There are as many reasons to test metals as there are metals:

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1 Testing Their Mettle Metals testing procedures ensure quality in raw materials and finished products BY BILL O NEIL, ADRIAN RIDDICK, FRANK LIO, PAUL KING, CHRIS WILSON, AND PATTY HARTZELL There are as many reasons to test metals as there are metals: product performance, safety, liability, and production costs to name just a few. It is essential these tests be performed accurately and consistently in order to obtain a true rawmaterial or finished-product profile. By testing these products, manufacturers are able to simulate real-life service conditions, check product quality, verify designs, and ensure proper manufacturing techniques. There are six major types of mechanical tests: tensile, hardness, bend, fatigue, torsion, and impact. Following is a description of each type of test, including how the tests are performed and why they are so important. Tensile Testing A tension test is probably the most fundamental type of mechanical test that can be performed on metal. Tensile testing is typically used to apply a proof load to a specimen or to take the specimen through failure. Tensile tests may be performed on samples prepared from raw materials or finished metal products such as reinforcement bars (rebar), fasteners, wire, cable, pipes, tubes, plates, sheets, or structural steel. Common results determined from tensile tests include Young s modulus, yield strength, tensile strength, elastic deformation, extension under load, specimen elongation, reduction of area, and strain hardening behavior. Fig. 1 During a tensile test, an extensometer records the strain on the specimen. How Tensile Tests Are Performed In tensile testing, a metal specimen is secured in a set of grips suitable for the specimen shape and pulled apart with a universal testing machine. The force needed to apply this tension load may be provided with an electromechanical drive or hydraulically powered actuator. Machines featuring an electromechanical drive use rotating ball screws to move a crosshead. As the machine s crosshead is moved, the grips are forced apart and tension is applied to the specimen. Hydraulic systems are able to apply greater forces by using a pump and hydraulic fluid to move an actuator. As the actuator moves, the grips are again forced apart and a tensile force is applied to the specimen. The strength of the metal specimen typically determines which style of testing machine is used. Electromechanical systems can accurately apply loads as low as 0.1 pounds, while standard hydraulically powered machines can apply loads up to 600,000 pounds. During a tensile test, the force applied to a specimen and the elongation of the specimen are measured. Typically, a load cell system is used to capture load while an extensometer records strain Fig. 1. From the load reading, specimen stress can be computed by dividing load by the cross-sectional area of the specimen (stress = load/cross-sectional area). From these measurements (load, stress, and strain), a test curve can be generated. Why Tensile Testing Is Important Because metal is commonly used as a structural element, it is imperative to know its precise characteristics. Therefore, by analyzing the test curve, important material properties such as the modulus of elasticity, yield strength, tensile strength, and breaking load can be quantified. This type of quality assurance testing is critical to ensuring that metal products meet the material or finished-product design criteria. Unsatisfactory results can indicate that the manufacturing process is not being properly controlled and needs to be examined. Hardness Testing Hardness is the resistance of a material to permanent indentation Fig. 2. It is an empirical test, not a material property. There are several different hardness tests that will each determine a different hardness value for the same piece of material. Hardness testing is one of the simplest, most effective, and most efficient means of mechanically characterizing a material due to minimal sample preparation and, in many cases, its nondestructive nature. Hardness is test-method dependent, and every test result must have a label identifying the method used. BILL O NEIL, ADRIAN RIDDICK, FRANK LIO, PAUL KING, CHRIS WILSON, AND PATTY HARTZELL are Product Managers at Instron, Canton, Mass. They can be contacted at info_news@instron.com. FALL

2 Fig. 2 A close-up of a Rockwell hardness testing instrument. There are three types of hardness tests Rockwell, micro-/macroindentation, and Brinell. Although they provide somewhat different information, each test involves resistance of a material to permanent indentation. How Hardness Testing Is Performed Fig. 3 A bend test on rebar. In three-point bend tests such as shown here, the force is applied centrally. In four-point bend tests, the force is applied at a defined distance on either side of center. The three main types of hardness testing typically performed on metals are Rockwell, micro-/macro-indentation (Knoop/ Vickers), and Brinell tests. Each test type involves pressing an indenter of particular geometry and properties into the test material at a precisely defined force for a specified length of time. The Rockwell test measures the depth of penetration of that force while the micro-/macro-indentation and Brinell tests determine the indentation dimensions. Brinell testing applications generally include cast iron, large steel framework, and aluminum. In the Rockwell test, a diamond cone or steel ball indenter moves down into position on the part surface. A minor load is applied and a zero reference position is established. The major load is applied for a specified time period, after which the major load is released, leaving the minor load applied. The resulting Rockwell number represents the difference in depth from the zero reference position as a result of the application of the major load. Rockwell testing is particularly suitable to homogeneous metals or alloys. Knoop testing is done with a diamond indenter that forms an elongated diamond-shaped indent. The indenter is pressed into the sample by an accurately controlled test force and maintained for a specific dwell time, normally seconds. After the dwell time is complete, the indenter is removed, leaving the elongated, diamond-shaped indent. The size of the indent is determined optically by measuring the longest diagonal. The Knoop hardness number is a function of the test force divided by the projected area of the indent. This testing is suited to metallographic mounts, thin materials, wires, and coatings. Vickers testing is performed similarly to Knoop testing, but the diamond indenter forms a square indent. The size of the indent is determined optically by measuring the two diagonals of the square indent. The Vickers hardness number is a function of the test force divided by the surface area of the indent. As with Knoop, this testing is suited to metallographic mounts, thin materials, wires, and coatings. The Brinell test involves applying a test force to a hard steel or carbide ball of fixed diameter. The test force is held for a predetermined length of time and then removed. The resulting indentation is measured across at least two diameters. A chart is then used to convert the averaged diameter measurements to a Brinell hardness number. Why Hardness Testing Is Important The most common uses for hardness tests are to verify the heat treatment of a part and to determine if a material has the necessary properties for its intended use. In addition, basic hardness testing equipment is relatively inexpensive, and the process can be performed quickly. Often hardness testing is the only nondestructive test alternative for effectively qualifying and releasing material or components for target applications. Hardness testing is paramount in metals heat-treating operations and case depth analysis. Bend Testing Bend tests are conducted to determine the ductility or 14 INSPECTION TRENDS

3 Fig. 5 A worker prepared to conduct a torsion test on a wire sample. Fig. 4 Two fatigue testing systems. These machines apply cyclic loads to the test specimen to determine how the material will perform under similar conditions when in service. strength of a material. Bend tests are divided into two categories: ductility tests and strength tests. Bending ductility tests determine the smallest radius around which a specimen can be bent without cracks forming on the outer surface. These tests are often used to test the ductility of welds. Bending strength tests offer a means of determining the modulus of elasticity in bending and the bending strength of flat metallic samples in the form of strip, sheet, or plate. How Bend Testing Is Performed Bend testing involves placing a metal specimen on a support span with two rollers. A force is then applied either centrally (three-point bend test), or at a defined distance on either side of the center (four-point bend test) Fig. 3. By modifying the diameter of the rollers, the height of the support base and the distance of the support span can alter the severity of the bend. General three- and four-point bend fixtures are available with adjustable support spans and interchangeable rollers to meet a variety of bend testing applications. In addition, specialized bend fixtures are available to conform to reinforcement bar and weld bend testing standards. There are cold bend fixtures designed for bend testing reinforcing bars up to a full 180-deg bend in accordance with ASTM A615 and ASTM A99. Guided weld bend test fixtures are also available that are designed to evaluate the ductility of plate specimens as well as welded ferrous and nonferrous materials, in accordance with ASTM A370 and ASTM E190. Why Bend Testing Is Important Once the material s resistance to bending and ductility is quantified via the testing curve, physical examination, and related calculations, the results are compared to applicable material standards and the expected material behavior. Unacceptable results can indicate that a change is necessary in the manufacturing process or that improper welding techniques were used. Fatigue Testing Fatigue testing can be thought of as applying cyclic loading and unloading to a test specimen in order to understand how that material will perform under similar conditions as part of a real design or component Fig. 4. The testing load can either be a repeated application of a fixed load (constant amplitude fatigue) or the application of a more complex loading history more closely representing the in-service conditions experienced by real world components (variable amplitude fatigue). The load application may be repeated millions of times, with speeds up to several hundred loading cycles per second. Fatigue falls into two basic categories: high-cycle fatigue (HCF) and low-cycle fatigue (LCF). High-cycle fatigue testing simulates loading conditions components typically experience at higher frequencies. An example is a component subjected to vibrational loading conditions, such as the structural mounting bracket that attaches the aircraft engine to the wing section. Low-cycle fatigue testing simulates the loading conditions of components that undergo either mechanically or thermally induced cyclic plastic strains that can cause failure within relatively few cycles. An example of this type of loading condition is the stress cycle a turbine blade experiences during the startup phase of an aircraft jet engine. How Fatigue Testing Is Performed High-cycle fatigue testing is typically performed in load control, with the test specimen being cycled between an upper and lower load level. These load levels may be in the same stress direction (for example, tension-tension fatigue testing) or may cycle from tension to compression (reverse stress loading). These tests are often conducted in a servohydraulic test machine at relatively high frequencies. This reduces the time necessary to complete the high numbers of cycles that may be necessary to fail the test specimen. Alignment of the specimen within the test frame is critical since any misalignment can result in bending stresses being imposed on the test specimen, resulting in premature failure. Accurate load measurement can also be a challenge at higher test frequencies due to the potential of introducing artifacts into the load measurement system from the inertia of the test fixtures. Such effects can be minimized with the use of suitably designed dynamic load cells with built-in inertia compensation. FALL

4 Fig. 6 In a Charpy-type impact test machine, a pendulum arm swings down and strikes the specimen while it is clamped in a horizontal position. Low-cycle fatigue testing is normally performed at lower test frequencies than HCF. This is due to the higher levels of plastic strain associated with typical LCF tests and the need to maintain isothermal conditions during the test. Low-cycle fatigue tests are normally performed in strain control using an extensometer attached to the specimen. In addition, the tests are often performed in nonambient temperature conditions to simulate the operating environment of the materials under test, such as Nimonic alloys used in the construction of high-performance turbine blades. A typical test setup may, therefore, be comprised of a load frame with a servohydraulic or servoelectric actuator, high-temperature furnace, and high-temperature gripping system with precision alignment adjustment. Thermo-mechanical fatigue (TMF) testing is an extension of LCF testing. Thermo-mechanical fatigue tests simulate the behavior of materials subjected to combined mechanical and thermal strains. The overall mechanical test setup is similar to an LCF test except that the temperature control system must be capable of rapidly heating and cooling the test specimen. In addition, the temperature control must be synchronous with the mechanical strain control in order to correctly simulate the reallife combined effects of dynamic changing mechanical and thermal strains. Why Fatigue Testing Is Important In many applications, materials are subjected to vibrating or oscillating forces. The behavior of materials under such load conditions differs from the behavior under a static load. It is therefore critical that designers be able to predict fatigue life the total number of cycles to failure under specified loading conditions using appropriate modeling tools in conjunction with the relevant material s fatigue data. Fatigue testing provides Fig. 7 A drop weight impact testing system. The drop weight tear test measures resistance to fracturing in materials such as structural steels and line pipe that require high impact energies and velocities. information on the fatigue characteristics of specific materials, and furthers the understanding of the underlying mechanisms giving rise to fatigue failures. Results from such tests enable the development of alloys with enhanced fatigue performance and improve the ability to design components that efficiently withstand fatigue service loads. Torsion Testing Many products and components are subjected to twisting or torsional forces during their operation. Products such as switches, fasteners, and automotive steering columns are just a few devices subject to torsional stresses. A torsion test can be conducted on most materials to determine their torsional properties. These properties include, but are not limited to, modulus of elasticity in shear, yield shear strength, ultimate shear strength, modulus of rupture in shear, and ductility. 16 INSPECTION TRENDS

5 How Torsion Testing Is Performed An electromechanical or hydraulically powered testing machine can be used for torsion testing. An electromechanical drive system transfers the rotational motion of a motor to the specimen, while a hydraulic system employs closed-loop servo control together with a hydraulic power supply to apply torsion loads. To ensure the most accurate results, both electromechanical and hydraulic systems should be fitted with a control system capable of collecting data at high frequencies. Standard torsion grips including drill-type chucks, collets, and sockets are used to secure specimens such as wires, fasteners, bars, columns, and components. Types of torsion tests include torsion only, which means applying only torsional loads to the test specimen; axial-torsion, or applying both axial (tension or compression) and torsional forces to the test specimen; failure testing, which twists the product, component, or specimen until failure; proof testing, which involves applying a torsional load and holding that torque load for a fixed amount of time; and operational testing, or testing complete assemblies or products to verify that they perform as expected under torsion loads. Torsion properties can be determined from a machined metal sample or from finished products and components, such as wire, bar, or cable Fig. 5. Specimens can be twisted to determine the shear modulus, peak torque, and breaking strength. For fasteners, a torsion test reveals the fasteners behavior under service load torques and can be used to determine the torsional breaking strength. large mass in a vertical direction, with rails to guide it during the free fall Fig. 7. Applicable standards for drop weight tear tests include API 5L3 and ASTM E436, E208, and E604. Many modern systems include full instrumentation and PC analysis to capture and analyze test data. This allows the user to view the test curve profile to ensure materials have the impact and fracture resistance properties needed. Why Impact Testing Is Important Impact resistance is one of the most important properties for a parts designer to consider and, without question, the most difficult to quantify. The impact resistance of a part is, in many applications, a critical measure of service life. More importantly, it involves the perplexing problem of product safety and liability. One must determine the impact energies the part can be expected to see in its lifetime, the type of impact that will deliver that energy, and then select a material that will resist such assaults over the projected life span. Conclusion There s no question that metals testing requires a certain commitment of time and resources: financial, technical, and human. Yet the benefits to be realized increased quality in raw materials and finished products, a reduction in potential liability, and streamlined production processes makes the process more than worthwhile. Why Torsion Testing Is Important Because many products and components are subjected to torsional forces during their operation, torsion tests are important in determining their resistance to torsional fatigue. They can also be used to measure how easy it is to turn one part with respect to another or for measuring the torque and force required to insert a screw into a base material. Torsion testing can also be used to investigate the product quality of devices, revealing information about the design, weld joints, and ultimate strength of these products. Impact Testing Impact testing determines an object s ability to resist high rate loading. An impact test determines the energy absorbed in fracturing a test piece at high velocity. In essence, impact testing involves one object striking another at a relatively high speed. How Impact Testing Is Performed Common metal impact tests include Charpy, Izod, and drop weight tear. The Charpy test has a specimen laid horizontally on two supports against an anvil. The sample is notched in the center, and the notch side is positioned away from the pendulum Fig. 6. When the pendulum swings through the gap in the anvil, it impacts the center of the sample with a radiused hammer. ASTM E23 and EN10045 are common standards for the Charpy test. In the Izod test originally conceived as a way to determine the suitability of various metals for use as cutting tools a pendulum with a known weight at the end of its arm swings down and strikes the specimen while it is clamped in a vertical position. ASTM E23 is a common Izod test standard. Drop weight tear tests measure the resistance to rapid progressive fracturing in structural steels and line pipe. These tests can require very high impact energies and velocities. Vertical-style drop weight towers are commonly used. This involves dropping a FALL