Tensile testing

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

Standardized tensile test samples are stretched at a defined cross-sectional area up to the break, the elongation and the way evenly and smoothly increased at a low speed. During the test the force on the sample and the change in length of the measuring section of the sample is continuously measured. Of the force by the cross- sectional area of ​​the undeformed specimen rated voltage:

Calculated from the change in length of one determines the total elongation with respect to the initial length of the test section:

The result of the tensile test is the nominal voltage / total strain diagram. From the technical material parameters can be read.

Material parameters

  • : Modulus of elasticity
  • Elastic limit: yield strength
  • : Lower yield strength
  • : Upper yield strength

Description of a Zugverfestigungskurve

At the beginning of stress, many materials behave linear- elastic, ie deformation from the initial length disappears when released back completely. The associated material parameter that describes the linear elastic deformation behavior of the modulus of elasticity and is the slope of the line so-called Hooke.

Upon reaching the yield point is plastic deformation (see Figure 1). From this point on the curve is strongly dependent on the material. Often the beginning of plastic deformation is not by a bending of the curve can be clearly identified (as in Figure 1). In these cases, instead, the yield strengths, stating the plastic deformation used used (often: for the yield strength at 0.2 % plastic deformation).

The Zugverfestigungskurve shown describes the schematic course of a ferritic- pearlitic steel with pronounced yield effects in path or strain control. Austenitic steels, quenched and tempered steels or ductile nonferrous metals show different curves. For non-metallic materials such as plastics, ceramics or composites usually appear significant other curves, since the microstructural processes of plastic deformation occur almost exclusively in metallic materials ( dislocation motion ). In comparison, there are, for example, in the permanent deformation of plastics to the dissolution and formation of secondary bonds ( hydrogen bonds, dipole -dipole and van der Waals forces).

Common to all materials that plastic deformations in relief remain. Only the elastic component of the total deformation disappears. Against this background, the amounts of the uniform elongation and elongation at break can be determined by relieving parallel to Hooke's straight from the Zugverfestigungskurve and the intersection is taken with the abscissa. For all strain characteristics it is accordingly plastic strain shares and it applies to the total strain always:

The maximum of the Zugverfestigungskurve designates one of the most important material parameters: the tensile strength. The corresponding strain characteristic value is the uniform elongation, as far the samples no macroscopic constriction ( cross-sectional narrowing ) show. Materials that do not fail when it reaches the tensile strength, showing a distinct constriction. The sample can then break the elongation at break (respectively) as described in the previous paragraph, can be determined.

Sample geometries

The tensile specimens are defined for metallic materials in DIN 50125 ( 2007-09 edition ).

Technical and physical experimentation

Tensile testing in the physical continuously the true cross-section, and the true length of the sample are measured and used to calculate the true stress and the true strain. For technical applications, the technical experimentation ( with respect to initial cross section and measuring length), for reasons of simplicity and better detection of the strain characteristics preferred. In addition, the tensile strength and the maximum tolerable force depending on the cross-sectional area corresponds. After reaching the tensile strength is also called the onset of material failure, as the fraction of a component in a technical application from here is unstoppable.

Most universal testing machines are used with a PC link or XY recorder for the experimental procedure ( Figure 4). The strain can be recorded on the crosshead of the machine or additional extensometer as extensometer or strain gages directly on the specimen. The determination of the sample elongation using the crosshead travel is falsified by the deformation of the machine under load and mechanical play in the power circuit to the sample. Extensometer work around this issue by changing the strain is measured directly at the sample outside of the power flow.

Standardize

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

A selection of current standards for tensile test:

  • Metals: EN ISO 6892-1, ISO 6892, ASTM E8, ASTM E 21, DIN 50154; DIN 50125
  • Plastics: ISO 527, ASTM D 638;
  • Fiber-reinforced composites: ISO 14129;
  • Soft Elastic Foams: ISO 1798, ASTM D 3574;
  • Rigid foams: ISO 1926, ASTM D 1623;
  • Rubber: ISO 37, ASTM D 412, DIN 53504;
  • Adhesives: ISO 6922;
  • Paper: ISO 3781, TAPPI T 456, ISO 1924, TAPPI T 494;
  • Fibers and filaments: ISO 5079, ASTM D 3822;
  • Yarns and threads: ISO 2062, ASTM D 2256, ISO 6939;
  • Textile fabrics: ISO 13934-1;
  • Nonwovens: ISO 9073-3.

For the technically relevant ceramic materials is often only minimal elongation observed at very large forces, which is why they are considered as tensile strength to break. To test the tensile strength of ceramic materials, therefore, the burst test is used.

Fracture mechanics Strain (materials science)
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