Nonlinear load exceedance model for cyclic load processes: development

Reversible deformation in nanocrystalline TWIP steel during cyclic loading by partial slip reversal and detwinning

Simulation of ductile fracture toughness test under monotonic and reverse cyclic loading

The leak-before-break design concept is based on J-R curves, which are obtained by J-R tests on various types of specimens and are known to be dependent on the cyclic load history. The J-R curves of standard specimens suggested by the American Society for Testing Materials are determined based on quasi-static tensile loading. However, seismic loading induces a reverse cyclic loading that alternately applies a tensile and a compressive load to nuclear plant piping. Therefore, it is very important to obtain the fracture toughness characteristics under reverse cyclic loading for the integrity estimation of nuclear plant piping. The objective of this paper is to study the effects of reverse cyclic loading on the fracture toughness characteristics of SA312 TP316L stainless steel, which is a nuclear plant piping material. J-R tests on a flat, equivalent stress gradient specimen with varying incremental displacement were carried out. The test results were reviewed by comparing the J-R test results under quasi-static loading. In addition, the safety margin of the nuclear plant piping was evaluated using a crack driving force diagram method. For the SA312 TP316L stainless steel, the results showed that the J-R curves were decreased with a decrease in the incremental displacement. When the incremental displacement was set to 0.25 mm, the unstable crack growth point value was about 73.0% of those for the quasi-static loading conditions.

SUMMARYWind loads are very important in the design of tall buildings as often the load combinations containing the wind load govern the design. As for reliability, however, because of the higher inherent uncertainty in the wind load in comparison to the gravity load, safety indices decrease as the ratio of wind load to gravity load increases. The safety indices in RC columns depend on the nominal wind to gravity load ratios. Due to the interaction between bending moment and axial force, a single ratio cannot be defined, because the eccentricity is not similar for wind and gravity loads. In this paper, the ratio of wind to gravity loads is considered separately for axial force and bending moments. Unlike conventional approach, here it is assumed that the wind and gravity loads' eccentricities are not equal, and the final load eccentricity used in either the design or the reliability analysis is a function of applied loads. The results demonstrate that the sensitivity of RC columns' safety indices to bending moment ratios is higher than that of axial force ratios. Furthermore, the variation of RC columns' safety indices with a low rebar percentage is very different from RC columns with high rebar percentages. Copyright © 2010 John Wiley & Sons, Ltd.

This study concentrates on experimental evaluation of the effect of fiber reinforcement on the bond response of steel bars under load reversals. Beam specimens with symmetrical spliced top and bottom steel bars were used in the investigation. The specimens were designed to experience splitting mode of bond failure. The test variables included the diameter of the spliced reinforcement db or the ratio of concrete cover to bar diameter c∕db, the concrete compressive strength: normal-strength concrete (NSC) and high-strength concrete (HSC), and the volume fraction of steel fibers. Without fiber reinforcement, the control specimens experienced very sudden and quick bond deterioration leading to a total loss of load resistance in the first cycle following bond splitting. The presence of steel fibers increased the peak bond strength, reduced the bond degradation or damage under cyclic loading, and resulted in substantially enhanced energy absorption and dissipation capacities. The level of improvement in cyclic performance increased with increase in c∕db. Also, the increases in average bond strength at bond failure due to fibers were higher for the HSC when compared to the NSC specimens. Design equations in which the bond strength is expressed as a function of fc′1∕4 were more accurate in predicting the test data for the plain unconfined specimens in comparison with fc′1∕2. Available expressions for predicting the bond strength of developed/spliced bars in fiber-reinforced concrete showed good agreement with the test results.

This study investigated the loading rate effect on the fracture resistance under cyclic loading conditions to clearly understand the fracture behavior of piping materials under excessive seismic conditions. J-R fracture toughness tests were conducted under monotonic and cyclic loading conditions at various displacement rates at room temperature (RT) and the operating temperature of nuclear power plants (NPPs), i.e., 316°C. SA508 Gr. 1a lo w-alloy steel (LAS) and SA312 TP316 stainless steel (SS) piping materials were used for the tests. The fracture resistance under a reversible cyclic load was considerably lower than that under monotonic load regardless of test temperature, material, and loading rate. Under both cyclic and monotonic loading conditions, the fracture behavior of SA312 TP316 SS was independent of the loading rate at both RT and 316°C. For SA508 Gr. 1a LAS, the loading rate effect on the fracture behavior was appreciable at 316°C under both cyclic and monotonic loading conditions. However, the loading rate effect diminished when the cyclic load ratio (R) was −1. Thus, it was recognized that the fracture behavior of piping materials, including seismic loading characteristics, can be evaluated when tested under a cyclic load of R = −1 at a quasi-static loading rate.

In order to investigate freeze–thawed red sandstone failure processes under cyclic loading and unloading conditions, real-time acoustic emission (AE) and scanning electron microscopy (SEM) techniques were used to reveal the fracture process of the saturated red sandstone after cyclic loading and unloading tests using uniaxial compression. The results show that the stress–strain curves of the freeze–thawed sandstones show signs of hysteresis and exhibit a two-stage evolution of “sparse → dense”. In the cyclic loading and unloading process, the modulus of elasticity in the loading process is always larger than that in the unloading process, while the Poisson’s ratio is the opposite, and the radial irreversible strain and cumulative irreversible strain are larger than those in the axial direction. As the number of freeze–thaw cycles increases, the rock specimens need more cycles of loading and unloading to make the crack volume compressive strain Δεcv+ reach the maximum value and tend to stabilize, while the crack volume extensional strain Δεcv− tends to decrease gradually. This study also shows that the growth phase of the cyclic loading and unloading process has more ringing counts and a shorter duration, while the slow degradation phase has more ringing counts with loading and less with unloading. In addition, the F-T cycle gradually changes the internal microcracks of the red sandstone from shear damage, which is dominated by shear cracks, to tensile damage, which is dominated by tensile cracks. This study’s findings contribute to our knowledge of the mechanical characteristics and sandstone’s degradation process following F-T treatment, and also serve as a guide for engineering stability analyses conducted in the presence of multiphysical field coupling.

Gamma processes and peaks-over-threshold distributions for time-dependent reliability

The evaluation of ductile crack initiation in steel welded structures under seismic loading is crucial for structural design or safety assessment to prevent ductile cracking induced brittle fracture. The main purpose of this study is to propose a transferable criterion for evaluation of ductile cracking in welded structures under large scale cyclic loading from small scale tensile test results. On the basis of the observation of ductile crack initiation behavior of round-bar specimen with/without circumferential notches tested in single tension, the main controlling factor for ductile cracking in two-phase steel used was found to be a nucleation of micro-voids in soft phase (Ferrite phase) near Ferrite-Pearlite interface after a large extent of plastic straining. The material damage concept under reverse loading which correlates the material damage for micro-voids nucleation to macro-scale mechanical parameters was proposed by taking into account two aspects of Bauschinger effect : (a)mechanical aspect which influences deformation and stress/strain behaviors, (b) material damage aspect caused by dislocation behavior. A new criterion for ductile cracking of structural members under cyclic loading was proposed on the basis of that proposed effective damage concept and “two-parameter criterion” which can be applied to the steel structures under increasing load in single direction. The validity of the advanced two-parameter criterion was verified by conducting cyclic loading tests in axial direction for round-bar specimen and cyclic 3-point bending tests for cross-shaped specimens. Consequently, the advanced two-parameter ductile cracking criterion was found to be a transferable criterion for the evaluation of critical loading cycle of structural members from small scale tensile test results.

The release of a homogeneous high dose to the tumour region has been one of the cornerstones of radiotherapy (RT) treatment since its early days. According to the organ type and to the cancer histology, different doses are required in order to inactivate malignant cells, thus stopping proliferation. However, radiation-induced cell killing is a stochastic process. Tumour control probability (TCP) models have been developed in order to assign a success rate to a given RT treatment. At the same time, there is the need to keep the risks of normal tissue toxicity at an acceptable level. Normal tissue complication probability (NTCP) models provide a means of doing this. Traditionally, TCP and NTCP models combine clinical outcomes with dosimetric information in terms of dose-volume histograms (DVH). Model parameters are derived by mathematical fits to clinical observations and are subsequently used to estimate the risk of tumour relapse or toxicity. In both types of models, all of the patient dosimetric information is condensed into the DVHs, which represents a potential limitation on their descriptive and predictive power. This choice, related to historical and practical reasons, does not allow the full complexity of the 3D dose distribution in the patient to be taken into account. Neglecting these aspects might be relevant in a modern RT setting, which often includes the presence of high dose gradient regions. This has motivated research on ‘advanced’ TCP and NTCP models, able to tackle the problem by looking at a different scale, e.g. in tumour sub-regions or at the single voxel level. This is relevant not only from the purely dosimetric point of view. Increasing evidence is reported on the heterogeneity of cancer tissues, suggesting that non-uniform dose distributions could result in improved survival, for instance if targeted to take into account sub volumes with high clonogen density or hypoxic radioresistant regions. Similarly, radiation-induced side effects are part of a complex biological response, which depends not only on cell killing, but also on the inflammatory response and in some cases on the interplay among different organs. Obviously, conventional NTCP models cannot describe this scenario, and the development of more advanced mathematical tools is needed. This review will be focused on the discussion of recent studies showing possible directions for moving the field of TCP and NTCP modelling forward. Without diminishing the role and usefulness of available models, the aim is to shed light on the benefits that might be achieved by ‘enhanced’ modelling. This could represent an important step in the gradual transition of radiation therapy towards a form of precision medicine.

Beam–column joints are extremely vulnerable to lateral and vertical loads in reinforced concrete (RC) structures. This insufficiency in joint performance can lead to the failure of the whole structure in the event of unforeseen seismic and wind loads. This experimental work was conducted to study the behaviour of ternary blend geopolymer concrete (TGPC) beam-column joints with the addition of hybrid fibres, viz., steel and polypropylene fibres, under reverse cyclic loads. Nine RC beam-column joints were prepared and tested under reverse cyclic loading to recreate the conditions during an earthquake. M55 grade TGPC was designed and used in this present study. The primary parameters studied in this experimental investigation were the volume fractions of steel fibres (0.5% and 1.0%) and polypropylene fibres, viz., 0.1 to 0.25%, with an increment of 0.05%. In this study, the properties of hybrid fibre-reinforced ternary blend geopolymer concrete (HTGPC) beam-column joints, such as their ductility, energy absorption capacity, initial crack load and peak load carrying capacity, were investigated. The test results imply that the hybridisation of fibres effectively enhances the joint performance of TGPC. Also, an effort was made to compare the shear strength of HTGPC beam-column connections with existing equations from the literature. As the available models did not match the actual test results, a method was performed to obtain the shear strength of HTGPC beam-column connections. The developed equation was found to compare convincingly with the experimental test results.

Behaviour of steel-fibre-reinforced normal-strength concrete slender columns under cyclic loading

A fully numerical model is presented for simulating the response of slender structures to artificial wind loading. The deterministic method in the time domain is adopted to evaluate the response of very large systems during turbulent gusts lasting 1 to 10 min. Particular attention is devoted to very high-wind-sensitive systems such as guyed masts, as well as television reticulated towers. For this kind of structure, interaction effects between the loading process and the response process cannot be neglected. This causes an adjunctive nonlinearity of the mechanical problem, which is coupled with the so-called geometric nonlinearity that arises from significant displacements occurring during the phenomenon. The wind loading is artificially generated by simulating the time series of the turbulent wind field in the portion of the atmospheric boundary layer that is of interest for real structures (up to about 400 m). The stochastic processes that simulate the wind speed at given points are generated a...

This study investigates the failure mechanisms of Reinforced Concrete (RC) columns infilled with Autoclaved Aerated Concrete (AAC) under reverse cyclic loading, simulating seismic conditions. Given the growing use of AAC for its sustainability benefits and thermal insulation properties, its impact on the seismic behavior of RC columns requires thorough evaluation. The study explores the structural behavior of AAC-infilled RC columns, focusing on their load-bearing capacity, deformation, and failure modes during cyclic loading. Experimental testing was conducted on RC columns with varying AAC infill dimensions, subjected to reverse cyclic lateral loads to observe the effects of cyclic loading on their load-displacement behavior, energy dissipation, and failure modes. The results indicate that AAC infill enhances the lateral load resistance of RC columns, but also introduces specific failure modes related to the interaction between the AAC and the surrounding RC. They also demonstrated that while AAC infill can enhance the overall stiffness and strength of RC columns, it also influences failure modes, including concrete spalling, AAC crushing, and reinforcement failure. The failure mechanisms observed include cracking and crushing of AAC blocks at the column interface, shear failure, and debonding between the AAC and RC. Additionally, cyclic loading leads to progressive degradation of both AAC and RC materials, with a reduction in strength and stiffness after repeated loading cycles. The study also reveals that the low density and high porosity of AAC influence the energy dissipation capacity and damping characteristics of the infilled columns. The findings suggest that while AAC offers benefits such as reduced weight and improved insulation, it also presents vulnerabilities in terms of seismic performance, particularly at the AAC-RC interface. This study concludes with recommendations for design improvements and reinforcement strategies to enhance the seismic resilience of AAC-infilled RC columns. It also provides valuable insights for the design of AAC-infilled RC structures in seismic-prone areas.

Stochastic processes If a classical particle moves in an external force field F(x ) its motion is deterministic and it could be called a ‘deterministic process’. If the particle interacts with randomly distributed obstacles its motion is still deterministic, but the trajectory depends on the random characteristics of the obstacles, and maybe also on the random choice of the initial conditions. Such a motion is an example of a ‘stochastic process’, or rather a ‘random process’. In a more abstract generalized definition, a stochastic process is a random variable X ξ that depends on an additional (deterministic) independent variable ξ, which can be discrete or continuous. In most cases it stands for an index ξ ∈ N or time t , space x or energy E . An almost trivial ubiquitous stochastic process is given by additive noise η( t ) on a time-dependent signal s(t ), i.e. X t = s(t ) + η( t ). As a consequence, X t is no longer continuous. The most apparent applications of stochastic processes are time series of any kind that depend on some random impact. A broad field of applications are time series occurring, for example, in business, finance, engineering, medical applications and of course in physics. Beyond time series analysis, stochastic processes are at the heart of diffusion Langevin dynamics, Feynman's path integrals [43], as well as Klauder's stochastic quantization [62], which represents an unconventional approach to quantum mechanics. Here we will give a concise introduction and present a few pedagogical examples.