Strain distributions, measurement

Physical Properties. Raman spectroscopy is an excellent tool for investigating stress and strain in many different materials (see Materlals reliability). Lattice strain distribution measurements in siUcon are a classic case. More recent examples of this include the characterization of thin films (56), and measurements of stress and relaxation in silicon—germanium layers (57). 

Many other examples of stress or strain measurements through Raman spectroscopy are still primarily qualitative [18, 27]. Much of this stems from the fact that Raman spectroscopy provides only limited additional information (generally only in the form of frequency shifts) from potentially complicated strain distributions. Furthermore, care must be taken when extracting stresses from measured Raman shifts as key mechanical properties such as Young s modulus (which is related to the compliance or stiffness matrix elements) may be diameter dependent in NWs [61]. Still, Raman mapping with submicron spatial resolution and careful polarization analyses may help clarify the piezospectroscopic properties of semiconductor NWs in ongoing research. 

A method for determining the particle size distribution from a single X-ray diffraction profile when strain is present was applied to co-precipitated nickel oxide on alumina and silica. Appreciable strain occurred in the NiO, possibly due to the pressure developed in the small particles to balance the surface tension forces and the distortion produced by the deformation of the f.c.c. structure into a rhombohedral form. Apart from errors in the size distribution created by neglected lattice strain, the measurement of strain itself is important because its correlation with catalytic activity has been suggested. 

For example, a delamination in a composite layup produces strain distribution in the surroimding area that is substantially different from that in a pristine zone [20]. Strain monitoring can be done with conventional electrical resistance strain gauges or with fiber optics. The latter offer the capability of having several measuring locations that can be individually addressed on the same fiber. Kesavan et al. [20] used conventional strain gauges to monitor delaminalion in a composite T-joint. The strain gauges were placed on the outside of the joint in zones deemed sensitive to strain redistribution due to delaminations. 

Distribution measurements show that the silver complex of dr-cyclo-octene is less stable than that of cycloheptene, presumably owing to more ring strain in the latter 129,130). tronr-Cyclooctene is considerably more strained than the cis isomer, and can be separated from it by extraction with 20% aqueous silver nitrate 32), but there are no quantitative measurements of the stability of the silver complex of the tram isomer. The interesting possibility of isomerizing cis to trom-cyclooctene via metal complexes has not yet been achieved. tronr-Cyclooctene has been resolved via its platinum(II) complex with the optically active amine l-phenyl-2-amino-propane (am), 7r-CgH]4PtCl2am. 29). 

Figure 35. Strain measurements (in %) at beam midspan, taken at two loads of 5 kN and 30 kN. The two drawings at the top are for the reference beam (conventional GL24h) the strains are linearly distributed over the beam height. At the bottom, the results for a selected bipartite beam with adhesive layer 027 are shown the interrupted linear strain distribution confirms the theoretical prediction of stress redistribution.

The punch height at fracture H) which is influenced by limit strain and strain distribution of a material has been used as a measure of stretch-bend formability. The conditions are selected in such a manner that the fracture takes place in the region of punch contact. All the testings are performed in the dry condition [59]. 

Typically, the yardstick for qualitatively measuring the internal resistance of an adhesive bond to an external load has been the determination of the strain distribution in the adhesive and adherends. This is a difficult task. Even in simple lap joints, the actual stress-strain distributions under load are extremely complex combinations of shear and tensile stresses, and are very prone to disturbance by non-uniform material characteristics, stress concentrations or locaUzed partial failures, creep and plastic yielding, etc. It is extremely difficult to accurately measure the strains in adhesive joints with such small glue Une thicknesses and such relatively inaccessible adhesive. Extensometers, strain gauges, and photoelasticity are being used with limited success." ... 

The mechanism of stress transfer from the plastic matrix to the wood particles and vice versa was investigated by Sretenovic et al. [68]. They measured the strain distribution around the particle using electronic speckle pattern interferometry (ESPI). They also conducted an analytical analysis and finite element modelling of strain and stress distribution in the wood plastic composite. 

The embedded BOTDA strain sensing fiber can measure the strain distributions within slope mass effectively. 

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