Hi

Can yiu share your full specimen dimension.

More over. Yiu follow ASTM D. 3039 for testing tensile properties.... Use teflon cloth while gripping the composite strip....

Best

Arunprakash Vincent

Dear Mr. Hossain,

This means that at high clamp pressure, the transverse stress from out of plane compression in the grips exceeds the transverse strength of the material and adhesive shear strength, leading to tab failures, while at low grip pressure, the tabs to grip interface is not strong enough.

In mechanical testing, you will likely need to compromise. Some of your options are as follow:

The first and most effective would be to manufacture thinner specimens. UD composites are very strong in tension and reducing the sample thickness will lower the failure load. If you keep the same width, the shear strength of the tab will be kept, but the sample failure load reduced. Also, as you have a relatively thick laminate, it might be sensitive to edge effects unless the sample is very wide, further increasing the failure load. As Mr. Prakash suggested, follow ASTM D3039 guidelines for specimen geometry.

It is unclear if what you call slippage corresponds to the tabs slipping in the grips or the specimen de-bonding and slipping between the tabs. In the first case, using more aggressively serrated grips may work. Often, for composites abrasive coated grips are recommended. However, if you use tabs, serrated grips may work better by having the grips penetrate the tabs. Some have successfully used metal screens between the gripo and tabs to increase the mechanical indentation in the tabs and prevent slippage. In the second case, better bonding surface preparation, a thinner layer of a stronger adhesive, tab bevelling (very shallow, 7-10 degrees) and softer tab materials (glass mat or +/-45 glass fibre composite) may help.

Search for FAA tabbing guide for more details.

Regards,

Dear Cormier,

Thank you very much for your answer. The slippage of tab is the second case you assumed. So, I think I sould try some stronger adhessive first to see how it behave. If that fails then will try other options you suggested. The tabbing material I am using is +/- 45 glass fiber composites and adhessive is DP 490 from Scotch-Weld.

Regards,

Dear Arunprakash,

The specimen dimension is 250mm length and 15mm width according to ISO 527-5. However, I will look into ASTM D 3039. \

Regards,

Milon

Dear Sir,

Some observations to be made:

1.Sometimes it also depends upon type of specimen material used.

2.Rough holding jaws/gripper to be preferred.

3.It will be better to consider a new design and fabricate a holder or gripper of your choice as per the extreme ends of the specimen before testing.

4.Impose a striations using hand files on the extreme ends of the specimen for gripping purpose.

Hope this information is helpful.

Dear Sir,

Some observations to be made:

1.Sometimes it also depends upon type of specimen material used.

2.Rough holding jaws/gripper to be preferred.

3.It will be better to consider a new design and fabricate a holder or gripper of your choice as per the extreme ends of the specimen before testing.

4.Impose a striations using hand files on the extreme ends of the specimen for gripping purpose.

Hope this information is helpful.

Dear Manjunatha,

Thanks for your valuable information.

regards,

Milon

Dear Mr. Hossain,

I hope you are successful with your modified specimens. Be aware that surface preparation before bonding is very important. Make sure you thoroughly clean the mating surfaces as they may have release agent left from the moulding operations. You should also lightly abrade both surface with an emery cloth before bonding. Try not to sand down to the fibres and make sure to remove any sanding dust before bonding (I use paint brushes with stiff bristles). Some advise not using solvent to clean after sanding as they may leave residues.

Also, tapering the tab towards the gauge section at a very shallow angle helps relieve the stress concentration at the tab tip and can help in preventing tab debond. ASTM D3039 provides recommendation about tab bevelling.

Regards,

Dear Laurent Cormier,

Thank you very much for your valuable tips. I will definitely follow that while prepare the samples.

Best regards,

Milon

Dear Md. Milon

I think carfule specimen preparation before testing is vital. Mr. Laurent has summarized it nicely.

Wish you good success.

Mohammed..

reinforce with glass fibre