We recently started using a tensile tester to enable us to measure tensile strength, easily and rapidly, in-house. The tensile tester is a part of our strategy of continuous improvement, and our aim for Clarity and Quality. Where applicable, we use this tensile tester to check incoming goods and fasteners that we have modified in our Production Department.
Tensile strength is the maximum mechanical tensile stress that an object can withstand before it starts to distort. Tensile strength is determined by both the material and the thickness of the object. Distortion means that part of the object becomes thinner and can withstand less stress at that vulnerable point. Under continuous stresses, it will become thinner even more quickly at that point and will be less able to withstand stress. Breakage then becomes inevitable.
From the viewpoint of establishing the correct use of fasteners, it is more interesting to determine the proportionality limit, elasticity limit and fluidity limit. These are moments that occur before the tensile strength limit is reached.
When a screw or bolt comes under tensile stress, an elastic distortion will occur first. This happens in proportion to the stress, up to the proportionality limit. Beyond this point, the material will distort proportionally faster than the increase in stress - up to the elasticity limit. Until the elasticity limit is reached, the material will return to its basic shape when the applied stress is removed. Above the elasticity limit, the material will distort ‘plastically’, which means that it will change its shape irreversibly. The fluidity limit is then reached, in which the object becomes increasingly longer and thinner, even under consistent stress. The material at this point is distorted permanently and the risk of breakage is high if the applied stress is not removed.
Extremely hard alloys often have very limited fluidity, making the fluidity limit difficult to determine. The hardness means that such alloys are already close to the tensile strength limit when they pass the elasticity limit. A 0.2% yield stress is often adhered to as a ‘theoretical fluidity limit’. In constructions, you aim to tighten the fasteners as close as possible to the proportionality limit - and in any event below the fluidity limit - to ensure the construction’s maximum strength and durability. Above the fluidity limit, the fastener will be subject to excessive stresses.
A good method to determine all these fastener properties, such as proportionality limit, elasticity limit, fluidity limit and tensile strength, is to conduct a tensile test. This is a destructive test in which the fastener being tested is placed in a tensile tester. The tensile tester then exerts stress on the screw or bolt. Various measurements are taken during the tensile test. The force measurement cell measures the tensile force, while the extensometer measures the extension of the fastener. The result is displayed graphically via the tensile tester’s software. A tension-elongation diagram (also known as the stress-strain curve) is displayed, in which the elongation is presented in percentages on the horizontal axis and the tension in MPa (or N/mm²) on the vertical.
Proportionality limit: up to the proportionality limit the material stretches proportionally with the stresses exerted on it.
Elasticity limit: up to the elasticity limit, the material will return to its original basic shape when the applied stress is removed.
Fluidity limit: at the fluidity limit, the material becomes fluid and the elongation continues to increase during consistent stress. Extremely hard materials and hard alloys often have very limited fluidity, which means that a theoretical fluidity limit is used, often of 0.2% yield stress, and sometimes lower.
Tensile strength: above the tensile strength the object starts to distort. It becomes narrow in places, which means that it can handle less stresses and will start to distort even more quickly. When the tensile strength has been reached, a breakage is almost unavoidable.
0.2% offset yield: the stress at which the test object is elongated by 0.2% of its length after the applied test stress has been removed.
Breakage: the object is no longer in one piece.
Our tensile tester
Our tensile tester is fully-programmable, and provides extensive and clear reports that can be exported to different file formats. Our tensile tester can reach 150 kN and accepts fasteners from M2 to M12 as test objects. In practice, this means that we can test standard steel classes reliably from 4.8, 8.8 via 12.9 to Unbrako 130. We do not only use the tensile tester for our own quality controls, but also for our customers’ projects. In certain cases, students from technical study programmes can even use our tensile tester.
Please contact us for more information.