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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.

Design of Axial Testing Machine:

This section explains the design of various components and their functions, and where they are generally sourced from. Some components can be used straight after they are bought, so there is no “design” process to explain for those.

Frame and fasteners

T-Slot extrusion that acts as the chassis/frame. The extrusion used to build this machine is made of 6560-T6 Aluminum, and is 40mm high (this dimension is also known as rail height), 20mm wide, and about 6” long.

• Since the steel plates are to be clamped upright on the frame, T-Slotted framing brackets and fasteners that are appropriately sized for the rail height are needed. Each plate is inserted between two brackets and fastened to the frame. The fasteners are M5 nuts. The brackets are 3/4” long, have an M5 mounting fastener sizeand can fit the rail height of this extrusion.

All these components can be purchased on McMaster-Carr. On the website, it is easy to find the right brackets and fasteners once the T-Slot of the desired dimensions is chosen.

Rectangular Plates

3 rectangular metal plates, hereby called the First, Middle and End plates, are fixed onto the frame using brackets and fasteners. Each plate has dimensions of 72mm x 38mm x 6.21mm. The holes or slots designed on each plate are all at the exact centre of the plate. At the bottom of each plate, there are two through-holes (5mm diameter that allow the plates to be fastened to the brackets along the thickness of the plate.

• The First plate has a through-hole in the middle to accommodate the M10 hex bolt.

• The Middle plate has a hex slot in the middle to hold the coupling nut.

• The End plate has a thin angled slot the runs vertically down from the top face to the middle. The slot allows space for the wire to be inserted, and is at about a 1deg angle from the vertical to prevent the wire from jumping straight out of the slot.

U-Plates and Pins

There are two U-Plates in the machine. Both U-Plates are made of cold rolled steel, and both have circular slots along their sides to hold a steel pin each. On one side of each U-Plate is a small hole of diameter 2mm, through which one end of the wire is threaded through and then tied.

• One U-Plate is welded to the Coupling nut and the other is welded to the End Plate. Both U-Plates therefore have different widths.

• The U-Plate welded to the Coupling nut has a few differences in its design. It has an angled slot on its front face to allow the wire to pass through freely. The front face of this U-Plate is extruded about 1.5mm higher than the height of the two side faces. This is due to the placement of the circular slots, as well as the pin that is to be inserted in them.

The circular slots are 6.35mm in diameter, but the circles are not placed entirely within the faces of the plate. Only about 3/4th of the circle lies below the top edge. The Pin used here therefore will have some material jutting out of the top. If the wire is to be wrapped around in a way that the free end passes over the top of the pin, then it needs a slot there to make sure the wire remains straight. This can only be done by extruding the front face to make it a bit taller.

It is highly important that the pins do not rotate, as this causes unnecessary friction to the wire that will be wrapped around them. In early prototypes of the machine, (a version of) these pins rotated about their own axes, which meant that the wire wrapped around them took longer to go into tension. This meant that once the wire did go into tension, there weren’t enough turns left in the coupling nut to pull the wire till it snapped. Another solution to this could have been lengthening either the Hex Bolt or the Coupling nut by about an inch, but this would also mean increasing the length of the frame to accomodate a wire that was long enough to show the effects of stretching clearly. To avoid investing in a completely new structure and have the coupling nuts made again, it was decided that the pin would have to be fixed.

Hex Bolt

A fully threaded, medium-strength Class 8.8 Steel, Hex Head Screw that is M10 x 1mm, 60mm long, is needed. This is perhaps the most important driving part of the machine, since this is part that is tightened (using a torque wrench) and ultimately stretches the wire. The bolt can be purchased on McMaster-Carr.

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Hex Coupling Nut

A 5/8” Hex coupling nut (Threaded M10 x 1.5mm), one end of which is welded onto a U-Plate. The other end of the coupling nut is inserted into a hex slot on the Middle plate, and it is at this end that the hex bolt is driven in.

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Metal Wire

A metal wire of a fixed length that is constrained between the two U-Plates. The wire is stretched by the mechanism and ultimately breaks. The wire can be made of any material, with the obvious fact being that different materials will break at different strain rates and show slightly different types of failure.

Throughout the whole process of developing a working prototype, a 0.2mm diameter Nichrome wire from the E3 Machine Shop was used for testing. This wire was taken from a spool of wire lying around in the machine shop for about three decades, so there wasn’t a lot of documentation to provide the exact composition of the wire. This is not a problem, because the general properties of Nichrome can be used for any calculations.

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Manufacturing Process:

This section shall discuss in brief the manufacturing processes for some of the above components. All of these parts were made in the E3 Machine Shop, or altered at the E5 WEEF Machine Shop at University of Waterloo. Since this wiki page was written during the prototyping stages of the machine, it was necessary to go back to the design stages and fix some of the problems that arose after manufacturing. This really made me appreciate the beauty of the design process all over again.

Rectangular Plates

Originally, the plates were all made of Aluminum 6061-T6. However, when the decision to weld a cold rolled steel U-Plate to the End Plate was made, the End Plate was made of Stainless Steel. This is because only similar enough materials can be welded together.

When the next set of prototypes are made, all the rectangular plates need to be made of Stainless Steel to aid uniformity. Regardless of the material, the plates and their features (holes and slots) are waterjet cut.

U-Plates

Originally, the U-Plates were supposed to be made of 11 GA (so the thickness was 1/8”) 1010 Cold Rolled steel. It was thought that a strip of the metal could be bent into the shape of a U and then machine the features. However, it was learnt from the Machine Shop that cold rolled steel cannot be bent. So an alternative was to waterjet cut the Us from a sheet of appropriate thickenss. The Machine shop did not stock 1/8” thick cold rolled steel at the time of prototyping, so the U-Plates (all there features included) were waterjet cut from 1/2” thick Cold Rolled 1010 Steel.

After testing the prototype, it was discovered that the top edges of the slot on the U-Plate welded to the coupling nut were too sharp, and caused unwanted stress buildup on the wire where it touched the slot, which skewed the real stretching. To fix this, the top edges of the slot were chamfered slightly using a drill press. This seems to satisfactorily fix the issue, but to make the design perfect, the front face will have to be extruded a bit higher than it already was. This will ensure that the only points that the wire coud accidentally touch are the two inner faces of the slot, which aren’t sharp like the top edges.

The U-Plate can be made of Aluminum too, as long as the rectangular plates are made of Aluminum too. However, the fact that Aluminum is a lot lighter than steel should also be considered.

Hex Nut

The Hex Coupling nut was made from Cold Rolled Steel Hex stock. The stock was cut into the desired length, i.e., 2”, and a hole was then drilled through the middle and threaded. In this case, the hole and thread were M10 x 1.5mm. The coupling nut had to be custom-made in the machine shop because a 2” long coupling nut was not available online or in stores.

Welding

Welding is a type of fabrication process where two or more parts are joined together by heating the materials (usually metals or thermoplastics) until they melt, and then allowing them to cool so that they fuse together. A weld “filler” material is sometimes added to the molten mixture of the parts in order to acheive a better welded joint. [1] To create a good weld, the metals need to have similar enough properties, such as:[2]

• Melting Point: The parts that are to be welded need to have the same (or similar enough) melting point so that the welder can use the same temperature to melt both metals.

• Coefficient of Thermal Expansion (CTE): The CTE of a metal specifies how the object’s size changes as the temperature changes. While welding two vastly dissimilar metals, this means that each metal would expand and contract at different rates as they are exposed to high heat and then allowed to cool. As the temperature changes around the welded joint, there are a lot of stresses acting on the weld, especially around the intermetallic zone (the space where the two metals fuse together with the weld filler material).

• Electrochemical Difference: The two metals have to be as close to each other as possible on the electrochemical scale. This ensures that the intermetallic zone does not corrode.

• Solubility: The solubility of both metals have to be similar too, because this ensures that they are compatible with each other in the molten state.

For this machine, there are four welds to be done.

• The Coupling Nut and its corresponding U-Plate are welded together.

  • The steel pin is first pressed into the circular slots in the U-Plate, and then welded to it.

• The End Plate is welded to its U-Plate.

  • The other steel pin is pressed into the slots of this U-Plate and then welded.

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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. HereAt this point, that 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 [31], 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. 

Assembly

Step 1: Assemble the rectangular metal plates on the frame

• The frame needs to be stable, so it can be fixed on a table using a c-clamp or held within the jaws of a vice. The vice is a better option because the c-clamp only fixes the bottom of the frame to the table and allows it to rotate.

• Place the First plate between two brackets on the edge of the frame.

• Use the T-Slot fasteners to secure the brackets to the frame.

• Use M5 bolts and nuts to fix the Plate to the bracket.

• Place the Middle plate between two brackets placed right after the end of the First plate’s brackets, and repeat the same procedure.

• Place the End plate on the other end of the frame, between two brackets, and repeat the procedure.

Step 2: Insert the Hex bolt and coupling nut

• Insert the Hex Bolt into the First plate such that its inner face coincides with the outer face of the plate. The other end of the bolt should pass through the slot on the Middle plate.

• Now insert the coupling nut from the opposite side of the Middle plate, until it meets the Hex bolt.

• Tighten the bolt until the face of the coupling nut lines up with the inner face of the Middle plate. Make sure that the head of the bolt still coincides with the outer face of the First Plate.

Step 3: Add the Wire

• Measure out a length of wire, free of kinks, long enough to run the length of the space between the U-Plate and the End Plate, and then some more to account for securing the wire to the pins.

• Tie one end of the end around the small hole on the side of the U-Plate welded to the Coupling nut. Wrap the remaining wire twice around the pin, and make sure the wire is not slack anywhere.

• The wire should now run over the top of the pin and pass through the slot on the U-Plate. Take the other end of the wire, pass it through the slot on the End Plate, wrap it twice around the pin on its U-Plate, and then tie the remaining wire around the hole on the side.

Make sure the wire is as taut as possible. This will help it to go into tension a lot quicker.

Step 4: Stretch the Wire

• Using a Torque wrench set at the appropriate Torque, tighten the bolt. This rotary motion of the bolt is transferred into a linear motion which pulls the wire towards the First Plate.

• The wire breaks after a certain point. Measure the length that the coupling nut moves, and this can help to calculate the strain of the wire.

• Look closely at the wire where it broke. You should see some necking, which is a type of failure where there is a large localized decrease in area of the part due to tensile loading the part. Here, the wire should have some material in the middle with surface area smaller than that of the actual wire. This bit of material is what tries to resist the load while the material around it tears/breaks, and is the last connective “tissue” that the wire has right before it breaks into two.

Next Versions

• A prototype of a newer version of the axial machine that includes keyless drill chucks as clamping devices for the wire has already been made.

• A load cell needs to be added to measure the torque acting on the bolt, and a vernier caliper needs to be installed to accurately measure the distance that the wire is stretched. This needs to be physically added to the prototype, as the CAD models are already completed.

References

[1] “What is welding? - definition, processes and types of Welds,” TWI. [Online]. Available: https://www.twi-global.com/technical-knowledge/faqs/what-is-welding . [Accessed: 11-Nov-2021].

[2] A. Mulkerin, “What is dissimilar metal welding?: Metal Welding Techniques,” APX York Sheet Metal, 28-Jan-2021. [Online]. Available: https://www.yorksheet.com/york-sheet-blog/dissimilar-metal-welding . [Accessed: 11-Nov-2021].

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