Fibre Strength & Composites Interface Flashcards
Compare two fibre lengths and explain which one is stronger and why?
For almost the same survival probability, the relatively short fibres (L=16.6 mm) can undertake higher strength than the long fibres (L=100 mm). Therefore, the short fibres are stronger. This is because the short fibres have a smaller volume, which has a less probability of encountering severe flaws/defects on the fibre surface
Describe the function of silane couple agent in glass fibre coating and its effect on the ultimate tensile strength of glass fibre reinforced composites
Silane couple agent has the ability to form strong bonding with glass fibre surface and polymer matrix respectively. This creates fibre-matrix interface with sufficient interfacial shear strength so that fibres will not be easily pulled out of the polymer when composites are subject to external loading. As a result, ultimate tensile strength of glass fibre reinforced composites can be significantly improved.
What is the weakest link theory?
It is necessary to know fibre strength distribution
instead of just an averaged value, a large number of
individual fibres have to be tested in order to conduct a
meaningful statistic analysis.
This becomes a problem of random variable and
probability distribution function is needed to express how
fibre strength spreads under applied stress which is known as weakest link theory.
The model assumes that a length of brittle chain-like structure is composed of n unit links of length in series. Fracture of the entire chain occurs when the weakest link fails.
What is the weakest link formula?
P[L] = 1 - exp [ -L/L[o] * (sigma[f]/sigma[o])^m ]
Where P[L] = failure probability L = Length L[o] = normally set to be a unit length sigma[f] = applied stress sigma[o] = characteristic strength (when 63.2% of sigma [f]) m = shape parameter
Survival of each link formula?
S[Lo] = 1 - P[Lo]
Describe how the change of the length would affect the Young’s modulus of glass fibres
Unlike the strength in brittle materials, the Young’s modulus is a structural property and is not dependent on the fibre length. Thus, the Young’s modulus remains the same for both long and short fibres
How fibre-matrix bonding is normally characterised and why the fibre-matrix interface is important to composite
properties
Fiber-Matrix Interface is created when fibres make
a contact with polymer. Responsible for stress
transfer from matrix to fibre reinforcement.
Poor bonding – fibre can be pulled out and can lead to low composites strength
Strong bonding – no fibre pull-out and can result in low composites toughness
From thermodynamic point of view how is the interface controlled?
Wetting is controlled by surface energy of liquid and solid. Underlying thermodynamics controls in impregnation of fibre bundles and wetting of fibre surface by liquid polymer. Enables a liquid-solid contact and avoids formation of Voids (trapped air and unwetted fibre surface).
What is used to form an interface?
Silane couple agent has the ability to form strong bonding with glass fibre surface and polymer matrix respectively. Probably the most important single chemical to glass fibre industry.
