Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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Materials Characterization Primer Fracture mechanics is used to determine the effect of defects on the load bearing capacity of materials and structures, to determine fracture toughness of materials under monotonically increasing loading, and to provide a quantitative assessment of material fracture toughness for design purposes. Applications include the determination of the effects of material variables on the fracture toughness and the determination of critical defect sizes in structural components. It is also used to provide an end of life point in a life prediction based on a fatigue crack growth and to provide an input for the evaluation in a structural reliability or fitness for purpose. Fracture mechnics is also used as a tool in failure analysis investigations. Samples Materials: Metals alloys, polymers, ceramics, some composites. Size and shape: Machined per stan- dard test procedures; standard shapes are the compact tension and three-point loaded bend. Preparation: Specimens are machined per instructions in standard test procedures and pre-cracked using cyclic loading before the test is conducted; side grooves can be machined in after the precracking is completed. FRACTURE MECHANICS TESTING John Landes University of Tennessee, Knoxville Limitations Materials with low toughness can be analyzed using linear-elastic parameters; materials with high toughness must be Estimated Analysis Time • Preparation time for machining samples may take a few hours. • The pre-cracking procedure is labor intensive and may also take a few hours. • Testing generally takes less than one hour. However, testing at temperatures other than room temperature requires heating or cooling and a soak time at temperature to ensure a constant temperature throughout the test sample. • The analysis of results can usually be done in less than one hour. analyzed using nonlinear, elastic-plastic parameters. Steels with ductile to brittle transition and tested in the transition regime must be analyzed using statistical methods. Brittle fracture behavior gives a distinct point of fracture; ductile fracture behavior results in a stable process without a distinct fracture point. The analysis requires a construction procedure given in the standard test procedures. The test is destructive. Related techniques Impact testing, such as Charpy Vnotch, which could provide an estimate of fracture toughness. Fatigue crack growth rate testing is required if the fracture toughness is to be used for the evaluation of structural life. Other fracture toughness measures include rapid load tests, crack arrest tests, and chevron-notched specimens. ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 33

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