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Composites Failure Criteria Quesiton...

Composites Failure Criteria Quesiton...

Composites Failure Criteria Quesiton...

Apologies for incoherent post. A colleague of mine who is currently working on major EU Aircraft MRB/Concessions project was telling me that they are using strain based approach i.e. failure criteria. They are working on composite honeycomb laminates in the flaps.

From my dealings with whatever EU aerospace projects, they seem to recommend a stress based failure criteria.

I also am aware of Max Strain Failure criteria, my question, is there any specific situations where using a strain based approach would be advantageous over stress based approaches?

I understand that the above does not give much to go by, but it is all the info I have. I would appreciate if experienced analysts would help me understand the above better.

Thanks in advance.

RE: Composites Failure Criteria Quesiton...

You can use either approach, but a strain based approach tends to be preferred for the following reasons (excerpt from upcoming book):

The use of an in-plane strain allowable is common for composite analysis. Conversely, metals typically use stress allowables. There are some advantages to using strain allowables, such as:

1. Ply strains are continuous through-the-thickness of a laminate, unlike ply stresses (See Figure X). The use of stresses may be confusing because ply stresses are discontinuous through-the-thickness.
2. Strain allowables tend to be more constant with varying laminate fiber percentages. This allows preliminary design values to be used in a more general way (See Section X). Also, rule of thumb allowables are based on strains. Stress allowables fluctuate to a greater degree because the effective modulus changes as well.
3. Structural arrangements, such as a stringer bonded to a skin, are strain compatible. In general, they do not operate at the same stress levels because the elastic properties may be different for each member (See Chapter X). Also, strain compatibility is enforced for general solid laminate beams and sandwich structures (may have different elastic properties for the flanges/facesheets). Therefore, the analysis output values are strains (See Chapters X and X).
4. Strain is non-dimensional.
5. Laminate strains are directly calculated via Classical Laminated Theory (CLT) (See Chapters X and X).
6. For a ply based failure analysis, a strain based approach is better suited to capture the nonlinear response for an epoxy matrix.
7. Long history of use in aerospace industry.

Note that for the bearing failure mode, a strength allowable is generally used (also typical for metals).


RE: Composites Failure Criteria Quesiton...

Hi Brian,
Thanks for your detailed response. Definitely helped in bettering my understanding. I am so looking forward to your book. A little disappointed it is pushed back to Q2/3 of this year, but I understand that the book needs to be as right as possible before release.

A couple of follow up questions.

Although strain based approach is simpler, how does industry account for the fact that Max Strain faiure criterion does not factor possible stress components interactions just like Max Stress failure criterion?

What are the uses of Strain Invariant Failure Theory and where is it typically used?

RE: Composites Failure Criteria Quesiton...

The interactive failure theories do not correlate to laminate test data. See many previous posts on this. Max fiber strain criteria works well for carbon fiber laminate level in-plane strength for fiber dominated layups.

SIFT is a completely different thing which sort of works for some resin dominated failure modes and does not at all work for fiber failure modes (in spite of the various published hype).

RE: Composites Failure Criteria Quesiton...


In new (well newish) European standard EN13121-3 (Glass reinforced plastics tanks and vessels) there are (at least) two checks to be made on the laminate at a specific point (in fact many specific points). One is ultimate limit state (ULS); the other is serviceability limit state (SLS)

The SLS checks if the agreed (with purchaser) level of strain will be exceeded - when a combination loading is applied.

Strain (and strain limits) are critical for plastics materials.

Both BS4994 and EN13121-3 both have strain limits - which vary according to glass type used.

Hopes this helps


Ed Clymer
Resinfab & Associates

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