Pull-out Mechanism Research Articles

The effects of heat treatment and coating roughness on the strength of alumina-zirconia fibres

The strengths of Al 2O 3–20 wt.% ZrO 2 fibres as-received, heat treated and coated with SnO 2 have been determined. The mean strength of as-received fibres was 1380 MPa for a gage length of 17 mm. The strengths of fibres heat treated at 500, 600 and 900 °C decreased by 5, 7 and 21%, respectively, relative to the as-received fibres. This could be because of a secondary glassy phase formation at the grain boundaries. It was found that the accuracy of the tensile strength distribution increased with the number of samples tested and at a sample number of 80, the variation in the tensile strength was about 2%. The thickness and roughness of the SnO 2 coating had a profound effect on the strength of the coated fibres. It was observed that as the coating thickness increased, roughness of the coating increased. This acted as a notch in order to decrease the strength of the fibres. Control of roughness at the fibre/coating and/or coating/matrix interphase is very important in the development of tough ceramic–matrix composites. As the interphase roughness increases, fibre debonding and pull-out decrease because of the increased compressive clamping stress. Hence for increased toughness in CMCs through fibre debonding and pull-out mechanisms, a relatively smooth interphase is necessary. The interphase roughness can be controlled either by selecting smooth fibres or by decreasing coating roughness.

Micro-mechanical approximation to the stress field around an inclined fiber of FRCC

Matrix spalling or crushing is one of the important mechanisms of fiber-matrix interaction of fiber reinforced cementitious composites (FRCC). The fiber pullout mechanisms have been extensively studied for an aligned fiber but matrix failure is rarely investigated since it is thought not to be a major affect. However, for an inclined fiber, the matrix failure should not be neglected. Due to the complex process of matrix spalling, experimental investigation and analytical study of this mechanism are rarely found in literature. In this paper, it is assumed that the load transfer is concentrated within the short length of the inclined fiber from the exit point towards anchored end and follows the exponential law. The Mindlin formulation is employed to calculate the 3D stress field. The simulation gives much information about this field. The 3D approximation of the stress state around an inclined fiber helps to qualitatively understand the mechanism of matrix failure. Finally, a spalling criterion is proposed by which matrix spalling occurs only when the stress in a certain volume, rather than the stress at a small point, exceeds the material strength. This implies some local stress redistribution after first yield. The stress redistribution results in more energy input and higher load bearing capacity of the matrix. In accordance with this hypothesis, the evolution of matrix spalling is demonstrated. The accurate prediction of matrix spalling needs the careful determination of the parameters in this model. This is the work of further study.

Large-Scale Simulation of Adhesion Dynamics for End-Grafted Polymers

The adhesion between a polymer melt and substrate is studied in the presence of chemically attached chains on the substrate surface. Extensive molecular dynamics simulations have been carried out to study the effect of temperature, tethered chain areal density (Σ), tethered chain length (Nt), chain bending energy (kθ), and tensile pull velocity (v) on the adhesive failure mechanisms of pullout and/or scission of the tethered chains. We observe a crossover from pure chain pullout to chain scission as Nt is increased. Below the glass transition, the value of Nt for which this crossover begins approaches the bulk entanglement length Ne. For the values of Nt and Σ used here, no crossover to crazing is observed.

Surface-tethered chains entangled in a polymer melt: effects on adhesion dynamics.

The adhesion between a polymer melt and substrate is studied in the presence of chemically attached chains on the substrate surface. Extensive molecular dynamics simulations have been carried out to study the effect of temperature, tethered chain density (Sigma), tethered chain length (N(t)), and tensile pull velocity (v) on the adhesive failure mechanisms of pullout and/or scission of the tethered chains. We observe a crossover from pure chain pullout to chain scission as N(t) is increased. The value of N(t) for which this crossover begins approaches the bulk entanglement length N(e) as the temperature is lowered.

Inertial effects in the pullout mechanism during dynamic loading of a bridged crack

Inertial effects in the mechanism of fibre pullout during the dynamic propagation of a bridged crack are examined by reposing simple shear lag models of pullout as problems of dynamic wave propagation. The only coupling considered between the fibres and the matrix is uniform, rate independent friction—no debond energy is included. Analytical solutions are found for the coupled waves propagating in the fibres and the matrix away from the fracture plane of the bridged crack as the bridging tractions increase with time. These solutions yield the time-dependent relationship between the crack opening displacement and the bridging traction. Engineering criteria for inertial effects being significant are deduced by comparing the dynamic bridging traction law with its counterpart for static loading, which is recovered as a limit of the dynamic case. The criteria are evaluated for two crack cases: the asymptotic limit of a long, fully bridged matrix crack propagating unstably through a fibrous composite under remote tension; and a finite crack partially bridged by stitches or rods that delaminates a double cantilever beam under the impetus of a flying wedge. In both cases, the rate of increase of the crack opening displacement appears to be sufficient for inertial effects to be pronounced in the bridging (pullout) mechanism. Expected trends of the significance of inertial effects with material and geometrical parameters are identified.

Computer Modeling and FEA Simulation for Composite Single Fiber Pull-Out

Using state-of-the-art computer-aided technologies, particularly CAD and integrated CAD/FEA technique to model and simulate the composite, the single-fiber pull-out problem is presented. The capability and effectiveness of using enabled CAD to generate solid modeling based composite fiber/matrix heterogeneous assembly and its application with the seamless integration of CAD/FEA to simulate fiber pull-out mechanisms and effects of material and geometric variation on stresses in fiber and matrix are demonstrated. The CAD/FEA model predictions are compared with an analytical solution. Parametric studies of fiber pull-out problems with different stiffness ratios of fiber and matrix, and with irregular fiber cross-sections, for which solutions are difficult to obtain analytically, are also presented.

Effect of whisker surface treatment on the mechanical properties of 20vol%SiCw/BAS glass-ceramic composites

20vol%SiCw/BAS (barium aluminosilicate) composites with as-received, acid-leached and heat-treated whiskers were fabricated by a hot-pressing technique and their mechanical properties were evaluated. The whiskers used in these composites were characterized using Auger electron spectrometry (AES) and scanning electron microscopy (SEM), and the observations were correlated with the mechanical properties. The results show that the oxygen content of the whisker surfaces was reduced by the hydrogen fluoride (HF) acid-leaching treatment and the surface geometry of whiskers was smoothed by the heat-treatment in an inert gas atmosphere. The corresponding composites show an obvious increase (18% and 31%, respectively) in toughness compared with the composites reinforced with as-received whiskers. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, crack bridging and whisker pullout for the composites with surface treated whiskers.

Analysis and Experimental Study for Steel Fibre Pullout from Cementitious Matrices

A model for the response of steel fibres embedded in a cementitious matrix and subjected to a pullout load, is presented. The pullout behaviour is dictated by the force transmission between fibre and matrix through an interfacial layer surrounding the fibre. Three distinct pullout load - bearing mechanisms are considered. The first one assumes perfect bond between matrix and fibre and is used for the description of the elastic stage when no real “slip” occurs. The second one is a transitional mechanism, which describes the behaviour at the onset of debonding, and the last one is a frictional dynamic pullout mechanism based on the matrix hydration shrinkage and a fibre - matrix misfit consideration. Pullout and pullthrough tests of steel fibres from cementitious matrices are also reported herein in order to investigate experimentally the pullout mechanisms, generate related data and use them complementary to the developed model. An attempt for the validation of the proposed model through comparisons between experimental data and analytical behaviour curves is also included.

Influence of Fibre-Matrix Adhesion on Mechanical Properties of Graphite/Epoxy Composites: III. Impact and Dynamic Mechanical Properties

An experimental investigation was carried out on the graphite/epoxy composites with two different fibre surface conditions to identify the effect of fibre-matrix adhesion on the impact and dynamic mechanical properties. The results obtained from Charpy and falling weight impact tests did not show a clear influence of fibre-matrix adhesion on the impact properties of the composites. This is attributed to the highly localised damage process of impact tests, in which the main damage mechanism is the fibre breakage and there are no extensive delamination and fibre pull-out mechanisms involved. The dynamic mechanical analysis (DMA) is a non-destructive method and it is different from the common methods for characterisation of fibre-matrix interfacial properties such as fibre pull-out and fibre fragmentation tests in which fibre-matrix interface debonding is produced. The results of DMA tests indicate the dynamic properties of the composites are not very sensitive to the change of fibre-matrix adhesion. During the DMA tests, there is no or little fibre-matrix debonding induced. Therefore, the DMA method may not be very applicable for evaluating the fibre-matrix interfacial adhesion in practical applications.

Mechanics Of Crack Bridging Under Dynamic Loads

AbstractA bridging law for fiber reinforced composites under dynamic crack propagation conditions has been derived. Inertial effects in the mechanism of fibre pullout during dynamic propagation of a bridged crack are critically examined for the first time. By reposing simple shear lag models of pullout as problems of dynamic wave propagation, the effect of the frictional coupling between the fibres and the matrix is accounted for in a fairly straightforward way. The solutions yield the time-dependent relationship between the crack opening displacement and the bridging traction. Engineering criteria and the role of material and geometrical parameters for significant inertial effects are identified.

Mechanics Of Crack Bridging Under Dynamic Loads

A bridging law for fiber reinforced composites under dynamic crack propagation conditions has been derived. Inertial effects in the mechanism of fibre pullout during dynamic propagation of a bridged crack are critically examined for the first time. By reposing simple shear lag models of pullout as problems of dynamic wave propagation, the effect of the frictional coupling between the fibres and the matrix is accounted for in a fairly straightforward way. The solutions yield the time-dependent relationship between the crack opening displacement and the bridging traction. Engineering criteria and the role of material and geometrical parameters for significant inertial effects are identified.

A Theoretical Model for Anchored Geosynthetics in Pull-Out Tests

A theoretical model is presented for the pull-out response of strip anchor reinforcement based on a nonlinear elastic (hyperbolic) shear stress-displacement relationship for the soil-reinforcement interface. The interface pull-out mechanism has been simplified by formulating the boundary conditions for the anchor section. Additional terms for the soil-reinforcement interaction, which incorporate the anchor section, are also introduced. A new concept of an anchorage factor (anchor-to-strip reinforcement capacities), the relative stiffness, and the displacement parameters are defined to explain the pull-out behaviour. A parametric study is carried out for a range of relative stiffness values, interface shear stresses, and anchorage factors. The normalised load-displacement relationship and the variations of pull-out force and displacements with distance along the reinforcement, at different anchorage factors, are presented. Experimental pull-out test results for planar reinforcements are also examined/compared with the model using a zero anchorage factor. The model helps to evaluate the pull-out displacements, stresses, and strains along the length of the strip anchor reinforcement.

Reactive compatibilization of polyamide-6 (PA 6)/polybutylene terephthalate (PBT) blends by a multifunctional epoxy resin

A multifunctional epoxy resin has been demonstrated to be an efficient reactive compatibilizer for the incompatible and immiscible blends of polyamide-6 (PA 6) and polybutylene terephthalate (PBT). The torque measurements give indirect evidence that the reaction between PA and PBT with epoxy has an opportunity to produce an in situ formed copolymer, which can be as an effective compatibilizer to reduce and suppress the size of the disperse phase, and to greatly enhance mechanical properties of PA/PBT blends. The mechanical property improvement is more pronounced in the PA-rich blends than that in the PBT-rich blends. The fracture behavior of the blend with less than 0.3 phr compatibilizer is governed by a particle pullout mechanism, whereas shear yielding is dominant in the fracture behavior of the blend with more than 0.3 phr compatibilizer. As the melt and crystallization temperatures of the base polymers are so close, either PA or PBT can be regarded as a mutual nucleating agent to enhance the crystallization on the other component. The presence of compatibilizer and in situ formed copolymer in the compatibilized blends tends to interfere with the crystallization of the base polymers in various blends. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 23–33, 2000

Pullout test model for extensible reinforcement

A formulation for the analysis of pullout test on highly extensible planar reinforcement is presented. The non-linear differential equation for pullout mechanism was expressed in non-dimensional form and solved numerically using the Gauss–Siedel technique. Parametric study was carried out for various ranges of relative stiffnesses, and relative bond resistances. Normalized load–displacement relations and the variations of pullout force and reinforcement displacements along the length of reinforcement are presented graphically. A method for the estimation of the interface interaction parameters from a pre-failure test is also given. The numerical predictions compare well with the available experimental pullout test results for various geotextiles, polymers and nylon geosynthetics. Copyright © 1999 John Wiley & Sons, Ltd.

Tensile properties and failure mechanisms of 3D woven GRP composites

Abstract Tensile tests were performed on glass reinforced polymer (GRP) composites with three-dimensional (3D) orthogonal, normal layered interlock, and offset layered interlock woven fibre architectures. The mechanical properties and failure mechanisms under tensile loading were similar for the three composites. Cracks formed at low strains within the resin-rich channels between the fibre tows and around the through-thickness binder yarns in the composites, although this damage did not alter the tensile properties. At higher applied tensile stresses the elastic modulus was reduced by 20–30% due to inelastic tow straightening and cracking around the most heavily crimped in-plane tows. Further softening occurred at higher strains by inelastic straightening of all the tows. Composite failure occurred within a localised region and the discrete tow rupture events that have caused tow lock-up and pullout mechanisms in other 3D woven composites were not observed.

Modeling the bundle bridging mechanism in 2D SiC/C/SiC composite materials

Abstract Mechanical tests have been carried out on various sizes of compact tension (CT) specimen made of two-dimensional woven SiC/C/SiC composite material. As a continuation of previous work, a model is proposed to estimate the magnitude of the bundle bridging mechanism in this composite. The results summarized below, which are compared with previous ones obtained from mechanical tests performed on dog-bone and double-edge notched specimens, show that the smallest size of CT specimen (W=20 mm) provides a satisfactory estimate of the ultimate tensile strength of the bundle of fibers (UTSBF). However, it has been noted that a more accurate estimate of the bundle strength could be achieved by a more precise model of the crack-opening angle and its effects on both bundle bridging and pull-out mechanisms.

Microstructure and room-temperature mechanical properties of Si3N4 with various α/β phase ratios

After polycrystalline silicon nitride samples with various α/β phase ratios were fabricated by uniaxial hot-pressing under vacuum, their microstructures, α-to-β transformations, and grain-growth mechanisms were then characterized by electron microscopy. Room-temperature fracture toughness (KIC) increased with increasing β-phase content, and a value of 7.0 ± 0.5 MPa ċ m1/2 was obtained for a sample that contained 60 vol% β phase. The increase in KIC is the result of an increase in elongated-grain fractions with increasing β content, which promotes energy absorption by crack deflection, as well as by grain debonding, pullout, and bridging mechanisms.

A Theoretical Model for the Pull-Out Response of Geosynthetic Reinforcement

Soil-reinforcement pull-out tests are essential for evaluating the strength, integrity, and effectiveness of the soil-reinforcement system. In this paper, a new pull-out test model that calculates the soil-geosynthetic reinforcement interface shear stress for highly extensible geosynthetic reinforcement is proposed. Based on a new bilinear interface shear model, the geosynthetic pull-out test results are calculated with regard to the variation of the mobilised geosynthetic tension with distance, geosynthetic pre-yield and post-yield behaviour, and the effective and extended length of the geosynthetic reinforcement. The resulting nonlinear equation for the soil-geosynthetic interface shear stress pull-out mechanism is nondimensionalised, expressed in a finite difference form, and solved numerically using the Gauss-Siedel technique. A parametric study is carried out for a range of relative stiffness values and interface shear stresses. The normalised load-displacement relationship and the variation of the pull-out force and reinforcement displacements, with distance along the reinforcement, are presented. The values calculated using the proposed model are compared with experimental pull-out test results for a needle-punched, nonwoven geotextile, polyester fibres coated with polyethylene, and nylon reinforcements.

CREEP CRACK GROWTH WITH SMALL SCALE BRIDGING IN CERAMIC MATRIX COMPOSITES

Time-dependent crack growth in ceramic matrix composites with linearly creeping fibers in an elastic matrix is predicted for bridged regions that are much smaller than the crack length. The relationship between crack openings and bridging tractions was modeled previously and accounts for the mechanics of fiber pull-out while fibers are creeping. The length of the bridged zone is determined by a fiber failure criterion and the condition that the stress intensity factor at the crack tip equals the matrix toughness. Solutions that relate constant crack velocities to applied stress intensity factors are presented. Two fiber failure criteria are considered: a critical crack opening and a critical total strain in the fiber at the crack plane. When fiber failure is governed by a critical crack opening, bridging lengths will decrease with increasing velocity. For the case of a critical strain, the steady-state bridge length will increase with increasing velocity. Asymptotic analytical solutions are presented for cases where the crack velocity is small. © 1997 Acta Metallurgica Inc.

Deformation Behavior of Poly(ether ester) Copolymer As Revealed by Small- and Wide-Angle Scattering of X-ray Radiation from Synchrotron

The deformation behavior of poly(ether ester) is studied by means of small- and wide-angle X-ray scattering (SAXS and WAXS). The material under investigation represents a polyblock thermoplastic elastomer of poly(ether ester) (PEE) type. It comprises poly(butylene terephthalate) (PBT) as hard segments and polyethylene glycol (PEG) as soft segments in a ratio of 57/43 wt %. Isotropic PEE bristles are drawn to five times of their initial length and subsequently annealed with fixed ends for 6 h at 170 °C in vacuum. The WAXS patterns were registered by a pinhole camera and a 2D area gas detector. These measurements were performed both under stress and during the subsequent relaxation in the absence of stress. The deformation was increased stepwise up to the breaking point of the sample (ca. 185%). SAXS patterns were obtained in the same deformation range by means of monochromatic X-ray radiation in the beamline A2 of the synchrotron DESY in Hamburg, Germany. SAXS patterns were registered by means ofa 2D Image-plate detector. Five deformation intervals were revealed by SAXS. In the first one (e = 0-50%) an ensemble of uncorrelated strained microfibrils exists and the corresponding layer line small-angle pattern is observed. These microfibrils scatter independently. In the second interval (e = 50-80%) interactions between neighboring microfibrils develop, a microfibrillar network is observed, and the layer line pattern transforms into a four-point diagram. In the third interval (e = 80-100%) two additional reflections show up and an unique six-point pattern is seen. Pull-out of tie molecules from crystallites begins to fibrillate the network. This pull-out mechanism is independently proved by WAXS. In the fourth interval (e = 100-130%) the mean long period of the four-point pattern decreases and the pattern itself vanishes. At last the fiber is completely fibrillated and only a two-point pattern remains visible. In the second, third, and fourth intervals, the microfibrils correlate in the transverse direction, which allows determination of the interfibrillar distance (so-called transverse long period). It decreases gradually with the progress of deformation in both the strained and relaxed state. In the fifth interval (e > 130%) the long period of the two point pattern remains constant, but its intensity decreases until the fiber breaks. Only a few microfibrils are simultaneously carrying the load. They are destroyed one by one, until the fiber breaks as a whole.