Maximum Load Level Research Articles

Failure analysis of adhesively-bonded metal-skin-to-composite-stiffener: Effect of temperature and cyclic loading

The aim of this research is to analyse the failure of a Fiber Metal Laminate (FML) skin adhesively bonded to a Carbon Fiber Reinforced Polymer (CFRP) stiffener, under quasi-static loading at different environmental temperatures (−55°C, Room Temperature RT and +100°C) and under fatigue loading at RT. This bonded joint was tested using stiffener pull-off tests, which is a typical setup used to simulate full-scale components subject to out-of-plane loading. The failure sequence for all test conditions consist of: (1) damage initiation at the noodle of the CFRP stiffener; (2) damage propagation by delamination from the noodle to the stiffener foot; (3) detachment of the stiffener from the skin. Increasing the temperature, decreases the joint stiffness (40% when compared to RT) and decreasing the temperature decreases the maximum load (50% when compared to RT). The fatigue life initiation of the joint presents a very large scatter but the fatigue life propagation presents more stable results. The fatigue threshold (no damage) is reached at approximately 30% of the maximum load level. The fracture surfaces indicate a predominant inter and intra-laminar failure of the composite under mixed mode I/II. The CFRP stiffener is the weakest link of the bonded FML-skin-to-CFRP-stiffener both for static and fatigue loading.

К оценке контактно-усталостной долговечности химико-термоупрочненных зубчатых колес

Introduction. The specification of the strength characteristics of the diffusion layer of carburized (C) and nitro-carburized (NC) involute gears is described. Regulated by GOST 21354-87, the dependence of the surface endurance limit of steel parts on hardness in the region of finite (up to 10-12 million cycles) life is overstated. The work objective is to determine the maximum allowable load level of the involute gearing under the conditions of alternative failure forms and the area of correct application of GOST 21354-87 recommendations. Materials and Methods. In the case-hardened parts, both deep and surface processes of contact fracture occur simultaneously. However, these processes have essentially different mechanisms of contact-fatigue durability with fundamentally different mechanisms of the contact-fatigue life. The high-end fracture mode is determined by a variety of factors, the key ones being the level of intensity and the properties of the near-surface layers of the material. The loading capacity of the diffusion layer is investigated on the basis of the experimental data by the strength assessment procedure of a structurally inhomogeneous material. Here, Pisarenko-Lebedev generalized criterion of limiting state is used. Research Results. Consideration of the effect of the plastic properties of the material and its variation within the diffusion layer has allowed significantly increase the forecasting validity of the loading capacity level of the carburized and nitro-carburized gears for contact strength. The occurrence of the threshold value of the working surface hardness of the wheels teeth is confirmed at the level of 60-61 Rockwell units. This level increase leads to the decrease in contact-fatigue life. The plasticity parameter impact on the capability level and the variation of this parameter value for different types of the thermo-chemical treatment (TCT) is assessed. In the finite life area, deep contact fractures are leading. And the limits of contact fatigue life upon surface pitting readings should be oriented to the allowed normal deep contact stress. Discussion and Conclusions . The obtained results are in qualitative agreement with the experimental data, but they exceed them a little in some cases. Studies on the plasticity parameter value and its variation depending on the chemical composition of the steel and the TCT type are needed. But already at this stage, the results obtained can be used in the design of heavy-loaded case-hardened gears to determine the surface hardness and the diffusion layer thickness. The application domain includes the tractor track gear transmissions, oil-producing beam engines, mining and handling facilities.

On the question of predicting the service life of lattice steel structural elements

Present paper deals with the evaluation of fatigue strength at crack initiation stage in the double angle elements of lattice steel structures. The current construction norms for steel structures do not include check for fatigue strength at crack initiation stage, and the calculation of elements endurance limit does not account actual stress-strain state in stress concentration zones, material deformation characteristics in these areas, and strength properties of steel. Experimental study of the stress-strain state kinetics in the areas of gusset plate breakage of double angle welded joints was carried out by strain gauge method. Results were obtained regarding the stress state stabilization under cyclic loading and stress intensity factor. During cyclic tests, maximum load level varied depending on the yield stress. The formation of fatigue cracks was detected in the zone of gusset plate breakage. The curve at fatigue cracks initiation stage, by deformational fracture criteria at 'stiff' loading, was described by the equations presented in the 'Standards of strength calculation of equipment and pipelines of nuclear power plants'. Calculation analysis of results for number of cycles before fatigue crack initiation has been made and good agreement with test results has been obtained.

Biaxial Loading of a Textile Ribbons Structure for an Inflatable Module of Space Habitats

The aim of the experimental campaign was to assess and observe the biaxial tensile mechanical response of a ribbons structure as part of an inflatable module of space habitats. The structure consists of orthogonal Zylon™ and Kevlar™ ribbons connected at crossover by Nylon thread seams. A clamping system was designed and manufactured to reproduce the biaxial loading condition supposed in the inflatable module application. The tests were assisted by a digital image correlation system. The images post-processing allowed the observation and measurement of the full field strain of the different connection schemes. The results of the tests show the excellent mechanical performance of the ribbons structure at the maximum biaxial load level without visible damage.

Fatigue Performance of Ballastless Track Slabs Reinforced with BFRP and SFCB

AbstractThe main objective of this research was to evaluate the behavior of fiber-reinforced polymer (FRP) bar-reinforced ballastless track slab subjected to a fatigue loading. Five ballastless track slabs were experimentally tested, which were reinforced with steel fiber–reinforced polymer composite bars (SFCBs), basalt fiber–reinforced polymer (BFRP) bars, and traditional steel bars, respectively. All slabs were loaded in three-point loading and designed to have the same initial stiffness of cross section. To simulate the wheel cyclic load, different maximum loads were selected, but the minimum load was the same for all slabs. At the same maximum load of 1.76 static wheel loads, the stiffness degradation and crack width in the SFCB slab were similar to those of the traditional RC slab and survived 3,000,000 fatigue cycles, showing good fatigue performance and a residual strength reduction of only approximately 3%. At the maximum load level of 60% of the ultimate load static capacity, both the SFCB slab ...

Amplified Seismic Loads in Steel Moment Frames

This study focuses on exploring the seismic axial loads for columns in steel moment resisting frames (SMRFs) under strong ground motions. For this purpose, the increases in axial loads are investigated at the maximum lateral load level and the corresponding lateral displacement. The results are presented in terms of maximum amplification factors (Ω0) of all frame columns under the selected ground motions and axial load-moment levels in columns. four typical steel moment resisting frames representing typical low, medium and high rise steel buildings are designed based on the seismic design requirement in ASCE 7-10 and AISC 341-10 . An ensemble of ground motions range from moderate to severe are selected to identify the seismic response of each frames. Two sets of ground motions corresponding to 10% and 2% probability of exceedance are used in nonlinear dynamic time history analyses.

Flexural experiment and stiffness investigation of reinforced concrete beam under chloride penetration and sustained loading

This paper presents an experimental study conducted to characterize the effect of corrosion and sustained load on structural behavior of reinforced concrete (RC) beam. The effects of loading level on corrosion of reinforcing steel, the flexural deflection and residual loading capacity of the test beams were investigated. In the first scenario, two non-corroded beams were tested to determine the maximum load levels that were required by the beams to reach their deflection limits and that were also tested for the sustained levels of the applied loads. For the second experiment, the corrosion was accelerated through the application of external direct current whilst the beams were under the load equivalent to 0%, 50%, 65% and 80% of the ultimate load. The results indicate that load levels have a significant effect on the rise of steel corrosion level, the decrease of flexural stiffness and the degradation of flexural capacity. The mechanical properties like the strength and ductility are significantly impaired due to simultaneous effect of loading and corrosion in comparison to the effect of the single parameter at a time. Considering the corrosion situation of different position of steel bar, the formula of the ratios of the different position of steel bar corresponding with different sustained loading levels was given. An equation was deduced to calculate the flexural stiffness of corroded reinforced concrete and can be served as a reference for deflection calculation of corroded reinforced concrete beams.

Spherical indentation of NiTi-based shape memory alloys

In this study, spherical indentation responses of room temperature superelastic (homogenized Ni50.8Ti49.2 and 400 °C 3 h aged Ni45.3Ti29.7Hf20Pd5) and nonsuperelastic (Ni49.9Ti50.1 and 600 °C 3 h aged Ni45.3Ti29.7Hf20Pd5) are revealed as functions of maximum loading level in nano and macro scales. Experimental results showed that for all alloys, the depth and work recovery ratio decrease and then saturate with hmax/R. In contrast, indentation hardness increases and then saturates with indentation depth. Both NiTi and NiTiHfPd alloys have higher hardness in austenite than in martensite. NiTiHfPd alloys have a higher indentation hardness than NiTi alloys which was attributed to the higher strength of NiTiHfPd alloys.

Push-Off Tests in the Study of Cyclic Behavior of Interfaces between Concretes Cast at Different Times

AbstractThe knowledge about the cyclic behavior and effects of fatigue on interfaces between concrete cast at different times, subjected to shear stress, is still very limited. This aspect is particularly relevant in structures subjected to important cyclic loads, such as railway bridges. In this context, first, a brief state of the art on the monotonic and cyclic behavior of interfaces between concretes cast at different times is presented. Then, the test campaign, which was aimed at obtaining experimental support for modeling the behavior of such interfaces, subjected to a high number of load cycles, is described, and the corresponding results are discussed and analyzed in terms of slip and crack opening, stress variations in the monotonic tests, failure mode, and number of resisting load cycles. It was further observed that the number of resisting load cycles increases as the maximum load level decreases. On the basis of this evidence, an experimental S-N curve was set for this type of interface.

A polytetrafluoroethylene porous membrane and dimethylhexadecylamine quaternized poly (vinyl benzyl chloride) composite membrane for intermediate temperature fuel cells

A composite material for phosphoric acid (PA) loaded membrane was prepared using a porous polytetrafluoroethylene (PTFE) thin film. N, N-Dimethylhexadecylamine partially quaternized poly (vinyl benzyl chloride) (qPVBzCl−) was synthesized as the substrate for the phosphoric acid loaded polymer membrane. SEM observation indicated that the pores were filled with the qPVBzCl−. The maximum PA loading level was calculated to be 4.67–5.12 per repeat unit on average. TGA results showed that resultant composite membrane was stable in the intermediate temperature from 100 °C to 200 °C. The composite membrane tensile stress was 56.23 MPa, and the Young's Modulus was 0.25 GPa, and the fractured elongation was 23%. The conductivity of the composite membrane after the PA addition (H3PO4@PTFE/qPVBzCl−) increased from 0.085 S cm−1 to 0.11 S cm−1 from 105 °C to 180 °C. The peak power density of the H2/O2 at 175 °C under low humidity condition (<1%) for H3PO4@PTFE/qPVBzCl− membranes was 360 mW cm−2.

Drilling and borehole techniques relevant to natural gas exploration and development: A collection of published research (2009–2015)

This work aims to investigate the mechanical behaviour of medium-grained sandstones under cyclic loading with different loading/unloading rates. This type of cyclic loading is called differential cyclic loading (DCL) and is considered for testing rock behaviour. In the experiments, constant amplitude and multi-level cyclic loadings were performed. Three loading modes were designed to consider different relationships between loading and unloading rates. Axial strain evolution and energy dissipation were analysed for different loading/unloading rates and maximum cyclic load level. The correlations between P-wave velocities and strengths of rocks deduced from this research are compared with existing published data. The relationships between final strength and axial strain at failure under different loading patterns were also discussed and a rough assessment of the remaining fatigue life is introduced using the predicted value by fitting the axial peak strain.

Seismic axial loads in steel moment resisting frames

During the 1994 Northridge Earthquake, many buildings with modern steel moment resisting frames (SMRFs) suffered from connection failures. One year later, similar damage has occurred in the 1995 Kobe earthquake in Japan. The unexpected seismic response of SMRFs resulted in comprehensive analytical and theoretical investigations and major changes in steel building design have been implemented consequently. One of the requirements in the subsequent seismic design codes is the stability check of the columns. Column yielding in a seismic force resisting systems (SFRSs) is not the desired damage mode and might result in column rupture or global buckling and threaten life safety. This study focuses on exploring the seismic axial loads for columns in SMRFs under strong ground motions. For this purpose, the increase in axial loads in low-, medium-, and high-rise SMRFs are investigated at the maximum lateral load level and the corresponding lateral displacement. The results are presented in terms of PHRs, average system overstrength factors (Ω0) of all columns in the frames under the selected ground motions, the distribution of Ω0 in the individual columns in the frame, and axial load levels in columns. The results indicate that axial load level remains below 0.4 in the columns for low- and medium-rise frames, whereas it may get as high as 0.95 in highrise frames.

Influence of Fatigue Loading in Shear Failures of Reinforced Concrete Members without Transverse Reinforcement

Shear fatigue of reinforced concrete members without transverse reinforcement has been observed to be potentially governing for the strength of some structural members subjected to large live loads of repetitive nature (as traffic, wind or wave actions). Although extensive experimental programmes have been performed in the past and a rational ap-proach to the problem can be performed on the basis of Fracture Mechanics, most design codes still ground shear fa-tigue design on empirical equations fitted on the basis of existing data. These empirical formulas show inconsistency amongst them and some neglect potentially relevant parameters as the ratio of maximum and minimum fatigue load levels. In this paper, a consistent design approach is presented, by using the principles of Fracture Mechanics applied to quasi-brittle materials in combination with the Critical Shear Crack Theory. This approach leads to a simple, yet sound and rational, design equation incorporating the different influences of fatigue actions (minimum and maximum load levels) and shear strength (size and strain effects, material and geometrical properties). The accuracy of the design expression is checked against available test data in terms of Wöhler (S-N) and Goodman diagrams, showing consistent agreement to experimental evidence. In addition, the estimate of the number of cycles until failure is shown to be significantly more accurate and with lower scatter than current empirical shear fatigue formulations of Eurocode 2 or fib-Model Code 2010.

Effect of Different Cyclic Signals on Three-Points-Bending Behavior of a Fibreglass and Epoxy Resin Composite Materials

The study of mechanical behavior during three point-bending fatigues has been systematically associated with a sinusoidal cyclic signal. The purpose of this work is to study and to analyze the behavior of composite materials, composed from glass fiber and epoxy resin under the effect of two other cyclic waves having respectively a square and a triangular signal. In the present work a new experiment is carried out to evaluate the cyclic signals (sinusoidal, square and rectangular waves) influence on the fatigue behavior. This material has been tested under bending in order to determine the maximum displacement which helps to fix the maximum loading level during tests. The tested samples damage and rupture caused by the application of the three cyclic waveforms has been observed by optical microscope, results are compared and discussed.

Elastic failure of locally supported silos with U-shaped longitudinal stiffeners

For practical considerations, thin-walled steel silos are often supported by a limited number of discrete equidistant supports around their circumference. In such cases, large loads are transferred to the limited number of supports, causing locally high axial compressive stress concentrations. A possible solution is to add a partial-height U-shaped longitudinal stiffener above each support. Such stiffeners create a more gradual transmission of vertical loads to the supports, increasing the maximum failure load. This paper aims to map the influence of the dimensions of such longitudinal stiffeners on the failure behaviour of a thin-walled silo. Both the parameters of the cross-section and the height of the stiffeners are discussed. All the results and the findings are based on geometrically and material nonlinear analyses — GMNA — performed with finite element software. The simulations indicate that, in general, thin-walled silos will fail by pure elastic buckling in the unstiffened silo wall above the terminations of the longitudinal stiffeners. However, this is only true if the cross-section of the stiffeners, and to a lesser degree the moment of inertia, is sufficiently large in order that the longitudinal stiffeners can absorb the supporting loads. In contrast, for longitudinal stiffeners with a small cross-section, the silo structure will fail by premature elasto-plastic collapse of the stiffeners itself at significantly lower load levels. Furthermore, the height of the stiffeners and the degree of support - the circumferential width of the supports and the stiffeners is equal to each other — are the most important geometrical parameters which are beneficial to reach a maximum load level for a specific silo. Finally, the buckling behaviour and the failure load are hardly influenced by radial width and the thickness of the longitudinal stiffeners.

European punching design provisions for double-headed studs

Double-headed studs are used as punching shear reinforcement in reinforced or prestressed slabs and footings. Owing to an improved anchoring behaviour, double-headed studs have a higher load capacity than stirrups. Double-headed studs are quick to install and are therefore more cost-effective than stirrups. Previously in Europe, national technical approvals specified the design of double-headed studs. Since the end of 2012, European technical approvals have been available. All approvals are based on the same design method, which is used in combination with BS EN 1992-1-1. It is necessary to distinguish between punching shear failure in slabs and foundations with and without shear reinforcement. In slabs and foundations with shear reinforcement, a failure can occur at maximum load level, in and outside the shear-reinforced zone. The European technical approvals provide design equations for all types of failure modes. To quantify the safety level of the design provisions for slabs, footings, and ground slabs with and without shear reinforcement, test evaluations based on a reliable data bank are shown. The paper also provides useful background information and additional design recommendations for applications, which are not covered by the design provisions of the European technical approvals.

Bending fatigue behavior of twill fabric E-glass/epoxy composite

Twill E-glass/epoxy composite was considered for its bending fatigue behavior. Displacement controlled bending fatigue tests with stress ratio R of 0.1 were conducted on standard specimens and damage development in the composite was continuously monitored through the decrease of bending moment during cycling. The specimens were subjected to different fatigue loadings with the maximum loading level up to 75% of the material ultimate flexural strength. Early damage was observed after hundreds of loading cycles causing degradation of material stiffness with cycling. The amount of stiffness reduction was observed to be a function of the magnitude of the fatigue loading applied to the specimen. For some selected specimens, after 1 million cycles, fatigue tests were stopped and residual properties were measured. Different levels of reduction on material strength and elastic modulus were found to depend on the level of fatigue loading. Finally detailed discussion is made to correlate the found fatigue data and obtain general description of the material fatigue behavior useful for composite component design.

Static and Fatigue Behavior of Sandwich Panels with GFRP Skins and Governed by Soft-Core Shear Failure

Sandwich panels composed of glass fiber–reinforced polymer (GFRP) skins and low-density polyurethane core are investigated for potential structural applications such as lightweight insulated roofing or cladding systems. Ten 1,220×318×78-mm panels, including three control specimens, were tested under high-cycle fatigue at maximum load levels of 45–70% of the ultimate static load. Fatigue tests were carried out to failure, thereby enabling the development of the complete fatigue life (S-N) curve and stiffness degradation. Digital imaging correlation was used to map shear deformations and showed that shear contributes about 90% of the deflection. Both static and fatigue failure modes were due to core shear failure. The study showed that fatigue threshold was 37% of the ultimate static capacity, and that to sustain at least 2 million cycles, service load should be limited to 45% of ultimate. The maximum design service live and snow loads, as well as wind load based on the most critical conditions in Canada, were all below the fatigue threshold of the panel. The panels had a strength safety factor of 2.1 relative to the factored maximum design live and snow load, and 6.5 relative to the factored maximum wind load. Deflection limits may govern for some service load conditions, but is not critical when only wind load considered. Stiffness degradation reached a maximum of 15–20%, but residual deflections upon unloading were insignificant.

Алгоритм гнучкого масштабування для хмарних систем з обмеженням на час обслуговування

In the Cloud systems that solve the tasks in real-time mode there is imposed the additional requirements on the scheduling algorithms and scalability. The scheduling algorithm A* has been modified to improve the scalability parameter. Using The Load Balancing mechanism allows to route the incoming requests on the available system resources. To increase the flexibility parameter we suggest to set the maximum and minimum load level for the each priority level value, according to which the allocation is performed and the resources have released. The number of virtual machines is depended on the current load of the system resources and thresholds. The algorithm takes into account the installation of the new machine and the tasks transfer to it, that is may be significant in real time. We create the model of real-time systems, and the experiments show the flexibility of the suggested algorithm.

Functionalization of 3D covalent organic frameworks using monofunctional boronic acids

Co-crystallizing a monomer capable of forming a three-dimensional covalent organic framework (3D COF) with a truncated analog represents a robust strategy to functionalize the pores of these crystalline polymer networks. Here we elaborate this approach by demonstrating that monofunctional arylboronic acids serve as effective truncation/functionalization agents for COF-102, a boroxine-linked 3D network derived from the dehydration of a tetrahedral tetrakis(boronic acid) monomer. The COF-102 network forms under typical solvothermal conditions, even in the presence of a large excess of 4-tolylboronic acid, which is incorporated into the polymer's boroxine linkages up to a maximum loading level of ca. 33 mol%. This finding indicates the maximum truncation level for the COF-102 network and suggests that framework crystallization is irreversible. At high feed ratios of the monofunctional boronic acid, the isolated COF-102-tolyl powders are initially contaminated by significant amounts of tris(4-tolyl)boroxine, which is removed through a solution-based activation process to provide COF-102-tolyl samples with high functionalization density, long-range order, and permanent porosity. We also demonstrate the generality of this truncation study by evaluating several other readily available arylboronic acids, each of which are incorporated into the COF similarly. Together these findings demonstrate the simplicity and generality of this truncation/functionalization approach, as well as its fundamental limits.