Extreme Loading Conditions Research Articles

Damage monitoring of carbon fiber reinforced silicon carbide composites under random vibration environment by acoustic emission technology

Carbon fiber-reinforced silicon carbide (C/SiC) composites are widely used in high-temperature thermo-structural applications. They are subjected to extreme loading conditions, such as random vibrations, which are likely to damage the structure. Structural micro-damage identification during vibration is very difficult, owing to the randomness of the environmental vibration and the complicated response it causes in structures. This study aims to determine a method for monitoring the damage properties of a C/SiC structure under a random vibration environment using acoustic emission (AE) technology. First, a pencil break experiment is conducted to verify the feasibility of the AE technology. Then, an AE monitoring experiment of the structural damage in a vibration environment is systematically conducted. Two types of experiments are designed for simulating the damage formation process inside the structure. In addition, the parameter characteristics of typical AE signals in the random vibration test are analyzed, and the relationships between the AE signal parameters and vibration loading are obtained. Lastly, the different stages of material damage development and damage types in each stage are provided to reveal the damage evolution processes of C/SiC composites. The results indicate that AE technology can be effectively applied to investigate the damage behaviors of C/SiC composites in random vibration environments.

Mechanical and tribological behavior of friction stir welded joint on AA 8011 at diverse strengthening condition through post processing

Promoted research work of aluminium alloys 8011 were taken for friction stir welding under optimal parametric conditions and further taken for post processing like heat treatment and heat treatment then short peening was carried. The enrichment of weldment strength was directly depended on secondary processing in order to remove internal stress and increase the surface properties further it will be taken for mechanical test and tribo meter test. The standard test procedure was carried out for hardness, yield and tensile strength. Also wear and friction test was carried under influenced process parameters like load conditions are 10, 20, 30 and 40 N at a settled sliding distance of 1000 m and 2000 m under dry sliding condition at a room temperature and average values were recorded. The obtained better hardness and ultimate tensile strength average value was obtained welded AA8011 followed by heat treatment then short peening samples due to strengthening and ultra-fine grain size. The outcomes exposed for all the test samples on the tribo meter test, wear rate increments with increment in applied load and coefficient of friction expressions decrement at most extreme load conditions. Wear rate increments with increment in sliding distance and coefficient of friction decline with increment in sliding distance. From this investigation, it is found that the heat treated then shot-peened test sample shows better mechanical and tribological properties among other test samples because of enhancement in the surface strength of the post processing after welding.

Thermo-structural fatigue and lifetime analysis of a heat exchanger as a feedwater heater in power plant

Today, the use of shell and tube heat exchangers has become widespread and they are used in various industries under very diverse operating conditions. Specific operating conditions make it possible to consider and simulate the operating terms and failure conditions of these converters. In this study, the design of a shell-and-tube counter-flow heat exchanger in AutoCAD software is first considered, and the system is then meshed and simulated in ANSYS 2019 software. Simulation results of temperature, pressure, heat flux and fluid velocity within the system are reported in order to understand the system performance. Failure conditions are evaluated according to the ASME VIII Boiler and Pressure Vessel Code, and the results of equivalent thermal stress analysis and system lifetime under two extreme loading conditions are reported. The highest equivalent thermal stresses under these extreme load conditions occur at the joints of the tubes and tubes sheet and is equal to 641 and 931 MPa, respectively. Also, the lifetimes of tubes and tube sheets are 105 and 104 cycles respectively for the valley and peak load conditions.

Fire Evaluation of RC Frames Strengthened with FRPs Using Finite Element Method

Reinforced Concrete (RC) structures may confront with extreme loading conditions. Sometimes, structures are not only under extreme loading such as earthquakes but also, they may be subjected to fire. Therefore, investigation of reinforced concrete structures which are the most common ones is essential. In this study, experimental RC frame is considered to validate in ABAQUS finite element software. RC frame is subjected to both earthquake and fire loading condition to assess the seismic behavior of structure under extreme conditions. FRP techniques is also consider evaluating the seismic behavior such as load capacity, ductility, energy absorption and stiffness. In this regard, two different approaches including reinforcing and wrapping are employed. In this research, the pattern of retrofitting and reinforcing are the novelty of this work. In fact, the effect of using steel bars, BFRP bars and sheets are evaluated. The new method for combination of steel-BFRP bars and different BFRP sheet’s angle as divergence and convergence are investigated. After carrying out the load-displacement diagrams, the seismic parameters of RC frames are compared and the optimized method and model is presented.

A scuffing criterion of steels based on the friction-induced adiabatic shear instability

The modified scuffing equations based on the adiabatic shear instability induced by friction effects are postulated. With the present equations, an alternative softening mechanism of the near-surface material undergoing adiabatic shear failure is proposed. This mechanism is dominated by the frictional work, rather than the plastic-deformation work. Based on the mechanism, a quantitative criterion is developed for scuffing prediction, where scuffing failure results from the competition between the thermal softening and work hardening during adiabatic shear. Accordingly, the effect of materials properties associated with scuffing can be thoroughly understood, and quantitative evaluation of the material performance and efficient exploration of new materials subjected to extreme contact loading conditions are possible.

Reliability Analysis of k-out-of-n Systems of Components with Potentially Brittle Behavior by Universal Generating Function and Linear Programming

Due to initial cracks, careless construction, and extreme load conditions, components with brittle behavior may exist in a structural system. The presence of brittle behavior of components usually is accompanied by a low strength. However, existing methods for calculating the reliability of structures of components with brittle behavior are rather complicated or impossible. By means of decomposing the entire system into a set of subsystems, this paper proposed a method to estimate the bounds on failure probability of k-out-of-n system of components with potentially brittle behavior by using universal generating function (UGF) and linear programming (LP). Based on the individual component state probabilities and joint probabilities of the states of a small number of components, the proposed method can provide the bounds for the failure probability of a system with a large number of components. The accuracy and efficiency of the proposed method are investigated using numerical examples.

Improving the damage potential of W-Zr reactive structure material under extreme loading condition

Projectiles made of reactive structure materials (RSM) can damage the target with not only kinetic but also chemical energy, but the enhanced damage potential of RSM may become compromised if extreme loading condition disintegrates the projectile before the target is reached. In this work, a ductile coating of Ni was introduced to a tungsten-zirconium (W-Zr) alloy, a typical brittle RSM, to preserve the damage potential of the projectile. Detonation driving tests were carried out with X-ray photography and gunpowder deflagration driving tests were carried out with high-speed photography for the coated and uncoated RSM samples, respectively. The craters on the witness target were analyzed by scanning electron microscopy and X-ray diffraction. The Ni coating was found to effectively preserve the damage potential of the W-Zr alloy under extreme loading conditions, whereas the uncoated sample fractured and ignited before impacting the target in both detonation and deflagration driving. The crack propagation between the reactively brittle core and the ductile coating was analyzed based on the crack arrest theory to mechanistically demonstrate how the coating improves the structural integrity and preserves the damage potential of the projectile. Specifically, the Ni coating envelops the W-Zr core until the coated sphere penetrates the target, and the coating is then eroded and worn to release the reactive core for the projectile to damage the target more intensively.

Mechanism of mooring line breakage of floating offshore wind turbine under extreme coherent gust with direction change condition

With an extreme load condition, the mooring system of a floating offshore wind turbine (FOWT) will be led to failure, such as mooring line breakage. However, the induced FOWT mooring line breakage in extreme gust still requires further study for design optimization in the future. In this paper, an aero-hydro-cable-servo time domain coupled simulation have been carried out of a NREL’s 5 MW OC4-DeepCwind semi-submersible type FOWT for investigate the mooring system response under extreme coherent gust with direction change (ECD) condition. The platform is assumed to be installed at 50 m depth location in the South China Sea. The practical ECD is simulated by a fast conversion between two wind conditions with different mean wind speeds and wind direction. In addition, the gust characteristics that can generate snap tension of mooring lines were identified, and the consequence of the induced ECD accident is investigated. ECD condition with rise time of 10 s is prone to cause a snap tension of the mooring line, and it may eventually lead to cascading mooring line breakage and potential catastrophic collision events.

Fracture studies on carbon steel straight pipes having off-centred circumferential through-wall crack under finite compliance

The level-3 Leak-Before-Break (LBB) assessment of nuclear piping system is based on simplified fracture mechanics calculations. A postulated through-wall circumferential crack is shown to be stable even under extreme loading conditions. An important aspect of this fracture assessment, which is generally not considered, is the reduction in load at the cracked section that occurs due to the local and global compliance of the piping system. In this study, the effect of piping system compliance, crack size and crack location on the load carrying capacity of SA-333 Gr. 6 carbon steel pipes was analysed. Experimental investigations were performed on 8״ NB Schedule 100 pipes (Outer diameter 219 mm and wall thickness of approx. 12.6 mm) under quasi-static four point bending loads. Out of the six fracture tests, one specimen had a through-wall circumferential crack at the centre (along the length of pipe) and the remaining five tests had off-centred cracks. While the initial crack size showed a weak influence on the load carrying capacity of a stiff piping system, the effect of crack location was, however, quite significant. Calculations based on twice-elastic slope (TES) method and limit analysis of uncracked piping system (in some cases) for evaluation of load carrying capacity of pipes under finite compliance may lead to large safety margins.

Life-cycle cost (LCC) based design procedure to determine the optimal design parameters and target reliability under nonlinear deformation for marine installations

ABSTRACT This study proposes a procedure to determine the optimal design parameters and target reliability based on Life cycle cost (LCC) based analysis. An iterative design procedure is performed for different levels of reserve strength ratio (RSR), and LCC is calculated. A case study is carried out for installation of the bottom fixed jacket support structure for offshore wind turbines (OWT) system subjected to extreme loading condition. To determine the ultimate capacity of the structure, static pushover analysis is integrated with the proposed design methodology. An evolutionary based particle swarm algorithm is selected for the structural design optimization of offshore jackets. The RSR evaluated from pushover analysis assists to determine the probability of failure based on the first order reliability method (FORM). Based on a set of LCC values, which are the collective sum of capital expenditure (CAPEX), operational cost (OPEX), and risk cost (RISKEX), the optimal design is reached.

Finite Element Evaluation of RC Frames Strengthened with FRPs under Fire and Lateral Loads

Reinforced concrete is one of the most common structures in the world. These structures may be under extreme loading conditions such as earthquake or they may confront with other flooding, impact or even the fire load. In the term of fire load, RC structures are very sensitive and due to the brittle behavior, the fast collapsing happens. In this research, two experimental setups are validated: (1) RC frame and (2) RC column. The Reinforced Concrete (RC) frame is modeled using finite element ABAQUS software and validated with experimental program. The RC frame validation is subjected to lateral and vertical loads. On the other side, RC column is also modeled and validated using ABAQUS software. The RC column is subjected to fire load. The results of validations show that the simulation techniques are accurate to perform the further investigation. The RC frames are retrofitted by FRP sheets and reinforced using steel or FRP bars. The angle of FRP sheets is both convergent and divergent. The fire is applied at the inside surface of the frame by 200, 400, 600, 800 and 1000°C and lateral displacement-control load is considered, concurrently. Force-displacement diagrams have been evaluated to check the load capacity, ductility, stiffness, seismic factor and crack propagations. The results show that using FRP has better performance at the lower fire temperature.

Design, qualification & manufacture of ITER gravity supports

As one of the key components to support all the magnet coils, the GS faces engineering challenge to its operational safety throughout the design, qualification and manufacturing process as a result of extreme loading condition. The structural safety of GS was confirmed by both the FEM analysis and the semi-prototype engineering test after a long time of design, qualification, manufacture and assembly. Welding the cooling pipe to the flexible plate without obvious deformation as well as tightening uniformly and precisely all the tie rods to clamp the plates were carried out successfully during manufacturing. The result of final vacuum leakage test indicates that the GS can not only meet the ITER vacuum requirement but also have no slow out-gassing. The first set of GS which has passed the ITER acceptance test is to be delivered to ITER construction site soon.

Low Cycle Fatigue Behavior of Elbows with Local Wall Thinning

There have been a number of studies concerning the integrity of high-strength carbon steel pipe elbows weakened by local pipe wall thinning, the latter can be typically caused by flow accelerated erosion/corrosion. In particular, the focus of several recent studies was on low cycle fatigue behavior of damaged elbows, mainly, in relation to strength and integrity of piping systems of nuclear power plants subjected to extreme loading conditions, such as earthquake or shutdown. The current paper largely adopts the existing methodology, which was previously developed, and extends it to copper-nickel elbows, which are widely utilized in civil infrastructure in seismically active regions. FE (finite element) studies along with a full-scale testing program were conducted and the outcomes are summarized in this article. The overall conclusion is that the tested elbows with various severity of local wall thinning, which were artificially introduced at different locations, demonstrate a strong resistance against low cycle fatigue loading. In addition, elbows with wall thinning defects possess a significant safety margin against seismic loading. These research outcomes will contribute to the development of strength evaluation procedures and will help to develop more effective maintenance procedures for piping equipment utilized in civil infrastructure.

Optimization of a cantilever steel frame structure of bottom platform by finite element method

The finite element analysis of a cantilever steel frame structure of the bottom platform in civil engineering is carried out. Based on ANSYS and the basic characteristics of steel frame structure, parametric modelling is carried out, and the stress and deformation of the structure under extreme load conditions are analysed. The first-order optimization algorithm is used to optimize the beam cross-section size, which significantly reduces the total weight of the structure, thus providing a basis for the optimal design of the engineering structure.

Investigation of Sandwich Panel Under High Temperature Loading Using Finite Element Analysis

Structural and nonstructural members are always encountered with extreme loading conditions. In case of nonstructural members, the failure happens much sooner due to lack of load capacity. The nonstructural elements are not designed for structural loading conditions. However, they might be impacted to other loading types such as fire. In recent years, the use of 3D panel is one of the most common partitions in the world. In this research, the effect of fire on 3D panel partitions is presented. To achieve this goal, ABAQUS finite element software is employed to simulate the 3D panel. The 3D panel is embedded inside steel frame. In order to have an accurate result, two validations are performed including steel frame and the steel frame with 3D panel. In the study, 5 different fire loadings 200, 400, 600, 800 and 1000°C are applied into the model. Results include load-displacement diagram and stiffness and ductility factors. The results indicated that as fire load increases, the stiffness and ductility factor are reduced.

Simulation of Erosion and Redeposition of Plasma Facing Materials Under Transient Plasma Instabilities

Deposition of plasma energy during off-normal fusion reactor operational events delivers a transient heat flux of up to 100 MJ/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to the plasma-facing materials (PFMs). Understanding the exact material response to the extreme energy loading conditions plays a key role in establishing a realistic computational tool that simulates the fusion plasma-material interaction. Surface damage can occur due to vaporization, melting, spallation, and liquid splatter. However, splashing mechanisms such as boiling and splattering, which result from various liquid instabilities, appear to be the main mechanism contributing to the melt layer erosion. The primary focus of this article is melting and resolidification and the effect of redeposition of the eroded material on surface erosion. A set of selected PFMs was exposed to a plasma heat flux of up to 40 GW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over a deposition duration of 200 μs. The source of the high energy plasma used in this article is the Surface InteRaction Experiment at North Carolina State (SIRENS) plasma source, which used to simulate disrupted plasma conditions. The underlying erosion mechanisms involved in the formation, ejection, and solidification of molten droplets are investigated using the basic plasma equations and a plasma fluid model implemented in the simulation code. The net erosion and redeposition thickness due to erosion of the vapor and melt layer have been evaluated post-plasma exposure and compared to the experimental measurements.

Flexible riser-bend stiffener top connection analytical model with I-tube

The flexible riser top connection to the floating unit is a critical region considering extreme loading and fatigue lifetime assessment and is generally protected by a bend stiffener to limit the curvature in this region. The top connection usually interface the floating unit with two main configurations: i) end-fitting and bend stiffener directly connected to a riser balcony or ii) riser connected to the floating unit in the end of an I-tube, which reduces the end-fitting bending loading, and bend stiffener assembled to a bellmouth with a given inclination in relation to the I-tube longitudinal axis. The traditional modeling approach considers the riser/bend stiffener system attached to the floating unit, representative of the first configuration. A more realistic modeling approach, capturing the complex interactions of flexible riser/bend stiffener with I-tube interface can be employed for preliminary assessment with less conservatism. In this work, a large deflection analytical beam model is developed for the riser top connection with I-tube considering the bellmouth transition region with a straight rigid surface followed by a curved section. The riser follows a nonlinear bending behavior described by a bilinear moment vs curvature function and the bend stiffener polyurethane material exhibits nonlinear elastic symmetric response represented by a power law function. It is assumed that there is no gap between the riser and the bend stiffener and the riser is fixed in the end-fitting position. The mathematical formulation of the statically indeterminate system results in three systems of coupled differential equations combined with the corresponding multipoint boundary conditions to be numerically solved by an iterative procedure. A case study is carried out with a 7” flexible riser protected by a bend stiffener connected to an inclined I-tube bellmouth. The system is subjected to extreme loading conditions and the influence of the sleeve shape and I-tube length on the riser curvature distribution, including the end-fitting position, and contact forces between the riser/sleeve and riser/bend stiffener sections are assessed.

Weak phases production and heat generation control fault friction during seismic slip

The triggering and magnitude of earthquakes is determined by the friction evolution along faults. Experimental results have revealed a drastic decrease of the friction coefficient for velocities close to the maximum seismic one, independently of the material studied. Due to the extreme loading conditions during seismic slip, many competing physical phenomena occur (like mineral decomposition, nanoparticle lubrication, melting among others) that are typically thermal in origin and are changing the nature of the material. Here we show that a large set of experimental data for different rocks can be described by such thermally-activated mechanisms, combined with the production of weak phases. By taking into account the energy balance of all processes during fault movement, we present a framework that reconciles the data, and is capable of explaining the frictional behavior of faults, across the full range of slip velocities (10−9 to 10 m/s).

Low and ultra-low-cycle fatigue behavior of X52 piping steel based on theory of critical distances

The cyclic failure observed in structural components such as pipelines subjected to extreme loading conditions highlights some limitations concerning the application of existing fatigue damage models. The evaluation and prediction of this type of failure in these steel components under large-scale plastic yielding associated with high levels of stress triaxiality are not sufficiently known nor explored. This fatigue domain is conventionally called ultra-low-cycle fatigue (ULCF) and damage features are representative of both low-cycle fatigue (LCF) and monotonic ductile fracture. Thus, in order to understand the ULCF damage mechanisms both monotonic and LCF tests are required to get representative bounding damage information to model the material damage behaviour under such extreme loading conditions. This paper aims at exploring the Theory of Critical Distances (TCD) in the LCF and ULCF fatigue regimes, including the application of the point, line and area methods. The application of the TCD theories has not been explored so far in the ULCF fatigue regimes, despite its promising results in the LCF and high-cycle fatigue. An experimental program was carried out on several specimens’ geometries made of X52 piping steel. In detail, smooth plane specimens and notched plane specimens were cyclic loaded under tension/compression loading in order to obtain fatigue lives within the range of 101–104 cycles. In addition, cyclic bending tests on notched plane specimens were also incorporated in this study. Finite element simulations of all small-scale tests were conducted allowing to derive elastoplastic stress/strain fields along the potential crack paths. The numerical data were subjected to a post-processing in order to find characteristic lengths that can be treated as a fatigue property according to the TCD. A unified strain-life relation is proposed for the X52 piping steel together with a characteristic material length, consisting of a practical relation for pipeline strain-based design under extreme cyclic loading conditions.

Assessment and accounting for extreme impacts for validation of strength and service life of highly loaded machines and structures

The paper presents four groups of problems that are critical for assessment and validation of strength and service life of technical systems operating under extreme loading conditions: (1) developing constitutive equations valid in wide ranges of strains, strain rates and temperatures; (2) analysis of the kinetics of local states of stresses and strains in notch zones for elastic, limited elastoplastic and severe plastic strains; (3) justified selection of the criterion of ultimate damage and fracture; (4) assessment of the local stresses and strains at the crack tip according to the formulation of the nonlinear fracture mechanics. The results of studies of the problems of deformation, damage accumulation and fracture under extreme coupled thermomeachanical loading conditions that are relevant for highly loaded machines and structures employed in nuclear and thermonuclear power engineering as well as in aircraft, rocket and space engineering industries are described