Henning Agerskov

Fatigue in steel structures under random loading

Abstract Fatigue damage accumulation in steel structures under random loading is studied. The fatigue life of welded joints has been determined both experimentally and from a fracture mechanics analysis. In the experimental part of the investigation, fatigue test series have been carried through on various types of welded plate test specimens and full-scale offshore tubular joints. The materials that have been used are either conventional structural steel with a yield stress of f y ∼360–410 MPa or high-strength steel with a yield stress of f y ∼810–1010 MPa. The fatigue tests and the fracture mechanics analyses have been carried out using load histories, which are realistic in relation to the types of structures studied, i.e. primarily bridges, offshore structures and chimneys. In general, the test series carried through show a significant difference between constant amplitude and variable amplitude fatigue test results. Both the fracture mechanics analysis and the fatigue test results indicate that Miners rule, which is normally used in the design against fatigue in steel structures, may give results, which are unconservative, and that the validity of the results obtained from Miners rule will depend on the distribution of the load history in tension and compression.

An analytical model for fatigue life prediction based on fracture mechanics and crack closure

Abstract Fatigue in steel structures subjected to stochastic loading is studied. Of special interest is the problem of fatigue damage accumulation and in this connection, a comparison between experimental results and results obtained using fracture mechanics. Fatigue test results obtained for welded plate test specimens are compared with fatigue life predictions using a fracture mechanics approach. In the calculation of the fatigue life, the influence of the welding residual stresses and crack closure on the fatigue crack growth is considered. A description of the crack closure model for analytical determination of the fatigue life is included. Furthermore, the results obtained in studies of the various parameters that have an influence on the fatigue life, are given. A very good agreement between experimental and analytical results is obtained, when the crack closure model is used in determination of the analytical fatigue lives. Both the analytical and experimental results obtained show that the Miner rule may give quite unconservative predictions of the fatigue life for the types of stochastic loading studied.

Improvement of Fatigue Life of Welded Structural Components by Grinding

To improve quality and reduce lead-time in production of large two-stroke diesel engines, several producers have requested welded steel constructions to substitute cast iron structures. Change from cast iron designs requires new easy-to-weld designs, with special focus on high stressed transitions between thick and thin plate members, in order to obtain weight reduction corresponding to improved material properties of steel. Burr grinding is an obvious method for improvement of fatigue strength in production of large diesel engine structures. The present investigation compiles results reported in IIW-1629-03 (ex-doc. XIII-1966–03) with new fatigue tests on large-scale burr ground steel S275 specimens with plate thickness of 50 mm. All specimens have been subjected to constant amplitude loading, some in pure bending and some in pure membrane loading. 3D linear elastic finite element stress evaluation of burr ground notches have shown almost same endurance limit for pure bending — and membrane loading. This result is important for evaluation of real structures subjected to both bending and membrane loading. Although the number of tests carried out is limited, the results obtained show that burr grinding improves fatigue strength significantly. In some cases, crack initiation moved from the weld toe to the non-ground surface between the ground areas at the weld toes, due to the grinding.

An Investigation on the Influence of Root Defects on the Fatigue Life of the Welded Structure of a Large Two-Stroke Diesel Engine

The crankshaft housings of large two stroke diesel engines are welded structures subjected to constant amplitude loading and designed for infinite life at full design load. A new design of the so-called frame box has been introduced in the engine using butt welded joints of thick plates, welded from one side only, with no access to the root side. Linear elastic fracture mechanics applied to three-dimensional finite element models has been used to assess this new design from the fatigue viewpoint. The methodologies used, from coarse models of the complete engine structure to refined sub-models of the welded joints, are described and results presented. In addition, large-scale test specimens with controlled “lack of fusion” weld root geometry were manufactured and fatigue tested to develop “S-N” curves and determine threshold stress intensity factor range values. These were established for opening mode loading both under the influence of residual stresses from production and in stress relieved specimens.

The Fatigue Behavior of Steel Structures under Random Loading

Fatigue damage accumulation in steel structures under random loading has been studied in a number of investigations at the Technical University of Denmark. The fatigue life of welded joints has been determined both experimentally and from a fracture mechanics analysis. In the experimental part of the investigation, fatigue test series with a total of 540 fatigue tests have been carried through on various types of welded plate test specimens and full-scale offshore tubular joints. The materials that have been used are either conventional structural steel or high-strength steel. The fatigue tests and the fracture mechanics analyses have been carried out using load histories, which are realistic in relation to the types of structures studied, i.e. primarily bridges, offshore structures and chimneys. In general, the test series carried through show a significant difference between constant amplitude and variable amplitude fatigue test results. Both the fracture mechanics analysis and the fatigue test results indicate that Miner’s rule, which is normally used in the design against fatigue in steel structures, may give results, which are unconservative, and that the validity of the results obtained from Miner’s rule will depend on the distribution of the load history in tension and compression.

Improvement of Fatigue Life of Welded Structural Components of a Large Two-Stroke Diesel Engine by Grinding

The crankshaft housings of large two-stroke diesel engines are welded structures subjected to constant amplitude loading and designed for infinite life at full design load. A new design of the so-called frame box has been introduced in the engine using butt weld joints of thick plates, welded from one side only, with no access to the root side. Various investigations on the fatigue life of the structural components of this new design have been carried out. The present investigation concentrates on the improvement in fatigue life which may be obtained by grinding of the weld toes. The tests performed showed a significant increase in fatigue life due to the grinding, ranging from a factor of approx. 2.8 to ∞, depending on the load level. Although the number of tests was limited, the results indicate a favourable change in slope of the S-N curve, from m ~ 3.0 for the test series without grinding to m ~ 6.0 for the test series with grinding. In one of the test series, it was observed that in most cases crack initiation moved from the weld toe to the non-ground surface between the ground areas at the weld toes. Tests were made on steel S 275, on centrally and eccentrically loaded test specimens.