Tensile test

Various rubber test sticks

The tensile test is a standardized method of material testing to determine the yield point , tensile strength , elongation at break and other material parameters . It is one of the quasi-static , destructive test methods .

In the tensile test, standardized specimens with a defined cross-sectional area are stretched until they break , with the stretching or the path being increased evenly, without jolts and at a low speed. During the test, the force on the sample and the change in length in the measuring section of the sample are measured continuously. From the force and the cross-sectional area of ​​the undeformed specimen, the nominal stress becomes : ${\displaystyle F}$ ${\displaystyle \Delta L}$ ${\displaystyle S_{0}}$ ${\displaystyle \sigma _{n}}$

${\displaystyle \sigma _{\mathrm {n} }={\frac {F}{S_{0}}}}$

calculated, the change in length is used to determine the total elongation with reference to the initial length of the measuring section : ${\displaystyle \Delta L}$ ${\displaystyle \varepsilon _{\mathrm {t} }}$ ${\displaystyle L_{0}}$

${\displaystyle \varepsilon _{\mathrm {t} }={\frac {\Delta L}{L_{0}}}}$

The result of the tensile test is the nominal stress / total elongation diagram . The technical material parameters can be read from this.

Material parameters

• ${\displaystyle E}$: Young's modulus
• Elastic limit
  • ${\displaystyle R_{p}}$: Yield strength
  • ${\displaystyle R_{eL}}$: Lower yield point
  • ${\displaystyle R_{eH}}$: Upper yield point
• ${\displaystyle R_{m}}$: Tensile strength
• ${\displaystyle A_{g}}$: Uniform elongation
• ${\displaystyle A_{5}}$or : elongation at break of the tensile specimen ( marked as in the diagram ) ${\displaystyle A_{10}}$ ${\displaystyle A}$
• ${\displaystyle A_{L}}$ Lüders expansion
• ${\displaystyle Z}$: Constriction of the fracture

Description of a tensile strength curve

Figure 1 Schematic technical stress-strain diagram with pronounced yield strength ${\displaystyle R_{eH}}$

Figure 2 Schematic technical stress-strain diagram with continuous start of flow

At the beginning of a load, many materials behave in an approximately linear-elastic manner, i.e. H. the deformation compared to the original length disappears completely when the load is removed. The associated material parameter, which describes the linear-elastic deformation behavior, is the modulus of elasticity and corresponds to the slope of the so-called Hooke's straight line . ${\displaystyle E}$

${\displaystyle E={\frac {\Delta \sigma }{\Delta \varepsilon }}}$

When the yield point is reached, the first recognizable plastic deformation begins (see Figure 1). From this point on, the course is heavily dependent on the material. Often the beginning of plastic deformation cannot be clearly identified by a kink in the curve (as in Figure 1). In these cases, the yield strengths are used instead , stating the plastic deformation used (often: for the yield strength at 0.2% plastic deformation). ${\displaystyle R_{eH}}$ ${\displaystyle R_{p}}$ ${\displaystyle R_{p0,2}}$

The tensile strength curve shown describes the schematic course of a ferritic-pearlitic steel with pronounced yield point effects in the case of displacement or strain control. Austenitic steels, quenched and tempered steels or ductile non-ferrous metals show different curves. In the case of non-metallic materials such as plastics, ceramics or composite materials, significantly different curves usually occur, since the microstructural processes of plastic deformation occur almost exclusively in metallic materials ( dislocation movement ). In comparison, it is z. B. in the permanent deformation of plastics to the dissolution and formation of secondary bonds ( hydrogen bonds , dipole-dipole and van der Waals forces ).

All materials have in common that plastic deformations remain when the load is removed. Only the elastic part of the total deformation disappears again. Against this background, the amounts of uniform elongation and elongation at break can be determined by relieving the strain from the tensile strength curve parallel to Hooke's straight line and reading the intersection with the abscissa . All elongation parameters are therefore plastic elongation components and the following always applies to the total elongation: ${\displaystyle \varepsilon _{\mathrm {e} }}$ ${\displaystyle \varepsilon _{\mathrm {t} }}$

${\displaystyle \varepsilon _{\mathrm {t} }=\varepsilon _{\mathrm {e} }+\varepsilon _{\mathrm {pl} }}$

The maximum of the tensile strength curve denotes one of the most important material parameters: the tensile strength . The associated elongation parameter is the uniform elongation, as the samples up to this point have not shown any macroscopic constriction (cross-sectional tapering). Materials that do not fail when the tensile strength is reached show a clear constriction. If the specimen breaks, the elongation at break ( or ) can be determined as described in the previous paragraph. ${\displaystyle R_{m}}$ ${\displaystyle A_{5}}$ ${\displaystyle A_{10}}$

Sample geometries

The tensile tests for metallic materials are defined in DIN 50125 (edition 2009-07).

Technical and physical experimentation

Universal testing machine with PC connection

Round sample of an AlMgSi alloy after the break test

In the physical tensile test, the true cross-section and the true length of the specimen are measured continuously and the true stress and the true elongation are calculated from this. For technical applications, technical experimentation (with reference to the initial cross-section and measuring length ) is preferred for reasons of simplicity and better recording of the elongation parameters . In addition, the tensile strength also corresponds to the maximum bearable force depending on the cross-sectional area. Once the tensile strength has been reached, one speaks of the onset of material failure, since the breakage of a component in a technical application can no longer be stopped from here. ${\displaystyle S_{0}}$ ${\displaystyle L_{0}}$

Universal testing machines with PC coupling or XY recorders are usually used to carry out the tests. The strain can be recorded directly on the sample via the crosshead travel of the machine or additional strain sensors such as an extensometer or strain gauges . The determination of the specimen elongation based on the traverse path is falsified by the deformation of the machine under load and mechanical play in the frictional connection to the specimen. Extensometers avoid this problem by measuring the strain directly on the specimen outside of the force flow.

Norms

The tensile test is mainly used for metallic and synthetic ( plastics ) materials and is standardized differently.

A selection of current standards for tensile tests:

• Metals: DIN EN ISO 6892-1, ISO 6892, ASTM E 8, ASTM E 21, DIN 50154;
• Plastics: ISO 527, ASTM D 638;
• Fiber-reinforced composites: ISO 14129, ASTM D 3039 / D3039M;
• Flexible foams: ISO 1798, ASTM D 3574;
• Rigid foams: ISO 1926, ASTM D 1623;
• Rubber: ISO 37, ASTM D 412, DIN 53504;
• Adhesives: DIN EN 15870;
• Paper: ISO 3781, TAPPI T 456, ISO 1924, TAPPI T 494;
• Fibers and Filaments: ISO 5079, ASTM D 3822;
• Yarns and twisted threads: ISO 2062, ASTM D 2256, ISO 6939;
• Textile fabrics: ISO 13934-1;
• Nonwovens: ISO 9073-3.

In the case of technically relevant ceramic materials , only a minimal elongation with very high forces can often be observed, which is why they are considered to be tensile until they break. Therefore, for testing the tensile strength of ceramic materials is the burst test used.

literature

• Karl-Eugen Kurrer : History of Structural Analysis. In search of balance , Ernst and Son, Berlin 2016, pp. 168–170 and pp. 382ff, ISBN 978-3-433-03134-6 .

Web links

Wiktionary: Tensile test  - explanations of meanings, word origins, synonyms, translations

• Summary of tensile test (PDF; 37 kB)
• Video of tensile tests - Karlsruhe University of Applied Sciences
• Video of fully automated strain-controlled tensile test with optical evaluation of several strain zones
• Overview of hydraulic clamping of tensile specimens