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An approach to achieve “zero leakage” is discussed with respect to the experience in Germany, where strict regulations for landfill lining and capping systems have been developed and issued because of large environmental problems related to landfills that accumulated in the 1970’s and 1980’s. Using a thick, high quality HDPE geomembrane (GMB), which is installed free of residual waves and wrinkles in intimate contact with a compacted clay liner (CCL) or geosynthetic clay liner (GCL) of very low permeability, by a qualified, experienced, well equipped and properly third-party controlled installer and which are protected by heavy protection layers designed with respect to long-term performance of the GMB, may result in a liner or capping system of practically no leakage. This is demonstrated by analyzing results of measurements obtained from permanently installed leak detection systems in combination with HDPE GMBs. The survey is based on 32 German landfills with 1,276,500 m² of installed HDPE GMB.
Welded plate girders are used in heavy steel construction, industrial buildings and bride construction. Residual stresses are present in all plate structures. They are mainly caused by welding. In addition, they influence the load bearing capacity of these welded components. However, Eurocode (EC) does not provide any specific residual stress patterns for consideration of residual stress impact on load capacity. Hence, the decision for a particular problem has to be made by the designer. Many codes, including EC 3, permit the use of non-linear finite element analysis (FEA) for the design of structures. This contribution gives an overview on recent research topics: modeling, experimental measuring and evaluation of residual stresses as well as welding simulation tools and simplified methodologies for assessment of weld residual stresses.
Welded plate girders are used in heavy steel construction, industrial buildings and bride construction. Residual stresses are present in all plate structures. They are mainly caused by welding. In addition, they influence the load bearing capacity of these welded components. However, Eurocode (EC) does not provide any specific residual stress patterns for consideration of residual stress impact on load capacity. Hence, the decision for a particular problem has to be made by the designer. Many codes, including EC 3, permit the use of non-linear finite element analysis (FEA) for the design of structures. For that purpose, the contribution gives an overview on recent results derived from welded I-girders made from mild steel S355 and high-strength steel S690QL. Weld residual stresses have been investigated by X-ray diffraction (XRD) on small-scale specimen. In addition, residual stresses of welded component-like I-girders have been studied using the sectioning technique. The results confirmed the beneficial effect of certain edge preparation of the plates by flame cutting prior to welding. High-strength steels like the S690 can contribute to both: increased load capacity and decreased influence of weld residual stresses.
Welded I-girders are used in many applications in steel construction, especially a tailored mixture of different plate thicknesses is difficult to manufacture by rolling. Residual stresses are mainly caused by
welding and it is in most cases impractical to anneal the structures. Eurocode 3 (EC 3) does not provide any
specific residual stress pattern. Hence,
the decision for a particular problem has to be taken by the designer. Stability failures are often decisive in the design of steel beam and column members. Many standards, including EC 3, permit the use of non-linear FEA for the design of structures. Load influencing imperfections are mainly geometric deviations from the
ideal shape and residual stresses(both due to assembly or weld manufacturing). Currently the residual stresses are considered by simplified “robust” procedures fixed in common standards(such as EC 3. Models mostly based on specimens from mild steel. Information about high-strength steels is less
available for realistic assessment of residual stresses in full-scale I-girders. Hence, the scope of this work is the measurement of residual welding stresses on component-like I-girders S355 and S690 by sectioning method and global structural welding simulation.
Welded plate girders are used in heavy steel construction, industrial buildings and bride construction. Residual stresses are present in all plate structures. They are mainly caused by welding. In addiiton, they influence the load bearing capacity of these welded components. However, Eurocode (EC) does not provide any specific residual stress patterns for consideration of residual stress impact on load capacity. Hence, the decision for a particular problem has to be made by the designer. Many codes, including EC 3, permit the use of non-linear finite element analysis (FEA) for the design of structures. Recent developments in the last years, enabled the use of computerized models instead of laboratory experiments. In this scope, the FE-model should include all relevant factors properly. This important if considering that weld residual stresses can be a critical assessment factor. In addition, measuring of residual stresses is difficult, time consuming and expensive, it is therefore common to use founded distribution functions (e.g. Swedish BSK 99). Welding simulation tools offer new possibilities for a realistic assessment of weld-induced stresses and deformations. However, the modeling and the computational effort for large structural components is still not in a practicable range and a simplified methodology is in needed. As a result, a new approach (suitable for capacity analysis) is presented and detailed in the present contribution.
Independent from the interpass temperature residual stresses in longitudinal direction were lower than in transverse direction. This applies for the surface as well as for the bulk of the welds.
Residual stresses in the bulk were in general lower compared to stresses found on the surface.
Compressive residual stresses as a result of the martensite formation were formed in the bulk weld metal, only.
Independent from the LTT filler used high interpass temperatures were beneficial to reduce the stresses mainly in longitudinal direction.
Lower interpass temperatures tend to narrow the tensile zone in the weld metal but they also prevent the formation of compressive residual stresses.
Independent from the interpasstemperature residual stresses in longitudinal direction were lower than in transverse direction. This applies for the surface as well as for the bulk of the welds.
-Residual stresses in the bulk were in general lower compared to stresses found on the surface.
•Compressive residual stresses as a result of the martensite formation were formed in the bulk weld metal, only.
•Independent from the LTT filler used high interpasstemperatures were beneficial to reduce the stresses mainly in longitudinal direction.
•Lower interpasstemperatures tend to narrow the tensile zone in the weld metal but they also prevent the formation of compressive residual stresses.
Adaption of Heat Control Concepts for Welding Stress Optimization in High-strength Steel Components
(2016)
High-strength structural steels from 690 MPa are increasingly applied at present. Major reasons are lightweight design trends and potential cost reductions. The structural design of welds and their manufacturing become more challenging with increasing material strength. By reasons of a higher yield ratio of these steels, the development of high residual stresses has to be avoided, since they are detrimental to the components safety and performance. Local restraint stresses and welding loads due to external shrinkage restraints occur. Frequently, this leads to critical tensile residual stresses in the weld and HAZ. In this study, influences of welding process parameters and restraint conditions on the residual stress state in welded components of high-strength steels were investigated. Multilayer GMAW tests under free shrinkage and experiments under well-defined restraints in special in-house developed testing facilities were accomplished. The tests permitted analyses of the resulting local residual stresses measured by means of X-ray diffraction and global reaction stress build-up while welding and cooling. Significant effects were found for heat control, seam configuration and restraint condition. Besides high restraints, elevated preheating and interpass temperatures lead to increased welding stresses. An adaption of welding parameters considering heat control, weld run sequence and seam configuration proved to be beneficial.
The use of high-strength steels is wide spread in vehicle and crane manufacturing due to light weight reasons. These steels are used for impact of crash loaded components and therefore it is important to investigate high-strength welds at dynamic impact. Investigations of welds at high strain rates have been rarely conducted in the past.
To determine the dynamic impact behaviour of hybrid laser-arc welds, the Split Hopkinson Pressure Bar (SHPB) technique was used. The base material was a quenched and tempered fine-grained structural steel with yield strength of 1100 MPa. The influence of two variables, cooling time t8/5 and strength of filler material, on the impact behaviour was studied. A matching and an under matching filler material were used. The cooling time t8/5 was varied to influence the microstructure in the heat affected zone (HAZ) and to analyse the effect on hardening and softening. Hardness measurements and optical microscopy was used to analyse the weld microstructure before and after the SHPB test. The investigations showed a correlation between the overall hardness of the weld, influenced by filler material and cooling time, on the maximum stress level during dynamic impact.
Fast digital radiography was used to observe the crack development during single pass bead-on-plate gas tungsten arc welding with a minifocus X-ray source and a digital detector array with 75-micrometer pixel size. The sample material were 5 mm thick AlMgSi plates. An acquisition rate of 12 frames per second and an exposure time of 40 ms per frame were used for real-time observation of the hot crack propagation during welding. The basic spatial resolution of the images is about 80 µm. A 3D laminographic reconstruction of the acquired 2D radiographic images gives access to the volumetric extent of the cracks in the welded sample. The parallel use of a high-speed camera during welding allows the real-time inspection of surface cracks. The development of surface cracks was compared with the crack’s distribution in the sample volume and these results were correlated to the used welding parameters.