Composites- Impact Failure Mechanisms and Toughening

Question 1

Standard in service loading vs impact loading

Answer 1

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.

Question 2

The two regimes of impact events and how they are defined

Answer 2

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

Question 3

What are the failure processes in the two regimes dominated by?

Answer 3

Low-velocity: dominated by bending
Ballistic: dominated by material removal by shear or shock waves

Question 4

Impact sources in aerospace, manufacture and maintenance, combat environments

Answer 4

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

Question 5

What is impact damage in composites largely influenced by?

Answer 5

The relatively weak interfacial bonding between the matrix and fibres

Question 6

Issues that are relatively unique to impacting of composite materials

Answer 6

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)

Question 7

What are the 3 primary modes of impact damage in composites?

Answer 7

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.

Question 8

What does the severity of the 3 modes of impact damage depend on?

Answer 8

The nature of the fibre, matrix and the interface between them.
The construction and geometry of the composite.
The impacting object and impacting conditions.

Question 9

Damage caused by low velocity impact event

Answer 9

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

Question 10

What do ply splits from low velocity impacts do?

Answer 10

Initiate and steer delamination

Question 11

How do ballistic impacts work?

Answer 11

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

Question 12

What do you see after ballistic impacts?

Answer 12

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).

Question 13

Factors affecting damage tolerance in composites

Answer 13

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

Question 14

What is improving the damage tolerance of composites mostly centred on?

Answer 14

Delaying or preventing the initiation of delamination

Question 15

7 methods for toughening composites

Answer 15

Modifying the matrix
Interleaving
Using angle plies
Dispersing plies
Hybridising
Using woven fabrics or 3D composites
Edge modification

Question 16

Problem with high performance matrices

Answer 16

Such as those used in aerospace. Have relatively low toughness. Thermosetting resins tend to be brittle in comparison to thermoplastics as they are often high cross-linked. Restricted chain motion means impact energy can’t be easily dissipated and may instead break the polymer chains

Question 17

How does crosslink density affect matrix?

Answer 17

Higher crosslink density between main backbone chains means shorter chain lengths between crosslinks and results in a more stiff and rigid polymer.

Question 18

Modifying the matrix: add toughening agent

Answer 18

Could add elastomeric, e.g CTBN (carboxyl terminated butadiene acrylonitrile),
Thermoplastic, e.g PES (poly ether sulphone),
Particulate: adds stress concentrations encouraging matrix to yield, the particles act as stress concentrators causing blunt crack growth, initiate local failure sites and local yielding.

Question 19

Modifying the matrix: other ways to improve toughness

Answer 19

Use a less cross-linked matrix: cure at lower T/shorter time or use lower functionality matrices, increases molecular weight between crosslinks/decreases monomer functionality.
Use more flexible matrix: aliphatic, cycloaliphatic, instead of aromatic.
Use a thermoplastic matrix: e.g PEEK, PEI, but processing is more difficult.
Can use combination of these to improve toughness further (butnreduce other mechanical properties

Question 20

Modifying the matrix: compromise

Answer 20

Most matrix modifications improve the toughness whilst simultaneously reducing other mechanical properties like stiffness, strength and Tg.
Lots of research being done to try and fix this problem

Question 21

Interleaving

Answer 21

Layers of tough, ductile polymer or adhesive are interleaved between the load bearing composite plies. Significant improvements in the interlaminar fracture toughness can be achieved. Reduces delamination but also ff so mechanical properties reduced.
Alternatives are to only interleave on selected layers, or only interleave at delamination prone areas such as at the edges

Question 22

Using angle plies

Answer 22

Delamination encouraged when ply splits intersect at a high angle, elf in cross-ply there is 90° angle between cracks promoting delamination. Angled plies act to reduce the angle difference between splits when moving from ply to ply. Can use as many angles plies as needed however complexity is increased and anisotropy reduced

Question 23

Dispersing plies

Answer 23

Having different angle plies dispersed throughout a laminate instead of grouping them together improves delamination resistance. Remember symmetrical laminates are required to avoid tension-bending coupling. Having many thin plies improves delamination resistance, but thinner plies and the composites made from them are harder to manufacture. Having a thin composites also improves delamination resistance, but mechanical properties may be compromised if composite is too thin.

Question 24

Hybridising

Answer 24

Hybrid laminates contain more than 1 type of reinforcing fibre. Take advantage of properties of each

Question 25

Intraply hybrids

Answer 25

Weave tows of relatively tough glass/polymer with relatively stiff carbon. Resulting composite stiffer than in only glass/polymer, tougher than only carbon. Example is carbon with dyneema

Question 26

Interply hybrids

Answer 26

Combine stiffer 0° carbon plis with tougher 90° glass plies. Resulting composite has improved delamination resistance as 90 transverse plies are tougher. Negligible effect on composite stiffness and strength as these are controlled by 0 layers

Question 27

Using woven fabrics or 3D composites

Answer 27

Interlacing or interlooping fibres means failure by splitting and delamination is less likely. Z-pinning 3D composite weaving results in ultimate delamination resistance

Question 28

Edge modification

Answer 28

At the edges of laminates with differently angled plies, separation stresses act to delaminate the composite. Can control this by:
Ply termination- transverse plies terminated before they reach edge of laminate improves delamination resistance,
Edge notching- series of narrow, shallow notches introduced into the edge of the laminate improves delamination resistance, stress path is diverted away from separating plies (out of plane), but notches are stress concentrators in plane

Question 29

What can you do if you want to add holes to a composite afternit has been made?

Answer 29

Design it so that reinforcement is not in the regions the holes will be put in