View source for ICIT – Limits of Fracture Mechanics Evaluation

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<span style="font-size:1.2em;font-weight:bold;">ICIT Limits of fracture mechanics evaluation</span>
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==Requirements for the loading conditions==

In order to use the [[instrumented Charpy impact test]] to solve problems in materials development and optimisation, the loading conditions (see also [[Stress | stress]]) must be adapted so that as many [[Plastics | plastics]] and composite materials as possible are assessed under exactly the same loading conditions [1].

Of particular importance is the knowledge of the relationships between individual stress conditions with regard to a standardisation of the method with the aim of a [[Fracture Mechanical Testing | fracture mechanical characterisation]] of the toughness [2, 3].

==Influence of the notch depth, the pendulum hammer speed and the support distance==

In addition to the influence of the notch depth (see also: [[J-Integral Evaluation Methods (Overview) | J-integral evaluation methods (overview)]]) and the pendulum hammer velocity ''v''<sub>H</sub> (see: [[ICIT ‒ Influence of Pendulum Hammer Velocity | ICIT ‒ Influence of pendulum hammer velocity]]) on the shape of the impact loads (F)‒deflection (f)-diagrams and the material behaviour, the influence of the [[Support Distance |support span]] of the test specimen bearings on the measured variables must also be taken into account.

While fracture mechanically evaluable F‒f diagrams were obtained for polypropylene in the entire ''a''/''W'' and ''s''/''W'' range for ''v''<sub>H</sub> = 1 ms<sup>-1</sup> in [1], for a large number of materials, in particular high-impact polymer materials and modified, i.e. filled and reinforced polymer materials, depending on the material structure, a limit of the fracture mechanical evaluation possibility is already reached at room temperature. Even a higher electronic reinforcement cannot provide any material information, as the maximum impact load is too small in comparison with the superimposed vibration (see: [[ICIT – Experimental Conditions|ICIT ‒ experimental conditions]]). A further problem for filled and reinforced plastics is that often no unstable [[Crack Propagation | crack propagation]] occurs.

To derive the general relationships, two material examples for chlorinated PVC ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PVC-C) and polyamide 6 ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PA 6) were explained in [1]. '''Figure 1''' shows the ''F''–''f'' diagrams for selected ''a''/''W'' and ''s''/''W'' ratios at the lowest possible pendulum hammer velocity (''v''<sub>H</sub> = 0.5 ms<sup>-1</sup>) at room temperature.

[[file:ICIT-limits_1.jpg]]
{| 
|- valign="top"
|width="50px"|'''Fig. 1''': 
|width="600px" |Limit of the fracture mechanics evaluation possibility for chlorinated PVC ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PVC-C) (RT; ''v''<sub>H</sub> = 0.5 ms<sup>-1</sup>)
|}

'''Figure 2''' shows the F-f diagrams recorded in [[Instrumented Charpy Impact Test|instrumented Charpy impact test]] for polyamide 6 ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PA 6) for different ''a''/''W'' and ''s''/''W'' ratios at a pendulum hammer speed of  ''v''<sub>H</sub> = 0.5 ms<sup>-1</sup> at ''T'' = 273 K.

[[file:ICIT-limits_2.jpg]]
{| 
|- valign="top"
|width="50px"|'''Fig. 2''': 
|width="600px" |Limit of the fracture mechanics evaluation possibility of the impact load‒deflection diagrams of polyamide 6 [[Plastics – Symbols and Abbreviated Terms | abbreviation: PA 6]] with variation of the ''a''/''W'' and ''s''/''W'' ratio
|}

==Limits of the fracture mechanics evaluation possibility==

It is clear from the examples shown:

# the maximum impact load ''F''<sub>max</sub> decreases with increasing ''a''/''W'' and ''s''/''W''
# the inertial load ''F''<sub>1</sub> is independent of a/W and ''s''/''W''
# the period of the inertial oscillation period ''τ'' increases with increasing ''s''/''W''
# with increasing ''a''/''W'' and ''s''/''W'', the relationship ''F''<sub>max</sub> > ''F''<sub>1</sub> becomes increasingly difficult to fulfil and at the same time the better fulfilment of the relationship ''t''<sub>B</sub> > 2.3…3 ''τ''  becomes meaningless.

While for PVC-C the limit of the fracture mechanical analysability of the diagrams is reached for ''a''/''W'' = 0.45 and ''s''/''W'' = 7, for example, for the PA 6 material this is already at lower stress conditions (''T'' = 273 K; ''v''<sub>H</sub> = 0.5 ms-1; ''a''/''W'' = 0.45; ''s''/''W'' = 6).

==Optimisation of the signal shape for fracture mechanics evaluation==

With regard to obtaining optimum signal shapes for a fracture mechanics evaluation in compliance with the control conditions formulated for [[ICIT – Experimental Conditions | ICIT – Experimental conditions]] according to Eq. (1) to Eq. (3) for the [[Instrumented Charpy Impact Test | instrumented Charpy impact test]], the demand for ''s''/''W'' = 4, a low ''a''/''W'' ratio and a low pendulum hammer velocity is derived from the results presented. This provides generally applicable [[Stress | loading conditions]] for a large number of applications, if the applicability of the fracture mechanics concepts for describing toughness is also proven for low ''a''/''W'' ratios. A further advantage of using low [[Support Distance | support distances]] of the abutments lies in the restriction of the possibilities of pull-through. This is particularly important for plastics with high impact strength, which naturally exhibit high deflections.

==See also==

* [[Instrumented Charpy Impact Test | Instrumented Charpy impact test]] (ICIT)
* [[MPK-Procedure MPK-ICIT]]
* [[Impact Test | Impact test]]
* [[ICIT – Extended Stop-Block Method | ICIT – Extended stop-block method]]
* [[Impact Loading Plastics | Impact loading plastics]]


'''References'''

{|
|-valign="top"
|[1]
|[[Grellmann,_Wolfgang|Grellmann, W.]]: Beurteilung der Zähigkeitseigenschaften von Polymerwerkstoffen durch bruchmechanische Kennwerte. Habilitation (1986), [https://de.wikipedia.org/wiki/Technische_Hochschule_Leuna-Merseburg Technischen Hochschule Merseburg], Wiss. Zeitschrift TH Merseburg 28 (1986), H. 6, S. 787–788 ([https://www.polymerservice-merseburg.de/fileadmin/inhalte/psm/veroeffentlichungen/Habil_Grellmann_Inhaltsverzeichnis.pdf Inhaltsverzeichnis], [https://www.polymerservice-merseburg.de/fileadmin/inhalte/psm/veroeffentlichungen/Habil_Grellmann_Kurzfassung.pdf Kurzfassung])
|-valign="top"
|[2]
|ISO 179-2 (2020-05): Plastics ‒ Determination of Charpy Impact Properties ‒ Part 2: Instrumented Impact Test
|-valign="top"
|[3]
|[[MPK-Procedure MPK-ICIT]] (2016-10): Testing of Plastics – Instrumented Charpy Impact Test: Procedure for Determining the Crack Resistance Behaviour Using the Instrumented Impact Test
|}

[[Category:Fracture Mechanics]]
[[Category:Instrumented Impact Test]]