When all else fails, they say, in situations where there is nothing more to be done, because we have done what we could. Yes, but when are all the ropes cut? In fact, do all ropes break at the same time? Not at all. There are easier and harder to pour ropes. What we can expect from them is determined by their tensile strength. A bit of mechanics after metallurgy.
The concept of tensile strength
Tensile strength is an important concept in engineering, particularly in materials science, mechanical engineering and structural engineering.
Tensile strength is the mechanical stress required to break a rope, wire, girder or similar structural element.
Symbol s, unit Pa (pascal).
The tensile strength of the material is only its strength under constant load provides information that, under dynamic stresses, materials will tear at much lower stress levels.
The tensile strength of the as a function of temperature changes (usually decreases), at higher temperatures under constant load the material undergoes a steadily increasing deformation (permanent flow).
Tensile strength and elongation are determined by MSZ 5360-71 are defined according to the standard.
Determination of tensile strength
To determine the tensile strength, the material shall be subjected to mock test a static tensile test is carried out.
This is a destructive method, in which the workpiece, trained to the shape of the test specimen, is gradually stretched on a tensile testing machine with constant tension until the specimen breaks.
During the process, the elongation of the workpiece is measured as a function of the change in load, which means that the pulling force is slowly increased and the machine picks up the stress-strain diagram.
This elongation-strength diagram is one of the results of the tensile test, which barcode diagramare called. The process of stretching and breaking can be traced and divided into stages.
- The first stage is the elastic deformation stage. At this stage, no deformation occurs that would prevent the sample from regaining its original shape.
- The second stage is the uniform deformation. Then the plastic deformation is the same throughout the drawn sample.
- The third stage is the contraction, when the sample narrows at one point and then breaks at that point.
The Hooke Law
Hooke's law is a law governing the relationship between the elastic deformations of solid bodies and the forces or stresses that produce them, according to which the amount of elastic deformation within a certain limit (elastic limit) is proportional to the force that creates it.
Important measurable voltages
There are three important stresses to consider in the failure of materials under constant load:
- River crossing (Re): the stress that the material can withstand without permanent deformation. This point cannot always be defined in concrete terms, and is therefore considered instead for some materials as the stress as the river's edge, where the residual deformation is 0.2%. Below the yield point, all deformation is reversible. Beyond the yield point, the material suffers a permanent deformation (elongation in the case of steel).
- Tensile strength (Rm): the maximum stress that a material can withstand without breaking, or the amount of stress that can be of the maximum force point of a break diagram and the ratio of the cross section of the sample.
- Tear: can be read from the discontinuity diagram Tension, where the material breaks.
Interesting facts, brittle materials have no yield point and no hardening phase, and the maximum stress and tensile strength are the same.
If the metals cold working or drawn, their tensile strength is increased.
Typical strength values for some materials
For interest, the table below shows the typical strength values of some materials.
| Name of material | Flow limit (MPa) | Maximum stress (MPa) | Density (g/cm³) |
| Structural steel A36 | 250 | 400 | 7,8 |
| High-strength alloy steel A514 | 690 | 760 | 7,8 |
| High-strength prestressed steel wire | 1650 | 1860 | 7,8 |
| Piano wire (steel) | about 2000 | 7,8 | |
| High density polyethylene (HDPE) | 26-33 | 37 | 0,95 |
| Polypropylene | 12-43 | 19,7-80 | 0,91 |
| Corrosion resistant steel AISI 302 - (cold rolled) | 520 | 860 | |
| Cast iron 4,5% C, ASTM A-48 | 130 | 200 | |
| Titanium alloy (6% Al, 4% V) | 830 | 900 | 4,51 |
| Glass | 50 (on pressure) | 2,53 | |
| Marble | N/A | 15 | |
| Concrete | N/A | 2-5 (on draw) (6)-20-60-(200) (for pressure) |
2,4 |
| Caterpillar | 500 | ||
| Bone | 130 |
