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As a composite designer how do you deal with Matrix Microcracking

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TLAR

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I have not seen this topic discussed before. Search function turns up zero results
 

wsimpso1

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Where did you find the topic in the first place? I expect that you could start there. Then tell us about it.

It sounds like a fatigue topic, and that is usually kept at bay with factor of safety. Most of us already use FOS of 2.0 for composites, and most of us approach limits only rarely. It might already be precluded in practice.

Billski
 

TLAR

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I’m still getting schooled up on it. But it is a topic that I am gonna get to the bottom of for sure.
Not being smart but not precluded in practice.
I started to study up on Micro and it’s history, and stumbled across it. I assumed you guys knew all about it.
The matrix fractures but not the fiber.
I will read up more and report back
 
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Vigilant1

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From what I recall, it is a bigger factor with polyester than with epoxy, and in environments where caustic/corrosive agents can get in and make trouble with the resin, fibers, or resin/fiber bond. So, those things would be a bigger worry to marine applications.
Resins (including epoxy formulations) that can accommodate a larger strain before fracture are less prone to trouble than less flexible ones.
I don't hear much about microfrractures in airplane composites, and there are lots of highly stressed planes with lots of hours. But, I am not an expert.
 

Vigilant1

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Read the article in Composites World
? "Meet my friend, he's in Texas.'?

ETA: This article? Micro-Cracking
Interesting. Says that some composites using brittle resins (esp polyesters) may exhibit microcracking at a strain of 0.2%, and that this is just 10% of their ultimate strength.
Epoxies are (generally) less brittle and also adhere better to the reinforcing fibers than polyesters do. So, it would be useful to find a technical paper describing the effect in epoxies. And, also useful to know if this microcracking occurs in actual use in actual airplanes, with actual detrimental results.

ETA again: Lots more technical info in this article from 2001, for those interested. Primarily deals with microcracking in epoxy matrix composites (E-glass and CF). I can't evaluate the importance or relevance of the information presented, except in the most superficial manner.
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.36.1505&rep=rep1&type=pdf
 
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TLAR

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I will just shoot from the hip and state it’s root cause is probably a resin mismatch. I don’t see a lot of information about matching resin to the fiber it’s supposed to restrain.
 

wsimpso1

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Seems that the article asked and provided the answer several ways.

First, it said that microcracking occurs with brittle resins and indicated polyester. Hmm. Then it gave the solution, which is to use modest fraction of ultimate strength. So if you are using polyester resins, which tend to be low strength and low toughness, you also have to use large FOS. Sounds HEAVY to me.

Typically, polyester resins have strain to failure that are less than fiber strain to failure. For instance, most glass cloth has failure strain around 1.5-2%, graphite around 1% and polyester resins have failure strains in the 0.3-0.5% range. Resin failure sets part (bulk) strength at a modest fraction of fiber strength. UGH! Then polyester has some other poor characteristics - just don't go there.

Airplanes are typically made of directional cloths and either vinylester or epoxy resins. These resins have strain to failure in the range of 4 - 7%. Strength of the composite tends to come at fiber limits and at much higher bulk strength than with polyester resin. In airplanes, most of us are working with FOS of 2 over first fiber failure, which means that first fiber to fail is at half of its failure load when the airplane is at limit load and the resin is at around 10% of its strength. Matrix microcracking was postulated to not be much of an issue at these levels of loading in resins...

Sounds to me like you simply do not use polyester resin.

Now look at successful composite airplane designs. Anybody know of any composite structure airplanes using polyester resin? Just epoxy and vinylester in my knowledge. We fair and finish with epoxy too.

Billski
 

TLAR

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Drilling down on this topic is like going down in the sewers of NYC, it goes ten different ways.
I will just have to keep Drilling
 

Map

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I used a program for analyzing composite laminates for a while that gave the results separated in margins to fiber failures and resin failures. It analyzes the layup by checking the stress between the layers of fabric, which depends on the fiber angles.
What Wsimpso1 wrote is correct.
In a properly designed laminate, the fibers take most of the stress, the resin just keeps things together and does not get close to cracking. So in airplanes, it is not really an issue and fibers are either at 0 deg for normal stress and +-45 for shear. If there are loads in other directions, add some 0/90 deg fabric.
 

TLAR

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Thanks Map
There must be a program for everything!
Thanks for your help
 

peter hudson

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I've come across it with composite pressure vessels when higher strength epoxy resins were used. The pressure proof tests cause matrix failure, but since the loads were always tensile the structures still held the pressure.

It also comes up on some resins due to temp alt humidity cycle testing for aircraft structures. Those resins were to be avoided.
 

Vigilant1

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Most of the published studies indicate the microcracking occurs most in BID reinforcements, with minimal cracking in UNI plies.
That doesn't speak to the importance of the phenomenon, just where it may occur.
 

TLAR

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So this is precluded in practice.
Is there a takeaway here? In layman’s terms? No large reinforcement direction changes, 45deg per ply?
 

Vigilant1

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So this is precluded in practice.
Is there a takeaway here? In layman’s terms? No large reinforcement direction changes, 45deg per ply?
My plan: Unless folks with more practical experience than I have start worrying about it, I won't worry about it. These composite materials aren't cutting edge anymore, there are planes with many, many hundred of hours on them and commercial acft with dedicated continuous testing programs to detect material degradation.
I will learn more about the subject to assure I don't inadvertently stumble into the grey area by, say, choosing a "better" epoxy that has different properties than the standard ones, etc. I think significantly varying a layup schedule from a proven one might introduce more risk than the risk posed by microcracking.
 
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