: X-ray diffraction data to evaluate the welding residual stresses variation in welded joints under cyclic loads
This article presents the results for the measurement of the residual stresses by means of X-ray diffraction in butt-welded specimens subjected by cyclic loads. To do it a procedure was proposed and a study case was developed. The material studied was AISI 1018 steel. The process used to join the plates was semiautomatic welding. Residual stresses were measured at zero, 2 500, 5 000, 7 500 and 10 000 load cycles respectively. For this a diffractometer XRD; PANalytical Empyrean model was utilized to obtain a residual stresses and a MTS810 servohydraulic testing machine to mechanical test. The mechanical characterization of the joint was performed to determine the parameters of the fatigue load cycle. The residual stresses were measured before starting to apply the cyclic loads and subsequently they were measured again every 2 500 load cycles. As the main result of the application of this procedure to the joint studied, it was obtained that in the analyzed position, the residual stresses were compression and increased in absolute value with the action of the cyclic loads. For each diffractometric are showed a data of the position and the light intensity. Also are showed the incidence angle and the distance between adjacent lattice planes.
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The specimen used in this investigation has a "dog bone" shape. To build the specimen, two AISI 1018 steel of 4,7 mm thick plates were joined by semiautomatic welding. The measurement of residual stresses was performed on an X-ray diffractometer (XRD; PANalytical Empyrean model) with CrKα1 radiation (λ= 2.28976 Å) operated at 45 kV and 40 mA. A Material Testing System MTS810 hydraulic machine was used to do the cyclic loading tests. To determine the intensity peak of the light to be used to determine the residual stresses, a sample of the base material was first run on the diffractometer, using radiation CuK utilizing a detector Pixel 1d in a geometry Bragg Brentano. The interval of the angle of sweeping used for each sign went from 5º to 140º with a step of sweeping of 0,016º and 10 seconds for step in a continuous mode. The incident ray passed through a little grate monocromatic of 0,04 rad, a little divergence grate of ½º, a mask of 10 mm, and a little grate anti dispersion to 1º. The diffracted ray dried through a little grate monocromatic of 0,04 rad, a little divergence grate of ½º and a beta nickel filter. Before explained was in order to select the angles of the peaks the determination of residual stress. The selected peak was 65,005º. These data when utilizing cupper's wavelength. After Chrome's radiation was utilized with another configuration to measure residual stress. In this case, an Euler's cradle of five axes, while in the optics of diffraction a Xe proportional detector and a collimator of parallel plates was used. In the incidental optics beam a Parallel beam X-ray lens was employed. By means of the Bragg´s law is possible to know the peak of the intensity of the light to be analyzed when a Cr radiation is used. This allowed knowing that the peak of the intensity of the light to be analyzed with the angle of incidence equal to 106º. After knowing the peak to be analized, the welded specimen was then placed in the support device in the diffractometer and in this way the residual stresses were determined in the location shown. After obtaining the value of the residual stresses, the specimen was disassembled from the diffractometer and it was placed in the MTS810 testing machine to apply a first block of 2 500 load cycles. After this, the action of the cyclic load was stopped, and residual stress was measured again. This procedure was repeated every 2 500 load cycles until reaching 10 000 load cycles. Three welded specimens was tested at tensile test under conditions of force control to determine to determine the yield limit and the break point for the welded joint. In this way, the average yield and ultimate limit, that were equal to 360 and 703 MPa, respectively. With the yield limit value was calculated the parameters of the load cycle. The maximum load was 80% of the elastic limit of the material. That is a maximum load Pmax = 36 845 N and minimum load Pmin = 3 684,5 N. The cycle asymmetry coefficient was 0,1.