Fatigue, yield & fracture criterion for fibre reinforced plastics in comparison to metal 1

[goutam066]

Hello All,

I would like to request from the interested folks, if anyone can suggest me fibre reinforced plastic CAE. I would like to know following;
-To predict fracture in Plastic, which theory we need to follow, max principal strain or max von Mises strain?
- Can we define a clear Yield strain (proof strain) for FRP? if it is, then what should be plastic strain?
- For fatigue, should we take linear stress or do a nonlinear analysis, may be at high operating temp E value may not be appropriate for the given load. Can we take stress criterion ( a factor of UTS) if we don't fatigue data? Which stress combination we need to take? Principal or von Mises?

Appreciate if I can get answers with theoritical backing. Thanks to everyone!

 

[Onetripper]

What a great excuse to dust off my old Fracture Mechanics book ( By T.L. Anderson). While reading ch9 it states that "ASTM committee D20 has developed a standard method for K1c testing of plastics [10]." (pg 362) On the same pg there is a nice stress stain diagram that shows how yield strength is defined for plastics. As far as "predicting" cracks, I have been raised on the pedagogue of always assume the crack exist, the real question is at what stress does it go bang!

 

[SWComposites]

Pure I reinforced plastics are very different from fiber reinforced plastics. And the latter highly depend on whether it is discontinuous or continuos fiber material.

For continuous fiber reinforced plastics, stress-strain is usually linear to failure, the is no "yield" and neither max prin or Von mises criteria are appropriate; need to use something like max stress or strain in the fiber directions (and ignore most of the failure criteria stuff in text books - it is wrong)

 

[goutam066]

@SWComposites Thanx for your reply. What target should we use for glass filled plastics (Ex. PA6 GF30, PA66 GF30 etc)? whether it should be 0.2% strain or 20% of UTS or some other standard value. Thr is a great variation of mechanical properties with change in temperature(-40 to 140 deg C)so , it is getting very difficult to fix a standard target for every temperature range.

 

[SWComposites]

Target?

 

[goutam066]

I am talking about failure criterion. Like for steel we fix yield point as target or 0.2% Proof strain for ductile material.