View source for Tensile Test Compliance

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<span style="font-size:1.2em;font-weight:bold;">Tensile test compliance</span>
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==Machine compliance==

In [[Tensile Test|tensile tests]] to characterise the [[Deformation|deformation behaviour]] or elastic properties of [[Plastics|plastics]], it may not be possible to use measuring devices for direct [[Measure|measurement]] of the strain on the test [[Specimen|specimen]] (strain extensometer, clip-on extensometer or optical sensors; see [[Tensile Test#Tensile test, path measurement technique|tensile test path measurement]]). This is the case, for example, if measurements are to be taken in a temperature control chamber or if the deformation behaviour of plastics with graduated glass or carbon fibre contents is to be compared. When performing conventional tensile tests with a constant [[Crosshead Speed|crosshead speed]] in accordance with ISO 527-2 [1], the test conditions and the determination of [[Material Value|characteristic values]] are influenced by various factors. Due to the applied [[Stress|stress]], the various components of the [[Material Testing Machine|material testing machine]] are deformed, which is also known as [[Machine Compliance|machine compliance]] and is of great importance for traverse path measurement.

The deformation of the machine crossbars and spindles as well as the drive slip (see: [[Drives Materials Testing Machines|drives for materials testing machines]]), the bending of the crosshead and the traverse are included in the measurement signal as Δ''L''<sub>F</sub>, whereby the absolute errors are relatively small here. A larger proportion is contributed by the [[Deformation|deformation]] of the deformation body of the force measuring cell (see: [[Electro-mechanical Force Transducer|electro-mechanical force transducer]] and [[Piezoelectric Force Transducer|piezoelectric force transducer]]) Δ''L''<sub>K</sub> and, in particular, the slip in the clamping jaws Δ''L''<sub>E</sub>.

The quality and operating principle of the clamping jaws significantly determine the measurement result in the deformation channel and thus significantly influence the elastic values, such as [[Elastic Modulus|modulus of elasticity]] or strain limits.

==Influence of test specimen clamping==

The importance of the quality of the specimen clamping becomes clear when comparing the wedge clamp and the parallel clamping device ('''Fig. 1'''). Due to the rollers of the wedge clamping device, the holding function is only achieved with increasing test load, which creates an additional path that enters the measurement signal and can cause strong start-up behaviour. In contrast, the parallel clamp only causes minimal misalignment, which is further reduced when pneumatically or hydraulically pressing clamps are used. However, the result of the test is also influenced by the type of clamp inserts fine or coarse file cut, hard rubber.

[[File:Z_nachgibeigkeit_1.jpg|300px]]
{| 
|- valign="top"
|width="50px"|'''Fig. 1''': 
|width="600px" |Comparison of wedge clamp a) and parallel clamp b)
|}

The path measurement signal Δ''L''<sub>M</sub> therefore consists of the sum of the individual deformation components of the test device Δ''L''<sub>M</sub> = Δ''L''<sub>P</sub> + Δ''L''<sub>F</sub> + Δ''L''<sub>K</sub> + Δ''L''<sub>E</sub> 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|electro-mechanical force transducer]], extension rods, [[Specimen Clamping|clamping jaws]] or jaw inserts) changes the value of the compliance ''K''.

==Determination of machine compliance==

Due to configuration dependency, [[Machine Compliance|machine compliance]] is not usually specified by the [[Manufacturer of Material Testing Machines|manufacturer of the universal testing machine]]. If it is, the compliance in mm/kN is usually only the reciprocal of the stiffness of the [[Load Framework|load frame]] without additional equipment and often corresponds only to a calculated value. Since, in the case of traverse path measurement, the compliance depends on the additional equipment used, some of which is developed in-house, many testing machine manufacturers have integrated software modules into the testing software that allow the specific compliance and a corresponding correction curve to be determined. This means that optimum displacement measurement and positioning accuracy can be guaranteed via the crosshead transducer even without the use of special strain gauges or displacement transducers (see: [[Tensile Test#Tensile test, path measurement technique|Tensile test path measurement technology]]). However, it should be emphasised at this point that even the best correction curves cannot replace high-precision strain transducers that [[Measure|measure]] the [[Deformation|deformation]] directly on the test [[Specimen|specimen]].

[[File:Tensile_Test_Compliance-2.jpg|350px]]
{| 
|- valign="top"
|width="50px"|'''Fig. 2''': 
|width="600px" |Tensioning the test clamp (a) and determining the compliance curve (b)
|}

To determine the yield strength ''C''<sub>t</sub> in the [[Tensile Test|tensile test]], a test [[Specimen|specimen]] with high [[Stiffness#Tensile stiffness|tensile stiffness]] ''E''<sub>t</sub> · ''A''<sub>0</sub> and minimal deformation under the test force to be used should be selected. The individual elements in the load path of the [[Material Testing Machine|universal testing machine]] (linkage, clamps) should be braced under a load of approx. 90 % of the nominal load of the [[Electro-mechanical Force Transducer|force measuring device]] ('''Fig. 2a'''). After the test specimen, e.g. made of steel, has been aligned and [[Specimen Clamping|clamped]], the [[Tensile Test|tensile test]] is performed at a low [[Test Speed|test speed]] up to the nominal load of the force measuring device and then unloaded again while recording the data of the traverse path and the sensor path ('''Fig. 2b'''). The corresponding correction curve Δ''L''<sub>T</sub>(F) can be calculated from the loading and unloading curve by regression. Subtracting the deformation Δ''L''<sub>F</sub> of the test specimen then produces the correction curve that can be used for the compliance correction online in the [[Tensile Test|tensile test]] or offline in post-processing using Excel® or Origin®. Depending on the test load, the self-deformation component is then subtracted from the measurement signal of the traverse path ('''Fig. 3'''). This significantly improves the [[Measuring Accuracy|accuracy]] and [[Tensile Test Control|control behaviour]], even in controlled tests.

[[File:Tensile_Test_Compliance-3.jpg|400px]]
{| 
|- valign="top"
|width="50px"|'''Fig. 3''': 
|width="600px" |Application of the correction curve in the [[Tensile Test|tensile test]] with traverse path measurement
|}

==See also==

* [[Machine Compliance|Machine compliance]]
* [[Specimen Clamping|Specimen clamping]]
* [[Stiffness]]
* [[Tensile Test Control|Tensile test control]]
* [[Compression Test Compliance|Compression test compliance]]


'''References'''

{|
|-valign="top"
|[1]
|ISO 527-2 (2025-06): Plastics – Determination of Tensile Properties – Part 2: Test Conditions for Moulding and Extrusion Plastics
|}

'''Additional literature'''

* [[Bierögel, Christian|Bierögel, C.]], [[Grellmann, Wolfgang|Grellmann, W.]]: Quasi-Static Tensile Test. In: [https://www.researchgate.net/profile/Wolfgang-Grellmann Grellmann, W.], [[Seidler, Sabine|Seidler, S.]]: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein, Volume VIII/6A3, Springer, Berlin (2014) pp. 76–143, (ISBN 978-3-642-55165-9; see [[AMK-Büchersammlung|AMK-Library]] under A 16)

[[Category:Stiffness Compliance]]
[[Category:Tensile Test]]