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A sandwich structure is a composite material consisting of thin skins encapsulating a cellular core. Such structures have proven to be excellent energy absorbents and are frequently found in various types of protection. Even so, few studies exist in the open literature on the response of the core material itself under extreme loadings such as blast and impact. Since a blast load is usually accompanied by numerous fragments, it is important to understand and be able to predict the ballistic impact resistance of the often highly inhomogeneous cellular core materials in design. In this study, the ballistic impact response of an aluminium foam with a complex cell structure has been investigated both experimentally and numerically. First, an extensive material test program involving compression tests on cubic specimens loaded in the thickness direction of the foam was carried out to reveal the mechanical properties of the material. In addition, several of the specimens were scanned before testing using X-ray Micro Computed Tomography (XRMCT) to map the multi-scale topology and morphology of the material. These data were later analysed to extract density-variation plots in many different material orientations. Second, ballistic impact tests were conducted using a gas gun where rigid spheres were launched towards aluminium foam plates, and the ballistic limit velocity and curve of the foam material were established. Finally, numerical simulations of both the material tests and the ballistic impact tests were carried out using LS-DYNA and different modelling approaches based on the XRMCT data. It will be shown that, independent of the modelling strategy applied, good agreement between the experimental impact tests and the numerical predictions can be obtained. However, XRMCT data are important if the final goal is to numerically optimise and improve the behaviour of inhomogeneous foams with respect to energy absorption, thermal isolation, or similar properties. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the possibility of predicting ductile fracture of pipeline steel by using the Gurson–Tvergaard–Needleman (GTN) model where the onset of void coalescence is determined based on in situ bifurcation analyses. To this end, three variants of the GTN model, one of which includes in situ bifurcation, are calibrated for a pipeline steel grade X65 using uniaxial and notch tension tests. Then plane-strain tension tests and Kahn tear tests of the same material are used for assessment of the credibility of the three models. Explicit finite element simulations are carried out for all tests using the three variants of the GTN model, and the results are compared to the experimental data. The capability of the simulation models to capture onset of fracture and crack propagation in the pipeline steel is evaluated. It is found that the use of in situ bifurcation as a criterion for onset of void coalescence in each element makes the GTN model easier to calibrate with less free parameters, all the while obtaining similar or even better predictions as other widely used formulations of the GTN model over a wide range of different stress states. [ABSTRACT FROM AUTHOR]

Guidelines have been worked out on how to design sub-sea pipelines in fishing-rich areas subjected to the possible interference by trawl gear or ship anchors. One topic of special interest for the offshore industry is pipelines first subjected to impact from an anchor before being dragged along the seabed. After removal of the load, the pipe will be straightened due to rebound and present axial forces. The material in the deformed impact zone will experience a complex stress and strain history, which subsequently can cause cracking, leading to leakage or full failure. To study these topics, full-scale testing is not straightforward and thus a simplified approach is chosen as a first step in the present study. Motivated by the observed local curvature in impacted pipelines, three-point bending tests of plate strips cut from a typical offshore pipeline have been carried out and the strips subsequently stretched to a straight position. One objective of these tests was to investigate whether cracking in the plate strip could occur after such a loading sequence. Material tests with specimens taken in different directions and at different locations in the actual pipe were carried out to calibrate an appropriate constitutive relation (taking anisotropy and kinematic hardening into account) and a simplified fracture criterion. Numerical simulations of the complete loading sequence were finally carried out and the predicted response was validated against the experimental data. [ABSTRACT FROM PUBLISHER]

Abstract: 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. [Copyright &y& Elsevier]

The Norwegian Public Roads Administration has initiated a large research project on a potential ferry-free coastal highway route E39 on the west coast of Norway. For the wide and deep fjords, a submerged floating tunnel (SFT) in reinforced concrete has been suggested as a possible solution. A potential hazard for such a structure is internal blast loading, which can be devastating to its structural integrity. To assess the blast performance of concrete structures, a shock tube has been used to generate blast loading against concrete slabs with and without reinforcement. The shock tube tests were filmed with high-speed cameras, and digital image correlation was used to measure the out-of-plane deformations. A finite element model using input from material tests was set up in ABAQUS/Explicit to recreate the slab experiments. Based on the models validated by the shock tube experiments, full-scale numerical simulations of blast loaded SFTs with circular and rectangular cross-sections were run. The results were used to assess the qualitative performance of each cross-section geometry with respect to blast loading, where the circular cross-section indicated a superiour behaviour. • Digital image correlation was used to measure deformations in concrete tests. • The data was used to calibrate a constitutive relation for concrete. • The calibrated model gave good results for shock loaded concrete slabs. • The model was used to make predictions for blast loaded submerged floating tunnels. • A circular cross-section appears to be preferable to a rectangular for blast loads. [ABSTRACT FROM AUTHOR]

In this paper, a discrete particle method was evaluated and used in numerical simulations of low-velocity penetration in sand. Hemispherical, blunt, and ogival-nosed impactors were tested at striking velocities below 5 m/s. The tests were conducted in a dropped-object-rig where the resisting force from the sand was measured continuously during the experiments. This provided a basis for comparison for the simulations. The shapes of the force-penetration depth curves were different for the various impactors, but the ultimate penetration depths were similar in all tests that were done with the same impact velocity. Three-dimensional discrete particle simulations were generally capable of describing the behavior of the sand. However, the peak resisting force was underestimated, which led to a slight overestimation of the ultimate penetration depth. This discrete particle method has previously been evaluated at high impact velocities. The results presented in this study supplement past results and show that the method can also be used to describe the overall response of sand subjected to low-velocity penetration. [ABSTRACT FROM AUTHOR]

An increase in material strength is commonly associated with better protection in blast-resistant design. However, higher material strength comes at the cost of reduced ductility, and it is still not clear how this trade-off affects the load carrying capacity of blast-loaded structures. This work presents an experimental investigation of the strength–ductility trade-off on the dynamic response of thin steel plates subjected to blast-like loading conditions. Two different plate materials were applied for this purpose; one dual-phase, medium-strength, high-hardening steel, and one martensitic, high-strength, low-hardening steel. Seven plate geometries with various pre-formed defects were tested for both materials, triggering distinctly different deformation and fracture modes in the plates. In total, six different blast intensities were applied, and all tests were recorded using two high-speed cameras synchronized with pressure measurements. 3D-DIC measurements were used to track the mid-point displacement and deformation profile of the target plates until fracture. For plates without defects, the high-strength, low-hardening steel resulted in the smallest deflections for a given load. However, for plates with pre-formed defects, the medium-strength, high-hardening steel showed a superior resistance to fracture. The geometry of the pre-formed defects was found to significantly influence the global deformation, the resistance against crack propagation, and the fracture mode. [Display omitted] • Thin, perforated steel plates were subjected to blast loading. • Pre-formed defects dominate the fracture mode. • Strength–ductility trade-off during large, plastic deformations. • Increased material strength reduced the resistance to ductile fracture. [ABSTRACT FROM AUTHOR]

• A novel calibration method of the modified Mohr-Coulomb fracture model is presented. • Localization analyses and two material tests are used in the model calibration. • Experiments on AA6016 specimens covering a range of stress states are performed. • FE simulations predict crack initiation and propagation with good accuracy. This paper presents a novel calibration procedure of the modified Mohr-Coulomb (MMC) fracture model by use of localization analyses and applies it for three tempers of an AA6016 aluminium alloy. The localization analyses employ the imperfection band approach, where metal plasticity is assigned outside the band and porous plasticity is assigned inside the band. Ductile failure is thus assumed to occur when the deformation localizes into a narrow band. The metal plasticity model is calibrated from notch tension tests using inverse finite element modelling. The porous plasticity model is calibrated by use of localization analyses where the deformation histories from finite element simulations of notch and plane-strain tension tests are prescribed as boundary conditions. Subsequently, localization analyses are used to establish the failure locus in stress space for proportional loading conditions and thus to determine the parameters of the MMC fracture model. Finite element simulations of notch tension and in-plane simple shear tests as well as two load cases of the modified Arcan test are used to validate the calibrated fracture model. The predictions by the simulations are in good agreement with the experiments, even though some deviations are seen for each temper. The results demonstrate that localization analyses are a cost-effective and reliable tool for predicting ductile failure, reducing the number of mechanical tests required to calibrate the MMC fracture model compared to the hybrid experimental-numerical approach usually applied. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The growing global demand for greener and more sustainable materials begs the question whether low-carbon concrete can withstand extreme loadings like blast and ballistic impact as well as standard concrete. Few, if any, studies have considered the protective potential of low-carbon concrete where the cement is partially replaced by low-emission alternatives. To investigate this, we fired ogival steel projectiles at 100 mm thick concrete targets. The mechanical properties of three different concrete materials were found using standardised material tests. The low-carbon alternatives were found to provide equally good protection compared with standard concrete. • Low-carbon concrete has the same ballistic protection ability as standard concrete. • Quasi-static material parameters are good indicators of ballistic performance. • Normal steel grid reinforcement does not affect ballistic resistance significantly. [ABSTRACT FROM AUTHOR]

• Sandwich panels were made of aluminium skins and a very inhomogeneous foam core. • The ballistic perforation resistance increased by 16% when the skins were added. • This increase is the same as the weight increase of the panel by adding the skins. • XRMCT data was used to distribute the foam's density variation in FE simulations. • The numerical model was able to accurately predict the ballistic limit velocity. Sandwich structures have proven to be excellent energy absorbents and are found in various protective solutions. In this study, an inhomogeneous aluminium foam was used as core material in a sandwich panel with skins of thin aluminium plates. Ballistic impact tests were conducted in a gas gun using spherical projectiles to reveal the ballistic response of the sandwich panel, and the results were compared with data from impact tests on the core material only. Numerical models, calibrated based on material tests and X-ray Micro Computed Tomography (XRMCT) data, of the ballistic impact tests were established in a non-linear finite element solver. The numerical model was found able to reproduce the complex response of the sandwich panel. [ABSTRACT FROM AUTHOR]

• Three different concretes were subjected to three different material tests. • The ballistic limits of 50 mm thick slabs of the three concretes were found. • A modified Holmquist-Johnson-Cook model was calibrated for all three concretes. • Axisymmetric simulations gave good conservative estimates for the ballistic limits. • Tensile strength is the governing parameter for the impact resistance of thin slabs. Concrete is a material frequently used in protective structures exposed to extreme loading. In this study, the ballistic perforation resistance of 50 mm thick plain concrete slabs impacted by 20 mm diameter ogive-nose steel projectiles is investigated both experimentally and numerically. Three types of commercially produced concrete with nominal unconfined compressive strengths of 35, 75 and 110 MPa were used to cast material test specimens and slabs. After curing, ballistic impact tests were carried out in a compressed gas gun facility to determine the ballistic limit curve and velocity for each concrete type. Alongside the impact tests, material tests were conducted to assess the mechanical properties of the materials. Finite element models using input from the material tests were established in LS-DYNA. Here, the constitutive behaviour of the three concrete types was predicted by a modified version of the Holmquist-Johnson-Cook (MHJC) model from the literature. Numerical simulations of the ballistic impact tests were finally carried out and the results were found to be in good agreement with the experimental data. The main objective of the study is to reveal the accuracy of the MHJC model in predicting the ballistic perforation resistance of concrete slabs impacted by ogive-nose steel projectiles using standard material tests and two-dimensional digital image correlation to calibrate the constitutive relation. [ABSTRACT FROM AUTHOR]

• 34 concrete pipes have been subjected to blast loading from C-4 charges. • Detonations from the inside are more damaging than detonations from the outside. • A new concrete model was calibrated based on inverse modelling of material tests. • Particles and 3rd order elements with node splitting were used for FE simulations. • Excellent qualitative results were obtained though the pressure was underpredicted. The E39 coastal highway route along the west coast in Norway is envisioned a future without ferries. A submerged floating tube bridge built in concrete has been suggested as a means of crossing wide and deep fjords, and an internal blast load against this structure could have disastrous consequences. To investigate the response of tubular concrete structures subjected to blast loading, standard off-the-shelf concrete pipes were tested using live explosives. The tests were monitored by high-speed cameras and pressure sensors. Three different positions for the explosive charges were used and the charge size was varied for each position. It was found that a contact charge detonated from the outside requires approximately twice the explosive amount to breach the pipe than a contact charge detonated from the inside, suggesting a significant confinement effect. Charges placed in the centre of the cross-section produced longitudinal cracking and fragmentation. Measurements show that the pressure inside the pipe is notably higher than immediately outside. As one would expect, reinforcement increased the blast capacity of the pipes significantly. Material test specimens (cubes and cylinders) were cut from the pipes and tested quasi-statically to determine the mechanical properties. The material tests were used to calibrate a concrete material model for finite element simulations, where the blast load was described by a particle-based method. The structural response from the simulations showed good agreement with the experimental data. This applied to the contact charges as well although no special measures were taken to account for the contact detonation. [ABSTRACT FROM AUTHOR]

In this study, an elastic-plastic model with yielding described by a newly proposed orthotropic yield criterion was used to model the unusual deformation of a strongly textured AA6060 alloy. Available experimental data from tension tests and results of crystal plasticity simulations were used to determine the anisotropy coefficients involved in the yield criterion. Virtual material tests using a recent polycrystalline model were performed to obtain flow stresses for loadings where experimental data were not available. The capability of the elastic-plastic model to account for the distinct anisotropy of the material is demonstrated through comparison of finite element simulations and experimental tests on both smooth and notched axisymmetric specimens of the AA6060 alloy. Specifically, for the smooth specimen, the model predicts that the minimum cross-section evolves from a circle to an ellipse while for the notched specimens, the minimum cross-section evolves from a circular shape to an approximately rectangular, or rhomboidal shape, respectively as observed in the experiments. This model can be easily implemented in finite element codes, requires reduced CPU time compared to crystal plasticity finite element simulations, and can be applied in simulations of large-scale structural applications. • Cazacu (2018) describes unusual deformation of anisotropic AA6060. • Calibration done with mechanical and virtual data obtained with polycrystal model based on new single-crystal model. • Effect of notch on geometry of final cross-sections well-described. [ABSTRACT FROM AUTHOR]

• Additive manufactured and die-cast AlSi10Mg showed different tensile test response. • A significant difference in microstructure was also noted. • Despite the differences the behaviour during ballistic impact were very similar. • Ductile hole growth with limited fragmentation was seen for both materials. • Conventional computational tools gave good predictions of the ballistic limit. Materials and structures made through additive manufacturing (AM) have received a lot of attention lately due to their flexibility and ability to customize structural components of complex geometry. One range of application not exploited so far is the use of additive manufactured metal plates for ballistic protection. In this study, plates of AlSi10Mg with dimensions 100 mm × 80 mm × 5 mm were manufactured in a powder-bed fusion machine. From the printed plates, material specimens were extracted and strained to fracture in uniaxial tension to reveal the mechanical response of the AM material. Metallurgical investigations were also conducted to study the microstructure of the as-built alloy both before and after testing. Next, the perforation resistance of the AM plates was disclosed in a ballistic range. During testing, the plates were impacted by 7.62 mm APM2 bullets at various velocities. In an additional test series, only the hard core of the same bullet inserted in a sabot was fired towards the plates. Based on high-speed camera images, the initial and residual velocities of the different bullets were measured, and the ballistic limit curves and velocities were determined. For comparison, the studies described above were repeated on a traditionally die-cast block of AlSi10Mg having the same chemical composition as the powder used in the 3D printing. Finally, based on the conducted material tests a standard constitutive relation and failure criterion, frequently used in ballistic impact simulations, were calibrated based on inverse modelling. Finite element models of the ballistic impact problems were established in ABAQUS/Explicit, and the numerical results were compared to the experimental data. Good agreement between predicted and experimental results was in general obtained, even though no special measures were undertaken concerning the fact that the target material was additively manufactured. [ABSTRACT FROM AUTHOR]