Axial Testing Machine

Project Objective

The Axial Testing Machine is a part of the broader Materials Testing Machines Project. The Axial Testing Machine was developed to help students understand topics such as Torque, Strain, Ultimate Tensile Strength and Failure in a more “hands-on” way.

Force is a linear quantity, so it either pushes or pulls on the object it is acting on. Torque, being the rotational equivalent of Force, rotates an object around a specific axis. Torque is also dependent on the perpendicular distance between the line of action of this force and the axis that the object is rotated around. Strain gives us a ratio of how much the length of an object changes after being loaded, with respect to its original length before loading. Ultimate Tensile Strength is the stress at which an object begins to fail. In other words, this is the point on the stress-strain curve at which an object has been (tensile) loaded so much that it cannot withstand an increased tensile load without deforming plastically and so it ultimately breaks. All three of these quantities have numerous applications, both in everyday life and in industry-specific situations, thus making them important tools for students to understand and control.

In the Axial Testing Machine, the mechanism is such that the Torque applied on a bolt causes a wire of a fixed length to stretch, and this way, the strain of the wire can be measured. The machine therefore gives students a way to relate the two topics in a very visual manner. The Ultimate Tensile Strength of the material of the wire is used as a way to determine the minimum torque that is needed to (theoretically) break the wire. Realistically, this minimum torque value might not always produce the most accurate results, and students can learn a lot by troubleshooting this and other aspects of the machine.

Calculation of Minimum Torque required to break wire

The minimum torque required to break wire needs to be calculated from the Ultimate Tensile Strength of the wire. “Minimum”, because Ultimate Tensile Strength, or UTS, is the stress at which the material begins to fail. At this point, the stress is the maximum amount of tensile force (tension) acting on the cross-sectional area of the wire, that the wire can take before breaking. Hence the minimum torque needed to break the wire can be calculated by finding the maximum tension.

The UTS depends on the kind of material, and is an empirical value that can be found on various materials datasheets. The only thing known about the wire is that it’s Nichrome, so there are a range of UTS values for the entire Nichrome family. The average of this range, 750 MPa [1], was used in the following calculation. The diameter of the wire is 0.2mm, and the cross-sectional area can be found using the formula for the area of a circle.

Ultimate Tensile Strength = Stress = Force / Area

We are solving for the maximum tensile force here.

750MPa = Force / (0.2^2 * pi / 10^6) 

Force = (750 MPa * 0.2^2 * pi) / 10^6 

Force = 94.24 N.  

The length of the wire is approximately 26.57cm. This means that the torque acting on the bolt, which in turn acts on the wire as a tensile force, acts over 26.57cm of the wire.

Min. Torque = Force * Length

Min. Torque = 94.24 N* 0.2657m

Min. Torque = 24.975 Nm. 

The following pages give an insight into the design and manufacture of the machine, as well as how it is assembled and ‘ultimately’, used.