Mechanical Properties of Materials | MechaniCalc
Check out our Stress-Strain Curve calculator based on the methodology Because the area decreases as a material is loaded, true stress is higher than. The resulting stress-strain curve or diagram gives a direct indication of the Percent Reduction in Area - The reduction in cross-sectional area of a tensile. True stress is the stress determined by the instantaneous load acting on the True Stress-Strain Curve.)ln( From the constancy-of-volume relationship,. T. A.
This ratio is the strain hardening ratio: If a material is loaded beyond the elastic limit, it will undergo permanent deformation.
After unloading the material, the elastic strain will be recovered return to zero but the plastic strain will remain. The figure below shows the stress-strain curve of a material that was loaded beyond the yield point, Y.
The first time the material was loaded, the stress and strain followed the curve O-Y-Y', and then the load was removed once the stress reached the point Y'. Since the material was loaded beyond the elastic limit, only the elastic portion of the strain is recovered -- there is some permanent strain now in the material. If the material were to be loaded again, it would follow line O'-Y'-F, where O'-Y' is the previous unloading line.
The point Y' is the new yield point. Note that the line O'-Y' is linear with a slope equal to the elastic modulus, and the point Y' has a higher stress value than point Y.
Therefore, the material now has a higher yield point than it had previously, which is a result of strain hardening that occurred by loading the material beyond the elastic limit. By strain hardening the material, it now has a larger elastic region and a higher yield stress, but its ductility has been reduced the strain between points Y'-F is less than the strain between points Y-F. Elastic and Plastic Strain Up to the elastic limit, the strain in the material is also elastic and will be recovered when the load is removed so that the material returns to its original length.
However, if the material is loaded beyond the elastic limit, then there will be permanent deformation in the material, which is also referred to as plastic strain. In the figure above, both elastic and plastic strains exist in the material. The elastic strain and plastic strain are indicated in the figure, and are calculated as: Ductility Ductility is an indication of how much plastic strain a material can withstand before it breaks.
A ductile material can withstand large strains even after it has begun to yield. Common measures of ductility include percent elongation and reduction in area, as discussed in this section. After a specimen breaks during a tensile test, the final length of the specimen is measured and the plastic strain at failure, also known as the strain at break, is calculated: It is important to note that after the specimen breaks, the elastic strain that existed while the specimen was under load is recovered, so the measured difference between the final and initial lengths gives the plastic strain at failure.
This is illustrated in the figure below: The percent elongation is calculated from the plastic strain at failure by: Ductile and Brittle Materials A ductile material can withstand large strains even after it has begun to yield, whereas a brittle material can withstand little or no plastic strain.
The figure below shows representative stress-strain curves for a ductile material and a brittle material. In the figure above, the ductile material can be seen to strain significantly before the fracture point, F. There is a long region between the yield at point Y and the ultimate strength at point U where the material is strain hardening. There is also a long region between the ultimate strength at point U and the fracture point F in which the cross sectional area of the material is decreasing rapidly and necking is occurring.
Stress–strain curve - Wikipedia
The brittle material in the figure above can be seen to break shortly after the yield 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 above is larger than the area under the stress-strain curve for the brittle material, the ductile material has a higher modulus of toughness -- it can absorb much more strain energy before it breaks. Additionally, because the ductile material strains so significantly before it breaks, its deflections will be very high before failure.
Mechanical Properties of Materials
Therefore, it will be visually apparent that failure is imminent, and actions can be taken to resolve the situation before disaster occurs. A representative stress-strain curve for a brittle material is shown below. This curve shows the stress and strain for both tensile and compressive loading.
Note how the material is much more resistant to compression than to tension, both in terms of the stress that it can withstand as well as the strain before failure. This is typical for a brittle material. Until this point, the cross-sectional area decreases uniformly and randomly because of Poisson contractions. The actual fracture point is in the same vertical line as the visual fracture point.
However, beyond this point a neck forms where the local cross-sectional area becomes significantly smaller than the original. If the specimen is subjected to progressively increasing tensile force it reaches the ultimate tensile stress and then necking and elongation occur rapidly until fracture. If the specimen is subjected to progressively increasing length it is possible to observe the progressive necking and elongation, and to measure the decreasing tensile force in the specimen.
The appearance of necking in ductile materials is associated with geometrical instability in the system. Due to the natural inhomogeneity of the material, it is common to find some regions with small inclusions or porosity within it or surface, where strain will concentrate, leading to a locally smaller area than other regions.
For strain less than the ultimate tensile strain, the increase of work-hardening rate in this region will be greater than the area reduction rate, thereby make this region harder to be further deform than others, so that the instability will be removed, i.
However, as the strain become larger, the work hardening rate will decreases, so that for now the region with smaller area is weaker than other region, therefore reduction in area will concentrate in this region and the neck becomes more and more pronounced until fracture. After the neck has formed in the materials, further plastic deformation is concentrated in the neck while the remainder of the material undergoes elastic contraction owing to the decrease in tensile force.
The stress-strain curve for a ductile material can be approximated using the Ramberg-Osgood equation. Brittle materials[ edit ] Brittle materials, which includes cast iron, glass, and stone, are characterized by the fact that rupture occurs without any noticeable prior change in the rate of elongation.
Therefore, the ultimate strength and breaking strength are the same. A typical stress—strain curve is shown in Fig.
Typical brittle materials like glass do not show any plastic deformation but fail while the deformation is elastic. One of the characteristics of a brittle failure is that the two broken parts can be reassembled to produce the same shape as the original component as there will not be a neck formation like in the case of ductile materials.
A typical stress—strain curve for a brittle material will be linear.