Failure In Tension Research Articles

MODELACIÓN NO LINEAL DE MUROS DE MAMPOSTERÍA EMPLEANDO ELEMENTOS DE CONTACTO

Se presenta la modelación de muros de mampostería simple y confinada sometidos a carga axial y lateral con el método de los elementos finitos para obtener la envolvente de resistencia a corte. Se utilizan elementos de contacto en las juntas para tomar en cuenta el comportamiento no lineal en la mampostería. Se tomaron en cuenta los dos tipos de falla más comunes: la falla en tensión (Modo I) y la falla a cortante (Modo II); en ambos casos se consideró el ablandamiento. En el rango elástico, se corroboró la consistencia de los resultados obtenidos con el modelo de elementos de contacto con los producidos con un modelo detallado y un modelo homogeneizado. En el rango no lineal, se utilizó un modelo trilineal de la superficie de fluencia que no incluye una reducción de la resistencia por el esfuerzo normal, a diferencia del esquema propuesto por otros autores que si la toman en cuenta. El efecto de esta consideración se estudió mediante la comparación de los resultados obtenidos con el modelo propuesto y los resultados numéricos y experimentales reportados en la literatura. Se comentan las principales diferencias. Finalmente, se modela un muro confinado ensayado en el laboratorio para comparar la envolvente de resistencia a corte experimental con la obtenida numéricamente. Los resultados obtenidos son bastante similares.

Evaluation of Solid-Solution Hardening of Fe-27 at. pct Al by Vanadium and Comparison to Precipitation Strengthening by Vanadium Carbides

The compressive yield stress of Fe-27Al-xV(-C) (x = 0 to 4 at. pct) at 1073 K (800 °C) has been determined. The increase of the yield stress of Fe-Al by increasing vanadium content is explained by solid-solution hardening. The experimentally observed values of the yield stress at 1073 K (800 °C) are compared with the strengthening given by theories evaluating the interaction between solute atoms and dislocations. The experimental results fit well the increase of the yield stress by the interaction of the solute atoms with screw dislocations. Further increase in yield strength in similar alloys due to vanadium carbides is documented. Precipitated carbides were identified by transmission electron microscopy and Kikuchi patterns. Although precipitation of vanadium carbides increases the compressive yield stress, they also could result in premature failure in tension as their highly anisotropic shape may facilitate crack nucleation.

Effect of PFM Firing Cycles on the Mechanical Properties, Phase Composition, and Microstructure of Nickel‐Chromium Alloy

The purpose of this study was to compare the mechanical properties of beryllium-free nickel-chromium (Ni-Cr) dental casting alloy before and after each porcelain firing cycle (once fired, twice fired, and thrice fired) and to relate these properties to the microstructural changes and changes in X-ray diffraction patterns of Ni-Cr alloy that occur after each porcelain firing cycle. Forty tensile bar specimens and 20 disc-shaped specimens of Ni-Cr alloy were prepared. These specimens were divided into four groups. The first group was not heat treated and tested in the as-cast condition, thus serving as control group. The second, third, and fourth groups were fired once, twice, and thrice, respectively. Tensile bar specimens were loaded to failure in tension using a universal testing machine. Values of ultimate tensile strength, 0.1% yield strength, and percentage elongations were determined. Microstructural study and hardness testing were done using an optical microscope and digital Vickers hardness tester, respectively, on disc-shaped specimens. Disc-shaped specimens were again used to obtain the X-ray diffraction patterns by using diffractometer Bruker D8 focus. Statistical comparisons of the mechanical properties and hardness of the alloy were made with ANOVA. Intergroup comparisons of the data in the as-cast and fired specimens were analyzed by applying Tukey's HSD multiple comparison tests. Before porcelain firing, the alloy exhibited higher ultimate tensile strength (548 MPa), 0.1% yield strength (327 MPa), hardness (192 HV), and lower elongation values (18%). After each firing cycle, there was a significant (p < 0.001) decrease in ultimate strength (464 MPa for three times fired specimens), 0.1% yield strength (284 MPa for three times fired group), and hardness (164 HV for three times fired group) and significant (p < 0.001) increase in elongation value (28% for three times fired group) of Ni-Cr alloy. The microstructure of the control group specimen exhibited heterogeneous microstructure, and after each firing, microstructure of the alloy was gradually homogenized by formation of grain boundaries at the interdendritic interfaces. X-ray diffraction pattern shows that the alloy exhibited four strong diffraction peaks within the range of 2θ = 40° to 100°. After a third firing, intensity of these planes increased. Results of this study confirmed that nickel-based alloys become weaker after each firing process. After firing treatment, the microstructure of alloys showed decreased dendritic structure (i.e., homogenization, which was responsible for decrease in strength and an increase in ductility of the alloy); however, this decreased strength and hardness of Ni-Cr alloy after heat treatment was still superior to those of the most noble metal alloys used in dentistry. X-ray diffraction study showed that firing process led to relieving of stresses, which ultimately resulted in stability in the crystal structure of alloy.

Why the Smith theory over-predicts instant rupture velocities during fiber transverse impact

The effect of multi-axial loading on single Kevlar® KM2 fibers is explored with an emphasis on correlating the results to fiber/yarn transverse impact. A 0.30 caliber fragment simulation projectile (FSP) is slightly modified to act as a transverse loading indenter. Fiber failure angles are forced between ∼0° and 50° in efforts to deduce the deleterious effect caused by such angles. Said angles are also enforced in order to create the geometry that would be produced at an impact velocity causing immediate fiber/yarn rupture in transverse impact experiments. The effect of fiber angle around the FSP indenter is experimentally studied along with an analysis of the specific angle causing immediate fiber failure. It is shown that there exists a demonstrative reduction in fiber longitudinal failure strain of KM2 filaments due to this multi-axial stress state, thereby questioning the recurrent assumption that fiber performance within a body armor system is dominated by failure in pure tension.

A novel bioactive agent improves adhesion of resin-modified glass-ionomer to dentin

Bioactive glasses are surface-active and able to induce remineralization of dentin. Two resin-modified glass-ionomer cements (RMGICs) doped with bioactive glass (Biosilicate®) were used as restorative materials in dentin. Experimental powders were made by incorporating 2, 5, and 10 wt% of Biosilicate® in Vitremer® (VT) and Fuji II LC® (FL) powders. Commercial FL and VT were used as control materials. Six cylinders of each material were tested for failure in compression (1.0 mm/min), after 24 h storage in distilled water at 37 °C. For microtensile bond strength (MTBS) test, cavity preparations were performed on 30 noncarious human molars and restored with the tested materials. Teeth were stored in water (37 °C) for 24 h or 7 days, sectioned into beams and tested for failure in tension (0.5 mm/min). Data were analyzed by analysis of variance and multiple-comparison tests (p < 0.05). Analysis of debonded specimens was performed by scanning electron microscopy. Only incorporation of 2 wt% of Biosilicate® did not decrease compressive strength of the RMGICs. Two weight percent of Biosilicate® into RMGICs produced an increase in MTBS after 24 h for FL and after 24 h and 7 days for VT. Two weight percent of Biosilicate® particles into RMGICs did not affect compressive strength and improved MTBS to dentin.

Orientation gradients in relation to grain boundaries at varying strain level and spatial resolution

The resolution dependence of orientation gradients was studied in a well-annealed 99.9995% pure polycrystalline copper pulled to failure in tension. Owing to the well developed neck, different regions in the sample correspond to different tensile strains. Post-mortem characterization was performed using EBSD on cross-sections containing the tensile axis. Kernel average misorientation (KAM) was calculated as a metric to establish correlation between defect accumulation and microstructural features, with a threshold of 5° to focus on intra-granular gradients. The region with the lowest strain (2%) showed high KAM values adjacent to grain boundaries compared to the grain interior, regardless of the point spacing, i.e. the spatial resolution. However, in the region with the highest strain (13%) a strong dependence on resolution was found. For point spacings of 0.5μm or smaller, the same correlation of high KAM with locations near boundaries was found. At coarse spacings i.e. low spatial resolution, by contrast, the reverse was found in that the highest KAM values appear in the grain interiors, as previously observed in X-ray microscopy on the same sample which had a similar coarse resolution. An analysis of orientation gradients parallel to, and perpendicular to boundaries suggested that the latter tend to be the larger of the two. This helps to explain why boundary-adjacent points have low KAM values. The conclusion is that measurement of local orientation gradient requires a resolution that is comparable to the dislocation substructure.

A multiscale structural investigation of the annulus-endplate anchorage system and its mechanisms of failure

Background contextThe annulus-endplate anchorage system performs a critical role in the disc, creating a strong structural link between the compliant annulus and the rigid vertebrae. Endplate failure is thought to be associated with disc herniation, a recent study indicating that this failure mode occurs more frequently than annular rupture. PurposeThe aim was to investigate the structural principles governing annulus-endplate anchorage and the basis of its strength and mechanisms of failure. Study designLoading experiments were performed on ovine lumbar motion segments designed to induce annulus-endplate failure, followed by macro- to micro- to fibril-level structural analyses. MethodsThe study was funded by a doctoral scholarship from our institution. Samples were loaded to failure in three modes: torsion using intact motion segments, in-plane tension of the anterior annulus-endplate along one of the oblique fiber angles, and axial tension of the anterior annulus-endplate. The anterior region was chosen for its ease of access. Decalcification was used to investigate the mechanical influence of the mineralized component. Structural analysis was conducted on both the intact and failed samples using differential interference contrast optical microscopy and scanning electron microscopy. ResultsTwo main modes of anchorage failure were observed—failure at the tidemark or at the cement line. Samples subjected to axial tension contained more tidemark failures compared with those subjected to torsion and in-plane tension. Samples decalcified before testing frequently contained damage at the cement line, this being more extensive than in fresh samples. Analysis of the intact samples at their anchorage sites revealed that annular subbundle fibrils penetrate beyond the cement line to a limited depth and appear to merge with those in the vertebral and cartilaginous endplates. ConclusionsAnnulus-endplate anchorage is more vulnerable to failure in axial tension compared with both torsion and in-plane tension and is probably due to acute fiber bending at the soft-hard interface of the tidemark. This finding is consistent with evidence showing that flexion, which induces a similar pattern of axial tension, increases the risk of herniation involving endplate failure. The study also highlights the important strengthening role of calcification at this junction and provides new evidence of a fibril-based form of structural integration across the cement line.

Strengthening Slender Reinforced Concrete Columns Using High-Modulus Bonded Longitudinal Reinforcement for Buckling Control

AbstractThis paper introduces a model for strengthening slender reinforced concrete columns. The proposed technique aims at controlling second-order lateral deflections using longitudinal high-modulus bonded reinforcement, thereby altering the loading path to intercept the axial load-bending moment (P-M) interaction curve at a higher axial capacity. With the availability of high and ultra-high-modulus carbon fiber–reinforced polymer (CFRP) plates, this approach should be quite efficient according to Euler’s buckling rule, in which column strength is stiffness-controlled. This approach is different from the classical transverse-wrapping method for confinement, a technique that achieves strengthening by enlarging the (P-M) diagram in the compression-controlled region. The proposed model accounts for concrete nonlinearity in compression, cracking in tension, steel rebar plasticity, and certainly geometric nonlinearity, in addition to the possibility of premature CFRP-debonding failure in tension and the lowe...

Influence of different drying methods on microtensile bond strength of self-adhesive resin cements to dentin

Objective. This study investigated the effect of different drying methods of dentin surface on the bonding efficacy of self-adhesive resin cements (SRCs). Materials and methods. Three SRCs (RelyX U200, RU; Maxcem Elite, ME; and BisCem, BC) and one resin-modified glass ionomer cement (RelyX Luting 2, RL) were used. The characteristics of the materials were evaluated using thermogravimetric analysis and surface roughness and contact angle measurements. Human dentin surfaces were finished with 600-grit silicon carbide paper and assigned to three groups according to these drying methods: ethanol dehydration, drying by waiting for 10 s after blot-drying and blot-drying. The four cements were used for luting composite overlays to the dried dentin. After 24 h storage at 37°C and 100% relative humidity, stick-shaped specimens with a cross-sectional area of 0.8 mm2 were prepared and stressed to failure in tension at a crosshead speed of 0.5 mm/min (n = 27). Failure modes of fractured specimens were assessed by optical and scanning electron microscopy. Results. RL was the most hydrophilic, followed by BC and ME and then RU. All the luting cements luted to ethanol-dehydrated dentin showed zero bond strengths. For the three SRCs, drying by waiting produced higher microtensile bond strengths than blot-drying. RU showed the best bonding performance in the above two dentin conditions. RL showed significantly higher bond strength in blot-drying condition than in drying-by-waiting (p < 0.001). Conclusions. This study suggests that dentin surface moisture has a crucial effect on the bond strength of SRCs.

Investigation of a crash concept for CFRP transport aircraft based on tension absorption

Transport aircraft made of carbon fibre reinforced plastics (CFRP) have to provide an equivalent crashworthiness compared to today's aluminium aircraft designs. However, CFRP structures typically show brittle failure behaviour under complex loading conditions and little energy absorption, whereas aluminium structures provide comparably high energy absorption due to their ductile failure characteristics. Improved crashworthiness for CFRP fuselages can be obtained by the installation of special crash devices, which are designed for energy absorption by progressive failure in compression, tension or bending. The realisation of crashworthy CFRP fuselage designs with the focus on compression or bending absorbers is often associated with significant mass penalty compared to the purely static sizing of the corresponding fuselage structure. In this context, an alternative crash kinematics was developed and numerically investigated in which most of the kinetic energy is dissipated by tension absorption in the sub-cargo area of a fuselage structure. The numerical study was performed on the basis of a purely vertical impact with a two-bay fuselage section using the explicit finite element (FE) solver Abaqus/Explicit. The simulation results show for the developed crash kinematics several advantages, e.g. reduced mass penalty, with the tension absorption concept compared to crash concepts that use energy absorption by progressive crushing in the sub-cargo area.

Modeling combined tension-shear failure of ductile materials

Failure of ductile materials is usually expressed in terms of effective plastic strain. Ductile materials can fail by two different failure modes, shear failure and tensile failure. Under dynamic loading shear failure has to do with shear localization and formation of adiabatic shear bands. In these bands plastic strain rate is very high, dissipative heating is extensive, and shear strength is lost. Shear localization starts at a certain value of effective plastic strain, when thermal softening overcomes strain hardening. Shear failure is therefore represented in terms of effective plastic strain. On the other hand, tensile failure comes about by void growth under tension. For voids in a tension field there is a threshold state of the remote field for which voids grow spontaneously (cavitation), and the material there fails. Cavitation depends on the remote field stress components and on the flow stress. In this way failure in tension is related to shear strength and to failure in shear. Here we first evaluate the cavitation threshold for different remote field situations, using 2D numerical simulations with a hydro code. We then use the results to compute examples of rate dependent tension-shear failure of a ductile material.

Fatigue of ZK60 magnesium alloy under uniaxial loading

Uniaxial fatigue experiments were conducted on ZK60 magnesium (Mg) alloy in ambient air. They include fully reversed strain-controlled experiments with strain amplitudes ranging from 0.24% to 6.0%, strain-controlled experiments with a large compressive minimum strain of −9.4%, and stress-controlled experiments. For fully reversed strain-controlled experiments, fatigue process consists of crack initiation, small crack growth, and final failure in tension when the strain amplitude is lower than 3.0%. Final failure occurs under compression when the strain amplitude is larger than or equal to 3.5%, where the fracture surface shows characteristics identical to these observed from monotonic compression. A kink point corresponding to a strain amplitude of 0.35% is observed in the strain-life curve from the fully reversed strain-controlled fatigue experiments. This strain amplitude demarcates the influence of twinning–detwinning deformation on fatigue of the material. Asymmetric stress–strain hysteresis loops with a tensile mean stress are observed when the strain amplitude is larger than 0.35%. When the material is pre-compressed to −9.4%, the initial texture is changed from a strong basal texture to a strong (0001) or c-texture. The subsequent strain-controlled fatigue loading results in compressive mean stresses. The fatigue lives are similar to those of the basal textured state with identical strain amplitude for strain amplitudes larger than 0.4%. The Smith, Watson, and Topper fatigue parameter does not correlate the fatigue experiments with large compressive mean strains.

Characterization of Strength of Intact Brittle Rock Considering Confinement-Dependent Failure Processes

As technologies for deep underground development such as tunneling underneath mountains or mass mining at great depths (>1,000 m) are implemented, more difficult ground conditions in highly stressed environments are encountered. Moreover, the anticipated stress level at these depths easily exceeds the loading capacity of laboratory testing, so it is difficult to properly characterize what the rock behavior would be under high confinement stress conditions. If rock is expected to fail in a brittle manner, behavior changes associated with the relatively low tensile strength, such as transition from splitting to the shear failure, have to be considered and reflected in the adopted failure criteria. Rock failure in tension takes place at low confinement around excavations due to tensile or extensional failure in heterogeneous rocks. The prospect of tensile-dominant brittle failure diminishes as the confinement increases away from the excavation boundary. Therefore, it must be expected that the transition in the failure mechanism, from tensile to shear, occurs as the confinement level increases and conditions for extensional failure are prevented or strongly diminished. However, conventional failure criteria implicitly consider only the shear failure mechanism (i.e., failure envelopes touching Mohr stress circles), and thus, do not explicitly capture the transition of failure modes from tensile to shear associated with confinement change. This paper examines the methodologies for intact rock strength determination as the basic input data for engineering design of deep excavations. It is demonstrated that published laboratory test data can be reinterpreted and better characterized using an s-shaped failure criterion highlighting the transition of failure modes in brittle failing rock. As a consequence of the bi-modal nature of the failure envelope, intact rock strength data are often misinterpreted. If the intact rock strength is estimated by standard procedures from unconfined compression tests (UCS) alone, the confined strength may be underestimated by as much as 50 % (on average). If triaxial data with a limited confinement range (e.g., σ3 ≪ 0.5 UCS due to cell pressure limitations) are used, the confined strength may be overestimated. Therefore, the application of standard data fitting procedures, without consideration of confinement-dependent failure mechanisms, may lead to erroneous intact rock strength parameters when applied to brittle rocks, and consequently, by extrapolation, to correspondingly erroneous rock mass strength parameters. It follows that the strength characteristics of massive rock differ significantly in the direct vicinity of excavation from that which is remote with higher confinement. Therefore, it is recommended to adopt a differentiated approach to obtain intact rock strength parameters for engineering problems at lower confinement (near excavation; e.g., excavation stability assessment or support design), and at elevated confinement (typically, when the confinement exceeds about 10 % of the UCS) as might be encountered in wide pillar cores.

Modeling matrix cracking in composite rotor blades within VABS framework

This paper deals with modeling of the first damage mode (i.e. matrix micro-cracking) in helicopter rotor or wind turbine blades and how this effects the overall cross-sectional stiffness. The helicopter rotor or wind turbine rotor system operates in a highly dynamic and unsteady environment leading to severe vibratory loads present in the system. Repeated exposure to this loading condition can induce damage in the composite rotor blades. These helicopter rotor or wind turbine blades are generally made of fiber-reinforced laminated composites and exhibit various competing modes of damage such as matrix micro-cracking, delamination, and fiber breakage. There is a need to study the behavior of the composite rotor system under various key damage modes in composite materials for developing Structural Health Monitoring (SHM) system. Each blade is modeled as a beam based on geometrically non-linear 3-D elasticity theory. Each blade thus splits into 2-D analyzes of cross-sections and non-linear 1-D analyzes along the beam reference lines. Two different tools are used here for complete 3-D analysis: VABS for 2-D cross-sectional analysis and GEBT for 1-D beam analysis. The physically-based failure models for matrix in compression and tension loading are used in the present work. We detect the matrix cracking using two failure criterion: Matrix Failure in Compression and Matrix Failure in Tension which are based on the recovered field. We set the strain variable which drives the damage variable for matrix cracking and this damage variable is used to estimate the reduced cross-sectional stiffness. The procedure presented in this paper is implemented in VABS as matrix micro-cracking modeling module. To investigate the matrix failure model, three examples are presented which illustrate the effect of matrix cracking on cross-sectional stiffness by varying the applied cyclic load. Finally, the stiffness degradation of composite cross-section of rotor blade due to matrix micro-cracking is correlated to the life of composite rotor blade using damage accumulation model. At the end, an empirical equation is given for the Stress Intensity Factor (SIF) to simulate the matrix micro-crack growth in future.

Fractographic analysis of microalloyed steel failure in tension and impact loading

Metallographic and fractographic studies of the failure of microalloyed steel grade X42 (type 09G2S) are performed. Microhardness, flow stress, yield strength, elongation, and impact strength are measured. In tension the steel exhibits typical ductile failure at room temperature and cleavage fracture with impact loading. Grain elongation along the rolling direction plays an important role in crack propagation.

A Coupled Approach to Simulate the Explosion Response of a Buried Structure in a Soil-Rock Layered Medium

The paper presents an investigation on the response of a buried structure in a soft soil layer above a rock bed that is subjected to blast loading. A comprehensive approach to simulate the behavior of soft soil with a buried structure laying over a rock mass due to the dynamic (explosive) action in the rock mass is presented. The numerical algorithm was developed to simulate the shock wave propagation within the medium, considering both the bulk and deviatoric damage and taking into account the possible shear damage accumulation. It takes into account the contact conditions between the layers to simulate the shock wave transmission and the soil-structure interaction including their possible separation. The soil-lining interaction problem is solved by a combination of the variational-difference method (for the lining) and of the Godunov's method (for the soil). The coupling of these two approaches is performed by calculation of the contact stresses and velocities on the soil-lining boundary. Two types of the soil models were investigated – an ideal elastic plastic medium and an irreversible compressible medium with failure in bulk tension – and the results were compared with each other. The analysis of the free surface and interface velocities and displacements as well as the contact stresses and the damage at the media interface were carried out. The effects of the relative cover thickness on the interface and free surface behavior is studied. Analysis of the contact stresses at the soil-tunnel interface, was carried out. The effect of the charge location as well as the effect of the soil-rock interface on the tunnel's response was investigated.

Temperature-dependent strain localization and texture evolution of highly nanotwinned Cu

Strong differences in the plastic strain and texture development in high purity nanotwinned copper foils deformed to failure in tension at ambient and liquid nitrogen (LN) temperatures have been observed. High energy microdiffraction patterns for the room temperature (RT) sample showed two distinct deformation regions relative to its fracture surface: d &amp;lt; 500 μm and d &amp;gt; 500 μm; while for the sample deformed at 77 K, three distinct regions were observed: d &amp;lt; 120 μm, 120 μm &amp;lt; d &amp;lt; 1550 μm, and d &amp;gt; 1550 μm. The localized plastic deformation in the RT tensile test and the increased ductile deformation at LN temperature are discussed in terms of the dualistic nanotwins plus micro-scale columnar grain structure.

Numerical 3D finite element modelling and experimental tests of rounded dovetail connection

The mechanical behaviour of the rounded dovetail connection subjected to shear load is analysed experimentally and numerically. The brittle failure of the joist member is observed in the experimental investigation. The experimental results are used to validate a three-dimensional (3D) non-linear finite element model. The non-linear models use the Hill criterion to manage the plastic yielding of the wood material. The modified Hill failure criteria, managing the damage evolution in wood, are used to take into account the brittle failure in shear and tension. Besides, the models also incorporate the contact and the geometric non-linearity. The comparison with experimental results shows that the numerical models are in good agreement with real behaviour. The validated models can be used to predict the strength and failure position of the rounded dovetail connection for parametric studies. Le comportement mécanique des assemblages par queue d’aronde sollicités en effort tranchant est analysé expérimentalement et numériquement. La rupture fragile du membre solive est observée dans l’étude expérimentale. Les résultats expérimentaux sont utilisés pour valider un modèle numérique tridimensionnel (3D) non-linéaire par éléments finis. Les modèles non-linéaires utilisent le critère de Hill pour gérer le comportement plastique du matériau bois. Les critères de Hill modifiés, qui gèrent l’évolution des dommages dans le bois, sont utilisés pour tenir compte de la rupture fragile en cisaillement et en traction. Par ailleurs, les modèles intègrent également le contact et la non-linéarité géométrique. La comparaison avec les résultats expérimentaux montre que les modèles numériques sont en bon accord avec le comportement mécanique réel. Les modèles validés peuvent être utilisés pour prédire la force et la position de rupture d’assemblage par queue d’aronde pour des études paramétriques.

Biological and Biomechanical Evaluation of the Ligament Advanced Reinforcement System (LARS AC) in a Sheep Model of Anterior Cruciate Ligament Replacement: A 3-Month and 12-Month Study

The purposes of this study were to assess tissue ingrowth within the Ligament Advanced Reinforcement System (LARS) artificial ligament (LARS AC; LARS, Arc sur Tille, France) and to study the biomechanical characteristics of the reconstructed knees in a sheep model of anterior cruciate ligament (ACL) replacement. Twenty-five female sheep underwent excision of the proximal third of the left ACL and intra-articular joint stabilization with a 44-strand polyethylene terephthalate ligament (mean ultimate tensile failure load, 2,500 N). Animals were killed either 3 or 12months after surgery. Explanted knees were processed for histology (n= 10) or mechanical tests including tests of laxity and loading to failure in tension (n= 15). Well-vascularized tissue ingrowth within the artificial ligament was only observed in the portions of the ligament in contact with the host's tissues (native ligament and bone tunnels). Ligament wear was observed in 40% of explanted knees. The ultimate tensile failure loads of the operated knees at both time points were inferior to those of the contralateral, intact knees (144 ± 69 N at 3 months and 260 ± 126 N at 12 months versus 1,241 ± 270 N and 1,218 ± 189 N, respectively) (P < .01). In specimens with intact artificial ligaments, failure occurred by slippage from the bone tunnels in all specimens explanted 3 months postoperatively and in half of the specimens explanted 12 months postoperatively. This study provides evidence that the LARS AC has a satisfactory biointegration but that it is not suitable for ACL replacement if uniform tissue ingrowth is contemplated. Despite good clinical performance up to 1 year after implantation, none of the reconstructions approached the mechanical performance of the normal ACL in the ovine model. Partial tearing of the artificial ligament, which led to a significant decrease in ultimate tensile strength, was observed in 40% of cases in the ovine model. The LARS is not a suitable scaffold for ACL replacement. Further animal studies are needed to evaluate its potential for augmentation of ligament repair.

Inverse Hall–Petch relationship in nanocrystalline tantalum

Tantalum polycrystals (grain sizes varying from 2.5 to 30nm) generated by Voronoi tessellation were subjected to tension and compression under uniaxial strain loading at strain rates on the order of 108–109s−1 using molecular dynamics (MD) simulations. In contrast with MD simulations of FCC metals, the response in tension is significantly different from that in compression. In tension, fracture is initiated at grain boundaries perpendicular to the loading direction. It propagates along grain boundaries with limited plastic deformation, at a stress in the range 10–14GPa. This brittle intergranular failure is a consequence of the high strain rate imposed by MD, leading to a stress that exceeds the grain-boundary cohesive strength. Thus, grain-boundary separation is the principal failure mechanism. In compression, on the other hand, there is considerable plastic deformation within the grains. This occurs at stresses higher than failure in tension. The difference between tensile and compressive response for tantalum is attributed to the difficulty in generating dislocations, in contrast with FCC metals, where tensile failure occurs by void nucleation at grain boundaries associated with partial and perfect dislocation emission. In BCC tantalum, both grain-boundary sliding and dislocation emission are much more difficult.The compressive yield stress is found to increase with grain size in the 2.5nm<d <30nm region. This inverse Hall–Petch relationship is analyzed in terms of the contributions of dislocation motion and grain-boundary shear to plastic deformation. As the grain size is increased the contribution of grain-boundary sliding is decreased and plastic strain is accommodated by dislocation and motion. In tensile deformation, on the other hand, this behavior is not observed.