The Titanic, considered indestructible by engineers, may have sunk because of 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 cabin pressure caused cracks within the fuselage to turn into large fractures. These disasters may have been caused by equipment or material failure. And it is these kinds of tragedies that engineers seek to avoid through the process of materials testing.
Tough Enough?
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. Materials testing examines the toughness, flexibility and overall strength of a given substance.
Considering the magnitude of accidents such as the Titanic or the 1978 roof collapse at the Hartford Civic Center in Hartford, CT, one can see why materials testing is 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.
Concrete, steel, bricks and mortar are not the only materials subjected to 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 the National Aeronautics and Space Administration and Perdue Farms Inc. regularly test materials including plastic, ceramic, metal, textile, rubber, wood, paper, adhesives, asphalt, wire, cable, food, fasteners and concrete.
Which is Which?
There are five tests that materials generally undergo. Compression testing drops heavy loads on a material to measure its ability to stand up under crushing weight. Hardness and impact tests measure the ability of a substance to withstand intense force. Tensile testing subjects materials to rigorous stretching, and torsion testing looks at how a particular substance handles twisting and wrenching. Not all materials must necessarily undergo all five tests, but more manufacturers lately have been increasing the number and type of materials tests performed.
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 determine how substances will react to real-life situations.
Each type of testing is essential in gaging the strength of a material. Impact testing anticipates what the consumer does not. No matter the product, it may face an unexpected blow, collision or impact during its lifetime; some milk cartons invariably get dropped, for example, and car bumpers someArial collide with parking posts. A product is most likely to fail when it is subjected to a blow that hits with higher than expected force.
Impact testing simulates these conditions in an effort to prevent the product from breaking down. Impact testing helps 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 the 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 an effect on the final product. Impact testing also can identify incipient damage. For example, a nick in a blade could cause a stress concentration and a weak point, ultimately leading to failure.
To test impact, engineers use instruments that measure energy dissipation when subjecting a part to impact. Properties measured include yielding, ductility, and maximum and failure load. In many cases, a product's impact resistance is a critical measure of its service life.
Diamond Dents
As the name suggests, hardness testing gages the resistance of a material to permanent indentation. The test is straightforward when compared to impact testing, which is more difficult to quantify. To test hardness, engineers force 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 measures the hardness. Harder materials exhibit a smaller indent than softer materials.
Hardness testing is most often performed on metals, hard plastics and rubber. Hardness testing is used to complement tensile and compression testing for several reasons. It is fast, primarily nondestructive and can be performed directly on the component.
Some hardness testing can be done in seconds with a handheld device. The indent made by the hardness test can either be ground out, or is insignificant and does not affect the performance of the component. Because the testing is done to the component itself, each product or a sample of products can be tested before shipping to customers.
Hardness testing is key to ensuring top quality in various kinds of automotive components. Gears in the transmission of a car must have a high hardness surface to ensure durability, for example. Conversely, the core of these gears must be softer to successfully accommodate the transfer of power. If the gear is too hard all the way through, stress cracks can occur, leading to failure.
Vehicle crankshaft likewise must have hard surfaces with softer cores. And the hardness must extend to specific depths beyond the surface of the material. Because crankshafts are reground during engine overhauls, if the hardness is not deep enough, the part will not survive the regrinding process. Only hardness tests can provide the needed information.
Slow Torture
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 to impact testing's quick powerful jabs, tensile testing is 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. Tensile testing determines 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, used in conjunction with other testing, ensures a material's capacity to stand up to twisting and wrenching. Torsion testing not only measures the strength of a component, but also the joints or fixtures to which it 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 produces a twisting motion, are ideally suited for products that must withstand constant twisting, such as bolts, nuts and other fasteners, as well as switches and wire. 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 their use of torsion testing. For example, switches used to control fan speed in cars are being tested. 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 is imperative that materials undergo all testing needed to simulate 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. Expanded testing of material properties beyond tensile and compression tests can help ensure the personal safety of consumers and protect manufacturers against liability claims. While some accidents and disasters occur due to unforeseen forces, some mishaps can be avoided through materials testing.
