Tensile / Pull test standards / Specimens - 14 mil vinyl interior products

[tjlee]

Our company produces vinyl interior products (typically 14 mils thick)for the automotive industry. Seams are fused together using a RF weld process. We have internally developed 'pull testers' fitted with load cells to test the weld strength in newtons (50 - 100 N range). While our load cells and data displays are sent out for calibration, we have no way of knowing if the complete pull test 'fixture' is accurate (e.g. undetected loads/tension on the load cell during cycling). These units are in a production environment. I would GREATLY appreciate any leads on where I might obtain standards / specimens of a known tensile failure point/range to use for periodic verification.

[Kevin H]

I suspect that you will have difficulty in finding standards - I ran a mechanical testing lab for stainless sheet steel in a steel mill for 5+ years, and even with the longer history of steel had to develop standards internally.
In the hope that my program might help you devlop a similar one for your product I'm giving a detailed description of it.

We used commercially produced testing equipment to measure tensile properties of the sheet produced by the mill. We were unable to find a traceable standard material to use for verification purposes. What we did was have the testing machines calibrated by an ISO Guide 25 calibration lab on a regular basis (I left just prior to the upgrade to ISO Guide 17025 from ISO Guide 25), internally developed a standard used to verify the machines each turn prior to use - basically took a scrap coil, verified its test results throughout the length of the coil & across the width of the coil, had that coil cut into sheets about 4 ft. long, placed those sheets on a pallet and stored them in the lab so we could prepare standards when needed. We ran SPC charts to verify the results of tensile testing equipment each turn prior to use - took 2 standards, ran them, plotted the results on the chart using averages & range between the 2 individual specimens for 0.2 % offset yield strength, ultimate tensile strength, and % elongation. If results were within the spc limits, the machine was OK to use. If not, we had a work instruction describing what operators could do prior to getting the supervisor involved. The machines had to be verified as OK to use prior to running samples for verifying coil properties. As part of our program to verify the machines & determine capability, we also took part in interlaboratory tests run by an outside concern - received material to prepare and tensile test twice a year and had to report results back to the outside concern which published the overall test results twice a year.

One thought regarding accuracy would be to contact a calibration laboratory capable of verifying the equipment in place. For the tensile testing equipment, I used Tinius Olsen. I believe Instron also offers a similar service, and there are others as well. My thought for setting up your equipment would be to calibrate in place by an external service, secure a proposed standard material prior to their calibration, run the standard to develop base SPC data, and have the equipment recalibrated/checked by the service. Then use your internally developed standard to verify your equipment as required by your production practices.

By the way, do not expect to get the same results from each piece of test equipment - if you have adequate discrimination in your test, you'll end up with similar but different results.

Hope the above helps you towards a solution.

[Hershal]

Tinius-Olsen, Instron, and a few others do in fact build machines that perform tensile pulls.

There is also an ASTM Standard (I forget the number) that deals will various types of tensile pulls including an adhesive one. It deals primarily with fire retardents if I recall correctly, but the concept sounds the same as what you mention you need.

Hope this.

Hershal

[Caster]

Originally Posted by tjlee

Our company produces vinyl interior products (typically 14 mils thick)for the automotive industry. Seams are fused together using a RF weld process. We have internally developed 'pull testers' fitted with load cells to test the weld strength in newtons (50 - 100 N range). While our load cells and data displays are sent out for calibration, we have no way of knowing if the complete pull test 'fixture' is accurate (e.g. undetected loads/tension on the load cell during cycling). These units are in a production environment. I would GREATLY appreciate any leads on where I might obtain standards / specimens of a known tensile failure point/range to use for periodic verification.

This may be a bit late, but check out MSA version 3. There are now some examples and methods to perform gage studies on non replicable (destructive) tests. Known samples are not needed, apparently you can use your production parts.

I can offer little practical help, I have avoided doing GR&R on our tensile tester for years now.

Another source could be your tensile tester calibration teck. They get into many plants including the competion and are usually willing to share what they have seen elsewhere.

[Ken K]

I would GREATLY appreciate any leads on where I might obtain standards / specimens of a known tensile failure point/range to use for periodic verification.

It's highly unlikely you will find tensile standards any more. We used them back in the early '90's but the supply dried up when the samples could not be controlled to give accurate results from batch to batch.

You mentioned a range of 50-100 N for your range. Was that an internal or supplier developed specification? Have you been keeping control charts for these tests? I would think if your numbers have been consistant and within the tolerance your method would be suitable.

[DavidPhillips]

This reply is probably late, but have you considered simply using a calibrated spring? The force, displacement curve is linear and you are able to check the displacement measurement system as well at the same time. As most Standards for the accuracy of "pull" or tensile testing equipment only specifies +-0.5% of reading this is a perfectly adequate and cheap way of achieving what you want. As long as you dont overstretch the spring you can use it for years.

I dont understand the cost effectiveness of producing your own testing equipment when there are so many perfectly good and standardised devices on the market.

[atreyu915]

Originally Posted by Hershal

Tinius-Olsen, Instron, and a few others do in fact build machines that perform tensile pulls.

There is also an ASTM Standard (I forget the number) that deals will various types of tensile pulls including an adhesive one. It deals primarily with fire retardents if I recall correctly, but the concept sounds the same as what you mention you need.

Hope this.

Hershal

ASTM D 882 is the standard test method for tensile properties of thin plastic sheeting. We use a Com-Ten tensile tester. We have never had any trouble with it. An outside house calibrates it once a year. (load cell, Digital monitor controller).