Various Types Of Loading Conditions Research Articles

Creep Response of Neat and Carbon-Fiber-Reinforced PEEK and Epoxy Determined Using a Micromechanical Model

A micromechanical model is developed to study the creep phenomena with neat and carbon-fiber-reinforced PEEK (Polyetheretherketon) and epoxy. The model considers that the continuous elastic circular fibers form a regular array inside the matrix material. In this study, the fibers are considered to be linear elastic and anisotropic, while the matrix has a nonlinear viscoelastic behavior. The approach describes the time-dependent response of unidirectional viscoelastic composites subjected to various types of loading conditions. A comparison between the finite element analysis and the proposed micromechanical model shows a good agreement. Experimental tests validate the results obtained using the proposed theoretical model.

Reducing Beat Frequency Oscillation in a Two-phase Sliding Mode-controlled Voltage Regulator Module

During static and dynamic loading conditions, voltage regulator modules (VRMs) are expected to provide regulated voltage with minimal ripple even at high current requirement. Compared to regular power supplies, VRMs repetitively experience high-frequency loading conditions that is greatly dependent on the software running in the processor utilizing them. In the scenario that when the transient load frequency is near the VRM’s switching frequency, high-magnitude and low-frequency oscillations are observed at the phase currents. This phenomenon is called the beat frequency oscillation. In this study, the sliding mode control principle is employed to both the voltage and current share loops of the VRM to reduce the phase currents’ beat frequency oscillations. A fixed frequency sliding mode controller is derived and extensively evaluated using the PSIM simulator. Our results show that while maintaining equal load sharing among VRMs at less than 5% sharing error and various types of loading conditions, the sliding mode controller can reduce the beat frequency oscillation phenomenon to 20 kHz at maximum with reduced peak current values. The output voltage is also regulated within the desired ± 1.65% band.

Reducing Beat Frequency Oscillation in a Two-phase Sliding Mode-controlled Voltage Regulator Module

During static and dynamic loading conditions, voltage regulator modules (VRMs) are expected to provide regulated voltage with minimal ripple even at high current requirement. Compared to regular power supplies, VRMs repetitively experience high-frequency loading conditions that is greatly dependent on the software running in the processor utilizing them. In the scenario that when the transient load frequency is near the VRM’s switching frequency, high-magnitude and low-frequency oscillations are observed at the phase currents. This phenomenon is called the beat frequency oscillation. In this study, the sliding mode control principle is employed to both the voltage and current share loops of the VRM to reduce the phase currents’ beat frequency oscillations. A fixed frequency sliding mode controller is derived and extensively evaluated using the PSIM simulator. Our results show that while maintaining equal load sharing among VRMs at less than 5% sharing error and various types of loading conditions, the sliding mode controller can reduce the beat frequency oscillation phenomenon to 20 kHz at maximum with reduced peak current values. The output voltage is also regulated within the desired ± 1.65% band.

Capacity Evaluation of Ressalat Jacket of Persian Gulf Considering Proper Finite Element Modeling of Tubular Members

The capacity curve obtained from the pushover analysis of jacket-type offshore platforms gives their structural performance levels, ultimate capacity and ductility. Accurate estimation of structural capacity curve is of great importance. Accurate modeling of the global and local buckling of compression tubular members in a correct form is an effective part of studying the behavior of offshore jackets under all various types of loading conditions at any given time of their life. Modeling of compressive braces by shell or solid elements when the imperfections are applied leads to deformations due to local buckling based on buckling modes. This paper aims to achieve more accurate compressive behavior of compression members. The ABAQUS finite element software has been used for this purpose. Regarding to the results achieved from investigation of buckling in tubular members proper elements have been introduced to investigate the global and local buckling phenomena. Then pushovers results of Ressalat jacket with conventional modeling versus more accurate modeling proposed in this paper for compressive members have been compared as a case study. According to the results applying improper mesh size for compressive members can under-predict the ductility by 33% and under-estimate the lateral loading capacity up to 8%. Finally, ISO equations and Marshall strut theory have been applied to investigate critical buckling load and post-buckling response of tubular braces. The innovation of this paper is investigating the interaction of global and local buckling in the braces of jacket with 1-Dimentional elements using ISO equations and buckling envelope derived from the solid element results, which results in low computational costs.

Behaviour of Plain Concrete and Steel Fibre Reinforced Concrete (SFRC) under Biaxial Stresses- A Review

This paper provides general overview on the development of biaxial behaviour of plain concrete and steel fibre reinforced concrete (SFRC) under various types of loading conditions, namely, biaxial compression, biaxial tension-compression and biaxial tension. The biaxial behaviour including failure envelope, ultimate strength, failure mode and stress-strain relationship of plain concrete and SFRC are reported and compared. The effects of fibre volumetric fraction of fibre in SFRC on the biaxial behaviour are also discussed. Overall, previous researchers show that the inclusion of fibre enhance the biaxial behaviour of the concrete, agreed with the biaxial failure envelope developed. However, further experimental works and investigation is required to determine the relationship between the biaxial behaviour of SFRC and its fracture toughness, in addition to prove the analytical prediction of biaxial behaviour of SFRC especially in biaxial tension-compression and biaxial tension.

Simplified material model for analysis of asphalt core in embankment dams

Asphalt concrete exhibits time- and temperature-dependent viscoelastoplastic behavior which is difficult to model and implement in practical dam design analyses. The main goal of the paper is to develop a simplified material model for the various types of loading conditions experienced by an asphalt core in an embankment dam. The types of loading conditions are simulated by conducting long-term triaxial creep tests with stepwise loading corresponding to the slow construction of the asphalt core and dam embankment; unloading-reloading cycles corresponding to seasonal reservoir fluctuations, and dynamic cyclic tests corresponding to earthquake loading. The results of the long-term triaxial creep tests show that creep strains accumulate until a “creep-stable state” is reached when virtually no further creep is occurring at that level of sustained deviator stress and temperature, and a corresponding “creep-stable stress-strain modulus” is defined. Based on the test results, a simplified material model expressed in terms of deviator stress is proposed and formulated analytically with a plastic yield boundary (PYB). The unloading-reloading and dynamic behavior was found to be almost linearly elastic, and the unloading-reloading modulus and dynamic secant modulus to be in the order of 20 and 40 times the creep-stable modulus, respectively. The proposed simplified material model and equivalent secant moduli may be used in finite element analyses of the asphalt core and its interaction with adjacent gravel and rockfill zones in the dam.

The “Inverse Hall-Petch” effect on the impact response of single crystal copper

Based on the available experimental and computational capabilities, a phenomenological approach has been proposed to formulate a hypersurface in both spatial and temporal domains to predict combined specimen size and loading rate effects on the material properties [1–2]. A systematic investigation is being performed to understand the combined size, rate and thermal effects on the properties and deformation patterns of representative materials with different nanostructures and under various types of loading conditions [3–16]. The recent study on the single crystal copper response to impact loading has revealed the size-dependence of the Hugoniot curve. In this paper, the “inverse Hall-Petch” behavior as observed in the impact response of single crystal copper, which has not been reported in the open literature, is investigated by performing molecular dynamics simulations of the response of copper nanobeam targets subjected to impacts by copper nanobeam flyers with different impact velocities. It appears from the preliminary results that the “inverse Hall-Petch” behavior in single crystal copper is mainly due to the formation and evolution of disordered atoms and the interaction between ordered and disordered atoms, as compared with the physics behind the “inverse Hall-Petch” behavior as observed in nanocrystalline materials.

Investigation on Buckling of Cylindrical Shells and Influences of Boundary Conditions

Thin-walled cylindrical shells are widely used in many industrial sectors as light structural elements. Determination of their buckling strength under various types of loading conditions is a crucial work for engineering design. Due to the needs of research of crashworthiness, dynamic buckling of cylindrical shells subjected to the strong axial impact becomes a frontier issue in recent years. The axial impact is a very complex dynamic process because of the coupling of multiple effects. In this paper, the buckling mechanism of cylindrical shells subjected to axial impact and the influences of boundary conditions, and energy absorption properties have been investigated by experiments.

Serum levels of natriuretic peptides in children with various types of loading conditions

Objectives. To evaluate the influence of volume overload of the left (LV) and right ventricle (RV) and pressure overload of LV and restrictive physiology on levels of N-terminal proatriopeptide (ANPN) and N-terminal pro-brain natriuretic peptide (NT-proBNP). Design. We studied 41 children with atrial septal defect (ASD), 35 with patent ductus arteriosus (PDA), 27 with coarctation of the aorta (CoA), 25 with restrictive physiology caused by Mulibrey nanism, and 64 control children. We measured serum concentrations of natriuretic peptides and evaluated ventricular size and function with echocardiography. Results. In patients with ASD, PDA, and Mulibrey nanism, levels of both ANPN and NT-proBNP were higher than in controls but in children with CoA, only ANPN levels were higher. ANPN levels correlated with RV size in ASD and NT-proBNP levels with LV size in PDA. In patients with restriction, NT-proBNP levels correlated negatively with LV size. Conclusions. Correlation between echo measurements and levels of natriuretic peptides varied according to loading condition. Measurement of natriuretic peptide levels provides a supplemental method for non-invasive haemodynamic evaluation of children's heart disease.

Closed-form structural stress and stress intensity factor solutions for spot welds in commonly used specimens

Closed-form new structural stress and stress intensity factor solutions for spot welds in lap-shear, square-cup, U-shape, cross-tension and coach-peel specimens are obtained based on elasticity theories and fracture mechanics. The loading conditions for spot welds in the central parts of the five types of specimens are first examined. The resultant loads on the weld nugget and the self-balanced resultant loads on the lateral surface of the central parts of the specimens are then decomposed into various types of symmetric and anti-symmetric parts. Closed-form structural stress and stress intensity factor solutions for spot welds under various types of loading conditions are then adopted from the recent work of Lin and Pan to derive new closed-form structural stress and stress intensity factor solutions for spot welds in the five types of specimens. The selection of a geometric factor for square-cup specimens and the decompositions of the loads on the central parts of the U-shape, cross-tension and coach-peel specimens are based on the corresponding three-dimensional finite element analyses of these specimens. The new closed-form solutions are expressed as functions of the spot weld diameter, the sheet thickness, the width and the length of the five types of specimens. The closed-form solutions are also expressed as functions of the angular location along the nugget circumference of spot welds in the five types of specimens in contrast to the limited available solutions at the critical locations in the literature. The new closed-form solutions at the critical locations of spot welds in the five types of specimens are listed or can be easily obtained from the general closed-form solutions for fatigue life predictions.

Fatigue life evaluation for multi-spot welded structures

A procedure using a conventional stress value (ERS, equivalent radial stress) to evaluate fatigue behaviour is applied to spot welded joints. The ERS is based on a closed-form solution of a theoretical bi-dimensional model of the spot weld area, under various types of loading conditions; ERS can be determined for every spot weld of the jointed structure. The closed-form solution is also digitalized in order to define a spot element which can be used in FE simulation of multi-spot structures, providing profit both in accuracy and in computing time saving. Through the use of ERS-N curves, fatigue data collected on different joint geometries can be successfully mixed together. One of the main aspects evidenced is that progressive damage deeply influences fatigue behaviour: a simple numerical solution neglecting accumulated damage is unable to foresee the whole fatigue life. Indeed, the final value of ERS is given accounting the progressive damage effects of spot welds using a non-linear damage-step procedure. In the present paper the method has been applied to many experimental results: it is shown that a single criterion is used to deal with several different structures and materials.

Theoretical Framework for Modeling Spot Welds under Various Types of Loading Conditions

Theoretical Framework for Modeling Spot Welds under Various Types of Loading Conditions

Closed-form structural stress and stress intensity factor solutions for spot welds under various types of loading conditions

The theoretical framework and closed-form stress intensity factor solutions in terms of the structural stresses for spot welds under various types of loading conditions are presented based on elasticity theories and fracture mechanics. A mechanics description of loading conditions for a finite plate with a rigid inclusion is first presented. The loading conditions of interest are the resultant loads on the inclusion with respect to the center of the inclusion in a finite or infinite plate and the surface tractions on the lateral surface of a finite or infinite plate. The surface tractions on the lateral surface of the plate can be decomposed into a load-balanced part and a self-balanced part. The load-balanced part is statically in equilibrium with the resultant loads acting on the inclusion. The self-balanced part can be represented by the resultant loads on the lateral surface of the plate. The resultant loads on the inclusion and the self-balanced resultant loads on the lateral surface are then decomposed into various types of symmetric and anti-symmetric parts. Based on the stress function approach and the Kirchhoff plate theory for linear elastic materials, closed-form in-plane stress, moment and transverse shear force solutions are derived for a plate with a rigid inclusion subjected to various types of resultant loads on the inclusion and various types of resultant loads on the plate lateral surface. Based on the J integral for a strip model, closed-form analytical stress intensity factor solutions for spot welds joining two sheets of equal thickness are derived in terms of the structural stresses around a rigid inclusion in a plate under various types of loading conditions. The closed-form solutions presented in this paper are used as the basis to develop new analytical stress intensity factor solutions for spot welds in various types of specimens presented in a subsequent paper.

A numerical study of the effect of loading conditions on tubular hydroforming

In this work, the mechanics of tubular hydroforming under various types of loading conditions is investigated. The main objective is to contrast the effects of prescribing fluid pressure or volume flow rate, in conjunction with axial displacement, on the stress and strain histories experienced by the tube and the process of bulging. To this end, axisymmetric finite element simulations of free hydroforming (without external die contact) of aluminium alloy tubes are carried out. Hill’s normally anisotropic yield theory along with material properties determined in a previous experimental study [A. Kulkarni, P. Biswas, R. Narasimhan, A. Luo, T. Stoughton, R. Mishra, A.K. Sachdev, An experimental and numerical study of necking initiation in aluminium alloy tubes during hydroforming, Int. J. Mech. Sci. 46 (2004) 1727–1746] are employed in the computations. It is found that while prescribed fluid pressure leads to highly non-proportional strain paths, specified fluid volume flow rate may result in almost proportional ones for the predominant portion of loading. The peak pressure increases with axial compression for the former, while the reverse trend applies under the latter. The implication of these results on failure by localized necking of the tube wall is addressed in a subsequent investigation.

Effects of particulate reinforcement and strain‐rates on deformation and fracture behavior of Aluminum 6061‐T6 under high velocity impact

AbstractAluminum matrix composites (AMC) exhibit an attractive combination of mechanical and physical properties such as high stiffness and low density, which favors their utilization in many structural applications. Thus, increasing the structural applications of AMC is the driving force for the need to adequately understand their deformation and failure mechanisms under various types of loading conditions. In this study, plastic deformation of alumina particle reinforced Aluminum 6061‐T6 matrix composite is investigated and compared to that of an un‐reinforced Aluminum 6061‐T6 alloy at high strain‐rates under compressive loading. Dynamic stress‐strain curves are obtained using direct impact Split Hopkinson Pressure Bar (SHPB). Particulate reinforcement increases the deformation resistance of the aluminum alloy at high strain‐rates. Strain localization along narrow adiabatic shear bands is observed in both the reinforced and un‐reinforced alloy. Whereas the microstructure of shear bands in un‐reinforced alloy showed finer grain size compared to that of the bulk material, the shear bands observed in the AMCs are darker than the bulk material and the reinforcing particles are observed to be more closely spaced along the shear bands.

Role of atomic scale interfaces in the compressive behavior of carbon nanotubes in composites

Carbon nanotubes (CNT) are potentially promising fibers for ultra high strength composites. In order to fully harness the outstanding mechanical properties of carbon nanotubes as fiber reinforcements, it is essential to understand the nature of load transfer between fiber and matrix under various types of loading conditions that include tension, compression, torsion and a combination thereof. In this paper, we study the compressive behavior (buckling and post-buckling) of carbon nanotubes in the neat form, when they are embedded in polyethylene matrix and with interface chemical modifications using molecular dynamics simulations based on Tersoff–Brenner potential. It is observed that the critical load for buckling increases only very marginally for nanotubes embedded in polythene matrix (with non-bonded interface) compared to neat CNTs. When CNTs are chemically bonded to the matrix, the compressive behavior occurs in two phases; pre- and post-buckling phases. First, the critical stress for buckling is found to reduce because the change in chemical bonding induces deviation from perfect cylindrical structure. In the post-buckling phase, however, the nanotubes behave similar to short fibers and deform by crushing. The results are compared with continuum solutions, wherever applicable. It is shown that the continuum solutions should be applied carefully whenever the effect of nanoscale interfaces becomes a factor.

Stability of an advancing crack to small perturbation of its path

In this work we investigate the stability of a nominally straight two-dimensional quasistatically growing crack to a small perturbation of its path. Formulae for perturbations of stress intensity factors induced by slight deviation of the crack trajectory were developed by Movchan et al. (Int. J. Solids Struct. 35, 3419) Their solution is exploited to derive an equation for the perturbation of the crack path on the assumption that the crack advances in pure “opening” mode (i.e. local K II=0). Various types of loading conditions are considered, including a cracked body loaded by a pair of point body forces and a crack whose faces are subjected to given tractions acting in the direction normal to the crack boundary. The body is also subjected to a remotely maintained uniaxial stress, aligned with the direction of the unperturbed crack. The loading is assumed to advance as the crack advances, to maintain the critical value of Mode I stress intensity factor. Numerical computations of possible crack paths have been performed, extending results on crack stability obtained by Cotterell and Rice (Int. J. Fract. 16, 155). The results show that in the case of loading by point body forces the stability of the crack path depends on the positions of the points of application of the applied forces and the magnitude of the applied stress acting parallel to the crack. There exists a critical value of this stress such that the crack path is stable for values less than critical and unstable otherwise. It is shown that the crack is always unstable in the case of point force tractions applied normal to the crack faces.

Micromechanical characterization of the non-linear viscoelastic behavior of resin matrix composites

A micromechanical analysis is proposed for the determination of the overall response of unidirectional composites with non-linearly viscoelastic resins. The non-linear behavior is modeled by the single-integral constitutive equations developed by Schapery. The method is implemented for the prediction of the non-linear viscoelastic behavior of unidirectional glass/epoxy and graphite/epoxy composites subjected to various types of loading conditions. The predicted response is compared with a finite element solution, and good agreement between the two methods is shown to exist.