Fiber-bed drapability - A262
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| Document Type | Article |
| Document Identifier | 262 |
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Material |
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Introduction[edit | edit source]
Drapeability refers to the ability of a fabric or reinforcement mat to conform to a complex shape[1]. Draping with dry fabrics results in a preform, which can then be impregnated with a resin via liquid composite moulding (LCM) processes. Draping with prepreg thermosets allows for several plies to be stacked to form a pre-consolidated laminate preform before forming the material[2].
Scope[edit | edit source]
This page defines fibre-bed drapeability, explains its significance in composites processing, and outlines the deformation modes and reinforcement architectures that influence its behaviour. Common defects, methods to characterize drapeability, and practical manufacturing strategies are briefly discussed.
Significance[edit | edit source]
Drapeability is a process relevant material behaviour that affects the deposition and consolidation step, permeability, defect formation, and final part quality.
Prerequisites[edit | edit source]
Recommended documents to review before, or in parallel with this document:
Definition[edit | edit source]
Drapeability refers to the ability of a fabric or reinforcement mat to conform to a complex shape[1]. Highly drapeable materials conform more readily to complex geometries, ensuring a more accurate final part shape.
Deformation mechanisms[edit | edit source]
During the forming process, an initially planar material deforms into a three-dimensional shape by undergoing large local deformations to conform to the complex shape of a tool[2]. This deformation is governed by several mechanisms that affect local fibre directions, volume fractions, and thickness.
Intra-ply shear[edit | edit source]
Intra-ply shear is the main deformation mode for forming 3D shapes. It involves the rotation and compaction of intersecting fibre tows, a process that is highly dependent on temperature and processing rate for prepregs. As the fabric shears, the warp and weft tows rotate along their intersection until they physically contact each other, resulting in a sudden rise in shear resistance. This phenomenon is known as "shear locking" and beyond this threshold the fabric buckles out-of-plane.
Intra-ply tensile loading[edit | edit source]
Intra-ply tensile loading extends the material parallel to the fibres and acts as one of the main sources of energy dissipation, with stiffness increasing as the weave straightens.
Ply/tool or ply/ply shear[edit | edit source]
Ply/tool or ply/ply shear describes the relative sliding movement between adjacent composite layers or against the tool surface, where the friction changes based on the process pressure, temperature, and rate.
Ply bending[edit | edit source]
Ply bending is essential for forming curved geometries. This mechanism involves the bending of individual layers, which is facilitated by the internal fibres sliding past one another.
Compaction/consolidation[edit | edit source]
The compaction and consolidation mechanism forces fibres to pack more densely, particularly around complex features. This reduces the material's thickness, increases the fibre volume fraction, and minimizes void content, depending on the applied temperature and processing rate. However, the compaction of the fibre architecture decreases the preform's local permeability, which alters and restricts resin flow paths during consolidation [A158].
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| a) Intra-ply shear | b) Intra-ply tensile loading | c) Ply/tool or ply/ply shear |
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| d) Ply bending | e) Compaction |
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Wrinkling[edit | edit source]
Wrinkling is the most common defect observed in the draping process. It occurs when a fabric is sheared beyond its locking angle or when inter-ply shear transfers compressive forces between adjacent layers, resulting in out-of-plane buckling[2].
Undesirable fibre orientations[edit | edit source]
If a material does not shear smoothly or is forced over a complex geometry, the fibres can distort into undesired orientations, decreasing the mechanical performance of the final part.
Bridging[edit | edit source]
Bridging occurs when the fabric fails to tightly conform to the radius of a concave corner. This bridging effect creates gaps that result in resin-rich corners, which are highly prone to porosity and voids after curing[3].
The influence of reinforcement architecture[edit | edit source]
Different fibre architectures offer distinct drapeability and dimensional stability characteristics[3]. High dimensional stability helps the material resist in-plane distortion during handling. However, good drapeability and resistance to shear are mutually exclusive, requiring a strategic compromise based on the specific forming process details.
The reinforcements in a unidirectional (UD) ply conform easily along the fibre axis, but lack transverse integrity, making them prone to splitting when draped over complex geometries. For non-crimp stitched fabrics, the stitching locks the fibres in place. This restricts in-plane shear and reduces their overall drapeability compared to standard weaves. Chopped reinforcement mats utilize short, randomly oriented fibres that can easily shift independently, offering high drapeability at the expense of structural strength.
Woven fabric is the most common material architecture used in the forming of complex geometries[3]. It offers a good balance between dimensional stability and drapeability, and it is available in a variety of patterns that offer distinct properties.
Plain weave[edit | edit source]
In a plain weave, each warp and weft yarn continually passes over and under successive yarns. Plain weaves have the highest number of interlaces per unit area, making them the most dimensionally stable weave, but at the expense of having the lowest drapeability.
Satin weave[edit | edit source]
Satin weaves have the fewest interlaces, resulting in the lowest dimensional stability, but the highest drapeability. A higher harness number results in fewer interlaces, meaning that an eight-harness satin weave is more drapeable than a five-harness satin weave.
Twill weaves[edit | edit source]
Twill weaves offer intermediate drapeability and stability compared to plain and satin weaves. In this pattern, one or more warp yarns alternately weave over and under two or more weft yarns.
Methods to characterize drapeability[edit | edit source]
The primary motivation for drapeability testing is to understand material behaviour during shaping and to rank materials accordingly[2]. However, these testing methods currently lack universal standardization. For general composite property and composite characterization tests, refer to Testing - A178.
The picture frame test is used to measure intra-ply shear. A hinged square rig forces a cross-shaped fabric sample into a state of perfectly uniform shear. Another method for measuring intra-ply shear is the bias extension test. A rectangular strip with fibres angled at ±45° is subjected to tension, and measurable zones of shear are observed. The ply pull-out experiment characterizes interfacial friction, demonstrating how layers slide against the tooling or adjacent layers during forming. Cantilever or axial buckling tests yield out-of-plane stiffness values and demonstrate how a material conforms to tight, single-curvature radii.
Practical manufacturing strategy[edit | edit source]
Flat ply collation is a manufacturing strategy that involves dividing a laminate into pre-collated, segmented ply packs rather than being laid directly onto the mould[3]. These segmented packs are assembled on a flat surface, transferred to the tool, and vacuum-formed in stages to mitigate out-of-plane wrinkling. Splicing is utilized at the boundaries to connect the segmented packs and accommodate any severe geometrical transitions.
Related pages
| Page type | Links |
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| Introduction to Composites Articles | |
| Foundational Knowledge Articles | |
| Foundational Knowledge Method Documents | |
| Foundational Knowledge Worked Examples | |
| Systems Knowledge Articles | |
| Systems Knowledge Method Documents | |
| Systems Knowledge Worked Examples | |
| Systems Catalogue Articles | |
| Systems Catalogue Objects – Material | |
| Systems Catalogue Objects – Shape | |
| Systems Catalogue Objects – Tooling and consumables | |
| Systems Catalogue Objects – Equipment | |
| Practice Documents | |
| Case Studies | |
| Perspectives Articles |
References
- ↑ 1.0 1.1 [Ref] Peters, S. T., ed. (1998). Handbook of Composites. Springer US. doi:10.1007/978-1-4615-6389-1. ISBN 978-0-412-54020-2.CS1 maint: date and year (link)
- ↑ 2.0 2.1 2.2 2.3 2.4 [Ref] Empty citation (help)
- ↑ 3.0 3.1 3.2 3.3 [Ref] Campbell, F.C. (2004). Manufacturing Processes for Advanced Composites. Elsevier. doi:10.1016/B978-1-85617-415-2.X5000-X. ISBN 9781856174152.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
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