Chitoshi Miki

High strength and high performance steels and their use in bridge structures

Abstract This paper reviews the histories of the development and use of high strength and high performance steels for bridge structures in Japan.

Fatigue strength of cope hole details in steel bridges

Cope holes found at the intersections of steel structural members to access crossing welds may cause high stress concentration and decrease the fatigue strength of the detail. In this study, fatigue strength and local stresses for cope hole details existing in I-section beams bent in-planes were investigated by fatigue tests, stress measurements and stress analyses. The experimental results showed the fatigue strength of cope hole details was extremely low due to the local stress around the cope holes. In particular, when shear deformation was induced in a gap of flange plate inside a cope hole, the local stress was greatly increased. Using an analytical approach, the relationship between the magnitude of local stress and some influential parameters was investigated and a simple method to estimate the local stress for cope hole details is proposed.

Fatigue strengths of fillet-welded joints containing root discontinuities

Abstract In order to examine the fatigue performance of fillet-welded joints containing root crack or root-gap defects, fatigue tests are carried out. The specimens consist of two series: the first series are non-load-carrying cruciform, load-carrying cruciform and longitudinally welded joints. The second series are all load-carrying cruciform joints and have four kinds of details with various root-gap sizes and leg length. The test results indicate that the fatigue strength of joints which contain defects is equal to or higher than that of joints with no defects.

FRACTURE TOUGHNESS OF COLD WORKED AND SIMULATED HEAT AFFECTED STRUCTURAL STEEL

Abstract Deterioration of fracture toughness due to both cold working and welding are studied to investigate the combined effect which is seen in the welded part of cold worked members. It is known that fracture toughness is degraded in cold-worked steels by strain aging and is also degraded in the heat affected zone (HAZ). In this experimental study, CTOD tests are carried out using simulated HAZ specimens which are pre-strained before heat treatment. Although multi-pass welding produces a complex distribution of microstructures in HAZ, two types of thermal cycles are used to simulate the HAZ. One is to simulate intercritically reheated coarse grain HAZ (IRCG HAZ), the most degraded part in the local brittle zone. Another is to simulate subcritical HAZ (SC HAZ), the least degraded part in HAZ. In these two types of simulated microstructures, the effects of cold working are discussed based on fracture mechanics. It can be observed that the fracture toughness in IRCG HAZ is fairly low and the variation of fracture toughness with increasing pre-strain does not exist in that particular region. The fracture toughness in SC HAZ is not so crucial as seen in IRCG HAZ, but is strongly affected by pre-strain before heat treatment.

Weld Repair for Fatigue-Cracked Joints in Steel Bridges by Applying Low Temperature Transformation Welding Wire

When repairing fatigue damage by welding, the fatigue strength of cracked joints needs to be restored and also improved to prevent the reoccurrence of cracks. In this study, a new welding wire called low temperature transformation (LTT) welding wire, which enables the introduction of compressive residual stresses around a weld bead and the reduction of weld deformation, was applied to the weld repair. Its applicability was investigated by fatigue tests with compact tension specimens and a plate girder specimen, The compact tension specimens revealed that compressive residual stresses introduced by LTT welding wire have positive effects on crack initiation and propagation behaviour. The plate girder specimen indicated that the fatigue lives of cracked joints are restored and also improved by weld repair with LTT welding wire. Therefore, weld repair with LTT welding wire makes it possible to improve the fatigue strength of cracked joints.

Fatigue strength improvement by hammer peening treatment—verification from plastic deformation, residual stress, and fatigue crack propagation rate

This study investigates the fatigue strength improvement by the hammer peening treatment. The fatigue life is evaluated by dividing into the crack initiation life and the crack propagation life. The improvement effects of peening treatment are verified from the viewpoints of stress concentration, residual stress distribution, plastic deformation, and crack propagation behavior. From the measurement of residual stress and plastic deformation, it is indicated that the region where the compressive residual stress introduced by the peening treatment exists gives close agreement with the plastic (hardening) region. The fatigue test results show the reduction of stress concentration improves the crack initiation life significantly. The crack propagation rate retards even if the fatigue crack propagates over the region where the compressive residual stress has originally existed. Therefore, the hammer peening treatment increases both the crack initiation and propagation lives.

Fatigue strength improvement of out-of-plane welded joints of steel girder under variable amplitude loading

This study investigates improvement effects of fatigue strength by the postweld treatments, which are hammer peening treatment and additional welding with low-temperature transformation welding material. High cycle fatigue tests with constant and variable amplitude loadings are performed on out-of-plane gusset-welded joints of girder specimens. Test results indicate that this treatment can improve fatigue life under both the constant and variable amplitude loadings. The damage accumulations (D) of as-welded and cleaning obtained by Miner’s rule with the mean curve of constant amplitude loading are conservative (D > 1.0). However, since the beneficial effects of the compressive residual stress on fatigue life should decrease due to the relaxation of compressive residual stress, the D in specimens prepared by cleaning plus pneumatic peening, cleaning plus electric peening, and additional welding with low-temperature transformation welding material are extremely nonconservative (D ≈ 0.2–0.6). The modified Miner’s rule based on the results of fatigue test with constant maximum stress, which is the same as the maximum stress in the variable amplitude loading, is appropriate to the improved welded joints.

Improvement Effects of Fatigue Strength by Burr Grinding and Hammer Peening Under Variable Amplitude Loading

This study investigates improvement effects of fatigue strength by burr grinding and hammer peening treatments. High-cycle fatigue tests with constant and variable amplitude loadings are performed on out-of-plane gusset plate specimens. Test results indicate that the reduction of stress concentration by the burr grinding greatly improves the fatigue life under both the constant and variable amplitude loadings. For the variable amplitude loading, the damage accumulation D obtained by Miner’s rule with the mean curve of constant amplitude loading is conservative (D > 1.5). However, under the variable amplitude loading, since the beneficial effects of the peening treatment on the fatigue life decrease due to the relaxation of compressive residual stress, the damage accumulations in specimens prepared by cleaning and subsequent hammer peening are extremely non-conservative (D ≈ 0.2). In addition, the stress patterns do not affect the fatigue life significantly.

Fatigue evaluation for longitudinal-to-transverse rib connection of orthotropic steel deck by using structural hot spot stress

Orthotropic steel decks have serious fatigue problems especially on the connections between longitudinal and transverse ribs. In this paper, experimental and analytical works were conducted to evaluate the fatigue performance of connections between longitudinal U-ribs and transverse ribs with and without the slit on the transverse rib web. Finite element analyses of the specimen for all possible loading cases clarified the influence surfaces for structural hot spot stress (SHSS) working on weld toes of the connections. Fatigue tests with the loading case that generates the maximum and minimum SHSS on the connections were conducted. Fatigue test results for the slit connections evaluated by SHSS show agreement with previous fatigue data of similar connection details. No fatigue crack was initiated on the connection without the slit after 4.6 million cycles compared to fatigue failure of the slit connection at 0.7 million cycles.

Technique for Determining Bridge Displacement Response Using MEMS Accelerometers.

In bridge maintenance, particularly with regard to fatigue damage in steel bridges, it is important to determine the displacement response of the entire bridge under a live load as well as that of each member. Knowing the displacement response enables the identification of dynamic deformations that can cause stresses and ultimately lead to damage and thus also allows the undertaking of appropriate countermeasures. In theory, the displacement response can be calculated from the double integration of the measured acceleration. However, data measured by an accelerometer include measurement errors caused by the limitations of the analog-to-digital conversion process and sensor noise. These errors distort the double integration results. Furthermore, as bridges in service are constantly vibrating because of passing vehicles, estimating the boundary conditions for the numerical integration is difficult. To address these problems, this paper proposes a method for determining the displacement of a bridge in service from its acceleration based on its free vibration. To verify the effectiveness of the proposed method, field measurements were conducted using nine different accelerometers. Based on the results of these measurements, the proposed method was found to be highly accurate in comparison with the reference displacement obtained using a contact displacement gauge.