BS 1007-5:2010 pdf download – Advanced technical ceramics — Ceramic composites — Methods of test for reinforcements Part 5: Determination of distribution of tensile strength and of tensile strain to failure of filaments within a multifilament tow at ambient temperature
4 Principle A multifilament tow is loaded in tension. The test is performed at a constant displacement rate up to failure of all fibres. Force and cross-head displacement are measured and recorded simultaneously. When required, the longitudinal deformation is derived from the cross-head displacement using a compliance correction. From the force-displacement curve, the two-parameter Weibull distribution of the rupture strain and the distribution of the rupture strength of the filaments are obtained by sampling the non-linear parts of the curve at discrete intervals j, which correspond to an increasing number of failed filaments in the tow. The test duration is limited to reduce time dependent effects.
5 Significance and use
The measurement of strain directly on the tow is difficult, so it is usually achieved indirectly via a compliance measurement that includes contributions of the loading train, grips, tab materials, etc. When it is possible to measure the tow elongation directly (by using a suitable extensometer system) this correction is not needed. The calculation of the results in Clause 10 also applies in this case by setting the load train compliance equal to zero. The evaluation method is based on an analysis of the non-linear increasing and decreasing parts of the force- displacement curve, which are caused by progressive filament failure during the test. The occurrence of these stages is promoted by a higher stiffness of the loading and gripping system. This method of evaluation is only applicable when the force-displacement curve shows these non-linear parts. The distribution of filament rupture strains does not depend on the number of filaments in the tow and is hence not affected by the number of filaments that are broken before the test. The determination of the filament strength distribution and of the elastic modulus necessitates knowledge of the initial cross sectional area of the tow. Because the number of unbroken filaments within the tow prior to the test is usually unknown, the values for the filament strength and for the elastic modulus necessarily represent lower bounds to these quantities. Also, the variation in filament diameter, which affects the strength values, is not accounted for.
6 Apparatus
6.1 Tensile testing equipment The test machine shall be equipped with a system for measuring the force applied to the specimen and the displacement, or directly the tow elongation. Additionally, the machine shall be equipped with a system for measuring the crosshead displacement with accuracy better than 1 µm.
6.2 Load train The grips shall align the test specimen with the direction of the force. Slippage of the test specimen in the grips shall be prevented as well as avoiding pre-damage due to gripping.
6.3 Data recording A calibrated recorder shall be used to record force-displacement curves. The use of a digital data recording system combined with an analogue recorder is recommended.
7 Test specimens
7.1 General Specimens with a gauge length of 200 mm shall be used to establish the filament strength and filament rupture strain distributions. Specimens with gauge lengths of 100 mm and 300 mm shall be used to determine, if necessary, the load train compliance. Examples of two types of test specimens are given below.
7.2 Window type specimen A window type specimen is shown in Figure 2. A stretched tow is fixed between two identical sheets of material (e.g. cardboard plates), each containing a central window. When the displacement is not measured directly on the tow, the height of the window defines the gauge length. NOTE This type of specimen has the advantage of easy handling.