Oluschinski: Created page with "{{Language_sel|LANG=ger|ARTIKEL=Biegefestigkeit}} {{PSM_Infobox}} Flexural strength FORCETOC ==Determination methods== The flexural strength ''σ''_{f_M} is determined in a three-point or four-point bending test [1, 5] on plastics or short fibre-reinforced plastic composites. The test on rigid and semi-rigid plastics, i.e. thermopl..."

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Created page with "{{Language_sel|LANG=ger|ARTIKEL=Biegefestigkeit}} {{PSM_Infobox}} Flexural strength __FORCETOC__ ==Determination methods== The flexural strength ''σ''fM is determined in a three-point or four-point bending test [1, 5] on plastics or short fibre-reinforced plastic composites. The test on rigid and semi-rigid plastics, i.e. thermopl..."

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{{Language_sel|LANG=ger|ARTIKEL=Biegefestigkeit}}
{{PSM_Infobox}}
Flexural strength
__FORCETOC__

==Determination methods==

The flexural strength ''σ''fM is determined in a [[Bend Test|three-point or four-point bending test]] [1, 5] on [[Plastics|plastics]] or [[Short-fibre Reinforced Plastics|short fibre-reinforced plastic composites]]. The test on rigid and semi-rigid plastics, i.e. thermoplastic [[Moulding Compound|moulding compounds]], extrusion compounds and casting compounds, is carried out in accordance with ISO 178 [1–4] in a [[Bend Test#The three-point bending test method|three-point bending test]]. For fibre-reinforced plastics, the three-point or [[Bend Test#The four-point bending test method|four-point bending test]] (method A or B) in accordance with ISO 14125 [5] can be used.

==Definition of flexural strength==

The definition of flexural strength is identical in both standards and states that flexural strength corresponds to the maximum bending stress that is withstood by the [[Specimen|test specimen]] (see also: [[Test Piece|test piece]]) during a [[Bend Test|bend test]] [1–5].

In contrast to ISO 14125, where primarily only the deflection at the point of flexural strength ''s''M or the deflection at flexural strength is determined as the deflection at the force at which flexural strength is achieved, ISO 178 requires the flexural strain ''ε''fM at flexural strength to be determined. The flexural strength is determined in the three-point bending test according to '''Eq. (1)''' [1–4].

{|
|-
|width="20px"|
|width="500px" | <math>\sigma _{fM}\ = \frac{3 F_{M}\cdot L}{2b\cdot h^{2}}</math>
|width="50px" |(1)
|}

In the four-point bending test, the flexural strength is calculated according to '''Eq. (2)''' [5], where ''L'' is the [[Support Distance|support span]] in both equations and ''b'' and ''h'' represent the cross-sectional dimensions of the test specimens.

{|
|-
|width="20px"|
|width="500px" | <math>\sigma _{fM}\ = \frac{F_{M}\cdot L}{b\cdot h^{2}}</math>
|width="50px" |(2)
|}

In the case of three-point bending according to ISO 178, the dimensionless flexural strain for flexural strength is calculated using '''Eq. (3)''' or, when multiplied by 100, as flexural strain in percent. Based on the simplified bending theory of the undeformed test specimen, this flexural strain corresponds to the symmetrical [[Peripheral Fibre Strain|peripheral fibre strain]] on the tension (strain) or compression (contraction) side of the test specimen ('''Fig. 1''').

{|
|-
|width="20px"|
|width="500px" | <math>\varepsilon _{fM}\ = \frac{6s_{M}\cdot h}{L^{2}}</math>
|width="50px" |(3)
|}

The corresponding bending strain ''ε''fM can be calculated as an approximate peripheral fibre strain according to ISO 14125 as a dimensionless parameter using '''Eq. (4)''' and the geometric boundary conditions of the four-point bending test from the measured deflection ''s''M [5].

{|
|-
|width="20px"|
|width="500px" | <math>\varepsilon _{fM}\ = \frac{4,7s_{M}\cdot h}{L^{2}}</math>
|width="50px" |(4)
|}

In this case, too, the bending strain can be expressed as a percentage value by multiplying it by a factor of 100.

==Evaluation of bend tests==

According to [1], bending strength can only be determined if the maximum of the flexural stress-flexural strain curve lies within the deflection interval of 0 ≤ ''s'' ≤ ''s''C or the strain interval 0 ≤ ''ε''f ≤ 3.5 % which corresponds to a standard deflection sC of 6 mm for test specimens with a thickness of 4 mm. For larger deflections of up to approx. 9 mm and when a deflection of 6 mm is reached without a maximum occurring, the standard flexural stress ''σ''fC must then be determined ('''Fig. 1'''), which is not comparable with the flexural strength.

[[File:Biegefest_1.jpg|450px]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Determination of characteristic [[Material Value|values]] for [[Plastics|plastics]] in [[Bend Test|bending tests]] according to ISO 178 [1]
|}

The limit value of 6 mm no longer exists in the latest edition of ISO 178 [3], meaning that this regulation no longer applies. The ISO 14125 standard does not specify such a deflection limit ('''Fig. 2'''), but all stress and bending strain values must be evaluated using correction equations in order to approximately compensate for the influence of excessive deflections in comparison with bending theory. As a result of support spacing reductions, support friction, and horizontal forces, the measured [[Peripheral Fibre Strain|peripheral fibre strains]] are too large, and the bending strengths are generally too small [6].

[[File:Biegefest_2.jpg|450px]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |Determination of [[Material Value|characteristic values]] for [[Plastics|plastics]] in [[Bend Test|bending tests]] according to ISO 14125 [5]
|}

==See also==

* [[Bend Test|Bend test]]
* [[Bend Test – Yield Stress|Bend test – Yield stress]]
* [[Bend Test – Test Influences|Bend test – Test influences]]
* [[Bend Loading|Bend loading]]
* [[Strength]]

==References==

{|
|-valign="top"
|[1]
|ISO 178 (2010-12): Plastics – Determination of Flexural Properties
|-valign="top"
|[2]
|ISO 178 (2013-04): Plastics – Determination of Flexural Properties
|-valign="top"
|[3]
|ISO 178 (2019-04): Plastics – Determination of Flexural Properties
|-valign="top"
|[4]
|ISO/DIS 178 (2026-03: Plastics – Determination of Flexural Properties
|-valign="top"
|[5]
|ISO 14125 (1998-03): Fibre-reinforced Plastic Composites – Determination of Flexural Properties (Technical Corrigendum Corrigendum1:2001-07 + AMD 1:2011-02)
|-valign="top"
|[6]
|[[Bierögel, Christian|Bierögel, C.]]: Bend Test on Polymers. In: [[Grellmann, Wolfgang|Grellmann, W.]], [[Seidler, Sabine|Seidler, S.]] (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 133–143 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see [[AMK-Library]] under A 22)
|}

[[Category:Bend Test]]

Flexural Strength - Revision history