Preloading History Research Articles

On the influence of drained cyclic preloadings on the cyclic behaviour of Zbraslav sand

This article experimentally evaluates the influence of directional drained cyclic preloadings on the subsequent undrained monotonic and cyclic response of Zbraslav sand. The cyclic tests were performed on dense samples and under triaxial conditions. The experiments considered several different preloading directions, magnitude of the maximum deviatoric stress reached during the drained preloading stage and deviatoric stress amplitudes during the undrained shearing stage. The experimental results suggested that the rate of strains and pore water pressure accumulation, and therefore, the number of cycles to reach initial liquefaction or failure conditions are remarkably affected by the direction of the drained preloading history. In addition, the preloading direction plays a major role on the contractive/dilative response during the subsequent undrained monotonic loading. In particular, directional drained cyclic preloadings toward positive deviatoric stresses at constant mean effective stress and isotropic compression led to more dilative responses. Contrary, preloadings toward negative deviatoric stresses at constant mean effective stress led to more contractive responses. The results suggest that these major variations cannot be associated to the rather small changes in the relative density of the samples but likely due to fabric changes and induced anisotropy caused by the drained preloading history.

Numerical Analysis of Experiments on Damage and Fracture Behavior of Differently Preloaded Aluminum Alloy Specimens

A large amount of experimental studies have shown significant dependence of strength of ductile metals on stress state and stress history. These effects have to be taken into account in constitutive models and corresponding numerical analysis to be able to predict safety and lifetime of engineering structures in a realistic manner. In this context, the present paper deals with numerical analysis of the influence of the load path on damage and fracture behavior of aluminum alloys. A continuum damage model is discussed taking into account the effect of stress state and loading history on damage criteria and on evolution equations of damage strains. Experiments with the biaxially loaded H-specimen have been performed and different preloading histories have been taken into account. Evolution of strain fields is monitored by digital image correlation, and fracture modes are visualized by scanning electron microscopy (SEM). In addition, numerically predicted stress states are used to explain occurrence of different stress-state- and preloading-path-dependent localization behavior in critical specimens areas, as well as damage and fracture modes, revealed by SEM. The experiments with newly developed biaxially loaded specimens and corresponding numerical simulations show that the preloading history remarkably affects the occurrence of width and orientation of localized strain fields, as well as evolution of damage mechanisms and fracture modes. Therefore, characterization of materials must be based on an enhanced experimental program including biaxial tests with different loading histories. The observed damage and failure behavior can be predicted by the proposed continuum model taking into account stress-state-dependent damage criteria and damage strains.

Review on Strengthening Reinforced Concrete Columns Using Reinforced Concrete Jackets

This paper highlights the review of the effects of concrete surface treatment and loading on the structural behavior of reinforced concrete (RC) columns retrofitted with RC jackets. The concrete surface treatment aspect is assessed based on the surface roughness, use of dowel bars, use of shear connectors, and applying bonding agent between the column core and the jacket. However, the loading aspect is evaluated based on preloading history and the applied loading pattern. The latest researches and recommendations for concrete jacketing are presented. It is found that sand-blasting with the use of dowel bars or shear connectors helps the retrofitted column to behave monolithically. Preloading does not have a significant effect on the retrofitted column. However, it is recommended to apply the load on the full retrofitted cross-section.

Permanent accumulated rotation of an offshore monopile wind turbine in sand during a storm

Offshore wind turbines exposed to storm situations are subjected to static and dynamic loads from the same direction over a considerable period of time. Such cyclic loading can potentially result in soil degradation, leading to an undesired permanent rotation of the wind turbine. This paper presents a workflow to predict the permanent accumulated rotation of an offshore monopile wind turbine in sand during an extreme storm event incorporating the use of fully nonlinear irregular waves versus linear waves in current practice. The fully nonlinear irregular waves are realized from a potential flow solver OceanWave3D previously validated at up to near-breaking wave conditions. Given the wave kinematics, the aero-hydro-elastic code HAWC2 is used to calculate horizontal loading and bending moment acting on the embedded pile head. The irregular load series is then decomposed into a set of constant-amplitude load parcels using rainflow counting. Eventually, the permanent accumulated rotation is predicated using the method proposed by LeBlanc et al. (2010b) with Miner’s rule-based superposition. In this paper, a case study of the DTU 10MW wind turbine supported by a monopile at 33 m water depth in sand is presented, where the pile is primarily laterally loaded. The simulation results suggest the importance of taking accumulated rotation into design. The permanent accumulated rotation is primarily decided by soil capacity, loading characteristics and pre-loading history. Furthermore, the results show that wave nonlinearity has only limited influence on the permanent accumulated rotation.

Liquefaction of sand under monotonic and cyclic shear conditions: Impact of drained preloading history

In-situ sand deposits inevitably undergo a preloading history caused by factors such as overlying structures, adjacent construction work, or repeated sedimentation and erosion; this will in turn affect their mechanical behavior and liquefaction susceptibility during subsequent loading. An experimental program was developed to explore the potential influence of the drained preloading history on the undrained response of sand subjected to both monotonic and cyclic loadings. The results indicate that the shear strength, dilatancy, and stiffness characteristics in monotonic tests depend on the relative directions of the drained preloading and subsequent undrained shearing. Two typical types of behavior are observed and identified in the cyclic tests, i.e., cyclic mobility and residual deformation accumulation failure, depending on the stress state prior to the undrained cycling. The cyclic resistance is quantified based on a failure criterion defined in terms of the triggering of flow behavior, and it is compared to that determined using a criterion of specified strain threshold. In particular, the direction–wise (compressional or extensional) influence of the preloading history on the liquefaction resistance and pore pressure generated in sand during cyclic loading agrees well with that observed during monotonic loading. Furthermore, a correspondence between the cyclic and monotonic tests can be derived, and a unified approach is proposed to evaluate both the monotonic and cyclic responses, irrespective of the drained preloading history.

Fatigue Behaviour of Textile Reinforced Cementitious Composites and Their Application in Sandwich Elements

Using large lightweight insulating sandwich panels with cement composite faces offers great possibilities for the renovation of existing dwellings. During their lifetime, these panels are subjected to wind loading, which is equivalent to a repeated loading. To guarantee the structural performance of these panels during their entire lifetime, it is necessary to quantify the impact of these loading conditions on the long term. The fatigue behaviour was, therefore, examined in this paper both at the material level of the faces and at the element level as well. plain textile reinforced cementitious composite (TRC) specimens were subjected to 100,000 loading cycles by means of a uniaxial tensile test, while sandwich beams were loaded 100.000 times with a four-point bending test. Results show that the residual behaviour is strongly dependent on the occurrence of cracks. The formation of cracks leads to a reduction of the initial stiffness. The ultimate strength is only affected in a minor way by the preloading history.

Effect of prior deformation on the subsequent creep and anelastic recovery behaviour of an advanced martensitic steel

The creep and anelastic recovery characteristics of a 10%Cr steel have been systematically investigated at 600°C after subjecting the test material to various prior deformation histories. Constant-load forward creep tests on specimens, either with a tensile or compressive preloading history, indicated that over- and reverse-preloading respectively decreases and increases the early primary creep rate of the steel. The extent of decrease (or increase) in early primary creep rate is also found to be directly proportional to the magnitude of stress during prior loading while such a correlation is not clearly evident for material deformation in the secondary and tertiary stages. Specifically, the creep rate in the secondary and tertiary stages is lower for specimens with a compressive prior loading while the rupture time is notably shorter for tensile pre-loaded specimens. The observed effect of prior loading on the early primary creep behaviour can be explained by considering micro-backstress development (as a consequence of dislocation pile-up formation during the prior loading phase) that subsequently introduces a kinematic hardening effect to the material's viscoplastic response. The second set of experiments involve monitoring the anelastic recovery behaviour immediately after accumulation of a similar amount of time-dependent strain either under forward creep (load control mode) or stress relaxation (strain control mode) condition in completely unloaded 10%Cr steel specimens at 600°C. Experimental observations indicate that the higher the stress magnitude during the prior loading phase, the greater and faster the anelastic recovery at zero stress. Further findings show the mode of prior deformation (creep or relaxation) to also not noticeably influence the subsequent anelastic recovery behaviour. The observed anelastic recovery characteristic can be mechanistically interpreted by consideration of the time-dependent material back-flow due to the relaxation of dislocation bows and pile-ups generated during the prior deformation.

Strain accumulation due to packages of cycles with varying amplitude and/or average stress – On the bundling of cycles and the loss of the cyclic preloading memory

In order to predict permanent deformations by means of a high-cycle accumulation (HCA) model, a random cyclic loading, i.e. a loading with frequently changing amplitudes, has to be grouped into packages of cycles each with a constant amplitude. Based on a series of drained triaxial tests on fine sand, in which the same cycles have been applied either in an order with frequently changing amplitudes or in packages of cycles, it is demonstrated that such bundling is conservative. Predictions by the HCA model of Niemunis et al. [19] are confronted with experimental data and with other approaches for the prediction of permanent deformations under packages of cycles, among them the frequently cited procedure of Stewart [26]. An effect not captured in the HCA model or in any other approach yet has been detected in another series of tests with a change of the average stress between bundles of cycles: The monotonic loading associated with this change can partially or fully erase the memory of the sand regarding its cyclic preloading history.

A variational formulation for thermomechanically coupled low cycle fatigue at finite strains

In this paper, a constitutive model suitable for the analysis of Low Cycle Thermo-Mechanical Fatigue in metals is elaborated. The model is based on finite strain elastoplasticity coupled to continuum damage theory. It is embedded into a thermodynamical framework allowing to consistently capture the interplay between mechanics and thermal effects. It is shown that the fully coupled constitutive model can be re-written into a variationally consistent manner such that all (state) variables follow jointly and naturally from minimizing an incrementally defined functional. By discretizing this time-continuous functional in time by means of implicit integration schemes a numerically efficient implementation is proposed. In order to predict the temperature increase caused by plastic deformations realistically, the pre-loading history of the considered specimen is accounted for by non-zero initial internal variables. A comparison of the results predicted by the novel constitutive model to those corresponding to experiments (Ultimet alloy) shows that the predictive capabilities of the final model are excellent.

The effect of drained pre-shearing on the undrained behaviour of loose sand with a small amount of fines

Stress history is recognised to play a major role in determining stress–strain behaviour of soil in undrained shearing. Most experimental studies on the effects of stress history on undrained behaviour are mainly limited to clean sand. In this paper, an experimental study carried out to investigate the effect of stress history on the undrained behaviour of loose sand with a small amount of fines is presented. Four series of triaxial compression tests, with different types of drained stress histories to near-failure prior to commencement of undrained shearing, were conducted. The experimental results indicate that drained pre-shearing to near-failure affects significantly the undrained behaviour of loose sand. In general, the drained pre-shearing improves the subsequent undrained behaviour of loose sand to the extent that liquefaction may not occur. It is shown that the effect of drained pre-shearing cannot be explained by the reduction in void ratio induced by drained pre-shearing. However, for specimens subjected to drained pre-shearing, \( p_{{{\text{d}}0}}^{\prime } \)/\( p_{{{\text{u}}0}}^{\prime } \) can be used as a parameter for analysing the effects of preloading history. It is also shown that for different preloading histories that brought the same change in void ratio or state parameter, drained pre-shearing to near-failure is the most effective, whereas pre-compression alone is the least effective in improving subsequent undrained behaviour of loose sand.

Speed and reloading effects on pavement rutting

This paper concentrates on two factors that affect the permanent deformation (rutting) of unbound pavement layers. These factors are the loading speed and stress history. The loading speed has a simultaneous two-way effect: the permanent response of the unbound material itself, and the change in the stress state depending on the resilient properties of the upper-bound layers. The modelled examples show that the dominating factor in the speed effect is the change in stress state due to the change in the resilient properties of the bound layers, whereas the speed effects on the unbound material itself have a smaller role. In addition, the effect of loading speed depends greatly on the temperature, varying from 10–15% at 10°C to 20–25% at 25°C. The measured effects can be estimated reasonably well with modelling. Unloading—reloading cycles at various load levels have little effect on the permanent deformation. The effect is usually so small that it can be neglected in calculations, but the preloading history itself has a clear effect on the permanent deformation.

Probing the Elastic-Plastic, Time-Dependent Stress Response of Test Fasteners Using Finite Element Analysis

Abstract The evolution of global and local stress/strain conditions in test fasteners under test conditions is investigated using elastic-plastic, time-dependent finite element analyses (FEA). For elastic-plastic response, tensile data from multiple specimens, material heats and test temperatures are integrated into a single, normalized flow curve from which temperature dependency is extracted. A primary creep model is calibrated with specimen- and fastener-based thermal relaxation data generated under a range of times, temperatures, stress levels, and environments. These material inputs are used in analytical simulations of experimental test conditions for several types of fasteners. These fastener models are constructed with automated routines and contact conditions prescribed at all potentially mating surfaces. Thermal or mechanical room temperature preloading, as appropriate for a given fastener, is followed by a temperature ramp and a dwell time at constant temperature. While the amount of thermal stress relaxation is limited for the conditions modeled, local stress states are highly dependent upon geometry (thread root radius, for example), preloading history and thermal expansion differences between the test fastener and test fixture. Benefits of this FE approach over an elastic methodology for stress calculation will be illustrated with correlations of stress corrosion cracking (SCC) initiation time and crack orientations in stress concentrations.

Strain accumulation in sand due to cyclic loading: drained triaxial tests

Our aim is the prediction of the accumulation of strain and/or stress under cyclic loading with many (thousands to millions) cycles and relatively small amplitudes. A high-cycle constitutive model is used for this purpose. Its formulas are based on numerous cyclic tests. This paper describes drained tests with triaxial compression and uniaxial stress cycles. The influence of the strain amplitude, the average stress, the density, the cyclic preloading history and the grain size distribution on the direction and the intensity of strain accumulation is discussed.

The Effects of Preloading History on the Undrained Behaviour of Saturated Loose Sand

ABSTRACT This paper presents a comprehensive analysis of the effects induced by drained and undrained preshear and, in general, by preloading history on the undrained behaviour of loose sand prepared by moist tamping. The results are interpreted from the point of view of understanding the evolution of elastic anisotropy and the shape of the yield surface. The undrained behaviour of loose sand is shown to be a powerful and expeditious means for analysing the effects of stress-induced anisotropy. At first the reliability of this approach is evaluated through a comparison of the undrained effective stress paths with the stress states corresponding to a stiffness change along drained stress paths of samples subjected to the same preloading history. Subsequently the effects induced by isotropic preconsolidation, by drained preshear at different maximum stress ratios and along different stress paths, by undrained preshear and, finally, by drained cyclic preshear are analysed in detail on samples with similar void ratios at the beginning of the undrained loading phase; the results are discussed in terms of the evolution of the yield surface and elastic anisotropy. This paper is consequently both a further development in the analysis of the effects of preloading history on the undrained behaviour of saturated loose sand and another step towards a better understanding of the mechanisms governing the evolution of mechanical anisotropy in loose sands along triaxial stress paths.

Theoretical investigation of the undrained stability of shallow submerged slopes

The problem of the criteria of stability of the equilibrium state of an infinitely long slope, with constant inclination, is tackled and the conditions of spontaneous collapse of the slope are proposed. An experimental programme is described to define the stability locus for loose Hostun RF sand. The validity of the instability line concept introduced by Lade for isotropically consolidated virgin specimens is corroborated. It is further shown that this instability line can still be defined when specimens are anisotropically consolidated, although then the slope of the line is greater. It is shown that an instability line cannot be defined for specimens pre-loaded in drained conditions and subsequently reloaded undrained, as the level at which peak occurs in undrained conditions is dependent on the pre-loading history. The results are examined in the light of elastoplastic strain-hardening theory. By using a model to allow for mixed isotropic–kinematic hardening, all the experimental phenomena observed can be accurately reproduced. The validity of the model is further corroborated in tests made to highlight some peculiar model predictions. The model is then used to predict the onset of instability in subaqueous slopes. The relevance of the in situ stress conditions, the preloading history and the kinematic constraints on the collapse mode are discussed. A possible explanation of why flow slides are not as common as one could predict on the basis of the collapse surface or instability line concepts is given. On étude d'abord le problème du critère de stabilité de l'état d'équilibre d'une pente infiniment longue, avec inclinaison constante, et on discute les conditions d'instabilité spontanée de la pente. Un programme experimental est décrit épour la détermination de la zone de stabilité pour le sable d'Hostun RF. La validité du concept de ligne d'instabilité introduit par Lade pour des échantilIons consolidés isotropiquement est vérifiéve. On montre de plus que on peut définir cette ligne d'instabilité même pour des échantillons avec consolidation anisotrope quoique dans ce cas la pente de la ligne soit plus grande. On montre que on ne pent pas définir la ligne d'instabilité pour des échantillons préchargés en conditions drainées et en suite recharges en conditions non drainées, le niveau auquel le pic se vérifie en conditions non drainées étant trés dépendant de l'histoire de chargement. Les résultats sont examinés à la lumière de la théorie de l'élastoplasticité. On montre qu'à l'aide d'un model avec écrouissage mixte isotropique-cinématique on peut reproduire avec soin tous les phénomènes expérimentaux observés. La validité du model est vérifiée avec des essais pour mettre en éidence certaines predictions particuliéres. Le model est alors utilisé pour définir le début de l'instabilité des pentes sous-marines. On discute l'importance des conditions de la contrainte in situ, de l'histoire de chargement et des conditions cinématiques sur le mode d'effondrement. Une explication possible pour laquelle les 'flow slides' ne sont pas aussi fréquent que prédis sur la base de la surface d'effondrement ou du concept de la ligne d'instabilité est finalement donnée.

Influence of Pre-Loading History on the Fracture Toughness of A533B-1 Steel

Influence of Pre-Loading History on the Fracture Toughness of A533B-1 Steel