Impact of conical tungsten projectiles on flat silicon carbide targets: Transition from interface defeat to penetration

Normal impact of conical tungsten projectiles on flat silicon carbide targets was studied experimentally and numerically for half apex angles 5° and 5–15°, respectively, and comparisons were made with cylindrical projectiles. A 30 mm powder gun and two 150 kV and four 450 kV X-ray flashes were used in the impact tests. The numerical simulations were run with the Autodyn code in two steps. In the first, the surface loads were determined for different impact velocities under assumed condition of interface defeat. In the second, these surface loads were applied to the targets in order to obtain critical states of damage and failure related to the transition between interface defeat and penetration, and the corresponding critical velocities. In the impact tests, interface defeat occurred below a transition velocity, which was significantly lower for the conical than for the cylindrical projectiles. Above the transition velocity, the initial penetration of conical projectiles differed markedly from that usually observed for cylindrical projectiles. It occurred along a cone-shaped surface crack, qualitatively corresponding to surface failure observed in the simulations. The transition velocity for the conical projectile was found to be close to the critical velocity associated with this surface failure.     

Ductile tearing assessment of high-strength steel X-joints under in-plane bending

This study reports an experimental investigation of a fatigue-cracked, pre-notched circular hollow section X-joints fabricated from high strength steels (with the yield strength higher than 800 MPa) subjected to brace in-plane bending. The circular hollow section X-joint entails a prefabricated V-notch near the weld toe at the crown position. The experimental procedure applies a fatigue pre-cracking cyclic load followed by a monotonic brace in-plane bending, which leads to brittle through-thickness crack propagation after some amount of ductile tearing. The ductile tearing assessment, integrating the fracture resistance curve obtained from the small-scale fracture specimens and the crack extension in the large-scale tubular joint, predicts closely the load level at which unstable crack extension takes place. The generic level 2A curve outlined in the BS7910 provides an un-conservative estimate on the failure load of the X-joint specimen. The parametric numerical investigation reveals that the strength definition for the cracked joints imposes a significant effect on the shape of the failure assessment curve.     

Pressure drop and heat transfer during two-phase flow vaporization of propane in horizontal smooth minichannels

This study examined the two-phase flow boiling pressure drop and heat transfer for propane, as a long term alternative refrigerant, in horizontal minichannels. The pressure drop and local heat transfer coefficients were obtained for heat fluxes ranging from 5–20 kW m^?2, mass fluxes ranging from 50–400 kg m^?2 s^?1, saturation temperatures of 10, 5 and 0 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and lengths of 1000 mm and 2000 mm, respectively. The present study showed the effect of mass flux, heat flux, inner tube diameter and saturation temperature on pressure drop and heat transfer coefficient. The experimental results were compared against several existing pressure drop and heat transfer coefficient prediction methods. Because the study on evaporation with propane in minichannels was limited, new correlations of pressure drop and boiling heat transfer coefficient were developed in this present study.     

Effects of dynamic mechanical properties on the ballistic performance of a new near-β titanium alloy Ti684

A comprehensive study of the newly developed near-β titanium alloy Ti684 has been carried out to determine the influence of the dynamic strength, dynamic hardness and critical failure strain on the ballistic impact properties. Two heat treatments of Ti684, namely β solution-treatment and α + β solution-treatment followed by aging, were carried out and the results were compared with Ti–6Al–4V. Ballistic impact tests were conducted on 7 mm thick front plates with a 20 mm thick A3 steel backing plate, using 7.62 mm armor piercing projectiles. The ballistic performance was evaluated by measuring the residual depth of penetration (DOP) in the A3 steel backing plates. It was found that the DOP values did not show obvious corresponding relation with both dynamic strength and dynamic hardness. The 800 °C solution +550 °C aged Ti684, which had the maximal dynamic strength, presented the worst ballistic performance, with a maximum DOP of 12.5 mm. In addition, the Ti–6Al–4V plate in the study with highest dynamic hardness did not show the best ballistic performance, having a DOP of 11.86 mm. However, as the critical failure strain increased, the DOP of the A3 steel backings were observed to decrease. This relationship was revealed from post ballistic microstructural observations.     

The influence of binder tow density on the mechanical properties of spatially reinforced composites. Part 1 – Impact resistance

This paper examines the influence of binder tow stitch density on the impact performance of advanced composite structures. Spatially reinforced composite reinforcements with multi-axis, multi-layer structures were woven on a specially developed loom. The binder tow stitch density, which was used to consolidate the structure, was varied in the range of 1–4 binder tow stitches/cm² (10 × 10 mm to 5 × 5 mm binder tow stitch spacing). A drop weight impact test (6.7 J/mm of composite thickness) was used to damage the samples. Both the depth of penetration and the damage area were measured after impact. The analysis of the results has shown that as the binder tow stitch density was increased the extent of damage decreased. The weave architecture, in terms of the relative position of the ±45° tows, was also shown to be a significant factor, the nearer the off-axis tows are to the impact surface the greater was the damage area.     

Evaluation of crack growth behavior and probabilistic S–N characteristics of carburized Cr–Mn–Si steel with multiple failure modes

The unexpected failures of case-hardened steels in long life regime have been a critical issue in modern engineering design. In this study, the failure behavior of a carburized Cr–Mn–Si steel under very high cycle fatigue (VHCF) was investigated, and a model for evaluating the probabilistic S–N curve associated with multiple failure modes was developed. Results show that the carburized Cr–Mn–Si steel exhibits three failure modes including the surface flaw-induced failure, the interior inclusion-induced failure without the fine granular area (FGA) and the interior inclusion-induced failure with the FGA. As the predominant failure mode in the VHCF regime, the interior failure process can be divided into four stages: (i) the small crack growth around the inclusion, (ii) the stable macroscopic crack growth outside the FGA, (iii) the unstable crack growth outside the fish-eye and (iv) the momentary fracture outside the final crack growth zone. The threshold values are successively evaluated to be 2.33 MPa m^1/2, 4.13 MPa m^1/2, 18.51 MPa m^1/2 and 29.26 MPa m^1/2. The distribution characteristics of the test data in transition failure region can be well characterized by the mixed two-parameter Weibull distribution function. The developed probabilistic S–N curve model is in good agreement with the test data with multiple failure modes. Although the result is somewhat conservative in the VHCF regime, it is acceptable for safety considerations.     

Fatigue failure investigation on anti-vibration springs

Fatigue failure investigation on anti-vibration springs, involving both metal and rubber materials, is presented. Rubber-to-metal bonded springs are widely used in industry as anti-vibration components giving many years of service. Recently a need to improve time and cost efficiencies has caused an unexpected early fatigue failure of the component with no immediate explanation. The required total fatigue life was 1.25 million cycles but only 0.7 million cycles achieved. There was an urgent need to investigate the causes of the fatigue failure and to modify the component design accordingly to meet the customer requirement and the supply schedule.The investigation, based on the actual fatigue loads, is carried out on these failed and modified products using a method of continuum mechanics. To simplify the simulation, a non-linear quasi-static analysis is carried out and then the residual stresses are superimposed to obtain the effective stress range to predict the metal crack initiation. For the rubber parts a three-dimensional effective stress criterion is employed to predict the fatigue crack initiation. The fatigue failure is taken as visual crack observation (normally 1–2 mm).The fatigue crack initiation for the metal parts of the failed component is predicted at 225 K cycles under specified fatigue load against total metal broken at 700 K cycles from the test. For the modified part the minimum total fatigue life for the metal parts of the component, estimated conservatively, is 2.1 million cycles against 1.75 million cycles from the test without any crack observed. The rubber fatigue crack initiation is predicted at 90 K cycles against crack onset around 79 K cycles and crack length 40 mm at 145 K cycles from the test. From design point of view it is important to optimize the rubber profile under this very tight allowable space to provide the maximum support of the metal interleaves and at the same time to meet the minimum requirements of the manufacture process. It is shown that this approach can be employed at a design stage for both metal and rubber fatigue evaluations on anti-vibration springs.     

Experimental study on influence of section thickness on mechanical behavior of die-cast AM60 magnesium alloy

Influence of section thickness on mechanical behavior of die-cast AM60 magnesium alloy has been experimentally studied. Tension, compression and shear tests with this material were performed on a universal test machine at strain rates from 5 × 10⁻⁴ s⁻¹ to 5 × 10⁻² s⁻¹. Specimens were cut from plates with five as-cast section thicknesses of 6.5 mm, 5.2 mm, 3.9 mm, 2.6 mm and 1.3 mm. According to the test results, flow stress becomes less sensitive to section thickness with larger section thickness, and the influence of strain rate on flow stress is also decreasing with larger section thickness. At different stress states, the tested material follows the von-Mises yield criterion. And stress state is found to be the main factor influencing the fracture behavior.     

Plastic deformation of polyurea coated composite aluminium plates subjected to low velocity impact

The demand for protective measures for structures is on the rise due to the increasing possibility of structural damage due to threats such as natural disasters, collision of vehicles, and blast and ballistic impacts. Application of an elastomer as a composite material with other base materials such as aluminium, steel and concrete has been considered as one of the measures to mitigate such threats. However, very limited work has been conducted in this area, especially on the feasibility of polyurea (elastomer) as a composite material against low velocity impacts. The focus of this research is to investigate the behaviour of polyurea coated composite aluminium plates subjected to rigid blunt-nosed projectile impact. AA5083-H116 aluminium alloy plates with polyurea coatings of 6 mm and 12 mm thickness were investigated. A blunt cylindrical projectile of high strength steel travelling in the velocity range of 5–15 m/s impacted at the centre of the 300 mm × 300 mm square plates. A polyurea coating was used to absorb part of the impact energy and provide protection to the plates as an energy damping material through application on the impact side of the plates. In addition, uncoated aluminium plates of the same thickness were used in the test program. A gas gun mechanism was used to fire a 5 kg projectile, and laser displacement monitoring equipment was used to record the out-of-plane deformation history of the plate during the impact. The complete test setup has been modelled numerically using the advanced finite element (FE) code LS-DYNA. The models were validated with the experimental results. Deformation time histories obtained from both the experimental and numerical studies for the plates were used to compare the ability of polyurea to effectively mitigate the damage resulting from low velocity impact. The polyurea coated plates showed a considerable reduction in out-of-plane deformation when compared to the uncoated plates. These findings indicate that polyurea can be utilised as an efficient energy absorbing/damping material against low velocity impact damage.     

Ballistic behavior of high hardness perforated armor plates against 7.62 mm armor piercing projectile

In this paper, some of the important defeating mechanisms of the high hardness perforated plates against 7.62 × 54 armor piercing ammunition were investigated. The experimental and numerical results identified three defeating mechanisms effective on perforated armor plates which are the asymmetric forces deviates the bullet from its incident trajectory, the bullet core fracture and the bullet core nose erosion. The initial tests were performed on the monolithic armor plates of 9 and 20 mm thickness to verify the fidelity of the simulation and material model parameters. The stochastic nature of the ballistic tests on perforated armor plates was analyzed based on the bullet impact zone with respect to holes. Various scenarios including without and with bullet failure models were further investigated to determine the mechanisms of the bullet failure. The agreement between numerical and experimental results had significantly increased with including the bullet failure criterion and the bullet nose erosion threshold into the simulation. As shown in results, good agreement between Ls-Dyna simulations and experimental data was achieved and the defeating mechanism of perforated plates was clearly demonstrated.     

Impact of thick PMMA plates by long projectiles at low velocities. Part II: Effect of confinement

A hybrid experimental–numerical investigation of the penetration process in unconfined and confined thick polymethylmethacrylate (PMMA) plates was carried out. The confinement was applied by insertion of the polymeric plate into a conical steel ring. The response of such plates to the impact of long hard steel projectiles having an ogive-head shape in the range of velocities of 165 < V₀ < 260 (m/s), was investigated experimentally. The results show that unconfined targets were perforated and broken due to combined effect of penetration and cracking. By contrast, the confined targets were not perforated and could withstand repeated impacts due to suppression of the brittle damage mechanism by the confinement. The tests were modeled using 3D explicit finite element analyses. A good agreement regarding the trajectory of the projectile and the depths of penetration was obtained. The numerical results show that the confinement introduces a negative triaxiality and even some plasticity within the confined plates prior to impact. The increase of plastic failure strain of the PMMA at negative triaxiality reduces the ductile damage during penetration, while the hydrostatic pressure reduces significantly the brittle fracture mechanism. The resisting force to the penetration depends on the failure strain–triaxality relationship, and does not necessarily increase with higher confinement levels.     

Failure of spent nuclear fuel canister mock-ups at isostatic pressure

This paper describes isostatic pressure tests of two large-scale mock-ups of Swedish copper/ductile cast iron canisters for geological disposal of spent nuclear fuel and the associated finite element analysis and micro-structural analysis. The key objective of the study was to demonstrate the safety margins against the very high pressure loads that could occur in a future ice age (44 MPa) and to determine the associated failure mode. The mock-ups were loaded to 130 and 139 MPa, respectively, in the tests. The dominant failure mode was plastic collapse in combination with some ductile tearing of manufacturing flaws. The observed deformations and the final failure loads are in good agreement with the finite element calculations. The post-test micro-structural analysis by radiography, ultra-sound techniques and fractography showed that the manufacturing defects in regions with tensile stresses propagated about 10 mm in stable tearing prior to plastic collapse, but arrested when entering regions with compressive stresses.     

Three-dimensional assessment of low velocity impact damage in particle toughened composite laminates using micro-focus X-ray computed tomography and synchrotron radiation laminography

Results are presented studying the contribution of particle toughening to impact damage resistance in carbon fibre reinforced polymer materials. Micro-focus X-ray computed tomography and synchrotron radiation computed laminography were used to provide a novel, multiscale approach for assessing impact damage. Thin (1 mm thick) composite plates containing either untoughened or particle-toughened resin systems were subjected to low velocity impact. Damage was assessed three-dimensionally at voxel resolutions of 0.7 μm and 4.3 μm using SRCL and μCT respectively; the former being an innovative approach to the laterally extended geometry of CFRP plates. Observations and measurements taken from μCT scans captured the full extent of impact damage on both material systems revealing an interconnected network of intra- and inter-laminar cracks. These lower resolution images reveal that the particle-toughened system suppresses delaminations with little effect on intralaminar damage. The higher resolution images reveal that the particles contribute to toughening by crack deflection and bridging.     

A detailed study of the relationship between fatigue crack growth rate and striation spacing in a range of low alloy ferritic steels

It was shown that the measured average fatigue striation spacings predicted the fatigue crack growth rates for low alloy ferritic steels to within a range of ±2% to 35% with an overall average error band value of ±10.1%. When we consider the fatigue stress range this average error was reduced to only some ±4%. This was good news to both failure analysts and other workers involved in the field of component remnant life and life extension since such predicted fatigue stress ranges use real fracture characteristics observed at some point on the actual component fracture surface.These findings were applied to a real cracking problem recently reported in a steam raising plant, viz., a cracked attemperator reducer weld. In this case an NDT assessment indicated that the maximum crack depth was 7 mm while the lower bound critical crack depth was estimated at 10 mm. As such, remnant life assessments can be estimated for a series of fatigue stress ranges through the use of a reported 450 °C fatigue crack growth law for C–Mn steels.Remnant life estimates of a 7 mm deep crack for a range of stress ranges varied from 3000 to 4000 starts where the chances of the real remnant life values being greater than the calculated values was only 1 in 2. However when a realistic failure probability which reflected the serious implications of a failure event of E?4 was taken the remnant life values were reduced to around 100 starts or some 6 months of normal service.     

Very high cycle fatigue properties of bainitic high carbon–chromium steel

Fatigue properties of bainitic 100Cr6 (SAE 52100, JIS SUJ2) steel are investigated in the high cycle and very high cycle fatigue (VHCF) regime. Fully reversed tension–compression fatigue tests are performed with ultrasonic fatigue testing equipment. Specimens are grinded which leads to surface compression stresses and increased surface roughness. About 1/3 of the specimens failed after crack initiation at interior Al₂O₃? or TiN-inclusions and 2/3 failed after surface crack initiation at scratches or cavities. When inclusions are considered as cracks, failures can occur at minimum stress intensity range of 2.8 MPa m^1/2, and maximum stress intensity range without failure is 3.3 MPa m^1/2. Facets are visible close to the inclusion in some specimens, and the stress intensity range at the border of the facet is approximately 4.5 MPa m^1/2. Murakami’s model can well predict the endurance limit at 10⁹ cycles for internal failures considering the area of the inclusion in the evaluation. Surface fatigue crack initiation can lead to failure above 10⁸ cycles. When scratches are considered as cracks, minimum stress intensity range of 2.5 MPa m^1/2 can propagate surface cracks to failure. Fracture mechanics approach showed several similarities to literature results of the same material tested in tempered martensite condition.     

Two-phase heat transfer and pressure drop of LNG during saturated flow boiling in a horizontal tube

Two-phase heat transfer and pressure drop of LNG (liquefied natural gas) have been measured in a horizontal smooth tube with an inner diameter of 8 mm. The experiments were conducted at inlet pressures from 0.3 to 0.7 MPa with a heat flux of 8–36 kW m⁻², and mass flux of 49.2–201.8 kg m⁻² s⁻¹. The effect of vapor quality, inlet pressure, heat flux and mass flux on the heat transfer characteristic are discussed. The comparisons of the experimental data with the predicted value by existing correlations are analyzed. Zou et al. (2010) correlation shows the best accuracy with 24.1% RMS deviation among them. Moreover four frictional pressure drop methods are also chosen to compare with the experimental database.     

Research on fatigue behavior and residual stress of large-scale cruciform welding joint with groove

Fatigue fracture behavior of the 30 mm thick Q460C-Z steel cruciform welded joint with groove was investigated. The fatigue test results indicated that fatigue strength of 30 mm thick Q460C-Z steel cruciform welded joint with groove can reach fatigue level of 80 MPa (FAT80). Fatigue crack source of the failure specimen initiated from weld toe. Meanwhile, the microcrack was also found in the fusion zones of the fatigue failure specimen, which was caused by weld quality and weld metal integrity resulting from the multi-pass welds. Two-dimensional map of the longitudinal residual stress of 30 mm thick Q460C-Z steel cruciform welded joint with groove was obtained by using the contour method. The stress nephogram of Two-dimensional map indicated that longitudinal residual stress in the welding center is the largest.     

Impact behavior of entangled steel wire material

Entangled steel wire (Q195F) with total porosity of 36.3 ± 1.3 to 61.8 ± 2.4% and pore sizes of 15–825 µm have been investigated in terms of the porous morphologies, impact deformation and failure behavior. The results reveal that the impact toughness increases with the decrease of the porosity. The sintered entangled steel wire materials with 61.8 ± 2.4% porosity exhibit an average of 11.8 J/cm² impact toughness. With 36.3 ± 1.3% porosity, the sintered materials have an average of 45.5 J/cm² impact toughness. Impact absorbing energy and impact toughness have been obtained by Charpy impact testing. Essential impact deformation and failure mechanisms such as pore edges (i.e. fibers) bending, bulking, rotating, yielding, densification and fracture, as well as break (or avulsion) of sintering points in the steel wire framework contribute to the excellent energy-absorbing characteristics under impact loading condition.     

Experimental investigation and correlation of two-phase frictional pressure drop of R410A–oil mixture flow boiling in a 5 mm microfin tube

This study presents experimental two-phase frictional data for R410A-oil mixture flow boiling in an internal spiral grooved microfin tube with outside diameter of 5 mm. Experimental parameters include the evaporation temperature of 5 °C, the mass flux from 200 to 400 kg m^?2 s^?1, the heat flux from 7.46 to 14.92 kW m^?2, the inlet vapor quality from 0.1 to 0.8, and nominal oil concentration from 0 to 5%. The test results show that the frictional pressure drop of R410A initially increases with vapor quality and then decreases, presenting a local maximum in the vapor quality range between 0.7 and 0.8; the frictional pressure drop of R410A–oil mixture increases with the mass flux, the presence of oil enhances two-phase frictional pressure drop, and the effect of oil on frictional pressure drop is more evident at higher vapor qualities where the local oil concentrations are higher. The enhanced factor is always larger than unity and increases with nominal oil concentration at a given vapor quality. The range of the enhanced factor is about 1.0–2.2 at present test conditions. A new correlation to predict the local frictional pressure drop of R410A-oil mixture flow boiling inside the internal spiral grooved microfin tube is developed based on local properties of refrigerant–oil mixture, and the measured local frictional pressure drop is well correlated with the empirical equation proposed by the authors.     

Fatigue lifetime and tearing resistance of AA2198 Al–Cu–Li alloy friction stir welds: Effect of defects

The fatigue strength and failure mechanisms of defect-free (“sound”) and flaw bearing friction stir butt-welds of 3.1 mm-thick AA2198-T8 Al–Li–Cu alloy have been investigated via S–N curves at R = 0.1 using cross weld specimens. The fatigue strength of sound welds is only reduced by 10–15% at the aimed lifetime of 10⁵ cycles compared to the base material. Joint Line Remnant (JLR) bearing welds have a similar fatigue strength as sound welds and the JLR is not the crack initiation site. Kissing Bond (KB) bearing welds that have undergone a weld root polishing show a reduction in fatigue strength by 17% compared to sound welds. For specimens loaded at or above yield strength of the weld nugget the crack systematically initiates from the KB during the first cycle, which is interpreted further using fracture mechanics. The strongest reduction, about 28% in fatigue strength, is found for welds with an initial gap between the parent sheets (GAP welds) along with initiation at intergranular surface microcracks. Kahn tear tests show a reduction in tearing resistance for the flaw bearing welds with a similar ranking as for the fatigue strength.