Tensile testing of metals

What “tensile testing of metals“? Let's make a simple experiment. Let's take a little spring, eg, such, which are used in ballpoint pens. Smoothly stretch it a little and let it go. The spring will return to its original length.. Let's repeat the experience, but this time we will pull the spring harder. At first, the spring will lengthen evenly with increasing force., and then suddenly starts lengthening much faster. Letting go of the spring – it no longer returns to its original length. The spring has received an irreversible increase in its length and is no longer suitable for the previous application..

Tensile test

Long ago, engineers developed a similar test - tensile test - to assess the mechanical properties of metals. Metal sample, often round bar (sometimes rectangular), stretched on a special machine. Tensile test requirements for metals, as well as the requirements for specimens for tensile testing is determined by GOST 1497-84. GUEST 7564-97 sets the rules for cutting samples for tensile tests from finished products or semi-finished products.

Tearing machine

For tensile testing of metals, special machines are used.. Such machines are called "tensile testing machine" or "tensile testing machine". These machines ensure that the specimen is reliably centered in their grippers., smooth loading and unloading of a specimen under tension, slow rate of elastic and plastic deformation of the sample. The load is applied along the bar axis, as shown schematically in the figure. Requirements for tensile testing machines are determined by GOST 7855-84.

diagramma-rastyazheniyaPicture – Strain diagram for tensile testing of metals

In a tensile test with increasing tensile force, the rod becomes longer and longer and this change in length is denoted as Δl, where the sign Δ denotes “change, increment", and l is the initial length of the sample. clear, that a force F of magnitude 50 kg, attached to each of two different rods - thin and thick – from the same material will give them a different increase in length. The thin rod will stretch, naturally, more.

Voltage

To compare the mechanical properties of materials regardless of the diameter of the samples, the term "stress" is used., which simply means the amount of effort, divided by the cross-sectional area of ​​the sample. When the same stresses are applied to a thin rod and a thick rod, they both lengthen by the same amount. clear, that with this effort, attached to the thick rod, will be more than effort, attached to a thin rod - more than just the same amount of time, how much its cross-sectional area is greater than the cross-sectional area of ​​a thin rod. Since stress is effort per unit area, then its unit of measurement is N / mm2 or kg / mm2 (kgf / mm2), where H is Newton, SI unit of force. Ten newtons are equal to one kilogram (more precisely 1 N = 9,8 kg(kgf)).

Tensile strain diagram

When, in a tensile test, the bar is stretched along its axis, then the applied forces are called "tensile forces", and the car, which causes these forces - "tensile testing machine" or "tensile testing machine". The figure shows a typical deformation diagram, which is obtained by tensile testing of metals. The applied voltage is plotted along the vertical axis. The change in sample length is plotted along the horizontal axis., but not in units of length, in relative units Δl / l, as shown in the picture 1. This unit is called "deformation". Tensile strain diagram is more commonly referred to as "tensile diagram".

Stretch diagram

Using a GOST stretch diagram 1497-84 defines the mechanical properties of metals: proportional limit, yield point (physical and conditional), temporary resistance, relative extension, relative narrowing. Below we will briefly review the most important ones..

Elastic section of the tensile diagram

The stretch diagram can be divided into two areas, as shown in the picture 1 - elastic area and plastic area. When the stress in the metal bar increases, the rod is lengthened, as well as a spring. They say, that deformations occur in the bar. Until, until these stresses and strains are too great, removing the load on the bar returns it to its original length. These deformations are called elastic..

Yield point

At the end of the elastic section of the tensile diagram, the stresses in the bar reach a certain critical level, which is called the "yield point", metal "for rent", just like the spring, described above. The deformation of the sample passes into the plastic region of the deformation diagram.

Temporary resistance

When testing metals in tension on the plastic section of the tensile diagram - after passing the yield point, two important phenomena are observed:
1) to continue the deformation of the sample for a given strain increment, a smaller increase in stress is required, than in the elastic region;
2) when unloading a sample – stress relief – residual, irreversible elongation of the bar as shown by the arrow line AB. The rod is loaded to point A, and then the load is removed: the bar is elongated from its original length by a percentage, which is calculated as B × 100. As shown in the figure, the increase in voltage, which is required for the continuation of plastic deformation reaches a maximum in the plastic region and then drops slightly before the stresses break the bar into two parts. This maximum stress is commonly referred to as "ultimate strength" or more commonly "tensile strength".

Relative extension

In addition to the yield point and ultimate strength, the deformation diagram gives one more measure of the mechanical properties of the metal - "relative elongation". Elongation characterizes the plastic properties of the metal. Elongation is the increase in sample length, which occurs after passing the yield point and until the very destruction of the rod. It is sometimes called permanent elongation., so it remains in the sample after its destruction and can be easily measured. The residual elongation of the specimen in the figure after, how elastic deformations relaxed, denoted by point C. By simply multiplying the deformation at point C by 100 we obtain the value of the relative elongation of the sample.

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