A stress test It is an experimental test that is carried out on a sample of material to determine how well it resists tensile stresses. Thanks to it, it is possible to know many of the mechanical properties of the material and determine if it is appropriate for a particular design..
The sample is usually a cylinder called test tube. This is subjected to a tension, consisting of applying two opposite forces at the ends that stretch the bar and deform it. The test continues exerting increasing efforts, until the specimen finally breaks.
Note the magnitude of the forces and the deformation that they produce in the specimen, from small forces that do not cause permanent deformation, to the stress that causes the part to break..
There the data collection ends and a stress-strain graph is drawn up, which will serve to analyze the behavior of materials such as metals, ceramics, cement, wood and polymers..
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The experiment is done using special machines like the one shown in figure 1, which provide the necessary effort to load and then unload the material to evaluate the deformation..
As for the specimen, it is a tube with a constant cross section, cylindrical, rectangular or square, whose dimensions are standardized. The ends are wider to facilitate clamping to the sample holder, as seen in figure 2 left.
The initial length Lor The calibrated region is measured and marked on the test tube. It is then clamped to the testing machine and the testing machine begins..
Materials have different behaviors under stress, shown in the following graph for which steel was used. The forces applied on the vertical axis are denoted by the Greek letter σ and the strain on the horizontal axis, called ε.
Strain has no dimensions, since it is the quotient between the change in length of the specimen ΔL = LF - Lor and the initial length. A) Yes:
ε = ΔL / Lor
For its part, the magnitude of the stress σ is the force / cross-sectional area ratio.
In the graph two important regions are distinguished: elastic zone and plastic zone.
When the tensile stress σ is small, the deformation is proportional, which is known as Hooke's law:
σ = Y ε
Once the effort ceases, the body returns to its original dimensions. This is the elastic region in color in Figure 3, which extends to the point called proportionality limit. Up to there the material obeys Hooke's law.
The constant of proportionality Y is the Young's modulus, characteristic of the material and which can be determined from tension and compression tests.
Young's modulus has units of pressure, in the international system [Y] = N / m ^ 2 = Pa. Strain, as already mentioned, is dimensionless, therefore the stress σ also has dimensions of force due to unit of cross-sectional area and in SI its unit will be the pascal: [σ] = N / m ^ 2 = Pa.
From the limit of proportionality and increasing the stress, one advances in a region where the deformation is reversible but does not obey Hooke's law. It ends at the point from which the body is permanently deformed, called elastic limit.
The material then enters the region of plastic behavior. Once the elastic behavior zone is exceeded, the steel enters the region of the Yielding effort or creep, in which the specimen deforms but does not break, although the stress remains constant at σY.
Once the yield zone is exceeded, the deformation increases with the applied stress, but no longer in a linear manner..
The material undergoes changes at the molecular level and strain hardening occurs. Therefore we see that increasing efforts are needed to achieve a deformation.
The limit of this zone is in the last effort. The material is considered broken at this point, although the specimen is still in one piece. From there, the load necessary to produce deformation is reduced and the specimen progressively thins (stricture) until it finally fractures (figure 2, right).
This curve and its regions are called conventional fracture stress. But above it is a dashed curve, called true fracture stress, which is obtained by recording the instantaneous or true length of the specimen, instead of working with the original length to find the strain, as explained at the beginning.
Both curves, the true and the conventional one, coincide in the zone of small efforts until the zone of yield. In any case, it is foreseen that the material works in the elastic range to avoid permanent deformations that prevent the good functioning of the manufactured part..
So among the most important data obtained from the test is the stress σY which defines the elastic limit.
The material used as a model in the above description is steel, which is widely used in construction and industry. But there are many materials such as concrete, concrete, various metals, alloys and wood, which are also used extensively..
Each one has a characteristic stress-strain curve, and according to its response to stress or traction, they are classified into two categories: brittle or ductile..
In the following graph σ versus ε (stress-strain) brittle materials are compared (brittle) and ductile (ductile), although it is necessary to clarify that the same material can have one or the other response depending on factors such as temperature. Materials tend to be brittle at low temperatures.
The notable difference between the two is that brittle material lacks or has a very small yield region. As soon as the elastic limit is exceeded, the specimen breaks. On the other hand, ductile materials absorb more energy before breaking, because they have a large plastic zone.
The stress test is useful to classify the material, being preferable according to the application the use of ductile materials, since they absorb more energy and are capable of deforming long before fracturing.
It should also be noted that although some materials are brittle under stress, they may resist other stresses better, as we will see below..
-Gray cast iron: brittle in tension, stronger in compression.
-Bronze: ductile.
-Concrete: fragile depending on the type of mixture, but very resistant in compression. When it is going to be put under tension it requires reinforcement by means of steel bars.
-Wood: depending on the origin, it is moderately ductile.
-Steel: brittle when high in carbon.
-Methacrylate: ductile with increasing temperature.
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