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Created page with "{{Language_sel|LANG=ger|ARTIKEL=Biegeversuch Nachgiebigkeit}} {{PSM_Infobox}} Bend test compliance __FORCETOC__ ==The machine compliance of the materials testing machine== In bending tests to characterise the deformation behaviour or elastic properties of plastics, it may not be possible to use measuring devices for direct measurement of the deflection of the test specimen..."

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{{Language_sel|LANG=ger|ARTIKEL=Biegeversuch Nachgiebigkeit}}
{{PSM_Infobox}}
Bend test compliance
__FORCETOC__

==The machine compliance of the materials testing machine==

In [[Bend Test|bending tests]] to characterise the [[Deformation|deformation]] behaviour or elastic properties of plastics, it may not be possible to use measuring devices for direct measurement of the deflection of the [[Specimen|test specimen]] (extensometers or optical sensors). This occurs when measurements are taken in a temperature chamber or when the [[Deformation|deformation]] behaviour of [[Plastics|plastics]] with varying glass or carbon fibre content is to be compared. When performing conventional bending tests with constant [[Crosshead Speed|crosshead speed]] in accordance with ISO 178 [1], the test conditions and the determination of characteristic [[Material Value|values]] are influenced by a wide variety of factors. Due to the external [[Stress|stress]] imposed, the various components of the [[Material Testing Machine|universal testing machine]] are deformed, which is also known as [[Machine Compliance|machine compliance]] (see also: [[Tensile Test Compliance|tensile test compliance]]) and has a significant influence on the crosshead path measurement. The deformation of the test machine's crossbars and spindles, as well as the slip of the [[Drives Materials Testing Machines|drive (motor)]] and the bending of the crosshead and traverse, are included in the deflection measurement signal as Δ''f''F, whereby the absolute errors are relatively small. The largest contribution is made by the deformation of the [[Deformation|deformation]] body of the force measuring cell Δ''f''K and, in particular, by the impression of the [[Support Distance|supports]] and the test stamp in the test [[Specimen|specimen]] Δ''f''W. The quality of the supports has a significant influence on the measurement result and thus significantly influences the elastic [[Material Value|characteristic values]], such as the [[Elastic Modulus|modulus of elasticity]] or strain limits. The deflection signal Δ''f''M therefore consists of the sum of the individual deformation components of the test equipment Δ''f''M = Δ''f''P + Δ''f''F + Δ''f''K + Δ''f''EW and thus essentially determines the compliance of the test system. Any configuration change made by the user of the testing machine ([[Electro-mechanical Force Transducer|force transducer]], extension rods, roller support and roller radius) influences the value of the compliance ''K''.

==Compliance in the three-point bending test==

Due to configuration dependencies, machine compliance is not usually specified by the [[Manufacturer of Material Testing Machines|manufacturer of the material testing machine]]. If it is known, the compliance in mm/kN is usually only the reciprocal of the [[Stiffness|stiffness]] of the load frame (see also: [[Machine Compliance|machine compliance]]) without additional equipment. This value usually corresponds only to a calculated value, e.g. using the finite element method (FEM). Since, in the case of crosshead path measurement, the compliance depends on the additional equipment used, some of which is developed by the testing laboratory, many testing machine manufacturers have integrated software modules into the testing software that allow the compliance and a correction curve to be determined. This guarantees optimum deflection measurement and positioning accuracy via the crosshead encoder even without the use of special deflection sensors. These correction curves cannot replace high-precision strain gauges, which measure the [[Deformation|deformation]] directly on the test specimen.

To determine the compliance ''C''t in the [[Bend Test|bending test]], a test [[Specimen|specimen]] with high bending stiffness ''E''f·''I''y (see [[Stiffness|stiffness]]) and minimal self-deformation must be used with the test force to be applied. The individual elements of the load train of the [[Material Testing Machine|universal testing machine]] (linkage, bending device) should be braced under a load of approx. 90 % of the nominal load ''F''N of the [[Electro-mechanical Force Transducer|force measuring cell]] used ('''Fig. 1a''').

[[file:Compliance_1.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Determination of the compliance curve in the tree-point bend test
|}

After applying the test specimen made of steel, for example, the [[Bend Test|bending test]] is performed at a low [[Crosshead Speed|test speed]] up to the nominal load of the [[Electro-mechanical Force Transducer|electro-mechanical force sensor]] and then unloaded again while recording the data of the traverse path ''s''0 and the sensor path of the centre deflection ''s''1 ('''Fig. 1b'''). The corresponding correction curve Δ''f''T(''F'') can be calculated from the loading and unloading curve by means of averaging and regression. The correction curve for the compliance correction can then be calculated by subtracting the deformation Δ''f''F of the test [[Specimen|specimen]]. This can be used online in the [[Bend Test|bending test]] or offline in post-processing using Excel® or Origin®. Depending on the test load, the self-deformation portion is then subtracted from the measurement signal of the traverse path ('''Fig. 2''').

[[file:Compliance_2.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |Application of the correction curve in the [[Bend Test|bending test]] with traverse path measurement
|}

==See also==

The WIKI lexicon: Polymer Testing & Diagnostics provides more detailed explanations of the following terms in this context:

* [[Stiffness]]
* [[Specimen Compliance|Specimen compliance]]
* [[Machine Compliance|Machine compliance]]
* [[Compression Test Compliance|Compression test compliance]]
* [[Tensile Test Compliance|Tensile test compliance]]

'''References'''

{|
|-valign="top"
|[1]
|ISO 178 (2019-04): Plastics – Determination of Flexural Properties
|}

[[Category:Bend Test]]
[[Category:Stiffness Compliance]]

Bend Test Compliance - Revision history