A Preface
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The procedures used to develop this document as well as those used for further maintenance are described in Part 1 of the ISO/IEC Directive. In particular, attention should be paid to the different approval criteria required for different types of ISO documents. This document has been drafted in accordance with the editorial rules of Part 2 of the ISO/IEC Directive (see iso.org/directives).

ISO 178-2019 – Determination of bending properties of Plastics

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This document is prepared by ISO/TC 61, Plastics, Subcommittee SC 2, Mechanical Properties.

ISO 178-2019 – Determination of bending properties of Plastics

The sixth edition eliminates and replaces the technically revised fifth edition (ISO 178:2010). It also incorporates amendments to ISO 178:2010 / Amd.1:2013. Compared with the previous edition, the main changes are as follows:

– Differentiation of calibration requirements according to test type;

– Introduction of deflectometer;

– Resumption of compliance correction procedures;

— A new annex D has been added, showing the relationship between tensile modulus and bending modulus.

Any feedback or questions regarding this document should be directed to the user’s national standards body. A full list of these agencies can be found at iso.org/members.html.

ISO 178-2019 – Determination of bending properties of Plastics

1 Scope
This document specifies a method for determining the bending properties of rigid and semi-rigid plastics under specified conditions. The preferred sample selection was defined, but parameters for the size of the standby sample were included for use where appropriate. Includes a series of test speeds.

This method is used to study the bending behavior of the specimen and to determine the bending strength, bending modulus and other aspects of the bending stress/strain relationship under defined conditions. It is suitable for free support beams loaded at mid-span (three-point loading test).

The method is applicable to the following material ranges:

– Thermoplastic forming, extrusion and casting materials, including filled and reinforced compounds in addition to unfilled types; Rigid thermoplastic sheet;

– Thermoset molding materials, including filling and reinforcing compounds; Thermoset sheet.

According to ISO 10350-1[5] and ISO 10350-2[6], this document applies to fiber reinforced compounds with fiber length ≤7.5 mm before processing. For fiber length > 7.5 mm long fiber reinforcement (laminate), see ISO 14125[7].

This method is generally not applicable to rigid porous materials or sandwich structures containing porous materials. In this case, ISO 1209-1[3] and/or ISO 1209-2[4] can be used.

Note 1 For certain types of textile fiber reinforced plastics, a four-point bending test is used. This is described in ISO 14125.

ISO 178-2019 – Determination of bending properties of Plastics

The method is carried out using specimens that may be molded to specified dimensions, processed from the central portion of a standard multipurpose specimen (see ISO 20753) or processed from finished or semi-finished products, such as moulds, laminated plates or extruded or cast plates.

This method specifies the selected size of the sample. Tests performed on specimens of different sizes or prepared under different conditions can yield incomparable results. Other factors, such as the speed of the test and the conditioning of the specimen, can also affect the results.

Note 2 Especially for injection molded semi-crystalline polymers, the thickness of the oriented skin layer depends on the forming conditions and also affects the bending performance.

This method is not suitable for the determination of design parameters, but it can be used for material testing and quality control testing.

ISO 178-2019 – Determination of bending properties of Plastics

2 Normative references
The following documents are referenced in the text in such a way that some or all of their contents constitute the requirements of this document. For dated references, only the cited version applies. For undated references, the new version of the reference (including any revisions) applies.

ISO 291, Plastics — Standard atmosphere for conditioning and testing

ISO 293, Plastics — Compression molding of thermoplastic material specimens

ISO 294-1:2017, Plastics. Injection molding of thermoplastic material samples. Part 1: General principles and molding of general-purpose and bar specimens

ISO 295, Plastics — Compression molding of specimens of thermoset materials

ISO 2602, Statistical interpretation of test results-mean estimates-confidence intervals

ISO 2818, plastics — Preparation of specimens by machining

ISO 750-1, Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1: Tensile/compression testing machines — Calibration and verification of force measurement systems

ISO 9513, Metallic materials — Calibration of extensiometer systems for use in uniaxial testing

ISO 10724-1, Plastics — Injection molding of thermoset powder molded plastics (PMC) specimens — Part 1: General principles and molding of multipurpose specimens

ISO 16012, Plastics — Determination of linear dimensions of specimens

ISO 20753, Plastics — Specimen

ISO 178-2019 – Determination of bending properties of Plastics

3 Terms, definitions and symbols
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminology databases for standardization at the following addresses:

– ISO online browsing platform: available at iso.org/obp

– IEC Electronic Encyclopedia: available at electropedia.org

3.1 Test speed v

The relative rate of movement between the sample support and the loaded edge

Note 1: It is expressed in millimeters per minute (mm/min).

3.2 Bending stress σf

Nominal stress on the outer surface of the specimen at the middle span

Note 1: It is calculated based on the relationship given in Equation (5).

Note 2: It is expressed in megapascals (MPa).

3.3 Bending stress σFB at fracture

Bending stress of the specimen at fracture

Note 1: It is expressed in megabascals (MPa).

Note 2: See Figure 1, curves a and b.

ISO 178-2019 – Determination of bending properties of Plastics

3.4 Flexural strength σfM

Maximum bending stress sustained by the specimen during bending test (3.2)

Note 1: It is expressed in megabascals (MPa).

Note 2: See Figure 1, curves a and b.

3.5 Bending stress σ under conventional deflection

Bending stress at conventional deflection, sC (3.7)

Note 1: It is expressed in megabascals (MPa).

Note 2: See also Figure 1, Curve c.

3.6 Deflection s

The distance of the top or bottom surface of the mid-span specimen from its original position during bending

Note 1: It is expressed in millimeters (mm).

3.7 Conventional deflection sC

The deflection (3.6) is equal to 1.5 times the sample thickness H

Note 1: It is expressed in millimeters (mm).

ISO 178-2019 – Determination of bending properties of Plastics

Note 2: Using a 16-h span L, the conventional deflection corresponds to a flexure strain of 3.5% (3.8).

3.8 Bending strain εf

Nominal fractional change in wafer length on the outer surface of the specimen at the mid-span

Note 1: It is expressed as a dimensionless ratio or percentage (%).

Remark 2: It is calculated from the relationship given in equations (6) and (7).

Figure 1 — Typical curves of bending stress σf versus bending strain εf and deflection s

ISO 178-2019 – Determination of bending properties of Plastics

3.9 Bending strain εFB at fracture

Bending strain of the specimen at fracture

Note 1: It is expressed as a dimensionless ratio or percentage (%).

Note 2: See Figure 1, curves a and b.

3.10 Bending strain εfM under bending strength

Bending strain under maximum bending stress

Note 1: It is expressed as a dimensionless ratio or percentage (%).

Note 2: See Figure 1, curves a and b.

3.11 Flexural modulus Flexural modulus Ef

Ratio of the stress difference σf2 − σf1 to the corresponding strain difference εf2 (= 0,002 5) − εf1 (= 0,000 5)

Note 1: It is expressed in megabascals (MPa).

Remark 2: The bending modulus is only an approximation of Young’s modulus.

Note 3: Please refer to Equation (9).

ISO 178-2019 – Determination of bending properties of Plastics

3.12 Rigid plastics

Plastics with an elastic modulus (3.11) in bending hinges or, if not applicable, in stretching, greater than 700 MPa under the given conditions

[Source: ISO 472:2013, 2.884, modified — entry comments have been omitted.

3.13 Semi-rigid plastics

A plastic with an elastic modulus in a bending hinge (3.11) or, if this modulus is not applicable, between 70 MPa and 700 MPa under tension for the given conditions

[Source: ISO 472:2013, 2.909, modified – entry comments have been omitted.

3.14 Span L between sample supports

The distance between the contact point between the specimen and the specimen holder

Note 1: It is expressed in millimeters (mm).

Note 2: See Figure 2.

ISO 178-2019 – Determination of bending properties of Plastics

3.15 Bending strain rate r

Rate of increase in bending strain (3.8) during the test

Remark 1: It is expressed as a percentage per minute (% ⋅ min−1).

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