Two main groups are angle-interlock and orthogonal-interlock fabrics in which several layers of threads were held together by separate interconnecting threads. Both weft and warp threads can be used for it. These interweave constructions have very good structural integrity and are crash-resistant. Multiple sheddings of different size can be formed simultaneously. Shuttles work in four insertion levels Figure 2 , allowing any number of fabric layers to be created on top of each other.
This unique arrangement of flexible shed formation and weft insertion guarantees a smooth processing when brittle fibres are used. However, the solid advantage is the endless loop of carbon weft yarns that creates closed-loop trajectories along the fabric structure for high-quality serial production with entire near-net-shape geometry. Figure 2. As investigation objects should not be too complex for identification of principal effects, we chose a control rod and a pipe connector which could be used in one T-joint assembly Figure 3. The control rod consists of loops at the ends connected by a flat section.
It is used to connect components and thereby transmit forces. The purpose of the pipe connector is to connect the central rod to the pipe as a movable assembly Figure 4. The woven part of both the components looks the same Figure 5. For the near-net-shaped 3D fabrics to be developed, fundamental generic elements were the base. Six individual variants of textile blanks comprising differences in additional reinforcement rovings as stuffer yarns will be discussed below. Figure 3.
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Possible application of the investigation objects as T-joint assembly, for example, as a hexapod element. Figure 4. Figure 5. Planar generic basic element left and thereof derived control rod right. Figure 6.
Forming the textile component blank into a pipe connector. From these elements, various fabric constructions were developed to create better functional properties for a control rod. The control rod Figure 5 was the base for functional variant 1. The load direction of the component is oriented in weft direction. Only carbon threads were used as weft material.
The basic structure of the fabric layers is tubular arranged around the entire component geometry Figure 7 , top. The weft insertion direction is indicated by the colour of the weft threads of the individual fabric layers. The weft threads shown in green are inserted into the fabric from left to right and the weft threads shown in black are inserted from right to left side.
More precisely, the different colours show the basic weft threads changing the fabric layers crosswise at the transition between the area of connection ring and central web.
Figure 7. Basic development in weave construction for all functional variants 1 schematic warp section.
Figure 7 , bottom, depicts the interconnection of the fabric layers by crossed interlock wefts through-thickness orthogonal-interlock. The fabric layers are linked by a second weft system, the so-called binding weft, highlighted in red. Both weave designs together were used as basic structure in variant 1a and in all textile blanks of other variants. For additional reinforcement, extra stuffer weft threads were wound around both the entire control rod and each connecting ring in one or two loops Figure 8.
At the transition from the connection ring to the central web, the reinforcing weft threads crossed each other once. The reinforcing carbon threads were put between the fabric layers without ondulation for straight load trajectories. They were held only by warp threads in the selvedge of the fabric. Figure 8. Extended development approaches in weave construction in variants 1b—1d schematic warp section. Starting point for the structural design of this reinforcement structure was a component consisting of two round connecting loops at the ends joined by a wider flat section Figure 9.
The central web was designed as a thin layer because the intended application as pipe connector forbids thick materials that need to be bent around a pipe. It was also important to ensure that the contact area between the connector and the pipe was as large as possible for evenly spread force transmission.
Hence, the CF rovings were spread into the bulging area of the textile. The textile blank can be used both for the control rod and, especially, for the pipe connector of the possible application Figure 3. Figure 9. Generic basic element with spatially expanded area element left and two functional elements derived therefrom as pipe connector. The contour lines of the functional variant 2 run no longer in a straight line but form an arc in the area of the central web.
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A 3D-woven structure was developed which had four different fabric sections, as depicted in Figure 10 , for the technical implementation of the weaving process. The textile reinforcement structure of section 1 was made of carbon weft threads, and the other sections were made of glass weft threads for economic reasons. Figure To keep the central web structure as thin as possible despite introducing additional rovings, the weave structures of the fabric sections with the high-performance fibres were modified.
Changes were executed step by step in a way that the central web bulges out in a circular arc in the horizontal direction during weaving. For this, the 96 carbon rovings of fabric section 1 were divided into two groups of weft threads in a ratio of The 64 carbon rovings of the first group were looped around the connection ring as a collective without ondulation. Thus, the carbon rovings were condensed effectively by the supporting fabrics and the full strength potential of the carbon fibres was well utilized.
One-third of the carbon rovings in fabric section 1 was used for binding of the warp threads and formation of the fabric layers in the area of the connection ring as well as in the central web area. The four-section 3D-woven structure could not only be produced problem-free, but it also improved handling during consolidation. Furthermore, it could be easily converted into the desired component shape. The reinforcement structure was then consolidated as a control rod with a flat central web for subsequent component testing.
Again, the textile structure was produced assisted by two supporting fabrics and one interstice fabric. To facilitate formation of the textile component blank into a pipe connector during consolidation Figure 6 , the weave structure was adapted in fabric section 1. A further change in the weave structure was made at the transition from the connection ring to the central web. Here, the number of weaves of the carbon rovings was reduced by overleaping more warp threads.
By adapting the weave structures, a smooth weaving process was feasible and thus damage to the load-carrying carbon rovings was avoided during weaving. This is evident from the results of the tensile test which are discussed below Figures 11 and Influence of weave and amount of carbon rovings additionally introduced into the textile blank for reinforcement on the component strength variants 1a—1d. Influence of weave and amount of carbon rovings additionally introduced into the textile blank for reinforcement on the component strength variants 1a, 1d, 2 and 3.
Control rods were manufactured exemplarily with the 3D weave reinforcement structures variant 1a—1d, 2 and 3 as described above.
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Structural sketches of the reinforcement concepts are shown in Table 1. For the design of the component, a sharp-edged transition between the central web and the connecting ring was deliberately chosen in order to clearly define the properties of the weaving variants. In a subsequent development step later on, it would be essential for a component design to make the geometric transitions even more suitable for force flow.
However, this was not the primary object of the comparative consideration of technical and sustainable improvements. Table 1. Matrix of variants of 3D-woven reinforcement structures. A low-viscosity, epoxy resin—based matrix system was used to ensure full impregnation of the partly compact fabric structures.
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The textile reinforcement structures were formed with a specially made consolidation tool Figure 13 for comparative tests of the components. An even and full resin impregnation was achieved for all components with the VAP consolidation process. In addition, smooth and uniform component surfaces as well as perfectly shaped component contours were attained. Consolidation forming tool for functional variants 1 left and 3 right.
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To work out the effect of the different weave structures on the mechanical properties of the CFRP components, a total of six 3D fabric samples were initially consolidated for component testing. Four sub-variations of functional variant 1 and the two optimized fabric constructions of functional variants 2 and 3 were manufactured as detailed in Table 1. The various 3D fabric structures selected for functional variant 1 differed in terms of their selvedge formation and the reinforcement concept applied at the sharp-edged transmission point.
Failure behaviours of the complex fabric constructions and the corresponding reinforcement yarns were analysed with tensile tests. A suitable specimen holder was manufactured for component testing, whose pins are intended for clamping the control rod into the test fixture and the other for connecting the specimen grips to the test fixture Figure Samples for tensile test of 12K carbon roving weft and air-textured glass yarn tex warp were taken from both the original yarns and the yarn sections from the woven fabric. The air-textured tex glass yarn for the warp had a breaking force of N tensile strength approximately MPa.
For the original 12K carbon roving from the bobbin, a breaking force of N approximately MPa was determined, while carbon rovings taken from woven fabrics showed a reduced breaking force of N approximately MPa.
Buckling and abrasion stresses occurring during weft insertion obviously led to filament breakages and thus to slight damage to the carbon rovings used as weft. Tensile tests of the blank textile structures themselves were not possible.
The carbon rovings slipped out of the textile laterally under load and the fabric fell apart. The consolidated components for tensile testing were made using the basic textile blanks of variant 1a and improved versions with additional stuffer threads using three different methods Table 1 :. The control rods were reinforced by a number of reinforcement loops placed around the entire control rod which work as stuffer wefts in the central web variants 1b and 1c.
The total number of reinforcement rovings in the weave was increased and placed in near-net shape variants 2 and 3. The tensile tests of the consolidated components had interesting results: the standard components with a reinforcement structure consisting of two integrated connecting rings variant 1a did break out at the critical transition from the connection ring to the central web but not at the outer fabric selvedge.
At this transition point, a strong edge existed and the yarn path of the carbon rovings was strongly bent. Due to the kink in the forming tool, the carbon rovings were deliberately misaligned. That fact was the motivation to optimize the 3D architecture of the above-outlined transition point exclusively through internal weave variants. Construction details, CF weft content and maximum breaking force related to the mass of the CF rovings used interwoven and stuffer wefts are represented in Table 1. All specimens failed during the tensile test at the intended critical point, which was the transition between the connection ring and the central web, due to a deliberately chosen, strongly bent yarn path of the carbon rovings.