Effect of yarn cross-sectional shape on resin flow through inter-yarn gaps in textile reinforcements

Axial flow through gaps between aligned straight yarns with realistic cross-sectional shapes, described by power-ellipses, was analysed numerically. At a given fibre volume fraction, equivalent gap permeabilities have a maximum at minimum size of elongated tapering parts of the gap cross-section and a ratio of gap width to height near 1. When the yarn spacing is given in addition to the fibre volume fraction, calculated maximum and minimum values for the equivalent permeability of inter-yarn gaps, which occur at near-rectangular and lenticular cross-sections, differ by factors of up to 3.3. Novel approximations for the shape factor and the hydraulic diameter in Poiseuille flow were derived as a function of the fibre volume fraction, the yarn cross-sectional aspect ratio and the exponent describing the shape of the power-elliptical yarn cross-section. This allows the equivalent gap permeability to be predicted with good accuracy for any fibre volume fraction and yarn cross-section

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

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 characterisation of textile compaction response: A benchmark exercise

This paper reports the results of an international benchmark exercise on the measurement of fibre bed compaction behaviour. The aim was to identify aspects of the test method critical to obtain reliable results and to arrive at a recommended test procedure for fibre bed compaction measurements. A glass fibre 2/2 twill weave and a biaxial (±45°) glass fibre non-crimp fabric (NCF) were tested in dry and wet conditions. All participants used the same testing procedure but were allowed to use the testing frame, the fixture and sample geometry of their choice. The results showed a large scatter in the maximum compaction stress between participants at the given target thickness, with coefficients of variation ranging from 38% to 58%. Statistical analysis of data indicated that wetting of the specimen significantly affected the scatter in results for the woven fabric, but not for the NCF. This is related to the fibre mobility in the architectures in both fabrics. As isolating the effect of other test parameters on the results was not possible, no statistically significant effect of other test parameters could be proven. The high sensitivity of the recorded compaction pressure near the minimum specimen thickness to changes in specimen thickness suggests that small uncertainties in thickness can result in large variations in the maximum value of the compaction stress. Hence, it is suspected that the thickness measurement technique used may have an effect on the scatter

Injuries during Equestrian Vaulting

Background: Vaulting is the least studied equestrian sports regarding the occurrence of injuries. As its sequences of motion do not compare to riding, vaulting must be assessed separately. Material and Methods: This retrospective, questionnaire-aided survey was aimed to gain insight into the overall frequency of injuries among equestrian vaulters. The second part of the study looked into the knee injuries that occurred. Survey forms were sent to 60 vaulting and equestrian clubs all over Germany, making for a response rate of 63%. Results: 95% of 624 responding athletes were female. The pool of participants consisted of both amateur and professional level vaulters with a mean age of 15 years. The survey showed a mean number of 4.1 injuries sustained during the observation period, i.e. the entire time an athlete had been active in the sport up to the data collection. The lower extremities were the most commonly injured area with a total proportion of 45%, followed by injuries to the spine and the head with 30%, and the upper extremities with 25%. Contusions accounted for the highest number of reported injuries. Other frequently reported injuries included muscle strain to the head and spine, fractures to the upper extremity and ligament damage to the lower extremity. 14% of the participants experienced at least one knee injury. The medial collateral ligament (27%) was found to be most prone to lesions, followed by the anterior cruciate ligament (23%) and the medial meniscus (22%). Half of all knee injuries occurred during dismounts, especially when swing-offs or flanks led to faulty landings. Conclusions: The results show that the lower extremity is the most commonly affected area. The ligamentous injuries affecting the lower extremity mainly result from dismounts. A specific training aimed at improving landing techniques might therefore prove beneficial in preventing injuries. The frequency of contusions and fractures to the upper extremity suggests that these injuries are related to falls

Downhill skateboard aerodynamics

Computational fluid dynamics has been used to perform a detailed aerodynamic analysis of a downhill skateboarder. The modelled geometry is that of a British downhill skateboarder, acquired using non contact laser scanning. Three helmets used by the skater were also compared, which revealed how each helmet directed the flow of air around and onto the body of the skateboarder. A study was also conducted into the influence of drafting, a tactical manoeuvre used in the race by skateboarders. This revealed how drafting skaters should position themselves behind a lead skater to minimize the drag force acting upon them