Sheng Bao

Piezomagnetism and fatigue: II

The cumulation of damage in test specimens of AISI 1018 steel, subjected to repeated cycles of tension and compression leading to fatigue failure at Nf cycles, is correlated with the evolution of stress–strain (σ − e) hysteresis and piezomagnetic (B–e) hysteresis. Specifically, the σ − e hysteresis loop areas, when plotted as a function of the number of loading cycles N, show systematic variations that can be identified with the three principal stages of fatigue: initial accommodation (i.e. strain softening or hardening), N < N2; accretion of damage, N2 < N < N3, and terminal failure (crack coalescence and growth); N3 < N < Nf. Data from 49 fatigue trials, spanning the range 1219 ≤ Nf ≤ 250 200, show that the transitional cycles N2 and N3 have an approximately invariant relation to final fatigue failure at Nf: i.e. N2/Nf ≈ 12% and N3/Nf ≈ 90%. Piezomagnetic hysteresis develops in parallel with stress–strain hysteresis and also exhibits transitions at N2PM and N3PM corresponding to N2 and N3. Detailed analyses of eight fatigue trials yield the approximately invariant ratios N2PM/Nf ≈ 12% and N3PM/Nf ≈ 93% where 3561 ≤ Nf ≤ 189 629.

Magnetomechanical behavior for assessment of fatigue process in ferromagnetic steel

In this work, the change of magnetization as a function of applied stress has been investigated for test specimens of AISI 1018 steel. The various stages of fatigue damage process are characterized by the magnetomechanical measurements recorded by an APS 428D fluxgate magnetometer. Of great significance is the fact that the stress-magnetic field hysteresis loop area changes systematically with the progression of fatigue. The magnetomechanical hysteresis demonstrates conspicuous changes in the initial stage of fatigue loading, then reverts to a relatively stable phase, and finally, drastic variations appear again as the cyclic loadings approach terminal failure. This work demonstrated that it is possible to correlate the progress of fatigue in ferromagnetic steels with the nondestructive evaluation technique of the magnetomechanical effect.

Asymmetric buckling of offshore pipelines under combined tension, bending and external pressure

The seamless steel pipes inevitably incorporate various initial geometric imperfections introduced in the manufacturing process. The offshore pipelines close to the touch-down point in the sagbend have to withstand the combined actions of axial tension, bending and external pressure during deepwater installation. In this paper, a two-dimensional (2D) theoretical formulation is further developed to investigate the asymmetric buckling behaviour of offshore pipelines with arbitrary initial geometric imperfections under complicated loading conditions. In addition, numerical simulations based on a 3D finite-element model of the imperfect tube using the software ABAQUS are carried out. Close agreement between the results demonstrates the effectiveness and practicality of this theoretical method. The theoretical model is then employed to study the effects of some important influencing factors such as initial geometric imperfections represented by initial ovality and wall eccentricity, ovality orientation, material properties represented by effective yield stress and strain-hardening parameter, diameter-to-thickness ratio, axial tension, and load cases on the buckling behaviour of the imperfect tubes. Based on extensive parametric studies, some significant conclusions were drawn.

Time-domain dynamic drift optimisation of tall buildings subject to stochastic wind excitation

Wind-resistant design of tall buildings has been traditionally treated using the equivalent static load approach. In order to account for the uncertainties in random wind excitation, it is necessary to develop a comprehensive and reliable dynamic optimisation technique in the time domain. The optimal lateral stiffness design problem of wind-excited tall buildings consists of (1) identifying the critical dynamic drift response in the time domain and (2) searching for the optimal distribution of element stiffness of the building subject to multiple drift design constraints. The critical time-history drift constraints of a wind-excited building are first treated by the worst-case formulation and then explicitly expressed in terms of element sizing variables using the principle of virtual work. The extreme value distribution and the Gaussian assumption are employed to formulate and simplify the probabilistic drift constraints, which are explicitly considered in the dynamic optimisation problem. The system reliability associated with the inter-story drift is estimated approximately by the bound approach to ensure that the most cost-efficient solution also attains an acceptable reliability level. A full-scale 45-story building example under wind tunnel derived time history wind loading is presented to illustrate the effectiveness and practicality of the reliability-based dynamic optimisation technique.

Magnetomechanical Measurements for Nondestructive Evaluation of Failure in Steel Structural Element

This research aims to explore magnetomechanical effect as a nondestructive method to monitor the failure process of steel structural elements subjected to compressive loadings. Experiments on steel tubular joints were carried out to study the relationship between the applied load and the magnetomechanical field. Results show that the mechanical response and the magnetomechanical response demonstrate similar behavior to the applied load in the whole loading progress. At the critical load of buckling, the load-magnetic field curve has a bifurcation point corresponding to the traditional bifurcation of load-displacement curve. This research demonstrates that the stress-induced magnetic filed can be utilized as an important indicator of impending failure in steel structural elements.