A thermodynamically consistent cohesive damage model for the simulation of mixed-mode delamination

This work is devoted to the formulation of a new cohesive model for mixed-mode delamination. The model is based on a thermodynamically consistent isotropic damage formulation, with consideration of an internal friction mechanism that governs the interaction between normal and shear opening modes

8-Node solid-shell elements selective mass scaling for explicit dynamic analysis of layered thin-walled structures

To overcome the issue of spurious maximum eigenfrequencies leading to small steps in explicit time integration, a recently proposed selective mass scaling technique, specifically conceived for 8-node hexahedral solid-shell elements, is reconsidered for application to layered shells,where several solid-shell elements are used through the thickness of thin-walled structures. In this case, the resulting scaled mass matrix is not perfectly diagonal. However, the introduced coupling is shown to be limited to the nodes belonging to the same fiber through the thickness, so that the additional computational burden is almost negligible and by far compensated by the larger size of the critical time step. The proposed numerical tests show that the adopted mass scaling leads to a critical time step size which is determined by the element in-plane dimensions only, independent of the layers number, with negligible accuracy loss, both in small and large displacement problems

Selective mass scaling for multi-layer solid-shell discretization of thin-walled structures

The computational burden of an explicit dynamic analysis of thin-walled structures discretized with solid-shell elements can be very high, since the stability condition leads to extremely low time steps because of the small thickness. A selective mass scaling procedure ([1], [2],[3]) can be introduced to overcome this limitation. The technique proposed in [4] for single-layer 8-node solid-shell elements is here generalized to the case of multi-layer shells. The idea is to modify the mass matrix, scaling down the highest structural eigenfrequencies, so that the critical time step is determined only by the in-plane size of the elements, as with standard four-nodes shell meshes. Moreover, the resulting critical time step is shown to be independent of the number of layers used for the throughthe- thickness discretization. The accuracy of the proposed procedure and the computational gain are tested with the aid of numerical examples

Blade cutting simulation with crack propagation through thin-walled structures via solid-shell finite elements in explicit dynamics

To simulate the crack propagation due to blade cutting of a thin-walled shell structure, we propose a numerical technique based on solid shell finite elements and explicit time integration. The limitation on the critical time step due to the small thickness along the out-of-plane direction is overcome through a selective mass scaling, capable to optimally define the artificial mass coefficient for distorted elements in finite strains: since the selective scaling cuts the undesired, spurious contributions from the highest eigenfreqeuencies, but saves the lowest frequencies associated to the structural response, and since the method preserves the lumped form of the mass matrix, the calculations in the time domain are conveniently speeded up. The interaction of the cutting blade with the cohesive process zone in the crack tip region is accounted for by means of the so-called directional cohesive interface concept. Unlike in previous implementations, through-the-thickness crack propagation is also considered. This is of critical importance in particular in the case of layered shells, where one solid-shell element per layer is used for the discretization in the thickness direction and it is a necessary ingredient for future possible consideration of delamination processes. We show by applying the proposed procedure to the cutting of a thinwalled laminate used for packaging applications that this is a promising tool for the prediction of the structural response of thin-walled structures in the presence of crack propagation induced by blade cutting

Directional cohesive elements for blade cutting simulations of layered shells

The blade cutting of thin layered shell involves three small geometrical scales, the scale of layer thicknesses, the scale of blade radius of curvature, the scale of fracture and delamination process zones, which need be resolved when a numerical simulation is carried out by means of a finite element discretization. Large deformations, material nonlinearity, contact, crack propagation and delamination make the problem highly nonlinear, so that an explicit dynamics approach based on the use of solid-shell elements is adopted to avoid convergence problems. A selective mass scaling technique [1,2] is developed to overcome the critical time step limitation, dictated by the layers thickness. The prescribed blade trajectory drives crack propagation, so that it is possible to adjust the mesh with element edges along the expected crack path. To model crack propagation accounting for the interaction between the sharp blade and the cohesive process zone, special “directional cohesive elements” [3] are placed between separating element edges. Crack propagation and delamination can be characterized by very small process zone sizes, depending on the type of material and on the layer thickness. Discretizations that are coarse with respect to these lengths may give rise to spurious oscillations and accuracy loss. Techniques for the mitigation of these problems are investigated. Numerical applications to engineering problems are used to assess the effectiveness of the proposed simulation approach

Explicit dynamics simulation of blade cutting of layered shells

The numerical simulation of blade cutting of thin layered shell is a challenging task, involving complex phenomena, such as large deformations, nonlinear material behaviour, contact, crack propagation, delamination. In particular, three small geometrical scales need be resolved: the scale of layer thicknesses, the scale of blade radius of curvature, the scale of fracture and delamination process zones. In view of the problem nonlinearity, an explicit dynamics formulation with solid-shell elements is adopted to avoid convergence problems. A selective mass scaling approach [1,2] is used to enlarge the critical time step size, dictated by the layers thickness. Crack propagation is modelled by inserting cohesive interfaces between adjacent elements placed along the prescribed blade trajectory. The problem of the interaction between the sharp blade and the cohesive process zone is addressed by using the so called “directional cohesive elements” proposed in [3]. Depending on the type of material and on the layer thickness, the problem may be characterized by very small process zone sizes. Techniques for the reduction of spurious oscillations in the presence of coarse discretization and for the improvement of overall accuracy are investigated. Numerical applications to engineering problems are used to assess the effectiveness of the proposed simulation approach