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NDT of cyclically loaded metal structures K.I. Yeryomin M.V. Nashchokin Magnitogorsk State Academy of Mining and Metallurgy Lenin av. 38 Magnitogorsk, 455000, Russia Contact

The necessity of giving industrial buildings and construc-tions safe service life is an important engineering problem. Durin operation cyclically loaded metal structures build up mechanical, corrosion and fatigue deteriorations; the change of structure loading conditions as well as of properties of steels and welded joint zones take place.

As a result of the inspections performed [1, 2] the operation process of the most of cyclically loaded metal structures such as crane secondary trusses (CST) of the oxygen converter steelmaking, deck girders of transporter cranes and others was found to be carried out with fatigue cracks existing in the welded joints, zones of a thermic effect of welding and in the basic metal. For evaluation of the residual life from positions of a failure mechanics first of all it is necessary to have data of the actual state of stress and strain (SSS) of the material in zones of possible fatigue failures. For the experimental investigations crane secondary trusses with spans of 24 and 36 meters and deck girders of the transporter cranes the Magnitogorsk Steel Plant, Orsk-Khalilovsk Steel Plant and Petropavlovsk TETs-2 have been chosen.

For the purpose of studying the actual state of stress and strain, evaluating the danger of the revealed defects, finding the most probable structure failure zones as well as evaluating the efficiency of the efforts made the static tests of the abov-mentioned structures were prepared and performed using tensometric apparatus - information-measuring system SIIT-3 operating in an automatic mode. While doing tests the tensoresistors were used with the base of 5 and 10 mm and resistance of 100-200 Om.

On the setting scheme the tensoresistors were united into rectangular tensosockets in accordance with the recommendations [3].

The tensosockets were disposed in the following way:

1. When there are cracks in the structure - on both sides of the cracks, at the apexes and on a supposed way of their spreading. Sometimes for the study of SSS at the crack apex the tensoresistors with a base of 2 mm arranged into chains were used [1].
2. At the places of the structure reinforcing - on the reinforcing units and along them, at the apexes of welded cracks.
3. In the structures without visible cracks - in supposed zones of the origin and spreading of the cracks.

As a result of the studies carried out the most heavily loaded parts at the crane secondary truss supporting joint of the division of a continuous casting of steel have been found to be an upper tension flange at the conjugation place with the supporting diaphragm and the wall. The basic stress swing in the flange is Ds_b1 = 0.36s_y. A higher swing of the basic stresses is in a zone of a stiffening rib reinforced by a plate Ds_b1 = 0.41s_y, Ds_b1 = 0.65s_y.

With double-sided loading of CST supporting joint the level of stresses becomes 2.0-2.5 times higher. In the result of the processing of experimental studies data the following has been established: at the primary truss upper chords of the trussed transporter cranes tensile stresses vary from 0.1s_y to 0.8 s_y. Tensile stresses at the primary truss lower chords of the trussed transporter cranes vary from 0.04s_y to 0.6s_y when the span is being loaded.

The most heavily loaded members of the lattice are bearing diagonals at the rigid supports in which compressive stresses reach 0.6s_y when the bracket on the side of the rigid support is being loaded. For the transporter crane with a composite system of the span the maximum values of the basic compressive stresses at the upper chord reach 0.7s_y when loading directly a rigid support by a loaded trolley; basic tensile stresses reach 0.2s_y when loading the bracket on the side of a flexible support. The maximum values of tensile stresses are at the points located at the crack encls 1.02s_y and 0.7s_y when loading the bracket on the side of a rigid support.

The most heavily loaded deck I-beams are the beams located in the middle of the lattice transporter crane bridges. These deck girders operate with the alternating load the stress reaching ±0.8s_y. The basic compressive stresses reach the yield point when a loaded trolleyis located on the bracket on the side of the rigid support.

The maximum stresses at vertical diaphragm fixing angles are horizontal components of the normal stresses which lead to cracking at the pick. These angles are overstressed and deformed by the alternating loading. The experiment showed that a considerable reducing of stresses at the welded crack mouth did not take place. In the result of the experimental works performed on studying the state of stress and strain of the cyclically loaded metal structures it has been established:

1. Zones of detected and possible fatigue failures in the investigated cyclically loaded metal structures are stress concentration zones, zones with the accumulated in the process of operation mechanical deteriorations as well as reinforcement zones performed leaving out the operation peculiarities of the structures during cyclic loading.
2. In the zones of fatigue failures of the investigated crane secondary trusses and transporter crane deck girders the swing of stresses from a cyclic loading of the structures reach Ds = 0.6¸1.0s_y which, taking into account the existing stresses from the dead weight and the stabilized residual welding stresses, proves the cyclic elastic-plastic, deformation of the material in the studied zones.
3. In the investigated crane secondary trusses and the transporter crane deck girders within one structure the material in each of the tested zones becomes deformed with individual on sign and value swing of stresses Ds and the coefficient of asymmetry of loading cycle R. With that the actual state of stress and strain of sections and joints of the structures changes in the process of operation owing to the appearance of deteriorations, reinforcing components, auxiliary fixings.

References

1. K.I. Yeryomin, S.A. Nischeta, M.V. Nashchokin. Study of the actual operation of cyclically loaded building metal structures. Magnitogorsk, 1996, 228 p,
2. A.B. Zlochevsky. Experimental methods in building mechanics. Moscow: Stroyizdat, 1983, 192 p, [book]
3. M.L. Daichik, N.I. Prigorovsky, G.H. Khurshudov. Methods and means of natural tensiometry. Reference book. Moscow: Mashinostroenie, 1989, 240 p, [book]

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