When do these macro cracks appear? After matrix forming process?

After the process and during the operation 

If you could fabricate the composite crack-free, you should ask yourself: what about the fiber-matrix interface? If you see these cracks only in matrix but not any damage in fibers, then you should check the interface bonding. Normally fibers should take over the load from matrix, as you know.

Please see this crack. 

I do not understand what do you want to refer by saying "what about the fiber-matrix interface?" !!!

Do you want to say the shear strength of the bonding is weak?

or do you want to say the curing time may be not suitable?

 It looks like a quite brittle matrix behavior, you should consider to optimize fiber design...

Is this "matrix cracking'?

Or it can be crack in painting? 

Another point, what do you mean by "optimizing the fiber design"? is it fiber volume ratio?

Jazak ALLAH khair :) 

It looks like a crack in matrix. "Optimizing fiber design" means distributing fibers much more effectively in matrix, using differently oriented fiber laminate layers, adding more fiber amounts in fabrication process etc.

Composites performance depends on raw materials reinforcements which are usually fibers, matrix properties, and interfaces, as well as manufacturing process.  If you are using unidirectional glass fibers as the reinforcements, and epoxy resin for this GFRP. This epoxy is probably very brittle.  If you can change, maybe use toughened epoxy.  On another hand, you may check the processing cycle for curing. If heating involved, maybe reduce the heating and cooling rate. If the rate it too high, resin shrinked fast cozing cracks.   Also heck whether it is over heat coz curing is a extho-thermic process. Over heat may coz swell of the structure at high temperature and shrinkage during  cooling.  If you are using glass fabrics ( such as plain weave, satin weave, 3D fabrics or braided fabrics or non-crimp fabrics), the generation of the cracks may also due to the fabrics micro structure.  But, I highly recommend checking the thermal history/ curing process. 

Thanks Danning, I use Polyester resin, and the fiber fabric mate is randomly distributed not unidirectional !

Hi,

I have a few comments and suggestion for you.

First, chop strand mats are not optimized fabrics. They provide reinforcements in all directions when you want to deal with loosely defined loads and result in an approximately isotropic material. Also, mats are often held together with a bonding agent that should be compatible with the resin type you use. Using incompatible bonding agent and resin may result in poor wetting of the laminate and weak parts.

Second, there is no scale on the image you provide, but it looks like the crack is fairly large and oriented. Also it looks like there is a gelcoat on the part so it is hard to see if there are other damages around the main crack. If the gelcoat has a much lower or much higher shrinkage than the resin, it may crack.

Third, to me this looks like a crack that results from mishandling of the part during unmolding or while moving it around. This is the kind of damage that result from applying to much force when trying to take the part out of the tool. One should always work the part out slowly and gradually while ensuring you don't use too much leverage. If the tool was prepared adequately, this should be easier. If mold preparation is poor. it may be almost impossible to take the part out without damage.

Fourth, polyester resin shrinks a lot during cure and excessive shrinking of the resin can cause cracking. However, these crack should be dispersed over the part with higher concentrations where the part is thicker and around features such as bends or holes. Finally, as stated before, resin curing is exothermic and if heating/cooling rates are too high, cracks may result from internal temperature gradients causing residual stresses. If I recall correctly, the faster the cure, the higher the shrinkage and therefore, the higher the residual stresses.