Universal Testing Machines (UTMs) are also commonly referred to as tensile testers, pull testers, or materials testing systems. UTMs are at the heart of material science and play a critical role in almost every manufacturing environment. A global market of $500 million is established for these types of machines, which include machines, accessories, software, and maintenance and repair services (MRO).
There are four major steps involved when specifying a Universal Testing Machine. While these machines are considered to be “universal” it still takes a considerable amount of foresight and planning in order to get the perfect machine for your lab.
Selecting the Right Test Frame
The test frame is the most critical component of the system because it determines the base capacity of the entire setup. The test frame is not necessarily upgradeable, so proper care must be taken to select enough capacity. The machine capacity is reliant on the application. The larger machines can fulfill a wide range of test procedures however they tend to operate at slower speeds and thus are not ideal for applications such as peel testing. The larger, floor-standing machines can weigh over 1,500 lbs. so proper planning must be undertaken for the shipping and final placement of the unit.
Settling in on a Suitable Controller
Control systems for Universal Testing Machines have evolved quite rapidly in recent years. Analog Vacuum Tubes and Paper Chart Recorders have been replaced with digital computers. Most UTM manufacturers offer two standard options for their controller units.
The standalone controller is ideal for highly repetitive testing such as in manufacturing quality labs. The interface has a pin pad and a small digital readout screen. Operators can select from a limited amount of tests and run the machine without a PC. Data is spitting out on the screen and hand-copied, or a series of raw data can be uploaded back to a PC for further analysis and documentation. These standalone type controllers tend to have fewer options in terms of servo-control, but they are also lower cost.
PC-Based Controllers are more typically suited for R&D settings where operators are constantly making changes to their test procedures and doing advanced analysis of the data results. The machine is completely operated through the PC. The PC-Based controller can either be embedded inside a PC desktop tower or can be embedded inside an external interface box. In some rare cases, the controller and data acquisition (DAQ) is embedded in the base of the machine. USB and Ethernet are the two most common data connection types. Using this type of controller allows for much easier collection and management of data and test results, while also giving the user the maximum amount of control options. PC-based controller and software packages are generally more expensive than their standalone counterparts, however, this is not always the case.
Identifying the Ideal Load Cell(s)
Over the years there have been many different types of devices used to measure load or force. The industry has settled in on strain gauge load cells. 4 individual strain gauges are typically wired in a Wheatstone bridge configuration which helps with stabilizing and calibrating the load cell(LC). These 4 strain gauges are packaged into a circular shell which is commonly known as a “pancake.”
As a rule of thumb, load cells work best between 5-95% of their posted range. If you have a 100 lb. load cell, it will not be able to register very well below 5lbs. and a more sensitive load cell will be needed. The situation is slightly different at the higher end where the risk of damaging the load cell increases substantially as the load approaches the maximum capacity of the LC.
Hydraulic machines with high capacity will use a pressure gauge transducer or a similar type of sensor instead of a Load Cell. These pressure gauges are slightly different in principle, however, they achieve the same result.
Grips, Fixtures, and Accessories
There is a myriad of grips and fixture options for Universal Testing Machines. The type of grip needed is based on the material constitution, sample geometry, and maximum anticipated force capacity. The Universal Grip Company offers the largest selection of grips in the world.
The majority of tensile testing applications will use either a vise grip or a mechanical wedge grip. Circular or square platens are used for compression testing. There are many more types of fixtures including bend test fixtures, puncture fixtures, and even special fixtures used to test food. Pneumatic grips are also popular because they help to standardize testing and increase throughput.
Aside from grips, there are many other add-ons including environmental chambers, extensometers, deflectors, and sample preparation tools. The environmental chambers are also known as a furnace or test ovens and can range in temperature from -70C to 350C, depending on the exact setup. Extensometers and deflectometer are used to measure elongation and compression and there are many types including regular contact, laser, and video extensometers.