration of solute and$\epsilon$is the strain induced in the lattice due to adding the impurity.

Particle/precipitate strengthening

Where the presence of a secondary phase will increase yield strength by blocking the motion of dislocations within the crystal. A line defect that, while moving through the matrix, will be forced against a small particle or precipitate of the material. Dislocations can move through this particle either by shearing the particle or by a process known as bowing or ringing, in which a new ring of dislocations is created around the particle.

The shearing formula goes as:

$\Delta \tau ={\frac {r_{\text{particle}}}{l_{\text{interparticle}}}}\gamma _{\text{particle-matrix}}$

and the bowing/ringing formula:

$\Delta \tau ={\frac {Gb}{l_{\text{interparticle}}-2r_{\text{particle}}}}$

In these formulas,$r_{\text{particle}}\,$is the particle radius,$\gamma _{\text{particle-matrix}}\,$is the surface tension between the matrix and the particle,$l_{\text{interparticle}}\,$is the distance between the particles.

Grain boundary strengthening

Where a buildup of dislocations at a grain boundary causes a repulsive force between dislocations. As grain size decreases, the surface area to volume ratio of the grain increases, allowing more buildup of dislocations at the grain edge. Since it requires a lot of energy to move dislocations to another grain, these dislocations build up along the boundary, and increase the yield stress of the material. Also known as Hall-Petch strengthening, this type of strengthening is governed by the formula:

$\sigma _{y}=\sigma _{0}+kd^{-{\frac {1}{2}}}\,$

where

$\sigma _{0}$is the stress required to move dislocations,

$k$is a material constant, and

$d$is the grain size.

Theoretical yield strength

Material	Theoretical shear strength (GPa)	Experimental shear strength (GPa)
Ag	1.0	0.37
Al	0.9	0.78
Cu	1.4	0.49
Ni	2.6	3.2
α-Fe	2.6	2.75

The theoretical yield strength of a perfect crystal is much higher than the observed stress at the initiation of plastic flow.

That experimentally measured yield strength is significantly lower than the expected theoretical value can be explained by the presence of dislocations and defects in the materials. Indeed, whiskers with perfect single crystal structure and defect-free surfaces have been shown to demonstrate yield stress approaching the theoretical value. For example, nanowhiskers of copper were shown to undergo brittle fracture at 1 GPa,a value much higher than the strength of bulk copper and approaching the theoretical value.

The theoretical yield strength can be estimated by considering the process of yield at the atomic level. In a perfect crystal, shearing results in the displacement of an entire plane of atoms by one interatomic separation distance, b, relative to the plane below. In order for the atoms to move, considerable force must be applied to overcome the lattice energy and move the atoms in the top plane over the lower atoms and into a new lattice site. The applied stress to overcome the resistance of a perfect lattice to shear is the theoretical yield strength, τ_max.

The stress displacement curve of a plane of atoms varies sinusoidally as stress peaks when an atom is forced over the atom below and then falls as the atom slides into the next lattice point.

$\tau =\tau _{\max }\sin \left({\frac {2\pi x}{b}}\right)$

where$b$is the interatomic separation distance. Since τ = G γ and dτ/dγ = G at small strains (i.e. Single atomic distance displacements), this equation becomes:

$G={\frac {d\tau }{dx}}={\frac {2\pi }{b}}\tau _{\max }\cos \left({\frac {2\pi x}{b}}\right)={\frac {2\pi }{b}}\tau _{\max }$

For small displacement of γ=x/a, where a is the spacing of atoms on the slip plane, this can be rewritten as:

$G={\frac {d\tau }{d\gamma }}={\frac {2\pi a}{b}}\tau _{\max }$

Giving a value of$\tau _{\max }$τ_maxequal to:

$\tau _{\max }={\frac {Gb}{2\pi a}}$

The theoretical yield strength can be approximated as$\tau _{\max }=G/30$.

Yield point elongation (YPE)

During monotonic tensile testing, some metals such asannealedsteelexhibit a distinct upper yield point or a delay in work hardening.These tensile testing phenomena, wherein the strain increases but stress does not increase as expected, are two types of yield point elongation.

Yield Point Elongation (YPE) significantly impacts the usability of steel. In the context of tensile testing and the engineering stress-strain curve, the Yield Point is the initial stress level, below the maximum stress, at which an increase in strain occurs without an increase in stress. This characteristic is typical of certain materials, indicating the presence of YPE.The mechanism for YPE has been related to carbon diffusion, and more specifically toCottrell atmospheres.

YPE can lead to issues such as coil breaks, edge breaks, fluting, stretcher strain, and reel kinks or creases, which can affect both aesthetics and flatness. Coil and edge breaks may occur during either initial or subsequent customer processing, while fluting and stretcher strain arise during forming. Reel kinks, transverse ridges on successive inner wraps of a coil, are caused by the coiling process.

When these conditions are undesirable, it is essential for suppliers to be informed to provide appropriate materials. The presence of YPE is influenced by chemical composition and mill processing methods such as skin passing or temper rolling, which temporarily eliminate YPE and improve surface quality. However, YPE can return over time due to aging, which is holding at a temperature usually 200-400 °C.

Despite its drawbacks, YPE offers advantages in certain applications, such asroll forming,and reducesspringback.Generally, steel with YPE is highly formable.

See also

• Plasticity (physics)
• Specified minimum yield strength
• Ultimate tensile strength
• Yield curve (physics)
• Yield surface

References

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