Firmness

firmness is a material property, which describes the mechanical resistance, which a material of a deformation opposes. From the stress-strain curve the technically relevant strength coefficients are determined. Depending upon material, material condition, temperature, load and rate of load application different strengtheningnesses can be achieved.

With the firmness one differentiatesa tensile stress between the terms

the elongation limit becomes thereby a certain plastic deformation, e.g. 0,2%,assigned. One writes then [itex] {p0,2 to R_}< /math>. (Pronounced) the yield strength plays only with un and low-alloy steel in certain conditions during heat treatment a role, in particular with structural steel.

Into the mechanical interpretation of construction units minimum value flows and/or. ensured value of the strengtheningnesses.

The minimum tensile strength is for example with one Steel (S235JRG2 - in former times St37-2), which in the steel structure use finds, depending upon quality with 370 N/mm ². Its minimum yield strength however with 235 N/mm ². If one became now in a traction test a sample of this steel, which has a cross section of 1 mm ², with Kraft to load, this would havewith at least 370 N lie around the sample to tear up. 370 N corresponds to the weight of a mass of 37,7 kg on earth. From it it can be concluded that with the attempt to lift with this steel wire a mass of 37,7 kg or more largely a failure of theMaterial any longer to be excluded cannot. In addition, with smaller loads the wire can be deformed lasting (plastic). Since this becomes mostly not certified, one uses the minimum yield strength (R e) during the mechanical interpretation of construction units frequently. This value describes the firmness of a materialup to the border of the flexible deformation. That is called with a traction power F z of 235 N on a sample with a cross section of 1 mm ² stretches this sample, it returns however, without deforming lasting (plastic), into their origin condition. Here leavesa mass of 23,955 kg (with 235 N/9.81 m/s ²) determine themselves, with whose weight this material in the traction test can be loaded, however flexibly behaves.

For safety reasons the characteristic values mentioned are divided in technical applications in principle still by a safety factor, that thoseUncertainties with the evaluation of the demand and the dispersion of the resistivities considers, in addition, depends on the possible damage with failure of the construction unit. In the steel structure the safety factor for the material usually is about 1,1. It is to be noted that the loads in each case by ownFactors to be secured (partial security concept).

Since the characteristic values are determined in each case in the traction test with one axle, construction units more oh victory to be however often stressed (e.g. It applies for waves on bend and torsion, whereby the bend actually strictly already more-oh-triumphs one to demand meant), with help of a strength theoryto determine a reference stress with one axle, which can be compared then with the well-known firmness.

With oscillation-stressed construction units a special case exists. One calls it the fatigue strength.