Nondestructive stress-strain

Both destructive and nondestructive measurements can be done on an Instron Material Tester. In this system, the sample is loaded in a test cell, and the compression or tension force is measured when the upper part of the cell is moved over a given distance (time). Within the elastic limit of the gel, the elastic modulus E (or gel strength) is obtained from the initial slope of the nondestructive stress/strain curve additional deformation results in the breakage of the sample, giving the characteristic parameters—yield stress and breaking strain. 

This photoelastic stress analysis is a technique for the nondestructive determination of stress and strain components at any point in a stressed product by viewing a transparent plastic product. If not transparent, a plastic coating is used such as certain epoxy, polycarbonate, or acrylic plastics. This test method measures residual strains using an automated electro-optical system. 

Most physical tests involve nondestructive evaluations. For our purposes, three types of mechanical stress measures (Figure 14.7) will be considered. The ratio of stress to strain is called Young s modulus. This ratio is also called the modulus of elasticity and tensile modulus. It is calculated by dividing the stress by the strain ... 

The indirect analytical nondestructive evaluation methods of interest are thermal analysis, finite element stress analysis, strain gauging, photoelasticity and brittle coatings. 

Other methods available for nondestructive evaluations are indirect in nature, but may be useful in certain situations. These methods will only be discussed in a cursory fashion and more detailed aspects can be obtained from the literature. It is useful to note that stress in any structure plays a vital role, and that the methods involved in determining stress in an engineering component or structure in the process conditions is of vital importance. The methods used in determining stress are finite element stress analysis, strain gaging, the photoelastic method and brittle coatings. 

Time-resolved fluorescence using 9-methylanthracene (9MAn) as a photoluminescent probe was used to detect residual stresses on polymer products. The fluorescence lifetimes of 9MAn in this nondestructive measurement were correlated with the stresses, not the strains, of the PVC samples containing carbon black and 9MAn. The test enabled the estimation of residual tensile stresses on... 

Fiber stress determination is of major importance for the modeling of composites and it is now weU established that Raman microspectroscopy, with its main advantage being its nondestructive nature, makes it possible in composites. First, Galiotis and Young demonstrated that Raman spectroscopy is an excellent method to follow the deformation of aramid and carbon fibers. This is a result of variation in the (stretching) vibrational wave number, as a consequence of the anharmonicity of the interatomic bonds. The relationship linking Raman wave number shifts (Av) to the tensile strain (Ae) is hnear, Av = TAe. 

Nondestructive tests have certain advantages Including the ability to measure the elasticity of the gel on products such as jellies containing particulates. Nondestructive tests can be used to blend raw pectins to a consistent jelly grade. Also, some reports have found nondestructive Instruments to be less expensive and more reliable (63). Mitchell (60) reported that large deformations outside the linear region of a stress vs. strain curve are more difficult to Interpret and more difficult to measure than small deformations because rupture occurs at a defect In the gel and large deformations are not as reproducible. 

Different nondestructive techniques are used to evaluate the stress level in products. They can predict or relate to potential problems. There is the popular electrical resistance strain gauges bonded on the surface of the product. This method identifies external and internal stresses. The various configurations of gauges are made to identify stresses in different directions. This technique has been extensively used for over half a century on very small to very large products such as toys to airplanes and missiles. 

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