Complex Loading Conditions Research Articles

Improved incrementally affine homogenization method for viscoelastic-viscoplastic composites based on an adaptive scheme

We proposed an adaptive incrementally affine method, a micromechanics-based mean-field homogenization scheme for viscoelastic-viscoplastic particle-reinforced composites, which is applicable for predicting the mechanical response under complex loading conditions. The formulation is based on an incrementally affine scheme using algorithmic tangent operators while adaptively adjusting the mean strain of each constituent at every step of the loading process to reflect the changes in tangent operators and the shape of the reinforcing particles. We found that its prediction beyond the viscoelastic regime can be further improved by enforcing consistency between the accumulated strain states of each phase and the concentration tensors, excluding accumulated affine strain and stress. It was inevitable that the plastic deformation of the composite was initiated earlier than what was predicted by the mean-field theory owing to the local stress concentration near the particles. Hence, we proposed a yield reduction method that enforces the earlier initiation of plastic deformation in the matrix phase when obtaining an effective mechanical response. We observed that the predictions of the adaptive incrementally affine scheme adjusted with the yield reduction agreed well with various numerical simulations of particle-reinforced composites considering viscoelastic, elastic-viscoplastic, and viscoelastic-viscoplastic matrices under uniaxial, cyclic, and biaxial loadings.

Bending and Crack Evolution Behaviors of Cemented Soil Reinforced with Surface Modified PVA Fiber.

To improve the flexural properties of cemented soils reinforced with fibers and avoid their brittle failure when subjected to complex loading conditions, a simple and cost-effective technique was explored to facilitate their application in retaining walls. In this study, how different fiber surface modifications, i.e., alkali treatment, acid treatment and silane coupling agent treatment, as well as different fiber contents, i.e., 0%, 0.25%, 0.5% and 1%, affect the bending properties of cemented soils was investigated by conducting three-point bending tests on notched beams. The digital image correlation (DIC) technology was used to examine the crack propagation process and the strain field distribution of cracks in specimens in the flexural tests. The results show that all fiber surface modifications increased peak strength and fracture energy, for example, the fracture energy of specimens AN1, AH1 and AK1 was increased by 180.4%, 121.5% and 155.4%, respectively, compared to PVA1. In addition, the crack tip strain, crack propagation rate and the initial crack width of the modified specimens were lower than those before modification. Lastly, scanning electron microscope (SEM) and mercury intrusion porosimetry tests were adopted to reveal the mechanism of bending performance in cemented soils reinforced by fiber surface modifications.

A Coarse-Mesh hybrid structural stress method for fatigue evaluation of Spot-Welded structures

A hybrid structural stress method is presented for significantly simplifying spot weld representations in fatigue evaluation of complex spot-welded structures while retaining a high degree of accuracy in structural stress computation. The method is formulated by extracting nodal forces and moments around a group of domain elements connected to a spot weld represented by a regular beam element. Through a systematic decomposition technique, existing closed-form solutions, previously only valid for modeling single-spot weld test specimens, can now be used for calculating the relevant structural stresses under complex loading conditions in structures, as validated its ability in correlating fatigue test data.

On the bending of MS1-P20 hybrid steels additively manufactured using laser powder bed fusion

Maraging steel (MS1)-tool steel (P20) bimetals additively manufactured using the laser powder bed fusion technique were studied under different heat treatment cycles and loading conditions. The hardening of P20 and aging of MS1 were performed sequentially on the hybrid samples. The interfacial characteristics and microstructural evolution of the bulk materials were studied using various advanced electron microscopy techniques. The post-processing procedures successfully produced a uniform martensitic structure throughout the MS1-P20 hybrid steels, leading to a less detectable interface under electron backscatter diffraction (EBSD) imaging. The mechanical performance of heat-treated hybrid steels was evaluated using complex loading conditions. 3-point and 4-point bending tests were performed to assess the impact of heat treatments on the mechanical performance of the hybrid steels. The heat-treated samples exhibited higher strength with relatively homogeneous hardness variations and deformed more uniformly in bending conditions.

An experiment-based cohesive-frictional constitutive model for cemented materials

Experiment-based damage laws at the contact bond level are often absent in the computational modelling of cemented materials using cohesive contact-bond models. In this paper, in-situ X-Ray Computed Tomography on element-scale cement bonds subjected to pure compression, shear and tensile loading was conducted to investigate their failure mechanisms and associated damage laws. A new cohesive-frictional constitutive model was subsequently developed using results obtained from X-Ray CT tests. The new constitutive model was capable of describing the behaviour of contact bonds subjected to complex loading conditions, including mixed-mode and compression damage. The new model was then implemented in an open-access Discrete Element Code (YADE) to simulate several challenging laboratory-scale tests (e.g., triaxial, Brazilian and Splitting tests) and very good agreements with the experimental data were achieved.

Experimental study on cracking behaviors of coarse and fine sandstone containing two flaws under biaxial compression

AbstractIn this paper, the effects of materials and confining pressures on the mechanical properties of flawed coarse and fine sandstone is first analyzed. Then, the evolution processes of spatiotemporal acoustic emission (AE) events of coarse and fine sandstone specimens containing two flaws subjected to biaxial compression are investigated to character the damage characteristics and the fracture mechanism. Third, the ultimate failure modes of coarse sandstone specimens are compared with those of the fine sandstone specimens. Finally, the Electron Probe Microscope Analysis (EPEI) technique is conducted to observe the main macrofracture surfaces of coarse and fine sandstone specimens. Besides, the distribution characteristics of two AE indexes are also employed to qualitatively reveal the failure mechanism of tested specimens. The experimental results can provide new insight into the mechanical responses and the cracking behaviors of flawed rocks in complex stress loading conditions.

Additively manufactured dual-functional metamaterials with customisable mechanical and sound-absorbing properties

ABSTRACT Acoustic metamaterials with broadband sound-absorbing capacity open up applications in aerospace, automotive, marine, defense, etc. For such applications, sound-absorbing materials that can withstand complex loading conditions are essential. Hence, to address acoustic and mechanical requirements simultaneously, we propose plate-reinforced dual-functional micro-lattice metamaterials (PDMMs) that exhibit elastic isotropy, dual crushing stages with a specific energy absorption up to 25.82 kJ/kg, and ultra-broadband sound absorption from 0.97 kHz to 6.30 kHz. The remarkable elastic isotropy lies in the topology-induced structural stiffness homogenising effect. The transition from single to dual plateau anti-compression stages is controlled by tailoring the structural local strength. On-demand broadband sound absorption is achieved by modulating the parallel coupling and cascade resonance effects, and the physical mechanism is revealed by examining impedance matching and system damping states. Overall, the presented novel metamaterials exhibit exceptional application potentials by overcoming the trade-offs usually found in traditional mechanical and acoustic metamaterials.

Anisotropic cyclic plasticity modeling for additively manufactured nickel‐based superalloys

AbstractThe additively manufactured materials show anisotropic mechanical properties. In the present paper, a constitutive model for a nickel‐based superalloy made by selective laser melting was established to characterize cyclic mechanical behaviors under multiaxial loading conditions. Detailed material tests revealed that the effects from the building orientation decrease with multiaxial cyclic loads. A cyclic constitutive model based on the Hill criterion was introduced for the superalloy and considered the orthotropic mechanical properties depending on loading cycles. The computation confirms the model can reasonably simulate the cyclic elastic‐plastic behavior under non‐proportional complex loading conditions and provides an applicable method for engineering applications.

Experimental study of hot spot stress for three-planar tubular Y-joint: II. Combined loads

The tripod foundation structure is typically used for offshore wind turbines, in which fatigue design under complex load conditions involving axial force, in-plane bending moment, out-of-plane bending moment, and torque is vital. Experimental data show that the hot spot stress (HSS) obtained using the Det Norske Veritas recommended method may lead to inaccurate evaluation of the fatigue life of multiplanar tubular joints. Therefore, this study investigates the HSS distribution along the weld seams of a three-planar tubular joint under combined load cases using model tests. This is in parallel to the experiment on basic loads reported in the companion paper. The geometric stress at key locations and the peak values of the stress concentration factor under 17 load cases were analyzed. The influence of torque on the HSS was statistically investigated using experimental data. An indirect method to evaluate HSS induced by complex load conditions is proposed based on the summary of the companion paper.

On the accuracy of general method adapted in EN 1993-1-1

In this paper, a safety assessment of the General Method (GM) in Eurocode 1993–1-1:2005 is carried out. The GM covers the stability verification of steel structures subjected to compression and/or bending even in cases where the structures have irregular shape, complex load and support conditions. Several research papers dealt with the accuracy of the GM, but within the method, all of them used the Ayrton–Perry formula type standard reduction factors calibrated for the fundamental (flexural and lateral-torsional) buckling modes. These studies compared the results of the GM to the results of the geometrically and materially nonlinear analysis with imperfections (GMNIA) applying characteristic numerical models. In this paper, firstly the exact imperfection factors of fundamental buckling modes are calculated with GMNIA, after that these factors are used within the GM. This way the effect of the inaccuracy of the standard calibrations is excluded, and the real accuracy of the GM will be reflected in the results. This study covers the fundamental case of coupled flexural and lateral-torsional buckling modes in case of hot-rolled IPE and H-type cross-sections.

A silanized MCNT/TPU-based flexible strain sensor with high stretchability for deformation monitoring of elastomeric isolators for bridges

Laminated elastomeric bearings are widely used in large bridges for adjusting the deformation and ensuring the safety of the bridge structure. Interestingly, the bridge bearing’s deformation contains rich information that reflects the complex load conditions of the bridge structure. Nevertheless, the structural health monitoring (SHM) of the bridge bearings has not been carried out effectively due to the special characteristics of bridge bearings’ deformation, including the large range and the existence of varied forms such as shear, compressive and rotation. Along these lines, a novel conductive polymer composite-based flexible strain sensor was fabricated by employing a simple solution-blending method. Specifically, the proposed strain sensor was composed of thermoplastic polyurethane (TPU), multiwall carbon nanotubes (MCNTs), and silane coupling agent KH550. The utilization of the KH550 not only promoted the dispersion of MCNTs within the TPU matrix but also enhanced the interfacial bonding between the MCNTs and the TPU matrix, resulting in the large strain range (∼150%) of the silanized MCNT/TPU strain sensor. The silanized strain sensor with 3.0 wt% MCNTs demonstrated extraordinary linearity, promising gauge factor (∼8.26), and great durability under a tensile strain up to the value of 150% through the application of consecutive stretch–release tests. Furthermore, the compressive bending sensing tests proved the stable and repeatable bending sensing capability of the proposed silanized strain sensor. Moreover, the silanized strain sensor could effectively monitor the shear and the compressive deformation of the laminated elastomeric bridge bearings. This work provides a new way for fabricating flexible strain sensors with enhanced strain range, which validates the feasibility of the strain sensor for monitoring the bearing’s deformation and provides an experimental basis for developing the SHM of laminated elastomeric bridge bearings.

Fast nonlinear mechanical features decoupling to identify and predict asphalt-based composites

Nonlinear matrices employed in asphalt-based composites exhibit a prominent nonlinear elastic-viscoplastic-viscodamageable mechanical response. The constitutive model for this type of matrices requires a significant experimental effort to identify the material constants. To alleviate the characterization effort without losing reliability in the mechanical prediction, this paper presents a fast procedure decoupling the elasto-viscoplastic response from the viscodamageable one and facilitating the identification of the material constants via efficient optimization. This procedure relies on a combined experimental–numerical also applicable to other rheologically complex materials like polyurea, thermoplastics or elasto-viscoplastic polycrystals. The experimental part is deliberately designed to only conduct cost- and time-effective monotonic loads at different strain rates in tension and compression. These pure monotonic results are used in the constitutive model to predict more complex loading conditions such as multiple load–unload–reload (LUR) cycles. To do this, an elasto-viscoplastic constitutive model suitable to describe creep-like phenomena and large irreversible deformations is proposed. This model incorporates pressure and strain rate sensitivity, which is essential to capture the compression–tension asymmetry in asphalt-based composites. A rate-dependent damage model is proposed to describe the degradation rate of the elasto-viscoplastic part. The stress and the consistent tangent modulus are derived. The model implementation for user-defined finite element subroutines are given for explicit or implicit solvers. The proposed decoupling facilitates an efficient identification of the constant parameters via an in-house Nelder–Mead-based optimization method. To accelerate the identification, all the stress–strain curves, either in tension or compression, are simultaneously used with applying physically-based constrains. The framework is validated with asphalt matrix at room temperature (24 °C) and verified under LUR conditions at different rates. The identified model predicts correctly the experimental observations, proving its applicability to investigate particle-based composites with highly nonlinear matrices.

Interaction of a parabolic notch with a generalized singularity

In this paper, the interaction of a parabolic notch with a generalized singularity is studied and its analytical solution is derived. The generalized singularity may represent a concentrated force or an edge dislocation. As an example, the parabolic notch interacting with a dislocation is studied in details, and the driving force on the dislocation due to the notch configuration is obtained in terms of the vector J-integral. It is found that a dislocation-free zone may exist beneath the notch root surface. The solutions developed in this study give, on the one hand, basic information about the state of stress induced in the notch neighborhood, and, on the other hand, can be used as building blocks to model damage at the notch root under complex load conditions.

Mechanical Characteristics of Prestressed Concrete Curved Transition Section of Composite Bucket Foundations for Offshore Wind Turbines

The composite bucket foundation (CBF) consists of a concrete curved transition section, a concrete beam-slab system, and a suction caisson and is increasingly used as the foundation for offshore wind turbines. The curved transition section transmits the upper load from the tower to the foundation, and its force and transmission characteristics are related to the safety of the entire wind turbine structure. The arced transition section has the characteristics of complex geometry, load conditions, and large curvature. It is difficult to determine its bearing characteristics and force transmission system. In this paper, the boundary conditions and loading device of the CBF model test are designed, and three 1:20 arced transition section model specimens are made. The mechanical characteristic experiments of CBF are used to analyze the failure process, failure characteristics, and seismic performance of the structure. Results show that the cracking effect of the arced transition section after prestressing is obviously better than that of a reinforced concrete arced transition section structure. The arced transition section specimens equipped with prestressed tendons can increase the structural cracking load ratio by about 35% for reinforced concrete members. The energy dissipation capacity of the specimens has been significantly improved, and the material properties can be fully utilized. The failure mode of the arced transition section structure under horizontal reciprocating load shows the characteristics of bending and shear failure.

A novel deep learning approach of multiaxial fatigue life-prediction with a self-attention mechanism characterizing the effects of loading history and varying temperature

A novel deep learning approach is established in this work to directly model the highly nonlinear mapping between the complex loading conditions (input) and the multiaxial fatigue life (output). An advanced deep learning mechanism, named as self-attention mechanism, is incorporated in this approach to characterize the effects of complex loading history and varying temperature on the fatigue life. Three typical examples are performed to verify the capability of proposed approach to achieve the mechanical and thermo-mechanical multiaxial fatigue life-predictions. The results demonstrate that both the effects of loading history and varying temperature on the multiaxial fatigue life are reasonably captured, and the predicted lives by the proposed approach are almost located within the scatter band of 1.5 times.

Dynamic fracture behaviors and fragment characteristics of pre-compressed flawed sandstones

Because of tectonic stress and dynamic disturbance, underground rock masses containing pre-existing flaws usually undergo complex loading conditions combining static pre-stress (SPS) and external dynamic loads. Comprehending the mechanical behaviors of flawed rocks under coupled static-dynamic loads is helpful to assess the stability of underground engineering. This study presented a series of coupled static-dynamic compression experiments on sandstone containing two non-coplanar flaws. The results indicated that the dynamic strength of the flawed sandstone increased first and then decreased with the increase in the SPS for a given dynamic strain rate (DSR), and the coupled strength of the flawed sandstone was less dependent on the DSR under the SPS of 60% of the uniaxial compressive strength than the other SPS values. Low SPS and DSR generally induced an asymmetrically X-shaped shear failure mode of the specimen, while this failure pattern could be observed in the tests with high SPS and DSR. Moreover, both a greater SPS and a higher DSR could lead to smaller location parameter values in the generalized extreme value fitting and a higher fractal dimension of the rock fragments, resulting in a more severe crushing destruction of the flawed sandstone. In addition, this study demonstrates that both the fracture process zones and the higher-order stress terms in the Williams series expansion, which are often neglected in traditional related studies, cannot be thoughtlessly ignored when analyzing the crack initiation, propagation, and coalescence of flawed rocks.

Multiple Earthquake Effects on Vulnerability of Horizontally Curved RC Bridges

ABSTRACT Bridges subjected to mainshock–aftershock sequences experience significant damage as a result of repeated shaking. Therefore, neglecting the effects of multiple earthquakes may lead to a considerable underestimation of bridges seismic vulnerability. The uncertainties in modeling bridge structural systems as well as the representation of seismic loads significantly affect the bridge vulnerability, especially for horizontally curved bridges, where complex loading conditions exist. In this study, fragility curves for horizontally curved bridges are developed for both mainshock only and mainshock–aftershock sequence cases. The seismic response of a typical four-span horizontally curved reinforcement concrete box-girder bridge is evaluated using the cloud analysis method and the effects of uncertainties on the probability of failure of the bridge components are examined and quantified using the Latin hypercube sampling technique. The results from this investigation indicate that considering the mainshock only could significantly underestimate the damage level even by around 50% and that incorporating uncertainties could change the bridge capacity by more than 45% of its estimated value.

Shakedown analysis of bounded kinematic hardening engineering structures under complex cyclic loads: Theoretical aspects and a direct approach

There exist two basic formulations of static shakedown theorem for bounded kinematic hardening (KH) model in literature. Whether the two formulations are equivalent, whether the conditions of loading-path-independence are satisfied, and how to employ the shakedown theorem to obtain the safety margin of realistic engineering structures under cyclic loading are some key issues to be solved and addressed. This paper presents theoretical aspects of the static shakedown theorem of KH and proposes a direct approach for shakedown analysis of KH structures. The presented direct approach allows for both the two shakedown formulations and considers the simultaneous influences of multiple load vertices. The two formulations are suitable for plastic models with different hardening laws and determine different shakedown load boundaries in the region of incremental collapse. Instead of dealing with a complicated problem of mathematical programming, the presented approach performs several linear elastic finite element iterative calculations, which ensures its high computational efficiency. Furthermore, the direct approach is implemented into Abaqus software platform so that it becomes a general tool to handle large-scale shakedown problems of engineering structures with complex geometry and loading conditions. An illustrative sample with analytical solution and three numerical examples from power plant engineering are adopted to validate these theoretical aspects and to present the potential of the direct approach.

Design of Triaxial Tests with Polymer Matrix Composites.

Multiaxial testing in composites may generate failure modes which are more representative of what occurs in a real structure submitted to complex loading conditions. However, some of its main handicaps include the need for special facilities, the correct design of the experiments, and the challenging interpretation of the results. The framework of this research is based on a triaxial testing machine with six actuators which is able to apply simultaneous and synchronized axial loads in the three space directions. Then, the aim was to design from a numerical point of view a triaxial experiment adapted to this equipment. The methodology proposed could allow for an adequate characterization of the triaxial response of a polymer-based composite with apparent isotropic behaviour in the testing directions. The finite element method (FEM) is applied in order to define the geometry of the triaxial specimen. The design pursues to achieve homogeneous stress and strain states in the triaxially loaded region, which should be accessible for direct measurement of the strains. Moreover, a fixing system is proposed for experimentally reproducing the desired boundary conditions imposed on the numerical simulations. The procedure to determine the full strain tensor in the triaxially loaded region is described analytically and with the help of FEM virtual testing. The hydrostatic component and the deviatoric part of the strain tensor are proposed for estimating the susceptibility of the polymer-based composite to fail due to the triaxial strain state imposed. Then, the loading scenarios that cause higher values of the deviatoric components in the triaxially loaded region are considered to be more prone to damage the region of interest. Nevertheless, the experimental failure is expected to be produced in the arms of the specimen which are uniaxially loaded, since in all of the loading cases the simulations show higher levels of stress concentration out of the triaxially loaded region. Thus, although the triaxial strength could not be accurately determined by the proposed tests, they can be utilized for observing the triaxial response before failure.

Mesh manipulation for local structural property tailoring of medical warp-knitted textiles

Medical meshes are used as structural reinforcement both in clinical surgery and tissue engineering. However, complex loading conditions often found in such applications result in a non-homogenous stress distribution, for which the uniform reinforcement provided by the meshes is not optimal. This study aims to design a textile reinforcement with a spatially heterogeneous, load-tailored fiber architecture. To this end, we developed a simple method of manipulating a standard uniform mesh by stretching in warp and weft directions to various extents in order to control fiber orientation and fiber volume fraction. Subsequent thermal treatment locked the manipulated configurations allowing further use of the meshes. Firstly, samples in five uniform configurations (two manipulated longitudinally (warp direction), two manipulated transversely (weft direction), one non-manipulated) were obtained and analyzed regarding their morphology as well as their mechanical properties under cyclic uniaxial loading. Significant effects of the manipulation on key characteristics of the pores such as angles, side lengths, aspect ratios, and fiber volume fraction were shown. Tensile testing demonstrated the range of tensile properties achievable with the simple manipulation of the mesh, not only in magnitude but also in the shape of the stress-strain response curve. Finally, local manipulation combining different mesh configurations was exemplarily applied to create a spatially heterogeneous load-tailored reinforcement to match local strain directions in tissue-engineered tubular heart valves. The proposed method enables the use of well-established uniform medical meshes to produce load-tailored non-uniform mesh reinforcement for many applications in an easy-to-implement manner.