An FFT-based spectral solver for interface decohesion modelling using a gradient damage approach

This work presents a fast Fourier transform (FFT) based method that can be used to model interface decohesion. The inability of an FFT solver to deal with sharp interfaces discards the use of conventional cohesive zones to model the interfacial mechanical behaviour within this framework. This limitation is overcome by approximating sharp interfaces (e.g. grain/phase boundaries) with an interphase. Within the interphase, the background plastic constitutive behaviour is inherited from the respective adjacent grains. The anisotropic kinematics of the decohesion process is modelled using a damage deformation gradient that is constructed by mapping the opening strains (in normal and tangential modes) to the associated projection tensors. The degradation (damage) of the interfacial opening resistances is modelled via a dimensionless nonlocal damage variable that prevents localisation of damage within the interphase. This nonlocal variable results from the solution of a gradient damage based regularisation equation within the interphase subdomain. The damage field is constrained to the interphase region by applying a relatively large penalisation on the damage gradients inside the interphase. The extent of nonlocality ensures that the damage is largely uniform in the direction perpendicular to the interphase, thus making its thickness the theoretical lengthscale for dissipation. To achieve model predictions that are objective with respect to the interphase thickness, scaling relations of the model parameters are proposed. The numerical performance is shown for a uniform opening case and then for a propagating crack. Finally, an application to an artificial polycrystal is shown.

Delamination resistance of composite laminated structures reinforced with angled, threaded, and anchored Z-pins

The delamination resistance of Z-pinned laminates is directly dependent on the strength of the pin–laminate bonding at the interface. In this paper, we investigate novel approaches to the Z-pinning technology in order to increase delamination strength via enhancing mechanical interlocking of the pins. Toward this end, we study the effect of pin insertion at an angle to the vertical in contrast to the conventional vertical pin insertion. Subsequently, a novel variety of pin, namely the threaded pin, is studied as a candidate for reinforcement which increases mechanical interlocking between the pin and the laminate as well as the epoxy-pin contact area, thus delaying delamination. In addition, the effect of anchoring reveals the length of smooth metal pins on to the surface of the laminate before curing on delamination strength is investigated. Experiments performed show increase in tensile pullout strengths when angled, threaded, or anchored pins are used. These experimental results for tensile pullout strengths validate nonlinear finite element models incorporating cohesive zones at the pin–laminate interface. In addition, fracture toughness and delamination resistance under mode-I loading are investigated by performing experiments on double cantilever beam specimens. Results demonstrate the superior delamination resistance properties for angled, threaded, and anchored pin inserts.

2-Scale Hierarchical Multiscale Modeling of Piezoresistive and Damage Response in Polymer Nanocomposite Bonded Explosive

This paper builds on previous work done [1, 2] to explore the effective piezoresistive response of polymer bonded explosive (PBX) materials where the polymer medium is reinforced with carbon nanotubes (CNTs). In the present work, the nanocomposite binder is modeled explicitly as a piezoresistive material whose properties are determined from the nanoscale through a micromechanics based 2-scale hierarchical model connecting the nanoscale to the microscale grain structure. Electromechanical cohesive zones are used to model the interface between the grains and nanocomposite binder in order to characterize interface separation and the resulting piezoresistive effect. The overall microscale piezoresistive effect is measured by using the volume averaged properties of the microscale RVE. The hierarchical framework developed here is used to explore key features of the NCBX microstructure such as the effect of grain conductivity, weight percentage of CNTs used and nanocomposite gage factor.

Micromechanical Analyses of Debonding and Matrix Cracking in Dual-Phase Materials

Failure in elastic dual-phase materials under transverse tension is studied numerically. Cohesive zones represent failure along the interface and the augmented finite element method (A-FEM) is used for matrix cracking. Matrix cracks are formed at an angle of 55 deg−60 deg relative to the loading direction, which is in good agreement with experiments. Matrix cracks initiate at the tip of the debond, and for equi-biaxial loading cracks are formed at both tips. For elliptical reinforcement the matrix cracks initiate at the narrow end of the ellipse. The load carrying capacity is highest for ligaments in the loading direction greater than that of the transverse direction.

2-Scale Hierarchical Multiscale Modeling of Piezoresistive Response in Polymer Nanocomposite Bonded Explosives

The current work aims to explore the effective piezoresistive response of polymer bonded explosive (PBX) materials where the polymer medium is reinforced with carbon nanotubes (CNTs). The effective piezoresistive response of these nanocomposite bound polymer explosives (NCBX) is evaluated using micromechanics based 2-scale hierarchical model connecting the CNT-polymer nanocomposite scale (nanoscale) to the explosive grain structure scale (microscale). The binding nanocomposite medium is modeled as electromechanical cohesive zones between adjacent explosive grains which are representative of effective electromechanical response of CNT-polymer nanocomposites. The hierarchical framework developed here is used to explore key features of the NCBX microstructure, e.g. ratio of grain to nanocomposite stiffness, ratio of grain to nanocomposite conductivities etc., and related to the NCBX effective piezoresistive response. The results obtained from the current work show dependence of effective NCBX piezoresistive properties on each of these microstructural features with and without interfacial damage between the explosive grains.