Double-notched Specimens Research Articles

Effects of age and loading rate on equine cortical bone failure

Although clinical bone fractures occur predominately under impact loading (as occurs during sporting accidents, falls, high-speed impacts or other catastrophic events), experimentally validated studies on the dynamic fracture behavior of bone, at the loading rates associated with such events, remain limited. In this study, a series of tests were performed on femoral specimens obtained post-mortem from equine donors ranging in age from 6 months to 28 years. Fracture toughness and compressive tests were performed under both quasi-static and dynamic loading conditions in order to determine the effects of loading rate and age on the mechanical behavior of the cortical bone. Fracture toughness experiments were performed using a four-point bending geometry on single and double-notch specimens in order to measure fracture toughness, as well as observe differences in crack initiation between dynamic and quasi-static experiments. Compressive properties were measured on bone loaded parallel and transverse to the osteonal growth direction. Fracture propagation was then analyzed using scanning electron and scanning confocal microscopy to observe the effects of microstructural toughening mechanisms at different strain rates. Specimens from each horse were also analyzed for dry, wet and mineral densities, as well as weight percent mineral, in order to investigate possible influences of composition on mechanical behavior. Results indicate that bone has a higher compressive strength, but lower fracture toughness when tested dynamically as compared to quasi-static experiments. Fracture toughness also tends to decrease with age when measured quasi-statically, but shows little change with age under dynamic loading conditions, where brittle “cleavage-like” fracture behavior dominates.

S-37 In-Vivo like Testing of Ceramic Ball Heads

Alumina (Al2O3) bearings in total hip arthroplasty lead to low wear rates, but catastrophic failure of Al2O3 femoral heads, while rare, remains a concern. In the present work, a composite of Al2O3 and niobium (Nb) was tested in vitro for potential use as an alternative femoral head material in vivo. Dense composite laminates of Al2O3 and Nb were fabricated by hot pressing, and their microstructure and mechanical properties were evaluated. The flexural strength of Al2O3–Nb laminates in four-point loading was 720 ± 40 MPa, compared with a value of 460 ± 110 MPa for Al2O3. Scanning electron microscopy and X-ray diffraction showed a well-bonded interface between the Al2O3 and Nb without measurable formation of an interfacial reaction phase. The interfacial shear strength between Al2O3 and Nb, measured by a double-notched specimen test, was 290 ± 15 MPa. The feasibility of fabricating prototype femoral heads (32 mm in diameter), consisting of an Al2O3 surface layer (2–3 mm thick) and a Nb core, by hot pressing was shown. The composite femoral head combined the low wear of an Al2O3 articulating surface with the safety of a ductile metal femoral head.

Investigation on Fatigue Limit Based on the Series Complicated Notches for Structural Steel

The rotating bending fatigue tests have been carried out to investigate the fatigue limit of specimen with double-notch which is constructed by the step and the blind hole, and the effect of stress concentrations at the double-notched bottoms on the fatigue limits are analyzed using three-dimensional elastic finite element method (3D-EFEM). There are fatigue limits of 17 groups on the single-notched specimen and double-notched specimen obtained in the fatigue tests. The stress field interactions are discussed between the stress field by the blind hole notch and the stress field by step using 3D-EFEM. The results obtained in this study are as follows: The fatigue limit of the double-notch specimen is insensitive to the distance between the blind hole and step for the low carbon structure steel with better ductility; while, for the high-strength steel, superposition and intensification of the stress concentration by the blind hole and step mutually may be avoided, and their adverse effects on the fatigue strength may be become to minimal, when taken appropriate distance between the blind hole and step. The results are significant for engineering design of the multi-notched parts, and studying fatigue strength theory.

Research of Fatigue Limit and Stress Concentration on Structural Steel with Double-Notches

The fatigue limit of parts and components that have the multi-notches is important data for the design and manufacture of machinery and traffic equipment which are operated under the high speed or pressure. In this paper the rotating bending fatigue tests have been carried out to investigate the fatigue limit of specimen with double-notch that is constructed of step and blind hole, and analyzed the effect of stress concentrations at the double-notched bottoms on the fatigue limits, using three-dimensional elastic finite element method. Firstly, the fatigue tests of 8 group specimens have been performed for examining the of fatigue limits of the single-notched specimen and double-notched specimen, respectively. Additionally, the stress field interactions between two stress fields by the blind hole notch and step are discussed using three-dimensional elastic finite element method. The main results obtained in this study are as follows: The fatigue limit of the double-notched specimen are down comparison with the fatigue limit of the single-notched specimen; the fatigue limit of the double-notch specimen is insensitive to distance between the blind hole and step for the low carbon structure steel with better ductility; for the high-strength steel, superposition and intensification of the stress concentration by the blind hole and step mutually may be avoided so that their adverse effects on the fatigue strength may be become to minimize, as take appropriate distance between the blind hole and step. The results are significant for the design of engineering design of the multi-notched parts, and the study of fatigue strength.

In vitro testing of Al 2O 3–Nb composite for femoral head applications in total hip arthroplasty

Alumina (Al 2O 3) bearings in total hip arthroplasty lead to low wear rates, but catastrophic failure of Al 2O 3 femoral heads, while rare, remains a concern. In the present work, a composite of Al 2O 3 and niobium (Nb) was tested in vitro for potential use as an alternative femoral head material in vivo. Dense composite laminates of Al 2O 3 and Nb were fabricated by hot pressing, and their microstructure and mechanical properties were evaluated. The flexural strength of Al 2O 3–Nb laminates in four-point loading was 720 ± 40 MPa, compared with a value of 460 ± 110 MPa for Al 2O 3. Scanning electron microscopy and X-ray diffraction showed a well-bonded interface between the Al 2O 3 and Nb without measurable formation of an interfacial reaction phase. The interfacial shear strength between Al 2O 3 and Nb, measured by a double-notched specimen test, was 290 ± 15 MPa. The feasibility of fabricating prototype femoral heads (32 mm in diameter), consisting of an Al 2O 3 surface layer (2–3 mm thick) and a Nb core, by hot pressing was shown. The composite femoral head combined the low wear of an Al 2O 3 articulating surface with the safety of a ductile metal femoral head.

Alumina–tantalum composite for femoral head applications in total hip arthroplasty

Dense composite laminates of alumina (Al 2O 3) and tantalum (Ta) were fabricated by hot pressing and tested in vitro for potential use as a femoral head material in total hip arthroplasty (THA). Al 2O 3–Ta composite laminates hot pressed at 1450 °C and 1650 °C had flexural strengths of 940 ± 180 MPa and 1090 ± 340 MPa, respectively, which were far larger than the values of 420 ± 140 MPa and 400 ± 130 MPa for Al 2O 3 hot pressed at 1450 °C and 1650 °C, respectively. The interfacial shear strength, determined by a double-notched specimen test, was 310 ± 80 MPa for the composite laminate hot pressed at 1650 °C, indicating strong interfacial bonding between Al 2O 3 and Ta. Scanning electron microscopy (SEM), energy dispersive X-ray (EDS) analysis, and X-ray mapping of polished sections of the hot-pressed laminates showed the presence of an interfacial region formed presumably by diffusion of O (at 1450 °C) or O and Al (1650 °C) from Al 2O 3 into Ta. Composite femoral heads of Al 2O 3 and Ta could combine the low wear of an Al 2O 3 articulating surface with the safety of a ductile metal femoral head.

Stress mapping of undamaged, strained, and failed regions of bone using Raman spectroscopy.

Stress differences via spectral shifts that arise among failed, strained, and undamaged regions of bone can be determined using Raman spectroscopy and double-notch specimens. A double-notch specimen is a model in which the early stages of fracture can be examined. At four-point bending, fracture occurs at one of the notches. Tissue near each notch is representative of bone in a state either directly before or after bone failure. Raman images are acquired among three regions: control, strained (root of unbroken notch), and failed (root of fractured notch). The center of gravities (CGs), a way to monitor wavenumber shifts, of the phosphate v(1) band are calculated. A PO(4) (-3) v(1) band shift most likely corresponds to a change in spacing between phosphate cations and anions. This spectral shift is converted into stress values using the dvdP coefficient, determined by applying known pressures/stresses and measuring the change in position of the PO(4) (-3) v(1) band. In comparison to control regions, the residual stress in strained and failed regions is significantly higher (p=0.0425 and p=0.0169, respectively). In strained regions, residual stress is concentrated near the corners of the unbroken notch, whereas in failed regions the high stresses are confined near the edge of the fracture.

Stress Reconstruction and Constitutive Parameter Identification in Plane-Stress Elasto-plastic Problems Using Surface Measurements of Deformation Fields

This paper deals with the identification of elasto-plastic constitutive parameters from deformation fields measured over the surface of thin flat specimens with the grid method. The approach for recovering the constitutive parameters is the virtual fields method. A dedicated algorithm is used for deriving the distribution of the 2D stress components from the measured deformation fields. A state of plane stress is assumed. Guesses of the constitutive parameters are input in the algorithm and updated until the stresses satisfy the principle of virtual work in the least squares sense. The advantage of this approach is that it can handle very heterogeneous plastic flows and it is much faster than classical finite element model updating approaches. An experimental application is provided to demonstrate it. Six mild steel double-notched specimens have been tested in a configuration combining tension and in-plane bending. The identified parameters are in good agreement with their reference counterparts. Stress fields are eventually reconstructed across the specimen all along the test for analyzing the evolution of the plastic flow.

A new impact test for the identification of a dynamic crack propagation criterion using a gas-gun device

The modelling of damage and fracture behaviour under high rates of loadings for metallic structures presents the more and more interests for engineering design, especially for crash phenomena. In order to perform a numerical simulation of such phenomena a crack propagation criterion must be identified using adapted laboratory tests. The objective of this paper is to present a new impact test intended for the identification of a cohesive crack criterion implemented into a home-made FEM code based on Extended Finite ElementMethod. Therefore, a double-notched specimen is impacted using a gas-gun device in order to obtain different crack paths depending on projectile speed. A post-impact macro-photographic observation allows to measure the crack path, the angles and the advancing length. These experimental results are used as input responses in the identification procedure for determining the crack cohesive criterion parameters. Some experimental results, for an aluminium alloy crack criterion identification, are presented to illustrate the proposed approach.

Tensile Fracture Behaviors in Double-notched Thin Plates of a Ductile Steel

Tension tests are carried out on smooth and double-notched specimens of SPCC steel plates with various ligaments and notch root radii. By observing the tensile fracture processes, initial cracks are found to occur at the notch roots, the center and the 2 axi-symmetrical inner areas of ligaments for specimens with acute, obtuse and intermediate notches, respectively. In order to clarify the effect of multi-axial stresses on the ductile fracture mechanisms, distributions of multi-axial stress components in the notched specimens are calculated by finite element method. In a specimen undergoing the maximum load, the mean stress shows the largest value at the region where initial cracks just occur. This behavior is discussed with reference to the effects of multi-axial stress components on the initiation and growth of micro-cavities in specimens. The result suggests that the mean stress component should be a dominant factor influencing the fracture behaviors of the ductile steel plates.

Investigation on elasto-plastic constitutive model coupled with damage for localization phenomena

On the basis of existing plasticity-based damage model for plasticity coupled with damage for localization analysis, constitutive parameter identification was carried out through a series of numerical tests at local level. And then improvements were made on the expressions of the evolution laws of damage. Strain localization phenomena were simulated with a typical double-notched specimen under tensions. Numerical results indicate the validity of the proposed theory.

Modeling 3D crack propagation in unreinforced concrete using PUFEM

Concrete is a quasi-brittle material, where tensile failure involves progressive micro-cracking, debounding and other complex irreversible processes of internal damage. Strain-softening is a dominate feature and advanced numerical schemes have to be applied in order to circumvent the ill-posdness of the Boundary-Value Problem to deal with. Throughout the paper we pursue the cohesive zone approach, where initialization and coalescence of micro-cracks is lumped into the cohesive fracture process zone in terms of accumulation of damage. We develop and employ a (discrete) constitutive description of the cohesive zone, which is based on a transversely isotropic traction separation law. The model reflects an exponential decreasing traction with respect to evolving opening displacement and is based on the theory of invariants. Non-negativeness of the damage dissipation is proven and the associated numerical embedded representation is based on the Partition of Unity Finite Element Method. A consistent linearization of the method is presented, where particular attention is paid to the (cohesive) traction terms. Based on the proposed concept three numerical examples are studied in detail, i.e. a double-notched specimen under tensile loading, a four point shear test and a pull-out test of unreinforced concrete. The computational results show mesh-independency and good correlation with experimental results.

Design Criterion of Fatigue Analysis on Plastic Basis by ASME Boiler and Pressure Vessel Code

The paper investigates the current rules for fatigue analysis on plastic basis of the ASME Boiler and Pressure Vessel Code. It addresses the question: When these rules are used for a vessel or component, is it designed with respect to crack initiation or failure? Using available fatigue test data, the question is answered for blunt double-notch plate specimens made of AISI 4340 steel. When the rules are applied to these specimens, it is found that they are designed with respect to the initiation of a crack of a certain length that decreases as the strain range increases. The plate specimens are not designed with respect to failure. It is concluded that for geometries that exhibit a strain range gradient away from the surface, the number of cycles allowed by the current rules includes the initiation phase but not the propagation phase of a fatigue crack. If this result is unsatisfactory, new rules have to be written.

Development of Evaluation and Application Techniques of SiC/SiC Composites for Fusion Reactors

Evaluation techniques and mechanical properties of silicon carbide composites (SiC⁄SiC composites) reinforced with highly crystalline fibers are reviewed for fusion applications. The SiC⁄SiC composites used were fabricated by means of the CVI method. The evaluation includes in-plane tensile strength by in-plane tensile test, transthickness tensile strength by transthickness tensile test and diametral compression test and shear strength by compression test using double-notched specimen. All tests were successfully conducted using small specimens for neutron irradiation experiment. As application technique, the novel tungsten(W) coating technique on SiC is reviewed. The W powder melted by high power lamp in a few seconds and formed coating on SiC. No thick reaction layers of WC and W5Si3, which are formed by the other coating methods, were formed by this method.

An Energy Model for Bifurcation Analysis of a Double-Notched Concrete Panel: Continuum Model

In this paper, the uni-axial tension of double-notched concrete specimens is analyzed by using a continuum plasticity model. As in a companion paper, the concept of minimization of the second-order energy is used as the criterion for judging a bifurcation. The energy computation is formulated in standard continuum plasticity. The analysis confirms that the unsymmetrical crack propagation (i.e. strains localizing on one side of the specimen) may occur either before or after the peak load. Influences on bifurcation of three factors, the notch depth to panel width, the local constitutive law, and the ratio of the panel width to panel length are investigated. A larger ratio of the notch depth to panel width, a steeper softening constitutive law, and a smaller ratio of the panel width to panel length, leads to an earlier bifurcation. These conclusions are consistent with those obtained from a simplified model.

Development of Graphite Jig-Assembly for High Temperature Interlaminar Shear Strength Testing with Double-Notched Specimen Configuration Subjected to Compressive Load

To measure inter laminar shear strength (ILSS) of C/C composites at high temperatures, a graphite jig was developed on double notched specimen configuration. ILSS of unidirectional C/C composites and 0/90 cross-ply of C/C composites (HTT2000°C and 3000°C in both composites) were measured using the assembly up to 1500°C. In all tests, the shear load showed a liner increase after the initial stage of the loading. The assembly developed was satisfactorily used to measure the ILSS of the C/C composites at high temperatures. All specimens showed catastrophic fracture, and the ILSS largely increased with testing temperature, at least, above 1250°C. In C/C composites, relationships between ILSS and testing temperature seem to depend not on orientation configuration of their reinforcement but on their heat-treatment temperature.

Microstructure and mechanical properties of superplastically joined yttria-partially-stabilized zirconia (Y-PSZ) ceramics

Joining of oxide ceramics has been recognized as a key issue in their successful utilization in a variety of demanding, high temperature applications. Superplastic joining of yttria-partially-stabilized zirconia (Y-PSZ) ceramics has been carried out to avoid the deleterious effects of joint interlayer materials. Microstructural characterization of superplastically formed joints indicate that the joint interface is indistinguishable from the bulk and that there is not grain growth across the interface. Compression tests conducted on double-notch specimens show that very good bonding was achieved in the superplastic joining process. From these experiments an underestimation of the shear strength was obtained as the specimens failed in the bulk. The room temperature flexural strength of joints measured had values up to 185 MPa.

Non-compact glide on the (010) plane in copper and corresponding dislocation structure

In a double-notch specimen cut from a copper single crystal, dislocation glide on a (010) non-compact crystal plane was generated by compressive creep in regions near a notch edge, outside the zone of shear. This glide probably substituted a successive glide of a/2<110>dislocations on two {111} planes in the bi-axial stress field. Dislocation structures on the (010) plane were investigated by transmission electron microscopy.

Interpretation of creep curves of double-notch copper single crystal specimens

Abstract Double-notch specimens were made from copper single crystals in order to realize a strain by shear on crystal planes {111}, {110}, {100} and {131}. Compressive creep tests at 773 K and 873 K yielded complicated creep curves, reflecting the complex stress and strain fields in the specimens. Analysis based on comparison of the curves with respect to Schmid factors of potentially active slip systems allowed identification of the bend and the shear component of strain in the zone of shear and the local compressive strain contribution to the curve. Under some creep conditions the shear creep curves were separated. It was concluded that the process of shear in the zone of shear is realized mostly by the usual 〈110〉{111} slip systems activated by the shear stress acting in the chosen non-compact shear plane, while the direct glide of dislocations on the non-compact shear planes is probably of minor importance.

Interpretation of creep curves of double-notch copper single crystal specimens

Abstract Double-notch specimens were made from copper single crystals in order to realize a strain by shear on crystal planes {111}, {110}, {100} and {131}. Compressive creep tests at 773 K and 873 K yielded complicated creep curves, reflecting the complex stress and strain fields in the specimens. Analysis based on comparison of the curves with respect to Schmid factors of potentially active slip systems allowed identification of the bend and the shear component of strain in the zone of shear and the local compressive strain contribution to the curve. Under some creep conditions the shear creep curves were separated. It was concluded that the process of shear in the zone of shear is realized mostly by the usual 〈110〉{111} slip systems activated by the shear stress acting in the chosen non-compact shear plane, while the direct glide of dislocations on the non-compact shear planes is probably of minor importance.