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The mechanical properties of a material affect how it behaves as it is loaded. The elastic modulus the the product affects exactly how much it deflects under a load, and also the toughness of the material determines the stresses the it deserve to withstand before it fails. The ductility the a material also plays a far-reaching role in determining when a product will break together it is loaded beyond its elastic limit. Due to the fact that every mechanical mechanism is subjected to loads during operation, it is essential to understand exactly how the products that make up those mechanical systems behave.

This page defines the mechanically properties of products that are relevant to the design and evaluation of mechanically systems.


Stress and StrainHooke"s Law
Strain EnergyStress-Strain Curve Approximation
Related Pages:
•Engineering Materials
•Strength the Materials
•Materials Database

Stress and also Strain

The relationship in between stress and also strain in a product is determined by subjecting a product specimen to a tension or compression test. In this test, a steadily boosting axial pressure is applied to a check specimen, and the deflection is measured together the fill is increased. These values can be plotted together a load-deflection curve. The deflection in the test specimen is dependence on both the material"s elastic modulus as well as the geometry that the specimen (area and also length). Since we room interested material habits without regard to geometry, the is beneficial to generalize the data to remove the result of geometry. This is excellent by converting the load values to tension values and converting the deflection values to stress, overload values:


In the equation because that stress, p is the load and A0 is the initial cross-sectional area the the check specimen. In the equation for strain, together is the present length of the specimen and L0 is the initial length.

Stress-Strain Curve

The worths of stress and strain figured out from the tensile test deserve to be plotted together a stress-strain curve, as shown below:


There are several points of attention in the diagram above:

P: This is the proportionality limit, which represents the maximum value of stress and anxiety at i beg your pardon the stress-strain curve is linear.E: This is the elastic limit, which represents the maximum worth of tension at which over there is no irreversible set. Even though the curve is no linear in between the proportionality limit and also the elastic limit, the product is quiet elastic in this region and if the fill is gotten rid of at or listed below this allude the specimen will go back to its original length.Y: This is the productivity point, which represents the value of stress over which the stress, overload will start to increase rapidly. The anxiety at the yield allude is referred to as the yield strength, Sty. For materials without a well-defined yield point, that is typically defined using the 0.2% offset method in i beg your pardon a line parallel come the linear part of the curve is drawn that intersects the x-axis at a strain value of 0.002. The suggest at i m sorry the line intersects the stress-strain curve is designated together the productivity point.U: This suggest corresponds come the can be fried strength, Stu, i m sorry is the maximum worth of tension on the stress-strain diagram. The ultimate stamin is additionally referred to as the tensile strength. After reaching the ultimate stress, specimens the ductile materials will exhibit necking, in i m sorry the cross-sectional area in a localized an ar of the specimen to reduce significantly.F: This is the fracture suggest or the break point, i beg your pardon is the suggest at i m sorry the product fails and also separates into two pieces.

Stress-strain curve are commonly needed when analyzing an engineered component. However, stress-strain data might not constantly be easily available. In this case, that is relatively straightforward to almost right a material"s stress-strain curve making use of the Ramberg-Osgood equation.

True Stress and also Strain

Engineers typically work with engineering stress, which is the force divided by the initial area the the specimen before loading: σ = P/A0. However, as a product is loaded, the area decreases. The true stress, , is the worth of stress in the material considering the really area of the specimen. Due to the fact that the area decreases together a product is loaded, true stress and anxiety is higher than design stress.

The figure listed below shows an design stress-strain curve as compared to a true stress-strain curve. Because the design stress is calculated as force divided by initial area (which is a constant), the engineering stress-strain curve has the exact same shape together the load-deflection curve. The engineering stress-strain curve drops after the ultimate strength is reached due to the fact that the pressure that deserve to be supported by the material drops together it starts to neck down. However, the stress value in the true stress-strain curve constantly increases together the stress, overload increases. This is due to the fact that the instantaneous value of area is used when calculating true stress. Even when the pressure supported through the product drops, the palliation in the specimen area outweighs the reduction in force, and also the stress continues to increase.


It should be noted that the design stress and also the true anxiety are essentially the same in the linear-elastic region of the stress-strain curve. Because engineers typically operate within this linear-elastic an ar (it is unusual to architecture a structure that is intended to operate beyond the elastic limit), it is precious to work with engineering stress together opposed come true stress.

Engineering strain is the readjust in length divided by the original length: ε = ΔL/L0. Rather of just calculating a single value the ΔL, think about that the readjust in size is divided among many tiny increments, ΔLj. The strain is likewise calculated in little increments: εj = ΔLj/Lj, where ΔLj is the adjust in length for an increment, and also Lj is the length at the start of the increment. Together these increments come to be infinitesimally small, the summation of the strains ideologies the true strain, :


If that is assumed that the volume is continuous throughout the deflection, climate true stress and also strain deserve to be calculation as:

True Stress:
True Strain:

where and also room the true stress and strain, and σ and also ε are the engineering stress and strain.

Hooke"s Law

Below the proportionality border of the stress-strain curve, the relationship in between stress and also strain is linear. The slope of this linear section of the stress-strain curve is the elastic modulus, E, additionally referred to as the Young"s modulus and the modulus of elasticity. Hooke"s regulation expresses the relationship in between the elastic modulus, the stress, and also the strain in a product within the linear region:

σ = E ε

where σ is the worth of stress and also ε is the value of strain.

Hooke"s law in Shear

Hooke"s law additionally has a kind relating shear stresses and also strains:

τ = G γ

where τ is the worth of shear stress, γ is the value of shear strain, and G is the shear modulus that elasticity. The elastic modulus and the shear modulus are related by:


where ν is Poisson"s ratio.

More information on Hooke"s law have the right to be uncovered here.

Poisson"s Ratio

As fill is applied to a material, the product elongates and the cross-sectional area is reduced. This reduction in cross-sectional area is dubbed lateral strain, and it is regarded the axial stress, overload by Poisson"s ratio, ν. Because that a one specimen this palliation in area is realized as a reduction in diameter, and also the Poisson"s proportion is calculate as:


Poisson"s proportion only applies within the elastic region of the stress-strain curve, and also it is typically around 0.3 for many metals. The theoretical maximum border of Poisson"s ratio is 0.5.

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Strain Hardening

After a material yields, it begins to experience a high price of plastic deformation. As soon as the material yields, it begins to strain harden which boosts the toughness of the material. In the stress-strain curve below, the strength of the material can be checked out to increase between the yield suggest Y and also the ultimate strength at suggest U. This boost in toughness is the an outcome of stress, overload hardening.

The ductile material in the figure below is quiet able to assistance load also after the ultimate strength is reached. However, ~ the ultimate stamin at point U, the rise in strength because of strain hardening is outpaced by the palliation in load-carrying capacity due to the decrease in overcome sectional area. Between the ultimate strength at suggest U and the fracture point F, the engineering strength that the material decreases and necking occurs.

In the stress-strain curve because that the brittle product below, a an extremely small an ar of strain hardening is shown between the yield point Y and the ultimate stamin U. Note yet that a brittle product may no actually exhibit any kind of yielding actions or stress, overload hardening at every -- in this case, the material would failure on the linear portion of the curve. This is an ext common in products such as ceramics or concrete.


Because the strain hardening region occurs between the yield suggest and the ultimate point, the ratio of the ultimate strength to the yield toughness is sometimes used as a measure up of the level of stress, overload hardening in a material. This proportion is the stress, overload hardening ratio:

strain hardening proportion = Stu / Sty

According to Dowling, common values of stress, overload hardening ratio in metals variety from roughly 1.2 to 1.4.

If a product is loaded past the elastic limit, it will certainly undergo permanent deformation. After unloading the material, the elastic strain will be recovered (return to zero) but the plastic strain will certainly remain.

The figure listed below shows the stress-strain curve that a material that was loaded beyond the yield point, Y. The an initial time the material was loaded, the stress and strain followed the curve O-Y-Y", and also then the pack was removed once the stress got to the point Y". Since the material was loaded past the elastic limit, just the elastic portion of the stress, overload is recovered -- over there is some permanent strain now in the material. If the product were to be loaded again, it would certainly follow heat O"-Y"-F, wherein O"-Y" is the previous unloading line. The suggest Y" is the brand-new yield point. Keep in mind that the line O"-Y" is linear with a slope same to the elastic modulus, and the allude Y" has actually a greater stress worth than point Y. Therefore, the product now has a higher yield allude than it had previously, i beg your pardon is a an outcome of stress, overload hardening that arisen by loading the material past the elastic limit.


By strain hardening the material, that now has actually a bigger elastic region and a greater yield stress, but its ductility has been lessened (the strain in between points Y"-F is much less than the strain between points Y-F).

Elastic and also Plastic Strain

Up come the elastic limit, the stress, overload in the product is likewise elastic and will it is in recovered when the fill is eliminated so that the product returns come its original length. However, if the product is loaded beyond the elastic limit, climate there will certainly be irreversible deformation in the material, which is also referred to as plastic strain.

In the figure above, both elastic and also plastic strains exist in the material. If the load is eliminated at the indicated point (σ, ε), the stress and also strain in the product will monitor the unload line together shown. The elastic strain and also plastic strain are indicated in the figure, and are calculation as:

Elastic Strain:εe = σ/E
Plastic Strain:εp = ε − εe

where σ is the anxiety at the suggested point, ε is the stress, overload at the indicated point, and also E is the elastic modulus.


Ductility is an indication of how much plastic stress, overload a material have the right to withstand prior to it breaks. A ductile material have the right to withstand huge strains even after it has begun to yield. Common measures of ductility include percent elongation and reduction in area, as disputed in this section.

After a specimen breaks throughout a tensile test, the last length that the specimen is measured and also the plastic strain at failure, additionally known as the strain at break, is calculated:


where Lf is the final length that the specimen after ~ break and also Lo is the initial size of the specimen. That is essential to note that after ~ the specimen breaks, the elastic strain the existed when the specimen was under pack is recovered, so the measured difference in between the final and also initial lengths gives the plastic strain at failure. This is portrayed in the figure below:

In the figure, it deserve to be seen that the plastic strain at failure, εf, is the strain staying in the material after the elastic strain has been recovered. The ultimate strain, εu, is the complete strain at failure (the plastic strain plus the elastic strain).

The percent elongation is calculated native the plastic stress, overload at failure by:


The percent elongation is a commonly provided material property, for this reason the plastic stress, overload at fail is commonly calculated indigenous percent elongation:

εf = eL / 100%

The ultimate stress, overload accounts because that both plastic and elastic stress, overload at failure:

εu = εf + Stu/E

Another important material residential or commercial property that can be measured throughout a tensile check is the reduction in area, i m sorry is calculated by:


Remember the percent elongation and reduction in area account for the plastic materials of the axial strain and also the lateral strain, respectively.

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In the figure above, the ductile material have the right to be watched to strain significantly prior to the fracture point, F. Over there is a long an ar between the productivity at suggest Y and also the ultimate toughness at point U whereby the product is stress, overload hardening. Over there is additionally a long an ar between the ultimate stamin at point U and the fracture point F in i m sorry the cross sectional area the the product is decreasing rapidly and also necking is occurring.

The brittle material in the figure over can be seen to break shortly after the productivity point. Additionally, the ultimate strength is coincident with the fracture point. In this case, no necking occurs.

Because the area under the stress-strain curve for the ductile material over is bigger than the area under the stress-strain curve for the brittle material, the ductile material has actually a greater modulus of toughness -- it have the right to absorb much more strain energy prior to it breaks. Additionally, due to the fact that the ductile material strains therefore significantly before it breaks, that deflections will be really high prior to failure. Therefore, it will be visually apparent that fail is imminent, and actions can be bring away to fix the situation prior to disaster occurs.

A representative stress-strain curve for a brittle product is shown below. This curve shows the stress and also strain for both tensile and also compressive loading. Note just how the product is much much more resistant come compression than to tension, both in regards to the tension that it can withstand as well as the strain before failure. This is usual for a brittle material.

See more: What Is 20/2 As A Mixed Number 20 2, Conversion Of Mixed Number 20 2

Strain Energy

When pressure is applied to a material, the product deforms and also stores potential energy, just like a spring. The strain power (i.e. The amount of potential energy stored due to the deformation) is same to the work expended in deforming the material. The full strain energy coincides to the area under the fill deflection curve, and also has units of in-lbf in us Customary units and also N-m in SI units. The elastic strain power can it is in recovered, therefore if the deformation remains within the elastic limit, then every one of the strain energy can be recovered.

Strain energy is calculation as:

General Form:U = work = ∫ F dL(area under load-deflection curve)
Within Elastic Limit:
(area under load-deflection curve)
(spring potential energy)

Note that there space two equations because that strain energy within th