Constant Mechanical Load Research Articles

Linking Genetics and Mechanics in Structural Protein Materials: A Case Study of an Alport Syndrome Mutation in Tropocollagen

Many proteins found in biological systems provide functional properties through the formation of hierarchical materials that feature distinct structures at multiple length-scales, from nano to macro. For example, collagenous tissues are composed of a complex assembly of triple helical collagen molecules and provide the basis for materials such as bone, tendon or connective tissues in organs. The breakdown of these materials due to defects, mutations or large forces can have serious consequences on the functioning of organisms. For example, Alport Syndrome is a severe genetic disease characterized by the breakdown of the glomerular basement membrane (GBM) around blood vessels in the kidney, leading to eventual kidney failure in most patients. It is the second most inherited kidney disease in the United States, and many other symptoms are associated with the disease, including hearing loss and ocular lesions. Here we probe the molecular level mechanisms of this disease, utilizing a bottom-up computational materiomics approach focused on the mutation associated with the most severe form of Alport Syndrome. Since the GBM is under constant mechanical loading due to blood flow, changes in mechanical properties due to amino acid mutations may be critical in the symptomatic GBM breakdown seen in Alport Syndrome patients. The purpose of this article is to review recent molecular dynamics simulations of structural and mechanical aspects of this disease at the single-molecule level (Srinivasan et al., Journal Of Structural Biology, 168(3), 503—510 (2010)), and to provide a general outlook into opportunities and challenges in the field of using molecular mechanics to improve our understanding of disease mechanisms (Buehler and Yung, Nature Materials, 8(3), 175—188 (2009)). We review a series of full-atomistic simulations in explicit solvent, which reveal the effects of single-residue glycine substitution mutations in a short segment of a collagen type IV tropocollagen molecule. Major changes are observed at the single-molecule level of the mutated sequence, including a bent shape of the structures after equilibration, with the kink located at the mutation site and a significant alteration of the molecule’s stress—strain response and stiffness. We also provide a general discussion on computational approaches to study the link between genetics and functional mechanical properties of tissues.

A Direct Method on the Evaluation of Ratchet Limit

This paper describes a new linear matching method (LMM) technique for the direct evaluation of the ratchet limit of a structure subjected to a general cyclic load condition, which can be decomposed into cyclic and constant components. The cyclic load history considered in this paper contains multiload extremes to include most complicated practical applications. The numerical procedure uses the LMM state-of-the-art numerical technique to obtain a stable cyclic state of component, followed by a LMM shakedown analysis, to calculate the maximum constant load, i.e., the ratchet limit, which indicates the load carrying capacity of the structure subjected to a cyclic load condition to withstand an additional constant load. This approach is particularly useful in conjunction with the evaluation of the stable cyclic response, which produces the cyclic stresses, residual stresses, and plastic strain ranges for the low cycle fatigue assessment. A benchmark example of a holed plate under the combined action of cyclic thermal load and constant mechanical load is presented to verify the applicability of the new ratchet limit method through a comparison with published results by a simplified method assuming a cyclic load with two extremes. To demonstrate the efficiency and effectiveness of the method for a complicated cyclic load condition with multiload extremes, a composite thick cylinder with a radial opening subjected to cyclic thermal loads and a constant internal pressure is analyzed using the proposed ratchet limit method. Further verification by the ABAQUS step-by-step inelastic analysis demonstrates that the proposed new method provides a general-purpose technique for the evaluation of the ratchet limit and has both the advantages of programming methods and the capacity to be implemented easily within a commercial finite element code Abaqus.

A comprehensive analysis for the shakedown of a Bree plate made of functionally graded materials

The shakedown of a functionally graded (FG) plate subjected to coupled constant mechanical load and cyclically varying temperature is analyzed comprehensively. The material of the plate is composed of an elastoplastic matrix and elastic particles, and the particle volume fraction varies through the thickness. The distributions of the effective mechanical and thermal properties of the composites through the thickness are evaluated with mean-field approaches and described with an exponential law. The temperature dependence of the material properties is taken into account. The distribution of temperature change and the shakedown of a typical FG Bree plate are analyzed. The comparison with the results of its homogeneous counterpart and that without considering the temperature dependence of the material properties exhibits marked qualitative and quantitative difference. The effect of the temperature dependence of the elastic properties of materials is also investigated. Since FG structures are usually subject to severe coupled thermal-mechanical loadings, the approach developed and the results obtained are significant for the analysis and design of such kind of structures.

Alport Syndrome mutations in type IV tropocollagen alter molecular structure and nanomechanical properties

Alport Syndrome is a genetic disease characterized by breakdown of the glomerular basement membrane (GBM) around blood vessels in the kidney, leading to kidney failure in most patients. It is the second most inherited kidney disease in the US, and many other symptoms are associated with the disease, including hearing loss and ocular lesions. Here we probe the molecular level structure-property relationships of this disease using a bottom-up computational materiomics approach implemented through large-scale molecular dynamics simulation. Since the GBM is under constant mechanical loading due to blood flow, changes in mechanical properties due to amino acid mutations may be critical in the symptomatic GBM breakdown seen in Alport Syndrome patients. Through full-atomistic simulations in explicit solvent, the effects of single-residue glycine substitution mutations of varying clinical severity are studied in short segments of type IV tropocollagen molecules. The segments with physiological amino acid sequences are equilibrated and then subjected to tensile loading. Major changes are observed at the single molecule level of the mutated sequence, including a bent shape of the structures after equilibration (with the kink located at the mutation site) and a significant alteration of the molecules' stress-strain responses and stiffnesses. These results suggest that localized structural changes at amino acid level induce severe alterations of the molecular properties. Our study opens a new approach in pursuing a bottom-up multi-scale analysis of this disease.

Static and kinematic shakedown analysis of FG plate subjected to constant mechanical load and cyclically varying temperature change

Abstract The static and kinematic shakedown of a plate made of functionally graded materials (FGMs) is analyzed. It is subjected to coupled constant mechanical load and cyclically varying temperature. The material is composed of elastoplastic matrix reinforced with linearly-elastic particles. The relevant mechanical and thermal properties of the plate are determined with self-consistent scheme and mixture theories. It is shown that the distributions of the material properties through the thickness of the plate can be approximated with exponential rules, provided the volume fraction of particles varies linearly through the thickness. The shakedown of an Al/SiC FG Bree plate is analyzed. The boundaries between the shakedown area and the areas of elasticity, incremental collapse and reversed plasticity are determined. The shakedown of its homogeneous counterpart with averaging material properties is also analyzed. The comparison between the results obtained in the two cases exhibits distinctly qualitative and quantitative difference, indicating the importance of a proper shakedown analysis for the FG plate. Since FG structures are usually subjected to coupled severe thermal–mechanical loads, the approach developed and the results obtained are significant for the analysis and design of FG structures.

Analysis of shakedown of FG bree plate subjected to coupled thermal-mechanical loadings

The static and kinematic shakedown of a functionally graded (FG) Bree plate is analyzed. The plate is subjected to coupled constant mechanical load and cyclically varying temperature. The material is assumed linearly elastic and nonlinear isotropic hardening with elastic modulus, yield strength and the thermal expansion coefficient varying exponentially through the thickness of the plate. The boundaries between the shakedown area and the areas of elasticity, incremental collapse and reversed plasticity are determined, respectively. The shakedown of the counterpart made of homogeneous material with average material properties is also analyzed. The comparison between the results obtained in the two cases exhibits distinct qualitative and quantitative difference, indicating the importance of shakedown analysis for FG structures. Since FG structures are usually used in the cases where severe coupled cyclic thermal and mechanical loadings are applied, the approach developed and the results obtained are significant for the analysis and design of such kind of structures.

Comparative study of thermal post-buckling analysis of uniform slender & shear flexible columns using rigorous finite element and intuitive formulations

Thermal post-buckling analysis of uniform, isotropic, slender and shear flexible columns is presented using a rigorous finite element formulation and a much simpler intuitive formulation. The ends of the columns are axially restrained to move and consequently any temperature rise above the stress free condition of the column produces an equivalent constant compressive mechanical load that causes the column to buckle at a critical temperature. Further increase in temperature beyond critical temperature results in the thermal post-buckling phenomenon. As a result of constraints imposed on the axial displacement at the ends of the column, the post-buckling phenomenon is governed by the von-Karman strain displacement relation applicable to one dimensional problems. Empirical formula for ratio of nonlinear axial load to critical load (equivalent constant mechanical load for a given temperature rise) as a function of the central deflection are obtained using both the rigorous finite element and intuitive formulations for various boundary conditions. The boundary conditions considered are the classical such as hinged–hinged, clamped–clamped and clamped–hinged conditions and nonclassical boundary conditions like the hinged–guided or the clamped–guided conditions. Post-buckling analysis results pertaining to nonclassical boundary conditions are meagre in the literature. It is observed that results obtained from both the formulations are in excellent agreement for all boundary conditions considered. Also the accuracy and simplicity of the intuitive formulation is aptly demonstrated to slender and shear flexible columns.

Fatigue response of a PZT multilayer actuator under high-field electric cycling with mechanical preload

An electric fatigue test system was developed for evaluating the reliability of piezoelectric actuators with a mechanical loading capability. Fatigue responses of a lead zirconate titanate (PZT) multilayer actuator with a platethrough electrode configuration were studied under an electric field (1.7 times that of the coercive field of PZT material) and a concurrent mechanical preload (30.0 MPa). A total of 109 cycles was carried out. Variations in charge density and mechanical strain under the high electric field and constant mechanical loads were observed during the fatigue test. The dc and the first harmonic (at 10 Hz) dielectric and piezoelectric coefficients were subsequently characterized using fast Fourier transformation. Both the dielectric and the piezoelectric coefficients exhibited a monotonic decrease prior to 2.86×108 cycles under certain preloading conditions, and then fluctuated. Both the dielectric loss tangent and the piezoelectric loss tangent also fluctuated after a decrease. The results are interpreted and discussed with respect to domain wall activities, microdefects, and other anomalies.

Alport Syndrome mutation changes molecular structure and nanomechanics of type IV tropocollagen

AbstractAlport Syndrome is a genetic disease characterized by the breakdown of the glomerular basement membrane (GBM) around blood vessels in the kidney, leading to kidney failure in most patients. It is the second most inherited kidney disease in the US, and many other symptoms are associated with the disease, including hearing loss and ocular lesions. Here we probe the molecular level mechanisms of this disease utilizing a bottom-up computational materiomics approach focused on the mutation associated with the most severe form of Alport Syndrome. Since the GBM is under constant mechanical loading due to blood flow, changes in mechanical properties due to amino acid mutations may be critical in the symptomatic GBM breakdown seen in Alport Syndrome patients. Through full-atomistic simulations in explicit solvent, the effects of a single-residue glycine substitution mutation are studied in a short segment of a collagen type IV tropocollagen molecule. Major changes are observed at the single molecule level of the mutated sequence, including a bent shape of the structures after equilibration with the kink located at the mutation site and a significant alteration of the molecule’s stress-strain response and stiffness.

CREEP PARAMETERS OF SPRUCE WOOD IN HIGH TEMPERATURE ENVIRONMENT

The paper presents partial results of an ongoing project dealing with thermal diffusion and mechanical behavior of spruce wood in high temperature environment (up to 180°C). The objective of this part of study was to obtain rheological properties of spruce wood which will be used in future modeling of wood performance in high temperatures. Oven dry samples (30 x 30 x 700 mm 3 ) underwent constant mechanical loading in bending parallel to grain (σ max = 7.3 MPa, load span 600 mm) at three temperature levels, namely 120°C, 150°C and 180°C, respectively. Defl ection of the samples was measured using optical non-contact method. Four rheological parameters of Burger’s model, describing immediate elastic, delayed visco-elastic as well as plastic (permanent material change) behavior, were determined which show the strong infl uence of the temperature on creep propagation. Experimental setup, results and further application in the modeling of high temperature treatment are discussed.

Evidence for freely chosen pedalling rate during submaximal cycling to be a robust innate voluntary motor rhythm

Freely chosen pedalling rate during cycling represents a voluntary rhythmic movement. It is unclear to what extent this is influenced by internal (e.g. loading on the cardiopulmonary system) and external (e.g. mechanical loading) conditions. It is also unclear just how robust a voluntary motor rhythm, the freely chosen pedalling rate, actually is. The present study investigated (N = 8) whether or not the freely chosen pedalling rate during submaximal cycling was affected by separate increases in loading on the cardiopulmonary system (changed by exposure to acute simulated altitude of 3,000 m above sea level) and mechanical loading (changed by exposure to increased power output and thereby pedal force). We also investigated (N = 7) whether or not the freely chosen pedalling rate and another voluntary motor rhythm, unimanual unloaded index finger tapping rate, shared common characteristics of steadiness and individuality over a 12-week period. Results showed that the freely chosen pedalling rate was unaffected by increased loading on the cardiopulmonary system at constant mechanical loading, and vice versa. Further, the pedalling rate was steady in the longitudinal perspective (as was the tapping rate), and like tapping rate, pedalling rate was highly individual. In total this indicated that freely chosen pedalling rate primarily is a robust innate voluntary motor rhythm, likely under primary influence of central pattern generators that again are minimally affected by internal and external conditions during submaximal cycling.

Testing system for ferromagnetic shape memory microactuators

Ferromagnetic shape memory alloys are a class of smart materials that exhibit a unique combination of large strains and fast response when exposed to magnetic field. Accordingly, these materials have significant potential in motion generation applications such as microactuators and sensors. This article presents a novel experimental system that measures the dynamic magnetomechanical behavior of microscale ferromagnetic shape memory specimens. The system is comprised of an alternating magnetic field generator (AMFG) and a mechanical loading and sensing system. The AMFG generates a dynamic magnetic field that periodically alternates between two orthogonal directions to facilitate martensitic variant switching and to remotely achieve a full magnetic actuation cycle, without the need of mechanical resetting mechanisms. Moreover, the AMFG is designed to produce a magnetic field that inhibits 180 degrees magnetization domain switching, which causes energy loss without strain generation. The mechanical loading and sensing system maintains a constant mechanical load on the measured specimen by means of a cantilever beam, while the displacement is optically monitored with a resolution of approximately 0.1 microm. Preliminary measurements using Ni(2)MnGa single crystal specimens, with a cross section of 100x100 microm(2), verified their large actuation strains and established their potential to become a material of great importance in microactuation technology.

Combustion of pressed specimens of heterogeneous systems subjected to a constant axial mechanical load

The growth dynamics of the height of burning pressed specimens with the specimen ends subjected to a compressive force of constant magnitude is studied using the heterogeneous Ti+C+20% TiC system as an example. It is found that in the examined range of compressive forces 0.1–2 kg/cm2, the growth of the height of the burning specimens with time obeys a linear law; under a compressive force of ≈0.1 kg/cm2, the specimen height increases by ≈100%, and under a force of ≈2 kg/cm2, it increases by about 25%. The method used to measure the growth dynamics of the specimen height during combustion proved a useful tool for the experimental determination of both the burning time of the specimens and the delay in the transfer of combustion through an obstacle.

Finite element crack closure analysis of a compact tension specimen

Crack closure behaviour of a compact tension (CT) specimen subjected to a constant amplitude cyclic mechanical load, with load ratio R = −2 and the maximum load approaching and exceeding the yield strength based limit load of the specimen, has been examined by performing large strain elastic–plastic finite element (FE) crack growth analyses assuming kinematic hardening. Results from static analyses show that the near crack tip stress and plastic strain fields do not change significantly with further loading after crack closure takes place. Results from crack growth analyses show that the crack closure type depends on the maximum stress intensity factor (SIF) and the crack tip constraint conditions, and that the crack opening/closing loads can be correlated by the maximum SIF, for given crack tip constraint conditions (plane stress/strain). The equation given in the R5 procedure for the crack closure factor, q 0, is conservative compared with the results obtained from the FE simulation.

Fatigue crack growth law for ferroelectrics under cyclic electrical and combined electromechanical loading

New data sets of crack propagation in lead-zirconate-titanate DCB specimens under cyclic electric loading combined with a constant mechanical load have been obtained. Both an increasing mechanical load as well as an increasing field amplitude resulted in an enhanced crack propagation rate. The experiment was modelled with a Finite Element Analysis that used special crack tip elements and assumed a finite permeability of the crack. The calculations revealed a dielectric crack closure effect, explaining the experimentally observed threshold of fatigue crack growth for the electric load. Fracture quantities suitable for cyclic loading by electric fields above the coercive field were discussed and a Mode-IV intensity factor considered as appropriate. The resulting correlations were applied to the experimental results and a power law relationship for the crack growth rate versus the range of the Mode-IV intensity factor was found.

Nonlinear Response of the Macro Fiber Composite Actuator to Monotonically Increasing Excitation Voltage

A nonlinear model for a piezoelectric continuum subjected to a monotonic increase in electric field under constant mechanical load is investigated. A general model is derived and then specialized for a piezoceramic-based material with anisotropic actuation and mechanical properties, for example, the Macro Fiber Composite (MFC) actuator. This formulation includes the linear-elastic mechanical response, as well as linear and second-order piezoelectric behavior and a second-order electromechanical cross-term. Using previously published work by the same authors (Williams, R.B., Inman, D.J. and Wilkie, W.K. 2004. “Tensile and Shear Behavior of Macro Fiber Composite Actuators,” Journal of Composite Materials, 38(10):855—870) dealing with the mechanical response of the MFC, this set of material properties is determined experimentally in order to characterize the response of this device to given external loading conditions.

Effects of Electric Fatigue on Piezoelectric Ceramics

The fatigue behavior of piezoelectric ceramics PZT-5 subjected to three-point bending was investigated under a cyclic electric load coupled with a constant mechanical load. The direction of poling was perpendicular to that of the mechanical load. The fatigue life curves under high and low electric loading were compared. Non-linear relationship was observed between dS/dN and E/Ec.

Does Unilateral Pedaling Activate a Rhythmic Locomotor Pattern in the Nonpedaling Leg in Post-Stroke Hemiparesis?

Recent investigation in persons with clinically complete spinal cord injury has revealed that locomotor activity in one limb can activate rhythmic locomotor activity in the opposite limb. Although our previous research has demonstrated profound influences of the nonparetic limb on paretic limb motor activity poststroke, the potency of interlimb pathways for increasing recruitment of the paretic limb motor pattern is unknown. This experiment tested whether there is an increased propensity for rhythmic motor activity in one limb (pedaling limb) to induce rhythmic motor activity in the opposite limb (test limb) in persons poststroke. Forty-nine subjects with chronic poststroke hemiparesis and twenty controls pedaled against a constant mechanical load with their pedaling leg while we recorded EMG and pedal forces from the test leg. For the experimental conditions, subjects were instructed to either pedal with their test leg (bilateral pedaling) or rest their test leg while it was either stationary or moved anti-phased (unilateral pedaling). In persons poststroke, unilateral pedaling activated a complete pattern of rhythmic alternating muscle activity in the nonpedaling, test leg. This effect was most clearly demonstrated in the most severely impaired individuals. In most of the control subjects, unilateral pedaling activated some muscles in the nonpedaling leg weakly, if at all. We propose that, ipsilateral excitatory pathways associated with contralateral pedaling in control subjects are increasingly up-regulated in both legs in persons with hemiparesis as a function of increased hemiparetic severity. This enhancement of interlimb pathways may be of functional importance since contralateral pedaling induced a complete motor pattern of similar amplitude to the bilateral pattern in both the paretic and nonparetic leg of the subjects with severe hemiparesis.

The linear matching method applied to the high temperature life integrity of structures. Part 1. Assessments involving constant residual stress fields

Design and life assessment procedures for high temperatures are based on ‘expert knowledge’ in structural mechanics and materials science, combined with simplified methods of structural analysis. Of these, R5 is one of the most widely used life assessment methods internationally with procedures based on reference stress techniques and shakedown calculations using linear elastic solutions. These have been augmented by full finite element analysis and, recently, the development of a new programming method, the linear matching method (LMM), that allows a range of direct solutions that include shakedown methods and simplified analysis in excess of shakedown. In this paper, LMM procedures are compared with calculations typical of those employed in R5 for cyclic loading problems when the assumption of a constant residual stress field is appropriate including shakedown and limit analyses, creep rupture analysis and the evaluation of accumulated creep deformation. A typical example of a 3D holed plate subjected to a cyclic thermal load and a constant mechanical load is assessed in detail. These comparisons demonstrate the significant advantages of linear matching methods for a typical case. For a range of cyclic problems when the residual stress field varies during the cycle, which include the evaluation of plastic strain amplitude, ratchet limit and accumulated creep strains during a high temperature dwell periods, the corresponding LMM and R5 procedures are discussed in an accompanying paper.

The linear matching method applied to the high temperature life integrity of structures. Part 2. Assessments beyond shakedown involving changing residual stress fields

In an accompanying paper, the application of the linear matching method to the high temperature life assessment of structures was discussed and compared with assessment procedures, typical of those used in R5, for a range of cyclic problems within shakedown. These included shakedown and limit analyses, creep rupture analysis and the evaluation of rapid cycle creep deformation. In this paper, procedures based on the linear matching method are presented for a range of cyclic problems at higher stress ranges beyond shakedown involving changing residual stress fields. These include the evaluation of the plastic strain amplitude, ratchet limit and accumulated creep strains during high temperature dwell periods. The results are compared with calculation methods typical of the R5 methodologies. As in the accompanying paper, a 3D holed plate subjected to cyclic thermal load and constant mechanical load is assessed in detail as a typical example. The results demonstrate the flexibility of the linear matching method compared with current typical applications of high temperature assessment procedures.