Experimental investigation on low-temperature fatigue crack propagation in offshore structural steel A131 under random sea ice loading

An apparatus of low-temperature controlling for fatigue experiments and its crack measuring system were developed and used for offshore structural steel A131 under conditions of both low temperature and random sea ice. The experimental procedures and data processing were described, and a universal random data processing software for FCP under spectrum loading was written. Many specific features of random ice-induced FCP which differed with constant amplitude FCP behaviours were proposed and temperature effect on ice-induced FCP was pointed out with an easily neglected aspect in designing for platforms in sea ice emphasized. In the end, differences of FCP behaviours between sea ice and ocean wave were presented.

Effect of phase-transformation on mechanical-behavior of dual-phase steel plate

The mechanical behavior of dual phase steel plates is affected by internal stresses created during martensite transformation. Analytical modelling of this effect is made by considering a unit cell made of martensite inclusion in a ferrite matrix. A large strain finite element analysis is then performed to obtain the plane stress deformation state. Displayed numerically are the development of the plastic zone and distribution of local state of stress and strain. Studied also are the shape configuration of the martensite (hard-phase) that influences the interfacial condition as related to stress transmission and damage. Internal stresses are found to enhance the global flow stress after yield initiation in the ferrite matrix. Good agreement is obtained between the analytical results and experimental observations.

Fatigue crack-growth rate in ferrite martensite dual-phase steel

An empirical study is made on the fatigue crack growth rate in ferrite-martensite dual-phase (FMDP) steel. Particular attention is given to the effect of ferrite content in the range of 24.2% to 41.5% where good fatigue resistance was found at 33.8%. Variations in ferrite content did not affect the crack growth rate when plotted against the effective stress intensity factor range  which was assumed to follow a linear relation with the crack tip stress intensity factor range ΔK. A high  corresponds to uniformly distributed small size ferrite and martensite. No other appreciable correlation could be ralated to the microstructure morphology of the FMDP steel. The closure stress intensity factor , however, is affected by the ferrite content with  reaching a maximum value of 0.7. In general, crack growth followed the interphase between the martensite and ferrite.

Dividing the fatigue crack growth process into Stage I and II where the former would be highly sensitive to changes in ΔK and the latter would increase with ΔK depending on the  ratio. The same data when correlated with the strain energy density factor range ΔS showed negligible dependence on mean stress or R ratio for Stage I crack growth. A parameter α involving the ratio of ultimate stress to yield stress, percent reduction of area and R is introduced for Stage II crack growth so that the  data for different R would collapse onto a single curve with a narrow scatter band when plotted against αΔS.

On post-instability processes in adiabatic shear in hot rolled steel

EXPERIMENTS carried out using a split Hopkinson torsional bar have shown that only one shear band develops in specimens of hot rolled steel which break during testing. We observed, however, that in specimens which were not deformed to failure, several fine shear bands appeared. We believe that these formed during the loading cycle before the appearance of the final shear band and were not due to the effect of unloading. So we developed a numerical model to study the evolution of shear banding from several finite amplitude disturbances (FADs) in both temperature and strain rate. This numerical model reveals the detailed processes by which the FADs evolve into a fully developed shear band and suggests that beyond instability, the so-called shear banding process consists of two stages: inhomogeneous shearing and true shear-banding. The latter is characterized by the collapse of the stress and an abrupt increase of the local shear strain rate.

Microstructure of shear localization in low-carbon ferrite pearlite steel

A study has been made of the microstructure of the thermally assisted band in a low carbon ferrite-pearlite steel, resulting from high speed torsional testing with an average strain rate of about 1500 s−1. Metallographic examination showed that there are several fine shear bands distributed over a deformed region (the gauge length of the specimen). The width of these bands is estimated to be of the order of magnitude of 50 μm, and the spacing between them is roughly about 100 μm. Detailed scanning electron microscopy studies indicate that damage of the microstructure within the band is very apparent, as evidenced by microcrack initiation and coalescence along the shear deformation band. However, there is no evidence that the material in the band had become microcrystalline or non-crystalline.

Mixed-mode fracture mechanisms near the fatigue threshold of aisi-316 stainless-steel

Near threshold, mixed mode (I and II), fatigue crack growth occurs mainly by two mechanisms, coplanar (or shear) mode and branch (or tensile) mode. For a constant ratio of ΔK_I/ΔK_II the shear mode growth shows a self-arrest character and it would only start again when ΔK_I and ΔK_II are increased. Both shear crack growth and the early stages of tensile crack growth, are of a crystallographic nature; the fatigue crack proceeds along slip planes or grain boundaries. The appearance of the fracture surfaces suggest that the mechanism of crack extension is by developing slip band microcracks which join up to form a macrocrack. This process is thought to be assisted by the nature of the plastic deformation within the reversed plastic zone where high back stresses are set up by dislocation pile-ups against grain boundaries. The interaction of the crack tip stress field with that of the dislocation pile-ups leads to the formation of slip band microcracks and subsequent crack extension. The change from shear mode to tensile mode growth probably occurs when the maximum tensile stress and the microcrack density in the maximum tensile plane direction attain critical values.

Pressure Drop Studies of Gas-Liquid Bubbly Flow in Vertical Upward Pipeline

This paper describes the experimental and theoretical studies of gas-liquid bubbly flow in vertical upward pipeline carried out at Institute of Mechanics, Chinese Academy of Sciences. Bubbly flow in a vertical pipe with a 3 m long and 5 cm inner diameter plexiglass pipe was experimentally investigated, and studies carried out on the relationship between superficial velocities of the liquid and gas phases and pressure gradient is described. The developed drift-flux model applied to gas-liquid bubbly flow is presented, and the results are compared against the experimental data measured by ours in air/water vertical pipes.

Formation and Structure of Composite Coating of HAD and Micro-Plasma Oxidation on A3 Steel

Composite coatings were obtained on A3 steel by hot dipping aluminum(HAD) at 720 degreesC for 6 min and micro-plasma oxidation (MPO) in alkali electrolyte. The surface morphology, element distribution and interface structure of composite coatings were studied by means of XRD, SEM and EDS. The results show that the composite coatings obtained through HAD/MPO on A3 steel consist of four layers. From the surface to the substrate, the layer is loose Al2O3 ceramic, compact Al2O3 ceramic, At and FeAl intermetallic compound layer in turn. The adhesions among all the layers are strengthened because the ceramic layer formed at the At surface originally, FeAl intermetallic compound layer and substrate are combined in metallurgical form through mutual diffusion during HAD process. Initial experiment results disclose that the anti-corrosion performance and wear resistance of composite coating are obviously improved through HAD/MPO treatment.

Pipeline Corrosion Analyzed by Fuzzy Reliability Methods

Fuzzy reliability methods are used to study the corrosion of pipelines. Three methods are used. They consist of using fracture failure modes, failure assessment diagram (FAD) and residual strength for establishing fuzzy reliability. Calculations are made by application of JC, improved GA-JC and Mente-carlo methods. Examples for oilfield injecting water pipeline show the residual strength well agree with field data. Mente-carlo methods appear to yield results that have better agreement with field data.

Oscillatory Flow Induced Hydrodynamic Forces upon a Pipeline near Erosive Sandy Seabed

An experimental study of the properties of hydrodynamic forces upon a marine pipeline is presented in this paper, in the equilibrium scour conditions for various Keulegan-Carpenter numbers and various initial relative gaps between pipeline and the erosive sandy seabed. The tests are conducted in a U-shaped oscillatory water tunnel with a sand box located at the bottom of the test section. According to the experimental results, the maximum horizontal forces on the pipelines with an initial gap to seabed will decrease to some extent due to scouring process. For engineering appliances, it seems safer to estimate wave induced forces on pipelines under the assumption that seabed is plane. However, it should be noticed that great changes would be brought to the frequency properties of lift forces because of the sandy scour beneath the pipeline, which occurs for certain KC numbers.

The Effects of Backfill Soil Properties on Wave-induced Pore Pressures around a Trenched Pipeline

Offshore pipelines are always trenched into seabed to reduce wave-induced forces and thereby to enhance their stability. The trenches are generally backfilled either by in-site sediments or by depositing selected backfill materials over the pipeline from bottom-dump barge. The actual waves in shallow water zone are always characterized as nonlinear. The proper evaluation of the wave-induced pressures upon pipeline is important for coastal geotechnical engineers. However, most previous investigations of the wave–seabed–pipe interaction problem have been concerned only with a single sediment layer and linear wave loading. In this paper, based on Biot’s consolidation theory, a two-dimensional finite element model is developed to investigate non-linear wave induced pore pressures around trenched pipeline. The influences of the permeability of backfill soil and the geometry profiles of trenches upon soil responses around pipeline are studied respectively.