Modelling the seam architectures for textile performing
Sewing of textile preforms offers a lot of opportunities for the future production of FRP parts. Although out of plane properties and damage tolerance abilities of the preform improve by stitching, stitching degrades the in plane properties of the preform. The choice of material and process parameters is crucial to realise the benefits of stitching. This demands modelling and simulation of stitched textile structures. Currently simple textile structures like multi layer woven fabrics and multi layer non-crimp fabrics can be modelled. This paper shows a concept for a mathematical modelling of stitching architectures including the generation of the thread surfaces. The model can help in realising two functions. On the one hand different material and process parameters can be taken into account and on the other hand the computed surface description including geometrical dependencies can be transferred into a structural mechanic model (e.g. WiseTex).
Mechanical performance of carbon fibre-reinforced composites based on stitched preforms
A comparative assessment of the influence of pure assembly seams based on a thin (11 tex) polyester yarn in a zigzag geometry on the resulting mechanical performance of a non-crimped fabric (NCF) carbon fibre-reinforced epoxy composite manufactured by vacuum-assisted resin transfer moulding is presented. This study was aimed at generating a solid foundation regarding the overall performance level of stitched NCF composites and at identifying critical property changes. The comprehensive evaluation of the mechanical composite properties includes static as well as dynamic tests of the in-plane properties as well as a characterisation of the interlaminar properties such as apparent interlaminar shear strength (ILSS) and compression after impact (CAI). It is demonstrated that mechanical properties such as the tensile and compression stiffness and CAI strength are not degraded by the chosen stitching parameters, whereas the tensile and compression strength, ILSS as well as the tensile fatigue behaviour are reduced as a result of pronounced localised fibre ondulations. A direct comparison to properties of a commonly used 5H satin woven fabric composite verifies that the overall performance of these particular stitched NCF composites must be enhanced with regard to the identified key criteria to meet the level required for aircraft applications and in order to maintain the performance advantage of NCF composites as compared to standard woven fabrics in general. Promising approaches include the use of different yarn materials based on soluble thermoplastics and/or modified stitching parameters.
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.