| Comment | The three and four point flexure standard EN ISO/F-DIS 14,125 was based on ISO 178 with input from ASTM D790, JIS K7074, EN 63, CRAG 200, EN Aerospace 2562 and 2746. Composite materials are divided into four classes related to their structure and ratio of Young's Modulus to through-thickness Shear Modulus. Care is required in determining the failure mode. A four point loading arrangement has been included to delay the initiation of compression failure by reducing the load on the central loading roller(s). In some cases failure of the compression face can be avoided by use of a 0.2 mm thick pad located beneath the centre loading roller, following Japanese practice. However, flexure tests would not be recommended for generating design data.Flexure testing is a cheap and easy method of obtaining data equivalent to tensile data. However, for a laminated material where the layers are either different formats or at different orientations the tensile and flexural moduli will not be equal. There are two further difficulties in applying this ""simple"" method. In a flexure test a proportion of the measured deflection occurs in shear, rather than in flexure, resulting in a modulus lower than achieved by a tensile test even for a ""homogeneous"" material. The effect is proportional to the ratio of the tensile Young's Modulus to the through-thickness Shear Modulus and increases with decreasing test span. For composite materials this modulus ratio is much higher (eg 5-50), than for polymers (eg 2.5), resulting in a larger error in the modulus of the order of 10% at the same span (ie 16 x specimen thickness) In order to obtain an equivalent, or comparable value, the span must be increased in proportion. The span/thickness ratio is 40 for a unidirectionally reinforced carbon-fibre specimen. The other difficult concerns the tendency for different failure modes to occur. For example, materials with mat reinforcement will fail in tension, while a fully unidirectional specimen will often fail on the compression face through local micro-buckling assisted by the local compression and shear loads. |
