Frank Lio

Product Manager, Instron Corporation

The Twists, Turns, Trials and Tribulations of Your Average Material

The Titanic, considered indestructible by engineers, may have been sunk by something as basic as weak rivets. The Hyatt Regency Skywalk in Kansas City, MO collapsed at its base under the weight of pedestrians. Aloha Airlines Flight 243 literally had its roof torn off, as cracks within the fuselage turned to large fractures caused by cabin pressure. These disasters may have all been caused by equipment or material failure. It is possible, however, that tragedies such as these could be avoided through the process of materials testing.

What is materials testing? It’s the process of twisting, turning, hitting, and squeezing materials to make sure they stand up to intense stress, strain and impact. In other words, materials testing examines the toughness, flexibility and overall strength of a given substance. Considering the magnitude of an accident like the Titanic or the 1978 roof collapse at the Hartford Civic Center arena in Hartford, CT, one can see why materials testing is so important. That’s not to say that the rivets of the Titanic or the strength of the arena roof weren’t tested; however, the likelihood of projects being built with faulty materials is greatly reduced by materials testing.

It’s not just concrete, steel, bricks and mortar that are subjected to materials testing. Virtually anything and everything can be tested, from the clothes on your body, to the food on your plate, to the plastic bottle you drink from. Organizations as diverse as NASA and Purdue Farms Inc. regularly test materials such as plastic, ceramic, metal, textile, rubber, wood, paper, adhesives, asphalt, wire, cable, food, fasteners and concrete.

There are five different tests that materials generally undergo. One is compression testing, which basically measures a material’s ability to stand up to heavy crushing loads. In other words, heavy loads are dropped on specimens. The next two types of tests, hardness and impact, measure the ability of a substance to withstand intense force. Fourth is tensile testing, which tests materials by rigorously stretching them, and the fifth is torsion testing, which looks at how a particular substance can handle twisting and wrenching. Not all materials undergo all five tests, but each type of material tested really goes through the wringer, so to speak.

Compression and tensile testing are the oldest and most common materials tests performed. As the testing industry grows, and the demands of customers increase, additional materials testing is needed to ascertain how substances will react to real life situations.

Each type of testing is essential to gauging the strength of a material. Impact testing anticipates what the consumer does not. No matter what the product, it will inevitably face some type of unexpected blow, collision, or impact during its lifetime. It could be something as mundane as dropping a carton of milk at the grocery store or as jarring as a car bumper hitting a parking post. A product is more likely to fail when it is subjected to a blow that hits with higher than expected force.

The purpose of impact testing is to simulate these conditions in an effort to prevent the product from breaking down. Often, it is necessary to perform impact testing to ensure personal safety. One of the most dangerous actions of a saw blade is premature or unexpected failure while in use. Impact testing is used to simulate and measure sawing motion and action of a saw blade in use. Determining the maximum load of a saw blade allows manufacturers to see whether substituting different materials or manufacturing processes have any effect on a final product. Impact testing can also identify incipient damage. For example, a nick in blade could cause a stress concentration and a weak point, which could ultimately lead to failure.

To test impact, engineers use instruments that measure the energy dissipated when subjecting a part to impact. Engineers test and measure properties of material including, yielding, ductility, and maximum and failure load. In many cases, a product’s impact resistance is a critical measure of its service life.

As the name suggests, hardness testing gauges the resistance of a material to permanent indentation. The test is pretty straightforward compared to impact testing, which is more difficult to quantify. To test hardness, engineers use a machine that forces a shaped indentor, often a diamond because of its hardness, into the surface of the test material. The depth or size of the indent left by the diamond, in effect, measures the hardness. Harder materials will exhibit a smaller indent than softer materials.

Hardness testing is most often performed on metals, hard plastics, and even rubber. Hardness testing is used to complement tensile and compression testing for several reasons. It is fast, primarily, non-destructive, and can be performed directly on the component. Some hardness testing can be done within seconds with a hand-held device. The indent made by the hardness test can either be ground out, or can be so small as to not affect the performance of the component. Because the testing is done to the component itself, each product or a spot check of products can be tested before shipping to the customer. For example, the automotive industry is able to utilize hardness testing for products to ensure top quality. Gears in the transmission of a car must have a high hardness surface to ensure durability in a non-ambient environment. Conversely, the core of gears need to be softer because the cyclically power that is transferred. If the gear is too hard all the way through then stress cracks will occur.  Stress cracks lead to failure, thereby rendering the product useless. Performing hardness tests on the gears to ensure top quality.

Also, crankshafts used in various vehicles, must have a hard surface with softer core.  Crankshafts not only need to be hard on the surface but depth of surface must also be tested. Because crankshafts are reground during an engine overhaul, if the surface is not deep enough the part will not survive being reground time and time again. Only a hardness test can provide the needed results for crankshaft testing.

To test the flexibility of a substance and its overall ability to stand up to tension, engineers perform tensile tests. These tests differ from impact and hardness tests in that applied force is absorbed slowly. In comparison with impact testing’s quick powerful jab, tensile testing is more like slow torture. The tensile test involves mounting the specimen in a machine and subjecting it to tension by stretching it. The tensile capacity of the material is recorded as the specimen increases in length. The purpose of tensile testing is to determine the elastic limit, elongation, tensile strength, and yield or breaking point of the material.

Even after materials have been crushed, hit, rammed, and stretched, all the properties and limitations may not yet be discovered. Torsion testing is used in conjunction with other testing to ensure the capacity of a specimen to stand up to twisting and wrenching. Torsion testing not only measures the strength of the component, but the joints or fixtures to which the product is attached. Unlike other types of tests, torsion testing does not necessarily measure the material, but rather the entire component.

Torsion tests, done on a tabletop machine that simulates a twisting motion, are ideally suited for products that must withstand constant twisting, such as bolts, nuts and other fasteners, switches, wire, medical devices, and biomaterials. Torsion testing is the only way to see how a product will react to being twisted. Stretching or squeezing a wire will not yield the results needed to know how much wrenching that wire can undergo.

The automotive and aerospace industries are increasing use of torsion testing. For example switches used to control fan speed in cars. Torsion tests are used to compare the rotational force to the voltage output to ensure that the electrical signal is active precisely when the switch is in its detent position.

It has become imperative that materials undergo all the testing that best simulates the environment in which that material will be used. Performing a test that identifies how a material will react in real life situations demonstrates the appropriate applications of that material. Expanding testing of material properties beyond tensile and compression tests helps to ensure the personal safety of consumers, while protecting against the liability of manufacturers. While some accidents and disasters due to unforeseen forces are inevitable, there are mishaps that can be avoided through materials testing.