Test Specimen Machining for Destructive Testing

February 15, 2012

Test specimens machined at LTIDestructive 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 test specimensTurning 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 Test SpecimensMilling 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 test specimensGrinding 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.

Tagged as: specimens, test coupons, test specimen machining, test specimens

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Metals Testing and Certified Test Reports

September 6, 2011

Metals testing is generally performed to ensure the integrity of the metal product, part or sample that is being evaluated. Testing and the accompanying Certified Test Report are often required by industries such as aerospace, military, medical and others where safety and reliability are of utmost importance. Many businesses use metals testing as a quality assurance measure to be sure their manufacturing process is meeting their requirements. Customers will frequently request testing and a certification before purchasing products to be sure the items will meet their needs.

Metals testing can be performed on raw materials or on end products before use, during service or when a failure occurs. Metals testing methods are classified as destructive or nondestructive testing, referring to whether or not the test sample remains intact or is altered in order to perform the analysis.

Destructive metals testing is usually performed on a test specimen or coupon that is cut and machined to specification or on a sample that was dissolved from the original product in a chemistry lab. In all cases, the item is destroyed, hence the name destructive test. Destructive testing is further classified into mechanical testing, physical testing, metallurgical testing and chemical analysis. These test methods or processes are used to achieve a range of results:

  • Verify mechanical properties
  • Examine microscopic product features
  • Identify flaws and defects
  • Reveal elemental makeup or composition
  • Determine susceptibility to corrosion

Nondestructive testing of metals does not harm the sample, even though it may be exposed to water, penetrants or particles during the test process. Common types of nondestructive testing include X-ray inspection or radiography, ultrasonic testing, eddy current testing, liquid penetrant inspection, magnetic particle inspection, visual inspection, hydrostatic pressure testing and positive material identification. Nondestructive tests can be performed on all types of metal products including tubing, pipe, castings, plate, fasteners and other component parts. These tests primarily identify surface and subsurface defects or inconsistencies.

Metals testing serves many purposes and answers many questions, but always includes a Certified Test Report with documented test results.

Tagged as: certified test report, destructive testing, metal testing, metals testing, nondestructive testing, test report

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About Laboratory Accreditation

April 11, 2011
laboratory certificate of accreditation

What is accreditation? Accreditation, in terms of a testing or calibration laboratory, means that a recognized agency or organization (e.g. A2LA, PRI/Nadcap, NAVLAP) has assessed the competency, procedures, processes and equipment of the laboratory using set criteria and has determined that the company meets international standards. Accreditation is official recognition and public notification that the [...]

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Why is Materials Testing performed?

November 30, 2010
tensile testing for material strength

  Materials testing is performed for a variety of reasons and can provide a wealth of information about the tested materials, products or specimens. Many products are used in critical applications where a failure could result in extensive damage or injury. Testing is often required for component materials that go into aerospace, nuclear, defense, medical, [...]

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