This work aims at studying experimentally and reproducing numerically the failure mechanisms of a ship structure constitutive material when submitted to airblast loading. With this aim in view, a physically motivated approach has been developed and applied in order to describe the transition of behaviour between dense metal plasticity and micro-porous metal plasticity in the context of dynamic plasticity and failure. The viscoplastic material is supposed to be initially exempt of micro-voids. Subjected to a monotonic loading involving a positive or null stress triaxiality, it behaves elastic-(visco)plastically. As soon as the conditions of plastic strain, plastic strain rate, temperature and stress triaxiality for the germination of a given volume fraction of voids are satisfied, the material behaviour becomes pressure dependent. Its damage-plastic yielding is consequently described using a micro-porous metal plasticity potential. The latter is proposed in the form of a modified version of the Gurson-Tvergaard-Needleman model allowing for describing cavity growth and further fracture under shear loading. The 3D constitutive equations are implemented as user material in the engineering finite element computation code Abaqus®. Numerical simulations are conducted and compared with experiments considering airblast loaded ship structures. The numerical results show clearly the influence of the hole nucleation criterion related constants and of the loading conditions (temperature, strain rate) on the damage and further fracture of the material.

This work contributes to the simulation of the fluid-structure interaction (FSI) undergone by submarines subjected to a far-field underwater explosion.In such events, two physical phenomena occur.After a few milliseconds, an acoustic shock wave (fast excitation, linear acoustic fluid) first reaches the ship before a subsequent, slower, fluid motion also affects the ship over a longer time scale.These two phenomena can be studied in a decorrelated way [1].Numerical methods for solving the two stages of the FSI problem were proposed in [2] and applied to complex and realistic geometries (with several millions of degrees of freedom on the ship surface for the first stage).Regarding the first stage, the original goal of [2] was to implement the coupling of time-domain FEM for the ship and a "Z-BEM" approach that combines a fast boundary element method (BEM) implemented in the Laplace domain with the convolution quadrature method (CQM) for the discrete-time response of the whole fluid domain.The latter method was accelerated with a Fast Multipole Method and the use of a high frequency approximation, the BEM solutions at complex frequencies used by the CQM being computed in parallel [3].At the time, a heuristic iterative approach for coupling the FEM and the Z-BEM failed to converge, however.The Z-BEM was proved very effective on large and realistic models [3], but could only be used at the initial stage of the complete FSI analysis in [2]; then, the FEM was also used for a surrounding fluid region.While workable, this treatment entailed significant meshing difficulties.In the present follow-up work, we revisit the issue of coupling the transient Z-BEM (for the fluid) and the transient FEM (for the ship) towards solving the first (fast) stage of the FSI computation, in order to be able to use only the BEM for the (unbounded) fluid domain.To replace the previouslytried Neumann-Neumann (NN) coupling iterations (which failed to converge), we propose an iterative coupling scheme based on Robin-Robin (RR) iterations.By applying function analysis arguments to the continuous acoustic-elastodynamic FSI problem as well as the auxiliary acoustic and elastodynamic problems, we both (i) explain why the previous NN iterations applied to the transient FSI problem had to diverge, and (ii) prove that our proposed RR iterations are convergent for the FSI problem.We demonstrate our convergent coupled Z-BEM/FEM method on several examples, and highlight treatments allowing to accelereate the computation: reuse of compressed BEM operators at each coupling iteration, and convergence acceleration methods that reduce the number of coupling iterations (Aitken method, relaxation).Finally, the efficiency of this method to accurately simulate realistic problems will be illustrated with examples from the naval engineering industry.

In the last few years, the mechanical response of hollow thermoplastic microsphere-elastomer matrix composites has been investigated. The large majority of the studies focuses on their compressive properties and particularly on the stress-strain response. In the present paper, large strain uniaxial tension experiments are conducted on thermoplastic microsphere filled polyurethane elastomer. Six volume fractions are considered. Thanks to a two-camera setup and digital image correlation measurements, the remarkable volumetric behaviour is highlighted. First, the hydrostatic pressure vs. volume change response during a loading-unloading cycle consists of a hysteresis loop, and its size is directly related to the volume fraction of microspheres in the composite. Second, this loop admits a complex shape with several extrema. This remarkable response can be considered as the macroscopic signature of the complex microstructural phenomena involved during deformation.

In this paper, slamming experiments at constant velocity are presented. The hydraulic shock test-rig enables fast experiments at quasi-constant velocity during a water entry of a wedge. This approach provides results more suited for validation of numerical models. The objective of this paper is to discuss the influence of panel stiffness on pressure and strain, for an aluminum wedge with a deadrise angle of 30° and for various velocity levels. Information on wet surface expansion velocity is inferred from the detection of the threshold pressure on the placed sensors. The experimental evolution of the maximum pressure and wet surface expansion velocity is in good agreement with analytical results and can be predicted by semi-analytical formulas.

The goal of this work is to predict ductile failure of pipe-ring notched AISI 316L specimens, where notches mimic the geometry of corrosion defects. Uncoupled damage models are used to that end. The Johnson-Cook and LouHuh criteria are calibrated. The uncoupled damage models are calibrated using experimental results from testing pipe-ring notched specimens. Calibration was achieved using a hybrid experimental-numerical approach. Six notch shapes are studied. Experimental matrices are designed to determine these shapes, and the pipe-ring notched specimen is inspired from the literature. Calibration of the uncoupled damage models require a ductile crack initiation indicator. The first indicator was based on the derivative curves of the force versus the connectors displacement curve. The second was based on the raw images of the gage section. The third was based on a percentage of the connectors displacement at fracture. Results show that ductile fracture depends on the Lode paramater. As a consequence, the Lou-Huh criterion is more effective at predicting ductile fracture than the Johnson-Cook criterion.

Dynamic crack propagation in elastomer membranes is investigated; the focus is laid on cracks reaching the speed of shear waves in the material. The specific experimental setup developed to measure crack speed is presented in details. The protocol consists in (1) stretching an elastomer membrane under planar tension loading conditions, then (2) initiating a small crack on one side of the membrane. The crack speed is measured all along the crack path in both reference and actual configurations, including both acceleration and deceleration phases, i.e. non steady-state crack propagation phases. The influence of the prescribed stretch ratio on crack speed is analysed in the light of both these new experiments and the few previously published studies. Conclusions previously drawn for steady-state crack growth are extended to non steady-state conditions: stretch perpendicular to the crack path governs crack speed in intersonic crack propagation regime, and the role of the stretch in crack direction is minor.

Summary Three‐dimensional (3D) rapid transient acoustic problems are difficult to solve numerically when dealing with large geometries, because numerical methods based on geometry discretization (mesh), such as the boundary element method (BEM) or the finite element method (FEM), often require to solve a linear system (from the spacial discretization) for each time step. We propose a numerical method to efficiently deal with 3D rapid transient acoustic problems set in large exterior domains. Using the ‐transform and the convolution quadrature method, we first present a straightforward way to reframe the problem to the solving of a large amount (the number of time steps, M ) of frequency‐domain BEMs. Then, taking advantage of a well‐designed high‐frequency approximation, we drastically reduce the number of frequency‐domain BEMs to be solved, with little loss of accuracy. The complexity of the resulting numerical procedure turns out to be O (1) in regard to the time discretization and for the spacial discretization, the latter being prescribed by the complexity of the used fast BEM solver. Examples of applications are proposed to illustrate the efficiency of the procedure in the case of fluid‐structure interaction: the radiation of an acoustic wave into a fluid by a deformable structure with prescribed velocity and the scattering of an abrupt wave by simple and realistic geometries.

This paper is devoted to the modeling of crack propagation under impact loadings in thick shells composed of metallic materials using X-FEM. The proposed thick shell element is based on the existing Q4 gamma 24 4-node shell element for fast transient loadings that is enriched in the present work using X-FEM. The shell is considered to be always cut by a through the thickness crack. This element is hence enriched only with jump Heaviside functions on all degrees of freedom (displacements as well as rotations). The mass matrix corresponding to all added DOF is simply a copy of the usual continuous DOF diagonal mass matrix. The crack is discretized explicitly with a simple 1D mesh that lives on the shell's mid-surface and independently of the finite element mesh. Plasticity as well as stress field used for crack propagation criterion is evaluated using 5 Simpson points across the thickness. The crack propagation criterion is based on the measure of an equivalent stress at the crack tip and can predict both tensile and shear driven fracture as well as transitions between those two regimes. Comparisons with elastoplastic crack propagation experiments involving fracture under transient loadings show that the method is able to reproduce experimental fracture quite well.

Residual stresses can be beneficial or detrimental to mechanical structures. In this work, a rectangular plate specimen with a cold expanded hole was designed to study the influence of these stresses on low cycle fatigue of high-strength steel. Three separated measurements of the stresses were made using different techniques: indentation method, X-ray diffraction method and contour method. The results show good agreement with finite element modelling of the cold-expanded hole problem.

This paper deals with the numerical simulation of the dynamic failure of a ship structure steel plate under near-field air-blast loading. Various energetic levels of air-blast loading, involving variable explosive mass and charge-plate distance, were tested leading to the bulge of the loaded plate for the lowest energy level and to the failure of the loaded plate for the highest level. A modified version of the Gurson–Tvergaard–Needleman potential was used to reproduce the response of the material along the damage-plasticity process at stake. The 3D constitutive equations were implemented as user material in the engineering finite element computation code ABAQUS®, and numerical simulations were conducted and compared with experiments considering air-blast loaded plates. Several crucial numerical issues are addressed concerning notably the use of ABAQUS® conwep function, the hourglass control and the influence of the model constants. Numerical results clearly show the interest of the adopted modelling for the description of salient stages of dynamic structural failure.

This article presents a physically motivated approach, which has been developed in order to describe the transition of behavior between dense metal plasticity and microporous metal plasticity in the context of dynamic plasticity and adiabatic conditions. Considering that void germination requires a certain amount of plastic deformation, a ‘primary’ hole nucleation criterion as well as a statistical law governing the ‘secondary’ hole formation kinetics has been proposed. In a consistent way, the hole nucleation criterion accounts for the accelerating effects of stress triaxiality and the delaying effects of temperature and strain rate. In addition, a modification of the GTN model was proposed, allowing for describing cavity growth under shear loading. The 3D constitutive equations were implemented as user material in the engineering finite element computation code Abaqus®. Numerical simulations were conducted considering a single finite element under uniaxial tensile loading and simple shear then notched cylindrical samples under remote uniaxial tensile loading. The numerical results clearly show the influence of the hole nucleation criteria on the ductile damage and failure.

This work aims at studying experimentally and reproducing numerically the failure mechanisms of a ship structure material when submitted to severe conditions of strain and strain rate. Laboratory tests and airblast experiments were accordingly carried out and a constitutive model has been built describing the salient effects of strain hardening, thermal softening, viscoplasticity and void growth induced damage. Numerical simulations were conducted considering plates subjected to various airblast loading conditions. The numerical results show clearly the influence of the damage and further fracture related model parameters, as well as the limitation of the fluid/structure interaction model used in the present work.

A phenomenological modelling approach has been developed, based on some salient physical effects regarding void growth vs. plastic straining, to describe the transition behaviour between dense metal plasticity and micro-porous metal plasticity. Considering that void germination requires a certain amount of plastic deformation, a 'primary' hole nucleation criterion has been proposed, as well as a statistical law governing the 'secondary' hole kinetics. In a consistent way, the hole nucleation criterion accounts for the accelerating effects of stress triaxiality and, conversely, the delaying effects of temperature and strain rate. In this work, a modification of the GTN model has also been proposed, overcoming its inability to predict damage growth and fracture for zero and low triaxiality, shear-dominated deformations. In this respect the kinematic mean stress related shift mechanism has been introduced and quantified in the expression of the GTN plastic potential, enabling thus the damage growth under shear and under small negative triaxialities. The 3D constitutive equations have been implemented as user material in the engineering finite element computation code Abaqus (R). Numerical simulations have been conducted considering a single finite element under simple shear on one hand and a notched cylindrical sample under remote uniaxial tensile loading on the other hand. The numerical results show clearly the influence of the hole nucleation criterion related constants on the damage and further failure of the material.

Consistent constitutive modelling of material behaviour and further reliable numerical prediction of the response of structures under severe loading necessitate the knowledge of the microstructural mechanisms at the origin of failure.The present work deals notably with the identification of the microstructural damage mechanisms of a high purity (ferritic-pearlitic) mild steel employed as structural material in military ship building. With this aim in view, an extensive campaign of experiments has been carried out, including interrupted and until fracture tests on smooth and notched, axi-symmetric and plane specimens. Initial and post-mortem microstructures of the samples have been observed using a scanning electron microscope (SEM) in order to reveal the damage mechanism. The latter is double: quasi spherical cavity nucleation and growth inside the soft ferritic matrix and microcracking at the (soft)ferrite-(hard)pearlite interphase. Conditions for initiation and evolution of these two kinds of damage appear as being different as expected. The various steps of diffuse damage, microcracking and macro cracking yielding ultimate failure are also observed. Fractographies obtained from tensile tested samples and explosion loaded plates are also compared.Moreover, the material behaviour has been modelled, describing the salient effects observed experimentally, namely strain and strain rate hardening, and thermal and damage softening. The parameters identification was accomplished using an inverse method based methodology. Finite element numerical simulations involving far-field underwater explosion loading implemented as user subroutine in the FE computation code ABAQUS has also been performed, leading to a satisfying agreement between experimental and numerical results. (C) 2010 Elsevier Ltd. All rights reserved.

The problem of three-dimensional liquid–solid impact is considered. A numerical method has been developed to predict the hydrodynamic loads acting on the entering body. The proposed approach is based on the Wagner theory and the boundary element method. In order to validate the numerical simulation, an original experimental study has been performed. It consists of a series of impact tests carried out with a hydraulic machine. Three specimens have been tested: an elliptic paraboloid, a wedge with conical ends and a square pyramid. An excellent agreement between theory and experiments has been observed.

Dans un contexte de vulnerabilite militaire des bâtiments de surface et sous-marins, cette etude, realisee dans le cadre d'une these CIFRE DCNS-LIMATB, a pour objectif la prediction numerique de la reponse de structures navales soumises a explosion au contact. Dans cette perspective, ce travail vise la description de l'endommagement et de la rupture dynamique de materiaux metalliques. Le materiau de l'etude est un acier doux ferrito-perlitique constitutif des coques de bâtiments de surface. Pour etudier les effets couples du taux de triaxialite des contraintes, de la deformation, de la vitesse de deformation, et de la temperature, une vaste campagne experimentale a ete menee, incluant des tests mecaniques de laboratoire ainsi que des essais d'explosion en air libre. Les observations microstructurales d'eprouvettes et de plaques sollicitees ont parallelement revele la coexistence de deux mecanismes distincts d'endommagement : la croissance de cavites spheriques et une microfissuration. Les resultats experimentaux et les constats micrographiques ont alors servi de support a une modelisation avancee du comportement elasto-thermo-viscoplastique du materiau de l'etude et ont permis de construire un formalisme original au travers d'un modele multi-surfaces et multi-mecanismes rendant compte notamment des effets retardants de la vitesse et de la temperature sur la cinetique de l'endommagement ductile, ainsi que des consequences de l'endommagement, isotrope (par croissance de cavites) et anisotrope (par microfissuration). Des modeles tridimensionnels ont finalement ete implantes en tant que lois utilisateur dans le code industriel de calculs par elements finis ABAQUS. Plusieurs configurations d'essais d'explosion en air libre ont ete simulees en utilisant la carte CONWEP d' ABAQUS, destinee a reproduire l'interaction fluide/structure durant l'explosion et dont les limites ont ete mises en evidence. Les performances des modeles sont jugees encourageantes et des pistes d'amelioration sont donnees.

This paper deals with the accuracy of several analytical models for the prediction of the hydrodynamic force and pressure distribution acting on a body entering initially calm water. The problem of water entry is important for the analysis of slamming loads undergone by boats operating in waves, as well as of the steady behaviour of high-speed planing vessels. The considered models are briefly described and the obtained results are compared to those of numerical computations and experimental observations for a number of two-dimensional and axisymmetric cases of water impact.