Internal Load Transfer Research Articles

Synchrotron X-ray diffraction studies of the internal load transfer in Ni–CrC metal matrix composites

Strengthening in metal matrix composites (MMCs) is primarily due to the load transfer from the compliant matrix to the stiff reinforcement. While internal load transfer has been studied for conventionally manufactured MMCs, the extent to which it may be affected by the preexisting defects in additively manufactured MMCs remain elusive. In this study, we performed uniaxial compression loading on cold sprayed Ni–CrC particulate-reinforced MMC. We observe significantly enhanced strength and ductility compared to the brittle behavior previously reported in tension for the same MMC. In contrast to the absence of load transfer in tension, the presence of initial defects did not preclude the internal load transfer in the composites under compressive loading. In-situ high-energy X-ray diffraction analysis revealed a relatively constant load partitioning in the elastic regime, which aligned well with predictions by the Eshelby's inclusion model. Furthermore, an internal load transfer was observed from the Ni matrix to the CrC reinforcement upon plastic deformation of the Ni matrix. Finite element modeling further confirmed this and demonstrated that localized tensile stress at the interface in the transverse direction resulted in the interfacial debonding and partial relaxation of the matrix. We also report reinforcing particle fracture upon further compression which in turn triggered the eventual failure of the MMC.

Chirality in topologically interlocked material systems

The present study focuses on the mechanical chirality in plate-type topologically interlocked material systems. Topologically interlocked material (TIM) systems are a class of dense architectured materials for which the mechanical response emerges from the elastic behavior of the building blocks and the contact-frictions interactions between the blocks. The resulting mechanical behavior is strongly non-linear due to the stability-instability characteristics of the internal load transfer pattern. Two tessellations are considered (square and hexagonal) and patches from each are used as templates. While individual building blocks are achiral, chirality emerges from the assembly pattern. The measure of microstructure circulation is introduced to identify the geometric chirality of TIM systems. TIM systems identified as geometrically chiral are demonstrated to possess mechanical chiral response with a force-torque coupling under transverse mechanical loading of the TIM plate. The chiral length is found to be constant during the elastic response, yet size-dependent. During nonlinear deformation, the chiral length scale increases significantly and again exhibits a strong size dependence. The principle of dissection is introduced to transform non-chiral TIM systems into chiral ones. In the linear deformation regime, the framework of chiral elasticity is shown to be applicable. In the non-linear deformation regime, chirality is found to strongly affect the mechanical behavior more significantly than in the linear regime. Experiments on selected TIM systems validate key findings of the main computational study with the finite element method.

Review on Load Transfer Mechanisms of Asphalt Mixture Meso-Structure.

Asphalt mixture is a skeleton filling system consisting of aggregate and asphalt binder. Its performance is directly affected by the internal load transfer mechanism of the skeleton filling system. It is significant to understand the load transfer mechanisms for asphalt mixture design and performance evaluation. The objective of this paper is to review the research progress of the asphalt mixture load transfer mechanism. Firstly, this paper summarizes the test methods used to investigate the load transfer mechanism of asphalt mixtures. Then, an overview of the characterization of load transfer mechanism from three aspects was provided. Next, the indicators capturing contact characteristics, contact force characteristics, and force chain characteristics were compared. Finally, the load transfer mechanism of asphalt mixtures under different loading conditions was discussed. Some recommendations and conclusions in terms of load transfer mechanism characterization and evaluation were given. The related work can provide valuable references for the study of the load transfer mechanism of asphalt mixtures.

Load Transfer Behavior and Failure Mechanism of Bird’s Nest Anchor Cable Anchoring Structure

To research the internal load transfer behavior and failure mechanism of a bird’s nest anchor cable anchoring structure based on a pull-out test, a bond-slip failure model is established on the basis of statistical damage theory, and the distribution formula of shear stress at anchorage agent–rock interface is deduced. Combined with theoretical analysis, bird’s nest anchor cable pulling out test and particle flow code (PFC) numerical simulation test, as well as axial force distribution of the cable and shear stress distribution of its interface, help reveal its load transfer behavior and failure mechanism. Results show that: (1) The established bond-slip model can reflect the internal load transfer behavior and failure process of bird’s nest anchor cable anchorage structure. (2) The shear stress of the anchorage agent interface increases exponentially to the peak value and then decreases exponentially to the residual strength. The process is repeated at every location of the anchorage agent interface. The curve of the axial force and shear stress of the bird’s nest anchor cable is a negative exponential distribution with anchorage depth, and the maximum value occurs at the load end. (3) The crack of the anchorage agent interface extends from the load end to the other end and finally cuts through the whole interface. Rock mass generates radial cracks by the split effects of the bird’s nest. The failure mode is a combination of the debonding slip of the interface and the shear failure of the rock mass. The shear stress distribution and failure mode of the anchor structure are basically consistent according to laboratory tests and simulation tests, and PFC2D better reflects the internal load transfer behavior, failure mechanism, and failure process of the bird’s nest anchor cable under tensile loads.

Gender Differences in Cervical Spine Motions and Loads With Head Supported Mass Using Finite Element Models.

While many studies have been conducted to delineate the role of gender in rear impact via experiments, clinical investigations, modeling, and epidemiological research, the effect of the added head mass on segmental motions has received less attention. The objective of the study is to determine the role of the head supported mass on the segmental motions and loads on the cervical spinal column from rear impact loading. The study used finite element modeling. The model was subjected to mesh convergence studies. It was validated with human cadaver experimental data by applying the rear impact acceleration pulse to the base of the spine. At all levels of the subaxial spinal column, a comparison was made between male and female spines and with and without the use of an army combat helmet. For this purpose, segmental motions, forces, and bending moments were used as biomechanical parameters. Results showed that female spines responded with increased motions than males, and the presence of a helmet increased motions and loads in males and female spines at all levels. Numerical data are given. Head supported mass affects spine responses at all levels. The present computational modeling study, from one geometry for the male spine and one geometry for the female spine (limitations are addressed in the paper), provided insights into the mechanisms of the internal load transfer with the presence of head supported mass, prevalent in certain civilian occupations and active-duty Service members in the military.

Load transfer mechanism of precast concrete piers with demountable connections

To investigate the replacement or rehabilitation of precast concrete piers, a novel demountable steel-concrete composite connection with notched perfobond connectors was proposed for the pier-pile cap joint. In the proposed connection, a bolted mechanical connection is used to connect the longitudinal reinforcing bars and steel plate, which is bolted to the pile cap through pre-stressed anchor bolts. Pseudo-static capacity tests of two half-scale specimens were conducted with different axial compression ratios, and the experimental results were verified by nonlinear finite element analysis. The lateral force-displacement curves, failure modes of connection joint, stress of steel plates and connector-concrete slip were compared and discussed. Furthermore, the internal load transfer mechanism and characteristics were revealed. The results indicate that the precast pier with the proposed connection realizes the rapid assembly and demountability. Specimens present flexural failure, and damage appears at the bottom of the pier columns but above the steel jacket. The connection steel plates and bolted connection remain intact as expected. The stiffness, capacity and failure process in numerical analysis agree well with the experimental results. The notched perfobond connectors set on the base plates effectively transfer the force from the concrete column to the base plate, and enhance the base plate-concrete connection and bending resistance of the base plate.

Load redistribution in eutectic high entropy alloy AlCoCrFeNi[formula omitted] during high temperature deformation

The load redistribution between and within phases in eutectic high entropy alloy AlCoCrFeNi2.1 was measured using in-situ neutron diffraction during tensile deformation at 973 K. The load partitioning between phases is reversed compared to lower temperatures, with L12 becoming the stronger phase. The evolution of the orientation-specific stresses and strains in the L12 phase suggests that cube slip dominates the response. The low strength, internal load transfer and ideally plastic response of the B2 phase indicate a change in deformation mechanism compared to lower temperatures.

Mechanics of tubes composed of interlocking building blocks

Topologically interlocking material (TIM) systems are composed of convex polyhedral units placed such that building blocks restrict each other’s movement. Here, TIM tubes are considered as rolled monolayers of such assemblies. The deformation response of these assembled tubes under diametrical loading is considered. This investigation employs experiments on additively manufactured physical realizations and finite element analysis with contact interactions. The internal load transfer in topologically interlocking tubes is rationalized through inspection of the distribution of minimum principal stress. A thrust-line (TL) model for the deformation of topologically interlocking tubes is established. The model approximates the deformation behavior of the assembled tubes as the response of a collection of Mises trusses aligned with paths of maximum load transfer in the system. The predictions obtained with the TL-model are in good agreement with results of finite element models. Accounting for sliding between building blocks in the TL-model yields a predicted response more similar to experimental results with additively manufactured tubes.

Multiscale modeling of load transfer characteristics in crosslinked epoxy nanocomposites

A multiscale modeling approach is proposed to account for the interfacial load transfer characteristics of epoxy nanocomposites. The localized stress evolutions in nanocomposites are examined with molecular dynamics (MD) simulations: the particle phase stress and the regional stress evolution in matrix medium. Plus, detailed atomic motions in the matrix phase are thoroughly studied with respect to the crosslink conversion. The distinct structural features in a highly crosslinked system induce a cohesive motion under the mechanical loading, thereby playing a favorable role in the internal load transfer from the matrix medium to the filler region. Further, interfacial load transfers with the curing conversion of epoxies are taken into account in the equivalent FE model with the weakened interface concept

A New Method for Compaction Quality Evaluation of Asphalt Mixtures with the Intelligent Aggregate (IA).

To provide a new method for the evaluation of the compaction quality of asphalt mixture, a real-time data acquisition and processing system (RDAPS) for the motion state of aggregate with a small volume and high precision is developed. The system consists of an intelligent aggregate (IA), analysis software and hardware equipment. The performance of the IA was tested by regarding data sensitivity, high-temperature resistance, and mechanical properties. A new evaluation method was proposed for evaluating the compaction quality of AC-25 and SMA-25 asphalt mixtures based on an IA. The results show that the best transmission baud rate for the IA was 9600 bps, and the corresponding signal transmission distance was 380 m. Only one IA was needed to complete the state data collection for the aggregate within the asphalt mixture in a circular area, with the IA layout point as the center of the circle and a radius of 5 m. The IA conducted reliable data transmission up to 200 °C; however, its compressive strength decreased with increasing temperature until reaching stability. Traditional aggregate could be replaced by an IA to withstand external forces and internal load transfer. Embedding an IA into AC-25 or SMA-25 asphalt mixtures did not have a significant impact on the original mechanical properties of the mixture. The effect of the gradation type of the asphalt mixture on the IA motion state was not significant. When the compaction degree met the specification requirements, the motion data of the IA did not reach a stable state, and the interlocking effect between aggregates in the asphalt mixture could be further optimized. An evaluation method is proposed based on the IA for the compaction quality of AC-25 and SMA-25 asphalt mixtures with the compaction degree as the main index and the spatial attitude angle and spatial acceleration of the IA as the auxiliary indexes.

Improved strength and toughness of bioinspired Bouligand architecture composite by discontinuous carbon fiber

Strength and toughness are a pair of incompatible contradictions for most structure materials, and exploring the combined optimization strategy of strength and toughness of materials is a frontier research topic in the field of lightweight structure materials. Here, inspired by natural organisms, a novel hierarchical structure design scheme of the lightweight material that combines the helical layered Bouligand architecture inside the mantis shrimp rods with the “brick-mortar” staggered laminated structure of shell nacre was proposed. The deformation failure and energy absorption characteristics of this composite were compared with those of a single helical laminated structure material under bending load. It shows that, in the process of deformation and failure, the samples with both “brick-mortar” staggered laminated structure and helical laminated structure can improve their internal stress distribution and load transfer efficiency, alleviate local stress concentration and delay material failure, so as to realize the optimization of material strength and toughness. These research results have positive guiding significance for the internal structure design and performance optimization of lightweight and high-strength/toughness materials.

Dry Spinning of Continuous Graphene Oxide/TPU Composite Fibers with Excellent Strength and Toughness

Graphene fiber has broad application prospects in aerospace, biomedical and other fields because of its excellent mechanical properties. In this paper, graphene oxide (GO) and thermoplastic polyurethane (TPU) composite fibers have been prepared by two comparable methods including wet spinning and dry spinning. The fibers obtained by dry spinning show better strength and toughness compared with wet spinning. Further research results show that the dry spinning process can effectively control the internal microstructure of the fiber, improve the internal stress distribution and load transfer efficiency, then realize the optimization of its macro mechanical properties. This study will provide useful guidance for the optimization strategy of internal microstructure and the improvement of preparation process of graphene fiber.

Human Thoracolumbar Spine Tolerance to Injury and Mechanisms From Caudo-Cephalad Loading: A Parametric Modeling Study.

The aims of this investigation were to delineate the internal biomechanics of the spine under vertical impact vector and assess the probability of injury. Male and female whole-body human finite element models were used. The restrained occupants were positioned on the seat, and caudo-cephalad impacts were applied to the base. Different acceleration-time profiles (50-200 ms pulse durations, 11-46 g peak accelerations) were used as inputs in both models. The resulting stress-strain profiles in the cortical and cancellous bones were evaluated at different vertebral levels. Using the peak transmitted forces at the thoracolumbar disc level as the response variable, the probability of injury for the male spine was obtained from experimental risk curves for the various pulses. Results showed that the shorter pulse durations and rise times impart greater loading on the thoracolumbar spine. The analysis of von Mises stress and strain distributions showed that the compression-related fractures are multifaceted with contributions from both the cortical and cancellous bony components of the body. Profiles are provided in the paper. The intervertebral disc may be involved in the fracture mechanism, because it acts as a medium of load transfer between adjacent vertebrae. Injury risks for the shortest pulse was 63%, and for the widest pulse it was close to zero, and injury probabilities for other pulses are given. The present modeling study provides insights into the mechanisms of internal load transfer and describes injury risk levels from caudal to cephalad impacts.

Reinforcement of bio-apatite by zinc substitution in the incisor tooth of a prawn

Various material-strengthening strategies have evolved in the cuticle and the feeding tools of arthropods. Of particular interest is the crustacean mandible, which is frequently reinforced with calcium phosphate, giving a minerology similar to that of human bones and teeth. We report here a biological strengthening method of apatite by Zn substitution, found in the incisor teeth of the freshwater prawn Macrobrachium rosenbergii. Nanoindentation measurements show a clear positive correlation between the Zn/Ca ratio and the stiffness and hardness of the composite. In the incisor, Zn-substituted apatite forms an internal vertical axis, extending from the sharp outer edges of the tooth to its basal segment. The substitution level in this zone (up to 40%) is very high compared with the levels achieved in synthetic ceramics (<20%). Finite element simulation suggests that the high-Zn axis acts as a unique internal load transfer element, directing stress from the biting cusps to the more compliant underlying layers. In light of the considerable research being invested in developing synthetic calcium phosphate derivatives for human bone and tooth grafts, the innovative mineralogy of the M. rosenbergii incisor may inspire beneficial biomimetic applications. Statement of SignificanceThe controlled incorporation of impurities into biominerals is a widespread phenomenon in biomineralization that may pave the way to new classes of biomimetic materials. The present study reveals a biogenic mineral of zinc-substituted apatite found in the incisor teeth of a prawn. A clear correlation between zinc substitution level and stiffness and hardness, suggests that zinc substitution serves to mechanically reinforce the bioapatite. The spatial arrangement of the high-zinc apatite unveils a material-level adaptation strategy for tooth fortification, in which the rigid high-Zn structure servs as an internal load-transfer element that transmits the stress directly from the tooth's sharp cusps to the more compliant underlying layers.

In-situ synchrotron high energy X-ray diffraction study of micro-mechanical behaviour of R phase reorientation in nanocrystalline NiTi alloy

This study investigated the mechanisms and temperature dependence of the R phase variant reorientation deformation in a nanocrystalline NiTi wire by means of in-situ synchrotron high-energy X-ray diffraction technique during tensile deformation. The NiTi wire exhibited a single stage B2 → R transformation upon cooling with the R → B19′ martensitic transformation completely suppressed down to the liquid nitrogen temperature. The R phase variant reorientation deformation in tension was revealed to proceed in a Lüders-manner, as evidenced by sudden changes of diffraction peak intensity and increases of lattice elastic strains of the R phase during the deformation. The variant reorientation obeys the principles to yield maximum crystallographic strain in the direction of the applied load. Thus, the variants with larger d-spacing values aligned to the axis of tension grew at the expense of those of lower d-spacing values. The increases of lattice elastic strains are attributed to the internal load transfer among the different crystallographic variants during the R phase reorientation. The crystallographic strain of R phase reorientation was found to increase with decreasing the deformation temperature. This corresponds directly to the continued evolution of the structure of the R phase of increased structural distortion relative the B2 phase with decreasing the temperature. This reflects the second order nature of the R phase.

Scaling, Growth, and Size Effects on the Mechanical Behavior of a Topologically Interlocking Material Based on Tetrahedra Elements

AbstractThe present study is concerned with the deformation response of an architectured material system, i.e., a 2D-material system created by the topological interlocking assembly of polyhedra. Following the analogy of granular crystals, the internal load transfer is considered along well-defined force networks, and internal equivalent truss structures are used to describe the deformation response. Closed-form relationships for stiffness, strength, and toughness of the topologically interlocked material system are presented. The model is validated relative to direct numerical simulation results. The topologically interlocked material system characteristics are compared with those of monolithic plates. The architectured material system outperforms equivalent size monolithic plates in terms of toughness for nearly all possible ratios of modulus to the strength of the material used to make the building blocks and plate, respectively. In addition, topologically interlocked material systems are shown to provide better strength characteristics than a monolithic system for low strength solids.

Internal load transfer in an interpenetrating metal/ceramic composite material studied using energy dispersive synchrotron X-ray diffraction

The mechanism of internal load transfer in a newly developed interpenetrating metal/ceramic composite is studied for the first time in this work using energy dispersive synchrotron X-ray diffraction. The composite was fabricated by infiltration of Al12Si melt in an open porous alumina preform, fabricated using a mixture of two different polymer waxes as pore formers, by gas pressure infiltration. Several diffraction planes of all three crystalline phases of the composite – alumina, silicon and aluminum solid solution are investigated. Lattice strains, both along and transverse to the loading direction, in each phase are calculated from the shifts in the measured diffraction peak positions. Results show that until about 100 MPa applied compressive stress, all three phases undergo elastic deformation. At higher applied stresses aluminum undergoes plastic deformation and correspondingly load is transferred to alumina. The load carrying capability of alumina is however limited by incipient damage in this phase. At applied compressive stresses higher than 187 MPa, load carried by the alumina phase continuously decreases and correspondingly more load is again carried by the aluminum solid solution. Throughout, the fraction of load carried by silicon remains almost constant, suggesting no significant load transfer within the metallic alloy itself.

Mechanisms of Cervical Spine Disc Injury under Cyclic Loading.

Study DesignDetermination of human cervical spine disc response under cyclic loading.PurposeTo explain the potential mechanisms of intervertebral disc injury caused by cyclic loading.Overview of LiteratureCertain occupational environments in civilian and military populations may affect the cervical spine of individuals by cyclic loading. Research on this mechanism is scarce.MethodsHere, we developed a finite element model of the human C4–C5 disc. It comprised endplates, five layers of fibers, a nucleus, and an annulus ground substance. The endplates, ground substance, and annular fibers were modeled with elastic, hyperviscoelastic, and hyper-elastic materials, respectively. We subjected the disc to compressive loading (150 N) for 10,000 cycles at frequencies of 2 Hz (low) and 4 Hz (high). We measured disc displacements over the entire loading period. We obtained maximum and minimum principal stress and strain and von Mises stress distributions at both frequencies for all components. Further, we used contours to infer potential mechanisms of internal load transfer within the disc components.ResultsThe points of the model disc displacement versus the loading cycles were within the experimental corridors for both frequencies. The principal stresses were higher in the ground matrix, maximum stress was higher in the anterior and posterior annular regions, and minimum stress was higher along the superior and inferior peripheries. The maximum principal strains were radially directed, whereas the minimum principal strains were axially/obliquely directed. The stresses in the fibers were greater and concentrated in the posterolateral regions in the innermost layer.ConclusionsDisc displacement was lower at high frequency, thus exhibiting strain rate stiffening and explaining stress accumulation at superior and interior peripheries. Greater stresses and strains at the boundaries explain disc injuries, such as delamination. The greater development of stresses in the innermost annular fiber layer (migrating toward the posterolateral regions) explains disc prolapse.

Flatfoot and normal foot a comparative analysis of the stress shielding

Objectivethis study aims to develop a comprehensive 3D FE model of the foot to investigate the effect of soft tissue stiffness on the plantar pressure distributions and the internal load transfer between bony structures. Methodsthe stress shielding occurring on the plantar surface of a flatfoot was investigated and compared with the mechanical behavior of a healthy foot, trough baropodometric analyses and the FE models. Resultsthe flatfoot evidences a more intensive stress-shielding map with significant values of pressure acting on the medial plantar fascia. ConclusionClinically and radiographically, symptomatic adult flatfoot is a complex abnormality involving all three dimensions and multiple joints within the foot.

Finite element simulation on posterior tibial tendinopathy: Load transfer alteration and implications to the onset of pes planus

BackgroundPosterior tibial tendinopathy is a challenging foot condition resulting in pes planus, which is difficult to diagnose in the early stage. Prior to the deformity, abnormal internal load transfer and soft tissue attenuation are anticipated. The objective of this study was to investigate the internal load transfer and strain of the ligaments with posterior tibial tendinopathy, and the implications to pes planus and other deformities. MethodsA three-dimensional finite element model of the foot and ankle was reconstructed from magnetic resonance images of a 28-year-old normal female. Thirty bones, plantar fascia, ligaments and tendons were reconstructed. With the gait analysis data of the model subject, walking stance was simulated. The onset of posterior tibial tendinopathy was resembled by unloading the tibialis posterior and compared to the normal condition. FindingsThe load transfer of the joints at the proximal medial column was weaken by posterior tibial tendinopathy, which was compromised by the increase along the lateral column and the intercuneiforms during late stance. Besides, the plantar tarsometatarsal and cuboideonavicular ligaments were consistently over-stretched during stance. Particularly, the maximum tensile strain of the plantar tarsometatarsal ligament was about 3-fold higher than normal at initial push-off. InterpretationPosterior tibial tendinopathy altered load transfer of the medial column and unbalanced the load between the proximal and distal side of the medial longitudinal arch. Posterior tibial tendinopathy also stretched the midfoot plantar ligaments that jeopardized midfoot stability, and attenuated the transverse arch. All these factors potentially contributed to the progress of pes planus and other foot deformities.