The Hinge Behind the Wallet - Part 2


From the start we knew the hinges were strong.  We would pull on them and hang stuff from them and we couldn’t break them.  But just how strong are they?  Over the last two years we have worked to quantify that question by testing our hinges in tension and in fatigue. 

Tensile Testing Results

Tensile testing was completed per the ASTM D3039 standard.  Essentially, we used a machine to rip a wallet apart in tension.  We also did this will smaller samples so as to not run through so many wallets…  Each sample was 0.50 mm thick (the same thickness as our wallets) by 25 mm wide.  When pulling the full wallet apart, we found that it broke at 1,300 lbs. of load.  That’s equivalent to hanging two Harley Davidson Motorcycles from your wallet!  Compare that to a leather wallet that rips at just 300 lbs. of load.  Take a look here to see the full testing video.  

When testing the smaller samples, we compared them to unhinged samples of carbon fiber to see the reduction in strength due to our hinge process.  The average strength for the unhinged samples was 1066 lbs. and the hinged samples averaged 840 lbs.  This is a reduction in strength of only 22%.  When comparing this to the strength of a bonded or fastened joint, this is extremely impressive.  Needless to say, if you are ever in a pinch and need a really strong hinge, feel free to use your wallet!

 CF-Lex™ Hinge Tensile Testing Instron


 Fatigue Testing Results

Fatigue strength is trickier to test than tensile strength.  In order to get meaningful data you need to at least test each sample through 20,000 cycles.  Any volunteers to open and close their wallet 20,000 times?  Didn’t think so.  Instead we turned to our trusty friends (robots) to help out.  The fatigue testing was performed by Force Technology in Denmark. The samples were tested by bending one set of hinges to 180 degrees and another set to 270 degrees.  These hinges were 0.50 mm thick and were cycled 20,000 and 50,000 times.  Samples were then tensile tested at these markers to determine the life of the hinge and the reduction in strength over cycle times.

The results were shocking! The largest reduction in strength for any one sample batch was 11% after 50,000 cycles.  However, some samples resulted in an increase in strength of up to 8%! Therefore, we concluded that after 50,000 cycles there is no clear reduction in strength.  You can sleep soundly knowing that if you were to open and close your wallet once a day for 135 years it would still be just as strong as it was on the day you bought it, possibly even stronger! 

Open and Close Wallet GIF using CF-Lex™

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