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Created page with "{{Language_sel|LANG=ger|ARTIKEL=Rissverzögerungsenergie}} {{PSM_Infobox}} A-Bild-Technik __FORCETOC__ ==Definition and significance== The modification of plastics with the aim of improving their mechanical properties is achieved technically by using inorganic fillers such as chalk, talc and glass balls or fibres such as short glass fibre..."

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{{Language_sel|LANG=ger|ARTIKEL=Rissverzögerungsenergie}}
{{PSM_Infobox}}
A-Bild-Technik
__FORCETOC__

==Definition and significance==

The modification of [[Plastics|plastics]] with the aim of improving their mechanical properties is achieved technically by using inorganic [[Particle-filled Thermoplastics|fillers]] such as chalk, talc and glass balls or [[Fibre-reinforced Plastics|fibres]] such as short glass fibres (E-glass) or C-fibres. The structure that forms at the interface between the matrix and the additive is of crucial importance here. To influence these interface properties, [[Fibre–Matrix Adhesion|coupling]] agent are often used, which induce physical and/or chemical interactions in the [[Phase Boundary Surface|boundary]]. A sensitive method for detecting changes in the deformation behaviour at the interface is the [[Instrumented Charpy Impact Test|instrumented Charpy impact test]] (ICIT), in which so-called crack propagation energies AR appear in the impact load (F)–deflection (f) diagrams.

These crack propagation energies characterise a reduction in the crack propagation speed and are an expression of dominant stable [[Crack Propagation|crack growth]]. This means that it is no longer possible to evaluate [[Toughness|toughness]] using linear-elastic [[Fracture Mechanics|fracture mechanics]] concepts. This first requires the quantification of the crack propagation energy and subsequently the determination of [[Crack Resistance (R) Curve|crack resistance curves]] (R curves) to describe the qualitatively different material behaviour.

With regard to the use of conventional [[Material Value|characteristic values]] of [[Notched Impact Test|notch impact toughness]] to characterise changes in the deformation behaviour in the interface, it becomes clear that possible misinterpretations arise not only from the well-known fact (see: [[Instrumented Charpy Impact Test|instrumented Charpy impact test]]) that

* large forces (loads) and small deformations (deflections) may, under certain circumstances, deliver the same notch impact energy as a small force and a large deformation,

but also that

* the determined notch impact energy is composed of completely different energy components that must be assigned to different mechanisms.

==Application examples from material development==
===PA/GF fibre composites – Influence of the a/W ratio===

'''Figure 1''' shows the impact load (F)–deflection (f) diagrams for PA/GF fibre composites (see: [[Short-fibre Reinforced Plastics|short-fibre reinforced plastics]]), whereby the notch depth (''a'') – specimen width (''W'') ratio was varied. The relationship between impact load and deflection becomes non-linear as the ''a''/''W'' ratio increases, with the maximum impact load ''F''max decreasing and the deflections ''f''max and ''f''GY increasing.

[[File:Crack Propagation Energy-1.jpg|400px]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px"|Impact load (F)–deflection (f) behaviour in ICIT with crack propagation energy in PA-glass fibre composites for ''a''/''W'' ratios: (a) 0.2, (b) 0.3 and (c) 0.45
|}

Parts a to c of '''Figure 1''' clearly show that crack propagation energy is a sensitive variable for evaluating deformation behaviour, but is unsuitable as an evaluation parameter because it is not a material parameter due to its geometry dependence (''a''/''W'' ratio) (see: [[Geometry Criterion|geometry criterion]]) and can therefore be used for quality control but not as a target variable in material development.

===PE chalk composites – Influence of a coupling agent===

'''Figure 2''' shows the impact load (F)–deflection (f) diagrams recorded using the [[Instrumented Charpy Impact Test|instrumented Charpy impact test]] for a PE-HD chalk composite and a PE-chalk composite modified with stearic acid with the same filler volume fraction (''φ''v = 0.15) (see also: [[Particle-filled Thermoplastics|particle-filled plastics]]).

The use of surface-modified additives (coupling agents) changes the requirements for evaluation concepts used to describe [[Toughness|toughness]] behaviour (see: [[Levels of Knowledge in Fracture Mechanics|levels of knowledge in fracture mechanics]]), as there is no longer any dominant unstable [[Crack Propagation|crack propagation]] in the material, which is documented in the F-f diagram by the lack of a pronounced load drop after ''F''max and the formation of ''A''R.

[[File:Crack Propagation Energy-2.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px"|Impact load (F)–deflection (f) behaviour in ICIT with crack propagation energy in PE/chalk composites: (a) low coupling agent content and (b) optimal PE/chalk composite
|}

The [[J-Integral Concept|J-integral]] values were determined using the [[SUMPTER and TURNER – J-Integral Estimation Method|estimation method developed by SUMPTER and TURNER]]. '''Figure 2''' shows that, with the same [[Crack Toughness|crack toughness]] values as resistance to unstable crack propagation, ''J''Id of 2.6 Nmm-1 can result in very large differences in crack propagation energy. Knowledge of the crack propagation energy is of exceptional practical importance, as this energy component clearly represents a toughness reserve.

===PP-GF composites – Influence of fibre modification===

'''Figure 3''' shows the influence of fibre modification with a coupling agent (HV) on the toughness behaviour of GF-reinforced polypropylene. The fibre content of the PP-GF composite was ''φ''v = 0.13 and the coupling agents used were 0.2 %, 0.5 % and 1 % HV.

[[File:Crack Propagation Energy-3.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px"|Impact load (F)–deflection (f) diagrams for PP-GF composites with (a) 0 %, (b) 0.2 %, (c) 0.5 % and (d) 1 % HV coupling agent
|}

After reaching ''F''max, the F-f diagrams show non-negligible crack propagation energies. Up to a coupling agent content of 0.5 %, there is an increase in the maximum impact load, the maximum deflection increases with increasing HV and the crack propagation energy components reach a maximum at 0.2 % coupling agent. At a bonding agent content of HV > 0.2 %, a significant decrease in specific crack propagation energy is observed with a simultaneous increase in crack opening displacement velocity (see: [[Levels of Knowledge in Fracture Mechanics|levels of knowledge in fracture mechanics]]) ('''Fig. 4''').

[[File:Crack Propagation Energy-4.jpg|400px]]
{|
|- valign="top"
|width="50px"|'''Fig. 4''':
|width="600px"|Relationship between J values, specific normalised crack propagation energy, crack opening displacement velocity and coupling agent content
|}

However, the J-integral values only show a significant increase in toughness at 0.5 % HV.

Due to the different conclusions that can be drawn from observing the material behaviour before reaching ''F''max on the one hand and after reaching ''F''max on the other, a fracture mechanical evaluation using the conventional methods described is no longer possible, and it is necessary to switch to determining characteristic values at stable crack growth in order to quantify the energy dissipative processes using fracture mechanical material parameters (see: [[Fracture Mechanics|fracture mechanics]]).

The recording of the [[Crack Resistance (R) Curve|crack resistance (R) curves]] required for this is explained under [[Crack Resistance Curve – Experimental Methods|crack resistance curve – experimental methods]].

Microfractographic investigations of the PP-GF composites have shown that the increase in energy dissipation is associated with fibre pull-out and severe fibrillation of the matrix with the formation of pointed structures (see: [[Ramps, Clods and Steps|ramps, clods and steps]]) ('''Fig. 4''') (see: [[Fibre-reinforced Plastics Fracture Model|fracture model of fibre-reinforced plastics]] and [[Fibre–Matrix Adhesion|fibre–matrix adhesion]]).

==See also==

* [[ICIT – Types of Impact Load–Deflection Diagrams|ICIT – Types of impact load–deflection diagrams]]
* [[Instrumented Charpy Impact Test|Instrumented Charpy impact test]]
* [[Short-fibre Reinforced Plastics|Short-fibre reinforced plastics]]
* [[ICIT – Experimental Conditions|ICIT – Experimental conditions]]
* [[Polymer Blend|Polymer blend]]

'''References'''

* [[Grellmann, Wolfgang|Grellmann, W.]], [[Seidler, Sabine|Seidler, S.]]: J-integral Analysis of Fibre-reinforced Injection Moulded Thermoplastics. J. of Polymer Engineering 11 (1992) 71–101, DOI: https://doi.org/10.1515/POLYENG.1992.11.1-2.71
* Seidler, S., [https://www.researchgate.net/profile/Wolfgang-Grellmann Grellmann, W.]: Zähigkeit von teilchengefüllten und kurzfaserverstärkten Polymerwerkstoffen. VDI-Fortschr.-Ber., VDI-Reihe 18 Nr. 92, VDI Düsseldorf (1991) (ISBN 3-18-149218-3; see [[AMK-Büchersammlung|AMK-Library]] under A 14)
* [https://de.wikipedia.org/wiki/Wolfgang_Grellmann Grellmann, W.], [https://de.wikipedia.org/wiki/Sabine_Seidler Seidler, S.] (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see [[AMK-Büchersammlung|AMK-Library]] under A 22)

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

Crack Propagation Energy - Revision history