Noncontacting measurements devices

The tensile test is the most commonly performed test. Artifacts in measurement are seen because of deformation of material in the grip region. To overcome this problem, dogbone-shaped specimens are used. Truly accurate measurements, however, require the use of extensometers, localized displacement-measuring devices. Extensometers may be contact or noncontact. For design purposes, it is also of interest to measure the lateral contraction of the plastic in response to the applied tensile stress. The ratio of lateral contraction to tensile extension is termed Poisson s ratio. 

IR measurement provides an accurate, noncontact method for measuring device temperature. Depending upon base-temperature stabilization time, the test time can range from as fast as 1 to over 5 min. 

Noncontacting Electrostatic Measurements These measurements are made by instruments that respond to the electric fields at their sensing electrodes. Gonsiderable care must be taken in the interpretation of the measurements. The three general types of devices are described as follows. 

Electronics has, in fact, been a very fertile area for SEM application. The energy distribution of the SEs produced by a material in the SEM has been shown to shift linearly with the local potential of the surface. This phenomenon allows the SEM to be used in a noncontact way to measure voltages on the surfaces of semiconductor devices. This is accomplished using energy analysis of the SEs and by direedy measuring these energy shifts. The measurements can be made very rapidly so that circuit waveforms at panicular internal circuit nodes can be determined accurately. 

Many industries regularly conduct repeat insult patch tests on human volunteers to evaluate topical irritancy. Groups of human volunteers are patched with test substance. One to five concentrations can be tested simultaneously, a wide enough range to yield results relevant to the usage. Cumulative skin irritancy is measured by applying patch applications each day for 3 weeks. Skin irritation is usually assessed visually, but blood flow and skin temperature can be measured objectively by laser Doppler flowmetry, ultrasound Doppler, heat flow disk measurement, sensitive thermocouple devices, or noncontact infrared radiative techniques. In these tests, dose-response curves can be obtained. Skin thickness can be measured with calipers as a measure of edema formation. 

Tissue Vibration. Noncontact methods are preferred for measuring the surface motion of tissues. Laser vibrometers are commercially available with sufficient bandwidth and resolution for most studies. A direct mass load on the skin, together with the skin s elasticity, forms a mechanical low-pass filter (see Simple Lumped Models in Sec. 10.3.1). If a device is to be mounted directly on the skin, it must be of low weight (e.g., <3 g) and possess a comparatively large attachment area (e.g., >S cm ), in order for vibration to be recorded without attenuation of the motion at 80 Hz. An upper frequency limit of 200 Hz is theoretically achievable (—3dB) with a transducer and skin mount weighing 3 g and an attachment area of 1.8 cm. ... 

Electrical conductivity measurements have also been developed as tomography (104). Eddy-current testing (ET) of CFRP laminates is feasible (105). ET is noncontact and NDT, as long as thermal effects fi om resistive heating and energy dissipation are sufficiently small. Superconducting quantum interference devices (SQUID) have been used for ET to detect damage in CFRP (106). However, SQUID... 

Ultrasound echo ranging transducers can be used in either wetted (contact) or nonwetted (noncontact) configurations for continuous measurement of Hquid level (Fig. 18.19(a)). An interesting appHcation of wetted transducers is as depth finders and fish finders for ships and boats. Nonwetted transducers can also be used with bulk materials such as grains and powders. Radio-frequency and electro-optic Hquid level transducers are usuaUy noncontact, echo ranging devices, which are similar in principle and appHcation to the nonwetted ultrasound transducer. 

A major advantage of this method is that it is a noncontact and nondestructive way of measuring. Additionally, it is not expected to influence the electrical or chemical structure of the material. The technique is very simple, but fast, accurate, and versatile it can, in principle, be used in many environments. However, experience has shown that for practical sensor applications Kelvin probes are not suitable due to the necessary instrumentation and the limits to miniaturization. Most probes are rather macroscopic with a reference electrode the size of a few millimeters or centimeters. In addition, this method of measurement is complicated in comparison with other methods used in gas sensing and requires specific conditions and equipment. As a result, these devices are not available on the sensor... 

The Arcan and losipescu specimens can also be used as a joint where the adhesive is in a thin layer between two substrates. The test methodology is very similar to the bulk version. The adhesive displacement can be measured by noncontacting devices or extensometers of the type proposed by Dean et al. (1996). For devices that measure the adhesive and adherend displacement, a correction must be applied to have only the adhesive displacement. The stress distribution is not perfectly uniform, and an FE analysis can be used to have the value at the center of the specimen where the strain is obtained. Grabovac and Morris (1991) and Wycherley et al. (1990) used the losipescu method where two shaped adherends are bonded together. Weissberg and Arcan (1988) proposed a joint test based on the Arcan configuration. More recently, Cognard et al. (2005) proposed an Arcan joint method to produce not only shear but also biaxial stress states. 

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