DANSK DSF/PREN ISO 178 PDF

1.1 This International Standard specifies a method for determining the flexural properties of rigid [4] and semi- rigid plastics under defined conditions. A standard test specimen is defined, but parameters are included for alternative specimen sizes for use where appropriate. A range of testing speeds is included.
1.2 The method is used to investigate the flexural behaviour of the test specimens and for determining the flexural strength, flexural modulus and other aspects of the flexural stress/strain relationship under the conditions defined. It applies to a freely supported beam, loaded at midspan (three-point loading test).
1.3 The method is suitable for use with the following range of materials:
. thermoplastics moulding, extrusion and cast materials, including filled and reinforced compounds in
addition to unfilled types; rigid thermoplastics sheets;
. thermosetting moulding materials, including filled and reinforced compounds; thermosetting sheets.
In agreement with ISO 10350, parts 1 and 2, this International Standard applies to fibre reinforced compounds with fibre lengths ¡Ü 7,5 mm prior to processing. For long fibre reinforced materials (laminates) with fibre lengths > 7,5 mm see [1].
The method is not normally suitable for use with rigid cellular materials and sandwich structures containing cellular material [2,3].
NOTE For certain types of textile-fibre-reinforced plastics, a four-point bending test is preferred. It is described in [2]
1.4 The method is performed using specimens which may be either moulded to the specified dimensions, machined from the central portion of a standard multi-purpose test specimen (see ISO 3167) or machined from finished and semi-finished products such as mouldings, laminates and extruded or cast sheet.
1.5 The method specifies preferred dimensions for the test specimen. Tests which are carried out on
specimens of different dimensions, or on specimens which are prepared under different conditions, may produce results which are not comparable. Other factors, such as the speed of testing and the conditioning of the specimens, can also influence the results.
NOTE Especially for semi-crystalline polymers the thickness of the oriented skin layer, dependent on moulding conditions and thickness, affects the flexural properties.
1.6 Flexural properties can only be used for engineering design purposes for materials with linear
stress/strain behaviour. For non-linear material behaviour the flexural properties are only nominal. The
equations given have been derived assuming linear elastic material behaviour. They also assume that the deflection of the specimen is small compared to its thickness. Even for the conventional flexural strain of 3,5%, which at l/h=16 corresponds to a deflection of 1,5 h for the preferred specimen 80 mm x 10 mm x 4 mm, these are strictly not valid. Therefore flexural tests are more appropriate for stiff and brittle materials than for very soft and ductile ones.