Loss Of Stiffness Research Articles

Design of Steel Columns in Fire using Second‐order Inelastic Analysis with Strain Limits

AbstractIn this paper, a structural steel fire design approach using second‐order inelastic analysis with strain limits is proposed and applied to the design of steel columns in fire. The proposed design approach employs beam finite elements to conduct a second‐order inelastic analysis. Within the proposed method, strain limits are used to account for the effects of local buckling; while global instability, the loss of strength and stiffness and thermal effects are fully considered through second‐order inelastic analysis carried out using beam finite elements. The proposed method is assessed against validated shell FE results. It is observed that the proposed design approach provides consistently accurate and safe capacity predictions for steel columns in fire.

Mechanical and chemical analysis of structural silicone adhesives with the influence of artificial aging

Structural sealant glazing systems, or bonded glazing, refers to façades in which the glass is structurally bonded to a façade frame and which is typically used in large façades for office buildings. The structural silicone bond lines are an essential element of such façades and a variety of environmental influences and aggressive media interact with the adhesive and may affect its performance. The ETAG 002 is commonly used for the verification of the technical applicability taking these effects into account. In this study, samples of an approved standard silicone adhesive and a high-modulus silicone adhesive are artificially aged according to ETAG 002 and exposed to a climate change test. The aim of this study is to determine the essential mechanical properties and to explain the changes due to aging by means of chemical analysis. Polymer degradations are determined by evaluating the molecular structure with Fourier-transform Infrared Spectroscopy and Thermo-Gravimetric analysis. In addition, water absorption is investigated with Karl Fischer titration. Changes in the chemical structure are particularly identified after aging in SO2 atmosphere as well as after UV exposure. Increased water contents are detected for all specimens conducted to high moisture. The mechanical properties as well as their characteristic temperature ranges are determinate by the Dynamic Mechanical Analysis and Differential Scanning Calorimetry. The results of the study show significant loss of the adhesive stiffness up to 30% after artificial aging, mainly after storage in cleaning agent and after climate change test. However, there are no significant chemical effects of both aging methods on the adhesives. On the other hand UV-radiation as well as the acid attack, interact with the polysiloxane and the filler of the adhesive. This shifts the crystallization temperature a bit but it does not influence the adhesive stiffness at room temperature. Finally, tensile tests and shear tests are performed on both reference and aged bonded glass–aluminum specimens. These test confirm the good durability of silicone adhesives and the results meet the requirements of the ETAG 002.

Stainless Steel Portal Frame Tests

AbstractExperiments on stainless steel frames were carried out within this research. In total, 5 portal frames made of austenitic stainless steel SHS/RHS were tested. The paper thoroughly describes the performance of the loading tests including the layout of auxiliary elements and measuring devices. The technical drawing of the experiment layout with a description is presented as well as several photos. The load‐bearing capacities of the frames are summarised in a table. The research aims for a global structure behaviour examination with a particular focus on stainless steel structures. Their behaviour is highly influenced by nonlinear material stress‐strain response. Material nonlinearity leads to loss of stiffness, which results in greater deflections. And greater deflections generate greater second‐order effects. Worldwide research show this particularity needs to be taken into consideration when conducting a global analysis of the structure.

Modal interactions of a dual-joint cylindrical shell system via Nonlinear Normal Modes

Bolted flange joints are commonly encountered in engineering structures. One specific challenge that is regularly encountered is the modeling of connecting interfaces between flanges. Indeed, for large amplitudes of vibration, due to discontinuity of the structure and mechanical contact of the connecting interface, clearances may occur between flanges and cause local stiffness loss, exhibiting complicated nonlinear dynamic behavior. In this paper, a simplified dynamic model for bolted flange joint via structural static analysis is proposed, in which the stiffness nonlinearity due to the non-smoothness of jointed interface is characterized by piecewise-smooth springs. The proposed simplified model has demonstrated good accuracy and precision in capturing the nonlinear dynamic characteristics of bolted flange connections. Following the simplified modeling technique and focusing on the stiffness nonlinearities, an analytical dual-joint system with free–free boundary conditions is established capturing the interaction of a longitudinal mode with two bending modes. The Harmonic Balance Method combined with Asymptotic Numerical Method (HBM–ANM) is utilized to estimate Nonlinear Normal Modes (NNMs) of the system, which provide an interpretation of the underlying nonlinear dynamic behavior, including bifurcation analysis, internal resonances characterization, and stability properties. Especially, internal resonances between the three modes can be triggered at high energy (oscillation amplitude) level, without having commensurate linear natural frequencies, leading to qualitative changes in periodic motions and internal loads environment of the system. The influences of linear frequency ratios and stiffness nonlinearity of each joint on the dynamics of the system are discussed in terms of NNMs. It is interesting to reveal that reducing the stiffness nonlinearity of joint may grow the complexity of the nonlinear dynamics of the system, from the perspective of motion stability, and trigger internal resonance with a complex topology. The studies in this paper provide an in-depth understanding of the nonlinear dynamic behavior of dual-joint systems, and the NNMs certainly represent a useful framework that could lead to a better dynamic design of jointed structures.

Configurational Entropy Effects on Glass Transition in Metallic Glasses.

Configurational entropy (Sconf) is known to be a key thermodynamic factor governing a glass transition process. However, this significance remains speculative because Sconf is not directly measurable. In this work, we demonstrate the role of Sconf theoretically and experimentally by a comparative study of a Zr-Ti-Cu-Ni-Be high-entropy metallic glass (HE-MG) with one of its conventional MG counterparts. It is revealed that the higher Sconf leads to a glass that is energetically more stable and structurally more ordered. This is manifested by ab initio molecular dynamics simulations, showing that ∼60% fewer atoms are agitated above Tg, and experimental results of smaller heat capacity jump, inconspicuous stiffness loss, insignificant structural change during glass transition, and a more depressed boson peak in the HE-MG than its counterpart. We accordingly propose a model to explain that a higher Sconf promotes a faster degeneracy-dependent kinetics for exploration of the potential energy landscape upon glass transition.

Damping Behavior of Hybrid Composite Structures by Aeronautical Technologies

Hybrid composite laminates are manufactured by using technologies and raw materials of the aeronautic sector with the aim to improve the damping behavior of composite structures. Matrix hybridization was achieved by laminating carbon fiber reinforced (CFR) plies with elastomer interlayers. Up to 10 different composite sandwich architectures were investigated by changing the stacking sequence, the thickness of the elastomer layers, and the elastomer typology, whereas the total number of the CFR plies was fixed to six for all the hybrid composites. Square panels with the size of 300 × 300 mm2 were autoclave molded with vacuum bagging, and rectangular samples were extracted for static and dynamic tests. Dynamic mechanical analyses were performed to measure the storage modulus and loss factor of hybrid materials, which were compared with static and dynamic performances of the composite structures under bending. Repeated loading–unloading cycles and free oscillation tests allowed us to the energy loss per unit of volume, and the acceleration damping, respectively. Results show that softest elastomer interlayers lead to big loss of stiffness without any positive effect in the damping behavior, which worsens as well. By using soft elastomers, complex architectures do not provide any additional benefit in comparison with the traditional sandwich structure with soft core and hard skins.

On the Stability of Underground Masonry Vaults: The Case of the Mairie d’Ivry Station of the Paris Metro

ABSTRACT An analysis is presented to study the stability of a shallow masonry tunnel of the Paris metro subjected to the spreading of the vault’s supports caused by nearby excavations. FEM numerical computations are carried out using a specifically developed model for the masonry that combines a homogenization technique with a damage model. Calculations display three hinges in the vault. Measures show that the masonry vault may withstand about 2 centimeters of support spreading. Soil–structure interaction analysis shows that the ground reaction is critical to ensure stability. The loss of stiffness and shear strength of the soil next to the sidewalls produces severe deformations and cracks in the structure. Based on the results some thresholds are proposed. The study also presents an analysis on the importance of the constitutive law for masonry and a brief discussion on the use of Eurocodes to investigate the stability of the vault.

Filament path optimization of Fused Filament Fabricated parts incorporating the effect of pre-fusion densities

Fused Filament Fabricated parts exhibit mechanical anisotropy induced by the filament extrusion pattern and possibly due to the intrinsic nature of feedstock material. Consequently, an optimized filament deposition strategy is desirable for improving the part’s functionality. The present work optimizes the in-plane filament paths of Fused Filament Fabricated parts to strengthen component load-bearing capacity, with a particular focus on obtaining production-ready design solutions through a direct imposition of the manufacturing constraints. To perform an effective in-plane filament path optimization, the present contribution also addresses, among several other aspects, the following: (i) the development and implementation of a new material model that incorporates the transverse stiffness loss in the absence of inter-filament fusion, and (ii) a comparative study between a two-step gradient-based minimization and a global metaheuristic minimization. The results indicate that the new material model yields more realistic filament patterns compared to the assumption of neglecting the transverse stiffness loss at low filament densities. Further, the comparison of optimization approaches suggests the preference for the two-step gradient-based approach due to its better efficiency, flexibility, and compatibility with the proposed material model.

Combination of Enzymes and Rutin to Manage Osteoarthritis Symptoms: Lessons from a Narrative Review of the Literature.

Osteoarthritis is the most common joint disorder affecting over 300 million people worldwide. It typically affects the knees and the hips, and is characterized by a loss in normal joint movement, stiffness, swelling, and pain in patients. The current gold standard therapy for osteoarthritis targets pain management using nonsteroidal anti-inflammatory drugs (NSAIDs). NSAIDs are associated with several potentially serious side effects, the most common being gastrointestinal perforation and bleeding. Owing to the side effects, NSAID treatment doses need to be as low as possible and should be continued for the shortest duration possible, which is problematic in a chronic condition like osteoarthritis, which requires long-term management. Numerous clinical trials have examined oral enzyme combinations as a potential new approach in managing pain in patients with osteoarthritis. Oral enzyme combinations containing bromelain in combination with trypsin, both proteolytic enzymes, as well as the plant flavonoid rutin, may be an effective alternative to typical NSAIDs. The aim of this narrative review is to summarize and discuss the evidence on the efficacy of oral enzyme combinations compared to the gold standard (NSAID) in the management of osteoarthritis symptoms. Nine randomized controlled trials identified in this review assessed the efficacy and safety of the oral enzyme combination containing bromelain, trypsin, and rutin in patients with osteoarthritis. Most of the studies assessed the impact of the oral enzyme combination on the improvement of the Lequesne Algofunctional index score, treatment-related pain intensity alterations and adverse events compared to patients receiving NSAIDs. Although largely small scale, the study outcomes suggest that this combination is as effective as NSAIDs in the management of osteoarthritis, without the adverse events associated with NSAID use.Infographic

Topology optimization of periodic mechanical structures with orthotropic materials based on the element-free Galerkin method

A topology optimization (TO) algorithm for periodic mechanical structures with orthotropic materials is established by using the element-free Galerkin method (EFGM) and imposing periodic constraints on design subdomains. The effectiveness of the presented TO algorithm is verified by two numerical examples compared with the finite element method (FEM). The comparisons demonstrate that the EFGM periodic structures without using any filtering techniques have smoother boundaries and fewer intermediate relative densities. These optimal periodic structures can be manufactured by 3D printing from the extracted "*.stl" files. The effects of the number of design subdomains, Poisson's ratio factor, and off-angle on minimum compliance and periodic structures are studied, and reasonable ranges of parameters are recommended to improve the mechanical performance of periodic structures. The results indicate that the increase of the number of design subdomains in any direction leads to inevitable stiffness loss, but the minimum compliance can be further reduced by decreasing Poisson's ratio factor and adjusting off-angle properly for specific number of design subdomains. Optimal periodic structures with orthotropic materials are of better mechanical performance to resist deformation and reduce von Mises stress than those with isotropic materials.

Medial Patellofemoral Reconstruction With a Hamstring Allograft

Background: Lateral patellar instability is common in young, active patients. Patients who fail conservative treatment may benefit from medial patellofemoral ligament (MPFL) reconstruction. Indications: Recurrent lateral patellar instability. Technique Description: Examination assesses patellar translation, eversion and range of instability. Diagnostic arthroscopy is performed for loose bodies, cartilage damage, trochlear dysplasia, and tracking. A longitudinal incision is made from superomedial to mid-body of the patella. The plane between the capsule and retinaculum is developed for palpation of the medial epicondyle and adductor tubercle. Electrocautery and rongeur are used to create a trough on the patella from centromedially to superomedially. Two suture anchors are placed at the proximal and distal trough. Fluoroscopy is utilized to identify Schöttle point with a perfect lateral radiograph. A 3-centimeter incision is made, and blunt dissection connects the 2 incisions in the developed plane. A guidepin is advanced at Schöttle point, and suture anchor sutures are shuttled through the plane, posterior to the guidepin. There should be loosening of tension with knee flexion. A semitendinosus allograft is whipstitched with terminal tapering. The whipstitched end is tunneled around the guidepin and brought back to the patella, so that both ends have an excess of 20 to 25 millimeters, and excess graft is trimmed from the free end prior to whipstitching. The doubled graft is sized. The midportion of the tendon is tagged and passed through both anchors. The graft is tensioned to the patellar trough and the graft ends are advanced through the developed plane. The femoral tunnel is reamed, and the graft is tensioned into the tunnel after nitinol wire placement with the knee in full extension. Isometry and lateral patellar translation are assessed, aiming for 1 quadrant with firm endpoint. The patella is proximalized and the tensioned graft is secured with an interference screw. Results: MPFL reconstruction is successful for the majority of patients, with 1.2% reporting instability, 3.6% apprehension, and 3.1% reoperation. Possible complications include patellar fracture, patellofemoral pain, and knee stiffness (loss of range of motion). Discussion/Conclusion: Lateral patellar instability is common, and MPFL reconstruction is typically successful for the majority of patients.

A Strong Correlation Between the Severity of Flatfoot and Symptoms of Knee Osteoarthritis in 95 Patients.

ObjectiveThe purpose of this study is to assess the association between the presence and severity of flatfoot and symptoms of knee OA.Methods95 participants with knee OA were recruited from a patient cohort at a regional hospital. Symptoms of knee OA, including knee degeneration, femorotibial alignment, pain, stiffness and dysfunction were assessed using the Kellgren-Lawrence (K-L) grading system, femoral-tibial angle (FTA), and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Participants were divided into groups with flatfoot (mild, moderate and severe) and without flatfoot based on the Clarke's angle. Linear regression and ordinal logistic regression were used for statistical analysis, as appropriate.ResultsHaving flatfoot was associated with a significantly increased risk of having a higher K-L grade (OR: 20.03; 95% CI, 5.88, 68.27; p < 0.001), smaller FTA (Beta: −2.96; 95% CI, −4.41, −1.50; p < 0.001), higher pain score (Beta: 0.47; 95% CI, 0.24, 0.69; p < 0.001) and greater loss of function (Beta: 0.25; 95% CI, 0.02, 0.48; p = 0.03). Severe grades of flat feet were associated with a higher K-L grade (OR: 0.19; 95% CI, 0.08, 0.44; p < 0.001), smaller FTA (Beta: 1.51; 95% CI, 0.66, 2.35; p = 0.001), higher pain score (Beta: −0.25; 95% CI, −0.39, −0.11; p = 0.001), greater stiffness (Beta: −0.24; 95% CI, −0.38, −0.09; p = 0.002) and greater loss of function (Beta: −0.27; 95% CI, −0.41, −0.14; p < 0.001).ConclusionThe results indicated that the severity of flattening is significantly associated with symptoms of knee OA. For the conservative management of knee OA, both flatfoot and its severity should be carefully considered.

Response of nonconforming RC shear walls with smooth bars under quasi-static cyclic loading

In this study, an experimental investigation is conducted to determine the behavior of RC shear walls found in old and existing buildings that do not comply with the design rules in modern earthquake standards. Scaled reinforced concrete shear wall specimens are built with smooth bars and low concrete quality. The dimensions of the shear wall specimens were selected with an aspect ratio bigger than two as 2500, 1050, and 150 mm for the height, length, and thickness, respectively. Four specimens are representative of nonconforming shear walls, and one wall is used as a reference specimen which was designed in accordance with recent building codes using deformed bars. The behavior of the shear walls is determined experimentally by displacement-lateral load relationship under lateral cyclic loading. The study used measurable parameters to investigate the behavior of the test specimens in terms of lateral force capacity, rigidity, ductility, dissipated energy, and displacement components contribution to the total lateral response of the walls. The results showed a substantial loss of stiffness, ductility and energy dissipation capabilities for the tested nonconforming shear walls. Moreover, it is proven in this study that these specimens are governed by the bar slip phenomena which demonstrated more than 80% contribution to the total lateral displacement capacity. In contrast, the reference shear wall exhibited a notable flexural behavior and plastic hinge formation. Additionally, the shear walls built with smooth reinforcement bars lost about 44% of their theoretical potential flexural capacity due to the observed bar slip failure.

Computer vision-based automated stiffness loss estimation for seismically damaged non-ductile reinforced concrete moment frames

Computer vision-based automated stiffness loss estimation for seismically damaged non-ductile reinforced concrete moment frames

Effect of voids on the impact properties of Non-Crimp fabric carbon/epoxy laminates manufactured by liquid composite Moulding

A great effort has been made to quantify the detrimental effect of voids on CFRP, nevertheless, the effect on impact properties has been barely studied and when it has been considered, it has always been for prepeg materials and under low impact energies (< 6 J). This work focuses on the quantification of the effect of voids on the impact properties of NCF plates manufactured by LCM. For this purpose, biaxial (0/90) NCF-Epoxy laminates with different void contents were tested by the drop weight impact method. Four LCM variants were used to obtain plates at different void levels: VARTM with controlled modified capillary number (0.58 ± 0.18 %), RTM with controlled modified capillary number (1.35 ± 0.28 %), uncontrolled RTM (2.44 ± 0.14 %), and WCM (4.34 ± 0.31 %). The high void content samples (WCM) recorded a reduction of 25.88% in ultimate fibre failure force Fff, and 9.57% in dissipated energy Edis as compared to the low void content samples (VARTM-Caopt*). Regarding the analysis of damage tolerance, the loss of stiffness after impacts ranging from 2.5 J to 20 J was studied. A slightly higher loss was observed in WCM plates compared to VARTM-Caopt*, the residual stiffness after 20 J impacts being 45.8% and 49.1%, respectively. Therefore, to achieve optimum impact properties it is necessity to develop controlled LCM manufacturing processes that approach zero-defect manufacturing.

Mapping soil liquefaction susceptibility across Europe using the analytic hierarchy process

Mapping the susceptibility of earthquake-induced soil liquefaction at the continental scale is a challenge. Susceptibility of soils to liquefaction is the tendency of certain geomaterials to undergo a severe stiffness degradation and loss of shear strength. The latter could be induced by cyclic loading induced by seismic events. The liquefaction surface evidence is a local phenomenon, and detailed geotechnical field investigations are not available for regional studies, not to mention at continental scales. The literature review shows earthquake-induced soil liquefaction evidence in several European countries, yet, a comprehensive picture of the susceptibility at the European scale is not available. This work aims to develop a methodology to assess the earthquake-induced soil liquefaction susceptibility in Europe using geospatial parameters weighted via the Analytic Hierarchy Process (AHP). The major outcome of the study is a novel Liquefaction Susceptibility map of Europe (LSE), based on the building of ten different European macro-units for earthquake-induced soil liquefaction. These European macro-units have been delineated in this work in order to be homogenous areas from the geological, physiographical, and geomorphological points of view. The adopted input parameters are the depositional environment of the sediments, the distance from water bodies (coast and rivers), and the compound topographic index (as a proxy of the soil saturation). The resolution of the LSE map is 900 × 900 m. The results have been cross-compared with soil liquefaction susceptibility maps available for a region of Greece (i.e. Thrace), Portugal, Bulgaria, and sites where liquefaction manifestations occurred across Europe. The LSE can be adopted to identify at large scale the areas susceptible to liquefaction including also the territories across Europe characterized by low seismicity and potentially affected by anthropogenic seismicity.

Investigation of Structural Behavior of Piles in Liquefiable Cohesionless Soils

Piled foundation design and behaviour under static and dynamic loading (wave motion, earthquake, wind, vibration loadings of machinery) conditions are study subjects that are focus of interest recently in the geotechnical engineering applications. Liquefaction can be described as strength and stiffness loss of a loose, saturated non-cohesive soil under undrained cyclic loading as a result of increasing pore water pressures which reduce effective stress. Large deformations and lateral flow occurring in the layers of liquefied soil during earthquake could lead to strength and stiffness lose which may result with pile buckling and considerably increased earthquake damage on the superstructure. Predicting the bearing capacity and the deformation shape of the piled foundations during the earthquake is essential for the economy and the structural safety of the design. In this study model pile tests are conducted in uniform sandy soil and pile structural capacity is investigated under the effects of relative density and degree of saturation of surrounding soil, and pile embedment depth. Steel rods were used to represent the piles in the model tests. Sand soil was placed in a cylindirical tank at different thicknesses to provide for different pile embedment depths. Soils were compacted at four different compaction level to provide relative densities in the range of 45-80%. Static incremental load has been applied on the upper plate of the pile system in the tests. While the increase in the relative density affects the structural capacity of the piles positively, surrounding soil being saturated has resulted with capacity losses. Experimental results show that there is a consistency between our experimental findings and literature about deformation shape and buckling length of piles in liquified soils.

Vibration-based damage detection in anisotropic laminated composite beams by a shear-deformable finite element and harmony search optimization

A numerical procedure to detect and quantify damages in laminated composite beams with arbitrary lay-up using limited vibration data and harmony search optimization is presented. Since any damage in a structure changes its vibration characteristics, they can be considered as the diagnostic parameters with the help of an optimization technique. For the present study, frequencies and mode shapes are used as the damage diagnostic parameters, and the harmony search algorithm is used as the optimization tool. For finite element analyses, a five-node, thirteen-degree-of-freedom shear-deformable beam element is employed. This element can take into account the elastic couplings among extension, bending, and torsion arising due to material anisotropy existing in generally laminated composites as well as the Poisson’s effect. The damage to the beam is introduced by a stiffness loss coefficient at the elemental level while mass is assumed to be unchanged. Two different objective functions to be minimized are considered for damage detection. The efficacy of the proposed method with and without the presence of noise is demonstrated by two numerically simulated composite beams including single and multiple damages. Although its accuracy somewhat decreases in case of multiple minor damages, the proposed method is successful to detect moderate and severe single and multiple damages in practice. To overcome this weakness, (i) the algorithm can be hybridized with other metaheuristics, (ii) objective function can be enhanced by combining it with other vibration characteristics such as curvature, flexibility, and so on, or (iii) two- or multi-stage damage identification methods can be considered.

Fatigue behaviour and damage mechanisms under strain controlled cycling: Comparison of Flax–epoxy and Glass–epoxy composites

Recent research showed that strain-controlled (constant strain-amplitude) cycling may be ideal for natural-fibre-composite (NFC) fatigue testing, since enforcing constant strain-rates eliminate the apparent modulus ‘improvement’ reported by several stress-controlled studies. This article re-examines Flax-reinforcement as an alternative to Glass by fatigue testing under constant strain-amplitude. Two Glass–epoxy crossply configurations ([0/90]3S, [±45]3S) are tested alongside four commonly-studied Flax–epoxy layups ([0]16, [0/90]4S, [±45]4S, quasi-isotropic [0/45/90/-45]2S). Fatigue endurance plots for all layups are modelled by linearised logarithmic strain-life relationships, for which unique parameters are identified. Evolution of several mechanical phenomena (stiffness damage, inelasticity, peak-stress) are evaluated as potential indicators of progressive damage, correlated with SEM-observed microstructure cracking. Flax–epoxy specimens have superior resistance to stiffness loss, lower inelastic strains , and withstand higher stresses per-unit-density, when compared to similar Glass-specimens. Flax remains a promising alternative to Glass for mechanical reinforcement against fatigue degradation in engineering structures.

Numerical analysis of a hydrodynamic air bearing with electromagnetic assistance

An air bearing system with active electromagnetic assistance is proposed, and its dynamic characteristics are numerically analysed. The system is based on herringbone groove hydrodynamic journal bearings. The air bearing system adopts electromagnetic force to assist in lifting the rotor and lowering the lift-off speed of the bearing and adjusting the dynamic stiffness. The characteristics of the bearings are examined using the two-way coupled fluid-structure interaction method for different operating environments. The fluid simulation is performed in ANSYS Fluent and coupled with the motion equations. The fluid-structure interaction analytical model is cross-examined with the published experimental results and is used to analyse the proposed system. The results demonstrate that the lift-off speed is lowered by 46.04% and that the electromagnetic force can compensate for the loss of stiffness at an altitude of 4000 m.