Realistic Loading Conditions Research Articles

07.14: Rack uprights under combined compression and bi‐axial bending: Experimental testing, numerical analysis and European standard approach

ABSTRACTPallet rack uprights are thin‐walled mono‐symmetric perforated open steel cross‐sections subject to combined axial compression and bending. Loads (pallets of goods to be stored) are transmitted to uprights through the beams and the beam‐upright connections. Bending about the major axis (cross aisle direction) is induced to the upright by the eccentricity of the beam end plate and the moment transmitted by the semi‐rigid beam to upright connection. The bending about the minor axis (down‐aisle direction) is induced by eccentricity of the axis of the beam and the axis of the upright. The nature of these components, with perforations and buckling failure modes, together with combined actions, is a challenge in structural design.The main objective of this investigation is to provide a comprehensive empirical, numerical and analytical analysis of how bending combined with axial compression influences in the design load of pallet rack uprights.This paper presents experimental tests performed on uprights under combined compression and bending, and the FEA including nonlinear geometric and material behaviour. Both results are compared to the current European Standard, EN 15512, for Steel static storage systems.One type of cross section has been analysed, with a length that can produce different type of failure modes, mainly local and distortional buckling. The uprights have been analysed with different axial – bending combinations, which cover the field of combinations possible in the real load conditions of the rack.

An ex vivo experiment to reproduce a forward fall leading to fractured and non-fractured radii.

Forward falls represent a risk of injury for the elderly. The risk is increased in elderly persons with bone diseases, such as osteoporosis. However, half of the patients with fracture were not considered at risk based on bone density measurement (current clinical technique). We assume that loading conditions are of high importance and should be considered. Real loading conditions in a fall can reach a loading speed of 2m/s on average. The current study aimed to apply more realistic loading conditions that simulate a forward fall on the radius ex vivo. Thirty radii from elderly donors (79y.o.±12y.o., 15 males, 15 females) were loaded at 2m/s using a servo-hydraulic testing machine to mimic impact that corresponds to a fall. Among the 30 radii, 14 had a fracture after the impact, leading to two groups (fractured and non-fractured). Surfacic strain fields were measured using stereovision and allow for visualization of fracture patterns. The average maximum load was 2963±1274N. These experimental data will be useful for assessing the predictive capability of fracture risk prediction methods such as finite element models.

Specimen Preparation Methods for Artificially Cemented Sand in Simple Shear and Hollow Cylinder Apparatuses

Portland cement can be mixed with sand to improve its mechanical characteristics. Considering the inherent anisotropy of most sands, it is not clear whether the added cement shall contribute to equal increases of strength and stiffness in vertical and horizontal directions or not. Literature review reveals that all the previous experimental studies on the cement-treated soil topics are limited to conventional triaxial device test results. The effect of cement addition to clean sand in more sophisticated stress-path controlled devices like Simple Shear Apparatus (SSA) and Hollow Cylinder Apparatus (HCA) simulating more realistic loading conditions, such as earthquake, have not been investigated yet. This paper presents innovative methods for testing artificially cemented sands through making modifications to the loading platens, specimen molds and specimen preparation methods of SSA and HCA. Undrained tests under different monotonic stress paths were carried out on different mixtures of Portland cement with clean sand to investigate the effect of principal stress rotations. The test results revealed that the cement mixture reduces the anisotropy, while it improves the mixture mechanical properties compared to the compacted uncemented sand.

Shake table assessment of gusset plate connection behaviour in concentrically braced frames

Diagonal bracing members and their connections to beams and columns are the key lateral resisting components in concentrically braced frames (CBFs). Although gusset plate connections are widely used to connect bracing and frame members in such systems, their design often involves significant simplifications and idealisations due to the complexity of their behaviour under seismic loading. A conventional approach, which utilises a standard linear clearance zone that permits out-of-plane brace deformation, is typically used in the design of gusset plates. This approach can result in overly large connections with cumbersome details. The desire to achieve an improved balance between the gusset over-strength, on the one hand, and a favourable overall frame performance coupled with practical connection detailing, on the other, has prompted proposals for an improved design approach. However, before new recommendations on the design of gusset plate connections can be provided for use in codified guidance, there is a need to assess the performance of such detailing alternatives under realistic earthquake loading conditions. Accordingly, in this study, the performance of different brace connection configurations and gusset plate designs are examined using shake table testing. The paper describes twelve single-storey full scale shake table tests, which were performed on the AZALEE seismic testing facility at CEA Saclay. In seven of these tests, the gusset plates at the end of the brace members were connected to both beam and column flanges, while in the other five tests these were connected to the beam flange only. Conventional gusset plate design with a standard linear clearance was used for six tests, whereas a more balanced design with a nonlinear elliptical clearance detail was used for the others. The experimental set-up, specimen details, and loading procedures are presented, together with a detailed account of the results and observations. The main findings and their implication on the performance at the local component, as well as the overall frame levels, are highlighted. In particular, it is shown that, provided a number of recommendations are followed, the balanced design approach using a nonlinear clearance can enhance the overall drift capacity, while maintaining control of the failure mode within the bracing member.

The imaging of failure in structural materials by synchrotron radiation X-ray microtomography

The initiation and propagation of short cracks are critical to the detailed understanding of the failure mechanisms of engineering materials and structures. Therefore, for a safety critical component, it is beneficial to be able to follow the progress of cracks in loaded structures and real environments ideally at sub-micron spatial resolution and/or sub-second time resolution. Here we review the great progress that has been made in the application of synchrotron radiation X-ray computed microtomography (SR-μCT) to the study of internal damage accumulation and evolution in structural materials including cast irons and steel, nickel superalloys, lightweight titanium and aluminum alloys as well as metallic, ceramic and polymer composite materials together with additatively manufactured or three-dimensional (3D) printed metallic materials. SR-μCT is shedding light on the complex interactions of cracks with intrinsic porosity, precipitates and intermetallic inclusions and grain structure, as well as macroscopic features such as holes and joints. Furthermore it is enabling qualitative and quantitative relations between the microstructural features and engineering performance to be established and validated under realistic service loading and environmental conditions. Here we introduce the basic 3D and time-lapse (4D) tomographic methods and discuss their strengths and limitations across a range of the applications, as well as identify opportunities for the future.

Novel Approach for Endurance Testing of Riblet-Structured Thin Sheets Under Realistic Loading Conditions in Active Drag-Reduction Systems

Abstract A novel fatigue testing device for bending loading was developed that resembles loading conditions in aerodynamic drag-reduction systems, creating transversal surface waves to modify the surfaces' turbulent boundary layer. The setup uses Lorentz force actuators and is employed in fully reversed loading (R = −1) at a testing frequency of around 100 Hz. Tests were run on riblet-structured Alclad 2024 T351 material. The damage mechanisms for this material are found to be similar for axial and flexural loading. In the high cycle fatigue regime up to a few 106 load cycles, the S/N-curves are also comparable for both loading modes. One distinct difference between the loading modes is a less pronounced threshold behavior in the S/N-curve for bending. Furthermore, a higher stress level has to be applied for fatigue failure in this case.

Thermal characteristics of externally venting flames and their effect on the exposed façade surface

In a compartment fire, Externally Venting Flames (EVF) may significantly increase the risk of fire spreading to adjacent floors or buildings, especially when combustible insulation materials are installed on the building façade. An increasing number of recent reports suggest that existing fire engineering design methodologies cannot describe with sufficient accuracy the characteristics of EVF under realistic fire load conditions. In this context, a series of fire safety engineering design correlations used to describe the main EVF thermal characteristics, namely EVF centreline temperature and EVF-induced heat flux on the exposed façade surface, are comparatively assessed. Towards this end, measurements obtained in a medium- and a large-scale compartment-façade fire test are employed; aiming to broaden the scope of the validation study, predictions of the investigated correlations are further compared to measurements obtained in 6 large-scale fire tests found in the literature. It is found that the correlation proposed in EN1991-1–2 (Eurocode 1) for the estimation of the EVF centreline temperature is under-predicting the measured values in large-scale fire tests. In addition, it is concluded that estimation of the local flame emissivity should take into account the specific fuel type used in each case.

Safe distance of cultural and historical buildings from subway lines

Construction and operation of subways (metros) closed to historical sites have been one of the main concerns of the world heritage protection bodies such as UNESCO. While metros alleviate condense traffic conditions and boost the tourism industry, the metro induced vibrations might damage cultural and historical structures (CHS). Although there have been various studies into the metros vibration characteristics and the CHS protection methods, there is still a lack of sufficient investigations into the measures by which a safe distance of the CHS to the metro can be derived. In response to this need, a thorough theoretical and experimental investigation was made in this research, aiming at developing a safe distance prediction graphs (SD). For this purpose, a finite element model of the track and the surrounding media was developed. The advantage of the model over the current ones is the consideration of the real (in situ) train loading conditions as an input. The model was validated by comparisons of its results with those of a comprehensive field measurement carried out in this research. New classifications of the CHSs and the track sub-structure form the aspect of metro-induced vibrations were developed in this research. Through parametric analyses of the model, the SD was developed for the first time as a function of metro characteristics, geo-mechanical properties of the media between the metro and the CHS, and the type of CHS. The effectiveness and practicability of the SD in construction of new subway lines were illustrated. It was shown that the SD graphs developed here can be taken as an effective tool for the design of subway lines in historical cities.

Künzel Model and Boundary Inverse Problem

In order to obtain material properties of a given system it is convenient in many casesto perform only noninvasive, i.e. boundary measurements. Our interest is then focused on buildingmaterials and their functionality when exposed to heat and moisture. To describe the underlying phenomena of heat and moisture transfer we use Künzel model with stochastic description of materialproperties for its relative simplicity and sufficient accuracy. For the inverse procedure weintend to utilize the Calderón’s problem framework which is regularly used in medical imaging asElectrical Impedance Tomography and is based on knowing Dirichlet-to-Neumann or Neumannto-Dirichlet map.Overall this work serves as a preliminary study of both aforementioned computational models andits goal is therefore to build up a solid foundations for further redefinition of both models in order tofit the realistic loading conditions for building structures.

Stable crack growth in composite laminates under various stiffness of the loading system

The work is devoted to experimental study of stable and unstable crack growth processes in composite materials. The eccentric tension tests of the compact specimens of glass-epoxy composite laminates in conditions of various stiffness of the loading system were carried out. Different measurement techniques of crack opening displacement were used in work. During the analysis of results of a tension test of compact specimens with crack, descending sections of the loading curves were considered by analogy with postcritical deformation stage, which could be found in test of smooth specimens of different materials. It is shown that behavior of composite specimens with crack under tension depends on the loading system parameters: crack grows process is stable when stiffness of the loading system is enough, crack opening displacement rate increase under reduction of stiffness of the loading system. This effect should be take into account during modeling and analyzing of fracture processes of composite materials, strength and survivability assessment of composite structures in real loading conditions.

Wear Behavior of MoS2 Lubricant Layers During Sheet Metal Forming

The increased use of high and ultra-high strength steel and materials with a high adhesion tendency leads to higher demands regarding the wear resistance of forming tools. A promising approach addressing both challenges is to apply MoS2 dry film lubricant coating systems to the tool surface. These coatings reduce friction and the use of drawing oils and can also be used as an abherent. Once these coatings are exhausted, they can be chemically removed and reapplied. However, the tool life of the coatings cannot be predicted properly, due to their inhomogeneous wear behavior which depends on the forming parameters and differs locally. Moreover, there are carryover effects of MoS2-particles to already ablated areas. Given the unpredictability of the wear of dry film lubricant coatings, it is not possible to plan the recoating cycles appropriately and thus, the potential of these coatings for commercial applications is limited.In this research, the wear behavior of MoS2-layers in deep drawing processes was investigated on a test stand with DP800+Z coil material which ensures realistic load conditions at the draw ring radius. Firstly, the wear pattern was determined and locally different contact pressures were recorded and assigned to respective wear areas. Due to bending effects of the sheet material, there are two major areas of wear on the draw ring radius. During wear tests, the wear-dependent coating thickness during wear initiation was determined by using 3D laser scanning microscopy and the friction force was evaluated.

응력 삼축성을 고려한 원자로 내부구조물 배플포머 집합체의 연성저하 평가

The study presents structural integrity assessment of ductility degradation of a baffle former assembly by performing finite element analysis considering real loading conditions and stress triaxiality. Variations of fracture strain curves of type 304 austenitic stainless steel with stress triaxiality are derived based on the previous study results. Temperature distributions during normal operation such as heat-up, steady state, and cool-down are calculated via finite element temperature analysis considering gamma heating and heat convection with reactor coolant. Variations of stress and strain state during long operation period are also calculated by performing sequentially coupled temperature-stress analysis. Fracture strain is derived by using the fracture curve and the stress triaxility. Finally, variations of ductility degradation damage indicator with the fracture strain and the equivalent inelastic strain are investigated. It is found that maximum value of the ductility degradation damage index continuously increases and becomes 0.4877 at 40 EFPYs. Also, the maximum value occurs at top and middle inner parts of the baffle former assembly before and after 20 EFPYs, respectively.

Experimental Assessment of Wind Loads on Vinyl Wall Siding

Wind-induced damage to multi-layer building wall systems, such as systems with vinyl siding, is common, especially in hurricane-prone areas. Wind load distribution through these multi layered walls and the amount of load reduction due to pressure equalization is expressed through Pressure Equalization Factors (PEF). The ASTM D3679 standard suggests a PEF of 0.36, which means a 64% reduction in the net pressure on the siding. This paper presents results from an experimental study conducted on a low-rise building subjected to realistic wind loading conditions at the Wall of Wind (WOW) experimental facility at Florida International University (FIU). Results from area averaged mean and peak pressure coefficients indicated that a very small portion of the total wind load is carried by the vinyl siding. However, PEF’s were found to be much higher when individual taps were considered. For instance, PEFs ranged from 71% to 106% for the case of pressure coefficients with negative sign (suction) and 39% to 110% for the case of pressure coefficients with positive sign (pressure). When a combined set of taps was considered, PEFs ranged approximately from 50% to 80% for the case of ‘suction’ and 15% to 75% for ‘pressure’. Based on the 1 m2 of tributary area used in ASCE 7-10 Standard, results show that the net load on vinyl wall siding can be obtained by reducing the net design load for the entire wall assembly by 25% and 60% for suctions and pressures, respectively. However, a smaller tributary area (< 1 m2) can experience a local peak load that can induce damage to connections, especially in the case of relatively flexible wall coverings, with no or very little load sharing between connection points. Results indicate that for smaller areas (~ 0.2 m2) the allowable percentage reductions should not be more than 15% and 25% for suctions and pressures, respectively. This study shows that the suggested ASTM PEF of 0.36 may lead to the underestimation of loads for the design of details affected by local loads. However, further research is needed to consider more cases when developing adequate design load guidelines for vinyl wall sidings.

Nonintrusive Near-Field Characterization of Spatially Distributed Effects in Large-Periphery High-Power GaN HEMTs

This paper introduces an improved nonintrusive near-field technique for in situ characterization of distributed effects in GaN high-power transistors. Compared with previous passive probing approaches which sense electric fields induced by drain bondwires, the proposed method employs the position-signal difference method to measure the $E$ -field fluctuations induced by transistor fingers. This allows a robust and detailed identification of in-circuit electrical quantities, such as voltages, currents, loading impedance, and output power, spatially distributed over individual transistor cells and fingers. The $E$ -fields needed for determining the distributed phenomena have been measured in situ above the fingers of a 100-W GaN power transistor at fundamental and second-harmonic frequencies, while the device operates under realistic loading conditions. The deduced in-circuit quantities are compared with their counterparts from an independently developed distributed in-house model of the same device for validation. The practical value of the proposed method is further demonstrated by uniquely identifying device damage at the finger level (enforced by laser cutting).

Analysis of the collapse of a greenhouse with vaulted roof

The failure of a multi-span steel greenhouse with vaulted roof covering an area of 7500 m2 is analysed numerically. Failure occurred during a day of heavy snow and moderate wind. The weather data at the location of the greenhouse and at the time of the collapse were not recorded. Weather data were available from meteorological stations in the broader area though. Numerical models of the complete 3-D structure and of a representative 2-D frame were developed to simulate the greenhouse structural behaviour under various combinations of actions. Linear, geometric and material nonlinear analyses were employed to investigate the causes of collapse. Based on the relevant provisions of the European Standard for greenhouse design and the Eurocodes, loading was varied to allow for an estimation of the real loading conditions at failure. Analysis confirmed the in- and out-of-plane failure modes that were observed after the incident. Based on all available data and the analysis results, it was shown that failure was the result of the combination of many factors that occurred simultaneously: an unfavourable combination of snow and wind actions, the heating system not operating as it should under such weather conditions and the absence of a complete row of internal columns.

A three-stage constitutive model for the creep behaviours of high-Cr steels at elevated temperatures

In this paper, a three-dimensional constitutive model is proposed to simulate the creep behaviours of high-Cr steels at elevated temperatures. In the model, the minimum creep rate and the average creep rupture time at different temperatures and stress levels are predicted by adopting two Larson–Miller parameters. The decrease of the creep rate during the primary creep stage is captured by introducing an internal variable representing the strain hardening effect. The material parameters of the model can be identified by using the conventional experimental results. Both the strain- and stress-driven algorithms are designed to solve the constitutive evolution equations. The response of high-Cr steels during the whole creep procedure can be predicted at a quantitative level by the current model. Further implementing the model into a finite element software, the global creep behaviours of high-Cr components under realistic loading conditions can be simulated.

차량 탑재형 레이더 시스템의 구조물에 대한 연구

The vehicle-mounted radar system (VMRS) including its electronic parts must be designed so that its performance is maintained under varying environmental conditions. The important aspects are typically weight and safety. Since many rotating VMRSs have been developed, discussion about the vibration and shock requirements for the transportation conditions has occurred: in addition, the dynamic unpaved, paved, and off-road effects have been emphasized with respect to lightweight designs. A lightweight-design VMRS should be capable of operating stably under the wind condition with the support of the vehicle structure. In this paper, a structural analysis regarding the support of the VMRS is performed, whereby the real-load conditions for three types of road and pressure were employed in terms of the wind condition. The structural analysis for the safety of the VMRS is performed, and the structural-integrity analytical processes of the VMRS are presented for different load conditions.

Contribution of fluid in bone extravascular matrix to strain-rate dependent stiffening of bone tissue – A poroelastic study

Osteoporotic fractures represent an increasing cost to society, and its diagnosis methods based on bone density still lack accuracy in identifying risk of fracture. This is why a better understanding of mechanical behavior of bone tissue is of importance, especially when it comes to relating experimental observations to realistic physiological fall loading conditions. This study aims at exploring the stiffening effect of pore fluid in bone extravascular matrix subject to high strain rate loading that is more realistic to simulate a physiological fall. A computational approach is used, where bone tissue microstructure extracted from micro-CT images is modeled using finite elements. The solid phase of bone tissue is modeled as a poroelastic material, a porous matrix filled with fluid. When the extravascular matrix experiences certain volumetric deformation, the fluid in pores presents load carrying capacity, which consequently varies the apparent stiffness of bone tissue. It is shown that effects of fluid stiffening in bone can be significant, depending on the chosen material properties, the amount of volumetric strain in tissue and the loading rate with respect to hydraulic conductivity and drainage conditions. It is also shown that such stiffening effect is influenced by bone microstructure, and is more significant in cortical bone than in trabecular bone.

Validation of a new computational 6-DOF knee simulator during dynamic activities

A new six-degree-of-freedom (6-DOF) joint simulator has recently been developed which facilitates testing of implants under more realistic loading conditions than has been possible previously. However, typical wear testing can be very time-consuming, taking weeks or months to complete. A validated computational model is an ideal complement to these types of long-running tests. In this study, a computational counterpart to the new 6-DOF joint simulator was developed and validated. Total knee replacement components were evaluated in both physical and computational simulations, and joint mechanics were compared between the experiment and the model. Kinematic comparisons were carried out for two total knee replacement designs, under loading conditions representative of three different activities of daily living: deep knee bend, gait, and stepdown. The model accurately reproduced the motions obtained in the physical simulator, and appropriately differentiated between activities and between implant designs. Root-mean-square differences in anterior-posterior translations and internal-external rotations were less than 1.7mm and 1.4°, respectively, for both implant designs and all three dynamic activities. Contact area, and peak and average contact pressure predicted by the model matched experimental measurements with a root-mean-square accuracy of 20mm2, 9MPa, and 1MPa, respectively. The computational model of the 6-DOF joint simulator will be a key tool in efficient evaluation of implant mechanics under loading conditions representative of the in vivo environment. These simulations may be used directly in comparison of devices, or may aid in facilitating optimal usage of the physical simulator through determining which activities and/or loading conditions best address specific clinical or design issues, for example, development of worst-case loading profiles for wear testing.

Performance of Suspended Nonstructural Components and Their Anchorage during Shake Table Tests

Nonstructural components and systems are typically connected to concrete structures by means of post-installed anchors. Despite their widespread use, very little is known about the behavior of post-installed anchors when used in a system and loaded during an earthquake. The study presented in this paper was conducted to investigate the seismic performance of suspended model non-structural components connected by post-installed anchors to a concrete slab. The concrete slab was mounted in a special loading rig on a shake table and subjected to a series of floor motions simulating the acceleration histories at specific floor levels of typical reinforced concrete buildings of variable heights. Simultaneously, the anchors supporting the component were set in predefined cracks, which opened and closed according to pre-calculated crack histories. This unique test setup allowed evaluation of the performance of anchors and attached components under realistic boundary and loading conditions.