Woven textiles are used extensively to produce composite materials; they are made into prepregs, infused with resins or even have resins applied with a bucket and a brush. These textiles are produced from yarn, strands or filaments by weaving “weft” or “fill” at 90 degrees to the “warp” at 0-degree direction.
The following definitions will be useful for understanding this article:
Bias: if a textile has a higher proportion of filaments in one direction rather than the other it is called “biased”. The bias can be quite subtle or it can be very strong where directionality is needed (e.g. woven materials intended as replacements for unidirectional fabric).
Drape: the ability of a fabric to conform to curvature and into features.
Crimp: fibres perform best when they lie straight, crimp is the “bend” created in the fibres as they go “over and under”.
Stability: this is the ability of the fabric to resist damage or distortion while being formed and handled.
Symmetry: when the weft threads go over and under an equal number of warp threads in a regular pattern. This means that the front and the back faces of the fabric have the same pattern and the fabric is symmetrical through the thickness.
There are a wide range of weave styles used but some are more commonly encountered in composites than others. Plain weave (a) for example is symmetric and stable but has the most crimp and poor drape properties. Twill weave (b) is a bit more open than plain weave, the crimp is lower but still moderately high, symmetry is maintained while drape is a bit better and stability is a bit worse. Hopsack styles involve putting multiple warp and weft threads together which creates a more open drapable fabric with lower stability and reduced crimp dependent on how many threads are woven together. Hopsack styles are usually symmetric such as the 2-end example shown in (c) but they can also be made irregular.
This explainer is mainly about specific issues when working with Satin fabrics. For more information on the different reinforcement types see our other explainer on reinforcement formats.
Satin weaves feature weft that crosses over multiple warp threads but under only one. This makes the fabric flat with minimal crimp and good drape; however it comes at the expense of stability and symmetry which can impact upon parts and test specimens in surprising ways.
The following examples use the “7781” style E-glass fabric that is commonly used in the aerospace sector however most of the principles can probably be applied to any satin weave. 7781 weighs approximately 300 g/m2 and is an “8-harness satin”; this means that the weft travels over 7 warp yarns and under 1. A visualisation of the 7781 satin viewed from the “upperside” and the “underside” are shown in (d) and (e) respectively. The warp and weft yarns are shaded for contrast as per the previous images. In reality, without the contrast, when working with cut samples of this fabric it is vitally important to keep track of which is the warp direction, rotating it by 90 degrees looks very much the same as turning it over.
In addition this weave style is slightly biased as it has on average 22.4 warp yarns per cm and 21.3 weft yarns per cm. This means that if all the plies are oriented (pointed) in the warp direction the tensile properties in the warp direction are slightly higher than the weft (see the example chart (f) below).
This difference can be reduced by rotating half the plies by 90 degrees. A combination of the lack of symmetry and the bias within the weave means that flatness can be difficult to achieve with this fabric, the number of plies must be even and it is important to have the same number facing “up” as facing “down” to create a symmetrical panel. If you are using a core material it may hold flat but if not you are likely to end up with (in a worst case) something resembling (g).
If we can agree that it is a good idea to always try to use an even number of plies and to mirror the stack we can now consider whether it matters if the upperside is facing out or if the underside is facing out as shown in the cross-section image (h).
We showed above that for 7781 the difference between the composite warp and weft strength in a tensile test are relatively small (<10%). For tests that involve bending this becomes more complicated as the fibres parallel to the test direction have a stronger effect the closer and flatter they are with the outer faces. This means that a laminate performs best in a 3-point bend in the warp direction if the laminate is constructed with the warp faces outwards. A weft direction laminate performs best with the weft-flush faces outwards. Example test results are shown in (i), this geometric effect is strong and it can overcome the warp bias that is present in the weave.
Be aware that you cannot overcome this difference by balancing the laminate with 90° plies because the effect is from the fibres closeness to the surface and there will always be a stronger and stiffer orientation in a flexural test.
7781 (and other satin weaves) also exhibit some strange behaviour when bonding to honeycomb cores. Honeycomb cores have directionality as shown in (j). The direction featuring parallel edges is known as the “L” or “Ribbon” direction. The direction at right angles to this is known as the “w” direction or the “expansion” direction in the case of some materials.
In order to get the best peel strength when bonding 7781 prepreg to honeycomb you should orient your “flush” face fibres to be parallel with the ribbon direction. This appears to give much higher (double) the bond strength when peeling along the “W” direction, this may be because the fibres are unstable enough in the satin fabric to move aside and grip the ribbon. Image (k) shows this point, the only difference between the two sets of samples was which fabric face was against the core.
In conclusion the satin weave can only produce flexurally unbalanced laminates and the directional differences in surface flatness lead to big differences in bonding. When designing with this fabric or evaluating material properties it is more important than usual to control carefully every aspect of the laminate construction.
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