Tensile tester in our quality room
As technical wholesaler we supply various types of fasteners that must satisfy a wide range of requirements. To guarantee quality, we continuously investigate whether the fasteners satisfy published specifications so that, as customer, you know exactly what you are buying. We have a special quality room for this, in which our measuring equipment is located, and a quality controller who oversees these sample tests and other checks.
We recently started using a tensile tester to enable us to measure tensile strength easily and rapidly in-house. The tensile tester fits in 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 reach before it starts to distort. Tensile strength is determined by the material and the thickness of the object. Distortion means that part of the object becomes thinner and can withstand less stress at that point. Under consistent stresses it will become thinner even quicker at that point and will be less able to tolerate stress. Breakage then becomes unavoidable. From the viewpoint of 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 tensile strength is reached.
When a screw or bolt comes under tensile stress, in the first instance an elastic distortion will occur, which is in proportion to the stress up to the proportionality limit. Above this, the material will distort faster than the increase in stress, up to the elasticity limit. Up to the elasticity limit, 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 is then 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 they are already close to the tensile strength 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 too high 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, the extensometer measures the extension of the fastener. The result is displayed graphically via the tensile tester’s software in a tension-elongation diagram (also known as the stress-strain curve), in which the elongation is presented in percentages on the horizontal axis and the tension in MPa (or N/mm²) on the vertical.
Example of a stress-strain curve.
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, 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 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 object. 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 in our own quality control, 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.