Loading Orientation Research Articles

Quasi-Static Mechanical Properties and Continuum Constitutive Model of the Thyroid Gland.

The purpose of this study is to obtain the digital twin parameters of the thyroid gland and to build a constitutional model of the thyroid gland based on continuum mechanics, which will lay the foundation for the establishment of a surgical training system for the thyroid surgery robot and the development of the digital twin of the thyroid gland. First, thyroid parenchyma was obtained from fresh porcine thyroid tissue and subjected to quasi-static unconfined uniaxial compression tests using a biomechanical test platform with two strain rates (0.005 s-1 and 0.05 s-1) and two loading orientations (perpendicular to the thyroid surface and parallel to the thyroid surface). Based on this, a tensile thyroid model was established to simulate the stretching process by using the finite element method. The thyroid stretching test was carried out under the same parameters to verify the validity of the hyperelastic constitutive model. The quasi-static mechanical property parameters of the thyroid tissue were obtained by a quasi-static unconstrained uniaxial compression test, and a constitutional model that can describe the quasi-static mechanical properties of thyroid tissue was proposed based on the principle of continuum media mechanics, which is of great value for the establishment of a surgical training system for the head and neck surgery robot and for the development of the thyroid digital twin.

Effect of loading orientation on plasticity in nano-laminated CoNiCrFeMn dual-phase high-entropy alloy: a molecular dynamics study

Dual-phase high-entropy alloys (DP-HEAs) have been proved to be a kind of promising materials that exhibit a combination of excellent strength and ductility. Previous studies have emphasized the effect of interface and phase volume fraction on mechanical performance in DP-HEAs. However, the deformation mechanisms such as interplays between dislocations and the constituent phases have not been fully understood. Particularly, the research concerning plastic anisotropy in DP-HEAs is still lacking. Here, molecular dynamics simulations are performed to probe the effect of loading orientation on plasticity in the nano-laminated face-centered cubic (FCC)/hexagonal close-packed (HCP) CoNiCrFeMn DP-HEA. Results reveal that a switch from strengthening to softening and back to strengthening is closely related to the activation of different slip systems when tailoring the inclination angles of the nanolaminates with respect to the tensile direction from 0° to 90°. Slip transfer across phase boundaries, phase transformation and the nucleation of shear bands dominate the plasticity in the samples with low, medium and high inclination angles, respectively. Furthermore, the evolution of microstructures, such as dislocations, stacking faults, and FCC/HCP phase are analyzed to study the underlying deformation mechanisms. These results can help understand the plastic anisotropy of DP-HEAs and design alloys with excellent mechanical properties for engineering applications.

Lateral Bearing Capacity of a Hybrid Monopile: Combined Effects of Wing Configuration and Local Scour

Pile foundations for offshore wind turbines are subjected to large lateral loads. By mounting wings on the perimeter of regular monopiles, winged monopiles have shown better performance in resisting deformation under horizontal loading. However, the hazardous effect of local scour on the lateral bearing capacity of winged monopiles installed in the sandy seabed has not been systematically evaluated. In this study, a modified Mohr–Coulomb model considering the pre-peak hardening and post-peak softening behavior of dense sand is adopted to simulate laterally loaded winged monopiles in the locally scoured sandy seabed, using three-dimensional finite element analyses. The effect of local scour depth on the lateral capacity of winged monopiles is examined and explained by soil failure mechanisms. The enhancement of lateral capacity with wings attached to the monopile is demonstrated to be more effective than extending pile embedment length. The effects of the relative density of sand and the wing load orientation are also discussed. Finally, the wing efficiency is evaluated to determine the optimal configuration of winged monopiles.

Role of anisotropy on strain localization at high-strain rates in Ti6Al4V

A systematic experimental investigation of the plastic anisotropy at high strain-rates of Ti6Al4V was carried out using a Split Hopkinson bar system. Dynamic tensile tests were conducted on flat specimens cut at seven orientations with respect to the rolling direction of the sheet. To quantify the effect of anisotropy on both the global response prior to necking as well as on the inclination of the strain localization bands that developed in the neck, both surface strains and local displacements were measured using high speed imaging in conjunction with digital image correlation. It was found that the material has a weak anisotropy in uniaxial yield stresses while it displays a strong anisotropy in Lankford coefficients; the inclinations of the bands in the neck depend on the loading orientation. Moreover, only for the rolling direction specimen, the neck contains two localization bands equally inclined to the loading axis. These observations confirm the correlations established in the theoretical study of Cazacu and Rodríguez-Martínez (2019) between the necking band angles and the variation in the uniaxial flow stresses with the loading direction. A constitutive model with yielding described with the orthotropic criterion of Cazacu and Barlat (2001) and a hardening law accounting for the strain rate effects was used to model the observed behavior. Comparison between finite element simulations, analytical calculations, and test data show that the experimental trends are captured.

Influence of loading orientation on deformation localization of irradiated tungsten

The plasticity of tungsten in low or medium temperature regimes is dominated by the screw dislocation mobility, which is well known to exhibit a strong non-Schmid effect and loading orientation dependence. As a typical plasma-facing material, many questions regarding the role of these plasticity features on the irradiation degradation performance of tungsten remain unanswered. Considering that deformation localization is a continuing critical problem in the nuclear application, in the current work, a systematic set of crystal plasticity calculations is designed to quantify the degree of deformation localization in irradiated tungsten, which is found to be sensitive to the loading orientation. The underlying physical mechanisms are carefully analyzed, highlighting the important role of the number of active slip systems, the non-Schmid effect controlled by the evolution of local stress state, and the high resistance stress induced by irradiation defects. Based on the micro-scale physical mechanisms, two physical indicators are proposed to provide guidance for predicting the orientation-dependent deformation localization in irradiated tungsten, which agrees well with the simulation results.

Prismatic-to-basal plastic slip transition in zirconium

We model three-dimensional dislocation ensembles in Zr pillars using newly developed mobility laws for dislocations on prismatic and basal planes. The effects of the loading orientation and temperature on the micro-scale mechanical response of single crystals are systematically explored. Easy dislocation glide is observed on prismatic planes, while hard glide occurs on basal planes. By selection of sample temperature and loading orientation, we show that the transition from prismatic to basal glide of dislocation ensembles can be controlled. We also show that the presence of dislocation cross-slip strongly promotes prismatic glide as a result of prismatic/basal cross-slip energy asymmetry. Cross-slip does not alter the occurrence of plastic slip transition but only induces a shift towards higher temperatures. The preponderance of plastic slip on basal or prismatic planes is found to be mediated by a transitional dislocation, which is composed of glissile segments on parallel prismatic planes connected by glissile super-jogs on basal planes. These findings provide a mechanistic understanding of experiments and highlight the significance of transitional prismatic-basal dislocations on the macroscopic characteristics of plasticity in HCP crystals.

Effect of Load and Fiber Orientation on Wear Properties of Additively Manufactured Continuous CFRP Composites under Dry Sliding Conditions

The present research investigates the wear properties of a continuous carbon fiber-reinforced additively manufactured polymer composite under dry sliding conditions. The effect of load and fiber orientation is examined on polymer composite specimens. The wear test of the additively manufactured polymer composite specimens is conducted on pin-on-disk test equipment. The result shows that the applied load and fiber orientation significantly affect the composite specimen’s wear properties. The wear and coefficient of friction (COF) increase with load. The minimum effect on wear and COF is observed for 0° fiber orientation, and the maximum effect is observed for 90° fiber orientation. Finally, morphological analysis is conducted using an optical micrograph of the worn-out surfaces to understand the failure type for different fiber orientations.

Triaxial compression behavior of large-scale jointed coal: a numerical study

Accurate estimation of the triaxial compression behavior of jointed coal is essential for coal mining. Few studies addressed the triaxial compression behavior of large-scale rock mass, especially with real joint geometry. We employed a numerical synthetic rock mass (SRM) method to study the triaxial compression behavior of jointed coal. Jointed-coal specimens were constructed based on in-situ joint measurements and microparameter calibration against laboratory experiments. A series of triaxial compression tests under different loading orientations and confining pressures were numerically performed to obtain joint and confining-pressure effects on the triaxial compression behavior and reveal the failure mechanism of jointed coal. Results suggest that the triaxial compression behavior of the jointed coal has strong joint and confining-pressure effects. Joints weaken the strength and elastic modulus, reduce the lateral deformation, and affect the geometries of the shear-rupture surface. An increase in the confining pressure causes the peak and residual strength increase significantly. With an increase in the confining pressure, the elastic modulus increases sharply at low confining pressure, the mechanical behavior transitions from brittleness to ductility, the failure mode transitions from shear-rupture surface to plastic flow, and the joint effect diminishes and even disappears. The jointed coal fails by means of a shear-rupture surface under triaxial compression loading with a confining pressure (which is not too high), and the geometries of the shear-rupture surface vary with the distribution of joints.

Crystal plasticity constitutive modeling of tensile, creep and cyclic deformation in single crystal Ni-based superalloys

A microstructure-sensitive crystal plasticity constitutive framework is proposed for simulating the tensile, cyclic, and creep response of single crystal Ni-based superalloys. In this framework, a non-Schmid model is used to account for the orientation- and temperature-dependent yield anisotropy. A model for the evolution of the slip system-level backstress is developed to simulate the cyclic response. The model accounts for the effect of microstructural features like precipitate volume fraction and size, and matrix channel width, on the associated hardening mechanisms. A physical model for creep deformation has also been developed that accounts for dislocation climb normal to the slip plane and the microstructure evolution due to rafting and isotropic coarsening of the precipitates. Application of the model is demonstrated by predicting the orientation- and temperature-dependent thermo-mechanical deformation of two single crystal superalloys, CMSX-4 and PWA-1484. The model is calibrated to the experimental tensile, cyclic and creep response for these alloys. Finally, the model is qualitatively validated by predicting the creep-fatigue interactions for different loading orientations and a range of thermo-mechanical conditions for CMSX-4 and PWA-1484.

Nucleation of superlattice intrinsic stacking faults via cross-slip in nickel-based superalloys

Superlattice intrinsic stacking faults (SISF) are the main culprit for the low temperature creep deformation of modern nickel-based superalloys used in jet engines. While these faults were identified over fifty years ago, their nucleation mechanism remains unclear. This work provides the first ever experimental evidence, via transmission electron microscopy, of a SISF nucleating from a cross-slip event in a polycrystalline alloy. Such an instance was identified in a grain with a near-〈001〉 tensile loading orientation. In the nucleation mechanism proposed, cross-slip allows the two dissimilar a2〈110〉 dislocations required to form a SISF to meet on adjacent planes at a precipitate interface. The concept of a nascent fault is introduced: the initial stacking fault that forms on a crystallographic plane and the dislocations of which continue to form coplanar faults as they glide away. This nucleation mechanism and the subsequent dislocation evolution are detailed taking into consideration the shear stresses on the individual Shockley partials and the full dislocations involved, as well as the stress orientation dependence of the energy barrier for cross-slip. These findings will guide future characterisation efforts in the field and inform the modelling of more realistic predictive models of creep behaviour.

Interface mediated deformation and fracture of an elastic–plastic bimaterial system resolved by in situ transmission scanning electron microscopy

A wide variety of today’s engineering material systems consist of multiple layered constituents to satisfy varying demands, e.g. thermal barrier- or hard coatings, thermal- or electrical conduction or insulation layers, or diffusion barriers. However, these layers are commonly only of the order of a few hundred nanometers to microns thick, which renders conventional mechanical investigation of interfacial failure quite challenging, especially if plastically deforming constituents are involved. Herein, we present an in situ study of the mechanical deformation of a WTi-Cu model interface, commonly encountered in the microelectronics industry, utilizing transmission scanning electron microscopy. This approach elucidated the interplay between plastic deformation and fracture processes when loading either perpendicular (mode I) or parallel to the interface (mode II). Under mode I purely ductile failure in the Cu phase, exhibiting dislocation slip facilitated void nucleation and coalescence, was observed with an initiation value for dislocation propagation of Jdislocation≈15 J/m2. Mode II loading exhibited nucleation and propagation of an interface crack, with the initiation value for crack extension as Jcrack≈8.8 J/m2. The results are discussed with respect to the frameworks of classical fracture mechanics and dislocation plasticity, providing fundamental insight into the failure behaviour of elastic–plastic interfaces with respect to loading orientation.

Performance-Based Fibre Design for Ultra-High Performance Concrete (UHPC)

The paper presents a method to establish a performance-based fibre design of high-strength micro steel fibres for ultra-high-performance concrete (UHPC). The performance-based fibre design considers effects of fibre layout, fibre orientation, and type of loading (quasi-static and cyclic) and expands the current approach using experiences and suitability testing results. The performance-based fibre design is based on a so-called utilization rate, which is determined via pullout tests of high-strength micro steel fibres in UHPC under quasi-static as well as high cyclic loading with varying orientations and embedment depths. The utilization rate for a straight fibre pullout is 0.27 on average considering the measured tensile strength of the fibre and 0.50 considering the manufacturers specifications. For inclined fibres, additional bending stresses occur at the exit point of the fibre channels, leading to a significant increase in local tensile stress. Therefore, the utilization rate of inclined fibres under quasi-static loading is approximately 60–70% higher than in the case of straight embedded fibres (comparing it to the measured tensile strength). Comparing the utilization rate to the manufacturer’s specification, it increases to approximately 1.00. Under cyclic loading, the additional bending stresses in inclined fibres result in a local increase of the load amplitude, leading to a reduced fatigue resistance and premature fibre rupture, underlining the feasibility of a performance-based fibre design.

Einfluss von Eiskollisionslasten am Propeller auf die Kontaktzustände in den Antriebsstranggleitlagern von Schiffen

Journal bearings in drive trains of ships in Arctic and Antarctic operations show wear and fatigue damage resulting from collision between the propeller and drift ice. The damage requires costly drivetrain maintenance in dry dock, which can be reduced by loadappropriate design and predictive maintenance measures However, this requires a reliable life time calculation of marine drive train bearings subjected to stochastic ice loads. In the following investigations, the influence of different load characteristics of a propeller-ice collision on the contact states in the drive train bearings of ships is investigated in order to be able to estimate the influence of different load characteristics in terms of the development of future life time models. For the investigations, a MBD and EHD model of the drive train of the research vessel SA Agulhas II was built. The results show that the load characteristics “load duration”, “load height” and “load orientation” and the operating parameter “oil temperature” have a high influence on the contact states in the drive train bearings and must be taken into account for future life time calculations.

Fracture analysis of healthy and osteoporotic femora using clinical CT images, phantomless densitometry, and linear FE method.

Computational fracture analysis has become a growing branch of orthopedic research. Particularly, the associated methods provide reliable tools for the analysis of 3D CT-based models of bone. This paper reports the results of such analyses for 15 human femora (healthy and osteoporotic) under different loading orientations (85 different analysis cases). A new method was developed for the calculation of the density distribution in the models from ordinary clinical CT images without calibration phantom. This method, along with a strain-energy-based linear finite element (FE) analysis scheme, was used to predict the fracture strength and pattern of 10 cadaveric femora, for which the mechanical testing results and calibrated FE models were already available. The very good agreement and consistency between different sets of results showed the reliability and accuracy of the new density calibration method, as well as the linear analysis scheme. Accordingly, the method was applied to five new clinical images, gathered from two clinics that used different scanners with different protocols. The strength and fracture pattern of each one of these specimens were analyzed under 15 different loading conditions. A consistent behavior was found for variation of the fracture load and pattern of all specimens with the loading orientations, while very clear contrasts were observed between the strength amplitudes of healthy and osteoporotic specimens. The proposed methods can be easily applied to ordinary daily (even archived) clinical CT scans to conduct fast and reliable fracture analysis of human femora for general bone research and opportunistic studies of osteoporosis and trauma.

Experimental, numerical and analytical investigations of masonry corners: Influence of the horizontal pseudo-static load orientation

The present work aims to expand the knowledge of the behaviour of masonry corners, which are capital to obtain an integral seismic response in masonry buildings. In particular, the influence of the seismic load orientation (from π/4 to π/2) is investigated experimentally, numerically and analytically. Both units and interfaces have been subjected to a material characterisation process, following which pseudo-static 1:4 scaled experiments on a tilting table have been conducted on a symmetric dry-joint masonry corner. The experimental results have also been simulated using a discrete element model. Finally, a new analytical limit analysis model has been developed, which considers both experimental and numerical observations and accounts for rocking-sliding and flexural mechanisms. In general, a good agreement is found between the three approaches, both in terms of collapse mechanism and load multiplier.

Distribution of dislocation-twin interaction in TWIP steel under different loading modes

Twinning induced plasticity (TWIP) steel with nanotwinned structure enables it to have adjustable strength and elongation. However, the dislocation reactions at the twin boundary (TB) in nanotwinned materials have obvious tension-compression asymmetry and orientation effects, and the corresponding mechanism of dislocation-TB interactions is not yet fully understood. In this study, the tension-compression asymmetry and orientation effect of dislocation-TB interactions in twinned TWIP steel is investigated using the discrete dislocation dynamics simulations and Schmid's law. The different types of dislocations dominate the TB migration in the twinned TWIP steel during tension and compression with the same loading orientation, and the asymmetric dislocation reactions at TB are attributed to the different leading partials produced after the dissociation of dislocation under different loading modes (tension or compression). Moreover, the complex dislocation-TB interactions, such as dislocations traversing TB, cross slip, dissociation, pile up and so on, are determined by considering the orientation effect of dislocation activation and the asymmetry of dislocation dissociation. The research advances the understanding of asymmetric behavior of twin and its influence on the macro hardening in plastic deformation of TWIP steels.

Effect of FDM infill patterns on mechanical properties

One of the advantage of additive manufacturing technologies is the possibility to produce hollow section products, which are lighter, cheaper, faster to create and still they can have the required mechanical properties. Most of the times a 3D printed part's most important feature is the freedom of shape, and by setting the appropriate infill we can ensure the proper resilience as well. In this paper the different infill patterns and volume related percentages are compared by using the commercial Fused Deposition Modelling (FDM) technology. For the investigation non-standardized bending test were made with two loading orientation. From the results, the relation between the mass of the product and manufacturing time can be stated clearly, the pattern and percentage, as well as the decrease of the resilience in case the use of hollow areas in products.

Strain localization analysis in materials containing randomly distributed voids: Competition between extension and shear failure modes

Strain localization is often a precursor to the ductile failure of materials. This paper investigates plastic strain localization phenomena in the case of random microstructures, namely cubic cells made of an elastic-perfectly plastic matrix embedding distribution of identical non-overlapping spherical voids. The consideration of random microstructures allows for a better representation of the interaction between voids and greater diversity of failure modes than single-void (or unit) cells. The cells are simulated by means of the finite element (FE) method for proportional stress loading paths with controlled stress triaxiality and Lode parameters. Strain localization is detected by Rice’s criterion computed at the cell level. This criterion is shown to accurately capture the onset of localization and the type of failure mode, either extension or shear banding. Moreover, the influence of the loading orientation, i.e. the orientation of the principal axes of the applied stress tensor with respect to the microstructure cube, is systematically studied. Significant anisotropy of failure behavior is observed, especially in the case of single void unit cells, which can be attributed to the intrinsic anisotropy of the simulation cells. Finally minimal failure strain values at localization with respect to all loading orientations are found. A zone of reduced ductility is observed under generalized shear loading conditions.

On the applicability of the cumulative strain energy density for notch fatigue analysis under multiaxial loading

In this paper, the cumulative strain energy density, also called fatigue toughness, was used to predict the fatigue life of notched members subjected to multiaxial loading. The relationship between the total energy absorbed to fracture and the number of cycles to failure was established from uniaxial strain-controlled fatigue tests. Multiaxial fatigue tests were performed on notched round bars made of high-strength steel under proportional bending-torsion considering different normal stress to shear stress ratios and different loading orientations. The fatigue lives predicted with the proposed model were compared to those obtained with other stress-based and energy-based models. Overall, the proposed model led to good predictions, and showed better predictive capabilities than the other tested models.

On the micromechanics of void mediated failure in HCP crystals

Three-dimensional voided cell calculations are carried out to investigate the effect of initially flat voids on the effective behavior of hexagonal crystals. The crystalline matrix is described using a set of constitutive relations that account for slip and twinning. In some materials, the limiting case of penny-shaped cracks better represents incipient void formation. Cell boundary conditions are such that internal necking of the intervoid ligament or void impingement are the only possible failure modes. The overall behavior of the cell is analyzed under various stress states and two loading orientations, with special attention given to the evolution of damage-relevant microstructural variables. Particular emphasis is placed on crystallographic aspects of void growth and coalescence, which often prove difficult to represent using phenomenological anisotropic plasticity models.