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Composite materials with textile reinforcements constitute a class of materials used as substitutions for conventional materials in certain specific applications (aeronautics, Sports and leisure, transportation) where the ratio performance to weight must to be optimized. The expected mechanical performances are function of the nature of constituents (fibre and resin) and of the quality of the Interface, i.e. the mechanical coupling between reinforcement and matrix. In the case of glass fibre-based composites, a test of calcination (or weight loss when burnt) at 625 degrees C with weighing of samples before and after dwelling in the oven (NF T 57-102 and ISO 1172) allows the measure of the percentage of non-volatile filler mass at the testing temperature, the identification of each layer, the designation of the degree of homogeneity of the composite by using different volume samples, etc. The degree of porosity (or void fraction) and its distribution in the composite depends on many factors such as the type of resin (nature, accelerator, catalyst, rheological characteristics, volatile components), the orientation, the structure and the mass of the reinforcement (weave structure, lubricating agent, stacking sequence), and the technological parameters linked to the manufacturing processes (temperatures, time, pressures and depressions, hygrometry conditions). In practice, the rate of porosity of industrial parts is located between 1 % and 10 %, according to the implementation process chosen. A high porosity rate translates essentially into high decreases in mechanical properties (up to 50 % for a 5 % void volume), especially for resistance to shear and failures at right angle with the fibre direction. The method for measuring densities (hydrostatic balance, pycnometer or specific gravity hydrometer) is often used because of its simplicity (no sophisticated equipment required, low cost and measuring time of the order of a minute). The fibre mass proportion is here determined by calcination or dissolution. The flexion tests of composites are covered by the Standards T 57-105, T 57-302, ASTM D 790, JIS K 7074 and standard EN ISO 14125. These tests are largely used in industry because of their simplicity. However, the flexion does not father a simple (or pure) constraint state within the materials tested: the tensile constraints are maximal in the convex part, the compression constraints are maximal in the concave part and the shearing constraints are maximal along the neutral axis of the test sample. (bilingual document: French/English)