Load-controlled Conditions Research Articles

Crack growth behaviour of P91 steel under trapezoidal loading at high temperature

The increased incentives to utilize renewable energy sources so as to supplement the energy requirements of the present civilization pose a challenge to the power plants running on fossil fuels, as they are required to operate in a ‘flexible’ manner instead of operating at a constant load as intended during their design and inception. This flexible mode of operation at elevated temperature causes a combined creep and fatigue type of loads to be applied on the components. This allows for a comprehensive and effective understanding of synergistic creep-fatigue crack growth behaviour to assess long term failure of components and accessories in plant. The use of P91 steel has been widely prevalent in power plant components owing to its high creep resistance, superior mechanical properties and excellent thermal conductivity, cost-effectiveness, and minimal coefficient of thermal expansion when compared to austenitic steels and Ni-based alloys. The American Society for Testing and Materials (ASTM) has proposed a standard for experimentally evaluating creep-fatigue crack growth, E 2760-19, wherein compact specimens, C(T) are tested under load-controlled conditions. The creep-fatigue crack growth regime and mode of fracture undergone by P91 steel at 600 °C were studied using C(T) specimens at two different force ratio 0.1 and 0.5 and with two different hold time 10 and 600 sec. The Creep fatigue crack growth behaviour has been characterized using stress intensity factor range parameter ∆K and (Ct)avg parameter to study the effect of hold time and force ratio.

Anisotropic plasticity mechanisms in a newly synthesised high entropy alloy investigated using atomic simulations and nanoindentation experiments

This work used atomic simulations and nanoindentation experiments to investigate hardness, modulus alongside sub-surface crystal defects and dislocation mediated plasticity mechanisms leading to anisotropic pile up and local entropy variation in high entropy alloys. The experimental campaign began from Thermo-Calc phase prediction of Ni25Cu18.75Fe25Co25Al6.25 HEA which followed experimental synthesis of the material using arc melting method and experimental nanoindentation using a Berkovich indenter under load-controlled conditions. Through MD simulations, the value of hf/hmax in monocrystalline HEA was consistently found to be larger than 0.7 which suggested pile-up behaviour to dominate and sink-in behaviour to be unlikely. In the case of (110) and polycrystalline HEA substrates, the elastic work in the indentation hysteresis loop was seen to be larger than the (100) and the (111) orientations which explains that the (110) orientation substrate showed least elastic modulus and hardness while the (111) monocrystalline HEA showed the highest elastic modulus and hardness. From the simulations, a “lasso” type loop on the (110) orientation and cross-over of shear loops on the other orientations accompanied by dislocations of type 1/6 < 112 > (Shockley), 1/2 < 110 > (perfect), 1/3 < 001 > (Hirth), 1/6 < 110 > (Stair rod) and 1/3 < 111 > (Frank partials) were seen to manifest an early avalanche of competing plasticity events. The defects accompanying these dislocations in the sub-surface were identified to be FCC intrinsic stacking faults (ISF), adjacent intrinsic stacking faults (quad faults), coherent ∑3 twin boundary and a coherent twin boundary next to an intrinsic stacking fault (triple fault). The EBSD analysis applied to the MD data showed that the (210) orientation and the< 110 > family of directions were seemed to be preferable to plastically deform the FCC phased Ni25Cu18.75Fe25Co25Al6.25 HEA.

Experimental and numerical investigation of ratcheting behavior in seamless carbon steel 90° elbow pipe with small dimensions under constant internal pressure and in-plane cyclic bending

The purpose of this study is to identify the effects of mean fatigue load, amplitude fatigue load, internal pressure and loading history on ratcheting behavior of seamless A234 WPB steel elbow pipe. So, in this paper the ratcheting behavior of seamless 90° elbow pipe of A234 WPB steel under constant internal pressure and in-plane cyclic bending is investigated experimentally and numerically. At first, in order to the confirm of elbow pipe material, a series of experimental tests were performed on elbow specimen, including: chemical composition analysis, metallography testing, hardness testing and uniaxial tensile testing. In the next step, the fatigue tests were performed on elbow pipe specimens at ambient temperature (25 °C) under asymmetric cyclic loading with load-control conditions. According to the experimental fatigue data, concluded that the increasing of mean load, amplitude load and internal pressure, increases the accumulation of ratcheting strain in axial direction at intrados and extrados locations, and also in hoop direction at crown location. So that the increasing of mean load or amplitude load has the greatest effect on the ratcheting strain at intrados and extrados locations. Also, it was found that the loading history affected to ratcheting behavior and premature failure of pressurized elbow pipes. Finally, the finite element simulation was carried out by ANSYS software based on the nonlinear kinematic/isotropic hardening model, and the FE simulation results were compared with experimental fatigue data and it was found that the numerical results are in good agreement with experimental fatigue data.

Load-control vs. displacement-control strategy in fatigue threshold analysis of adhesives: Effects of temperature

For the design of adhesive joints for service in fatigue, parameters such as the rate of fatigue crack growth and the fatigue threshold energy release rate are of extreme importance. These parameters can be obtained through fatigue fracture tests using different strategies, namely Constant load control and constant displacement control. Despite the extensive studies conducted on the fatigue performance of bonded joints, the role of testing methodology on the fatigue fracture behaviour of adhesives is less considered, especially in conjunction with the effect of temperature in these tests. Accordingly, in this study, a structural polyurethane adhesive suitable for automotive applications is characterized in terms of its fatigue behaviour in pure mode I using DCB (double cantilever beam) specimens. Fatigue tests were performed at different temperatures under both constant load control and constant displacement control conditions. A compliance-based method was used to measure the crack size throughout tests. The threshold energy release rate was calculated for different test temperatures and the two testing strategies were compared. It was seen that a constant displacement control methodology is superior to a constant load control methodology when testing for the threshold energy release rate, due to its shorter duration and low sensitivity to initial testing conditions. Differences observed between fatigue testing methodologies were exacerbated by the effect of temperature on the adhesive.

Effect of substrate surface finish and particle velocity on fatigue performance of cold spray coated A6061 aluminum alloy

Cold spray coatings made with powder of the same material as the substrate can be used to improve fatigue performance and/or to repair damaged parts. The effects of substrate surface finish and particle impact velocity on the bond strength and fatigue performance of Al-6061-T6 substrates coated with Al-6061 powder by using high pressure cold spray (CS) were investigated. To evaluate the effect of particle velocity, coatings were prepared by using both nitrogen (N2) and helium (He) as carrier gases. A new method for substrate surface preparation was introduced by low angle spraying (LAS) with the same powder prior to coating. The bond bar specimens in polished, semi-polished, grit-blasted (GB) and low angle sprayed (LAS) conditions were coated and tested for bonding strength. The microstructural characteristics and microhardness profiles of the coated samples were also evaluated. Fatigue tests on uncoated and coated samples were conducted under fully-reversed axial loading in load-controlled condition. The failure mechanism was identified by evaluating the fracture surfaces using scanning electron microscopy. The results showed that both bonding strength and fatigue performance of the coated specimens prepared by using He-gas were significantly higher than those prepared by using N2. Among the N2-sprayed coatings, the bonding strength of on LAS substrates were significantly higher than that of the other surface conditions, and the bonding strength of coatings on GB substrates was superior to that of polished and semi-polished samples. However, the fatigue performance of LAS-coated samples was inferior compared to uncoated samples due to cracks initiating in the coating and propagating into the substrate. The fatigue behavior of GB-coated specimens was similar to the uncoated samples due to delamination of the coating which resulted in stopping the cracks that initiated in the coating.

On the evaluation of overload effects on the fatigue strength of metallic materials

In this paper, experimental and numerical studies are performed to quantify the influence of overloads on the fatigue behaviour of the quenched and tempered steel 42CrMoS4 and the wrought aluminium alloy EN AW-6082 T6. The main objective is to incorporate the influence of overloads in the fatigue assessment. Three series of fatigue tests per material were performed, each under load control conditions, at R ═—1:i) constant amplitude loading (CAL) tests with no overloads; ii) CAL tests after five overload cycles of the magnitude OL 1; iii) CAL tests after five overload cycles of the magnitude OL 2. The respective overload levels are selected to achieve 75 % of the static strength of the test samples either estimated according to the FKM Guideline (2020) for OL 1 or determined directly by specimen testing for OL 2. During the overload cycles, the local notch strains were monitored by means of the digital image correlation (DIC) technique. X-ray diffraction was applied to quantify the residual stresses in the specimens due to manufacturing, after applying overload cycles, as well as after completing tests on runouts. Fatigue test results demonstrate that, depending on the material and the overload level, the fatigue strength can either decrease or increase or, in some cases, be only slightly affected by overloads. To explain the experimental findings, residual stresses determined from both experimental measurements and elastic-plastic finite-element analyses are included in the calculations of the notch stresses and strains and a fatigue damage parameter. The latter is then involved in the damage calculations to quantify the effect of overloads in terms of the magnitude and number of cycles to failure.

Fatigue Properties of Carbon Steel S45C and Die-cast Aluminum Alloy

The butt dissimilar joints of die-cast aluminum alloy and carbon steel by means of friction stir welding were prepared for investigating fatigue properties of the joints. The FSW tool used in this study has cemented carbide thread probe and a shoulder with scroll groove made of alloy tool steel. Fatigue tests were carried out under a load-controlled condition with a stress ratio R=-1 and R=0.1 in air at room temperature. From the experimental results, it was found that fatigue strength of the joint at 107 cycles were 117 MPa under R=-1 and 72 MPa under R=0.1, respectively. The fatigue fracture was initiated from the location approximately 6-7 mm distant from the welding interface. This site was thought to be the border between the stir zone and base metal. In the base metal region, a lot of silicon particles were distributed as acicular shape. On the other hand, in the stir zone, silicon particles were distributed as fine grain shape.

Quantification of energy and cost reduction from decreasing dissolved oxygen levels in full-scale water resource recovery facilities.

Aeration systems often lack the efficiency to maintain a desired residual dissolved oxygen (DO) concentration in the tank in part because little consideration is given to the dynamic daily and seasonal loading conditions. Although advanced aeration controllers exist, the majority of plants have DO set points typically based on common practice and literature values rather than site-specific conditions, which can result in DO set points higher than those necessary to meet treatment objectives. DO set point reduction strategies have primarily been proposed through either static or dynamic simulations. In this study, the substantial improvements associated with DO set point reduction are demonstrated at full scale. A yearlong characterization of full-scale aeration dynamics captured the effect of diurnal and seasonal fluctuations on oxygen transfer and energy demand and so facilitated the estimation of the potential savings of DO reduction strategies. Full-scale validation provided direct evidence of DO reduction strategies inducing an overall enhancement of oxygen transfer efficiency along the different bioreactors, while confirming that energy savings as high as 20% were feasible. This study quantifies the influence of oxygen transfer efficiency on operating choices and site-specific conditions (control strategy, loading conditions, and influent flow variability). PRACTITIONER POINTS: We quantified the energy reduction and cost savings associated with a DO reduction in an aeration tank. For each 0.2mg/L of DO decreased, the average power demand reduction per unit water treated exceeded 17%. Field measurements of dynamic alpha values eliminate the uncertainty in estimating aeration energy and cost savings from DO variations.

Systematically Increased External Loads Secured Inferior to Younger and Older Adults’ Center of Mass Improves Postural Control without Compromising Functional Motor Performance

Few studies have investigated the effect of external loads secured inferior to the center of mass (COM) on postural control and motor performance in younger and older adults. In the present study, we investigated the effect of systematically increased external loads secured inferior to the COM on young (N = 15, age [years]: M = 26.67, SD= 3.45) and older adults’ (N = 15, age [years]: M = 67.4, SD= 7.69) center of pressure displacement and velocity, and also dart throwing performance in four different load conditions: 1) no load (control condition), 2) loading with 5% of body mass, 3) loading with 10% of body mass, and 4) loading with 20% of body mass. Overall, older adults had higher COP displacement and velocity in both anterior/posterior (AP) and Medio-lateral (ML) directions and poorer dart throwing scores than younger adults. Despite no significant difference in dart throwing performance for the different load conditions within each age group, loading with 10% and 20% of body mass reduced AP COP displacement relative to control for both study groups, with 20% body mass also reducing AP COP velocity relative to control for both study groups. In conclusion the present findings reveal that external loads secured inferior to older and younger adults’ COM may enhance postural control without compromising motor performance. The findings were discussed in the context of fall-prevention and athletic performance.

The hypoferremic response to acute inflammation is maintained in thalassemia mice even under parenteral iron loading

BackgroundHepcidin controls iron homeostasis by inducing the degradation of the iron efflux protein, ferroportin (FPN1), and subsequently reducing serum iron levels. Hepcidin expression is influenced by multiple factors, including iron stores, ineffective erythropoiesis, and inflammation. However, the interactions between these factors under thalassemic condition remain unclear. This study aimed to determine the hypoferremic and transcriptional responses of iron homeostasis to acute inflammatory induction by lipopolysaccharide (LPS) in thalassemic (Hbbth3/+) mice with/without parenteral iron loading with iron dextran.MethodsWild type and Hbbth3/+ mice were intramuscularly injected with 5 mg of iron dextran once daily for two consecutive days. After a 2-week equilibration, acute inflammation was induced by an intraperitoneal injection of a single dose of 1 µg/g body weight of LPS. Control groups for both iron loading and acute inflammation received equal volume(s) of saline solution. Blood and tissue samples were collected at 6 hours after LPS (or saline) injection. Iron parameters and mRNA expression of hepcidin as well as genes involved in iron transport and metabolism in wild type and Hbbth3/+ mice were analyzed and compared by Kruskal–Wallis test with pairwise Mann–Whitney U test.ResultsWe found the inductive effects of LPS on liver IL-6 mRNA expression to be more pronounced under parenteral iron loading. Upon LPS administration, splenic erythroferrone (ERFE) mRNA levels were reduced only in iron-treated mice, whereas, liver bone morphogenetic protein 6 (BMP6) mRNA levels were decreased under both control and parenteral iron loading conditions. Despite the altered expression of the aforementioned hepcidin regulators, the stimulatory effect of LPS on hepcidin mRNA expression was blunt in iron-treated Hbbth3/+ mice. Contrary to the blunted hepcidin response, LPS treatment suppressed FPN1 mRNA expression in the liver, spleen, and duodenum, as well as reduced serum iron levels of Hbbth3/+ mice with parenteral iron loading.ConclusionOur study suggests that a hypoferremic response to LPS-induced acute inflammation is maintained in thalassemic mice with parenteral iron loading in a hepcidin-independent manner.

Fatigue response of friction stir welded joints of Al 6061 in the absence and presence of inserted copper foils in the butt weld

The present study examines the impact of inserted copper foil on n mechanical characteristics and fatigue strength of aluminum 6061 butt joints welded through the Friction Stir Welding (FSW) process. Thin copper foils were inserted between the faying surfaces of the sheet joints alloying Al 6061 locally with copper foil over FSW operation. Test specimens were then cut off from the welded plates to examine the quasi-static and fatigue behavior of the joints. The load-controlled condition tests were conducted on the specimens with a stress ratio of R = 0.1. Insertion of copper foils of 100 μm and 200 μm in the stirred zones, improved tensile strengths respectively 48 % and 31 %. The fatigue life of alloyed joints was improved as compared to as-welded joints in the absence of foils. A three-dimensional finite element (FE) analysis was employed to simulate the maximum stress and strain values in the weld zones. Weld regions due to their different mechanical properties experienced different stress magnitudes. The simulated stress and strain components were then used to predict uniaxial fatigue lives of FSW joint samples through the Smith-Watson-Topper (SWT) approach.

Single layer graphene controlled surface and bulk indentation plasticity in copper

The impact of graphene reinforcement on the mechanical properties of metals has been a subject of intense investigation over the last decade in surface applications to mitigate the impact of tribological loadings or for strengthening purposes when dispersed into a bulk material. Here, the effect on the plastic indentation response of a single graphene layer grown on copper is analyzed for two configurations: one with graphene at the surface, the other with graphene sandwiched under a 100 nm thick copper cap layer. Nanoindentation under both displacement and load control conditions show both earlier and shorter pop-in excursions compared to systems without graphene. Atomic force microscopy reveals much smoother pile-ups with no slip traces in the presence of a surface graphene layer. The configuration with the intercalated graphene layer appears as an ideal elementary system to address bulk hardening mechanisms by indentation testing. Transmission electron microscopy (TEM) cross-sections below indents show more diffuse and homogeneous dislocation activity in the presence of graphene. 3D dislocation dynamics simulations allow unraveling of the origin of these 3D complex phenomena and prove that the collective dislocation mechanisms are dominantly controlled by the strong back stress caused by the graphene barrier. These results provide a quantitative understanding of the impact of graphene on dislocation mechanisms for both surface and bulk applications, but with an impact that is not as large as anticipated from other studies or general literature claims.

Numerical Analysis of Pipeline Uplift Resistance in Frozen Clay Soil Considering Hybrid Tensile-Shear Yield Behaviors

Pipeline uplift resistance plays an essential role in the design of buried pipelines in permafrost regions or artificially frozen ground. The uplift resistance of a frozen soil is dependent on the soil’s mechanical properties, which are needed to characterize plastic zones in the frozen soil that surrounds pipes. Owing to the presence of unfrozen water, frozen soils (especially frozen clay) display complex mechanical properties that vary with temperature. There are limited amount of research studies with a focus on frozen soil–pipe interactions that consider temperature change within the literature. In this study, numerical modeling is conducted to investigate frozen soil–pipe interactions at different temperatures. A pipe is simulated with vertical displacement-controlled or vertical load-controlled conditions. The temperature-dependent mechanical parameters of frozen clay soil are examined in the simulation. Both tensile yield and shear yield behaviors are included in the modeling with the consideration of temperature-dependent failure criterions. Simulated stress paths in monitoring points are collected and displayed. The results from the Mohr–Coulomb model with the Rankine tensile cut off, and the hyperbolic Drucker–Prager model are compared and analyzed. The results indicate that hyperbolic Drucker–Prager model is effective for analyzing soil plastic zones surrounding a pipe. The applied hyperbolic Drucker–Prager model with reduced strength parameters can produce a more conservative uplift resistance–displacement relation when compared with that from the Mohr–Coulomb model. Thermally induced plastic strain may have a significant impact on the uplift resistance analysis, since a significant thermally induced uplift displacement is expected if there is a heat transfer process from a buried pipe to the surrounding frozen soil.

Ratcheting response of spherical vessels under load-controlled condition based on the Chaboche viscoplastic model

Ratcheting response of spherical vessels under load-controlled condition based on the Chaboche viscoplastic model

Failure mode transitions of unconfined granular media from dry to unsaturated to “quasi-fully” saturated states

Recent research has shown discrepancies between the prevailing mathematical representations of near-surface shear strength and the observed shear strengths. This investigation focuses on three granular materials, i.e., 1) poorly-graded, medium-fine silica-quartz sand, 2) an engineered silica-quartz mix of 3.38-mm and 0.638-mm sub-angular particles, and 3) an angular fused quartz sand. Specimens were tested under load-controlled conditions at variable saturations in order to identify and quantify the influence of suction on the granular structures and failure modes. All three materials exhibited localized radial particle force chain buckling failures in unconfined drained dry (UDκ) conditions and classical shear failures in the unconfined drained unsaturated shear (UDP) conditions. In unconfined drained suction failures (UDS) conditions, the poorly-graded, medium-fine silica-quartz sand exhibited a bulging and sloughing failure without weeping, while the other two materials wept and then held loads before failure. Thus, it is suggested that the pore fluid had a predominate lubrication (strength weakening) effect, and the assumption of structure stiffening (strength increase) from matric suction may not be valid at near-surface conditions for sub-angular silica-quartz materials but is valid for the angular fused quartz.

Quantitative relationship between microstructural factors and fatigue life of Ti-5Al-2Sn-2Zr-4Cr-4 Mo (Ti-17) fabricated using a 1500-ton forging simulator

The microstructures, tensile properties, and fatigue lives of the forged Ti-17 using a 1500-ton forging simulator subjected to different solution treatments and a common aging treatment under both load- and strain-controlled conditions to evaluate high cycle fatigue and low cycle fatigue lives, respectively were examined. Then, the tensile properties, microstructures, and relationships between fatigue lives and the microstructural factors were discussed. The fatigue limit under load-controlled conditions increases with increasing solution treatment temperature up to 1143 K, which is in the (α + β) region. However, it decreases with further increase in the solution treatment temperature to 1203 K in the b region. The fatigue ratio at fatigue limit is increasing with decreasing solution treatment temperature, namely increasing the volume fraction of the primary α phase, and it relates well qualitatively with the volume fraction of the primary α phase when the solution treatment temperature is less than the b transus temperature. The fatigue life under strain-controlled conditions to evaluate the low cycle fatigue life increases with decreasing solution treatment temperature, namely increasing the volume fraction of the primary α phase. The fatigue life under strain-controlled conditions to evaluate the low cycle fatigue life relates well quantitatively with the tensile true strain at breaking of the specimen and the volume fraction of the primary α phase for each total strain range.

Evaluating the fatigue limit of metals having surface compressive residual stress and exhibiting shakedown

AbstractThe objective of the study described in this article is to evaluate the effect of shakedown of surface compressive residual stresses introduced by shot peening on fatigue limit of stainless steel. First, the tension‐compression fatigue tests were conducted on ASTM CA6NM specimens under controlled load and displacement conditions to acquire a fatigue limit diagram under various compressive mean stress. The results showed that shakedown of negative mean stress occurs under controlled displacement. We then carried out in‐plane–bending fatigue tests under controlled load conditions on welded ASTM 309 stainless steel specimens with surface compressive residual stress introduced by ultrasonic shot peening. The results provide a fatigue limit of 415 MPa, which agrees with the value of 404 MPa calculated based on a modified Goodman line considering shakedown. Therefore, it is suggested that the surface layer is restricted by the internal bulk that creates controlled displacement conditions and the shakedown of surface compressive residual stress occurs.

High-cycle fatigue of rotating cantilever round copper tube under load-controlled condition

A cantilever rotating bending testing machine accepting only rod specimen with a fixed clamping diameter of 9 mm is modified for tube specimens using two developed adapters. One end of a ϕ15 mm copper tube is expanded into ϕ18 mm and forms the tube specimen. Two adapters fitting for ϕ15 mm and ϕ18 mm tube ends respectively are designed and fabricated. The ϕ15 mm adapter is connected to a shaft subject to an applied load and the ϕ18 mm adapter is connected to a motor, respectively through two collets. Both tube ends are adhered to the inner wall of the adapters with superglue to prevent slippery under the rotation. Finite element analysis is used to estimate the maximum applied stress in the tube specimen i.e. adjacent to the transition for a known counter weight. Only one fatigue test result i.e. at 2.2 × 106 cycles for applied stress of 205 MPa is successfully obtained from the experiment. The collets failed to clamp the adapters for applied stress higher or lower than 205 MPa due to the present of oxide powders resulting from the wear of the collect as it is a 15-year-old machine.

Seismic performance of end-perforated concrete beams strengthened with polymer sheets

This paper presents the results of eight seismic performance tests on end-perforated reinforced concrete beams strengthened with carbon-fibre-reinforced polymer sheets. The beams had different sizes of perforations and strengthening arrangements and were subjected to hybrid load–displacement control loading conditions. The ultimate load resistance, hysteretic performance, energy dissipation capability, ductility and stiffness degradation of the beams with and without strengthening were investigated. The test results show that the beams strengthened with polymer have better seismic performance, and that beam perforation size has a significant influence on seismic behaviour. An increase of hole diameter considerably decreases the hysteresis performance, energy dissipation capacity and ductility of the beams.

Nonlinear thermomechanical behaviors of thin functionally graded sandwich shells with double curvature

Analytical closed-form solutions for thermos-mechanical stability and explicit expressions for free- and forced- vibration of thin functionally graded sandwich shells with double curvature resting on elastic bases are investigated for the first time in this study. A core layer of ceramic and two cover layers of functionally graded materials constitute the shell structure. Governing equations are derived from the classical shell theory using Hamilton’s principle admitting Volmir assumption and von Karman nonlinear displacement fields. Theoretical solutions are achieved by using the Bubnov-Galerkin procedure in solving differential equations. Parametric studies showing the effects of temperature-dependent features, material constituents, initial geometry imperfections, external thermos-mechanical loadings, elastic bases, and geometry configuration on static and dynamic behaviors of the shells are performed. Thin functionally graded sandwich spherical, cylindrical, and hyperbolic paraboloid shells are studied. Snap-through phenomena under load-control conditions are recognized in thin functionally graded sandwich cylindrical shells. The fourth-order Runge-Kutta method is employed to numerically solve dynamic problems and four analogies are drawn to validate theoretical formulations.