There are many types of testing machines. The most common are universal testing machines, which test materials in tension, compression or bending. This month 50 Years of Quality takes another look at universal testers. EXTEND THE LIFE OF MATERIALS TESTING EQUIPMENT, MAY 2003 Is the performance of your material test equipment limited by aging components and outdated controls? If you have an aging mechanical dial, digital display or chart recorder, or an outdated computer control system, now might be the perfect time to upgrade your testing equipment. An upgrade allows you to retain your existing load frame, grips and load cells, while replacing older electronics and recording devices with a new controller and advanced testing software. The result can be a revitalized system at a fraction of the cost of new testing equipment. Upgrading equipment enables the integration of your testing procedures with your business operation. Automated test control increases the repeatability of test results, while PC storage, automated calculation and networking capabilities enable statistical analysis and data archiving, leading to improved productivity and more reliable results. IDENTIFY UPGRADE CANDIDATES Before purchasing an upgrade, evaluate your system by performing a visual inspection of the frame, operating the machine, and gathering data about its capacity and transducers. If the frame of your testing system can’t be calibrated, exhibits signs of backlash or has broken parts, retrofitting the system’s electronics will not improve its performance. The first step in any evaluation is to determine if the system is mechanically or hydraulically powered. Mechanical testers use an electromechanical drive system to supply torque power to the load frame. A standard system consists of a servocontrolled drive motor, timing belts and two screw columns. Hydraulic systems use a hydraulic fluid and a pumping unit—often called a hydraulic power pack—to supply the forces necessary for tension, compression or cyclic testing. On mechanical systems, all gears, pulleys, belts, chains and critical frame parts should be visually inspected before retrofitting the system. Start by walking around the testing frame and viewing the columns from all angles. Columns are often bent from off-center loading or incorrect lifting. If the columns are bent, machine alignment is jeopardized and uneven loading will occur. Next, remove the cover that encloses the mechanical assembly and electronics. Look for worn gears, broken teeth, frayed belts and loose chains. If parts are severely damaged, consider the cost and availability of replacement parts. For hydraulic equipment, screw columns, crosshead pockets and critical frame parts should be visually inspected before an upgrade is purchased. As with mechanical testers, start by walking around the testing frame to check for bent columns. Static universal testing machines may include stay plates to hold the columns in place. Check to ensure that these plates are not bent. Replacing bent stay plates may require complete disassembly of the frame. If the testing frame includes inhead gripping pockets, they should be inspected for wear. Incorrect use of grips and filler plates can result in crosshead deformation. Severe deformation will affect the gripping and loading of specimens. A repair requires the crossheads to be removed from the frame so the pockets can be remachined. Finally, look for hydraulic fluid leaking from the piston assembly. Machines that are leaking oil may not be able to reach capacity. This fix could be as simple as replacing the seal, or it could require a new piston and cylinder assembly. TENSILE TESTING BASICS, TIPS AND TRENDS, JANUARY 2005 There are many types of testing machines. The most common are universal testing machines, which test materials in tension, compression or bending. The primary use of the testing machine is to create the stress-strain diagram. Once the diagram is generated, a pencil and straight edge or computer algorithm can be used to calculate yield strength, Young’s Modulus, tensile strength or total elongation. There are two classes of testing machines, electromechanical and hydraulic. The electromechanical machine uses an electric motor, gear reduction system and one, two or four screws to move the crosshead up or down. A range of crosshead speeds can be achieved by changing the speed of the motor. A microprocessor based closed-loop servo system can be implemented to accurately control the speed of the crosshead. A hydraulic testing machine uses either a single- or dual-acting piston to move the crosshead up or down. In a manually operated machine, the operator adjusts a needle valve to control the rate of loading. In a closed-loop hydraulic servo system, the needle valve is replaced by an electrically operated servo-valve for precise control. In general, the electromechanical machine is capable of a wide range of test speeds and long crosshead displacements, whereas the hydraulic machine is a cost-effective solution for generating high forces. TENSILE TESTING BASICS, DECEMBER 2006 A tensile test is a static measurement of the effects of tensile force on a material or component, or to determine the bond strength of two materials that have been assembled together. By applying a tensile force on a material, one can find the tensile strength, yield point, yield strength, percent elongation, reduction in area and the modulus of elasticity. Tensile test results are used to indicate strength, ductility, stiffness and the correct parameters for heat treatment or processing. In general, tensile testers or universal testing machines apply the load mechanically by a screw and gears, or hydraulically with a pump and motor. A load cell device or pressure transducer is used to indicate the mechanical load applied to the test specimen. An extensometer is a device for measuring the extension or elongation of the test specimen. A computer that automatically runs the test machine—either by a selected strain rate, load rate or position rate—controls most new equipment. In addition, the computer can automatically calculate all of the properties needed by the operator. Tensile test specimens are normally shaped like dog bones—the center portion of the specimen is smaller in cross section than the two ends. Tensile specimens may be round or rectangular depending on the stock on which they are obtained. In most cases, the final results are reported in terms of pounds per square inch. Pounds per square inch is equal to the force divided by the cross sectional area. Most mechanical tests have been derived from testing metals. However, in materials testing, as the load is applied and the specimen is stretched, a stress vs. strain curve is plotted. Many material properties can be found in this test such as yield strength, ultimate tensile strength, percent elongation and modulus of elasticity. For most materials, the initial portion of the test, the relationship between the applied force, or load, and the strain or elongation of the specimen shows a linear relationship. Referred to as the proportional limit, Hooke’s Law or modulus line, it is where the material, if unloaded, would not show any permanent strain remaining when the stress is completely removed. Beyond this point is the yield point when strain occurs without an increase in stress. For metals and plastics the departure from the linear elastic region cannot be easily identified. Therefore, an offset method is used to determine the yield strength. MATERIALS TESTING MADE EASY, NOVEMBER 2007 Equipment has changed dramatically throughout the years, and the past five years have been no exception. In Meredith Platt’s five years with Instron (Norwood, MA), she has seen a huge increase in the number of people looking at reproducibility and reliability studies. Platt, a 3300 product manager, says that it is important to measure how accurate the gage is, and then use that analysis to see whether the parts are good. The FDA monitors many of the companies Platt works with, so it is crucial to have everything running smoothly. Because of Instron’s application knowledge and product knowledge, they can provide input on the system and check its accuracy, Platt says. Generally most problems occur because of an operator issue or test configuration. A system’s success may depend on procedures, and how good the standard operating systems are. Software is important, as it is generally the operator’s main contact with the system. Software improvements over the past five years can improve test flow and prevent operators from missing steps along the way, Platt says. Usability, productivity, throughput and safety are all important to customers. “People are willing to spend extra money on systems that they know are safer,” Platt says. “The future benefit of equipment in general can’t put a price tag on safety.” However, price is still a consideration. “Most people are looking for a good value,” Platt says. “They want equipment they can trust, but they don’t want to pay a fortune for it.” Equipment is not the only thing that has changed—operator levels have dropped, says Platt. When Instron was founded, Ph.D.s were the ones using the equipment. Now, the Ph.D.s are not the ones operating the machinery. CHOOSE THE RIGHT EXTENSOMETER FOR EVERY MATERIALS TESTING APPLICATION, OCTOBER 2009 In materials and component testing the range of applications where extensometers are used is extremely diverse. As a result, the technical requirements for these devices are multifaceted, and there is no single device that satisfies all needs. The requirements for an extensometer are determined primarily by the characteristics of the material to be tested. This includes its shape and dimensions, test requirements and the formal standards that must be met. These define the gage length, accuracy, test sequence and environmental conditions. Having said this, the right choice of extensometer cannot be limited to the basic material characteristics such as specimen dimensions, stiffness, strength and plasticity alone. It also is necessary to decide whether an extensometer can be connected directly to the specimen without influencing the load measurement or mechanically damaging the specimen itself. Very thin specimens such as foils can be sensitive to clamping forces, while very small wire specimens do not provide enough visible area for reliable noncontact measurements. A high stiffness in the initial extension range, followed by high plasticity traditionally requires more than one extensometer. The first measures small strains—typically up to 5 millimeters—very accurately in the elastic range, and the second measures very high extensions—typically greater than 500 millimeters. Specimens with very smooth surfaces, or made of transparent materials, are not suitable for noncontact measurements without first fixing measuring marks on the surface of the specimen. One important consideration is the behavior when the specimen fails. Metals and hard plastics will slip through the knife edges of a contact extensometer without damaging them, and rotatable knife edges should be used to further reduce the risk of damage even if the surface of the specimen is particularly rough. High extension or flexible specimens can damage or destroy the knife edges and even the extensometer itself due to whiplash, splintering or delamination of specimens. For these applications noncontact measurement is a must.
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