Stiffness Criteria Research Articles

Arterial stiffness as a factor of structural and functional cardiac remodeling in obesity

Aim. To analyze the association of parameters characterizing the degree of arterial stiffness and echocardiographic criteria for cardiac remodeling in patients with abdominal obesity.Material and methods. The study included 194 patients (men aged 46 to 55 years (49,0±2,3 years)), without hypertension (24-hour average blood pressure (BP) 117,5±5,5/73,0±4,1 mmHg), diabetes and cardiovascular diseases, with abdominal obesity (waist circumference >94 cm, body mass index 31,3±3,5 kg/m2). Lipids and glucose concentrations were evaluated, and glomerular filtration rate was estimated using the CKD-EPI equation. We conducted 24-hour monitoring of blood pressure and arterial stiffness parameters (aortic pulse wave velocity (PWV), augmentation index (AIx) and systolic BP in the aorta), and echocardiography.Results. Left ventricular (LV) hypertrophy was detected in 14 (7,2%) patients, LV diastolic dysfunction — in 36 (18,6%) patients. The correlation of the average aortic PWV and the AIx with the LV mass index and the left atrial volume was shown. Patients with a high aortic PWV exceeding the 75th percentile of distribution (8,2 m/s) were characterized by a higher incidence of hypertrophy (18,8% vs 4,9%,p<0,01) and LV diastolic dysfunction (50,0% vs 12,3%, p<0,001). Patients with/with-out LV hypertrophy and diastolic dysfunction were characterized by higher values of average 24-hour aortic PWV, AIx and systolic BP in the aorta. According to the regression analysis, the predictors of LV diastolic dysfunction were age, waist circumference, aortic PWV, and AIx.Conclusion. The relationship of parameters characterizing the degree of arterial stiffness, primarily, aortic PWV and echocardiographic parameters of the structural and functional cardiac remodeling in obese patients was revealed. Patients with a high aortic PWV (>8,2 m/s for men aged 46-55 years) are characterized by a higher prevalence of LV hypertrophy and diastolic dysfunction, as well as left atrial dilatation. This association is probably a reflection of one of the many pathogenesis links of HF and supraventricular cardiac arrhythmias in obese patients.

Application of Supersonic Shear Imaging to the Baveno VI Criteria and a Combination Model with Spleen Stiffness Measurement to Rule Out High-Risk Varices in Compensated Advanced Chronic Liver Disease

Recently, Colecchia et al. reported that by adding a spleen stiffness (SS) criterion sequentially to the Baveno VI criteria, screening endoscopy could be safely avoided. We aimed to compare the Baveno VI criteria, SS values and a sequential combination of the Baveno VI and SS values, measured by supersonic shear imaging (SSI), as approaches for safely avoiding screening endoscopy for high-risk varices (HRV). Between April 2017 and July 2018, we enrolled 274 compensated advanced chronic liver disease patients who had successfully undergone liver stiffness (LS) and SS measurements with SSI and esophagogastroduodenoscopy (EGD). 52 HRV patients were included, and we analyzed risk factors for HRV and compared proportions of patients who were spared EGD when Baveno VI criteria, SS cut-off and the combination of the two approaches were used. The AUROC values for estimating HRV by platelet count, LS and SS were 0.701, 0.757 and 0.844, respectively, and all three measures were found to be independent predictors of HRV. The SS cut-off value for excluding HRV was ≤ 27.3 kPa. The percentages of patients spared EGD were 18.6 % for Baveno VI, 28.8 % for SS cut-off and 36.1 % for the sequential combination of Baveno VI and SS cut-off. Less than 2 % of HRV patients were missed when using all of the criteria. The Baveno VI criteria can be applied to LS measurement by SSI. SS measurement by SSI is an excellent predictor of HRV. Screening endoscopy can be safely avoided when Baveno VI criteria and SS cut-off are applied together.

Low-Temperature Characteristic Evaluations of Base and Hybrid Asphalt Binders

Abstract Polymerized asphalt is broadly being applied for producing durable asphalt pavement all over the performance characteristics of asphalt binders, especially their low-temperature behaviors. This research study used two base binders and four hybrid binders (including styrene-butadiene-styrene and crumb rubber) to explore the lowest potential temperatures of resisting the cracks. The results indicated that the time-temperature principle needed to be properly evaluated to determine the 60th second as the exact loading duration to finalize the thermal cracking temperatures for hybrid asphalt binders. In addition, ΔTc, defined as the numerical difference between the critical low temperature determined from stiffness criteria (300 MPa) and the m value (0.300), which was used to characterize the crack potential of an aged binder, should be adjusted according to the characteristic demands of hybrid binders. Moreover, the conducted correlations between stiffness values at the low temperatures and phase angles at high temperatures were feasible to explore the viscous-elastic characteristics of base binders and hybrid binders.

Economic and Technical Viability of Using Shotcrete with Coarse Recycled Concrete Aggregates in Deep Tunnels

This work analyzes the technical and economic viability of using coarse recycled aggregates from crushed concrete in shotcrete, as a primary lining support in tunnels. Four incorporation ratios of coarse natural aggregate (CNA) with coarse recycled concrete aggregates from concrete (CRCA) were studied: 0%, 20%, 50% and 100%. The mechanical properties of the dry-mix shotcrete were obtained in an independent experimental campaign. Initially, the technical viability of CRCA shotcrete was validated for deep rock tunnels, based on the convergence-confinement method. Two cases were studied to determine the equivalent thickness for each combination of replacement ratio using CRCA shotcrete: (i) similar stiffness and (ii) similar yield stress. Subsequently, an economic assessment was performed. The stiffness criterion increased the thickness below 10% in both the 20% and 50% replacement ratios, which shows their technical viability with very marginal cost increase (<5%). On the other hand, the maximum pressure criteria required higher increments, close to 30% in the 50% replacement ratio. A full replacement was proven to be impracticable in both analyses.

A study on the lightweight design of lift structure by composite materials

The purpose of this study is to reduce the weight of elevator components in order to save energy and apply the findings to the new drive-type elevator system. Structural design has been carried out for two lightweight elevator models in which the damped aluminum laminate (DAL) and carbon fiber reinforced plastics (CFRP) are utilized for the elevator walls, respectively. The structural designs of the new elevator walls are based on the bending stiffness of the existing steel walls. The aluminum elevator model is designed to exhibit a bending stiffness similar to that of the existing steel elevator, while the CFRP model is designed to possess 20% of the existing wall bending stiffness considering that the tensile strength in the fiber direction is about nine times higher than that of the existing structural steel material. DAL and CFRP are applied to the elevator walls, respectively, and aluminum sandwich structures are applied to the ceiling and platform of two kinds of lightweight elevator models. It has been ascertained that the aluminum elevator model is about 40% lighter than the existing steel elevator, and the CFRP elevator model is about 50% lighter than the existing elevator. In order to evaluate the structural integrity of the newly designed elevator model, the criteria presented in the elevator inspection guideline are applied. The designed elevator model is confirmed to satisfy the strength and stiffness criteria presented in the elevator inspection parameter.

Observations of Laboratory and Natural Slow Slip Events: Hikurangi Subduction Zone, New Zealand

AbstractSlow slip events (SSEs) are recognized as an important component of plate boundary fault slip, and there is a need for laboratory friction data on natural samples to guide comparisons with natural SSEs. Here, we compile a comprehensive catalog of SSEs observed geodetically at the Hikurangi subduction zone offshore northern New Zealand, and compare it with results of laboratory friction experiments that produce laboratory SSEs under plate tectonic driving rates (5 cm/yr). We use samples from Ocean Drilling Program Site 1124 seaward of the Hikurangi subduction zone to represent the plate boundary that hosts shallow SSEs at Hikurangi. We find that laboratory SSEs exhibit a similar displacement record and range of stress drops as the natural SSEs. Results of velocity step tests, which can be used to evaluate frictional instability based on the critical stiffness criterion, indicate that the slow slip activity at Hikurangi is a form of stably‐accelerating slip. Our laboratory SSEs provide an alternative method of quantifying (in)stability by direct measurement of the unloading stiffness during the stress drop. The observed dependence of laboratory SSE parameters on effective normal stress is consistent with critical stiffness theory; however, depth‐increasing projections based on laboratory data do not match observations from natural SSEs. These differences are likely related to changing temperature and fault rock composition downdip but also complications related to scaling and/or limited sampling. Scientific drilling recently undertaken at the Hikurangi subduction zone should serve to improve and guide future studies of the role of frictional properties for the occurrence of SSEs.

The shear stiffness criterion for rock joints considering rock wear behaviour

Rock is a material that is affected by wear, and the curvature of the asperities on a rock joint surface increases with the degree of wear after shearing. Based on the Greenwood and Williamson (GW) model, a new model considering the change of asperity curvature is proposed to explain the wear behaviour of rock joints. First, the shear stiffness formula for a joint surface is derived when the asperity curvature is constant, which shows that the shear stiffness increases with increase of asperity curvature. According to the Mohr–Coulomb criterion, the yield position of a single asperity under normal force and tangential friction force is discussed. Then, the critical normal force for a single asperity at a specific friction coefficient is obtained, which shows that the normal force corresponds to the curvature radius of the asperity. A rough surface model with multi-level curvature radius is proposed. With increase of normal force, the higher-order asperities gradually fail and the curvature radius become larger. A specific pressure value excites a specific radius of curvature, and the larger the pressure, the larger the radius of curvature. The relation between the normal force and the curvature radius is proposed and a shear stiffness formula considering the change of curvature radius of the asperity is derived. The proposed model is verified on the basis of the published experimental results. The calculation results of the proposed model can reflect the test results well: for a given joint surface, with increase in normal force the joint surface gradually becomes smooth; for different joint surfaces, with increase in roughness, the joint surface is more easily smoothed.

Static performance of a new GFRP–metal string truss bridge subjected to unsymmetrical loads

A unique lightweight string truss deployable bridge assembled by thin-walled fiber reinforced polymer (FRP) and metal profiles was designed for emergency applications. As a new structure, investigations into the static structural performance under the serviceability limit state are desired for examining the structural integrity of the developed bridge when subjected to unsymmetrical loadings characterized by combined torsion and bending. In this study, a full-scale experimental inspection was conducted on a fabricated bridge, and the combined flexural–torsional behavior was examined in terms of displacement and strains. The experimental structure showed favorable strength and rigidity performances to function as deployable bridge under unsymmetrical loading conditions and should be designed in accordance with the stiffness criterion, the same as that under symmetrical loads. In addition, a finite element model (FEM) with a simple modeling process, which considered the multi segments of the FRP members and realistic nodal stiffness of the complex unique hybrid nodal joints, was constructed and compared against experiments, demonstrating good agreement. A FEM-based numerical analysis was thereafter performed to explore the effect of the change in elastic modulus of different FRP elements on the static deformation of the bridge. The results confirmed that the change in elastic modulus of different types of FRP element members caused remarkable differences on the bending and torsional stiffness of the hybrid bridge. The global stiffness of such a unique bridge can be significantly enhanced by redesigning the critical lower string pull bars using designable FRP profiles with high elastic modulus.

Implications of basement rock alteration in the Nankai Trough, Japan for subduction megathrust slip behavior

The Nankai Trough is an exceptionally well-studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip. Outcrop studies of exhumed analogues for the modern subduction thrust, as well as seismic reflection images of the active subduction zone, suggest that the basaltic basement participates in plate-boundary shearing at depths of a few km and greater. We obtained altered basaltic upper basement from the modern Nankai Trough seaward of the trench, recovered by drilling on IODP Expedition 333. We performed laboratory friction experiments on bare surfaces and gouge powders of this material, in addition to and in combination with other materials for comparison. The altered basalt exhibits predominantly velocity-strengthening frictional behavior, indicating a tendency for stable slip. For bare surface experiments, few instances of velocity-weakening friction occur and are restricted to slip rates of <10−6 m/s. Thin sections and XRD analyses of the starting material indicate that the velocity-strengthening behavior is likely associated with the presence of clay minerals, mostly Mg-smectite, which coat the rims of larger, stronger grains. Based our friction data, we suggest that in addition to creep the altered Nankai basalts may also allow the possibility of slow slip events for two reasons: (1) velocity-weakening friction may be expected at low slip rates, but velocity-strengthening at higher rates will damp any potential slip instabilities; and (2) based on a critical stiffness criterion for accelerating (stable) slip, SSEs are capable of nucleating in the altered Nankai basalt despite velocity-strengthening friction. In light of these observations, and given the depth at which altered basement appears to be incorporated along the plate interface, we raise the possibility that some shallow slow slip events in the Nankai accretionary prism could originate from shearing in altered basalt.

3D modelling of angular spindle’s head for machining centre

The unified design of the angular spindle head of the machining centre for drilling-milling and boring type is considered. Three-dimensional modeling of the angular head assembly in CAD KOMPAS-3D is performed using a specialized application “Shafts and 3D mechanical transmissions” including a three-dimensional representation of the machining centre kinematic scheme. The photorealistic visualization module Artisan Rendering, which is integrated into the KOMPAS-3D CAD contour, was used to create presentations and promote this construction to the machine tool industry. A two-stage procedure for analyzing the structure by the stiffness criterion is proposed: at the early stages of the design by the matrix initial parameters method; at the stage of the working project by the finite element method in the APM FEM module. The procedure for determining the stress-strain state of the angular head spindle with end mills realized.

Development of lightweight emergency bridge using GFRP–metal composite plate-truss girder

Design of a lightweight emergency vehicular bridge comprising a GFRP–metal composite plate-truss girder and measuring 24 m in span is reported. The said bridge was designed based on optimization of an original 12-m bridge specimen. The bridge, so developed, is intended to be lightweight, structurally sound with modular feasibility, and representative of a construction that is less time consuming overall and fully exploits advantages offered by the use of inherent and complementary pultruded GFRP materials. Conceptual design and considerations of the large-scale structure were first described in detail. Subsequently, full-scale nondestructive tests were performed under on- and off-axis static loadings to evaluate the actual linearly elastic mechanical behavior of the prototype. Experimental results demonstrated that the bridge satisfactorily met the requirements of strength, overall bending stiffness, and torsional rigidity with regards to emergency-bridge applications. Being recognized as the most critical loading case for emergency bridges with major influence on load distribution among truss girders, the lateral live-loading distribution was assigned great importance during design of the unique bridge. Extrusion-type unidirectional GFRP profiles with high-longitudinal but low shear strengths are predominantly suitable for structures subjected to large axial forces, and are, therefore, appropriate for application in the proposed hybrid structural system. Favorable testing results demonstrated that the proposed improved version of the original conceptual design can appropriately be used as a truss girder for a new lightweight emergency bridge with a longer measured span. It is suggested that such a hybrid bridge, which demonstrates reasonably good linearly elastic behavior under service live loads, must also be designed in accordance with a stiffness criterion. Corresponding finite element and analytical analyses were performed and compared against experimental results whilst demonstrating good agreement. The elicited comparisons indicated that the established simplified analytical models and the finite element model (FEM) were both equally applicable for use in preliminary structural calculations and design of the improved bridge under states within its serviceability limit. Results reported herein are expected to make a valuable initial contribution, which in turn, could further lead to development of similar lightweight structural systems.

Design of Transverse Stiffeners in Plate Girders with Corrugated Web

This study reports investigations into the effect of relative flexural stiffness of intermediate stiffeners γ on the failure zone location in the corrugated web. The study also aimed at obtaining stiffness criterion for intermediate stiffeners that depends on the magnitude of the plate geometry parameter α. To achieve the goals of the study, experimental investigations were conducted into load displacement paths of four exemplary SIN girders. They were simply supported girders, made to full scale, and composed of pre-assembled units. The phenomena occurring in the experiment were represented using the Finite Element Method. For FEM numerical analysis of girders with intermediate stiffeners, models with the web height of 1000, 1250 and 1500 mm, made from 2; 2.5 and 3 mm thick corrugated sheet metal were used. Due to the analysis of 52 girder numerical models, it was possible to propose the stiffness criterion of intermediate stiffeners. The criterion was based on the assessment of shear buckling strength of the corrugated web. Using the regression method, dimensionless coefficients of the stiffener stiffness ks dependent on the optimum stiffness γ were determined. Based on estimated coefficients of the stiffener stiffness ks, the absolute minimum stiffness of intermediate stiffeners Ismin used in corrugated web plate girders was calculated. It was demonstrated that the use of an intermediate stiffener, the stiffness of which is greater than Ismin , additionally leads to a change in the location of the site of the web shear buckling.

Target Setting for the Stiffness of Local Mounting Area in the Vehicle Body for NVH and R&amp;H

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Performance criterion of local stiffness in BIW(Body in White) for NVH(Noise, Vibration, and Harshness) and R&amp;amp;H (Ride and Handling) are presented in this study. A process to develop the local stiffness of the vehicle body has been established for the performances. For handling performance, not only suspension bush stiffness but local stiffness in BIW structure is important. However, there is no good standard to describe handling performance using BIW local stiffness. In this paper, a new evaluation standard for local stiffness in BIW is developed and verified considering actual vehicle driving condition for handling performance. Also, new evaluation process regarding local stiffness in BIW for vibration transmissibility is presented. The new process is developed considering optimal relation between connecting bush stiffness and BIW local stiffness. It is shown that the interior noise of the vehicle is reduced by applying the evaluation process, which is verified through several cases.&lt;/div&gt;&lt;/div&gt;

A two-step risk assessment method for radiofrequency ablations of spine metastases

BackgroundSpine metastases (MTS) can be treated via Radiofrequency Ablation (RFA) electrodes. To bring these electrodes into vertebral MTS, pathways have to be created. This can be done via transpedicular hammering or drilling. However, this is challenging due to spatial constraints, and because MTS can alter bone density considerably. MethodIn this work a two-step method is presented that intends to offer cognitive and physical assistance. Step 1 comprises two visualization methods that depict safety margins between and in risk structures. For Step 2, the correlation between Hounsfield Units (HUs) and drilling forces was analyzed to support manual and robot-assisted RFAs. ResultsIn-depth descriptions of two clinical cases and detailed feedback from the local clinic of neuroradiology are used to present the capabilities of the proposed method. Furthermore, a stiffness criterion is presented to predict drilling force changes from the local distribution and homogeneity of HUs with an inaccuracy of less than 1 mm. ConclusionsThe combination of visualization and drilling force prediction shows potential to support manual and robot-assisted spine RFAs. However, limitations have to be addressed in the future. For example, it has to be carefully evaluated to which extent the proposed method can speed up the planning process and increase intervention safety.

Optimal design of flax fiber reinforced polymer composite as a lightweight component for automobiles from a life cycle assessment perspective

AbstractThe present study combines the generalized rule‐of‐mixture (ROM) model and the Ashby material selection method for the life cycle assessment (LCA) of flax fiber reinforced polymers (FRPs) and glass FRPs (GFRPs). The ROM model allows life cycle environmental impact predictions according to specific parameters of flax FRPs such as fiber format, volume fraction, manufacturing technique, and load‐bearing capacity. The comparisons applied in this study are constructed on two common composite structures: mat panels and injection molded struts with equal stiffness and strength as the design criteria. On the one hand, the parametric LCA predicts that the equal strength design criterion for flax FRPs contributes to consistent mass increases, subsequently resulting in higher life cycle environmental impacts compared to the reference GFRPs; on the other hand, under the equal stiffness criterion the flax mat polypropylene (flax mat‐PP) film helps with mass reduction in reference to the glass mat‐PP composite, leading to the 20–50% life cycle environmental impact reductions for most impact categories. The subsequent evaluation of the influences of the fiber volume fraction on flax FRPs shows different patterns. For the short flax fiber‐PP composite, a steady decrease of the life cycle CO2 emissions can be observed with the increasing fiber volume fraction. However, for the flax mat‐PP composite, depending on the tensile modulus of the flax fiber, the optimal volume fractions of the fiber change from 28 to 32% v/v, whereby the lowest life cycle greenhouse gas (GHG) emissions can be achieved.

Failure characterization of solid structures based on an equivalence of cohesive zone model

Cohesive zone models have been widely used to model interface crack initiation and propagation both in single-material media and bi-material systems. For single-material media with cohesive elements inserted into interface among segments, in order to ensure that the introduction of interface cohesive zone models does not affect the mechanical properties of single-material media before the softening stage of cohesive zone models, a selection criterion of stiffness of cohesive elements is proposed theoretically firstly based on the properties’ equivalence. Taking the softening stage into account, the mechanical responses of the overall stress-strain relationship of single-material media, for the cases of stable increase of strain and snap-back instability of strain, are both obtained, and the related energy mechanism are investigated. For bi-material systems with cohesive elements at interface between two materials, the thickness-dependent failure characteristics of systems in uniaxial tension are found, which is attributed to the difference of the releasing rate of elastic strain energy in the materials with different thicknesses. Furthermore, as a more complex application of cohesive elements, based on the selection criterion proposed, failure behaviors of the ceramic coating/substrate systems under three-point bending are modeled by finite element method and inserting cohesive elements into the coating segments and the coating/substrate interface simultaneously. The simulation results indicate the transition of dominated failure mode from coating cracking to interface delamination with increasing coating thickness, and show faster damage of thick coating systems, agreeing with experimental results. The effects of interface strength and toughness of cohesive elements on failure are also revealed. These results can provide guidance for the application of cohesive elements, and help us better understand the overall failure behaviors of interface systems.

Application of shear wave elastography for Baveno VI criteria and combined model using spleen stiffness measurement to rule out high-risk varices in advanced chronic liver disease.

Purpose In 2015, Baveno VI workshop suggested consensus recommendation about non-invasive surveillance of patients with compensated advanced chronic liver disease (cACLD) that the screening endoscopy could be avoided safely in patients with platelet count over 150,000 / mm3 and liver stiffness (LS) value less than 20 kPa using transient elastography (TE). Recently, Colecchia et al. reported that by adding spleen stiffness (SS) criteria to the Baveno VI criteria sequentially, screening endoscopy can be avoided safely in a greater proportion of patients with cACLD. The purpose of this study is to compare Baveno VI criteria, SS value and a sequentially combined model of Baveno VI criteria and SS value, using 2D-SWE, to avoid screening endoscopy for high risk varices (HRV) safely in cACLD patients. Materials and Methods We enrolled patients who successfully underwent LS and SS measurements by 2D-SWE and recently underwent esophagogastroduodenoscopy(EGD) from April 2017 to July 2018. And the PLT count, LS value, SS value, presence and grade of esophageal varices (EV), child-pugh classification grade and laboratory findings were reviewed retrospectively. Patients with a decompensation symptom (GI bleeding, hepatic encephalopathy, jaundice, ascites), bleeding history or ligation history of EV, portal vein thrombosis, hepatic failure, non-cirrhotic portal hypertension, transjugular intrahapatic portosystemic shunt (TIPS), or hepatic and systemic malignancy were excluded. We evaluated the clinical significance of platelet count, LS and SS value and analyzed the patient factors for the prediction of HRV. We also compared the ratio of EGD spared patients using Baveno VI criteria, SS cut-off and a sequential combination of Baveno VI criteria with SS cut-off. Results The AUROC values of platelet count, LS and SS for estimating the HRV were 0.701, 0.757, and 0.844, respectively. Platelet count, LS and SS value were found to be independent predictors of HRV. In the ROC curve analysis of SSM, the cut-off value of SS to exclude HRV was ≤ 27.3 kPa. The ratio of spared EGD was 18.6% for Baveno VI criteria, 28.8% for SS cut-off ≤ 27.3 kPa and 36.1% for sequential combination of Baveno VI criteria and SS cut-off. The ratio of missed HRV patients at all criteria was less than 2%. Conclusions SS measurement using 2D-SWE is an excellent predictor of HRV for cACLD patients. Screening endoscopy can be avoided safely when Baveno VI criteria and SS cut-off are applied together.

Plastic Effect on the Sliding Inception Between a Cylinder and a Rigid Flat

The effects of material plasticity and local slip on the sliding inception of asperity are studied in the present work. Firstly, a semi-analytical solution is derived under the full-stick condition to analyze the effect of material plasticity on sliding friction. Then, a friction model with contact stiffness criterion is proposed to study the cases from partial-slip condition to full-stick condition. Finite element simulations with the provided model are used to present the friction map. The friction coefficient of full-stick interface converges at a stable value, approximately 0.3. Plasticity saturation appears as the normalized contact interference $$\omega ^{*}$$ is larger than 3. A transition mechanism from slip-dominated to yield-dominated takes place in the sliding process. The simulation results are compared with the semi-analytical solution.

Producing negative active stiffness in redundantly actuated planar rotational parallel mechanisms

Generally, the Cartesian stiffness matrix of closed-loop mechanisms can be divided into the passive stiffness and the active stiffness. The passive stiffness was determined by the kinematic parameters and joint stiffness, while the active stiffness is sub-divided into the forced active stiffness KE, related to the external wrench, and active stiffness KIG generated by internal force. The passive stiffness is always positive definite, however, the active stiffness can be positive, semi-positive or negative definite. We exemplified it with a redundantly actuated planar rotational parallel mechanism (RAPR-PM), showing negative stiffness criteria for the active stiffness. Whether the elastic limbs are compressed or stretched, once the preload is known, the sign of the active stiffness can be determined from kinematic parameters. The results can be used to design variable stiffness mechanisms. Since some biological structures of animals, like fish, can be represented by a series of joint-linked RAPR-PMs, the conclusions in this paper can be extended to study the inherent stiffness mechanism of animals.

DESIGN AND DEVELOPMENT OF HUB AND NACELLE STRUCTURE STRUCTURES FOR WIND TURBINE APPLICATIONS

Wind turbine is one of the technology to produce elctricity. Wind turbine consists of wind blade, nacelle, hub and tower. Hub is the structure which connect wind blade to the shaft of the generator to generate electricty.Nacelle is the structure that cover the generator from its environment. Nacelle is not design for strength, so there in no stress analysis for this structure. The purpose of this research is to design and make prototype for hub and nacelle. There are three models for hub, slot model (2 design) and lap joint design, and 1model for nacelle design. hub is selected by the simplicity of process of production and assembly, and by stress analysis. Then the lap joint design is the selected one, so that will be make for prototype. The prototype is made of composite sandwich structure, which is teak wood as its core. The composite sandwich structures is designed for its stiffness criteria.