Destructive testing is performed to evaluate physical characteristics of materials, such as strength, ductility, hardness and composition, and to determine susceptibility to corrosion. The information is most often used in material selection or failure analysis. This testing is performed on samples of material known as specimens or coupons that are prepared by machining to exacting dimensions and other precise requirements.
The Process
The four main processes performed in the production of test specimens are sawing, milling, turning and grinding. Although sawing is the starting point for all specimens, one or more of the other three processes may be performed during the preparation of specific types of specimens.
Sawing
Specimens are rough cut to length by sawing from a specific location on the sample material or part, as stated in a required specification or dictated in a drawing provided with the order. During this step, the material is cut into a workable size, which is generally close in size to the final specimen. If welds are involved, parts may be etched to identify position of the center line or heat-affected zone.
Turning
Turning is performed on a lathe by rotating a part against a stationary cutting tool to center and turn in diameter. This is a required step for all round specimens including tensile, stress rupture, gleeble, jominy, rotating beam and fatigues, since rough-cut specimens are usually square in shape. In addition to further refining specimen dimensions, the turning process reduces the gage length and threads the ends when required by fixturing on test machines.
Milling
Milling is the process of cutting away material by feeding a work piece past a rotating cutter with multiple teeth. The machined surface may be flat, angular, or curved and can be milled to any combination of shapes. Specimens such as flat tensile, stress rupture, and hardness samples are precision machined by this process. Also, side, face and root bend specimens for weld qualifications and certain corrosion specimens are milled. Milling is also used to further refine charpy specimens for the next step, grinding.
Grinding
Grinding is the finish step for many types of specimens including charpy, hardness, compact tension and macro specimens used in metallurgical examinations. During grinding, a rotating abrasive wheel smooths the surface to give the material a refined look or to attain the desired finish. When required, this process can be used to create a high-quality surface finish to 32 Ra (roughness average) or better.
Machining at LTI
The Machine Shop at LTI prepares all types of test specimens to required ASTM specifications and customer requirements for mechanical and corrosion testing. Our machinists also rough cut and grind metallography specimens, prepare chips for ICP analysis and perform electrical discharge
machining (EDM). Most specimens are machined from metals, including metal matrix composites, hardened steels and nickel-base alloys, using the latest CNC turning, milling and grinding machines.
All machining is PRI/Nadcap accredited and LTI is on the NIST Qualified Manufacturers List for Charpy V-notch Impact verification specimens. Traceability of sample material and specimens is insured from receipt inspection through the final process.
LTI specimen machining is a time-saving convenience for testing customers. These services also are available to companies that perform testing inhouse. Refer to our Sample Size Guidelines when ordering.
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