Yield Point Behavior Research Articles

A Comparative Study of the Structural Behavior of Concrete Beams Reinforced with Different Configurations of GFRP and Steel Bars

This study examined experimentally and numerically the performance of five concrete beams reinforced with longitudinal and transverse bars made of Glass Fiber Reinforced Polymer (GFRP) or steel. All beams had the same dimensions of 2700 mm in length, 180 mm in width, and 260 mm in depth. The beams were classified into two groups with different variables and compared with a reference beam reinforced with longitudinal and transverse steel bars. The first group consisted of two beams with longitudinal GFRP bars and no stirrups, varying the main reinforcement ratio. The second group comprised two beams with longitudinal GFRP bars and transverse GFRP or steel stirrups, varying the stirrup type. The results indicated that the beams with GFRP bars improved their flexural strength for different ratios but had limited shear resistance when using GFRP stirrups because increased deflection causes the number and width of cracks to grow, reducing the shear strength. All the tested beams exhibited linear elastic behavior until failure, with GFRP being more brittle than steel due to no yield point or plastic behavior in GFRP. The numerical simulation of the five beams using ABAQUS software showed good agreement with the experimental data obtained in the laboratory.

Mechanical anisotropy of ultra strong-and-ductile lamellar dual-phase steel

Generally, high-strength metallic materials with large ductility are of technological importance in various engineering fields, including aerospace, fuel efficiency, and emission reduction. This work developed a new type of steel with exceptional yield strength (1.5∼2.0 GPa) and high total elongation (11∼26%) through a simple three-step process involving cold-rolling, warm-rolling and annealing treatments. Microstructural analysis revealed that the steel is composed of nano platelets made up of austenite and martensite. These platelets aggregate within elongated prior austenite grains, forming hierarchical lamellar structures. The study thoroughly investigated the anisotropic mechanical properties of the steel, particularly its strength and yield point behavior in rolling and transverse directions. The primary sources of strength were mainly originated from nano lamellae and high density of dislocations, while the anisotropic strengthening and yield point behavior was attributed to the textures and lamellar boundaries. With increasing warm-rolling reduction, the anisotropy level of dislocation strengthening gradually decreases, while the anisotropy level of boundary strengthening significantly increases. The discrepancy in yield point behavior along the rolling and transverse directions is ascribed to the differing number of boundaries per unit area of cross-section perpendicular to the tensile direction and the shear stress required for dislocation slip. This study provides a deep insight into understanding the anisotropy of strengthening and yield point behavior in nanolamellar steel so as to contribute to its future applications in engineering structural components.

Metastability mediated grain size control in PH 17-4 stainless steel fabricated using Laser-Powder Bed Fusion (LPBF)

Varying the chemical composition and cooling rates during additive manufacturing (AM) can enable the formation and, in some cases, the retention of metastable phases affecting the solidification pathways. Altering the solidification pathways directly affects the microstructure and in turn the mechanical properties of the parts. In this study, we show that modifying the solidification pathway through the deliberate retention of metastable austenite in PH 17–4 stainless steel (SS) leads to significant grain refinement (one order of magnitude smaller grains) and improvement in tensile strength (30% higher ultimate tensile strength) of parts printed using Laser-Powder Bed Fusion (LPBF). Nitrogen (N2)-atomized feedstock powder containing higher concentrations of austenite-stabilizing elemental nitrogen was used to print parts with retained austenite. Parts absent of retained austenite printed using argon-atomized feedstock powder were used for comparison of microstructure and tensile properties. The grain refinement has been attributed to the “crowding effect” observed due to the simultaneous growth and coexistence of metastable austenitic phase with the stable ferrite. We also show that employing a three-step heat treatment procedure can eliminate the unwanted yield point behavior associated with the softer austenite while preserving the superior tensile properties in N2-atomized samples.

Combining laminated bimodal heterostructure and dislocation source limitation for superior strength-ductility synergy

Many unremitting pursuits have been paid on designing microstructures to enhance the strength-ductility synergy of metallic materials. In this work, laminated bimodal heterostructure and dislocation source limitation were introduced together into pure Ni with various laminate thicknesses by electrodeposition and subsequent annealing to circumvent the limitations of the strength-ductility trade-off. The specimen, composed of alternative fine-grained and ultrafine-grained laminates with the optimized laminate thickness, had a combination of 601 MPa yield strength which was 4.8 times its Hall-Petch yield strength (124 MPa), and 24.5% reasonable ductility. The combination went beyond the range established by the reported pure Ni. When the fine-grained laminates started dislocation slip first and ultrafine-grained ones remained elastic, incompatible deformation between the two different laminates resulted in a high density of piled-up geometrically necessary dislocations (GNDs) against the interfaces, increasing the yield strength. Deformation incompatibility was also present in each of the individual laminates exhibiting bimodal structure, which further facilitated strengthening and strain hardening. Plentiful dislocation cells and walls were found to form inside the fine-grained laminates during work hardening, which could contribute to the production of GNDs. The carefully constructed heterostructures maximized the role of GNDs. Besides, dislocation source limitation induced further hardening, in the form of the yield-point behaviour, and ductilization which resulted from more room inside grains for the accumulation and interaction of dislocations. The superior strength-ductility synergy here paves a promising paradigm for designing high-performance structural metallic materials.

Grain-size effect on dislocation source-limited hardening and ductilization in bulk pure Ni

Dislocation source-limited hardening and ductilization is an effective strategy to obtain superior strength-ductility synergy in some engineering structural metals. Recent works demonstrated that the synergy could be enhanced by grain-size reduction. However, the mechanism of grain-size dependence is still a mystery. In this work, bulk pure Ni produced by electrodeposition and subsequent annealing, with grain sizes ranging from ∼20 nm to ∼20 µm, were methodically investigated to unravel the mechanism of the grain-size effect on dislocation source-limited hardening and ductilization. The high-density nano-twinning in the as-electrodeposited nanograined specimens exhibited better thermodynamic stability than the peers with random high-angle grain boundaries, leading to fine recrystallized grains with low-density dislocations. The low dislocation density enabled extra hardening beyond grain boundary strengthening via yield-point behavior with grain sizes ranging from ∼110 nm to ∼10 µm and extra ductilization over ∼500 nm. This work demonstrated that the prerequisite for dislocation source-limited hardening was that the dislocation density of the specimen should be lower than the size-dependent critical value of ((1.1 × 107/d) m–2, d is the grain size in unit of the meter) where a transition from forest-dominated hardening to dislocation source-limited hardening could occur. On the other hand, dislocation source-limited ductilization only worked when the grain size was comparable to/larger than the theoretical dislocation mean slip distance. Dislocation source-limited ductilization resulted from more room in grains for accumulation of dislocations and deformation nano-stacking faults enabling the higher work hardening rate. This work offered an altogether new avenue to obtain stronger and more ductile metallic materials through utilizing grain-size dependent dislocation source-limited hardening and ductilization.

Expanded vermiculite and polyvinyl acetate composite as gap filler for wooden objects conservation

• Gap fillers are composites used to fill the gaps caused by deterioration processes in wooden objects. • A gap filler composite of expanded vermiculite and polyvinyl acetate was studied for the conservation of wooden objects. • Vermiculite is an inert micaceous mineral. It can also act as a pH buffer. • The mechanical properties, shrinkage and expansion with varying relative humidity of the gap filler indicated the potential use of expanded vermiculite for the conservation of wooden objects stored indoors. Gap fillers are composites used to fill the gaps caused by deterioration processes in wooden objects. The materials commonly used in gap fillers are acrylic, vinyl, cellulosic, and epoxy resins mixed with fillers such as sawdust, glass microspheres, and paper. In this work, a gap filler composite of expanded vermiculite and polyvinyl acetate was studied for the conservation of wooden objects. Expanded vermiculite is a stable material that meets some criteria for gap filler composites. This study evaluated the properties of the proposed gap filler by comparing it to a model gap filler containing sawdust as a filler. The particle size distribution of the filler materials was measured, and the pH, modulus of elasticity, yield point, and expansion and contraction behavior as a function of the relative humidity of the air were measured. The proposed composite showed comparable properties to those of the sawdust and polyvinyl acetate composite; in some tests, the properties were better than those of the model. These data suggest potential use of expanded vermiculite as a filler in gap filler for the conservation of wooden objects.

Machine Learning Model for Monitoring Rheological Properties of Synthetic Oil-Based Mud.

The drilling fluid rheology is a critical parameter during the oil and gas drilling operation to achieve optimum drilling performance without nonproductive time or extra remedial operation cost. The close monitoring for rheological properties will help the drilling fluid crew to take quick actions to maintain the designed profiles for the drilling fluid rheology, especially when it comes to the flat rheology drilling fluid system, which is a new generation for harsh and specific drilling conditions that require flat profiles for the mud rheology regarding the temperature condition changes. The current study introduces a machine learning application toward predicting the rheology of synthetic oil-based mud (flat rheology type) for the full automation system of monitoring the mud rheological properties. Four models are developed, for the first time, to determine the rheological characteristics of flat rheology synthetic oil-based system using artificial neural networks. The developed models are capable of predicting the plastic and apparent viscosities, yield point, and flow behavior index from only the mud density and Marsh funnel as model inputs. The proposed models were trained and optimized from a real field dataset (369 measurements) with further testing the models using an unseen dataset of 153 data points. The predicted rheological properties achieved a high degree of accuracy versus the actual measurements and showed a coefficient of correlation range from 0.91 to 0.97 with an average absolute percentage error of less than 9.66% during the training and testing phases. Besides, machine learning-based correlations are proposed for estimating the rheological properties on the rig site without running the machine learning system for easy field applications.

Effect of coiling and annealing temperatures on yield point behavior of low-carbon steel

Low-carbon steel is widely used for household appliance and automotive panel steel because of its excellent plasticity. Unfortunately, yield point phenomena easily appear in the low-carbon steel produced by a continuous annealing process and cause degradation to the surface quality during processing. The effect of the coiling temperature (600–750 °C) and annealing temperature (740–820 °C) on the yield point behavior is studied. Tensile tests show that coiling temperature has a greater effect on yield point elongation (YPE) and aging index (AI) than the annealing temperature. Microstructure observations show that coiling temperature at 750 °C would make the micron-sized carbides appearing at the grain boundary disappear and a number of dispersed nanoscale carbides precipitate in grain interior, corresponding to the highest solid solution carbon content in the matrix of 750 °C coiled sample. The experimental results suggest that AI rather than YPE has a positive relationship with the solid solution carbon content of the low-carbon steel. And YPE has a positive relationship with the upper/lower yield strength.

Proposal to assess printability of bioinks for extrusion-based bioprinting and evaluation of rheological properties governing bioprintability

The development and formulation of printable inks for extrusion-based 3D bioprinting has been a major challenge in the field of biofabrication. Inks, often polymer solutions with the addition of crosslinking to form hydrogels, must not only display adequate mechanical properties for the chosen application but also show high biocompatibility as well as printability. Here we describe a reproducible two-step method for the assessment of the printability of inks for bioprinting, focussing firstly on screening ink formulations to assess fibre formation and the ability to form 3D constructs before presenting a method for the rheological evaluation of inks to characterise the yield point, shear thinning and recovery behaviour. In conjunction, a mathematical model was formulated to provide a theoretical understanding of the pressure-driven, shear thinning extrusion of inks through needles in a bioprinter. The assessment methods were trialled with a commercially available crème, poloxamer 407, alginate-based inks and an alginate-gelatine composite material. Yield stress was investigated by applying a stress ramp to a number of inks, which demonstrated the necessity of high yield for printable materials. The shear thinning behaviour of the inks was then characterised by quantifying the degree of shear thinning and using the mathematical model to predict the window of printer operating parameters in which the materials could be printed. Furthermore, the model predicted high shear conditions and high residence times for cells at the walls of the needle and effects on cytocompatibility at different printing conditions. Finally, the ability of the materials to recover to their original viscosity after extrusion was examined using rotational recovery rheological measurements. Taken together, these assessment techniques revealed significant insights into the requirements for printable inks and shear conditions present during the extrusion process and allow the rapid and reproducible characterisation of a wide variety of inks for bioprinting.

Nanoindentation of HMX and Idoxuridine to Determine Mechanical Similarity

Assessing the mechanical behavior (elastic properties, plastic properties, and fracture phenomena) of molecular crystals is often complicated by the difficulty in preparing samples. Pharmaceuticals and energetic materials in particular are often used in composite structures or tablets, where the individual grains can strongly impact the solid behavior. Nanoindentation is a convenient method to experimentally assess these properties, and it is used here to demonstrate the similarity in the mechanical properties of two distinct systems: individual crystals of the explosive cyclotetramethylene tetranitramine (HMX) and the pharmaceutical idoxuridine were tested in their as-precipitated state, and the effective average modulus and hardness (which can be orientation dependent) were determined. Both exhibit a hardness of 1.0 GPa, with an effective reduced modulus of 25 and 23 GPa for the HMX and idoxuridine, respectively. They also exhibit similar yield point behavior. This indicates idoxuridine may be a suitable mechanical surrogate (or “mock”) for HMX. While the methodology to assess elastic and plastic properties was relatively insensitive to specific crystal orientation (i.e., a uniform distribution in properties was observed for all random crystals tested), the indentation-induced fracture properties appear to be much more sensitive to tip-crystal orientation, and an unloading slope analysis is used to demonstrate the need for further refinement in relating toughness to orientation in these materials with relatively complex slip systems and crystal structures.

The Mechanical Properties of Minimally Processed RDX

We report for the first time the mechanical properties of RDX crystals in a conventionally processed, sub-millimeter form that have had no additional mechanical processing. Nanoindentation of RDX powders was used to measure the elastic modulus (19.1±1.9 GPa), hardness (0.741±0.098 GPa), and yield point (onset of plastic deformation) on the as-grown faces of seven different RDX crystals, selected to provide random orientations. Properties within each crystal showed narrow distributions, while the range of properties across all crystals is indicative of testing a variety of orientations. The elastic modulus and hardness are within the range of other published reports on bulk and mechanically polished RDX. The distribution in yield point behavior, with the onset of plasticity occurring between 0.1 and 0.7 GPa, indicates that powders of RDX likely contain a significant number of dislocation sources in the as-processed condition, suggesting that deformation sources are prevalent in the energetic component of plastic bonded explosives prior to incorporating into pressed forms.

Motion of a rigid bar in a rigid-viscoplastic medium: The influence of the model type on the solution behavior

The paper deals with rigid-plastic materials satisfying the vonMises plasticity condition under the assumption that their yield point in pure shear depends only on the equivalent strain rate (rigid-viscoplastic material models). The rigid-viscoplastic models are classified by the yield point behavior in pure shear as the equivalent strain rate tends to zero or infinity. All in all, four classes of rigid-viscoplastic material models are distinguished. For each of these classes of material models, the solution is constructed for the translational motion of an axisymmetric rigid bar along its symmetry axis in a rigid-plastic medium. It is assumed that the maximum friction law acts on the surface of contact between the bar and the rigid-viscoplastic medium. It is shown that the solutions provided by models of different classes qualitatively differ from each other. A qualitative comparison with experimental results known in the literature is carried out. It is shown that predicting the formation of an intensive plastic deformation layer near the friction surface, which is observed in experiments, is possible if the rigid-viscoplastic model contains the saturation stress (the stress, bounded in magnitude, to which the yield point in pure shear tends as the equivalent strain rate tends to infinity).

Stop and go: understanding yieldpoint behavior

Yieldpoints are critical to the implementation of high performance garbage collected languages, yet the design space is not well understood. Yieldpoints allow a running program to be interrupted at well-defined points in its execution, facilitating exact garbage collection, biased locking, on-stack replacement, profiling, and other important virtual machine behaviors. In this paper we identify and evaluate yieldpoint design choices, including previously undocumented designs and optimizations. One of the designs we identify opens new opportunities for very low overhead profiling. We measure the frequency with which yieldpoints are executed and establish a methodology for evaluating the common case execution time overhead. We also measure the median and worst case time-to-yield. We find that Java benchmarks execute about 100M yieldpoints per second, of which about 1/20000 are taken. The average execution time overhead for untaken yieldpoints on the VM we use ranges from 2.5% to close to zero on modern hardware, depending on the design, and we find that the designs trade off total overhead with worst case time-to-yield. This analysis gives new insight into a critical but overlooked aspect of garbage collector implementation, and identifies a new optimization and new opportunities for very low overhead profiling.

Detection of Yield Point Behavior by Acoustic Emission in thin Films

Fabrication of domain-boundary based thin-film devices needs to control the yield point of a strained/cooled ferroelastics. We show that such control is possible by acoustic emission measurements using an integrated detector system. Through molecular dynamics simulations, we find that in thin films, the potential energy reduction during the yield event can reach to 4 meV/atom, and generate strain of 0.005 at the surface which lead to detectable acoustic emission signals.

TiC-Nb 소결 복합재료의 연성-취성 천이 특성

Abstract In order to clarify the effect of Nb addition on the ductile-brittle transition property of sintered TiC, TiC-10 mol%Nb composites were researched using a three-point bending test at temperatures from room temperature to 2020 K, and thefracture surface was observed by scanning electron microscopy. It was found that the Nb addition decreases the ductile-brittletransition temperature of sintered TiC by 300 K and increases the ductility. The room temperature bending strength wasmaintained at up to 1800 K, but drastically dropped at higher temperatures in pure TiC. The strength increased moderately toa value of 320 MPa at 1600 K in TiC-10 mol% Nb composites, which is 40 % of the room temperature strength. Pores wereobserved in both the grains and the grain boundaries. It can be seen that, as Nb was added, the size of the grain decreased.The ductile-brittle transition temperature in TiC-10 mol% Nb composites was determined to be 1550 K. Above 1970 K,yieldpoint behavior was observed. When the grain boundary and cleavage strengths exceed the yield strength, plasticdeformation is observed at about the same stress level in bending as in compression. The effect of Nb addition is discussedfrom the viewpoint of ability for plastic deformation.Key wordsductile-brittle transition, TiC-Nb, strength, composites, bending test.

Formation and mechanical characterization of ionically crosslinked membranes at oil–water interfaces

Here we report on the preparation and mechanical characterization of a 2-D self-assembled membrane formed by ionically crosslinking the polyelectrolyte parts of a gradient amphiphilic copolymer at oil and water interfaces. To fabricate these membranes, chloroform solutions of styrene-acrylic acid copolymers were suspended as pendant drops in an aqueous embedding phase. Due to the amphiphilic nature of these molecules, the copolymer chains migrate to the oil-water interface creating an interfacial layer. Upon the addition of zinc acetate to the embedding phase, crosslinks between copolymer molecules are formed via zinc-carboxylate complexes. While ionically crosslinked block copolymer membranes were critically damaged after one expansion cycle, ionically crosslinked gradient copolymers formed durable membranes that maintained their physical integrity through multiple expansion-compression-expansion cycles. This difference in mechanical behavior is attributed to the fact that gradient copolymers are more effective interfacial modifiers and have a significantly different molecular alignment at the oil-water interface. Additionally by changing the incubation time from 20 to 30 minutes, the low-strain dilatational modulus of these membranes was significantly increased due to higher interfacial coverage and crosslinking density. Longer incubation times also led to a distinct yield point and plastic deformation behavior at larger strains. Further mechanical characterization of the membranes showed that they can be quite robust and that by replacing the internal oil phase with an aqueous solution, future testing of membrane filtration and permeation may be possible.

Rheology of Milk Foams Produced by Steam Injection

Rheology of milk foams generated by steam injection was studied during the transient destabilization process using steady flow and dynamic oscillatory techniques: yield stress (τ(y) ) values were obtained from a stress ramp (0.2 to 25 Pa) and from strain amplitude sweep (0.001 to 3 at 1 Hz of frequency); elastic (G') and viscous (G″) moduli were measured by frequency sweep (0.1 to 10 Hz at 0.05 of strain); and the apparent viscosity (η(a) ) was obtained from the flow curves generated from the stress ramp. The effect of plate roughness and the sweep time on τ(y) was also assessed. Yield stress was found to increase with plate roughness whereas it decreased with the sweep time. The values of yield stress and moduli-G' and G″-increased during foam destabilization as a consequence of the changes in foam properties, especially the gas volume fraction, ϕ, and bubble size, R(32) (Sauter mean bubble radius). Thus, a relationship between τ(y) , ϕ, R(32) , and σ(surface tension) was established. The changes in the apparent viscosity, η, showed that the foams behaved like a shear thinning fluid beyond the yield point, fitting the modified Cross model with the relaxation time parameter (λ) also depending on the gas volume fraction. Overall, it was concluded that the viscoelastic behavior of the foam below the yield point and liquid-like behavior thereafter both vary during destabilization due to changes in the foam characteristics. Studying the transient rheology of milk foams during destabilization contributes to our knowledge of the relationships between the changes in foam properties: texture and mouth feel during the consumption of hot foamed beverages.

High Strain Rate Torsion Properties of Ultrafine-Grained Aluminum

Mechanical properties of most metallic materials can be improved by reducing their grain size. One of the methods used to reduce the grain size even to the nanometer level is the severe plastic deformation processing. Equal Channel Angular Pressing (ECAP) is one of the most promising severe plastic deformation processes for the nanocrystallization of ductile metals. Nanocrystalline and ultrafine grained metals usually have significantly higher strength properties but lower tensile ductility compared to the coarse grained metals. In this work, the torsion properties of ECAP processed ultrafine grained pure 1070 aluminum were studied in a wide range of strain rates using both servohydraulic materials testing machines and Hopkinson Split Bar techniques. The material exhibits extremely high ductility in torsion and the specimens did not fail even after 300% of strain. Pronounced yield point behavior was observed at strain rates 500 s−1 and higher, whereas at lower strain rates the yielding was continuous. The material showed slight strain softening at the strain rate of 10−4 s−1, almost ideally plastic behavior at strain rates between 10−3 s−1 and 500 s−1, and slight but increasing strain hardening at strain rates higher than that. The tests were monitored using digital cameras, and the strain distributions on the surface of the specimens were calculated using digital image correlation. The strain in the specimen localized very rapidly after yielding at all strain rates, and the localization lead to the development of a shear band. At high strain rates the shear band developed faster than at low strain rates.

Analytical and rheological investigations into selected unifloral German honey

The physicochemical and rheological properties parameters of five German unifloral honey: false acacia, heather, sunflower, lime and rape (39 samples) were analysed to demonstrate the relation between botanical origin and material properties. The validity of the rheological constitutive equations by Herschel-Bulkley, Ostwald-De Waele and Newton was confirmed by varying temperature and crystalline state. Highly structured (crystalline) honey exhibits temperature dependent with all rheological anomalies (shear thinning, yield point and rheodynamic behaviour) and can described an average effective viscosity. Modern oscillation measurements were used to determine the crystallization behaviour of honey. The results showed that crystallization of honey is depended on botanic origin, temperature and storage time.

Mechanical deformation of high-purity sputter-deposited nano-twinned copper

Near classical yield points were reproducibly observed at room and liquid nitrogen temperatures during tensile deformation of 170 μm thick, high-purity copper foils synthesized by magnetron sputter deposition. Uniformly distributed mobile dislocations introduced by rolling to ∼20% reduction in thickness eliminated the yield point at both temperatures. The experimental observations clearly demonstrate that the observed yield-point behavior is a direct result of the very low initial dislocation density in these sputtered films as expected for “ideal” nanoscale microstructural materials.