ISO 178:2019 download free.Plastics-Determination of flexural propertles.
1 Scope
ISO 178 specifies a method for determining the flexural properties of rigid and semI’rigid plastics under defined conditions. A preferred test specimen is defined, but parameters are included for alternative speamen sizes for use where appropriate. A range of test speeds is included
The method is used to Investigate the flexural behaviour of the test specimens and to determine the flexural strength, ilexural modulus and other aspects of the lIexural stress/strain relationship under the conditions defined. It applies to a freely supported beam, loaded at midspan (three-point loading test).
The method Is suitable for use with the following range of materials:
— thermoplastic moulding, extrusion and casting materials, including Filled and ren1orced compounds in addition to unfilled types; rigid thermoplastics sheets;
— thermosetting moulding materials, including filled and reinforced compounds: thermosetting sheets.
In agreement with 150 10350-ill) and ISO 10350•21n1. this document applies to llbre’relnforced compounds with fibre lengths s7,5 mm prior to processing. For long-fibre-reinforced materials (Laminates) with fibre lengths >7,5 mm. see ISO 14125(21.
The method is not normally suitable for use with rigid cellular materials or sandwich structures containing celluLar material. In such cases, ISO 1209-1131 and/or ISO 1209-2141 can be used,
NOTl I For certain types of textile-Ilbre-re4nIorced plastic, a [our-point bending test Is used. This Is described
In ISO 14125.
The method is performed using specimens which can be either moulded to the specified dimensions, machined from the central section of a standard multipurpose test specimen (see ISO 20753) or machined from finished or semi-finished products, such as mouldings, laminates, or extruded or cast sheet,
The method specifies the preferred dimensions for the test specimen. Tests which are carried out on specimens oldifferent dimensions, or on specimens which are prepared under different conditions, can produce results which are not comparable. Other factors, such as the test speed and the conditioning of the specimens, can also influence the results.
ROTh 2 Especially for In)ectbon mouided semi-crystalline polymers, the thickness of the oriented skin layer which is dependent on the moulding conditions, also affects the flexural properties.
The method Is not suitable for the determination of design parameters but can be used in materials testing and as a quality control test.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (Including any amendments) applies.
ISO 291, Plastics — Standard atmospheres for conditioning and testing
ISO 293, Plast.cs — Compression moulding of test specimens of thermoplastic materials
ISO 294-1:2017, PlastIcs — injection moulding of test specimens of thermoplastic materials — Part 1:
General pri ,,ci pies, and moulding of multipurpose and bar test specimens
3.12
rigid plastic
plastic that has a modulus ojelasticity inflexure (3.11) or. it that is not applicable, then in tension, greater than 700 MPa under a given set of conditions
[SOURCE: ISO 472:2013, 2.884, modified — Note to entry has been omitted.]
3.13
semi rigid plastic
plastic that has a modulus of elasticity in flexure (111) or, If that Is nut applicable, then In tension. between 70 MPa and 700 MPa under a given set of conditions
ISOURCE: ISO 472:2013. 2.909, modified — Note to entry has been omitted.]
3.14
span between specimen supports
distance between the points of contact between the test specimen and the test specimen supports
Note Ito entry: It is expressed in miIlinwtres (mm).
Note 2 to entry: See FIgure 2.
3.15
flexural strain rate
rate at which thefkxural strum (III) Increases during a test
Note I to entry: It Is expressed In percent per minutc(%• mm-2).
4 Principle
A test specimen of rectangular cross-section. resting on two supports. Is deflected by means of a
loading edge acting on the specimen midway between the supports. The test specimen is deflected in
this way ala constant rate at mldspan until rupture occurs at the outer surface olthe specimen or until
a maximum strain of 5% (see 3.8) is reached, whichever occurs first. During this procedure, the force
applied to the specimen and the resulting deflection of the specimen at midspan are measured.
ISO 178 specifies two methods: method A and method B. Method A uses a strain rate of
I %/min throughout the test. Method B uses two different strain rates: I %fmin for the determination of the flexural modulus and 5 %/min or 50 %/min. depending on the ductility of the material, for the
determination of the remainder of the flexural stress-strain curve.
NOTE I The strain rates mentioned above are to b interpreted as nominal ones, Nominal test speeds are calculated using Formula [4). For the machine settIngs the best fitting ones are selected from Tabk1.
NOTE 2 For materials exhibiting nonlinear stress/strain behaviour. the Ilexural properties are only nominal. The formulae given have been derived assuming lInear elastic behaviour and are valid for defleclions of the specimen that are small compared to Its thickness. With the preferred specimen (which measures 80mm x 10mm * 4 mm) at the conventional Ilexural strain 013,5% and a spanio-thlckness ratIo, 1./h, of 16 the deflectIon is 1.511. FIexural testS are more appropriate for still and brittle materIals showing small defleetbons at break than for very soft and ductile ones.
5 Test machine
5.1 General
The machine shall comply with ISO 7500.1 and ISO 9513 and the requirements given In 52 to &4,
5.4.3 Deflection measurement
The machine shall be capable of continuously recording the crosshead displacement with an accuracy conforming to the class of ISO 9513 indicated in Table 2. This shall be valid over the whole range of deflections to be measured, Non-contact systems may be used provided they meet the accuracy requirements stated above. The measurement system shall not be influenced by machine compliance.
When determining the flexural modulus as indicated in type IV, the deflection-measuring system, in accordance with ISO 9513 Class 1, shall be capable of measuring the change In deflection to an accuracy of I % of the relevant value or better, corresponding to t 3,4 m for a support span. L, of 66mm and a specimen thickness, h, o14,C mm (see Figure 3).
For type lii tests the deflection-measuring system, in accordance with ISO 9513 Class 2, shall be capable of measuring the change In deflection to an accuracy of 2 % ol the relevant value or better, correspondIng to t6.8 jm for a support span, L, of 64mm and a specimen thickness, h, o14,l) mm.
Other support spans and specimen thicknesses will Lead to different requirements for the accuracy of the deflection-measuring system.
For the determination of the flexural modulus using the crosshead displacement as Indicated In Type Ill, the latter shall be corrected for the compliance of the machine. If the machine is equipped with built In routines for compliance correction these shall preferably be applied. If such routines are not avaiLabLe, the procedure given in Annex C shall be used.
NOTE Annes C also gives some explanallon of the possible sources of machine compliance
The use of a deflectometer further reduces errors introduced by the test setup and is therefore preferred.
Any deflection Indicator capable of measuring deflection to the accuracy specified above Is suitable.
In any one test specimen, the thickness within the central one third of the length shall not deviate by more than 2 % from Its mean value. The width shall not deviate from its mean value within this part of the specimen by more than 3 %. The specimen cross section shall preferably be rectangular, with no rounded edges, except as explained in the NOTE in 6.4.
The preferred specimen can be machined from the central part of a multipurpose test specimen complying with ISO 20753.
6.1.3 Other test specImens
If it is not possible or desirable to use the preferred test specimen, use a specimen with the dimensions given In Tabk.
NOTE Certain specifications require that test specliswns front sheets of thickness greater than a specified upper limit be reduced to a standard thickness by machining one face only, In such cases, it is conventional practice to place Lhc test specimen such that the original surface of the specimen is in contact with the two supports and the force is applied by the central loading edge to the machined surlace of the specimen.