Composites- Impact Failure Mechanisms and Toughening Flashcards
Standard in service loading vs impact loading
In service loading in tension, compression, shear or flexure is more predictable and easier to design for.
Impact loading events are more random and harder to design for.
The two regimes of impact events and how they are defined
Low-velocity impact
Medium to high-velocity impact (ballistic)
No clear definition available for when an impact event is low-velocity or ballistic. Rough rule is that ballistic impacts are >100m/s. Velocity is a convenient way to categorise impact but impact energy is what really matters. More accurate to consider impact momentum
What are the failure processes in the two regimes dominated by?
Low-velocity: dominated by bending
Ballistic: dominated by material removal by shear or shock waves
Impact sources in aerospace, manufacture and maintenance, combat environments
Aerospace: runway debris, hailstones, bird strike, ground service vehicles.
Manufacture/maintenance: accidental dropping of tools onto the composite, a problem in all industries.
Combat environments: ballistic projectiles and blast events
What is impact damage in composites largely influenced by?
The relatively weak interfacial bonding between the matrix and fibres
Issues that are relatively unique to impacting of composite materials
Relatively low impact events can lead to significant amounts of internal damage, particularly if laminate is thin and constrained.
Damage in (opaque) carbon fibre composites is often impossible to see from surface, not the case with (semi-transparent) glass or aramid fibres, barely visible impact damage (BVID).
Residual strength is significantly reduced after impact events, particularly in compression (compression after impact, CAI)
What are the 3 primary modes of impact damage in composites?
Same as for normal loading conditions.
Matrix cracking through transverse or shear loads.
Fibre fracture, through thickness shear or direct stresses.
Delamination, often where a large ply angle change occurs.
What does the severity of the 3 modes of impact damage depend on?
The nature of the fibre, matrix and the interface between them.
The construction and geometry of the composite.
The impacting object and impacting conditions.
Damage caused by low velocity impact event
Causes laminate to bend. Damage similar to flexural damage.
Relatively little front face surface damage, an indentation may or may not be visible.
Matrix cracking, delamination, fibre fracture.
Damage to back face is much more apparent. Failed fibres and delamination can often be seen
What do ply splits from low velocity impacts do?
Initiate and steer delamination
How do ballistic impacts work?
Material has much less time to respond. Energy is transferred to the composite at such a high rate that:
Shearing of the material as the projectile enters can cause a plug of material to be removed,
At very high energies the directly impacted material can only respond by atomising and being lost (the shock zone),
Bending is minimal and damage is very localised and very severe
What do you see after ballistic impacts?
Sheared fibre ends. Relatively minimal and localised delamination. Plug of material missing. No evidence of tensile failure (mirror, mist, hackle) or compressive failure (micro-buckling).
Factors affecting damage tolerance in composites
Matrix toughness
Fibre-matrix interfacial strength
Fibre orientation
Laminate stacking sequence (order of angles)
Laminate thickness (lots of thin layers stop crack propagation)
Support conditions
What is improving the damage tolerance of composites mostly centred on?
Delaying or preventing the initiation of delamination
7 methods for toughening composites
Modifying the matrix
Interleaving
Using angle plies
Dispersing plies
Hybridising
Using woven fabrics or 3D composites
Edge modification