Composites Flashcards
Define a composite
A material formed by the combination of two or more phases to achieve superior properties than either constituent acting alone
What are the two parts of a composite?
A continuous phase, the matrix.
A dispersed phase, the reinforcement.
What are the three main forms of reinforcement of composites.
Particle reinforced, Fibre reinforced, Structural.
What are the two subdivisions of particle reinforced composites?
Large Particle and Dispersion Strengthened composites.
What scale do we consider Large Particle Mechanics on?
Particle-matrix interactions cannot be treated on the atomic or molecular level; rather, continuum mechanics is used. For most of these composites, the particulate phase is harder and stiffer than the matrix.
What scale do we consider Dispersion Strengthened Composites on?
Particle-matrix interactions that lead to strengthening occur on the atomic or molecular level. The mechanism of strengthening is similar to that for precipitation hardening. Whereas the matrix bears the major portion of an applied load, the small dispersed particles hinder or impede the motion of dislocations.
What is an advantage of dispersion-strengthened composites over precipitation hardening.
A dispersion-strengthened composite will retain its strength/stiffness at high temperatures, whereas a precipitation hardened materials’ properties will change as the precipitate grows.
What is a structural composite?
A structural composite is normally composed of both homogeneous and composite materials, the properties of which depend not only on the properties of the constituent materials but also on the geometrical design of the various structural elements.
What are the two most common type of structural composite?
Laminar composites and sandwich panels.
What is the structure of a laminate.
Layers of two-dimensional sheets or panels that have a preferred high strength direction such as is found in wood and continuous and aligned fibre-reinforced plastics
What is the structure of a sandwich panel
Two outer sheets which are adhesively bonded to a thicker core. The outer sheets are constructed of a stiff and strong material, the core is lightweight (foam, wood and honeycombs).
What are the two main types of particle reinforced composite?
Large particle composites & Dispersion strengthened composites.
How do large particle composites make a material stronger?
For most of these composites, the particulate phase is harder and stiffer than the matrix. These reinforcing particles tend to restrain movement of the matrix phase in the vicinity of each particle. In essence, the matrix transfers some of the applied stress to the particles, which bear a fraction of the load. The degree of reinforcement or improvement of mechanical behaviour depends on strong bonding at the matrix particle interface.
How do dispersion strengthened composites make a material stronger?
The mechanism of strengthening is similar to that for precipitation hardening. Whereas the matrix bears the major portion of an applied load, the small dispersed particles hinder or impede the motion of dislocations.
Describe the stress strain response of an aligned fibre reinforced composite as well as the response on failure
Stage I: both fibers and matrix deform elastically; normally this portion of the curve is linear.
Stage II: the matrix yields and deforms plastically while the fibers continue to stretch elastically, because the tensile strength of the fibers is significantly higher than the yield strength of the matrix. The proportion of the applied load that is borne by the fibers increases.
The onset of composite failure begins as the fibers start to fracture
Why is carbon reinforced composite failure not catastrophic?
First, not all fibers fracture at the same time, since there will always be considerable variations in the fracture strength of brittle fiber materials. In addition, even after fiber failure, the matrix is still intact inasmuch as. Thus, these fractured fibers, which are shorter than the original
ones, are still embedded within the intact matrix, and consequently are capable of sustaining a diminished load as the matrix continues to plastically deform.
Describe the concept of the critical fibre length
Some critical fiber length is necessary for effective strengthening and stiffening of the composite material as the force transmitted from the matrix phase to the fibre phase tends to zero at the ends. For a fibre below the critical length the force is not effectively transmitted from the matrix to the fibre. Fibers which are about 15 times the critical fiber length are termed continuous and effectively transmit the load.
A high performance composite material can be made in which Tungsten carbide particles are used to reinforce a Cobalt matrix. What type of composite is this? Comment on the likely values of Young’s modulus, tensile strength and hardness of this composite compared to mild steel. You can assume that the particle volume fraction is about 70%. Therefore suggest an application for this composite.
Metal matrix composite or particulate reinforced composite
For a high volume fraction of particles, Young’s modulus, tensile strength and hardness all would be higher than mild steel
Used for tools, drills and other high wear applications
Why would having a very high fibre volume fraction not be a good idea?
Matrix protects fibres and transfers stress to fibres
Too little matrix and you will get inferior properties
Name 2 advantages and 2 disadvantages of glass-fibre reinforced composites.
Advantages: inexpensive to produce, relatively high specific strengths and chemically inert
Disadvantages: care must be exercised in handling the fibres to avoid damage, they lack stiffness compared to other composite fibres, limited in maximum temperature
