Mainly negative, since residual stress can lead to premature cracks and fracture. Anyway, you must consider that almost all of the composite materials have residual stress induced during manufacture, at least a little, and everybody have to bear with it.

It depends on the application. Your composite windshield is designed to have a significant residual stress, that is what keeps it from shattering like normal glass in an accident. Residual stress can also be used to change the elastic properties of a composite. If strength is the only issue then, as Victor Ponce stated, the effects of residual stress are mainly negative.

During processing of C/C composites, the residual stresses can induce stress graphitization which can be beneficial in some cases.

When the matrix has lower elongation at break than the reinforcement, residual stress might have positive effect - for example in fiberglass - polyester resin composites

As said earlier they have mainly a negative influence and are most often the result of polymerisation shrinkage. Nevertheless, sometimes stresses can be build in on purpose as in the case of pre-tressed fibre-composites. Fibres are pre-tensioned during curing of the composite and this tension is released afterwards. This technique builds in compressive stresses into the composite in order to enhance their strength, toughness and fatigue resistance.

Yes, this is quite correct. The effect is very case specific.

Pretension or prestrees during forming the composite is not desirable. To solve this problem, preforms are always produced with shape which was called net-shape. Therefore, somehow pre-stress was reduced. Also, related with this problem, boundary conditions may be the very important point. So, fiber in the composite must be straight and matrix must be continued medium in the composite. And, any shape must be carried out during preforming step. This is the way to eleminate the resudial stresses in the composite. For ballistic applications, I will tell later on.

Bye Bye

K Bilisik

It depends on the type of use of the composite materials and can we ask you that you make hardening or softening?

The residual stresses in composite materials have influence normally and it depends on the situation.

It has influence

Which, depend on specific problem.

It is impossinbe give an answer in advance

there are several factors that contributes for the formation of residual stress in the material; Among them, thermal expansion coefficient difference between the matrix and second phase in the composite material plays a mojor role; based on this & including other factors, the stress formation would be either +/-; for eg, consider a case , a crack is propagating in a compressive stress region, in that case, the stress intensity factor at the crack tip keeps on decreases , which is considered to be beneficial since it decreases the driving force for its further propagation; this may vary between applications to applications........

As I understand influence of residual stresses in composite materials is similar to ones in the metals and alloys. Compressive residual stresses is a positive factor, tensile stresses has negative influence.

We have used hole drilling method to get qualitive information about residual stresses in carbon-fiber-reinforced polymers. We measured displacements Ux after drilling of a hole at a defined distance from the hole's center using electron speckle interferometry (usually r=2.5r0, where r0 is a radius of holes, it came from standard dimensions of strain-gage rosettes) .

When we have deal with metals we know that bigger residual stresses cause bigger displacements.

I made measurements after drilling of a hole in two similar CFRP samples and received that displacements at the defined distance in sample A is higher then in sample B . And we decided that residual stresses in the sample A are bigger then in sample B.

We got the curves for samples A (red) and sample B (blue): dependance of the displacements Ux on the distance x from the hole r (see pic attached). They are show that stresses in the sample B is bigger then in the sample A.

Both curves fade more quickly compared with such curves obtained for isotropic metal samples.

What do you think about my conclusions?

Sorry for my english. If something is not clear, please, ask for clarification.

The main idea makes sense to me. Now, i must point out that this kind of measurement would be influenced by the material anisotropy, which is very important when talking about polymers. Heating-cooling-heating at a MDSC could also help you regardless of such property, just follow the general rules. Result can be improved by taking sample at different spots/places.

Residual stress have effects on composite materials, that can be positive or negative depending on the applications. For example, they can affect the final shape of a complex shape part, as stated in the following papers.

Article Compaction influence on spring-in of thin composite parts: E...

Article A New Class of Thin Composite Parts for Small Batch Productions

It depend on you.

Prestresed concrete is a technology.

"Residual stresses can have a significant effect on the performance of composite structures by causing fiber buckling or void formation during solidification, reducing strength and initiating transverse cracks or delaminations "

The above statement is taken from the following link :Article Optimal post-manufacturing cooling paths for thermoplastic composites

Its meaning is that the substantial residual stresses arising from manufacturing (after cooling from above to below the glass transition temperature of the resin) do influence the polymer matrix behavior (the weakest link of the composites). Given the fiber network the residual stress in the polymer matrix are positive (tensile stresses).

For instance, during fatigue tests the residual stresses make the matrix to rapidly reaching the Critical Damage State (CDS) where cracks and crazes saturates. Subsequently the other damaging processes can develop. Thus the higher the residual stresses withinn the matrix the earlier is the development of damage mechanisms including the fiber-matrix detachment, fiber rupture, ply rupture, delamination that drive to the catastrofic failure.