The Dupont Impact Tester (Dupont IT) is an essential piece of kit for any company that handles paint or coatings. It allows you to perform tensile tests on the materials you work with, ensuring they are strong enough for the applications you have in mind. What is a tensile test? A tensile test measures the mechanical properties of a material or substance. It is essentially pulling apart at constant speed until something breaks. A digital camera is often used to monitor the test and make sure the speed is constant and that no parts of the specimen become detached during the test. For example, if you are testing the pull strength of a steel cable, you would need a steel cable with a diameter of not less than 4 inches to use as a control. The sample needs to be at least 4 inches in diameter in order to reduce the effect of the testing machine, or the ‘jig', on the result. If the cable is too small, the measured result will be lower than the actual value due to deflection and friction within the machine. In this case, a bigger cable is required. The same goes for anything that has a similar effect – thread pitch, wall thickness, etc. A tensile test is a great way of ensuring that your work is as strong as it can be without risking failure during use. In the field of paint and coating, a tensile test is very often used to determine the adhesion, or bond, between the substrate and the topcoat. This is most commonly assessed by peeling the topcoat away, in stages, to reveal the bonds between the two layers. If the bond is insufficient, gradual, incremental removal of the topcoat will eventually result in a catastrophic failure of the sample. This is highly undesirable, both from a safety and an economic perspective. Why should you care about the pull strength of your materials? The materials you work with have a finite lifespan and it is important that you know how long they will survive both before and after you use them. The first part of the lifespan is when you make the material – the process of extracting raw materials from the earth and refining them into a useable product. After you have used the material and it has become degraded through wear or exposure to extreme conditions, its lifespan ends and you will need to replace it. This is where the importance of knowing its pull strength comes into play. Knowing the strength of your materials will help you predict how long they will last. If you are unable to peel away the topcoat of an adhesive in stages during a tensile test, the bond could fail at a single pull and you will need to reapply it or remove and dispose of it altogether. If this is the case, you may as well have not bothered with the test in the first place – your material will not have the desired lifespan and you will need to replace it sooner than expected. The other option is a complete failure of the adhesive, leaving you with a sticky mess on your hands and an unexpected replacement part cost. What is a jig? A jig is an accessory tool that is used to hold the sample in the right position for testing. It can either be fixed in place or have a clamping mechanism to hold the specimen in the desired position during the test. A jig can be constructed of metal, plastic or wood and is designed to fit on top of a test stand. The jig is usually about the same height as the specimen being tested so the test can be performed in a standardized fashion. A jig is used to position the sample so you can pull it apart more easily during the test. When using a jig, the height of the pull will be the same for all specimens so the results can be compared easily. If you are testing wires, rods, or any kind of small material, a jig is a great way of ensuring repeatable, precise results and reduced error. If you are testing large objects or blocks of material, it is often better to use a mechanical strength tester instead. Jigs are also very useful for positioning samples exactly where you need them so you can get the most accurate results from your testing. They can often be used as a temporary bonding agent to hold the sample in place during the test while the adhesive sets in. Measuring the pull strength of your materials There are a variety of methods you can use to measure the pull strength of your materials. One method is to use the center of mass of the cross-section as a fulcrum to apply force along the longest axis of the specimen. Once the force is applied, you should be able to see the specimen begin to elongate. The amount of force you need to apply will depend on the size and type of material you are testing but a common range is from 10 to 25 kN (2.2 to 5.5 tons). Once the sample begins to elongate, you need to measure the load you are applying using an external load cell. Once you have recorded the value, you can move on to the next step. Peeling away the topcoat in stages One of the most common and effective ways of measuring the peel strength of an adhesive is to use a razor blade or a scalpel to sequentially slice away a small amount of material at a time. When applied to a large area, this allows you to make incremental assessments of the bond strength between the layers. It is important to note that the sample needs to be completely flat before you begin, with the topcoat facing upwards. Even small imperfections in the surface will result in inaccurate readings. After you have completed the first pass with the blade, you can feel the sample with your hands and ensure the bond has been reduced to the desired level. If you are happy with the results of the test, you can apply a protective coating to further preserve the sample. What is a tensile test stand? The tensile test stand is the device you will need to perform your test. It consists of a sturdy table or surface, on which you will place your specimen and clamping mechanism. The top surface should be smooth and flat to ensure you get accurate results from your test. The tensile test stand should meet the following requirements:
Dupont Impact Tester
