Mixed‐Mode Delamination – Experimental and Numerical Studies

A micro-mechanical model is developed for laminated composite materials and implemented in the explicit finite element method. The objective of this study is to get an accurate and simple micro-model, which can be used in the displacement-based explicit finite element packages. The micro-mechanical model implemented in the explicit finite element code can be used for simulating the behavior of composite structures under various loads such as impact and crash. The stress-strain relation for the micro-model is derived for shell element. As a demonstration case of the developed micro-model a finite element model of Graphite/Epoxy tube structure is developed and simulated under axial crash.

Dynamic delamination modelling using interface elements

Laminated glass is widely used for architectural glazing and as windshield material in automotive industry. The architectural application of laminated glass is subjected to low-velocity impact by flying debris in hurricane, tornado, and blast events. The automotive application is subjected to low-velocity impact in traffic accidents. Circular samples of laminated glass are subjected to low-velocity impact at 1, 2, 3, and 4 m/s impact velocity approximately by means of Instron Dynatup testing machine. The impact tup force is recorded by data acquisition equipment of the machine. The low-velocity impact of laminated glass is modelled in explicit finite elements, using the commercial software ABAQUS. Three finite element models are examined: brick element model with 4 plies of elements per a glass layer and one per the interlayer, continuum shell element model with a brick element ply for the interlayer, and conventional shell element model excluding the interlayer shear. The glass layers are modelled by brittle damage material model, which is based on fracture toughness Charpy tests. The interlayer of the polymeric (PVB) material is modelled as elastic material with instantaneous Young’s modulus found in a creep test. The careful modelling of laminated glass low-velocity impact in explicit finite element code provides corresponding to the experiment results. The conventional shell element model underestimates the energy dissipation. The continuum shell element model could be very efficient if it is stable enough. The necessity of efficient laminated glass shell element is discussed. The finite element analysis gives an insight of the progressive failure of the laminated glass, explaining the place and the order of radial and circular crack appearance. The investigation of the laminated glass response to the low-velocity impact gives the opportunity to optimize the laminated glass layers for higher resistance.

The difficulty with an OOP situation compared to an in-position situation is that the inflation of the folded airbag is much more important. It has to be realistically computed, since it affects the impact of the dummy. Attaining a realistic simulation means starting with a correct geometry of the folded airbag and simulating the inflation with correct gas dynamics. Several commercial software packages exist that can simulate the inflation process of an airbag, e.g., the explicit Finite Element (FE) code LS-DYNA [5].

This paper presents detailed experimental information on mode-II delamination development in fibre/epoxy composite materials and provides observations about the process zone in the vicinity of the crack tip. It is shown that the energy dissipated in delamination propagation is spent on two ways (i) creating new fracture surfaces (delamination) and (ii) nonlinear shear deformation in the composite plies adjacent to the delaminating interface. Therefore, the nonlinear process zone is not restricted to the resin-rich interface between the layers, but also extends into the fibre/epoxy composite layers. This is different from the conventional assumption in modelling delamination using cohesive elements where the fibre/epoxy layers are fully linear-elastic and the process zone is lumped at the plane of fracture. Based on the accurately measured displacement field around the crack tip, the experimental traction-separation relation at the interface is found to be trapezoidal which is again different from the conventional bilinear cohesive law.

Chapter 2 - Strategies for dynamic failure analysis on aerospace structures

This paper presents a fast engineering tool for the analysis of the mechanical fields in cracked pavements. The pavement structure is reduced to three elastic and homogeneous equivalent layers resting on a soil. The soil is modelled by one layer, named shear layer, connected to Winkler's springs in order to ensure the transfer of shear stresses between the pavement structure and the springs. The whole four-layer system (3 pavement layers connected to the shear layer) is modelled using a simplified model (M4-5n) developed for the analysis of delamination in composite materials. For the case of 2D plane strain, a system of twelve equations is written analytically. These second order differential equations are solved by the finite difference method (Newmark). In the case of a pavement containing a vertical cracking through one layer, the half-analytical solutions are obtained in less than one second CPU. This tool, called M4-5nW, thus allows parametrical studies such as the distribution of mechanical fields (especially interface stresses) as function of the load position more or less distant from the vertical crack. This new modelling shows its effectiveness compared to the use of a conventional finite element code.

An isogeometric continuum shell element for non-linear analysis

Carbon fiber epoxy laminates are extensively used in the aerospace industry as they offer a combination of high performance with relatively low weight. However, damage may occur in these materials, either at the manufacturing level or during use, which may affect their structural integrity. Among the type of damage which may occur, delamination is a common defect which causes a reduction in compressive strength and thus limits the life of laminated composites. If undetected, delamination may propagate during use due to mechanical loads and in the worst case may cause fracture of the material. Therefore, delamination in composite materials needs to be efficiently detected and accurately quantified for safety and economic reasons. Because of their heterogeneity, multi-layered structure and anisotropy, the inspection of composite materials remains difficult with conventional NDT methods. A method making use of Foucault currents, and relying on the principle of electromagnetic induction, can be adapted for the detection and characterization of damage in composite materials. The use of Foucault currents in characterizing delaminated areas at interfaces between plies in carbon fiber/epoxy laminates is addressed in this paper. It is demonstrated that Foucault currents can accurately detect and quantify delamination in CFRP materials. Furthermore, the potential of this technique for monitoring defect propagation in composite materials is discussed.

A nonlinear higher order shear deformation shell element for dynamic explicit analysis:: Part I. Formulation and finite element equations

Simply-supported bridges are vulnerable to surface fault rupture as evidenced by several fault-crossing bridges in the 1999 Chi-Chi earthquake. To investigate the seismic collapse mechanism of simply-supported bridges crossing the fault, across-fault ground motions are firstly simulated in the present study. In particular, based on a previously developed fault model of the 1999 Chi-Chi earthquake, broadband across-fault ground motions at six fault-crossing bridges are simulated using the hybrid deterministic-stochastic method, in which the low- and high-frequency components are computed using the deterministic Green’s function method and the stochastic finite-fault modeling method, respectively. The simulation results indicate that the hybrid deterministic-stochastic method can give reasonable predictions to the across-fault ground motions. Furthermore, utilizing the explicit dynamic finite element (FE) code LS-DYNA, behaviors of a three-span simply-supported bridge under a selected pair of across-fault ground motions are numerically simulated. Numerical results indicate that the structural responses and collapse mechanisms are dominated by the low-frequency ground motions. The large differential static offset across the fault is the main reason for the collapse of the simply-supported bridges. This study contributes understandings for the across-fault ground motions and the collapse mechanism of some bridges in the 1999 Chi-Chi earthquake.

<div class="htmlview paragraph">In early design phases vehicle safety testing has revealed the occurrence of both spot weld failure and sheet metal tearing that contributed to below specification performance. This would require redesign and additional testing to meet the specification. Because of cost and weight requirements in vehicle development, it is desirable to create efficient body designs for vehicle safety performance.</div> <div class="htmlview paragraph">To develop more efficient structures, it is useful to be able to predict both connection and material failure in a variety of loading conditions using finite element (FE) models. Two new FE methodologies were developed separately to aid in predicting these phenomena. Since it is a widely used explicit FE code, LS-DYNA<sup>™</sup> was chosen as the program with which to implement the new material and spot weld models. One methodology uses a force based failure criteria to model failure in, and next to, spot welds. A second methodology was developed to predict sheet material failure using the Forming Limit Diagram (FLD) for ferrous materials. These methodologies were confirmed separately using simple FE models and compared to actual test results.</div> <div class="htmlview paragraph">After each methodology was confirmed separately, the two new FE techniques were used in concert to predict automotive structural behavior with combined spot weld failure and sheet metal tearing present.</div>

Parallel computation methods for large-scale nonlinear CSM

Parallel computation methods for large-scale nonlinear CSM

<div class="htmlview paragraph">This paper presents a comprehensive damage model capable of predicting crash behavior of aluminum structures under varying applied loading conditions. The damage model has been implemented in a general purpose explicit nonlinear finite element code and crash analysis has been carried out for aluminum tubes. The response obtained from the finite element analysis shows a close agreement with the experimental data. The finite element program containing the proposed generalized damage model can be used to analyze aluminum structures subjected to complex service loading conditions and identify associated failure modes to assess crashworthiness.</div>