Research group of Steel Structures
Laboratory of Steel Structures is a leading laboratory in field of design of welded structures for demanding applications in Finland. The laboratory is part of department of mechanical engineering and consequently the focus of study are structures subjected to dynamic loading. The laboratory was founded in 1974 as a response to an increasing industrial need for knowledge on advanced welded structures. An exceptional task required an exceptional person to take the lead, so the first professorship was granted to Erkki Niemi. Professor Niemi had the theoretical mind of a scientist but the practical reason of an engineer, which proved to be a fruitful combination, serving the demand of Finnish machine and construction industry. Through the laboratory's 40-year-history this particularly close relationship with industry has been a distin-guishing factor that has been carefully cherished. Another important element of success has also been the close co-operation with LUT's Laboratory of Welding Technology; this perpetual symbiosis between the two neighboring laboratories has been advantageous for both parties. Also the role of international col-laboration in research has increased remarkably during the years. The research group is well-known abroad as well due to its active role in the IIW (International Institute of Welding) and in other activities.
The laboratory is creating new purposeful knowledge for design and fabrication of steel structures for demanding applications, which means that the structures are typically made of high or ultra-high strength steels (UHSS), subjected to dynamic loading (fatigue) and can be used also at low ambient temperature. The goal of education is first of all encouraging to design of innovative and competitive welded structures but also providing the tools for this task in the form of analyzing skills.
MAIN RESEARCH AREAS
The main stress (pun intended) in research is still concentrated in fatigue analysis of welded structures and especially joints. Current emphasis in this field is divided in four subgroups: effect of different weld processes, effect of post-treatments (vs as-welded), development of a general S-N master curve and design of details. The investigated materials are currently mainly high and ultra–high strength structural and stainless steels, processes are MAG with several sub-variations, laser and laser-hybrid and the improvement methods optimal welding parameters, waiving, HfMIT, TIG-dressing, MIG-brazing and LTT-fillers. In Fig. 1 is illustrated a high quality weld based on waiving technology in robotic welding.
Figure 1. Investigation of the fatigue strength of high quality filled weld joint made of S960 QC steel. Sorry of missing figs are added asap.
In the past special emphasis was put on fatigue analysis of welded joints and development of advanced analysis tools, resulting e.g. in important contributions to the formulation of structural hot-spot stress method. This research work has been the basis for current IIW recommendations concerning this method.
The latest remarkable achievement is the master curve analysis, Fig. 2, which is a novel local stress approach for the determination of design curve for the fatigue assessment of welded structures. The method is a combination of effective notch stress method (ENS), SWT approach and local strain method and the method takes into the account the residual stress effect, sres (welding, peening, etc), the applied stress ratio effect, R, and the effect of material (ultimate) strength, Rm.
Figure 1. Novel master curve analysis of a butt joint developed by Timo Nykänen at LUT Sorry of missing figs are added asap.
The interest in strength of welded joints in terms of load carrying and deformation capacities is increasing in accordance with increasing use of high and ultra-high strength steels. The study includes the definition of the throat thicknesses and the softening effect on the joint capacities in room and at low ambient temperature.
Due to the laboratory's long-time co-operation with Finnish steel manufacturer Ruukki (currently SSAB Europe) and their strong investments in tubular products, tubular structures are also an important area of research. This comprises buckling tests and capacities of welded RHS joints with the aim of developing a design guide for joints made of high-strength steel. The sensitivity analysis of joints is made by FEA using material models developed using the great amount of test results for RHS joints. This field includes strong collaboration with CIDECT.
Since welding is not the only joining technique for steel structures in the world, there must be a spot reserved for mechanical connections. The goal of this study is searching for alternatives by studying the behavior of bolted joints in fatigue conditions. His research is focused on the phenomenon of fretting and ways how to avoid it.
The research methods are based theoretical models, FEA –based simulations and laboratory tests. Laboratory of Steel Structures has a wide-ranging array of test rigs of which many are well suitable for what is the laboratory's main expertise – full-scale experimental tests. All test rigs are naturally good for standard test specimens, but the key factor that sets this laboratory apart from all others is the ability to exert fatigue tests to prototypes that can weigh several tons, as illustrated in Fig. 3. This is made possible by heavy-duty test rigs (of which the largest is able to exert forces up to 5 MN, Fig. 4) and a well-trained and imaginative staff. The test rigs and other laboratory equipment are as follows:
- 5 MN test rig "MEGA" for static/dynamic - loading
- 1250 kN, 700 kN, 400 kN, 3 x 150 kN and 25 kN test rigs for static/dynamic-loading
- 1 MN static compression test rig (up to 7 m total length)
- K-joint test rig for RHS-truss members
- Over 150 m2 of T slot floor area
- ARAMIS optical 3D deformation measurement & analysis system
- Stresstech residual stress measurement system (x-ray diffraction)
- Laser shape measuring system (for determining the local weld geometry)
Figure 3. Full scale fatigue test Sorry of missing figs are added asap.
Figure 4. 5 MN "MEGA" testing rig. Sorry of missing figs are added asap.
The laboratory is coordinating the OWS (=Optimization of Welded Structures, Finnish HRO) –forum, have 36 company members from Finnish industry, which are designing or fabricating welded structures - many of them being market leaders in their own business area in the world. OWS is one forum of the Finnish Welding Society and tries to solve problems involved in design of welded structures common for all industrial partners.
Laboratory is active also in education of International Welding Engineers (IWE) as well International Welded Structures Designer (IWSD).
The research work is worked out generally as collaboration with several international universities and IIW has an important role as general research forum. Current professor Timo Bjork is the Finnish delegate in IIW Commission XV and expert in Commission XIII.
The Finnish Welding Society gave the award called "center of excellence in welding" for the remarkable research and teaching work in the field of welded structures to laboratory of Steel Structures in 2014.
The unit's task is to provide the best education in the field and to grant graduate and postgraduate degrees. Additionally, it is tasked with scientific research, focusing for example on:
- static and fatique strength of welded joints
- dynamically loaded structures
- stress concentrations of welded joints (finite element analysis, FEA)
- fatigue life calculation methods
- strength of joints in structures made of structural hollow sections at sub-zero temperatures
- stability of thin walled structures
- distortion of thin-walled structures.
Material Testing Machines for Specifying the Static and Fatigue Strength of Structures
- 150kN (3 pcs), 400kN, 700kN, 1200 kN and 5MN
- A servohydraulic material testing machines (test rigs) for static and dynamic loading
- 1000kN compression
A material testing machine for compression testing. Especially for beam structures (the maximum length of specimen is 7m).
- K joint
A specific testing machine for the K joints made of structural hollow sections, also enabling sub-zero environments up to ‑60°C.
- T-groove floor and T-groove table
For various full-scale test arrangements.
Other research devices
- Amplifiers, filters and sensors
For strain, displacement, pressure and other measurements
- Roentgen diffraction
For determining stress state on body surfaces, e.g. residual stress
- 3D-SLM imaging method
For example, for determining the shape of the weld toe line
- Laser measurement sensor
For specifying geometrical shapes
- Optical 3D Deformation Analysis
For specifying deformation of the test specimens or stturctures
LUT Mechanical Engineering
Professor Timo Björk
tel. +358 400 553 508
Laboratory Engineer Matti Koskimäki
tel. +358 400 729 731