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·Materials Characterization and testing
NDT of cyclically loaded metal structuresK.I. Yeryomin
Magnitogorsk State Academy of Mining and Metallurgy
Lenin av. 38
Magnitogorsk, 455000, Russia
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 .
The tensosockets were disposed in the following way:
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 Dsb1 = 0.36sy. A higher swing of the basic stresses is in a zone of a stiffening rib reinforced by a plate Dsb1 = 0.41sy, Dsb1 = 0.65sy.
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 0.1sy to 0.8 sy. Tensile stresses at the primary truss lower chords of the trussed transporter cranes vary from 0.04sy to 0.6sy 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 0.6sy 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 0.7sy when loading directly a rigid support by a loaded trolley; basic tensile stresses reach 0.2sy 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 1.02sy and 0.7sy 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 ±0.8sy. 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:
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