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Reinforcement properties - A213

From CKN Knowledge in Practice Centre
Reinforcement properties
Foundational knowledge article
Material properties-UakV3h9hweWZ.svg
Document Type Article
Document Identifier 213

Overview[edit | edit source]

This page provides links to composite reinforcement properties. In the Knowledge in Practice Centre (KPC), reinforcement properties are defined as properties that describe the composite reinforcement material.

The role of the reinforcement is primarily to be the load bearing constituent in the composite material by providing strength and stiffness. Reinforcement may also provide other secondary and unique properties such as: heat and electricity conduction (in the case of carbon), thermal and electrical insulation (in the case of glass), abrasion resistance (in the case of aramid).

Below are the reinforcement property pages found in the KPC:

Chemical Properties[edit | edit source]

Coming soon.

  • Surface reactivity (effect of sizing)
  • Degradation

Physical Properties[edit | edit source]

Fibre volume fraction[edit | edit source]

Fibre volume fraction (\(V_f\)) describes the volume fraction of fibres in the composite with respect to total composite volume (matrix + fibre + voids).

It is cacluated as follows\[V_f = \frac{v_f}{v} = 1 - V_m - V_v\]


\(v_f = \) Volume of fibres

\(v = \) Total composite volume (volume matrix + volume fibres + volume voids)

\(V_m = \) Volume fraction matrix

\(V_v = \) Volume fraction voids

Illustration of the composite material constituent phases. (Matrix - continuous phase, Reinforcement - dispersed phase)

The mechanical properties of a composite material increase with greater proportions of fibre reinforcement content (greater value of \(V_f\)).

Material datasheets often report the composite constituent amounts in weight percent instead of volume percent, and care must be taken to avoid confusion between volume and weight percentages. Conversion between weight and volume can be made if both matrix density and fibre density are known using the density - weight/volume relationships.

\(\rho_f = \frac{w_f}{v_f}\)

\(\rho_m = \frac{w_m}{v_m}\)

\(\rho_c = \frac{{w_f}+{w_m}}{{v_f}+{v_m}+{v_v}} = \frac{{\rho_f v_f}+{\rho_m v_m}}{v}\)


\(\rho_f = \) Fibre density

\(\rho_m = \) Matrix density

\(\rho_c = \) Composite density

\(w_f = \) Weight of fibres

\(w_m = \) Weight of matrix

\(v_m = \) Volume of matrix

\(v_v = \) Volume of voids

KPC page for fibre volume fraction coming soon.

Click here to visit the how to measure reinforcement content - foundational method document KPC page.

Fibre weight fraction[edit | edit source]

For the purpose of fabrication, it is natural to describe the constituents of a composite in terms of their proportions by weight. Constituents (fibre, matrix, fillers, etc.) are weighed before processing, and laminate weight per unit area may be a design specification.

Fibre weight fraction (\(W_f\)) describes the weight fraction of fibres with respect to the total composite weight (total weight of the composite = weight of fibers + weight of matrix)

The term weight fraction may be applied to any of the constituents. Similarly, resin weight fraction, filler weight fraction, etc.

Fibre weight fraction is calculated as follows\[W_f = \frac{w_f}{w}\]

Similarly, weight fraction matrix (\(W_m\))\[W_f = \frac{w_m}{w}\]


\(w_f = \) Weight of fibres

\(w_m = \) Weight of matrix

\(w = \) Total composite weight (weight fibres + weight matrix)

With the total weight of the composite\[w = w_f + w_m = 1 = \frac{w_f}{w} = \frac{w_m}{w}\]

As mentioned for fibre volume fraction, material datasheets often report constituent amounts in weight percent instead of volume percent, and confusion between volume and weight percentages should not be made. While care must be taken not to inadvertently interchange the two, conversions between weight and volume can be made if both matrix density and fibre density are known.

\(\rho_f = \frac{w_f}{v_f}\)

\(\rho_m = \frac{w_m}{v_m}\)

\(\rho_c = \frac{{w_f}+{w_m}}{{v_f}+{v_m}+{v_v}} = \frac{{\rho_f v_f}+{\rho_m v_m}}{v}\)


\(\rho_f = \) Fibre density

\(\rho_m = \) Matrix density

\(\rho_c = \) Composite density

\(v_f = \) Volume of fibres

\(v_m = \) Volume of matrix

\(v_v = \) Volume of voids

\(v = \) Total volume of composite

KPC page for fibre weight fraction coming soon.

Click here to visit the how to measure reinforcement content - foundational method document KPC page.

Fibre diameter[edit | edit source]

The small diameter combined with long length aspect ratio of fibrous reinforcement is an important reinforcement property cosideration for several reasons:

  • Small fibre diameter reduces the probability of imperfections (see yield and fracture to learn more about this effect.)
  • A high aspect ratio (fibre diameter to length) to effectively transfer the applied load to the load-bearing reinforcement
  • Increases flexibility to aid in processing

The following are typical fibre sizes of popular fibre reinforcements:

  • Carbon: ~5 μm diameter
  • Glass: ~20 μm diameter
  • Kevlar 49: ~12 μm diameter

Comparison of fibre diameters between carbon fibre and glass fibre reinforcement.

When considering packing factors, approximately 6 glass fibres fill the space of roughly 92 carbon fibres.

KPC page for fibre diameter coming soon.

Fibre tow size[edit | edit source]

A tow is a bundle or yarn of individual fibres that are then woven together into reinforcement textiles. Typically, smaller tows are better because they result in a more homogeneous material.

However, the larger the tow:

  • The faster it is to deposit the fibre material
  • The easier is it for resin to flow in the gaps inbetween the fibre tows
  • The harder it is for resin to saturate within the fibre tows between the individual fibres

Typical tow sizes found for composite fibre reinforcements:

  • 1k (thousand)
  • 3k
  • 6k
  • 12k
  • 24k
  • 50k

KPC page for fibre tow size coming soon.

Fibre areal weight[edit | edit source]

Areal weight (or fiber areal weight, AW ) refers to the mass/weight of fiber per unit area. (Typically in g/m2 gsm ) or ounces/yard2 (often just called ounces). Areal weight depends on tow size and fiber architecture (weaving, density, etc.).

Fibre areal weight can be applied in the determination of reinforcement content (\(V_r\) or \(V_f\) for fibres) of a composite\[V_r=\frac{AW_r \times N}{\rho_r \times h}\]


\(AW_r = \) Reinforcement areal weight per layer/ply [kg/m2]

\(N = \) Number of reinforcement layers/plies

\(\rho_r = \) Reinforcement (e.g. fibre, \(\rho_f\)) density [kg/m3]

\(h = \) Composite thickness [m]

KPC page for fibre areal weight coming soon.

Click here to visit the how to measure reinforcement content - foundational method document KPC page.

Fibre preform permeability[edit | edit source]

Illustration of fibre preform permeability for in-plane resin flow.

Permeability refers to the ease of flow through a porous material, in this case the resin flow through the interconnected porous space in the fibre preform. For resin infusion processes (e.g. RTM), the permeability of the fibre preform is an important property as liquid resin is infused into the fibre reinforcement preform.

Darcy’s law governs flow through porous medium\[Q = -\frac{KA}{\mu}\frac{\Delta P}{x}\]


\(Q = \) Volumetric flow rate

\(K = \) Preform permeability

\(A = \) Preform cross-sectional area

\(\mu = \) Resin viscosity

\(\Delta P = \) Pressure differential across preform

\(x = \) In-plane flow distance of pressure differential

KPC page for fibre preform permeability coming soon.

Physical properties coming soon[edit | edit source]

  • Density
  • Suface area

Mechanical Properties[edit | edit source]

Yield and fracture[edit | edit source]

The yield and fracture behaviour of reinforcement is heavily influenced by the geometry and size form of the reinforcement material. For example, reinforcement materials are often employed in a thin fibre form that results in enhanced high strength. Glass and carbon fibre reinforcement are notable examples of this.

For most materials, and especially pronounced for brittle materials, a small diameter fibre is much stronger than the bulk form of the same material. The reasoning, as fibre diameter decreases, the probability of critical flaws that lead to fracture and failure in the material diminishes [1].

Steel fibre strength vs. fibre diameter. Strength increases as fibre diameter decreases.

The critical stress \(\sigma\) for failure depends on the absolute value of crack length or size of the imperfection. The thinner the wire, the smaller the size of imperfection that can exist in the fiber. Therefore, the smaller the fiber, the higher the critical stress that can be supported by the material.

This phenomenon that sees a reduction in critical flaws allows for the use of typically brittle materials, such as glass and carbon, to be used as high tensile strength composite reinforcement when in a small diameter fibre form is used.

KPC page for reinforcement yield and fracture coming soon.

Mechanical properties coming soon[edit | edit source]

  • Textile mechanics

Property Measurement[edit | edit source]

For methods to obtain material property values, please see the Foundational Methods Documents page:

Link to Foundational Method Documents page

Other Material Properties[edit | edit source]

Related pages

Page type Links
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


  1. [Ref] Callister, William D. (2003). Materials Science and Engineering: An Introduction. John Wiley & Sons, Inc. ISBN 0-471-13576-3.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)

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The relationship between function, material, shape and process consisting of Equipment and Tooling and consumables


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