International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Strain Analysis of solid wood and glued laminated timber constructions by close range photogrammetryFranke, Bettina; Bauhaus-Universität Weimar, Germany
Hujer, Steffen; Bauhaus-Universität Weimar, Germany
Rautenstrauch, Karl; Bauhaus-Universität Weimar, Germany
Over the past years, timber engineering tried to evaluate the load-bearing capacity of structural members in areas of connections with mechanical connectors, opening and notches by methods of fracture mechanics. The assumption of the fracture mechanics parameters leads to problems in evaluating the load-bearing safety of structural units of full and board laminated timber in loaded areas and disturbance ranges using fracture mechanics. Parameters close to reality can be gained only from experimental investigations into the determination of the crack expansion and the crack length with subsequent FE- simulation. For application in timber construction, numerous theoretical and experimental investigations have been documented and they contain joints, notches and openings subjected to transverse tensile forces. Due to the insufficient appropriation of measured data gained by tactile position encoders in conventional measuring procedures, it is advisable to investigate crack growth with the help of photogrammetry. With the measuring system which was developed within a research project it is now possible compared with the procedures usually used, to measure the crack geometry for the determination of fracture mechanics parameters in the post-process exactly. Additionally a complete picture of the local deformations and the validity of a test can be increased clearly by the possibility of the varying number of discrete measuring points. The measuring system for the contactless and non-destructive determination of the coordinates of the measuring points consist of a high-resolution digital colour camera in combination with a telecentric measuring objective.
An especially developed procedure that is based on digital close range photogrammetry and image processing makes it possible to measure the progression of deformations, cracks and deterioration including the loading and relieving of the specimens. The validity of a test can be substantially increased by the applied measuring marks or a photo of the surface. The contactless determination of the coordinates is made by a photo sequence over the total duration of test and a following automated evaluation with the program OSPREY. As the evaluation is done in the post process, a frequency can be achieved in the test, of up to 2 Hz. Which leads to a good compression of the desired courses of the curves. The gathered displacements of the coordinates of the measuring marks or other geometrical marks are easily transferable for a following processing in a numeric simulation with an FE program. With the results achieved, which have an accuracy of few micrometres, new findings could be achieved in timber construction.
2 Development of the measuring system and the application routines
2.1 Components of the system
The selection of a suitable measuring objective is important for the measurement. The object to be measured should be represented as largely as possible allowing for the necessary measuring accuracy. The telecentric measuring objectives JENmetarTM 0.2x/12 and JENmetarTM 0.13x/12 used allow a maximum measuring field of 67 x 49 mm2 with an magnification of 1:5 and. 1:7.5 respectively. Both objectives have a parallel optical path in a range from 21 mm and 18 mm. This guaranteed a constant reproduction scale with the variation of the distance between objective and camera. On the basis of a pure 2D-measurement, the specimens can move slightly in the depth without creating a distortion of the measurement results. For a sufficient measuring tolerance, the shape of the measuring marks should constitute at least 10 pixels. A mark diameter of 2mm corresponds to approximately 120 pixels at a magnification of 1:7.5. Objectives with a smaller magnification provide a better clearness in the measuring field but they create fewer pixels in the figure of the marks and thus a higher measurement uncertainty.
Contrast and the evenly distributed illumination of the entire image field are decisive for the future results of measurement. Since the applied software requires a grey tone picture for the optical determination of the coordinates of the measuring points, the measuring field is illuminated by a source of infra-red light for the close infrared range. By means of an infrared passage filter, the complete visible spectral range of 400 - 800 nm is filtered away. The environment light has thus no more influence and the camera behaves, when simultaneously switching off the interpolation routines, like a grey tone camera with 5 million pixels. For marking the wood surface, the pretreatment of the surface with a white wax glaze in combination with transfer marks applied afterwards, turned out to be a good method after extensive analyses. A sufficient black-white contrast is obtained by this combination, which provides the necessary grey tones by the two dimensional infrared lighting. The transfer marks vary in diameter from 0.6 to 2.0 mm. A sufficient bond of the measuring marks with the surface could be proven on the basis of smaller test series. This procedure of marking the specimens does not interfere or even destroy their structure as penetrating marks such as pins or drawing pins do.
2.2 Application routine
The determination of the coordinates of the applied measuring points is carried out with the software program OSPREY. The basic procedure for the measurement with the CCD matrix camera is similar to the classical optical measurement with a tool microscope. The object to be measured is reduced in size and shown on the CCD matrix. The pixels on the CCD-chip of the camera are the solid measure, because their geometry is fixed. The program OSPREY serves the measurement of concretely geometry elements with the help of the coordinate measuring technique. The search range for the circle touching is stretched by a touching circular arc and a crowd of search rays arranged radial to the touching circular arc. From the points of outline, which found the individual search rays according to the used edge criterion, the centre of the measuring mark is computed.
In the current measuring tasks high-contrast object edges are touched, why the procedure -dynamic threshold value- is applied. With this criterion according to the analysis of the picture brightness in the measuring field the threshold value is determined automatically at the edge transition by means of the histogram. Experience has shown that the optimal separation between the dark object and the bright background is achieved. If the brightness of the background or the object changes, the threshold is automatically adapted to the new conditions. The dynamic threshold value is supplemented by the approximation of the edge to determine the edge place at the interface between the edge function and the dynamic threshold value subpixel exactly. With this procedure, a touching uncertainty of 1/5 pixel for the single point can be regarded as surely attainable.
The attainable reproducibility of the photogrammetric touching of the measuring marks with the system developed is, depending upon the objective used, subject to the standard deviation of 0.4 and 0.5 mm when the subpixel method is used. By means of calibration equipment for inductive displacement gauges the touching accuracy of the picture measuring system for individual measuring marks was determined to 1 mm. The calibration equipment used supplied a measuring resolution of 10-7m.
3 Fields of application of the measuring system
3.1 Modification of the measuring system with help of tests on determining the length and strain
Simultaneously, the influence of the creep effect occurring in the timber specimen under constant load stage was examined for succession of picture sequences as well as for a possible change of the camera position to enlarge the measuring field. As a result can stated that with timber specimens a change camera position during one load stage cannot be applied successfully, since the deformation state of the specimen does not stabilise sufficiently within a suitable period, an therefor several synchronised camera systems would be necessary for a possible enlargement of the measuring field.
The measurement of strain was first carried out at tension member following DIN 52188-1997. The dimensions and the arrangement of the point grid can be seen in figure 3. As a conventional measuring procedure a extensometer DD1 with as measuring accuracy of 0.05% was applied at the back.
For the evaluation of the measurement results the modules of elasticity for tension was determined in each case. In table 1, the attained results for the materials beech and spruce are compared. One to the relatively high modules of elasticity of wood and the smaller strain resulting from that, the good accuracy classes would, as expected, not be achieved at DIN-efficiency.
By increasing the load over the DIN range and additionally extending the measuring length, the accuracy of the determination of the longitudinal strain could be improves by up to one tenth. Due to the positions of spring wood and late wood in the cross section, the scattering of the accuracy of the modules of elasticity in additionally influenced by the inhomogeneity of the timber.
3.2 Measurement of the crack growth for the determination of fracture mechanics parameters of the material wood
In contrast to the usually common procedure to determine the fracture mechanics parameters at CT- samples in the post process, the measuring procedure developed in our project can measure the geometry of the cracks in the measuring photos exactly. In the experimental investigations, the crack propagation could be determined exactly to within 10mm. With the attained lengths of the crack and their transmission to an FE simulation by means of the program ANSYS(r), it was possible for the first time to determine the curves of the resistance to cracking for wood.
3.3 Examination of the transverse tensile range at the notched structure
The relocation and or the reduction of the perpendicular tensile stresses in areas which are further from the notch and less stressed can be shown with this new measuring procedure. In comparison with that, in figure 7, the flow of the perpendicular tensile stress is shown, which determined in an FE simulation with the program ANSYS(r). One can clearly see, that our measuring procedure allows the determination of strain gradient in a realistic way. Because of the size of the measuring field available at the moment, it was possible to obtain a complete image of the entire area subjected to transverse pull/ tensile stresses perpendicular to the grain and of the crack development. By additionally record the crack growth we could however in combination with a numeric simulation, determine curves of the resistance to cracking. To observe the cracks further and thus to compute the curve progression complete, an enlargement of the measuring field and or the parallel employment use of several measuring systems would be necessary.
3.4 Examination of kink band at compression tests with wood
Figure 8 shows the measuring field in the deformed condition in the one hand and the determined compression compared with there unloaded condition on the other hand. The formation of the kink band starts after 100s, when the maximum load is reached, which can be seen in a clear increase in the displacement in section 14. At 300 s as can be seen in figure 8, the kink band extends first into section 11 then into section 9 and 12 and the compression gets up to 2.7 mm.
4 Areas of future applications
In many fields of civil engineering the photogrammetry measuring system that has been developed within the researched project can lead to new experiences. The test series that have already been completed provide a lot of results which contributed a considerably to new findings in timber constructions and to opening new opportunities in the field of experimental analyses of stress and of experimental technique. Especially the cracking of concrete units seems to be a worthwhile field of work.
The application will extend to the continuation and the future development of the experimental investigations into the determination of the curves of the resistance to cracking for wood and to the determination of important parameters for the fracture mechanics simulations. This measuring procedure will also be used to evaluate the load bearing performance of connections with mechanical fasteners in timber constructions more exactly. Hereby the evaluation of the load bearing capacity of cross connections including of crack growth and the photogrammetry pictures of relative displacements of the fasteners is a main area of investigations.