Material quality control for composites
The highest performance material in the composite world should be CFRPs (carbon fiber reinforced plastics).
The prepreg, which is the fibers with impregnated resins, shall be the main composite materials as the high performance FRPs.
But the people in the initial stage to start the business in FRP industries may mainly think about;
"How to make the products as cheap and efficient as possible"
It should not be wrong for sure but we need to understand the other aspects for the business success.
One of them shall be;
" Material quality control regardless of thermoplastic or thermosetting resin system"
Most of you would agree about this point.
The basic process to make using the CFRPs is consist of the heating (for cross linking reaction in thermosetting resins ) or phase transition ( like melting for thermoplastic resins ) in order to achieve the product shape.
But please just suppose the condition that the material quality is NOT stable.
You should not achieve the stable product quality with unstable material condition.
Today I would like to discuss 4 important points for CFRP material quality control.
1. Weight
The areal weight is one of the popular and important items which we need to consider from the material quality control view point.
What is the areal weight per square meter?
This question can let us know the condition of the uniformity of the fibers and resin impregnations.
The measurement of the weight is relatively easy and chosen as the quality control item.
2. Mechanical properties
Many of you may think about it as the first thing.
The strength and young's modulus like tensile, compression and shear would be the popular evaluated items for material quality control.
Someone think about the bending test but it is not normal as the basic design and quality control because the bending mode is the combination of the tensile and compression. Of course, the load mode of the material's application is mainly bending, you could require the bending test as the quality control item.
Additionally, you may not know how to decide the threshold value for the quality control.
If you decide the threshold value, based on the minimum value by the small number of test specimens, the quality problem would be happened in the future.
The reason of the above is;
" The minimum value by the small quantity of the specimens " is NOT proper as the minimum one for quality control.
Because you may not answer the questions below;
– What is the variation of tests?
– How many batches have you evaluated? Is it enough batch number?
The purpose of the threshold value for the quality control is to define the threshold value to detect the abnormal quality condition. It is not equal to the design value.
The point above shall be the most important to talk about the threshold value for the material quality control.
3. Chemical properties
Actually it is very important for the quality control.
Many of the people, who are in charge of making the material spec, don't have enough knowledge regarding chemical analysis.
I can say the examples as the chemical analysis for the material quality control are DSC（ Diferential Scanning Chalorimetry ) or DMA ( Dynamic Mechanical Analysis )
The summary of them is below.
a. DSC
DSA can be used to understand the matrix resin condition below.
– The storage condition by checking the curing status by the exotherm peak area for thermosetting resins
– The heat history by checking the crystallinity for the crystalline thermoplastic resins
b. DMA
DMA is also useful for checking the complex viscosity by the heating profile to check the pressing program adequacy, especially thermoplastic resins (because the viscosity would be drastically changed if the molecular weight of the polymer is changed).
Such kind of the basic skills to use the chemical analysis is necessary for the composite designers.
4. Visual inspection
Surely the visual inspection requirement shall be clarified to avoid the foreign material inside of the laminates.
But someone requested the visual inspection like;
"The gap between the fiber strands"
"Any visual problem are now allowed"
They are very doubtful to control the material quality "in production" (not in trial).
Too severe quality requirement should lead to the quality issues for sure.
The threshold for the visual inspection should be determined by the background of the risk assessment regarding the molded parts damage.
I would like to mention the last point.
The point is;
"The designers should consider and decide the material quality requirement, and then talk with the material suppliers."
This concept is MUST in the composite industries because there are few global material specs for the composite materials. The designer should DESIGN the composite materials too.
Many of the material user seem to think that the material quality and requirement shall be decided by the material suppliers. But it must not work for the composites.
Additionally the material suppliers should propose their materials including the application or design concept; The point to achieve the unique performance by applying their materials.
If the distance between the designers and material suppliers, the composite world should be grown drastically in the near future.
