Elastic Limit Research Articles

Stresses at grain boundaries: The maximum incompatibility stress in an infinitely extended elastic bicrystal under uniaxial loading

In a material under stress, grain boundaries may give rise to stress discontinuities. Stress localization is crucial to materials' behavior such as segregation, precipitation, and void nucleation. Here, the stress state at a grain boundary perpendicular to a uniaxial external stress is studied systematically. The grain boundary with the most extreme stress discontinuity is determined for cubic materials within the elastic limit for a bicrystal model. Additionally, grain boundaries with negligible stress discontinuity are identified. The influence of the elastic tensor components, C11, C12, and C44, and grain orientation is studied quantitatively.

Novel hybrid laser forging and arc additive repairing process for improving component performances

A new hybrid laser forging and arc additive repairing process was developed to significantly improve the performance of repaired components, in which a leading gas metal arc was adopted to repair the partially damaged component, and a trailing short-pulse laser directly acted on the high-temperature solidified metal without coating (laser forging). Compared with arc additive repairing with post-treatment, this hybrid process performed the arc repair and laser forging synchronously. The laser forging region can be accurately determined using a multi-physical molten pool simulation. The molten metal flow was also studied, indicating that the high sulfur content introduced by the filler metal transfer had a significant influence on the Marangoni stress distribution and thus changed the molten metal flow patterns. The mechanism for laser forging without coating and its related physical effects were investigated. The laser shock pressure was significantly higher than the Hugonoit elastic limit of the high-temperature solidified metal, causing plastic deformation of the repaired layer. The high strain and severe plastic deformation induced by laser forging caused martensite formation and grain refinement, which improved the mechanical properties and electrochemical corrosion performance of the repaired layer.

Tensor decomposition for modified quasi-linear viscoelastic models: Towards a fully non-linear theory

We discuss the decomposition of the tensorial relaxation function for isotropic and transversely isotropic (TI) modified quasi-linear viscoelastic (MQLV) models. We show how to formulate the constitutive equation using a convenient decomposition of the relaxation tensor into scalar components and tensorial bases. We show that the bases must be symmetrically additive, i.e., they must sum up to the symmetric fourth-order identity tensor. This is a fundamental property both for isotropic and anisotropic bases that ensures the constitutive equation is consistent with the elastic limit. We provide two robust methods to obtain such bases. Furthermore, we show that, in the TI case, the bases are naturally deformation-dependent for deformation modes that induce rotation or stretching of the fibres. Therefore, the MQLV framework allows to capture the non-linear phenomenon of strain-dependent relaxation, which has always been a criticised limitation of the original quasi-linear viscoelastic theory. We illustrate this intrinsic non-linear feature, unique to the MQLV model, with two examples (uni-axial extension and perpendicular shear).

Mechanical deformation of elastomer medical devices can enable microbial surface colonization

Elastomers such as silicone are common in medical devices (catheters, prosthetic implants, endoscopes), but they remain prone to microbial colonization and biofilm infections. For the first time, our work shows that rates of microbial surface attachment to polydimethylsiloxane (PDMS) silicone can be significantly affected by mechanical deformation. For a section of bent commercial catheter tubing, bacteria (P. aeruginosa) show a strong preference for the ‘convex’ side compared to the ‘concave’ side, by a factor of 4.2. Further testing of cast PDMS materials in bending only showed a significant difference for samples that were manually wiped (damaged) beforehand (1.75 × 104 and 6.02 × 103 cells/mm2 on the convex and concave sides, respectively). We demonstrate that surface microcracks in elastomers are opened under tensile stress (convex bending) to become ‘activated’ as sites for microbial colonization. This work demonstrates that the high elastic limit of elastomers enables these microcracks to reversibly open and close, as ‘dynamic defects’. Commercial catheters have relatively high surface roughness inherent to manufacturing, but we show that even manual wiping of newly-cast PDMS is sufficient to generate surface microcracks. We consider the implication for medical devices that feature sustained, surgical, or cyclic deformation, in which localized tensile conditions may expose these surface defects to opportunistic microbes. As a result, our work showcases serious potential problems in the widespread usage and development of elastomers in medical devices.

An integrated 3D dynamic FE vehicle-track model in elasto-plasticity to investigate short pitch corrugation under cyclic wheel loads

The mechanism of rail short pitch corrugation has remained elusive in the past. The damage mechanisms of the corrugation are reported to be differential wear or plastic deformation. The former has been extensively studied, while the plastic deformation, especially under multiple wheel passages has been seldom studied. To uncover the facts behind it, an integrated dynamic vehicle-track model with the rail material treated in elasto-plasticity is developed. Further, a novel method which can simulate the material deformation under cyclical axle loads is proposed. This method is used to study the rail material response at corrugation. Our research found that for the cases studied, the rail material undergoes cyclic plastic deformation at corrugation peaks only for a limited number of cycles (2–4 cycles) before reaching the elastic shakedown limit. After that, no further residual stresses and strains accumulate. The plastic deformation at corrugation peaks weakens the corrugation amplitude, serving as an early corrugation attenuation mechanism. Conversely, work-hardening at corrugation peaks increases wear resistance at those peaks, promoting corrugation in the long term. The explanation of the corrugation development process under the interplays of the plastic deformation and wear has been validated by field corrugation data. Additionally, we propose a wear coefficient in the wear model to account for the work-hardening and change in the wear resistance. Experimental results of the hardness distribution show the similar characteristics to the numerical results.

Dynamic compression behavior of TiZrNbV refractory high-entropy alloys upon ultrahigh strain rate loading

In this study, the dynamic compressive response behavior of a body-centered cubic (BCC) single-phase TiZrNbV refractory high-entropy alloy (RHEA) was investigated under impact at speeds of 313–1584 m s–1 using two-stage, gas-gun-driven, high-speed plate-impact experiments; recovery sample analysis; and theoretical calculations. The strain rate and pressure were approximately 107 s−1 and 5.07–29.37 GPa, respectively. The results showed that the TiZrNbV RHEA had a Hugoniot elastic limit of 4.12–5.86 GPa and a spall strength of 1.84–2.03 GPa. The initial yield strength of the alloy showed a strong strain-rate dependence and could be described by the modified Zerilli–Armstrong model, while the phonon-damping effect was the main reason for its high strain-rate sensitivity. Microstructural analysis showed that the dynamic deformation of the TiZrNbV RHEA was controlled by the dislocation slip, dislocation proliferation, intersection of the deformation bands, and grain refinement. The analysis also showed that the intergranular, transgranular, and mixed-type cracks dominated the spall failure of the material. The dynamic Hall–Petch effect and pinning from the lattice distortion led to high dynamic yield strength. The critical strain rate for the phonon drag effect was positively related to the relative atomic mass and local strain field of the metals. Within the experimental loading range, the RHEA showed good structural stability, and simultaneously, the theoretical calculation method for the equation of state based on a cold-energy mixture could accurately predict its shock-response behavior. The valence-electron concentration (VEC) had a direct effect on the shock-compression properties of the HEAs; higher VEC implied more difficulty in compressing the HEAs. The findings of this study provide insights into understanding the mechanical response characteristics of RHEAs under extreme conditions such as high-speed impact and ultrahigh strain-rate loading.

Elastic limit tensile stress of UHPFRC: Method of determination and effect of fiber orientation

This paper discusses the influence of fiber orientation on elastic limit tensile stress of ultra-high performance fiber reinforced cementitious composites (UHPFRC). An original model relating elastic limit tensile stress to fiber orientation is proposed, and three determination methods of the elastic limit are compared. Five specimens are tested under uniaxial tension and characterized with digital image correlation, acoustic emission and displacement transducers. Before testing, the fiber distribution (local dosage and orientation) of each specimen is determined using a magnetic probe. After testing, three methods (offset with 2 different threshold values, and deformation modulus drop) are applied to determine the elastic limit tensile stress. The test results show that the proposed model estimates well the elastic limit tensile stress on the basis of fiber orientation. The modulus-drop method yields the most representative value for the elastic limit considering the underlying physical phenomena. It may be taken as definition of elastic limit tensile stress of UHPFRC.

Response of elastic/viscoelastic layers on an elastic half-space in rolling contacts: Towards a new modeling approach for elastohydrodynamic lubrication

The 3D quasi-static contact problem of a rigid sphere rolling on an elastic half-space covered by a thin viscoelastic/elastic layer is studied as representative of soft layered contacts in engineering, physics, and biomedical applications, as well as for its potential merit to model Elasto-Hydrodynamically Lubricated (EHL) contacts in specific EHL film behavior suggested in earlier research. The viscoelastic layer behavior is modeled with a standard linear solid (SLS) model with a single relaxation time. Two approaches are used, the foundation (Winkler) reduced model assuming unidirectional stress–strain behavior normal to the surface only and the Papkovich–Neuber potential model which, based on the complete Navier–Cauchy equations, accounts for non-local support and bending effects. The models are validated against literature and compared. Whereas coated/layered contact problem studies mostly consider relatively thick coatings, and results focus on the pressure distribution and the contact area size, in this paper we consider layers with a thickness much smaller than the contact radius, i.e. of the order of a thickness of a conventional EHL film. The details of the layer deformation, pressure profiles and subsurface stresses are presented and interpreted in terms of the underlying physics, in particular the thin layer limit common also in thin fluid layers. It is shown that two dimensionless parameters, the ratio of the elastic modulus between the layer and the substrate and the ratio of the layer thickness to the corresponding Hertz contact width, dominate the systematic response for elastic layered contact problems. And four dimensionless parameters with two extras, the Deborah number based on the Hertz contact width and the ratio of the two elastic limits of the SLS viscoelastic material, are needed and analyzed for viscoelastic layered contacts. The results presented provide a good framework for the understanding and interpretation of the effects of viscoelastic layers on the deformation and pressure distribution of contact problems. Finally, the capability of actually modeling EHL film behavior via a viscoelastic layer suggested by van Emden et al. (2017) is discussed.

Pseudo-static test on stainless steel frame with high-strength bolted extended endplate joint

Many studies have been conducted on stainless steel materials, components and joints; however, experimental research on the seismic response of stainless steel structures is rare. This study reports pseudo-static loading tests on S2205 duplex stainless steel and Q355B carbon steel frames with high-strength bolted extended endplate beam-to-column joints. The two frames have the same geometric configuration and equivalent elastic limits (approximately 350 MPa). The detailed test processes and failure modes are presented. The Q355B carbon steel frame failed with extensive plastic deformation at the end of the beam, whereas the S2205 duplex stainless steel failed from fracture of the weld between the beam flange and endplate. The test results were analyzed at the global level (lateral load capacity, energy dissipation, etc.), beam-column joints (hysteresis curves), and yielding sequence. Owing to the considerable strain-hardening of stainless steel, the S2205 stainless steel frame had a higher loading capacity, whereas the Q355B carbon steel frame had a higher ductility and energy dissipation capacity.

Experimental study on PSPC beam – Concrete encased CFST column frame installed with novel steel panel dampers

A concrete-filled steel tubular (CFST) column has a bearing capacity with a relatively small cross section. From the perspective of the assembly, a partially steel-reinforced precast concrete (PSPC) beam can combine the merits of both the reinforced concrete beam and steel beam. However, strong earthquake action puts forwards a dramatic seismic requirement for the high-rise frame. In this study, to improve the seismic performance of the structure in an economical way, the duplex assembled I-shaped steel panel damper (DAISPD) was introduced to the PSPC beam-concrete encased CFST column frame through the eccentric brace. A half-scaled two-storey two-bay substructure specimen was tested under cyclic loading. A series of seismic properties was then analysed, including the failure modes, hysteretic performance, and energy dissipation capacity. To further investigate the contribution of the damper, groups of numerical analyses were conducted on the key design parameters of the DAISPD. The introduction of the damper provides an adequate lateral stiffness and energy absorption to the frame, especially when the interstorey drift is within the elastic and plastic drift limits. The damage of the damper and the beam hinge mechanism occurs sequentially, which results in a two-stage failure mode. Considering the predamage contribution and the postdamage influence of the damper, it is recommended to design with a moderate yield force and a small height.

Self-deploying amorphous metal origami structures for environmental protection of lunar payloads

The development of small subterranean deployable structures provides a novel methodology for thermal insulation and radiation protection of scientific payloads for future lunar missions. This work showcases the fabrication and characterization of an origami-inspired self-deploying dome fabricated from amorphous metal foil, taking advantage of the high elastic strain limit of amorphous metals to store energy for deployment in the foil folds when the structure is in the stored configuration. Experiments are performed to understand the influence of material selection on stored energy and springback. A cylindrical structure is fabricated to develop a simulation model for origami-inspired amorphous metal structures and probe the structural integrity of amorphous metal foil structures during repeated loading cycles. Finally, a prototype deployable dome structure is manufactured to showcase the self-deployment strategy and investigate deformation under a simulated lunar regolith load.

基于单轴压缩物理试验和虚拟仿真的钙果力学模型构建

The mechanical parameters of Cerasus humilis are the basic data for subsequent studies on fruit deformation, damage, and movement characteristics during harvesting and transportation, but these parameters are rarely reported. Relevant mechanical parameters of whole fruit compression are calculated by comparing physical tests and virtual simulations. The orthogonal rotating combined experimental design was used to arrange the simulation tests, with the elastic modulus (E), yield limit (Ey), and tangent modulus (Et) as the influence factors and compression force as the result. Response surface optimization was employed to find the closest test point to the force–deformation curve of the physical test. The parameters of the pulp test point are as follows: E = 0.923 MPa, Ey = 0.0897 MPa, and Et = 0.478 MPa. Results show that the step on the force–deformation curve was not the beginning of the pulp yield, which was substantially earlier than the strain rate at the simulation step. The region of increased stress in the pulp first appeared at the junction with the core due to stress concentration. Combining virtual and physical tests to solve the mechanical parameters of fruits is more suitable than testing the standard pulp sample.

A discrete element analysis for general failure behavior of basalt

Discrete element method is a commonly used numerical method in rock mechanics and rock engineering. In this research, the flat-joint contact bond model is used to reproduce the macro-properties of basalt on an assembly of particles. A series of laboratory tests, including uniaxial compression test, Brazilian tension test, and fracture toughness test are used to simulate progressive degradation of rock mass strength in a size-independent model with a unique set of micro-properties. The work involves a detailed analysis for estimating compressive elastic modulus, Poisson's ratio, bimodularity, mode I fracture toughness, crack closure, crack initiation, and crack damage thresholds, as well as the tensile and compressive strengths. The constructed flat-jointed material yields a realistic uniaxial compression to tensile strength ratio and nonlinear stress-strain response through sustaining adequate particle interlocking and embedded pre-existing microcracks. The role of sample size on the compressive elastic modulus, crack initiation and crack damage stress limits is demonstrated in detail. A model size-independency analysis was also performed to examine the effect of particle size on the results. The numerical results presented in this study are within 10% of the laboratory results.

A Numerical Procedure for Shakedown Analysis of Thick Cylindrical Vessels with Crossholes under Dual Cyclic Loadings.

A modified numerical procedure for the shakedown analysis of structures under dual cyclic loadings, based on the Abdalla method, is proposed in this paper. Based on the proposed numerical procedure, the shakedown analysis of the thick cylindrical vessels with crossholes (TCVCs) under cyclic internal pressure and cyclic thermal loading was carried out. The effects of material parameters (elastic modulus and thermal expansion coefficient) and crosshole radius on the elastic shakedown limit of TCVCs are discussed and, finally, normalized and formularized. Furthermore, the obtained shakedown limit boundary formulation is compared with FEA results and is verified to evaluate the shakedown behavior of TCVCs under cyclic internal pressure and cyclic thermal loading.

Diamond-TiC composite with an ultrahigh Hugoniot elastic limit

The Hugoniot elastic limit (HEL) is widely adopted as an important criterion for assessing the dynamic strength of materials, representing the transition stress from elastic to plastic response prior to failure under shock compression. Nano-polycrystalline diamond currently holds the highest HEL of 208 (±14) GPa. Here, we report a diamond-TiC composite (∼11.5 wt. % TiC) showing an ultrahigh HEL of at least 195 (±3.5) GPa, which is comparable to that of nano-polycrystalline diamond. All measured velocity profiles on the diamond-TiC free surface exhibited a single-wave structure at shock pressures of 48–195 GPa. Moreover, the measured Us–Up (shock wave velocity–particle velocity) relation can be linearly fitted, indicating no elastic–plastic transition or solid–solid phase transition up to a shock pressure of 195 GPa. The diamond-TiC composite's compression ratio was similar to that of TiC but significantly higher than that of diamond. These extraordinary dynamic responses are intrinsically attributed to the unique microstructure in which diamond polycrystals are encased in a TiC matrix, providing protection against yielding. Our findings not only developed a mechanically reliable, lightweight, and high-performance armor material at low synthesis costs, but also provided new insights into the shock compression behavior of diamond composites.

Dynamic shock response of high-entropy alloy with elemental anomaly distribution

Elemental inhomogeneities formed during the actual preparation or heat treatment of high entropy alloys (HEAs) have a profound effect on the mechanical properties of HEAs. In this work, molecular dynamics (MD) was used to simulate the plastic response of CoCrFeNiMn HEAs with Mn element as the concentration fluctuation variable under shock loading. The results show that the unique physical shock properties, including shock wave propagation, dislocation evolution, defect collapse, and energy dissipation, are related to the anomaly arrangement of Mn elements. Compared to the uniform distribution of Mn elements, the gradient distribution of Mn leads to lattice mismatch between heterogeneous tissues, increasing the energy barriers of the dislocation mediated slip and reducing the probability of generating disordered atoms (from 27.9 to 11.5%), which results in anomalous shock strength in the Hugoniot elastic limit (HEL) (from 69.1 Gpa to about 72.9 Gpa). As the main product of shock energy conversion, energy dissipation is subjected to the complex interaction between the shock compression and the reflection unloading, and the gradient distribution of Mn element can significantly reduce the efficiency of shock energy conversion due to dislocation activity and crystal phase changes caused by lattice mismatch. Finally, when defective structures are introduced, the gradient dislocation nucleation and energy conversion fluctuations of the microstructure were observed during shock loading. This work can provide a new perspective for further understanding the influence of solid solutions with non-uniform element distribution on the plastic deformation mechanism of high entropy alloys subjected to extreme dynamic shock.

Fatigue Properties of Spring Steels after Advanced Processing.

This article deals with the effect of strain-assisted tempering (SAT) on the fatigue properties of 54SiCr6 steel used for spring steel wires in a wide variety of automotive applications, including coil springs. This steel spring wire is extremely strong, having a high elastic limit and yield point, giving the steel excellent energy accumulation and fatigue properties. This combination opens up new possibilities in helical and cylindrical coil spring design, resulting in the reduction of both size and weight. Lightweight coil springs lead to improvements in fuel consumption, stability and vehicle traction. A large plastic deformation and SAT were applied to enhance the yield point of the study material. Improvements in the static and cyclic properties of steel springs were investigated using tensile tests and 3PB fatigue tests at ambient temperature. In addition, an advanced laser shock peening (LSP) process was employed to increase the fatigue resistance of the SAT material. The results presented here show great improvements in the static and fatigue properties over commercial steel treatment. The material quality of the wires was evaluated to be insufficient for further processing with cold coiling.

Shock-induced reactive molecular dynamics simulation in sodium aluminosilicate hydrate: Wave propagation, mechanical response, and structural deformation

Sodium aluminosilicate hydrate (N-A-S-H) gels have gained attention due to their potential use as components of geopolymers to improve structural and mechanical properties. In this study, we investigated the propagation of shock waves in N-A-S-H gels subjected to impact velocities (Up) ranging from 0.1 to 3.0 km/s, as well as the resulting mechanical responses and structural deformations. Our results showed that when Up<0.4 km/s, only one elastic wave existed, and the Hugoniot elastic limit was estimated to be 4.1 GPa. Above this limit, a two-wave structure formed. The elastic and elastoplastic deformation mechanisms involved initial compaction and densification of the N-A-S-H gel structure, followed by bond angle bending. The Hugoniot Us-Up relationship was found to be linear in the elastoplastic region, with a linear parameter λ of approximately 2.75. These new atomistic insights into the shock compression of N-A-S-H gels will provide valuable guidance for future studies.

The Mechanical Properties of Blended Fibrinogen:Polycaprolactone (PCL) Nanofibers.

Electrospinning is a process to produce versatile nanoscale fibers. In this process, synthetic and natural polymers can be combined to produce novel, blended materials with a range of physical, chemical, and biological properties. We electrospun biocompatible, blended fibrinogen:polycaprolactone (PCL) nanofibers with diameters ranging from 40 nm to 600 nm, at 25:75 and 75:25 blend ratios and determined their mechanical properties using a combined atomic force/optical microscopy technique. Fiber extensibility (breaking strain), elastic limit, and stress relaxation times depended on blend ratios but not fiber diameter. As the fibrinogen:PCL ratio increased from 25:75 to 75:25, extensibility decreased from 120% to 63% and elastic limit decreased from a range between 18% and 40% to a range between 12% and 27%. Stiffness-related properties, including the Young's modulus, rupture stress, and the total and relaxed, elastic moduli (Kelvin model), strongly depended on fiber diameter. For diameters less than 150 nm, these stiffness-related quantities varied approximately as D-2; above 300 nm the diameter dependence leveled off. 50 nm fibers were five-ten times stiffer than 300 nm fibers. These findings indicate that fiber diameter, in addition to fiber material, critically affects nanofiber properties. Drawing on previously published data, a summary of the mechanical properties for fibrinogen:PCL nanofibers with ratios of 100:0, 75:25, 50:50, 25:75 and 0:100 is provided.

Hydrodynamics and structural mechanics of jet pumps in a Boiling Water reactor: CFD analysis

The aim of this work is to the failure of jet pump analysis using the hydrodynamics and structural analysis coupling in a typical BWR. Different structural problems in jet pups of Boiling Water reactors (BWR) have been reported in the last 20 years. The liquid in the jet pump is accelerated due to high differential pressure in the nozzle that induces vibrations in the slip joint of the diffuser. If the vibrations are out of the range of the natural frequency, they may produce a leakage in the slip joint or a rupture in the riser pipe. A leakage in the area of the slip joint of 5% is simulated to estimate the frequency of vibration of the jet pumps. The results show that as the frequency of the vibration increases the displacement of the jet pump and the stress increases and frequencies higher than 47.5 Hz exceed the modulus of the elasticity limit of the material.