Residual stresses determination in thin-walled structures by combining the hole drilling method and reflection hologram
interferometry is considered as a tool with some unique properties in the field of industrial inspection. The relations
used for converting experimentally derived parameters into stress values of interest are presented. Required input data
are obtained by simultaneous measurements of probe hole distortions in two principal strain directions on opposite sides
of thin plane specimen. Emphasis is made on obtaining high-quality interferograms with high fringe density around
small probe hole drilled in residual stress field. Such fringe patterns are capable of describing residual stress
components of high level in a presence considerable stress gradients. It is shown that a resolution of ten fringes can be
achieved over a hole of 1.5 mm diameter. Practical implementing developed technique is illustrated in the course of
residual stresses characterisation near different welded joints of thin aluminium plates.
Sophisticated technique for reliable quantitative deriving residual stress values from initial experimental data, which are inherent in combined implementing the hole drilling method with both holographic and speckle interferometry, is described in detail. The approach developed includes both possible ways of obtaining initial experimental information. The first of them consists of recording a set of required interference fringe patterns, which are resulted from residual stress energy release after through hole drilling, in two orthogonal directions that coincide with principal strain directions. The second way is obtaining a series of interrelated fringe patterns when a direction of either observation in reflection hologram interferometry or dual-beam illumination in speckle interferometry lies arbitrary with respect to definite principal strain direction. A set of the most typical both actual and analogous reference fringe patterns, which are related to both reflection hologram and dual-beam speckle interferometry, are presented.
There are some topical mechanical problems in solving of which remarkable features of various holographic interferometric techniques are capable of ensuring an actually new level of data obtained comparing with other experimental methods. One of these problems is a detailed quantitative description of local elasto-plastic strain history in contact interaction zone. Holographic interferometry is still a very complex and time-consuming method for strain/stress analysis, even if modern tools for computer-aided fringe patterns evaluation are available. That is why in most cases holographic techniques using for a predictive strain/stress analysis have to be supplemented with corresponding numerical techniques thus forming so- called combined or hybrid approach. The presentation is devoted to main questions related to a choice of appropriate numerical models which are capable of high-accurate reproducing a local strain history under regular cyclic loading. The main objective of constructing such a model is a determination of 3D local strain fields to within 3% in order to ensure developing new refined techniques for evaluation of fatigue damage accumulation. Holographic interferometry data are consecutively used during a whole step-by-step procedure, first, to choose an initial model and, then, to construct and to validate a set of advanced models.
The paper contains a brief description of main principles of compensation speckle interferometric technique with an emphasis is made on their implementing in non-traditional fields of material testing. The main essence of optical compensation consists of the fact that routine quantitative interpretation of fringe patterns is substituted with more accurate measuring the geometrical parameters of the interferometer set up corresponding to the compensation instants. Metrological basis of this system is the electronic speckle pattern interferometer combining two speckle fields with both an object and a reference speckle pattern is created by a ground glass diffuser. A visualization of fringe patterns related to a small difference between two deflection fields of plane specimen under pure bending is performed by digital subtraction of corresponding intensity fields derived from CCD-camera. Some examples of Poisson's ratio and elasticity modulus determination for composite materials and their evolution due to internal humidity level changing are also presented.
Some methodological and metrological problems related to both high-quality fringe patterns recording at a proximity of fatigue crack of different length and following local strain fields determination are the first subject of the paper. The second question considered is a choice of the way of local strains representation in a form which is the most suitable for a determination of fracture mechanics parameters making emphasis on J-integral evaluation.
New capability of combined application of time-averaged holographic interferometry and defocused speckle photography, allowing us a comparative analysis of local stresses corresponding to different vibration modes, is considered. The main aspects of the technique developed are illustrated by using as an example bending oscillations of thin square plate with a large central circular hole. Experimental results are compared with corresponding numerical data obtained by means of the MSC/NASTRAN software.
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