An experimental study on the dynamic fracture of extruded AA6xxx and AA7xxx aluminium alloys

The dynamic fracture behaviour of extruded AA6xxx and AA7xxx aluminium alloys is investigated using an instrumented Charpy test machine and V-notch specimens. The specimens are made from extruded flat profiles with a rectangular cross-section of 10 mm thickness and 83 mm width. The material is in T6 temper, i.e. the peak hardness condition. The alloys have either recrystallized or fibrous grain structure. For each alloy six different Charpy impact tests are carried out in two series. In Series 1, the notch is parallel to the thickness direction of the profile (i.e. through the thickness), while the notch is perpendicular to the thickness direction in Series 2 (i.e. lying in the plane of the flat profile). In each series, the longitudinal direction of the specimen is parallel, 45° or 90° to the extrusion direction. Comprehensive fractographic investigations are carried out for the different tests and alloys. It is found that the dissipated energy is practically invariant to specimen orientation and notch direction for the recrystallized alloy. For the fibrous alloys the dissipated energy is lower when the longitudinal direction of the specimen is 90° to the extrusion direction, i.e. when the notch is parallel with the fibrous grain structure. Further, the energy dissipation is higher for Series 2 than for Series 1 due to substantial delamination and secondary cracking in Series 2. The precipitate-free zones (PFZs) formed adjacent to the grain boundary are weak areas, preferable for crack initiation and growth. This is seen in the fracture surface as facets with high density of small dimples and is more pronounced for specimens with the notch parallel to the fibre direction.     

The effect of grinding direction on flaw character and strength of single crystal and polycrystalline ceramics

The effect upon the room-temperature strengths and fractures of flexure bars caused by grinding them either parallel or perpendicular to their tensile axis was investigated for selected single- and polycrystalline ceramics. Particular attention was given to the character of the flaws from which failure initiated. It was shown that grinding introduces two basic sets of flaws: one set forms at an average angle of 0°, and the other at an average angle of 90°, to the grinding direction. The angles of these flaws varied somewhat due to their formation on preferred fracture planes in single-crystal or larger-grain polycrystalline bodies as well as due to statistical effects. However, overall, the difference in these two sets of flaws was a major factor in the effect of grinding direction on strength. Flaws that formed approximately parallel with the grinding direction were typically more severe, and hence lead to lower strengths for grinding perpendicular to the bar axis, i.e. when the stress was perpendicular, or nearly so, to them.     

Microstructure and Electrical Transport Properties of Bi<Subscript>3</Subscript>TiNbO<Subscript>9</Subscript> High-Temperature Piezoceramics

High-density ceramic samples of the Bi₃TiNbO₉ ferroelectric phase with a layered perovskite-like structure have been produced by tape casting slurry technology and hot pressing processes. The samples have been characterized by X-ray diffraction and dielectric, piezoelectric, and pyroelectric measurements. The results demonstrate that the samples have a well-defined texture due to the preferential orientation of the basal planes of their platelike grains across the pressing axis or along the film casting direction. The Curie temperature of the samples has been determined to be T_C = 1180 K. The samples have anisotropic electrical transport characteristics. Their piezoelectric constants d₃₃ in directions perpendicular and parallel to the texture plane is 22 and 5 pC/N, respectively. The conclusion has been drawn that the synthesized materials are potentially attractive for producing high-temperature piezoelectric elements.     

Fibre orientation effects on the fracture of short fibre polymer composites: on the existence of a critical fibre orientation on varying internal material variables

The dependence of fracture toughness on fibre orientation, in short fibre reinforced polymers, was investigated using materials with different polymer matrix (polyamide 6.6, polyarylamide and polyoxymethylene), fibre sizing, fibre content, mean fibre length and fibre length distribution.To assess the dependence on fibre orientation, plates with unidirectionally oriented fibres were prepared and cut at various angles with respect to the direction of the aligned fibres. The fracture behaviour was investigated by single-edge notch three-point bending tests. In addition the stress-strain behaviour was examined by performing uniaxial tension and compression tests.Both the critical stress intensity factor K _C and the fracture energy G _C measured at fracture initiation were found to present a bi-linear relationship to the factor characterizing fibre orientation, with different slopes over different ranges of the orientation factor. This suggested the occurrence of a transition between different failure mechanisms with varying fibre orientation, namely matrix fracture and fibre debonding at low values of the fibre orientation factor, fibre breakage and pull-out at high values of the fibre orientation factor. This interpretation is supported by the observation of the crack growth direction (which varies with varying fibre orientation) and the analysis of the fracture surfaces. The slopes of the two linear branches of the toughness vs. fibre-orientation-factor plot and the critical fibre orientation angle depend on all internal variables investigated: constituent polymer matrix, degree of fibre-matrix adhesion, fibre content, mean fibre length and fibre length distribution.     

The influence of molecular weight and mould temperature on the skin-core morphology in injection-moulded polypropylene parts containing weld lines

The sandwich structure of injection-moulded polypropylene parts with and without weld lines has been investigated using optical microscopy. X-ray scattering, and differential scanning calorimetry. It was found that the skin-layer thickness is strongly dependent on whether the mould is injected through one gate or through two gates. Samples containing a weld line show a much lower skin-layer thickness than samples without a weld line. This difference, however, depends strongly on the molecular weight of the polypropylene. The skin-layer thickness also varies along the flow path and decreases with increasing mould temperature. While most of the polypropylene crystallizes in the crystallographic -modification, in the textured skin layer, some -modification occurs. The -crystallinity depends on the molecular weight but disappears in the weld line. X-ray investigations exhibited a bimodal crystal orientation in the skin layer, one with thec-axis oriented parallel to the flow direction, the second one with thea ^*-axis oriented parallel to the flow direction. The core exhibited only a weakc-axis orientation. In the skin, the metastable ₁-modification was found.     

Effects of Gravitational orientation on the microstructural evolution of gas tungsten arc welds in an Al-4 wt% Cu alloy

Gas tungsten arc (GTA) welds on Al-4 wt% Cu alloys were investigated to determine effects of gravitational orientation on the weld solidification behavior. A bead-on-plate welding was performed by varying the relation between the arc translation direction and gravity vector, e.g., parallel-up, parallel-down, and perpendicular orientations. A solidification rate (V _S) was calculated from the measured grain orientation, and a thermal gradient (G _L) was estimated from the observed weld pool shape following a linear relation. A primary dendrite spacing (₁) decreased continuously from the s-l boundary to the weld pool surface regardless of the gravitational orientations. Larger ₁ for the parallel-up weld was observed near the boundary and surface than that of the perpendicular and parallel-down welds, which is believed to be associated with a smaller G _L due to larger weld pool dimension and with different solidification morphology. A solidification morphology and orientation in the perpendicular and parallel-up welds was comparable with a loss of columnar directionality near the weld surface and a continuous grain orientation. However, the parallel-down weld exhibited more columnar structure near the surface, which might be associated with the larger G _L and relatively mild convection flows. Outward convection flows in the parallel-down weld might be inhibited because of its reverse direction with respect to the gravity vector. This resulted in abnormal S shape of the trailing s-l interface and the V _S, which was receded toward the weld pool center. Based on these findings, significant influence of gravitational orientation resulted in the variation on the weld pool shape associated with convection flows, which in turn affected solidification orientation/morphology and the primary dendrite spacing.     

Anisotropic microhardness in single-crystal and polycrystalline BaTiO3

The room-temperature hardness of single-crystal and dense polycrystalline BaTiO₃ was investigated by microindentation. The longer diagonal of the Knoop indenter was oriented either parallel or perpendicular to the poled axis of the material. The hardness of the unpoled sample was isotropic. However, hardnesses in the poled samples were anisotropic, with the highest hardnesses resulting when the longer diagonal was parallel to the poled axis. The hardness anisotropy may be due primarily to residual stresses caused by the piezoelectric coupling effect.     

Fracture mechanics of short glass fibre-reinforced nylon composite

The fracture behaviour of injection-moulded short glass fibre-reinforced, thermoplastic nylon 6.6 plaques has been studied under static loading using compact tension specimens and under impact loading using single-edge notched charpy specimens. The influences of specimen position as taken from the plaque mouldings, notch direction, notch sharpness and the rate of testing on the fracture toughness of this composite system were investigated. Results indicated that the fracture toughness is highest for the cracks perpendicular to the mould fill direction and is lowest for cracks parallel to the mould fill direction. A single fracture parameter, K _c, seems to be inadequate for fracture toughness characterization. Evaluation of the fracture toughness as a function of notch sharpness indicated that for notches perpendicular to the mould fill direction the fracture toughness is not affected by the sharpness of the initial notch. However, for cracks in the mould fill direction, sharpness of the initial notch had a significant effect upon the measured value of the fracture toughness. Results also indicated, that the fracture toughness is rate insensitive over the crosshead speed ranging from 0.5–50 mm min^–1. Finally, the specimen position, as taken from plaque mouldings, had no significant effect on the measured value of the fracture toughness.     

Tensile fracture of centre-notched angle ply (0/±45/0)S and (0/90)2S graphite — epoxy composites

The fracture behaviour of centre-notched (0/± 45/0)_S and (0/90)_2S laminates with increasing notch length has been studied. Two test series have been investigated: specimens of constant width (W=20 mm) and small notch length (2a 12 mm), and specimens with various notch lengths (5 2a 35 mm) and a constant relative notch length (2a/W=0.5). An X-ray technique showed that the damage at the notch tip, which is formed at increasing load, consists mainly of subcracks parallel to the fibres of the constituent layers. The damage zone causes the crack opening displacement (COD) to deviate from the original linearity. TheK _R curve concept has been applied assuming that the COD deviation from linearity is completely the result of original crack extension. This approach fails to describe the notch length effect, because a tangent point between theK _R andK curves was not found and because of a strong dependency of the maximum fracture resistanceK _Rmax on notch length. The fracture behaviour of 20 mm wide specimens could be explained with the point and average stress criteria, based on characteristic lengths which are independent of notch length. At various notch lengths at a constant 2a/W=0.5, however, the characteristic lengths increased with increasing notch length.     

A Modal of Twin Domains in YBa2(Cu1−<Emphasis Type="Italic">x</Emphasis>Co<Emphasis Type="Italic">x</Emphasis>)3O<Emphasis Type="Italic">y</Emphasis>

The existence of twin domains in YBa₂(Cu_1?xCo _x )₃O _y samples was confirmed by many experiments. However, the physical picture of the twin domains has not been reported up to now. By analyzing oxygen content, Cu and Co valences, and Co coordination in those samples, we propose a domain model, which consists of two pairs of parallel zigzag Co-O chains surrounding orthorhombic cells, one along (110) direction and the other along (110) direction. The zigzag Co-O chains are the twin walls, with neighboring walls perpendicular and the opposite ones parallel. The distance between parallel walls (i.e., zigzag Co-O chains) depends upon Co concentration of sample. The twin domain model predicts the geometry, size, and absence of systematic symmetry of the domains. The model also predicts the modulation periodicity of electron diffraction patterns, which depends on Co concentration. All the predictions by the model agree well with the experimental results reported in literatures. Furthermore, the model indicates the possible existence of two domains, which have incommensurable modulation periodicity.     

Transverse Magnetoresistance Behaviors of Thin Polycrystalline Bismuth Films

The magnetoresistance (MR) properties of several thin polycrystalline bismuth films have been measured over a wide temperature interval (0.42K T 292 K) and a magnetic field range (0 T B 45 T). In most cases, the magnetic field was oriented in the transverse direction, with the field parallel to the substrate of the film and also perpendicular to the current direction. These MR results are different from those in either the perpendicular or parallel field orientations. The anomalous behavior of the transverse magnetoresistance can be explained considering partially diffused scattering of the carriers at the top and bottom surfaces of the films. The data are fitted using a phenomenological model, based upon the theory of Way and Kao and also using the two carrier expressions of Pippard and Fawcett (P-F).     

Tensile strength and crack nucleation in boron fibres

Boron fibres of about 100m diameter have been tested to fracture in a micro-tensile testing machine. The fracture surfaces have been investigated using a scanning electron microscope and classified according to the fracture nucleation type. Comparison with the measured fracture stresses proved a significant correlation with the type of crack nucleation. Transverse cracks, nucleated at the edge of radial cracks along the fibre axis direction, are the most critical weakening feature. Also crack nucleation at the external fibre surface implies a low tensile strength, and is discussed in terms of a notch effect of the well-known nodular surface topography.     

Deformation mechanisms in oriented high density polyethylene

The deformation behaviour of single crystal texture HDPE has been examined in tension as a function of the orientation of the tensile axis with respect to the chain direction. Over most of the orientation range examined, it was found that slip processes parallel to the chain direction were the dominant modes of deformation. Fibrillar slip becomes relatively more important than chain slip as the strain increases and as ₀, the initial value of the angle between the tensile axis and the chain direction, decreases. Lamellar slip was only observed over a limited range of orientations due to the high initiation stress required for the process. At low ₀ values, lamellar separation accounted for a substantial part of the applied strain. Stress-induced martensitic transformation, which was observed in samples with ₀>26°, cannot account for an important fraction of the strain although the resolved shear stress required for the activation of the martensitic transformations is of the same order of magnitude as that required for chain slip.     

Thermal conductivity and mechanical properties of various cross-section types carbon fiber-reinforced composites

In this work, to study the characteristics of carbon fiber-reinforced composites with different fiber cross-section types, such as round, C, and hollow-shape, the thermal conductivity and mechanical properties were investigated and compared. The thermal conductivity was measured by means of steady-state method to the parallel and perpendicular direction of reinforcing fibers. The mechanical properties were evaluated by a variety of test methods i.e., flexural, interlaminar shear strength, and impact strength. As a result, it was found that the thermal conductivity was greatly depended on the cross-section type of the reinforcing fibers, as well as, the reinforcing orientation. Especially, the anisotropy factor (k _///k ) and the thermal diffusivity factor (_///) of C and hollow-type carbon fiber-reinforced composites showed about two times higher values than those of round-type one. Also, the mechanical results showed that C and hollow-type carbon fibers-reinforced composites had higher values than those of round-type one in all mechanical tested. These results were probably due to the basic properties of non-circular (C and hollow-type) carbon fiber which can improve interfacial binding forces and widen interfacial contact area between reinforcement and matrix, resulting in effectively transferring the applied stress.     

An experimental and theoretical study of crack propagation in crossply fiber composites

Rectangular specimens of a cross-ply fiberglass composit e (Scotchply 1002) in which sharp edge notches have been cut along one of the fiber directions parallel to a specimen edge have been tested in an “Instron” testing machine. Tensile stress was applied perpendicular to the direction of the notch. The commencement and growth of the crack has been observed. It has been found that if the notch is sufficiently close to one edge of the specimen the crack instead of propagating along the direction of its axis, i.e. perpendicular to the applied stress runs along the stress direction; when the notch is sufficiently far from the edge the crack propagates in the direction of its axis right across the specimen while at intermediate notch positions the crack runs along in its axial direction and then turns sharply through a right angle and continues in the direction of the applied stress. The theory of this behavior has been studied and it is shown that if a criterion of a critical tensile force rather than a critical energy density is used, the observed phenomena can, at least qualitatively, be accounted for. The comparable situation for a brittle isotropic specimen of “Plexiglass” is also described. This shows a quite different response and conventional linear fracture mechanics can be used to describe this behavior.     

Anisotropic magnetic properties of obliquely deposited Ni films

Magnetic properties of evaporated nickel films, deposited onto 75 m thick 300 H Kapton substrates by evaporation at oblique off-normal angles of incidence, were investigated by SQUID magnetometry. We found that, in the film plane, the direction of easy magnetization lay perpendicular to the incidence plane for films deposited at < 50°. At large s, the easy axis changed to the direction parallel to the incidence plane. The anisotropy, coercivity and squareness of the hysteresis loops increased with an increase in . The results may be qualitatively understood from the presence of an inclined columnar structure with shape anisotropy governing the demagnetization of the magnetic fields.     

Investigation of fatigue crack incubation and growth in cast MAR-M247 subjected to low cycle fatigue at room temperature

MC carbide particles (with Hafnium and/or Tantalum as constituent metallic element, M) were observed to crack extensively in a cast polycrystalline nickel-base superalloy, MAR-M247, when subjected to low-cycle fatigue loading at room temperature. High resolution secondary electron images taken on the surface of a double edge notch test specimen revealed that approximately half the carbide particles cracked in the highly-strained notch section of the specimen. These images further illustrated that the average surface area of cracked particles was approximately three times that of the uncracked particles. Additional analysis illustrated that the cracks within a large number of particles aligned nearly perpendicular to the loading direction. However, high aspect ratio particles (with aspect ratio \({>}3\)) were prone to incubate cracks aligned along its major axis, independent of the loading direction. Additionally, forward-scattered imaging often showed a high density of slip bands interaction with most of the particles which cracked. The life limiting crack growth in MAR-M247 was observed to be crystallographic in nature, as the crack grew along slip bands as measured by high-resolution electron backscatter diffraction, even after spanning many grains. Statistically representative microstructure models of MAR-M247 were generated and used in the crystal plasticity finite element simulations. As expected, there was a significant variation in the computed stress state among constituent carbide particles. The stress state of the carbide particles was found to be heavily influenced by the stress in surrounding grains and the orientation of the major axis of the particles with respect to applied load direction. For particles that intersect the free-surface, stress was found to be highly concentrated at the free surface and a positive correlation between the magnitude of free-surface area and the maximum principal stress was found. Additionally, high stress concentrations were observed in regions where carbide particles intersect grain boundaries.     

Highly textured bismuth-containing high-temperature superconductor ceramics obtained by uniaxial pressing in liquid medium: Fabrication and properties

A new method of fabricating textured ceramics with a composition of Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O_x is described, according to which the initial low-density ceramics is subjected to uniaxial pressing in a liquid medium followed by drying and annealing. Analysis of the X-ray diffraction and scanning electron microscopy data show evidence for a high degree of texture in high-temperature superconductor ceramics fabricated using the proposed method. The results of magnetic measurements indicate that the obtained material possesses anisotropy of the magnetic moment for the magnetic field H oriented parallel and perpendicular to the direction of uniaxial pressing. The textured ceramics also show high diamagnetic response for H parallel to the c axis of grains, which makes these ceramics promising materials for practical applications.     

Grain orientation and anisotropy in the physical properties of SrBi2(Nb1 − x V x )2O9 (0 ≤ x ≤ 0.3) ceramics

SrBi₂(Nb_{1 – x} V _x )₂O₉ (0 x 0.3 in molar ratio) ceramics have been fabricated via conventional sintering at elevated temperatures. Interestingly sintering the pellets in the 1320–1470 K temperature range yielded partially grain oriented ceramics. The orientation factor (f) monitored via X-ray powder diffraction studies was found to increase with increasing V₂O₅ content and reached 83% for x = 0.3. The increment in (f) was not that significant with increase in sintering temperature and its duration. The microstructural studies suggest that V₂O₅ has a truncating effect on the abnormal platy growth of SBN grains. The dielectric constant (_r) and loss (D) measurements as functions of both temperature and V₂O₅ content have been carried out along the directions parallel (_rp) and perpendicular (_rn) to the cold pressing axis of the pellet. The anisotropy (_rn/_rp) associated with _r was found to be 1.11 at 300 K and 2.1 at the Curie temperature, (T _c) respectively. Different dielectric mixture formulae that were employed to analyze the effective dielectric constants of these samples with varying porosity confirmed that the experimental value of _r was comparable with that obtained using Wiener's formula. Impedance spectroscopy was employed to rationalize the electrical behavior of these ceramics. The pyroelectric coefficients along the directions parallel and perpendicular to the pressing axis of the grain oriented (83%) SBN ceramic at 300 K were 0.13145 mC/m²K and 0.26291 mC/m²K respectively. The ferroelectric properties of these grain-oriented ceramics were better in the direction perpendicular to the pressing axis than those in the parallel direction.