GRP Specimen size when conducting 3 point flexural tests

[Rastanking]

Hi everyone,

I am an Undergraduate Mechanical Engineer conducting my final project which is to investigate the repaired strengths of GRP panels. I intend to create a number of panels, subject them to a impact test, repair the damage and then finally subject them to a 3 point flexural bend test.

However I am having problems in finding a specimen size, using British Standard 2746 this gives the size as a thickness of 3mm, a width of 15mm and a length of 20 x thickness. This does not allow me to perform the tests I need nor does it give me the necessary area to repair.

So my question is this, is there another BS or ASTM standard that will allow me to use non-standard size specimens and, if so, do the formulas for flexural stress, modulus etc hold true for the non-standard size plates?

I know that this is a lot of information but any help would be greatly appreciated.

Regards

Andy

 

[mmhaynes]

It depends greatly on just what properties you are trying to evaluate. The BS2746 is similar to the ASTM D2344 and it is used primarily as a screening test to downselect resin/matrix systems. It will give you Apparent Shear strength, and an idea (when using the proper formula) of the elastic modulus, but for evaluation of the repaired strength I think you want to use a different method to evaluate strength.
Yes,to answer your question, the formulae are the same for larger specimens, but the span to thickness ratio must be held (5:1 for glass reinforcing, 4:1 for Carbon reinforcing), the problems start when trying to define the width of the specimen relative to the thickness.
If you must use an accepted standard, I might suggest using the ASTM 7137 Compression after Impact test (6" X 4" X 0.15" specimen), and set up the specimens, impact 2/3 of them with varying impact energies, and repair a select set. Then test all including your controls (no repair/no impact) using the 7137 fixture. This will give you compressive strength of the laminate, compressive strength after impact and compressive strength after impact and repair.
The big question to answer before you start into the specimen manufacture and subsequent process, is what the actual loading direction of the final part is going to be- So is it a vertical panel that receives loading in the Y-axis only, or a horizontal panel that receives compressive loading as well as x-axis loading (such as an upper wing panel). Answering this might bring you to a standard that is applicable, however is sounds more likely that you will need to define a unique test for the application. I don't feel that a 3 point bend will work for your needs.

Mark Haynes
Senior Test Engineer
CTD