EN 1007-7-2010 pdf download.Advanced technical ceramics – Ceramic composites – Methods of test for reinforcements – Part 7: Determination of the distribution of tensile strength and of tensile strain to failure of filaments within a multifilament tow at high temperature.
A multifilament tow is heated to the test temperature and loaded in ten&on. The test is performed at a constant displacement rate up to failure. 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 distribution of the rupture strength of the filaments is obtained by sampling the nonlinear 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.
Two methods can be used. The first method (hot-end method) consists of heating the tow over its total length. The second method (cold end method) consists of heating only a part of the test specimen length, the temperature profile of which is used to define the gauge length. The application of this method requires the realisation of three different heated zone lengths.
6 Significance and use
The measurement of strain directly on the tow is difficult, so it is usually achieved indirectly via a compliance measurement which includes contributions of the loading train, grips, tab materials, etc. These contributions shall be taken into account to achieve a correct analysis. 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 7 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. These parts are caused by progressive filament failures during the test, which is encouraged by a high stiffness of the loading and gripping system. When the force-displacement curve does not show these non-linear parts, the evaluation method of this part of EN 1007 cannot be applied.
The distribution of filament failure strains does not depend on the number of filaments In the tow and is hence not affected by the number of filaments which are broken before the test. The determination of the filament strength distribution requires knowledge of the initial cross sectional area of the tow arid because the number of unbroken filaments within the tow prior to the test is usually unknown, the values for the filament strength represent a lower bound to these quantities. Also, the variation in the filament diameter, which affects the strength values, is not accounted for.
a) Hot end method: for the hot end method, the gauge length, defined as the uniformly heated length, equals the test specimen length.
NOTE 1 SubjectIng the whole length of a tow to temperatures well above 1 000 “C. however, makes it difficult to fix the ends of the test specimen into appropriate temperature proof extensions.
b) Cold end method: for the cold end method, the test specimen length is the sum of the cold and the hot parts of the tow, with the gauge length is defined as the uniformly heated length. The temperature gradient zones along the tow axis are neglected.
NOTE 2 In this method, the problems associated with heating the clamps are avoided by heating only a central part of the tow and by keeping the junction at the ends of the test specimen at room temperature. This allows similar test specimen designs and organic resins to be used as in the room temperature test method (EN 1007-5). The interpretation of the results can be complicated by the superposition of the contributions from the cold and the hot tow zones.
The ratio of the hot part of the tow and the test specimen length is to be adjusted so as to keep the grips at room temperature, whilst the uniformly heated zone shall not be too short, in order to ensure a significant influence of the hot part on the overall failure behaviour of the tow.
7 Apparatus
7.1 Test machine
The test machine shall be equipped with a system for measuring the force applied to the test specimen, which shall conform to grade 1 according to EN ISO 7500-1:2004. Additionally, the machine shall be equipped with a system for measuring the cross head displacement with an accuracy better than 1 tim.
7.2 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. The load train performance including the alignment system and the force transmitting system shall not change because of heating.
7.3 Adhesive
A suitable adhesive for fixing the tow ends to the grip, such as ceramic cement (hot-end method) or epoxy resin (cold-end method).EN 1007-7-2010 pdf download.