Fabrication and properties of WC–10Co cemented carbide reinforced by multi-walled carbon nanotubes

High-velocity parting-off has been applied to 80 mm bars of pearlitic 100CrMn6, resulting in shear localisation and white-etching bands in a severely deformed region below the fracture surface. Electron microscopy showed that going from the bulk material towards the fracture surface the grains become elongated and refined. The region below the fracture surface can be divided into three subzones: 50–100 μm below the surface grains are elongated, cementite lamellae are distorted, break up and the lamellar spacing decreases. <50 μm below the fracture surface the microstructure becomes a mix of cementite lamellae and carbides in a ferrite matrix. Within the white-etching band the microstructure consists of equiaxed ferrite refined to a grain size of 50–150 nm. Several twinned regions caused by the deformation can be observed. Selected area electron diffraction and low angle convergent beam electron diffraction indicate nanocrystalline cementite dispersed in the ferrite matrix.     

The influence of confinement on the penetration of ceramic targets by KE projectiles at 1.8 and 2.6 km/s

This paper presents the results of scale size experiments using a tungsten-alloy long-rod projectile fired against 97.5% Al₂O₃ ceramic targets at 1.8 and 2.6 km/s. Two targets were used, one having lateral steel confinement; the other without. The projectile overmatched the target, and residual projectile length and velocity were recorded using ballistic-syncro photography. Flash radiography was used during penetration of the unconfined target to obtain the penetration velocity. Manganin pressure gauges were also used to obtain additional data on the response of the ceramic target during penetration. Results from the eight experiments indicate that the confinement reduced the residual energy of the projectile at both impact velocities. Expressed in terms of the projectile impact energy, 55–56% was lost in the unconfined target at 2.6 km/s compared with 60% for the confined design. The same trend was found at 1.8 km/s with 68% and 72–73% for the unconfined and confined, respectively. Predictions using the QinetiQ GRIM2D hydrocode and a simplified form of the Johnson–Holmquist ceramic material model agreed well with the experiments for three out of the four test configurations. The predicted projectile erosion and retardation against the confined target at 1.8 km/s was excessively high. Analytical predictions using the Tate modified Bernoulli equation also gave reasonably accurate predictions for three of the tests, but values for the Tate target ‘strength’ extracted from experiments using a different target configuration were not accurate for the target design used in this paper.     

Numerical simulations of target hole diameters for hypervelocity impacts into elevated and room temperature bumpers

Normal impacts between 3.175 mm diameter 1100-O Al spherical projectiles and 1.6 mm thick 6061-T6 Al targets are considered for both room temperature and heated targets. The elevated target temperatures under examination are 110 and 210 °C, and the simulated impact velocities range from approximately 2 to7 km/s. The AUTODYN v.5 hydrocode is used to generate data that are compared to empirical data obtained from the literature, and to that obtained with the University of Denver Research Institute's (DRI) two-stage light gas gun. The effect of using applicable strength and fracture models currently available in AUTODYN to replicate both room and elevated temperature target hole diameters is also examined. It was found that the numerical simulations for both room and elevated temperature targets predict hole diameters that differ approximately 0.1–13% from the DRI empirical data.     

Mechanical properties of Friction Stir Processed 2618/Al2O3/20p metal matrix composite

The effect of Friction Stir Processing (FSP) on the mechanical properties of 2618 aluminium alloy reinforced with 20% of alumina particles aluminium alloy has been studied in the present paper. The material was processed into the form of sheets of 7 mm thickness after T6 treatment and was tested in tension and fatigue at room temperature.

Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone, in order to analyse the superplastic properties of the recrystallized material and to observe the differences with the parent materials as a function of the strong grain refinement due to the Friction Stir Process. The high temperature behaviour of the material was studied, in longitudinal direction, by means of tensile tests in the temperature and strain rate ranges of 400–500 °C and 10⁻³–10⁻¹ s⁻¹, respectively.

Fracture surfaces of the deformed fatigue test specimens were comprehensively examined in a scanning electron microscope equipped with field emission gun to determine the macroscopic fracture mode and characterize the fine-scale topography and microscopic mechanisms governing fatigue fracture.

The mechanisms governing fatigue life, cyclic deformation and fracture characteristics are analysed in function of magnitude of applied stress, intrinsic micro structural evolution and material deformation behaviour.     

Effect of thickness on the relationship between shear lip and energy in dynamic tear specimens

In two previous papers, a relationship was shown to exist between the fracture energy and shear lip in 16–25 mm thick dynamic tear (DT) tests of quench and tempered carbon and low alloy steels and their weldments over a wide range of temperatures, compositions and locations in the weldments. The relationship between shear lip and energy was found to be independent of specimen thickness for the 16–25 mm thicknesses. In this paper it will be shown that the relationship holds for 7–15 mm thicknesses as well. Data used in these studies have been acquired from samples of 350 WT, A517 Grade F, HY80, HY100 and HLES steels and submerged arc, shielded metal arc, flux cored arc and gas metal arc welds of these steels.     

Quasi-brittle fracture during structural impact of AA7075-T651 aluminium plates

The stress–strain behaviour of the aluminium alloy 7075 in T651 temper is characterized by tension and compression tests. The material was delivered as rolled plates of thickness 20 mm. Quasi-static tension tests are carried out in three in-plane directions to characterize the plastic anisotropy of the material, while the quasi-static compression tests are done in the through-thickness direction. Dynamic tensile tests are performed in a split Hopkinson tension bar to evaluate the strain-rate sensitivity of the material. Notched tensile tests are conducted to study the influence of stress triaxiality on the ductility of the material. Based on the material tests, a thermoelastic–thermoviscoplastic constitutive model and a ductile fracture criterion are determined for AA7075-T651. Plate impact tests using 20 mm diameter, 197 g mass hardened steel projectiles with blunt and ogival nose shapes are carried out in a compressed gas-gun to reveal the alloy's resistance to ballistic impact, and both the ballistic limit velocities and the initial versus residual velocity curves are obtained. It is found that the alloy is rather brittle during impact, and severe fragmentation and delamination of the target in the impact zone are detected. All impact tests are analysed using the explicit solver of the non-linear finite element code LS-DYNA. Simulations are run with both axisymmetric and solid elements. The failure modes are seen to be reasonably well captured in the simulations, while some deviations occur between the numerical and experimental ballistic limit velocities. The latter is ascribed to the observed fragmentation and delamination of the target which are difficult to model accurately in the finite element simulations.     

Impact flash and debris cloud expansion of high-pure metal foils

Results of 2 mm aluminum spheres perforating Al, Cu, Mo, Au, Sn, and Zn metal foils of a purity > 99.9 % with thicknesses between 0.1 mm and 2.0 mm, densities of up to 20 g/cm³, melting temperatures of 500 – 3000 K and specific heats of fusion of 20 – 350 kJ/kg at impact velocities between v_p = 4.5 km/s and v_p = 9 km/s are presented. The influence of target thickness, target material properties and impact velocity on the perforation hole diameter, impact flash duration and expansion velocity, fragmentation and debris cloud formation at nearly constant areal density is demonstrated. The dependence of impact crater pattern at witness plates on target material density, thickness, impact velocity and areal density ratio between projectile and target material is discussed. For tin and lead evidence is given for the ability of digital scanning electron microscope analysis as an effective tool for indicating change of aggregation from solid into liquid and for the determination of relative projectile and target material quantities.     

Experimental investigation of the ballistic resistance of steel-fiberglass reinforced polyester laminated plates

The present experimental study is undertaken to investigate the effect of target configuration on ballistic performance when struck by standard bullets of different velocities. At first, single mild steel plates, 1–8 mm thick, are tested, and the effect of thickness and mechanical properties of plate material are explored. Secondly, in-contact laminae comprising an 8 mm-thick target, and spaced laminae of the same total steel thickness, with spacing distances equal to or multiples of the bullet core diameter (6 mm) are tested and the effect of number, thickness, and arrangement of laminae sought.In addition, fiberglass reinforced polyester (FRP) is used as a filler material for targets with spaced steel laminae. The influence of FRP's physical and mechanical properties on the ballistic performance of steel-FRP targets is investigated.In order to perform the ballistic tests, a special setup is constructed, which consists of a launcher, a target clamp and a velocity-measuring device. In each experiment, the change in the projectile velocity (while penetrating the target) divided by the length of penetration is established as a measure of target performance.Results show that single targets are more effective than laminated targets of the same total thickness, regardless of the configuration or striking velocity. It is noted, however, that the difference in performance diminishes as the striking velocity increases. Moreover, the effectiveness of laminated targets, in contact or spaced, increases as the number of laminae comprising each target decreases. Ballistic performance of laminated targets is further enhanced by using the thickest lamina as the back lamina. Results also emphasize the dependence of target performance on mechanical properties.Steel-FRP targets show better performance than weight-equivalent steel targets. Performance of a steel-FRP target is further improved by increasing fiber weight fraction in the FRP.     

An experimental cum numerical technique to determine dynamic interlaminar fracture toughness

A method combining experimental and finite element analysis is developed to determine interlaminar dynamic fracture toughness. An interlaminar crack is propagated at very high speed in a double cantilever beam (DCB) specimen made of two steel strips which are bonded together by epoxy with a precrack of about 40 mm length. The face of the front cantilever is bonded to a large solid block and a special fixture is designed to apply impact load to the rear cantilever through a load bar. In the load bar, a compressive square shaped elastic stress pulse is generated by impacting it with a striker bar which is accelerated in an air gun. The rear cantilever is screwed to the load bar; when the incident compressive pulse reaches the specimen, a part of the energy is reflected into the load bar and the rest of it passes to the specimen. By monitoring the incident and the reflected pulses in the load bar through strain gauges, deflection of cantilever-end is determined. The crack velocity is determined by three strain gauges of 0.2 mm gauge length bonded to the side face of the rear cantilever. Further, the first strain gauge, bonded very close to the tip of the precrack, and the crack velocity determine the initiation time of crack propagation.

The experimental results are used as input data in a finite element (FE) code to calculate J-integral by the gradual release of nodal forces to model the propagation of the interlaminar crack. The initiation fracture toughness and propagation fracture toughness are evaluated for an interlaminar crack propagating with a velocity in the range of 850 to 1785 m/s. The initiation toughness and propagation toughness were found to vary between 90–200 J/m² and 2–13 J/m², respectively.     

The dependence of penetration velocity on impact velocity

Recent experimental measurements show that eroding long-rod penetration velocity is a linear function of impact velocity over a very wide range of impact velocities and for an interesting range of rod–target material combinations. These experiments all show that U=a+bV, where U and V are the penetration and impact velocity, respectively, and “a” and “b” are constants for given projectile and target materials. Numerical simulations also show that U=a+bV. The accumulation of these results suggests that a linear relationship between penetration and impact velocity may be fundamental over a very large range of impact velocities. A linear relationship between penetration and impact velocity has a number of implications. Some implications of this result for the Tate–Alekseevskii model are briefly examined in this paper.     

Microstructural and mechanical properties evolution of magnesium AZ61 alloy processed through a combination of extrusion and thermomechanical processes

In this work, the microstructures and tensile properties of a commercial magnesium alloy “AZ61” processed by a combination of hot extrusion and thermomechanical processing (TMP) were investigated. The TMP was consisting of two or three hot rolling steps with large reductions per pass, thus allowing significant grain refinement. The microstructural evolution has been studied by means of optical and scanning electron microscopes, as well as X-ray diffraction analysis. The as-cast material is extruded in the form of a cylinder with initial diameter of 250 mm to a final diameter of 110 mm (80% reduction in cross-sectional area). Then hot rolling regimes were performed at 300 °C with different percentage of strain per pass. Tensile and hardness tests were performed in the samples (as-cast, extruded, and rolled) at room temperature in order to evaluate the mechanical properties of the material. The results of experiments demonstrated that fine grain size might be achieved in magnesium alloy AZ61 by using a two-step processing route involving an initial extrusion step followed by thermomechanical processing with large reduction in thickness per pass. This two-step process, designed to achieve average grain sizes of 10–20 μm.     

Young’s modulus and interlaminar fracture toughness of SU-8 film on silicon wafer

SU-8 cantilever beams deposited on the polished sides or unpolished sides of silicon wafers were fabricated by MEMS and bulk micro-machining techniques. Bending tests were conducted to measure the Young’s modulus and interface fracture toughness. The results indicate that the Young’s modulus is increasing as the width of specimens is increasing because the specimen quality becomes better. When the width of SU-8 cantilever beam is 400 μm, the Young’s modulus is about 3.5 GPa. On the other hand, since the interface between SU-8 and silicon wafer does not suffer the attack of etchant, the measured interface fracture toughness has no clear dependence on the specimen width. The averaged interface fracture toughness is about 25–30 J/m² for the polished interface and 15–20 J/m² for the unpolished interface.     

Growth and structure in abalone shell

The growth and self-assembly of aragonitic calcium carbonate found in the shell of abalone (Haliotis) is described. This was accomplished through the close examination of laboratory-grown flat pearl samples and cross-sectional slices of the nacreous shell. Further understanding of the sequenced assembly has been obtained. It has been confirmed that the growth of the aragonite component of the composite occurs by the successive nucleation of aragonite crystals and their arrest by means of a protein-mediated mechanism; it takes place in the “Christmas-tree pattern” [Nature 49 (1994) 371]. It is shown that the protein layer is virtually absent where plates on a same plane abut (along lateral surfaces of tiles). This suggests a mechanism of c-axis aragonite growth arrest by the deposition of a protein layer of approximately 20–30 nm that is periodically activated and determines the thickness of the aragonite platelets, which are remarkably constant (0.5 μm). This platelet size was measured for animals with shell diameters of 10, 50, and 200 mm and was found to be constant. The overall growth process is expressed in terms of parameters incorporating the anisotropy of growth velocity in aragonite (V_c, the velocity along c axis, and V_ab, the velocity in basal plane). Comparison of laboratory-raised and naturally-grown abalone indicates growth regulated by the level of proteinaceous saturation. Naturally-grown abalone exhibits mesolayers (growth bands) 0.3 mm apart; it is proposed that they result from seasonal interruptions in feeding patterns, creating thicker (10–20 μm) layers of protein. These mesolayers play a critical role in the mechanical properties, and are powerful crack deflectors. The viscoplastic deformation of the organic inter-tile layers is responsible for the significant improvement of tensile strength over the tensile strength of monolithic aragonite.     

Cryogenic and elevated temperature hypervelocity impact facility

The hypervelocity impact facility at Space Research Institute (SRI), Auburn University has recently completed a facility upgrade that permits the impact testing of space materials within the cryogenic and elevated temperature range. Sample temperatures within the range of 40–450 K have been achieved for polymer films. These wide temperature range capabilities add to the facilities current testing experience with impact initiated plasma discharge testing for solar cell arrays. The facility utilizes a plasma drag gun to accelerate a variety simulated micrometeorite materials in the 50–150 μm range to velocities between 5 and 12 km/s. For each test 5–50 particles impact the surface of the target sample within an impact area of approximately 15 cm in diameter. The test chamber can accommodate samples up to a meter wide for ambient and heated tests, and 48 cm for cryogenic samples. The gun and test chambers are evacuated by He cryopumps and dry roughing pumps to produce a clean, oil free environment. Utilizing a streak camera and PMT detection system, the correspondence between individual particle size, speed and impact site can be determined. Standard post-analysis yields: micrographs of each impact site, dimensions of the pertinent impact characteristics, individual particle velocity and size estimates.     

Numerical simulation of plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl

Plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl have been simulated by using periodic unit cells representing the relaxed-constraint model recently proposed by Lebensohn et al. [Acta Mater. 46 (1998) 4701–4709] for the co-deformation of the lamellar compound of PST-TiAl. The unit cells contain both intermetallic phases, ₂-(Ti₃Al) and γ-(TiAl). Furthermore, the six orientation variants of the γ-phase are also considered. The constitutive behaviour of both phases is described by crystal plasticity, and the damage behaviour has been implemented by means of cohesive elements. The unit cells have been used as submodels for multi-scale finite element simulations of compression tests and fracture mechanics tests of notched micro-bend specimens. It is shown that the anisotropy of plastic deformation and damage in PST-TiAl can be well represented.     

A detector for filtering γ-ray spectra from weak fusion–evaporation reactions out of strong background and for Doppler correction: The recoil filter detector, RFD

A detector has been designed and built to assist in-beam γ-ray spectroscopy with fusion–evaporation reactions. It measures with high efficiency the evaporation residues that recoil out of a thin target into the angular interval from 1.8° to 9.0° at an adjustable distance of 1000–1350 mm from a target, in coincidence with γ-rays detected in a Ge-detector array. This permits filtering of such γ-rays out of a much stronger background of other reaction products and scattered beam. Evaporation residues are identified by their time-of-flight and the pulse height using a pulsed beam. The velocity vector of the γ-emitting recoil is also measured in the event-by-event mode, facilitating to correct the registered γ-ray energy for the Doppler shift, with the resulting significant improvement of the energy resolution. The heavy-ion detection scheme uses emission of secondary electrons caused by the recoiling ions when hitting a thin foil. These electrons are then electrostatically accelerated and focused onto a small scintillator that measures the summed electron energy, which is proportional to the number of electrons. The detector is able to operate at high frequency of the order of 1 MHz and detect very heavy nuclei with as low kinetic energy as 5 MeV. The paper describes the properties of the detector and gives examples of measurements with the OSIRIS, GAREL+ and EUROBALL IV γ-ray spectrometers. The usefulness of the technique for spectroscopic investigations of nuclei with a continuous beam is also discussed.     

Material property variations and defects of carbon/carbon brake disks monitored by ultrasonic methods

Several ultrasonic techniques were applied to carbon/carbon brake disks for the evaluation of spatial variations in material properties that are attributable to the manufacturing process. In a carbon/carbon brake disk manufactured by a combination of pitch impregnation and vapor infiltration methods, the spatial variation of ultrasonic velocity was measured and found to be consistent with the nonuniform densification behavior in the manufacturing process. Low frequency (e.g. 1–5 MHz) through-transmission scans were used for mapping out the material property inhomogeneity. These results were compared with that obtained by dry-coupling ultrasonics. A good correlation was found between ultrasonic velocity and material density on a set of small blocks cut out of the disk. Pulse-echo C-Scans (10–25 MHz) were used to image near-surface material property anomalies associated with certain steps in the manufacturing process. Ultrasonic velocities in the in-plane directions were affected more by the relative contents of fabric and chopped fiber, and less by the void content.     

Hertzian fracture of Pyrex glass under quasi-static loading conditions

Hertzian fracture tests were conducted using an Instron on Pyrex glass specimens with various surface conditions, including lubricants, employing steel, Al₂O₃, WC and Pyrex glass indentors of 0.79 to 12.7 mm radius under ambient air and high vacuum environments at cross-head speeds of 8.5×10^–6 to 2.1×10^–4m sec^–1. The results were not in strict accord with Auerbach's law, nor any of the existing energy-balance Hertzian fracture theories. Rather, they indicated that surface roughness and friction modified the Hertz stress field so that the maximum tensile stress at the surface occurred outside the contact circle. Further, they indicated that Hertzian fracture occurred by the direct, unstable growth into a cone crack of a pre-existing flaw at the displaced site of the maximum tensile stress, the flaw size responsible for the fracture decreasing with decrease in ball size (contact radius). Once a cone crack occurred, its length and growth were described reasonably well by Roesler's theory; however, his constant appears to be too high by a factor of about 5. A surface energy of @ 4 J m^–2 was derived from bend tests on specimens similar to those used in the Hertzian fracture tests. Using this value, the crack sizes which lead to fracture were estimated to range between 0.6 and 3.5 m for the conditions investigated here. The increase in the critical load for Hertzian fracture with indentation velocity was concluded to be due to kinetic effects of water vapour acting at the tip of the crack.     

Explosive gun launcher for impact studies

We have modified the design of an explosive gun launcher described at the Third Symposium on Hypervelocity Impact for use with pre-cast explosive charges and to provide performance information for steel projectiles. Other modifications include additional confinement of the detonation products, a longer barrel and longer conical transition from breech to barrel, provision for dynamic sealing of the access hole for the detonator wires, and additional buffering to prevent spall fracture of the projectile in the barrel. The modified gun launched a 12.1 gram steel projectile to a measured velocity of 3.2 km/s. Computational simulations have been performed to determine the effects of changes in the projectile density, confinement material density, and explosive type. The use of recyclable confinement material is discussed.     

Stress measurements in glass using shaped-charge jets

Stresses were measured in glass targets in the vicinity of a penetrating shaped-charge jet. Stress levels of approximately 0.3 GPa were measured 12–20mm away from a jet formed by a 35mm copper liner. High speed framing camera photographs showed that the penetration velocity in the glass was 2.57 km/s and the glass fracture velocity was 2.10 km/s.