There is no doubt that Tensile Strength Testing can be used in multiple applications. Its core concept, that of predicting how materials can behave if subjected to a specific load of force or tension, has allowed many manufacturers to determine which materials are composed to use for specific production cycles.
Tensile testing has also seen considerable use in processes like extrusion, where raw plastic is being melted to be molded into various shapes and profiles.
However, it does give rise to the question: why is tensile strength testing needed for extrusion companies?
The Nature of Plastic
To answer the need for tensile strength testing in extrusion companies, one only has to look at the very material that extrusion facilities process: plastic.
The word “plastic” has a rather broad definition as the material is essentially made up of various combinations of components. Essentially, any material made from organic polymers can melt or degrade under intense hit, can be molded into various figures and shapes, and maintain a rigid to elastic characteristic upon hardening, is essentially plastic.
Due to its unique profile, plastic can be formed into various shapes and be applied for several specific purposes. An extrusion company, for instance, might be tasked to produce several plastic objects that include bottles, boxes, tubes, cups, sheets, plates, and films. With further processes like thermo-hardening, plastic can even take on heat-resistant and impact-resistant characteristics.
A major portion of the applicable uses for plastic will include some form of tension bearing. Rope, for example, is made of several hundreds of polypropylene fibers. Thus, knowing the breaking point of the material under intense tensile stress is crucial.
The same goes for materials like plastic sheets, tubes, and boxes, which can be subjected to stress and other external hazards like heat and compression over a prolonged period. Thus, the ability to maintain form and integrity will become crucial when extruding plastic for these products.
On the other hand, not all types of plastic are designed to be subjected to tension. However, those applications that do feature some form of force that will bend, break, or deform the material will most definitely benefit from tensile strength testing.
Tensile Tests in Plastic
By design, the tensile test can be applied universally. What an operator needs to do is merely to place the sample material on the machine, set the proper standards, depending on the material, and take note of both observable and computer-drawn results.
This makes tensile tests relatively straightforward in execution with a high rate of reproducibility. Also, a tensile test will not only determine a material’s tensile strength but also its rate of elongation and modulus.
Lastly, the tensile testing machine can be tasked with performing other tests to determine other factors like compressive and flexural strength without major alterations. All that is needed is to input the proper settings to conduct these tests.
To ensure accuracy in each testing cycle, the samples will have to be prepared to meet specific sizes and dimensions. The tensile strength test will require samples to be shaped like a dumbbell or a bone. This simply means two block-like shapes at opposite ends with a narrow shaft at the middle.
This shape is ideal as it allows for the machine’s jaws to properly grip the sample. At the same time, the narrow shaft in the middle can make for a predictable baseline for failure. In essence, this is where the sample will most likely break under intense tensile stress. With a predictable point of failure, the potential to reproduce the test several hundreds of times within a day is high.
Once the machine has properly gripped the sample at both ends, the loading cells at each point will then apply the corresponding tensile force. When tensile stress is applied at both ends, the sample material will start to deform or elongate.
What the tester would be looking for at this phase is the material’s Ultimate Tensile Stress. This is the maximum level of tensile stress it can be subjected to before breaking.
The very nature of plastic also allows the tester to determine the level of Modulus for the material. This is simply the ratio between stress and strain being inflicted on the material.
Polymers tend to behave differently under certain conditions even if subjected to the same strain/stress ratio. For instance, suddenly introducing tensile stress can accelerate material degradation while a gradual increase in stress can result in gradual yet more manageable deformations.
Although the standard tensile test is effective for a lot of materials, it is essentially a quality control procedure that is done on a short-term cycle. It does not give a proper insight as to how materials will behave under tensile stress over prolonged periods.
For this reason, a variant of the test was introduced and named the Creep Test.
The premise here is that plastic can be greatly affected by other external factors. For instance, plastic tubes have to deal with sudden changes in temperature and moisture levels in a single day, which can weaken the material.
The creep test will perform the same processes as the standard tensile strength test. This time, however, the goal is to subject the material to not only tensile stress, but also intense environmental factors like heat, water, and even sonic vibrations over a longer period.
The tester’s goal here is to observe at what specific combination of conditions will the material start to manifest observable defects like fractures and stress marks over its body. From this test, one can determine the plastic sample’s rate of stiffness, i.e. its ability to maintain form and structural integrity over prolonged exposure to stress.
Even if not applicable in all projects, tensile stress will help an extrusion company understand how their products can behave if subjected to stress. This is even more important if they designed or intended for their material to maintain its stiffness over a long period.
With a tensile test, the extrusion company can predict at what point their products might break and for what specific reason. This will then help in coming up with ways to improve on their output. Alternatively, this could help them take advantage of the inherent strengths of the set of polymers they have chosen for a particular extrusion job.