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Calibration of material testing machines

The calibration of the material testing and tension/compression testing machines axial and lateral directional force measurement system (also called it vertical and horizontal force measurement systems, but this is not accurate), is carried out in-site in almost all cases.

First, because in most cases these devices due to their size and weight are not suitable for transport, or only very elaborately, other hand according to the the standard specifications for these devices after moving or transporting, etc. them, recalibrating is required on the new installation place. Complete description of the relevant test standards:

MSZ EN ISO 7500-1:2004
Verification of static uniaxial testing machines
Part 1: Tension/compression testing machines. Verification and calibration of the force-measuring system

According to the MSZ EN ISO 7500-1:2004 international standard, calibration includes the following tests:

Calibration of the force measurement system unit, the following metrological characteristics are determined::

  • systematic error of the machine
  • repeatability error of the machine
  • reversing error of the machine
  • zero position error of the machine

DID YOU KNOW…

The testing machine based on the specification requirements classified the following classes of accuracy:

View: TABLE 1: MSZ EN ISO 7500-1:2004 standard table 2.

A LITTLE THEORY ...

Grouping material testing and force measurement devices by their principle of operation:
  • mechanical (pendulum, sliding weight, scale-beam, spring, etc.).
  • hydraulic (manometer, pendulum manometer, etc.).
  • Electronic (pressure sensor, measuring amplifiers, etc.).

Tensile-compression, flexural testing
Determining the material properties the tensile test is generally the best, which is not other than modelling the uniaxial tensile stress. The tensile test specimen is representative sample from the material or material items. Specific requirements are related to the tensile test and the tensile test machine which performs the test.
During the tensile a stress-strain diagram is made, i.e. tensile-force function is registered.
The STRESS-STRAIN function can be recorded in the following ways:

  • Coarse measurement - capture motion gives the strain signal
  • Fine measurement – the strain signal is given by the position sensor which is fixed to the signal distance of the test specimen (usually precision, high-resolution strain gauge required)

In the former case, only the specific force measurement data is accurate, the latter case, the position data may be used for evaluation.

Tensile has four phases:

  • elastic deformation,
  • yield phase,
  • hardening phase,
  • contraction phase.

Picture1: View a stress-srain diagram

From the stress-srain diagrams and the test specimens before and after sizes, the following features can be calculated:

Strength characteristics:

  • Tensile strength: Calculated as the maximum force diveded by the original cross-sectional area results fictive voltage:Egyenlet1
  • Yield point: Typical voltage, can be calculated, from the phenomenon of the flow of force:Egyenlet2
  • Conventional yield strength: Voltage is calculated from the 0.2% permanent deformation of force:Egyenlet3
  • True stress: calculated by dividing the characteristic strength of the current effective cross-section:Egyenlet4

Deformation characteristics:

  • Uniform elongatiom:Egyenlet5
  • Elongation-to-break:Egyenlet6
  • Contraction:Egyenlet7
  • Logarithmic deformation:Egyenlet8

For plastic deformation technologies logarithmic deformation is used because in this case the sequential percentage deformations can be summed properly. The tensile test results are important measurement funds in the technical life. Scaling the elastic deformation and rigidity is based on the modulus of elasticity in tension (also known as Young's modulus) and for the transverse deformation is the Poisson's ratio.
Basisc of Static strength design: Yield stress (so-called maximum voltage reduced with the appropriate safety factor); conventional yield point and tensile strength.

In addition to tensile tests, compressive bending tests are common. It is often used for the classification of small deformation capacity pottery, cast iron, hard metals, rigid composites, etc...

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