Study of springback of green bodies using micromechanical experiments and the discrete element method

The Discrete Element Method (DEM) is today a commonly used tool to simulate compaction of particulate media. The main issue when using DEM in compaction problems is the description of the contact between two powder particles. If the material properties are known, analytical and semi-analytical methods can be used [1,2] but for many industrial applications, for instance spray dried granules, the mechanical behaviour is unknown. The compaction behaviour and green properties of a cemented carbide powder is studied in this work and the issue of the contact description is solved by performing experiments on the powder granules. Firstly, compression tests are made on the single granules giving information of the mechanical properties at low strains. To get information at high strains, which are needed in powder compaction simulations, nanoindentation tests are performed. The measured material parameters are used in a FE model of two spheres in contact and the resulting contact law is exported to a DEM program. The DEM program is used to investigate the compaction properties of a powder compact and especially the springback during unloading which is important for predicting the final shape of the product. The results are compared with presently performed experiments and the applicability range of the discrete element simulations will be discussed

Uncoupled and coupled approaches to predict macrocrack initiation in fiber reinforced ceramic matrix composites

International audienceLocalized fiber pull-out is one of the fracture features of fiber reinforced ceramic matrix composites. The onset of this mechanism is predicted by using Continuum Damage Mechanics, and corresponds to a localization of the deformations. After deriving two damage models from a uni-axial bundle approach, and criteria at localization, different axisymmetric configurations are analyzed through two different approaches to predict macrocrack initiation

Localization due to damage in fiber-reinforced composites

International audienceFiber pull-out is one of the fracture features of fiber-reinforced ceramic matrix composites. The onset of this mechanism is predicted by using Continuum Damage Mechanics, and corresponds to a localization of the deformations. Alter deriving two damage models from a uniaxial bundle approach, different configurations are analysed through analytical and numerical (F.E. calculations) methods. For one model some very simple criteria can be derived, whereas for the second one none of these criteria can be derived and the general criterion of localization has to be used

Biotransport of Organic Pollutants to an Inland Alaska Lake by Migrating Sockeye Salmon (Oncorhynchus nerka)

Persistent organic pollutants such as polychlorinated biphenyls (PCBs) and the pesticide DDT, known to harm wildlife, have been shown to reach pristine Subarctic and Arctic areas by global atmospheric transport. Another transport route for pollutant entry into these ecosystems is provided by migrating salmon. Pollutant transport was studied in a population of sockeye salmon (Oncorhynchus nerka) in the Copper River, Alaska during their 410 km spawning mighration. Pollutants accumulated by the salmon during their ocean life stage were not eliminated during migration, but were transported to the spawning lakes and accumulated in the freshwater food web there. The influence of the biotransported pollutants was investigated by comparing pollutant levels and compositions in atmospheric deposition as well in two different populations of arctic grayling (Thymallus arcticus). One grayling population was in the salmon spawning lake and the other in a nearby lake not hosting anadromous fish, but receiving pollutants only via atmospheric deposition. The grayling in the salmon spawning lake were found to have concentrations of organic pollutants more than two times higher than those of the grayling in the salmon-free lake, and the pollutant composition resembled that found in salmon. Thus, in the studied Alaska river system, biotransport was found to have a far greater influence than atmospheric input on the PCB and DDT levels in lake biota.Il a été prouvé que des polluants organiques persistants comme les diphényles polychlorés (PCB) et le pesticide D.D.T., dont on connaît les répercussions néfastes sur la faune, se répandent dans des zones subarctiques et arctiques vierges par le biais du transport atmosphérique global. Les saumons en migration offrent une autre voie de transport pour l'entrée des polluants dans ces écosystèmes. On a étudié le transport des polluants dans une population de saumon sockeye (Oncorhynchus nerka) remontant le fleuve Copper, en Alaska, durant la migration de frai de 410 km. Les polluants accumulés par le saumon au cours de sa vie océanique n'étaient pas éliminés durant la migration, mais étaient transportés dans les frayères des lacs où ils s'accumulaient dans le réseau trophique de l'eau douce. On a étudié l'influence des polluants bio-transportés en comparant les niveaux et les compositions de polluants dans les retombées atmosphériques ainsi que dans deux populations distinctes d'ombre arctique (Thymallus arcticus). Une des populations d'ombre se trouvait dans le lac où frayaient les saumons et l'autre dans un lac voisin, ne contenant pas de poissons anadromes, mais recevant des polluants uniquement par retombées atmosphériques. On a trouvé que l'ombre qui se trouvait dans le lac où frayaient les saumons avait des concentrations en polluants organiques deux fois plus grandes que l'ombre qui se trouvait dans le lac sans saumons, et la composition des polluants ressemblait à celle que l'on retrouvait dans le saumon. Ainsi, dans le réseau fluvial de l'Alaska qui faisait l'objet de cette étude, on a trouvé que le transport biologique avait une influence beaucoup plus importante que les retombées atmosphériques sur les niveaux de PCB et de D.D.T. dans le biote lacustre

Granite rock fragmentation at percussive drilling - experimental and numerical investigation

International audienceThe aim of this study is to numerically model the fracture system at percussive drilling. Due to the complex behavior of rock materials, a continuum approach is employed relying upon a plasticity model with yield surface locus as a quadratic function of the mean pressure in the principal stress space coupled with an anisotropic damage model. In particular, Bohus granite rock is investigated and the material parameters are defined based on previous experiments. This includes different tests such as direct tension and compression, three point bending and quasi-oedometric tests to investigate the material behavior at both tension and confined compression stress states. The equation of motion is discretized using a FE approach and the explicit time integration method is employed. EOI (Edge-On Impact) tests are performed and the results are used to validate the numerical model. The percussive drilling problem is then modeled in 3D and the bit-rock interaction is considered using contact mechanics. The fracture mechanism in the rock and the bit penetration- resisting force response are realistically captured by the numerical model

On the Tensile Strength of Granite at High Strain Rates considering the Influence from Preexisting Cracks

The dynamic tensile behavior of granite samples, when some preexisting cracks are introduced artificially, is investigated. Spalling tests using a Hopkinson bar are performed and strain rates of ~102 1/s are obtained in both specimen types (with and without initial cracks). This experimental technique is employed being of the same order as strain rates in rock materials during percussive drilling, the application of interest here. The dynamic tensile responses of both sample-sets are compared using the velocity profile measured on the free-end of the sample. Furthermore, an anisotropic damage model based on the concept of obscuration probability describes the response without preexisting cracks. Here, a term of cohesive strength in obscuration zones is added to accurately handle the softening behavior of the material in tension. Results from the spalling tests are used to validate the model prediction of the dynamic tensile strength and also to calibrate the cohesive model parameters. Damaged elements are numerically introduced in the finite element calculations simulating the spalling experiments performed on predamaged samples. The results are compared with the experimental ones. Good agreement is obtained showing that a two-scale approach may constitute a suitable method to simulate numerically the tensile response of predamaged granite

Determining the density distribution in cemented carbide powder compacts using 3D neutron imaging

Spray-dried refractory carbide and metal powder mixtures, containing tungsten carbide, is compacted and sintered during the production of conventional cutting tool inserts. Since friction between the pressing tool and the powder gives rise to density gradients in the powder compact, shrinkage during sintering is uneven. The shape of the sintered blank is important and can be predicted with finite element (FE) simulations. To validate the simulation of the pressing procedure, the density gradients in the powder compacts must be measured with a high spatial resolution. Since tungsten has a high atomic number, it is hard to penetrate with X-rays and even cold neutrons. We show here that using a polychromatic beam of thermal neutrons, along with beam-hardening correction, such measurements can be successfully realized. The obtained results show good agreement with corresponding FE-simulations. Also, deliberate differences in the compaction process could be verified with the neutron measurements

Norovirus Gastroenteritis Outbreak with a Secretor-independent Susceptibility Pattern, Sweden

Nonsecretors were highly susceptible to norovirus GI.3 in a foodborne outbreak