Loading Orientation Research Articles

Loading Orientation Effects on the Strength Anisotropy of Additively-Manufactured Ti-6Al-4V Alloys under Dynamic Compression

The microstructure of additively-manufactured metals depends on the direction of build, and is distinctly different from those of conventional metals. This work examines the effect of strain rate, heat treatment, and loading orientation relative to the build direction of Ti-6Al-4V samples that have been additively manufactured by direct metal laser melting to determine how the microstructure affects overall mechanical properties. The effect of rate dependence on additively manufactured Ti-6Al-4V was investigated by compressing cylinders of the material both quasi-statically in a screw-driven load frame (10−4 s−1 to 10−1 s−1), and dynamically in a split Hopkinson (Kolsky) pressure bar system (375 s−1 to 6000 s−1). The yield strength of the additively manufactured Ti-6Al-4V was observed to monotonically increase with increasing strain rates and the samples failed along a 45° direction through the thickness regardless of loading orientation. As in the case of traditionally forged metals, annealed additively manufactured Ti-6Al-4V samples exhibited lower yield strengths than their non-annealed counterparts at similar strain rates. For quasi-static loads, a clear dependence of response on loading orientation angle with respect to the material layering direction was seen, with the yield strength being greatest when loading was applied parallel to the build direction – a notable contrast to what is observed in tensile results in which the yield strength is lowest when tension is applied along the build direction. No clear relationship between the yield strength and loading orientation was observed in the dynamic tests, likely because the differences were within the measurement uncertainty of the method.

On the conversion of plastic work to heat in Mg alloy AZ31B for dislocation slip and twinning deformation

In this study the conversion of plastic work to heat of a hot-rolled magnesium alloy AZ31B is investigated using a mechanism-based approach. Shear-compression specimen loading orientations are selected to promote plastic deformation principally through either basal slip, combined pyramidal and prismatic slip, or extension twinning. Loading orientations were selected based on predictions from a crystal plasticity model. High strain-rate adiabatic deformation was accomplished using a compression Kolsky bar. During each experiment the deforming gauge section temperature rise is recorded using a single-point infra-red detector. From the specimen deformation, thermal measurements, and crystal plasticity calculations of mechanism activity, the conversion of plastic work to heat was inferred. Specimens where basal, prismatic, and pyramidal slip accounted for the majority of plastic deformation exhibited similar percentage of plastic work converted to heat. Specimens where extension twinning was highly active exhibited a comparatively lower fraction of plastic work converted to heat.

A natural energy absorbent polymer composite: The equine hoof wall

The equine hoof has been considered as an efficient energy absorption layer that protects the skeletal elements from impact when galloping. In the present study, the hierarchical structure of a fresh equine hoof wall and the energy absorption mechanisms are investigated. Tubules are found embedded in the intertubular matrix forming the hoof wall at the microscale. Both tubules and intertubular areas consist of keratin cells, in which keratin crystalline intermediate filaments (IFs) and amorphous keratin fill the cytoskeletons. Cell sizes, shapes and IF fractions are different between tubular and intertubular regions. The structural differences between tubular and intertubular areas are correlated to the mechanical behavior of this material tested in dry, fresh and fully hydrated conditions. The stiffness and hardness in the tubule areas are higher than that in the intertubular areas in the dry and fresh samples when loaded along the hoof wall; however, once the samples are fully hydrated, the intertubular areas become stiffer than the tubular areas due to higher water absorption in these regions. The compression behavior of hoof in different loading speed and directions are also examined, with the isotropy and strain-rate dependence of mechanical properties documented. In the hoof walls, mechanistically the tubules serve as a reinforcement, which act to support the entire wall and prevent catastrophic failure under compression and impact loading. Elastic buckling and cracking of the tubules are observed after compression along the hoof wall, and no shear-banding or severe cracks are found in the intertubular areas even after 60% compression, indicating the highly efficient energy absorption properties, without failure, of the hoof wall structure. Statement of SignificanceThe equine hoof wall is found to be an efficient energy absorbent natural polymer composite. Previous studies showed the microstructure and mechanical properties of the hoof wall in some perspective. However, the hierarchical structure of equine hoof wall from nano- to macro-scale as well as the energy absorption mechanisms at different strain rates and loading orientations remains unclear. The current study provides a thorough characterization of the hierarchical structure as well as the correlation between structure and mechanical behaviors. Energy dissipation mechanisms are also identified. The findings in the current research could provide inspirations on the designs of impact resistant and energy absorbent materials.

Numerical investigation of the stress concentration on 7075-T651 aluminum alloy with one or two hemispherical pits under uniaxial or biaxial loading

Numerical simulations were carried out in order to obtain the stress concentrations induced by simulated hemispherical pits with 500 µm of diameter on square plates of 100 mm2 of the aluminum alloy 7075-T651, for three different plate thickness: 0.7, 1.6 and 3 mm.The pertinence of this study is supported by the fact that modern industries using this aluminum alloy, such as the aeronautical, marine, mold tool manufacturing and arms industries, are affected by the combination of loading and corrosion, which leads to stress concentration and the failure of material. Numerical simulations were performed to investigate the effect of one and two pits on the surface of aluminum plates, for the transverse or longitudinal load orientation and under uni-axial and biaxial loading. In addition, the depth of the pits across the thickness of the plate together with the proximity of two pits under the last described conditions were investigated. It was found that the orientation of two pits in regards the applied load plays a significant role on the stress concentration under uniaxial loading. Finally, graphs for the stress concentration and the corresponding general tendencies were obtained associated with hemispherical pits and the mentioned parameters of the plates, together with the general conclusions concerning these numerical results.

The effect of crop parameters on mechanical properties of oil palm fruitlets

Oil palm fruitlets are highly susceptible to mechanical damage during harvesting and handling. In this study, the effect of crop parameters and two loading orientations on mechanical properties of oil palm fruitlets were investigated. The investigated crop parameters were four mass categories (11–11.9, 12–12.9, 13–13.9 and 14–14.9 g), two varieties (Tenera and Dura) and three ripeness stages (underripe, ripe and overripe). For impact loading, both vertical and horizontal loading orientations were studied. The investigated mechanical properties were rupture force, rupture energy, deformation at rupture and specific deformation. The mechanical properties were measured using a pendulum impact load test device. A total of 540 fruitlet samples extracted from 18 fresh fruit bunches (FFB) were employed. The results showed that the values of rupture force and energy for all mass categories under the vertical loading orientation were lower than their values under the horizontal loading orientation. The maximum rupture forces of 697.1 and 939.4 N were recorded for the highest mass category (14–14.9 g) under the vertical and horizontal loading orientations, respectively. The results also showed that the values of all mechanical properties belong to Tenera variety were higher than that of Dura variety under the both loading orientations. In terms of the ripeness stage, the rupture force and energy of oil palm fruitlets decreased as the ripeness stage increased. The maximum rupture force and energy of 940 N and 2.95 Nm were recorded for ripe stage under the horizontal loading orientation, respectively. The results from this study would be very useful in designing a proper harvesting and in-field transportation systems to minimize crop damage due to impact loading.

Finite Element Calculation of Anisotropy of Hole Expansion in a Thin Steel Sheet with Six Degrees Polynomial Yield Function

A sixth order yield function was used to analyze the anisotropic plasticity behavior of sheet metal forming. Based on a complete sixth order homogenous polynomial in plane stress, the yield function was implemented as user material subroutines in the FE code ABAQUS Explicit and Standard. The associated flow rule and isotropic hardening were assumed. Material parameter values in the yield function were decided by uniaxial yield stresses and plastic strain ratios along 7 different loading orientations and plane strain yield and equal biaxial stresses and plastic strain ratio. To show the superiority of the sixth order yield function, the hole expansion test by Kuwabara et al.[1] was considered. The results of finite element simulation using the sixth order yield function showed a better agreement with the test results than YLD2000-2D yield function with M=6.

Interplay between the effects of deformation mechanisms and dynamic recrystallization on the failure of Mg-3Al-1Zn

This work focuses on the relationship between deformation mechanisms, dynamic recrystallization (DRX), and failure mechanisms in Mg-3Al-1Zn under tension at relatively low temperatures (25–200 °C). The loading orientation was selected to favor either prismatic slip, basal slip, or extension twinning as the primary active deformation mechanism upon yielding. The tensile response was accurately simulated at various temperatures using visco-plastic self-consistent crystal plasticity modeling coupled with optimization under constraints. The relative deformation mechanism activities were predicted to gain insight into the role of microstructure evolution and DRX in failure. Extensive microstructural investigation was performed to identify characteristics of failure and categorize them according to slip, twin, and DRX activities, in an attempt to ultimately provide insights for achieving better formability at low temperatures. Failure processes were categorized according to four primary microstructural features observed during deformation prior to failure: (1) extension twinning without DRX, (2) extension twinning with DRX, (3) contraction twinning followed by double twinning, and (4) abundant DRX and high microcrack density along DRX regions. Prismatic slip was found to promote homogeneous DRX via the continuous DRX mechanism, resulting in ductile fracture. Similarly, basal slip allows for high elongation to failure but low yield strengths while promoting discontinuous DRX, whereas extension twinning contributed to quasi-brittle failure at low temperatures and minimal DRX activity. As the yield strength becomes orientation-independent at 150 °C, with increased elongation, transition from twinning-to slip-dominant microstructures, and higher DRX activity, the findings indicate that the flow anisotropy in Mg-3Al-1Zn can be mitigated beginning at 150 °C.

Investigation on anisotropy of mechanical properties of Callovo-Oxfordian claystone

Anisotropic mechanical properties of the sedimentary Callovo-Oxfordian clay rock were investigated with triaxial compression and extension, cyclic compression, and creep tests. The tested cylindrical samples differ in bedding orientation. The compression was accomplished by increasing axial stress at constant radial stress. The extension was carried out vice versa: radial stress was increased while axial stress was kept constant. Moreover, extension was also achieved by simultaneously increasing radial stress and decreasing axial stress under constant mean stress. Five independent elastic parameters were obtained at different deviatoric stresses during cyclic loading-unloading compressions. The deformation and strength were determined in relation to the load direction inclined to bedding and in relation to the load paths. The claystone exhibits elastoplastic stress-strain behaviour with volume compaction until failure. The strength is dependent on both load path and load direction to the bedding. For a given major load direction, the compressive strength is higher than the extensional one and the lowest strength occurs by extension at constant mean stress. In each load path, the maximum strength values are reached parallel and normal to the bedding, while the minimum occurs at the load orientation of about 45° during compression and of about 30° during extension. The elastic stiffness increases with deviatoric loading due to hardening. The deformability is time dependent.

Stiffness and energy storage characteristics of energy storage and return prosthetic feet.

Mechanical properties of prosthetic feet can significantly influence amputee gait, but how they vary with respect to limb loading and orientation is infrequently reported. The objective of this study is to measure stiffness and energy storage characteristics of prosthetic feet across limb loading and a range of orientations experienced in typical gait. This study included mechanical testing. Force-displacement data were collected at combinations of 15 sagittal and 5 coronal orientations and used to calculate stiffness and energy storage across prosthetic feet, stiffness categories, and heel wedge conditions. Stiffness and energy storage were highly non-linear in both the sagittal and coronal planes. Across all prosthetic feet, stiffness decreased with greater heel, forefoot, medial, and lateral orientations, while energy storage increased with forefoot, medial, and lateral loading orientations. Stiffness category was proportional to stiffness and inversely proportional to energy storage. Heel wedge effects were prosthetic foot dependent. Orientation, manufacturer, stiffness category, and heel wedge inclusion greatly influenced stiffness and energy storage characteristics. These results and an available graphical user interface tool may help improve clinical prescriptions by providing prosthetists with quantitative measures to compare prosthetic feet.

The Formation Mechanism of Nondefective Silicon Micropatterns Fabricated by Scratching Assisted Electrochemical Etching

Fabrication of nondefective silicon microstructures at specific region is of great importance but challenging, which is promising to boost the performance of microsensors, micro energy storage devices, and microfluidic devices. A low-cost approach to fabricate nondefective silicon microstructures based on scratching assisted electrochemical etching has been proposed. Shallow patterns together with silicon amorphous and defects layer were scratched by the diamond tip. The scratched area was utilized as sacrificial layer in the electrochemical etching process and removed rapidly. The introduction of electrochemical etching would change the distribution of holes in the groove area and substrate, thus the formation and morphology of the nondefective silicon structures are different. The influences of etching voltage, scratching load, etching duration, resistivity and crystal orientation of substrates on the silicon microstructures were discussed systematically. The depth, width and cross section shape of the silicon microstructures change with etching duration, voltage polarity and resistivity. But the crystal orientation has little effect on the width and depth of silicon microstructures. Nondefective silicon microstructures array with different pitch and height can be fabricated in specific location by the scratch assisted electrochemical etching approach, which can be used in high performance micro devices.

Effect of secondary crystal orientations on the deformation anisotropy for nickel-based single-crystal plate with notch feature

The effects of the secondary crystal orientations on the nickel-based single-crystal superalloy turbine blades were investigated. The stress concentration features were used for investigation of the optimal secondary crystal orientation leading to the higher strength of the single-crystal turbine blades. The crystal plastic finite element method coupled with micromechanics constitutive model is applied to study the effect of secondary crystal orientation on plastic deformation and mechanical behavior around the cooling holes and notches with the primary (load) orientation fixed at [001] direction. For nickel-based superalloy plates with holes or notches, the secondary crystal orientation effect on the strength needs to be clarified at various load levels. The maximum von Mises stress in the single-crystal alloy varies significantly with variation in the secondary crystal orientations. It was found that only two slip systems dominate the deformation process of the material owing to their favorable orientation with loading. The secondary orientation of 45° was identified with lowest resolved shear stress in the dominating slip systems and potential of producing higher strength for single-crystal turbine blades.

Low-cycle fatigue of single crystal nickel-based superalloy – mechanical testing and TEM characterisation

Low-cycle fatigue (LCF) is studied for a nickel-based single-crystal superalloy in this paper, with a focus on the effect of crystal orientation and temperature. Specifically, cyclic deformation of the alloy was compared for [001]- and [111]-oriented samples tested under strain-controlled conditions at room temperature and 825 °C. Either cyclic hardening or softening was observed during the LCF process, depending on the strain amplitude, crystallographic orientation and temperature. LCF life was also reduced significantly by changing loading orientation from [001] to [111] or increasing temperature to 825 °C. Employing a comprehensive study with transmission electron microscopy (TEM), a connection between microstructure and mechanical behaviour of the alloy is discussed. It was found that the processes of γ′-precipitate dissolution and dislocation recovery were responsible for cyclic softening. Alignments and pile-ups of dislocations in the γ matrix, which prohibited their movement and reduced the interaction of dislocations on different slip systems, contributed to cyclic hardening.

Flatness‐based control of a rigid body carried by multiple heavy ropes

AbstractFlatness‐based trajectory planning for the distributed‐parameter system consisting of a rigid body carried by multiple heavy ropes is discussed. The suspension points (controlled ends) of the ropes are assumed to be freely movable. The other ends of the ropes are parametrized by a flat output of the nonlinear boundary system at the load that describes the motion of the rigid body. This parametrization is then used to express the solution of the linearized partial differential equations for the heavy ropes under small angle approximations around a vertical equilibrium configuration by the means of operational calculus. It is shown that the system trajectories of the ropes depend on distributed delays and advances of the flat output trajectory at the load‐side boundary system. Evaluating the derived parametrization of the heavy ropes at the controllable ends yields the desired open‐loop controller by using a planned trajectory for the flat output. This control can be used to realize a desired transition of the load position and orientation in finite time. The introduced method is illustrated for the example of a linear infinite‐dimensional model of two heavy ropes carrying a rigid rod.

Coupling effect of twin boundary and void on the mechanical properties of bulk nanotwinned copper: an atomistic simulation

Molecular dynamics simulations are performed to analyze the uniaxial tensile deformation behaviors of nanotwinned copper bulk containing pre-existing multiple spherical/ellipsoidal voids. The effects of twin-boundary spacing, initial void size, void fraction, void shape, void orientation angle and loading orientation on the mechanical properties are examined by considering the stress-strain response and dislocation activity. Results illuminate that the coherent twin boundary can significantly enhance the strength of bulk copper. The critical stress for dislocation emission in the copper specimen depends mainly on the size of voids, which can influence the yield stress of nanotwinned copper. Less stress is required to emit dislocations from larger voids than smaller ones. In general, the larger initial void porosity is accompanied by smaller elastic modulus and critical stress. Furthermore, it is found that different dislocation patterns can be triggered under different loading orientations.

The influence of impacting orientations on the failure modes of cracked tunnel

Underground tunnels are often subjected to dynamic loadings, and the dynamic loadings may come from various directions. To study the influence of loading orientations on the failure modes of cracked tunnels, laboratory impact experiments were conducted by a drop weight testing apparatus with synchronized measuring system. Green sandstone was selected as the experimental material to fabricate the tunnel samples, a pre-crack emanating from tunnel roof was designed to simulate the natural crack around a tunnel. In the process of dynamic loading tests, strain gauges were employed to determine crack time-to-fracture and crack velocity. Numerical simulation was carried out by the finite element software ABAQUS and the finite difference software AUTODYN. Crack initiation fracture toughness (IFT) was evaluated by experimental-numerical technique and the failure behavior of tunnel model under different orientation impacts was presented in this paper. The studying results show that as impacts come from tunnel roof, tunnel failure mainly occurs near the crack tip and the tunnel bottom (or the tunnel foot); as impact come from tunnel sidewall, the tunnel failure only occurs at the two sides of arch shoulder. The dynamic initiation fracture toughness of the pre-crack in the tunnel models is greatly related to the impact loading orientations.

Suspended load path tracking control using a tilt-rotor UAV based on zonotopic state estimation

This work addresses the problem of path tracking control of a suspended load using a tilt-rotor UAV. The main challenge in controlling this kind of system arises from the dynamic behavior imposed by the load, which is usually coupled to the UAV by means of a rope, adding unactuated degrees of freedom to the whole system. Furthermore, to perform the load transportation it is often needed the knowledge of the load position to accomplish the task. Since available sensors are commonly embedded in the mobile platform, information on the load position may not be directly available. To solve this problem in this work, initially, the kinematics of the multi-body mechanical system are formulated from the load’s perspective, from which a detailed dynamic model is derived using the Euler–Lagrange approach, yielding a highly coupled, nonlinear state-space representation of the system, affine in the inputs, with the load’s position and orientation directly represented by state variables. A zonotopic state estimator is proposed to solve the problem of estimating the load position and orientation, which is formulated based on sensors located at the aircraft, with different sampling times, and unknown-but-bounded measurement noise. To solve the path tracking problem, a discrete-time mixed H2/H∞ controller with pole-placement constraints is designed with guaranteed time-response properties and robust to unmodeled dynamics, parametric uncertainties, and external disturbances. Results from numerical experiments, performed in a platform based on the Gazebo simulator and on a Computer Aided Design (CAD) model of the system, are presented to corroborate the performance of the zonotopic state estimator along with the designed controller.

A Muscle-Specific Rehabilitation Training Method Based on Muscle Activation and the Optimal Load Orientation Concept.

Training based on muscle-oriented repetitive movements has been shown to be beneficial for the improvement of movement abilities in human limbs in relation to fitness, athletic training, and rehabilitation training. In this paper, a muscle-specific rehabilitation training method based on the optimal load orientation concept (OLOC) was proposed for patients whose motor neurons are injured, but whose muscles and tendons are intact, to implement high-efficiency resistance training for the shoulder muscles, which is one of the most complex joints in the human body. A three-dimensional musculoskeletal model of the human shoulder was used to predict muscle forces experienced during shoulder movements, in which muscles that contributed to shoulder motion were divided into 31 muscle bundles, and the Hill model was used to characterize the force-length properties of the muscle. According to the musculoskeletal model, muscle activation was calculated to represent the muscle force. Thus, training based on OLOC was proposed by maximizing the activation of a specific muscle under each posture of the training process. The analysis indicated that the muscle-specific rehabilitation training method based on the OLOC significantly improved the training efficiency for specific muscles. The method could also be used for trajectory planning, load magnitude planning, and evaluation of training effects.

Strengthening mechanisms in directed energy deposited austenitic stainless steel

Microstructures and mechanical properties are evaluated in austenitic stainless steel structures fabricated by directed energy deposition (DED) considering the effects of applied loading orientation, build geometry, and distance from the deposition baseplate. Locations within an as-deposited build with different thermomechanical history display different yield strength, while those locations with similar history have approximately the same yield strength, regardless of test specimen orientation. Thermal expansion of deposited material near the baseplate is inhibited by the mechanical constraint imposed by the baseplate, promoting plastic deformation and producing a high density of dislocations. Concurrently, high initial cooling rates decrease away from the baseplate as the build is heated, causing an increased spacing of cellular solidification features. An analysis of strengthening mechanisms quantitatively established for the first time the important strengthening contribution of high dislocation densities in the materials (166–191 MPa) to yield strength that ranged from 438 to 553 MPa in the present DED fabricated structures. A newly adopted mechanistic relationship for microsegregation strengthening from the literature indicated an additional important contribution to strengthening (123–135 MPa) due to the cellular solidification features. These findings are corroborated by the measured evolution of microstructure and hardness caused by annealing the DED material. These results suggest that the mechanical properties of deposited austenitic stainless steels can be influenced by controlling thermomechanical history during the manufacturing process to alter the character of compositional microsegregation and the amount of induced plastic deformation.

Effects of seating load magnitude and load orientation on seating mechanics in 5°40′ mixed-alloy modular taper junctions

BackgroundMechanically-assisted crevice corrosion of modular tapers continues to be a concern in total joint replacements. Surgical factors that may affect taper seating mechanics include seating load magnitude and load orientation. Seating mechanics is defined as the seating load versus displacement behavior. In this study, mixed-alloy (CoCrMo/Ti-6Al-4V) modular head-neck 5°40′ taper junctions were seated over a range of axially-oriented loads and off-axis orientations, capturing load-displacement during seating. The goals of the study were to assess the effects of seating load magnitude and load orientation on seating mechanics and correlate those findings with the taper pull-off load. MethodsA testing fixture measured head-neck seating displacement as the load was quasistatically applied. Motion was captured using two non-contact differential variable reluctance transducers which were mounted to the neck targeting the head. Seating experiments ranged from 1000 N to 8000 N. Load orientation ranged from 0° to 20° at 4000 N. ResultsSeating load-displacement behavior at different seating loads showed a consistent characteristic behavior. Testing demonstrated increased seating displacement with seating load. Pull-off loads increased with seating load and were approximately 44% of the seating load across the range of seating loads investigated. Seating load orientation up to 20° had no significant effect on seating displacement and taper pull-off load. ConclusionIncreased seating load magnitude increased seating displacement, work of seating and pull-off loads in mixed-alloy 5°40′ head-neck tapers. Altering load orientation up to 20° off-axis had no significant effect. Direct measurements of seating mechanics provides insights into the locking of taper junctions.

Designed for resistance to puncture: The dynamic response of fish scales

Natural dermal armors are serving as a source of inspiration in the pursuit of “next-generation” structural materials. Although the dynamic strain response of these materials is arguably the most relevant to their performance as armors, limited work has been performed in this area. Here, uniaxial tension and transverse puncture tests were performed on specimens obtained from the scales of Asian carp over strain rates spanning seven decades, from 10−4 to 103 s−1. The importance of anatomical variations was explored by comparing the performance of scales from the head, middle and tail regions. In both loading orientations, the scales exhibited a significant increase in the resistance to failure with loading rate. The rate sensitivity was substantially higher for transverse loading than for in-plane tension, with average strain rate sensitivity exponents for measures of the toughness of 0.35 and 0.08, respectively. Spatial variations in the properties were largest in the puncture responses, and scales from the head region exhibited the greatest resistance to puncture overall. The results suggest that the layered microstructure of fish scales is most effective at resisting puncture, rather than in-plane tension, and its effectiveness increases with rate of loading. X-ray microCT showed that delamination of plies in the internal elasmodine and stretching of the fibrils were key mechanisms of energy dissipation in response to puncture loading. Understanding contributions from the microstructure to this behavior could guide the development of flexible engineered laminates for penetration resistance and other related applications.