Geometric modeling of 3D woven preforms in composite T-joints

A common method to fabricate net-shaped three-dimensional (3D) woven preforms for composite T-joints is to weave flat 3D preforms via a standard weaving machine with variation in binder yarn path and then separate the preform in the form of a bifurcation. Folding introduces fiber architecture deformation at the 3D woven bifurcation area. In this paper, a geometric modeling approach is proposed to represent the realistic fiber architecture, as a preprocessor for finite element analyses to predict composite structural performance. Supported by X-ray micro-computed tomography (mCT), three important deformation mechanisms are observed including yarn stack shifting, cross-section bending, and cross-section flattening resulting from the folding process. Furthermore, a set of mathematical formulae for simulation of the deformations in the junction region are developed and satisfactory agreement is observed when compared with mCT scan results

Experimental determination of the permeability of engineering textiles: Benchmark II

In this second international permeability benchmark, the in-plane permeability values of a carbon fabric were studied by twelve research groups worldwide. One participant also investigated the deformation of the tested carbon fabric. The aim of this work was to obtain comparable results in order to make a step toward standardization of permeability measurements. Unidirectional injections were thus conducted to determine the unsaturated in-plane permeability tensor of the fabric. Procedures used by participants were specified in the guidelines defined for this benchmark. Participants were asked to use the same values for parameters such as fiber volume fraction, injection pressure and fluid viscosity to minimize sources of scatter. The comparison of the results from each participant was encouraging. The scatter between data obtained while respecting the guidelines was below 25%. However, a higher dispersion was observed when some parameters differed from the recommendations of this exercise.The authors are grateful to J.M. Beraud from Hexcel Fabrics for his support that made possible this exercise. The contributions of J.B. Alms, N.C. Correia, S. Advani, E. Ruiz and P.C.T. Goncalves to the preparation of the guidelines document and templates are acknowledged by the participants of this benchmark.Vernet, N.; Ruiz, E.; Advani, S.; Alms, JB.; Aubert, M.; Barburski, M.; Barari, B.... (2014). Experimental determination of the permeability of engineering textiles: Benchmark II. Composites Part A: Applied Science and Manufacturing. 61:172-184. doi:10.1016/j.compositesa.2014.02.010S1721846

Repeatability and reproducibility of the measurement of prepreg tack following ASTM D8336: Results of a round-robin study

This study reports the findings from a round robin test exercise investigating ASTM D8336, a method for the measurement of tack of prepregs applicable to automated material placement technologies. Twelve participants tested two prepreg materials in a core study of three temperature and deposition/peel rate combinations, and five participants conducted an extended study based on a test matrix of four temperatures and four rates. In the core study, the repeatability coefficient of variation of obtained tack values ranged from 6.06 % to 15.08 %, while the reproducibility coefficient of variation ranged from 12.37 % to 32.37 %. The probable dominant causes for the observed variability are the limited accuracy of control and measurement of specimen temperature, of compaction force, and of humidity. Using time-temperature superposition, tack mastercurves were constructed from the extended study, identifying values of peak tack, the rate at peak tack and the width of the tack mastercurve, as well as the 95 % confidence and prediction bands. Across all materials and participants, 93.3 % of the repeats from the core study lay within the 95 % prediction bands, supporting the view 2 that a mastercurve provides a greater degree of useful information at a comparable effort for data acquisition

Die strukturell-funktionale Theorie

Daheim H. Die strukturell-funktionale Theorie. In: Endruweit G, ed. Moderne Theorien der Soziologie. Stuttgart: Enke; 1993: 23-86