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UNIVERSAL TESING MACHINES
- Load cells are commonly used in materials and product testing applications. This type of testing requires a method of controlling the sometimes complicated test routines. Universal test machines often fulfill this purpose and are available from many manufacturers.
- Universal testers provide a means to hold a sample and place it under a compressive or tensile load. They can be simple motorized machines, whereby the speed and direction arev controlled using manual controls, hand-operated machines where the operator uses a wheel or pump control to increase the force on the sample, right up to high-end computer-controlled test stands that can run sophisticated multistage test programs. They are available with hydraulic actuation or electro mechanical control using electric motors driving a ball screw arrangement, for a wide range of forces from 500 N up to 30 MN. They will perform complex calculations such as Young’s modulus and create and print reports. When used in conjunction with other instrumentation, such as extensometers, even more sophistication is possible. With the ever growing library of international test methods, universal testers are required to provide more functionality and more calculations. A few of the range of available universal test machines.
- Portable force measurement is catered for by a plethora of handheld instrumentation, from simple gram gauges to advanced force gauges that allow data storage and transmission as well as peak and trough calculations. Some will allow simple control functions for operating some test stands so that no computer is necessary.
ASM INTERNATIONAL AND THE ALLOY CENTER
ASM International has emerged as one of the strongest providers of numeric materials data sources, and those sources are generally in three formats or platforms, hard copy, disk (usually CD-ROM), and the Alloy Center on the Internet. As an example, one of the most extensive sources of high- and low-temperature data for aluminum alloys has recently been made available through a collaborative effort of ASM and the Aluminum Association in both the book Properties of Aluminum Alloys Tensile, Creep, and Fatigue Data at High and Low Temperatures 11 and a searchable CD of the same title. Other representative data sources from ASM
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TEMPERATURE CONTROL SYSTEM
How To Control The Temperature :
Temperature control is directed at maintaining electronic component temperatures within pre-scribed limits. This is accomplished by the proper selection and application of materials which are used to conduct heat from or to the components.
The primary modes of heat transfer are conduction, convection, and radiation. The dominant considerations for heat transfer material selection include thermal conductivity, chemical inertness, resistance to corrosion, and sometimes wear resistance, sublimation, thermal expansion, and the ability to resist ionizing and ultraviolet radiation
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EFFECT OF SPECIMEN GEOMETRY
The toughness, or resistance to crack growth, of a material is governed by the energy absorbed as the crack moves forward. In an extremely brittle material such as window glass, this energy is primarily just that of rupturing the chemical bonds along the crack plane. But as already mentioned, in tougher materials bond rupture plays a relatively small role in resisting crack growth, with by far the largest part of the fracture energy being associated with plastic flow near the crack tip.
A “plastic zone” is present near the crack tip within which the stresses as predicted would be above the material’s yield stress 'σY' . Since the stress cannot rise above 'σY' , the stress in this zone is 'σY' rather than that given. To a first approximation, the distance rp this zone extends along the x-axis can be found with (θ = 0) to find the distance at which the crack tip stress reduces to 'σY'
[ σy = σY = Ki/√2πrp ]
[ rp = K^2 i/2πσ^2/Y ]
This relation is illustrated. As the stress intensity in increased either by raising the imposed stress or by crack lengthening, the plastic zone size will increase as well. But the extent of plastic flow is ultimately limited by the material’s molecular or microstructural mobility, and the zone can become only so large. When the zone can grow no larger, the crack can no longer be
constrained and unstable propagation ensues. The value of Ki at which this occurs can then be considered a materials property, named Kic.
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