·Table of Contents
·Methods and Instrumentation
Non-Destructive quality control and determination of residual stresses in elements and members of machine-Building Structures using the methods of holography, Electron Speckle-Interferometry and Shearography
Lobanov L.M., Pivtorak V.A., Oleinik E.M. and Kiyanets I.V.
(E.O.Paton Electric Welding Institute, Kiev, Ukraine)
(National State University, Kiev, Ukraine)
To provide the high-quality manufacture of structures is one of the most important scientific-technical problems which becomes more and more actual with a complication of structures. This problem is multi-sided and can be solved in several directions. Among them, the responsible role belongs to the development and wide application of advanced methods and equipment for a non-destructive control and determination of residual stresses, whose effectiveness is defined, first of all, by their reliability and efficiency.
The E.O.Paton Electric Welding Institute has designed and manufactured small-sized devices, instruments and optical schemes which can be arranged on the objects being examined, not using a special vibroinsulation. In collaboration with the T.G. Shevchenko National University a small-sized thermoplastic camera has been developed, in which a new composition of a film of amorphous molecular semiconductors (AMS), which is deposited on a glass plate, is used for recording the holograms. One such plate with a coating allows recording of more than 1000 holograms without its replacement (record-erase cycle).
The application of fiber optics in optical schemes of interferometers reduced to minimum the amount of optical elements and allowed a special vibroprotection of the holographic devices to be refused. Thus, for example, for a nondestructive holographic quality control of adhesive-bonded thin-walled pipes (100 mm diameter and 4 mm wall thickness) made from composite materials, a laboratory compact unit (Figure 1), which is fastened rigidly to the element examined and includes a device for an elastic bending, a recording medium (thermoplastic camera) and a light guide with an optical element, was manufactured .
Fig 1: General view of holographic unit for nondestructive quality control of elements of tubular structures.|
The holographic method made it possible to perform a nondestructive control of the quality of flat aluminium element welds produced by a new promising method, a friction stir welding (FSW) . However, when FSW is used, it is very important to provide a reliable quality control of welded joints, because the hard-to-reveal microdefects can occur which cannot be revealed by the known methods of control (X-ray, ultrasonic). Mechanical loading of welded flat aluminium specimens was realized using a special device given in Figure 2. Interference fringe patterns, shown in Figure 2, characterize the presence of inner microdefects at separate regions of the weld made with the FSW method.
Application of loading with an inner pressure is very effective in quality control of welded high-pressure cylinders used in motor transport. Figure 3 presents: a general view of a welded thin-walled cylinder made from high-strength steel (Figure 3,a); interference fringe pattern obtained at inner pressure changing in the cylinder and characterizes a localization of displacements at the region of flaws revealed (Figure 3,b). With increase in the inner pressure a fracture of the thin-walled cylinder in the zone of defects was observed (Figure 3,c). The application of heat loading for the quality control of a thin-walled lining of the aircraft fuselage which has one-sided access is challenging. Corrosion of material at the inner side of the lining, especially in places of contact of rigid sections and lining, is one of hazardous defects at long-time service of aircrafts. Figure 4 shows a general view of 1700x1700x3.5 mm element of the aircraft lining with rivets and interference fringe patterns which characterize the defectless zones and regions of presence of a lining corrosion (thinning of walls above 45 %).
| Fig 2: General view of a device for loading specimens (a) and interference fringe patterns which characterize the defective regions of the specimen, where b - defect-free region, c - defective region.|
Fig 3: General view of a welded thin-walled cylinder (a), interference fringe pattern which characterizes the defective region (b) and fracture zone (c).|
The important problem in manufacture of different structures is residual stresses which lead in many cases to their failure. Different methods of determination of residual stresses and also the problems of their measurement are known . The method of holes found the widest application. To measure the field of displacements, caused by a relaxation of stresses in the zone of a drilled hole, a method of holographic interferometry was used . To determine the residual stresses we have designed and manufactured different compact holographic units which can measure the fields of displacements in the stress relaxation zone (Figure 5). With the technology developed the residual stresses were determined in a framework of fastening the aircraft engine turbines, made from a magnesium alloy, after their repair. The experiments showed that the cracks, associated with a gradient of residual stresses, are often initiated after the repair welding. The general view of the framework, fringe patterns which characterize the residual stresses and values of residual stresses are given in Figure 6.
| Fig 4: General view of element of aluminium lining of aircraft (a) and interference fringe patterns which characterize the defective regions, where: b - defect-free region, c -region with a lining corrosion.|
| Fig 5: Compact holographic units of different design for determination of residual stresses|
The further development of methods of the optical holography promoted the creation of the new methods of nondestructive control and determination of residual stresses based on electron processing of the optical information. These are methods of electron speckle-interferometry and shearography [4,5]. In our investigations the method of the speckle-interferometry was used to determine the residual stresses in welded joints. The residual welding stresses in a AMg6 alloy welded T-joint made by the argon arc welding were determined using the method of an electron speckle-interferometry. The general view of the element examined, a specklogram and value of residual stresses are given in Figure 7.
Fig 6: General view of a framework for fastening aircraft engine turbines and fringe patterns which characterize the value of residual stresses|
The method of electron shearography was used by us for the nondestructive quality control of the structure elements. Shearography allows determination of deformation without the numerical differentiation of data. In addition, its main advantage is a low sensitivity to vibrations. With this method the nondestructive quality control of adhesive-bonded elements of honeycomb was made. Shearograms which characterize the quality of honeycomb elements are shown in Figure 8.
Fig 7: General view of element examined, specklogram and value of residual stresses at the region of examination|
The above-mentioned optical methods, compact devices and instruments make it possible to perform the nondestructive quality control and determination of stressed state of elements and members of structures made from metallic and composite materials using the contactless method. The application of methods of electron speckle-interferometry and shearography opens up new opportunities for the nondestructive diagnostics of structures under the conditions of their manufacture and service.
Fig 8: Shearogram which characterizes the quality of the honeycomb panel element|
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