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

Issue link: http://amp.digitaledition.asminternational.org/i/211830

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Materials Characterization Primer MULTIAXIAL TESTING Howard A. Kuhn University of Pittsburgh Estimated Analysis Time Preparation of samples may take a few seconds for machining or blanking simple specimens to a few hours for machining complex configurations. Setup and alignment may take more time than uniaxial testing because multiple axes and loads are applied. Testing time is similar to that for uniaxial testing of yield properties, creep, and fatigue failure. In all cases, testing at temperatures other than room temperature requires a hold time at temperature to ensure uniformity throughout the sample. Limitations Test equipment and fixtures are specialized and not widely available. Test specimen configurations may be complex and costly to fabricate. Each actuator must be isolated in the test chamber for environmental and nonambient temperature testing. Testing is destructive. 34 Multiaxial testing is used to determine the effect of combined stresses on the load bearing capacity and deformation of materials for design, to develop quantitative formability criteria for failure under combined stresses, and to provide a platform for characterization of material failure mechanisms under multiaxial stresses. Applications include the determination of the effects of material variables on failure under combined stresses, and the determination of the effects of strain rate, temperature, and environment on material response under combined stresses. It is also used to formulate design limits for material failure by yielding, fracture, creep, and fatigue under combined stresses; to develop formability limits for sheet metals undergoing deformation processes; and to evaluate the effect of reinforcement orientation on failure of composites under combined stresses. Other applications include measurement of failure response of biomedical and biological materials to combined stresses, and evaluation of failure of geosynthetic fabrics under biaxial in-plane and out of plane loading. Samples Materials: Metals, alloys, polymers, ceramics, fabrics, and composites. Size and shape: A multitude of specimen shapes can be fabricated for various biaxial and triaxial stress testing. Common configurations include thin-walled tubes for testing under combined axial force and internal pressure, and cruciform specimens for biaxial testing with a machine having two independently controlled, orthogonal load axes. Specimen sizes are comparable to uniaxial tests. Multiaxial testing is also performed on components and assemblies. Specimen sizes range from nanoindentation to shipsize beam structures. Preparation: Specimens are machined from cast/wrought billets or plates, blanked from sheets, or consolidated ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 from powder metals or composite elements as well. Related techniques Simple tests, such as notched tension and uniaxial compression with friction, use simple specimens, but are limited in applied stress combinations. Hydraulic bulge and spherical punch tests use simple sheet specimens, but are limited in strain ratio capabilities. Porous and powder material response to combined shear and pressure is measured in shear cells. Geosynthetic fabrics are tested in specialty biaxial and out-of-plane test systems. Biological and cell structures are tested in micron-sized test apparatus. Building structures are tested under multi-axial seismic, thermal, wind, and hydrodynamic loads by specially designed machines for each application. Transport vehicles are tested under multi-axial dynamic loads on servo-hydraulically actuated test rigs.

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