HS vs IM fibres in CAI tests?

[RPstress]

We are having a bit of trouble with conflicting test results for CAI (basically 30 J impact on the usual ~4 mm thick QI made from UD fibre specimen). There's not much difference for IM fibres compared with the usual HS and we think there should be... Does anyone have experience which they could use to advise us about trends if not actually share?

 

[SWComposites]

RP - yes, have seen this several times. Compression strength is complicated. CAI strength is complicated^2. CAI comparisons vary depending on whether you hold impact energy constant, dent depth constant, or something else. Damage for a given impact energy is a function of resin toughness (higher is not always better, depends on which fracture mode is different), fiber-resin interface properties, fiber strain to failure, ply thickness, tow bundle size and shape, amount of resin within the tow bundle vs amount of resin in the ply interlayer, etc, etc. The impact damage is then a complicated mixture of delaminations, matrix cracks and fiber breaks. Then the residual strength is a function of the specific damage state, the damage size relative to specimen width, the fiber diameter, tow size, resin location (within tow or interlayer), resin content, etc (compression strength is a kink band micro buckling failure mechanism, not a simple material strength). So - to understand what is going on with your materials you need to investigate the impact damage characteristics in some detail, and investigate undamaged uni and QI laminate compression strength. Or just throw up your hands and write it all off to the mysteries of CFRP strength.

Cheers,

SW

 

[TheBigM]

Hi, have you been able to evaluate the impact damage area? You'll need some sort of NDT capability to do so but it might help to explain the results you're getting. The fibre size and interaction with the matrix might lead to differing amounts of impact damage.

M

 

[RPstress]

Thanks SW (I think).

Impact damage affected by:
Impact type:
Impact energy constant
Dent depth constant
Something else constant (damage area?)
Resin toughness
Fibre-resin bond
Fibre failure strain
Ply thickness
Tow size
Tow shape
Resin interlayer vs. resin in fibres (intralayer)
Etc., etc.
—Query: how does tow shape affect matters?

Impact damage consists of:
Delams
Matrix cracks
Fibre breaks

Strength due to:
'Specific damage state'
Damage width/CAI specimen width
Fibre diameter
Tow size
Resin 'location' (damage distribution between interlayer and intralayer resin?)
Resin content (could also also measured by fibre content?)
Etc.
—Queries: see ? in brackets above.

Compression strength 'is'
Kink band micro-buckling (not a simple material strength).—Is this still true in the presence of impact damage?

Investigate impact damage characteristics in some detail, and investigate undamaged uni and QI laminate compression strength. Or just give up.

Well, I know what I'd do. It'd cost money, but... I shall pass on your recommendations.


@TheBigM: We'll be measuring damage area by C-scan (only the dent depth is regarded as being changeable due to relaxation so damage size scanning can wait a bit without generating much misinformation at this stage). The size on the fibres is standard for epoxy, and is itself is a modified epoxy.


Thanks both.

 

[SWComposites]

Tow shape affects compression strength (micro buckling of fibers), and possibly the degree of fiber "nesting" within the tow thereby the amount of resin around each fiber.
Resin 'location' (damage distribution between interlayer and intralayer resin?) > correct, the specific amount of resin within a tow (around the individual fibers), around the tows (intralayer) and between plies (interlayer) has an affect on damage type, area and compression strength
Resin content (could also also measured by fibre content?) > yes, but as noted above, its not just the overall resin or fiber content; often need micro photographs of cross sections to understand subtle differences
Kink band micro-buckling (not a simple material strength).—Is this still true in the presence of impact damage? > yes, though the failure mode is complicated by sub laminate buckling due to the delaminations, and by matrix cracks

Yes, understanding differences in behavior costs money ............

 

[mrsdesign]

Hi,
There is some evidence in the literature that suggests a delaminated area increase in high strength fibre composites compared to HM fibre composites. I would suggest this is the case in the IM fibre panels you have. Therefore, the difference in the CAI could be better understood by bringing in the C-Scan data - as TheBigM suggested.
QinetiQ have done some good work with organic solvents in impact damage sites which can be used to get some really good low KeV X-Ray images showing the damage modes present. It could be worth a look?