This Machine Tests the Tensile Strength of Materials By Automatically Pulling Them Apart

Introduction

The field of materials science is integral to the way we live, and testing material strengths is a large focus of experiments. Xieshi over on Instructables designed and built a rig that can automatically stretch items and record where they break. This form of experimentation is for testing the tensile strength, which is how much force something can withstand while being stretched until it breaks. Xieshi constructed this as a continuation of his first version (that he made in 8th grade!), but with a few improvements.

Principles of Operation

As stated previously, the tensile strength of a material is tested by continually stretching it until the item snaps. At that point, the current force applied is recorded and the machine stops. Normally, materials include plastic threads, metal rods/strings, and simple structures that are meant to withstand tensile forces. They get held in place with a pair of vice grips on either side, and one side applies force by moving away from the other side slowly.

Designing the Rig

This time around, Xieshi used Fusion 360 instead of TinkerCAD to design the rig. He began by fashioning the stepper mount/bearing block, which sits on the rail and is for moving one vice grip linearly. The bearing is placed just in front of the lead screw and its job is to stabilize the screw as it's spinning.

Next, there is a moving block that lets a load cell attach, and it measures the force being exerted on the material by the vice grips. The other side of the load cell is connected to one vice grip, and they are able to move together, while the other grip is fixed in place on the other side.

Fabrication

The majority of the components for this project were 3D-printed in PLA with 100% infill, making them very rigid, although using ABS plastic or milling them from aluminum would let the rig test even tougher materials. Everything slots into a single aluminum extrusion that runs the length of the rig, and this lets certain components slide. Xieshi also designed a PCB with EagleCAD and had it fabricated and shipped.

Hardware Used

The components for this project are quite simple and readily available. An Arduino Nano is responsible for controlling nearly the entire system, as it directs the motor, writes to the display, and polls the load cell. In order to isolate the motor's control from the rest of the system, he set up an auxiliary ATtiny85 that receives step information from the Nano via I2C and signals the A4988 stepper driver.

The Program

To make all of this work, Xieshi had to write quite a lot of code. It starts by defining many constants, which include load factor calibration factors, speed multipliers, thresholds, and timings. States are stored with the use of several boolean variables, and they keep track of everything from the mode state to if the emergency stop function is active.

Once the pushbutton is pressed, the device starts to slowly stretch whatever is between the pair of vice grips while continually updating the current pressure on the LCD. Once a break has been detected, the rig stops and is ready to be reset for another test.

Final Thoughts

This project is impressive, especially when it has been designed and constructed by someone so young. This also has the potential to speed up tensile strength tests and make them more accurate and/or reliable.