9 Komponentensicherheit
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- 9 Komponentensicherheit (81) (entfernen)
The so-called cyclic R curve, i.e. the crack size dependence of the fatigue crack propagation threshold in the physically short crack regime, is a key parameter for bringing together fatigue strength and fracture mechanics concepts. Its adequate determination is of paramount importance. However, notwithstanding this relevance, no test guideline is available by now and only very few institutions have spent research effort on cyclic R curves so far. The aim of the present paper is to give an overview on the state-of-the-art. Besides an introduction into the basic principles, the discussion will concentrate on the experimental determination on the one hand and questions of its application on the other hand.
Any fracture mechanics based determination of the fatigue strength of weldments requires different input information such as the local weld geometry and material data of the areas the crack is passing through during its propagation. The latter is so far not a trivial task as the fatigue crack is usually initiated at the weld toe at the transition from the weld metal to the heat affected zone and it subsequently propagates through the different microstructures of the latter to eventually grow into the base material and to cause final fracture. This paper describes how the material input information has gained particularly for heat affected zone material by thermo-mechanically simulated material specimens for two steels of quite different static strength. The data comprise the cyclic stress-strain curve, the crack closure effect-corrected crack growth characteristics, long crack fatigue crack propagation thresholds, the dependency of the parameter on the crack length and monotonic fracture resistance. The substantial experimental effort was necessary for the validation exercises of the IBESS approach, however, within the scope of practical application more easily applicable estimating methods are required. For that purpose the paper provides a number of appropriate proposals in line with its check against the reference data from the elaborate analyses.
The work presents the procedure developed within the German research project IBESS, which allows for the fracture mechanics-based prediction of the fatigue strength of welded joints under constant amplitude loading. Based on the experimental observations of the crucial failure mechanisms, the approach focuses on the short crack propagation, where elastic-plastic fracture mechanics and the build-up of closure effects must be considered as well as the variability of the local geometry at the weld toe and the modelling of multiple crack interaction. Analytical solutions are provided for the approximation of the through-thickness stress profiles at the weld toe and for the determination of the crack driving force in the form of a plasticity-corrected stress intensity factor range ∆K_p. Proposals for the determination of the initial crack size and the crack closure factor are also included.
The approach is validated against a large number of experimental data, which comprises fatigue tests on individual cracks monitored by heat tinting and beach-marking techniques, as well as stress life curves. Three kinds of welded joints, two steels of significant different strengths and three stress ratios are considered. The results show that the procedure provides good estimations of the statistical distribution of the fatigue strength of welded joints both for the finite and infinite life regime. Furthermore, the predictions are compared with available benchmark data for structural steels.
The laser hybrid welding process offers many advantages regarding deep penetration, increased welding velocity and with the help of the supplied filler wire an improved bridgeability to gap and misalignment tolerances. High power laser systems with a power of approx. 30 kW are already available on the market. Nevertheless, multi-layer technology with an arc process is still used for welding of plates from a thickness from 20 mm. A potential cause is the process instability with increasing laser power. It is inevitable that gravity drop-out due to the high hydrostatic pressure at increasing wall thickness especially at welding in flat position and with a low welding speed. The surface tension decreases with increasing root width resulting from low welding velocities. To prevent such inadmissible defects of the seam a use of weld pool support is required. Usual weld pool support systems such as ceramic or powder supports require a mechanical detachment which is time-consuming. The electromagnetic weld pool support system described in this work shows an alternative weld pool support which works contactless. It is based on generating Lorentz forces in the weld pool due to oscillating magnetic field and induced eddy currents. This innovative technology offers single pass welds up to 28 mm in flat position and reduced welding velocity with a laser power of just 19 kW. It also leads to improved mechanical-technological properties of the seams because of the slow cooling rate. With usage of an electromagnetic weld pool support the limitation of the hybrid laser arc welding process in the thick sheet metal will be extend.
The hybrid laser arc welding (HLAW) process provides many advantages such as improved gap bridgeability, deep penetration and misalignment of edges, that is why the process is used increasingly in industrial applications e.g. shipbuilding, power plant industry and line-pipe manufacturing. The obvious encountered problem for single pass welding in flat position is the gravity drop-out at low welding velocities. With the usage of an electromagnetic weld pool support system, which is based on generating Lorentz forces within the weld pool, wide seams followed by reduced welding velocities could be achieved in this study leading to the realization of a gap bridgeability up to 1 mm, misalignment of edges up to 2 mm and a single pass weld up to 28 mm thickness with a 20-kW fibre laser. These developments expand the boundaries of the HLAW process for different industrial applications. As a result, less accurate preparation of the edges would be sufficient, which saves time for manufacturing.
Hybrid laser-arc welding of thick-walled ferromagnetic steels with electromagnetic weld pool support
(2018)
The hybrid laser-arc welding (HLAW) process provides many advantages over laser welding and arc welding alone, such as high welding speed, gap bridgeability, and deep penetration. The developments in hybrid laser-arc welding technology using modern high-power lasers allow single-pass welding of thick materials. This technology can be used for the heavy metal industries such as shipbuilding, power plant fabrication, and line-pipe manufacturing. The obvious problem for single-pass welding is the growth of the hydrostatic pressure with increasing thickness of materials leading to drop-out of molten metal. This phenomenon is aggravated at slow welding velocities because of increasing weld seam width followed by a decrease of Laplace pressure compensating the hydrostatic pressure. Therefore, weld pool support is necessary by welding of thick materials with slow welding velocities. The innovative electromagnetic weld pool support system is contactless and has been used successfully for laser beam welding of aluminum alloys and austenitic and ferromagnetic steels. The support system is based on generating Lorentz forces within the weld pool. These are produced by an oscillating magnetic field orientated perpendicular to the welding direction. The electromagnetic weld pool support facilitates a decrease in the welding speed without a sagging and drop-out of the melt thus eliminating the limitations of weldable material thickness.
The interaction of hydrogen with various tungsten-inert-gas-welded austenitic stainless steels’ (AUSS) microstructure is studied by means of desorption/absorption analysis and microstructure observations. One of the limitations of welding is created by the presence of hydrogen in the weld, which can shorten the steel’s service life. The local hydrogen concentration, trapping, and its distribution along the welded samples were studied by thermal desorption spectrometry and were supported by X-ray diffraction (XRD) and electronic microstructural observations. Hydrogen content demonstrated a dependence on the welding zone. It was found that hydrogen distribution, and accepted microstructure during welding, played a significant role in the trapping mechanism of 316L AUSS. XRD analysis revealed residual stresses which were caused due to the presence of hydrogen in c-phase. It was shown that the austenite microconstituents inside 316L can have a crucial effect in preventing hydrogen-assisted cracking phenomenon. The effects of AUSS microstructure on hydrogen absorption and desorption behavior are discussed in detail.
This paper represents the results for proposed optical flow method based on the Lucas-Kanade (LK) algorithm applied to two different problems. The following observations can be made:
- The estimated strain and displacement for conducted tensile test are generally very close to those measured with conventional DIC-technique.
- The LK technique allows measurement of strain or displacement without special selection of a region of interest.
Using a novel optical measurement technique together with the optical flow algorithm, a twodimensional deformation analysis during welding was conducted. This technique is the first to provide a measurement of the full strain field locally in the immediate vicinity of the solidification front. Additionally, the described procedure of the optical measurement allows the real material-dependent values of critical strain characterizing the transition to hot cracking during laser welding processes to be determined.
A simplified model for numerical simulation of laser metal deposition process with beam oscillation
(2018)
A model of laser metal deposition with beam oscillation has been developed. The proposed model consists of two coupled sub-models calculating the heat transfer in the deposited part and the free surface of the molten pool, respectively. The heat transfer simulation of the deposited part solves a three-dimensional quasi-stationary heat conduction problem. The free surface of the molten pool are determined by solving the Laplace-Young equation. The developed model enables the layer-by-layer prediction of the shape of the deposited part and the resulting temperature field. It is shown that for an oscillation amplitude equal to the beam radius the peak value of the heat flux decreases by about 53% and 73% in the case of lateral oscillation and circular oscillation, respectively. Lateral oscillating laser beam results in a higher penetration depth due to the higher thermal efficiency. The amplitude of the laser beam oscillation effects the shape of the deposited wall and the deposition rate. A good correlation between the numerically calculated and experimentally observed results is obtained.
Two-dimensional residual stress mapping of multilayer LTT weld joints using the contour method
(2018)
Low transformation temperature (LTT) weld filler materials offer an attractive alternative to cost intensive postweld treatments as they can mitigate detrimental welding residual stresses during the welding process. Compared to conventional weld filler materials, LTT alloys are characterized by a delayed martensite transformation at low temperatures, which can result in compressive residual stresses in the weld. The high strength of these filler materials makes them potentially applicable to high-strength steels as well as for a large amount of requested repair works in steel structures. The focus of the study is on the confirmation of the LTT idea with regard to the residual stress state for multipass weld lines processed by metal active gas welding. It is demonstrated that the contour method is a well-suited technique for measuring the residual stress in the weld joint as it gives an entire two-dimensional map of the residual stress state in the weld line, heat affected zone (HAZ), and base material. The technique was applied at different LTT alloys with varying chemical compositions. Additionally, the results are compared to residual stress maps that were determined by Neutron diffraction using the Strain Analyzer for Large Scale Engineering Applications, an
instrument referred to as SALSA, at the Institut Laue-Langevin in Grenoble. For all investigated specimens, compressive residual stress distributions were determined in the area of the weld joint and the HAZ. They are balanced by tensile residual stresses in the surrounding base material. However, it is shown that the size of theregion exhibiting compressive residual stresses and the absolute values of the compressive residual stresses depend on the chemical composition of the weld filler material.