Split Hopkinson Pressure Bar: M-Bond AE-10 Adhesive Won’t Stick!

I recently visited a lab to investigate customer complaints regarding the performance of M-Bond AE-10 strain gage bonding adhesive. The customer complained that when attempting to bond strain gages to a split Hopkinson Bar using M-Bond AE-10 that not only was the gage output unstable but the gages were “popping off”. The user had come to the conclusion that the adhesive was at fault. During my investigation I discovered a litany of problems created by user inexperience and misunderstanding of the steps / details required to achieve the high performance bond necessary to make an accurate strain measurement.

Test specimen surface roughness.The surface was too smooth. The customer had polished the surface to a mirror finish leaving no mechanical “tooth” for the adhesive system to grip.See our Instruction Bulletin 129-8 on Surface Preparation for Gage Bonding:http://www.vishaypg.com/docs/11129/11129B129.pdf
Proper adhesive mixing procedure.M-Bond AE-10 is a 100% solids adhesive system and must be thoroughly mixed to ensure proper cross linking.This must be followed by timely and carefully dispensing and application to the test article / strain gage.Failure to follow each of these steps will result in an inadequate bond.In this case, the adhesive was correctly mixed but improperly dispensed.The mixing rod is also the dispensing / application tool.After mixing, the rod must be cleaned and the adhesive drawn from the center of the mixing jar to avoid any possibility of dispensing partially mixed adhesive from the rod or inside wall of the jar.In this case, the user failed to clean the mixing rod and did not draw the adhesive from the center of the jar.The procedure can be found at:http://www.vishaypg.com/docs/11137/11137B137.pdf. The unused adhesive remaining in the jar will cure rock hard and should be retained as a verification sample.
M-Bond AE-10 requires a clamped cure.In other words, the strain gages must be clamped in place during the cure cycle to ensure a thin, void free glueline.This is accomplished by the application of uniform pressure, typically using a silicone rubber pad of the correct durometer and spring clamp.Other clamping methods can be employed but care must be taken to ensure correct, uniform pressure.In this case the customer used a patch of M-Coat F FN-2 Neoprene Rubber Sheet instead of the prescribed silicone rubber pad and wrapped the installation with a rubber strap tied around the bar to apply clamping force which resulted in a very low, non-uniform clamping pressure.See TechTip 610 Strain Gage Clamping Techniques http://www.vishaypg.com/docs/11090/tt610.pdf
The prescribed time / temperature cure schedule must be correctly followed.M-Bond AE-10 requires a minimum time / cure temperature of six hours at 75 degrees Fahrenheit.The cure time may be reduced at elevated temperatures but the adhesive must see a minimum of 75F for six hours to fully crosslink.Below 75F the adhesive will not fully cure.In this case, the temperature of the lab was approximately 70 degrees Fahrenheit.In this case, application of head from a heat lamp, electric blanket or some other heat source to ensure the specimen / adhesive was at or above 75F would have ensured the adhesive crosslinked properly.
Once the installation is completed, it is recommended the gages be checked electrically using our Model 1300 Gage Installation Tester a Model P3 Strain Indicator / Recorder, one of our Signal Conditioning Amplifiers or StrainSmart data acquisition systems to ensure the installed gages meet the required insulation resistance to ground, zero stability and zero return characteristics prior to taking any test data.For information on these instruments visit:https://www.micro-measurements.com/instruments