Distribution by Scientific Domains
Distribution within Polymers and Materials Science

Terms modified by Tensile

  • tensile behavior
  • tensile bond strength
  • tensile ductility
  • tensile elongation
  • tensile failure
  • tensile force
  • tensile loading
  • tensile measurement
  • tensile mechanical property
  • tensile modulus
  • tensile property
  • tensile specimen
  • tensile strain
  • tensile strength
  • tensile strength measurement
  • tensile stress
  • tensile test
  • tensile tester
  • tensile testing

  • Selected Abstracts

    Tensile and compressive damage coupling for fully-reversed bending fatigue of fibre-reinforced composites

    W. Van Paepegem
    ABSTRACT Due to their high specific stiffness and strength, fibre-reinforced composite materials are winning through in a wide range of applications in automotive, naval and aerospace industry. Their design for fatigue is a complicated problem and a large research effort is being spent on it today. However there is still a need for extensive experimental testing or large safety factors to be adopted, because numerical simulations of the fatigue damage behaviour of fibre-reinforced composites are often found to be unreliable. This is due to the limited applicability of the theoretical models developed so far, compared to the complex multi-axial fatigue loadings that composite components often have to sustain in in-service loading conditions. In this paper a new phenomenological fatigue model is presented. It is basically a residual stiffness model, but through an appropriate choice of the stress measure, the residual strength and thus final failure can be predicted as well. Two coupled growth rate equations for tensile and compressive damage describe the damage growth under tension,compression loading conditions and provide a much more general approach than the use of the stress ratio R. The model has been applied to fully-reversed bending of plain woven glass/epoxy specimens. Stress redistributions and the three stages of stiffness degradation (sharp initial decline , gradual deterioration , final failure) could be simulated satisfactorily. [source]

    A coupled damage,plasticity model for concrete based on thermodynamic principles: Part I: model formulation and parameter identification

    Giang D. Nguyen
    Abstract The development of a coupled damage-plasticity constitutive model for concrete is presented. Emphasis is put on thermodynamic admissibility, rigour and consistency both in the formulation of the model, and in the identification of model parameters based on experimental tests. The key feature of the thermodynamic framework used in this study is that all behaviour of the model can be derived from two specified energy potentials, following procedures established beforehand. Based on this framework, a constitutive model featuring full coupling between damage and plasticity in both tension and compression is developed. Tensile and compressive responses of the material are captured using two separate damage criteria, and a yield criterion with a multiple hardening rule. A crucial part of this study is the identification of model parameters, with these all being shown to be identifiable and computable based on standard tests on concrete. Behaviour of the model is assessed against experimental data on concrete. Copyright 2007 John Wiley & Sons, Ltd. [source]

    Tensile and lignocellulosic properties of Prosopis chilensis natural fabric

    G. Venkata Reddy
    Abstract The uniaxial natural fabric Prosopis chilensis was treated with NaOH (alkali), poly (vinyl alcohol) (PVA), and polycarbonate (PC) solutions. The Prosopis chilensis fabric belongs to Leguminosae family. The properties of ligno-cellulosic fabric and the effect of sodium hydroxide (NaOH) treatment were evaluated using thermal analysis by means of thermogravimetric and differential scanning calorimetry analysis, Autonated total reflection-fourier transform infrared spectroscopy, X-ray diffraction (XRD), and field emission scanning electron microscopy. Tensile properties of the untreated and fabric treated with NaOH, PVA, and PC were also studied to assess their performance. The fabric has good thermal resistance on alkali treatment. The FTIR method indicates lowering the hemi cellulose and lignin content by alkali treatment. Further, the XRD studies reveal that crystallinity of the fabric increases on alkali treatment. Tensile properties of the fabric were enhanced on treatments with NaOH, PVA, and PC treatments. 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source]

    Plasticizing Effects of Beeswax and Carnauba Wax on Tensile and Water Vapor Permeability Properties of Whey Protein Films

    Pau Talens
    ABSTRACT: The possible plasticizing effect of beeswax (viscoelastic wax) and carnauba wax (elastic wax) on tensile and water vapor permeability properties of whey protein isolate (WPI) films was studied. For the experiments, 3 groups of films with different WPI:glycerol ratios (1:1; 1.5:1; 2:1, 2.5:1, and 3:1) were prepared. The 1st group was made without the addition of wax, and the latter 2 groups were made with the addition of beeswax and carnauba wax, respectively, mixing 1 part of wax to 1 part of WPI. Lipid particle size, water vapor permeability, tensile properties, and thickness of films were analyzed and measured. The results show that the incorporation of beeswax produced a plasticizing effect in WPI:glycerol films, whereas carnauba wax produced an anti-plasticizing effect. The moisture barrier properties of WPI:glycerol films benefit from the addition of beeswax, by both increase of the hydrophobic character and decrease of the amount of hydrophilic plasticizer required in the film. [source]

    Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

    Kyung Jin Yoon
    The creep behavior of a commercial grade of Si3N4 was studied at 1350 and 1400C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 22.9 kJ/mol, and compression, 489.2 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ,18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution,precipitation of Si3N4. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution,precipitation creep of Si3N4 did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses. [source]

    Mechanical properties of injection molded long fiber polypropylene composites, Part 1: Tensile and flexural properties

    POLYMER COMPOSITES, Issue 2 2007
    K. Senthil Kumar
    Innovative polymers and composites are broadening the range of applications and commercial production of thermoplastics. Long fiber-reinforced thermoplastics have received much attention due to their processability by conventional technologies. This study describes the development of long fiber reinforced polypropylene (LFPP) composites and the effect of fiber length and compatibilizer content on their mechanical properties. LFPP pellets of different sizes were prepared by extrusion process using a specially designed radial impregnation die and these pellets were injection molded to develop LFPP composites. Maleic-anhydride grafted polypropylene (MA- g -PP) was chosen as a compatibilizer and its content was optimized by determining the interfacial properties through fiber pullout test. Critical fiber length was calculated using interfacial shear strength. Fiber length distributions were analyzed using profile projector and image analyzer software system. Fiber aspect ratio of more than 100 was achieved after injection molding. The results of the tensile and flexural properties of injection molded long glass fiber reinforced polypropylene with a glass fiber volume fraction of 0.18 are presented. It was found that the differences in pellet sizes improve the mechanical properties by 3,8%. Efforts are made to theoretically predict the tensile strength and modulus using the Kelly-Tyson and Halpin-Tsai model, respectively. POLYM. COMPOS., 28:259,266, 2007. 2007 Society of Plastic Engineers [source]

    The effect of biological studies of polyester composites filled carbon black and activated carbon from bamboo (Gigantochloa scortechinii)

    POLYMER COMPOSITES, Issue 1 2007
    H.P.S. Abdul Khalil
    The development of composites using various filler material increased significantly nowadays. Composite materials need to implement the biological and mechanicals impact in their life cycle. The carbon black (CB) and activated carbon (AC) from bamboo (Gigantochloa scortechinii) were used as filler in polyester composites. CB and AC were pyrolized to 700C and activated with ZnCl2. The composites were prepared with 40% filler loading. The degradation of the composites CB and AC (mechanical properties) to the soil burial test was determined. Tensile, flexural, impact and mass changes of the buried samples were investigated. The deterioration of the samples affected by the biological attack was confirmed by SEM studies. The assessment was done after 0, 3, 6, and 12 months of the biological exposure and the results were compared with the composites filled CaCO3 (commercial filler) and cast polyester resin (as control). POLYM. COMPOS. 28:6,14, 2007. 2007 Society of Plastics Engineers [source]

    Production of leather-like composites using short leather fibers.

    POLYMER COMPOSITES, Issue 6 2002

    Leather-like composites were prepared by addition of chemically modified short leather fibers (SLF) into a plasticized polyvinyl chloride (pPVC) matrix. The fibers were subjected to chemical modification by emulsion polymerization to achieve good interfacial adhesion between SLF and the pPVC matrix. The SLF with chemical modification were obtained from three different reaction conditions where these SLF have different percentages of grafted and deposited PMMA polymer onto the fiber surface. The incorporation of the SLF into the thermoplastic matrix was carried out using a torque-rheometer and the composites obtained were molded by compression. Tensile and tear mechanical tests were performed on composite samples, and the morphology of the fractured surfaces was analyzed using scanning electron microscopy (SEM). The results show that the incorporation by grafting of polymethyl metacrylate (PMMA) onto the fibers produced a significant improvement of their interfacial adhesion to pPVC, promoting the compatibilization between the fiber surface and matrix. The findings are discussed and interpreted in terms of enhanced adhesion at phase boundaries. Overall, the results confirm that it is possible to produce modified leather composites based on a pPVC matrix, which exhibit relatively high tensile strength, tear resistance and flexibility. These composites are very suitable candidate materials for applications in the footwear industry. [source]

    A phenomenological study of the mechanical properties of long-fiber filled injection-molded thermoplastic composites

    POLYMER COMPOSITES, Issue 5 2000
    V. K. Stokes
    Tensile and flexural tests on specimens cut from rectangular injection-molded plaques show that long-fiber filled thermoplastic composites are complex, non-homogeneous, anistropic material systems. Like all fiber-filled materials, they exhibit through-thickness nonhomogeneity as indicated by differences between tensile and flexural properties. The in-plane orientation of fibers in through-thickness layers causes the material to have in-plane anisotropic properties. However, these long-fiber filled materials exhibit an unexpectedly large level of in-plane nonhomogeneity. Also, the effective mechanical properties of these materials are strongly thickness dependent. The thinnest plaques exhibit the largest differences between the flow and cross-flow tensile properties. These differences decrease with increasing thickness. A methodology for part design with this class of materials is discussed. [source]

    Thermal, mechanical, and diffusional properties of nylon 6/ABS polymer blends: Compatibilizer effect

    Seung Phil Jang
    The thermal, mechanical, and water absorption properties of blends of nylon 6 (PA6) and acrylonitrile-butadiene-styrene copolymer (ABS) with and without the compatibilizer maleic anhydride (MAH) were studied. Polymers were melt-blended using a twin screw extruder, and injection molded into sheets. Tensile and impact properties, hardness, heat deflection resistance, and dimensional stability were enhanced by the incorporation of MAH. Synergistic effects were observed for tensile elongation and flexural properties. The melting temperature and the thermal stability were not significantly affected by the incorporation of MAH. The mechanical property enhancement by the introduction of compatibilizer was explained by the formation of a micro-domain structure in the blends. The equilibrium water uptake increased with increasing concentration of PA6, and the diffusion coefficient was determined from the water transport kinetics at different temperatures. Activation energy was extracted from the temperature dependence of the diffusion coefficient. No compatibilizer effect was observed in the swelling behavior. [source]

    Effect of spatial architecture on cellular colonization

    Yan Huang
    Abstract The spatial cell-material interaction remains vital issue in forming biodegradable scaffolds in Tissue Engineering. In this study, to understand the influence of spatial architecture on cellular behavior, 2D and 3D chitosan scaffolds of 50,190 kD and >310 kD MW were synthesized through air drying and controlled rate freezing/lypohilization technique, respectively. In addition, chitosan was emulsified with 19, 76, and 160 kD 50:50 poly lactide-co-glycolide (PLGA) using 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine (DMPC) as stabilizer. 2D and 3D scaffolds were formed by air drying and lyophilization as before. Tensile and compressive properties of films and scaffolds were analyzed in wet conditions at 37C. Alterations in the cell spreading, proliferation, and cytoskeletal organization of human umbilical vein endothelial cells (HUVECs) and mouse embryonic fibroblasts (MEFs) were studied. These results showed that the formed 3D chitosan scaffolds had interconnected open pore architecture (50,200 m size). HUVECs and MEFs had reduced spreading areas and circular morphology on 2D chitosan membranes compared with 3D chitosan scaffolds. The fluorescence photomicrographs for actin (using Alexa Fluor 488 phalloidin) and cytoplasm staining (using carboxyfluorescein diacetate-succinimidyl ester) demonstrated that the cells spread within 3D chitosan matrix. 2D and 3D emulsified chitosan and chitosan/PLGA scaffolds reduced the spreading of HUVECs and MEFs even further. Proliferation results, analyzed via MTT-Formazan assay and BrdU uptake assay, correlated with the spreading characteristics. The reductions in cell spreading area on emulsified surfaces were not detrimental to the viability and endocytic activity but to proliferation. The observed alterations in cellular colonization are in part due to the substrate stiffness and surface topography. In summary, these results suggest a significant influence of spatial architecture on cellular colonization. 2005 Wiley Periodicals, Inc. [source]

    The three-dimension finite element analysis of stress in posterior tooth residual root restored with postcore crown

    Gang Fu
    Some researchers have analyzed the stress of the anterior teeth after postcore crown restoration, but the stress of the posterior teeth after such restoration has not been reported. We used three-dimension finite element methods to analyze the stress magnitude and distribution of remaining dentin in posterior tooth residual root restored with postcore crown. The binding material, loading direction, number, length and material of posts were studied. Methods:, The models of residual root of maxillary first molar restored with postcore crown were created by CT scanning, mimics software and abaqus software. Different number, length and material of posts were used in the modeling. The posts were cemented with zinc-phosphate cement or composited resin. A load of 240 N was applied to the occlusal surface in four directions and tensile, shear, and von Mises stresses were calculated. Result:, (i) The maximum stress on remaining dentin changed irregularly as the number and length of posts changed. (ii) The maximum stress on remaining dentin decreased slightly as elastic modulus of the material of posts increased. (iii) The maximum stress on bonding layer and remaining dentin was lower when bonded with resin luting agent than with zinc-phosphate cement. (iv) The maximum stress on remaining dentin increased markedly as loading angle increased. Conclusion:, The number, length, material of posts, bonding material and loading angle all have influence on the magnitude and distribution of stress. The influence of loading angle is most apparent. [source]

    Instability investigation of cantilevered seacliffs

    Adam P. Young
    Abstract Wave action is a fundamental mechanism in seacliff erosion, whereby wave undercutting creates an unstable cantilevered seacliff profile and can lead to large catastrophic cliff failures, thus threatening coastal infrastructure. This study investigated the instability of two such failures that occurred in Solana Beach, California, by combining terrestrial LIDAR scanning, cantilever beam theory and finite element analysis. Each landslide was detected by evaluating the surface change between subsequent high resolution digital terrain models derived from terrestrial LIDAR data. The dimensions of failed cantilever masses were determined using the surface change measurements and then incorporated into failure stress analysis. Superimposing stress distributions computed from elastic cantilever beam theory and finite element modeling provided a method to back-calculate the maximum developed tensile and shear stresses along each failure plane. The results of the stress superposition revealed that the bending stresses caused by the cantilevered load contributed the majority of stress leading to collapse. Both shear and tensile failure modes were investigated as potential cliff failure mechanisms by using a comparison of the back-calculated failure stresses to material strengths found in laboratory testing. Based on the results of this research, the tensile strength of the cliff material was exceeded at both locations, thus causing the cliffs to collapse in tension. Copyright 2008 John Wiley & Sons, Ltd. [source]

    Shaking table tests on seismic response of steel braced frames with column uplift

    Mitsumasa Midorikawa
    Abstract Previous studies have suggested that rocking vibration accompanied by uplift motion might reduce the seismic damage to buildings subjected to severe earthquake motions. This paper reports on the use of shaking table tests and numerical analyses to evaluate and compare the seismic response of base-plate-yielding rocking systems with columns allowed to uplift with that of fixed-base systems. The study is performed using half-scale three-storey, 1 2 bay braced steel frames with a total height of 5.3 m. Base plates that yield due to column tension were installed at the base of each column. Two types of base plates with different thicknesses are investigated. The earthquake ground motion used for the tests and analyses is the record of the 1940 El Centro NS component with the time scale shortened by a factor of 1/,2. The maximum input acceleration is scaled to examine the structural response at various earthquake intensities. The column base shears in the rocking frames with column uplift are reduced by up to 52% as compared to the fixed-base frames. Conversely, the maximum roof displacements of the fixed and rocking frames are about the same. It is also noted that the effect of the vertical impact on the column associated with touchdown of the base plate is small because the difference in tensile and compressive forces is primarily due to the self-limiting tensile force in the column caused by yielding of the base plate. Copyright 2006 John Wiley & Sons, Ltd. [source]

    Syntactic Iron Foams with Integrated Microglass Bubbles Produced by Means of Metal Powder Injection Moulding,

    Jrg Weise
    Systematic tests for the production of pure iron (99%) foams with integrated microglass bubbles by means of metal powder injection moulding (MIM) have been carried out with variation of the glass bubble contents. Process parameters were optimized and the resulting materials characterized using density measurements, tensile and compression tests, metallographic sections, and scanning electron microscopy. The corrosion behavior of this novel material was characterized using potentiodynamic polarization measurements and immersion tests. [source]

    A Polycrystalline Approach to the Cyclic Behaviour of f.c.c. Alloys , Intra-Granular Heterogeneity

    Xavier Feaugas
    For several decades, the plastic deformation mechanisms of f.c.c. metals under cyclic loading have received considerable attention. The extensive work on this subject has gradually lead to the identification of the physical processes to be included in a formal scheme of fatigue behavior. Accordingly, we propose a review of the physical mechanisms of plastic deformation in f.c.c. metals and alloys to define the state-of-the-art and motivate future studies. The aim is to demonstrate the importance of a good knowledge of the heterogeneous nature of deformation at the intra-granular scale in defining a physical model of cyclic behavior. A large characterization of the different stages associated with the evolution of heterogeneous dislocation structures during tensile and cyclic loadings is given for an austenitic stainless steel AISI 316L. A unified view of these various structures is proposed in the form of a modified Pedersen's map [,max,=,f(,pcum), where ,max is the maximum plastic strain and ,pcum the cumulative plastic strain] in the case of tensile loading and different kinds of cyclic loading: uni-axial and multi-axial tests under stress or strain amplitude control. The specificities of each domain defined in the map are discussed in terms of long-range internal stresses in order to formalize, in a simple composite scheme, the intra-granular stress,strain field. The importance of taking into account this scheme and the nature of the different dislocations populations in a polycrystalline model is illustrated. [source]

    Vitrified Silica-Nanofiber Mats as Reinforcements for Epoxy Resins

    Oliver Weichold
    The effects of vitrified, electrospun silica nanofiber mats on the tensile and bending strength (see Figure) of epoxy resins are presented. The mats consist of randomly oriented, amorphous filaments of 600,800,nm diameter. The effects of annealing conditions and surface functionalization on the fiber/matrix adhesion is discussed. The results are compared to those of reference materials. [source]

    Roll-Bonded Titanium/Stainless-Steel Couples, Part 2: Mechanical Properties after Different Material-Treatment Routes

    S. Dziallach
    The accessible mechanical properties of a roll-bonded titanium/stainless-steel couple, consisting of grade-1 titanium and a low-carbon Cr-Ni-Mo-steel (1.4404), after different heat treatments are described. The mechanical properties, determined by tensile and stretch-forming tests, facilitate the optimum process widow for the heat-treatment parameters after roll bonding. The results of stretch-forming tests to determine the forming limits of the composite are shown. These tests also give important indications of the failure mechanism of the composite. Deep-drawing tests allow a first estimation of the deep-drawing functional area for a subsequent forming process to be made. [source]

    Tensile-Compressive Creep Asymmetry of Recent Die Cast Magnesium Alloys,

    S. Xu
    The tensile-compressive creep asymmetry of die cast magnesium alloys is experimentally explored and the possible deformation mechanisms are discussed. Creep tests were performed under tension and compression at 125,C and 150,C on die cast Mg alloys AM50, AE44 and AJ62A. Higher tensile than compressive creep strengths were observed for all alloys except for low pressure die cast AM50 at a low creep stress of 35 MPa at 125,C. An aging treatment of 250 hours at 180,C was employed for AM50 samples to obtain an over-aged microstructure that would minimize the effects of dynamic precipitation of ,-Mg17Al12 on creep. The creep data for the aged samples showed significant scatter, and the trend in tensile-compressive creep asymmetry of the aged samples is not clear for the short-term creep tests under high creep stresses. [source]

    Synthesis and Low Cycle Fatigue Behavior of In-situ Al-based Composite Reinforced with Submicron TiB2 and TiC Particulates,

    S.C. Tjong
    Low cycle fatigue behavior of in-situ aluminum based composite reinforced with submicron TiB2 and TiC particulates was investigated. This novel composite was prepared from the TiO2 -Al-B-C system via reactive hot pressing. The incorporation of carbon into such a system induces the formation of TiC particulate at the expense of brittle Al3Ti phase. The influence of submicron particulate formation on the tensile and fatigue properties of the composite is discussed. [source]

    Numerical and experimental investigation of mixed-mode fracture parameters on silicon nitride using the Brazilian disc test

    ABSTRACT Engineering applications of ceramics can often involve mixed-mode conditions involving both tensile and shear loading. Mixed-mode fracture toughness parameters are evaluated for applicability to ceramics using the Brazilian disc test on silicon nitride. Semi-elliptical centrally located surface flaws are induced on the disc specimens using Vickers indentation and compression loaded to fracture with varying levels of mode mixity. The disc specimens are modelled via 3D finite element analysis and all three modes of stress intensity factors computed along the crack front, at failure load. We present a numerical and experimental investigation of four widely used mixed-mode fracture criteria and conclude that the critical strain energy release rate criterion is simple to implement and effective for silicon nitride under mixed-mode conditions. [source]

    Evaluation of creep damage accumulation models: Considerations of stepped testing and highly stressed volume

    W. A. GRELL
    ABSTRACT Many components experience combined temperature and stress loading and are designed to withstand creep. In this study, experimental creep testing was performed under both static and stepped loading conditions with constant temperature for two specimen geometries (tensile and three-point bend). The objective of this study was to evaluate whether existing damage accumulation models accurately predict creep performance when considering step loading and stress gradients. Model predictions, based on static tensile creep data and using a highly stressed volume correction for the three-point bend specimens and the experimental average damage sum, agreed well with experimental data; differences were on average within 38% (static) and 2.2 h (stepped). Comparisons showed more accurate predictions using an exponential Larson,Miller parameter curve and the Pavlou damage accumulation model. Findings of the current study have applicability to component design, where complex geometries often contain stress gradients and it is desirable to predict creep performance from static tensile creep data. [source]

    Characterization of strength properties of thin polycrystalline silicon films for MEMS applications

    R. Boroch
    ABSTRACT The aim of this work is to characterize the strength properties of polycrystalline silicon (polysilicon) with the use of tensile and bending test specimens. The strength of thin polysilicon films with different geometry, size and stress concentrations has been measured and correlated with the effective size of the specimen and its stress distribution. The test results are evaluated using a probabilistic strength approach based on the weakest link theory with the use of STAU software. The use of statistic methods of strength prediction of polysilicon test structures with a complex geometry and loading based on test values for standard material tests specimen has been evaluated. [source]

    Cleavage fracture of RPV steel following warm pre-stressing: micromechanical analysis and interpretation through a new model

    S. R. BORDET
    ABSTRACT In this paper, the warm pre-stress (WPS) effect on the cleavage fracture of an 18MND5 (A533B) RPV steel is investigated. This effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following a prior loading at high temperature, has received great interest in light of its significance in the integrity assessment of structures, such as nuclear pressure vessels, subjected to thermal transients. Several loading cycles between room temperature (RT) and ,150 C are considered: Load-Unload-Cool-Fracture (LUCF), Load-Cool-Fracture (LCF) and Load-Cool with Increasing K-Fracture (LCIKF). All experiments complied with the conservative principle, which states that no fracture will occur if the applied stress intensity factor (SIF) decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. The experimental results indicate that an effective WPS effect is present even at small pre-load (Kwps= 40 MPa,m), and that a minimum critical slope (,,K/,T) in the LCIKF cycle has to be exceeded to induce cleavage fracture between RT and ,150 C. Numerical modelling was performed using mixed isotropic and kinematic hardening laws identified on notched tensile (NT) specimens, tested in tension to large strains (up to 40%), followed by large compressive strains. Detailed microstructural investigations on compact tensile (CT) and NT fracture test specimens were performed so as to determine the nature of the cleavage initiation sites, as well as the local mechanical conditions at fracture. Based on this local information, a new cleavage model was calibrated and applied to predict the probability of cleavage fracture after WPS: it is shown that the predictions are in good agreement with the experimental results. [source]

    Tensile and compressive damage coupling for fully-reversed bending fatigue of fibre-reinforced composites

    W. Van Paepegem
    ABSTRACT Due to their high specific stiffness and strength, fibre-reinforced composite materials are winning through in a wide range of applications in automotive, naval and aerospace industry. Their design for fatigue is a complicated problem and a large research effort is being spent on it today. However there is still a need for extensive experimental testing or large safety factors to be adopted, because numerical simulations of the fatigue damage behaviour of fibre-reinforced composites are often found to be unreliable. This is due to the limited applicability of the theoretical models developed so far, compared to the complex multi-axial fatigue loadings that composite components often have to sustain in in-service loading conditions. In this paper a new phenomenological fatigue model is presented. It is basically a residual stiffness model, but through an appropriate choice of the stress measure, the residual strength and thus final failure can be predicted as well. Two coupled growth rate equations for tensile and compressive damage describe the damage growth under tension,compression loading conditions and provide a much more general approach than the use of the stress ratio R. The model has been applied to fully-reversed bending of plain woven glass/epoxy specimens. Stress redistributions and the three stages of stiffness degradation (sharp initial decline , gradual deterioration , final failure) could be simulated satisfactorily. [source]

    The effect of overload on the fatigue crack growth behaviour of 304 stainless steel in hydrogen

    M. H. Kelestemur
    Fatigue crack growth (FCG) behaviour and its characteristics following tensile overloads were investigated for AISI 304 stainless steel in three different atmospheres; namely dry argon, moist air and hydrogen. The FCG tests were performed by MTS 810 servohydraulic machine. CT specimens were used for the tests and crack closure measurements were made using an extensometer. FCG rates of 304 stainless steel at both dry argon and moist air atmospheres have shown almost the same behaviour. In other words, the effect of moisture on FCG of this material is very small. However, in a hydrogen atmosphere, the material showed considerably higher crack growth rate in all regimes. In general, for all environments, the initial effect of overloads was to accelerate the FCG rate for a short distance (less than a mm) after which retardation occurred for a considerable amount of time. The main causes for retardation were found as crack blunting and a long reinitiation period for the fatigue crack. Regarding the environmental effect, the overload retardation was lowest in a hydrogen atmosphere. This low degree of retardation was explained by a hydrogen embrittlement mechanism. In a general sense, hydrogen may cause a different crack closure mechanism and hydrogen induced crack closure has come in to the picture. Scanning electron microscope and light microscope examinations agreed well with the above results. [source]

    Characterization of a Novel Fiber Composite Material for Mechanotransduction Research of Fibrous Connective Tissues

    Hazel R. C. Screen
    Abstract Mechanotransduction is the fundamental process by which cells detect and respond to their mechanical environment, and is critical for tissue homeostasis. Understanding mechanotransduction mechanisms will provide insights into disease processes and injuries, and may support novel tissue engineering research. Although there has been extensive research in mechanotransduction, many pathways remain unclear, due to the complexity of the signaling mechanisms and loading environments involved. This study describes the development of a novel hydrogel-based fiber composite material for investigating mechanotransduction in fibrous tissues. By encapsulating poly(2-hydroxyethyl methacrylate) rods in a bulk poly(ethylene glycol) matrix, it aims to create a micromechanical environment more representative of that seen in vivo. Results demonstrated that collagen-coated rods enable localized cell attachment, and cells are successfully cultured for one week within the composite. Mechanical analysis of the composite indicates that gross mechanical properties and local strain environments could be manipulated by altering the fabrication process. Allowing diffusion between the rods and surrounding matrix creates an interpenetrating network whereby the relationships between shear and tension are altered. Increasing diffusion enhances the shear bond strength between rods and matrix and the levels of local tension along the rods. Preliminary investigation into fibroblast mechanotransduction illustrates that the fiber composite upregulates collagen I expression, the main protein in fibrous tissues, in response to cyclic tensile strains when compared to less complex 2D and 3D environments. In summary, the ability to create and manipulate a strain environment surrounding the fibers, where combined tensile and shear forces uniquely impact cell functions, is demonstrated. [source]

    Spontaneous Outcropping of Self-Assembled Insulating Nanodots in Solution-Derived Metallic Ferromagnetic La0.7Sr0.3MnO3 Films

    Csar Moreno
    Abstract A new mechanism is proposed for the generation of self-assembled nanodots at the surface of a film based on spontaneous outcropping of the secondary phase of a nanocomposite epitaxial film. Epitaxial self-assembled Sr,La oxide insulating nanodots are formed through this mechanism at the surface of an epitaxial metallic ferromagnetic La0.7Sr0.3MnO3 (LSMO) film grown on SrTiO3 from chemical solutions. TEM analysis reveals that, underneath the La,Sr oxide (LSO) nanodots, the film switches from the compressive out-of-plane stress component to a tensile one. It is shown that the size and concentration of the nanodots can be tuned by means of growth kinetics and through modification of the La excess in the precursor chemical solution. The driving force for the nanodot formation can be attributed to a cooperative effect involving the minimization of the elastic strain energy and a thermodynamic instability of the LSMO phase against the formation of a Ruddelsden,Popper phase Sr3Mn4O7 embedded in the film, and LSO surface nanodots. The mechanism can be described as a generalization of the classical Stranski,Krastanov growth mode involving phase separation. LSO islands induce an isotropic strain to the LSMO film underneath the island which decreases the magnetoelastic contribution to the magnetic anisotropy. [source]

    The role of friction and secondary flaws on deflection and re-initiation of hydraulic fractures at orthogonal pre-existing fractures

    Xi Zhang
    SUMMARY In this study, we explore the nature of plane-strain hydraulic fracture growth in the presence of pre-existing fractures such as joints without or with secondary flaws. The 2-D plane-strain fracture studied can be taken as a cross-section through the short dimensions of an elongated 3-D fracture or as an approximate representation of the leading edge of a 3-D fracture where the edge curvature is negligible. The fluid-driven fracture intersects a pre-existing fracture to which it is initially perpendicular and is assumed not to immediately cross, but is rather deflected into the pre-existing fracture. The intersection results in branching of the fracture and associated fluid flow into the pre-existing fracture. Further growth results in opening and frictional sliding along the pre-existing fracture. Fracture propagation in an impermeable homogeneous elastic medium and fluid invasion into a pre-existing fracture are both driven by an incompressible, Newtonian fluid injected at a constant rate. The frictional stress on the surfaces of pre-existing fractures is assumed to obey the Coulomb law. The governing equations for quasi-static fluid-driven fracture growth are given and a scaling is introduced to help identify important parameters. The displacement discontinuity method and the finite difference method are employed to deal with this coupling mechanism of rock fracture and fluid flow. In order to account for fluid lag, a method for separately tracking the crack tip and the fluid front is included in the numerical model. Numerical results are obtained for internal pressure, frictional contact stresses, opening and shear displacements, and fluid lag size, as well as for fracture re-initiation from secondary flaws. After fracture intersection, the hydraulic fracture growth mode changes from tensile to shearing. This contributes to increased injection pressure and to a reduction in fracture width. In the presence of pre-existing fractures, the fluid-driven cracks can be arrested or retarded in growth rate as a result of diversion of fluid flow into and frictional sliding along the pre-existing fractures. Frictional behaviour significantly affects the ability of the fluid to enter or penetrate the pre-existing fracture only for those situations where the fluid front is within a certain distance from the intersecting point. Importantly, fluid penetration requires higher injection pressure for frictionally weak pre-existing fractures. Fracture re-initiation from secondary flaws can reduce the injection pressure, but re-initiation is suppressed by large sliding on pre-existing fractures that are frictionally weak. [source]

    Non-double-couple mechanisms in the seismicity preceding the 1991,1993 Etna volcano eruption

    A. Sara
    Summary The temporal evolution of the complete source moment tensor is investigated for 28 earthquakes that occurred at Mt Etna in the period August 1990,December 1991 preceding the biggest eruption of the last three centuries. We perform several tests to check the robustness of the results of inversion considering different frequency ranges and different groups of stations. As well as the selection of good-quality data, the error analysis, statistically significant at the 95 per cent confidence level, is employed to validate the findings of the inversion and to distinguish between physical solutions and artefacts of modelling. For events between 0.3 and 10 km depth, strike-slip mechanisms prevail on normal, inverse and dip-slip mechanisms; this is possibly due to the dyke-induced stress dominating the overall stress field at the surface, producing a continuous switch of the tensile and compressive axes. The regional E,W tension prevails at depth, as indicated by the prevalence of normal mechanisms. An increment of the non-double-couple components is observed immediately before the eruption and can be related to movements of fluids, even though, for some events, the complex interaction between tectonic stress and volcanic activity cannot be excluded. The source time functions retrieved are in general simple and short but some show complexities, as one would expect in volcanic seismicity. From the seismic scalar moment found, we extrapolate an empirical moment,magnitude relation that we compare with other relations proposed for the same area and computed for the duration magnitude and the equivalent Wood,Anderson magnitude. [source]