Matrix Interface Cracking Research Articles

Modeling of progressive debonding of interphase- matrix interface cracks in particle reinforced composites using VCFEM

The Voronoi cell finite element model (VCFEM) containing three phases of inclusion, interphase and matrix in an element has been proven to be very accurate and efficient in the analysis of inclusion reinforced composites. In this paper, we extended our previous study to include the matrix-interphase interface debonding problems. The traction-free at the debonded matrix-interphase interface and the traction reciprocity at the bonded matrix-interphase interface were introduced into the functional by displacement as Lagrange multiplier. A modified complementary energy functional was constructed. When the matrix-interphase interface was debonded according to critical normal stress law, additional nodes are added as node pairs with existing nodes whose stress exceeds the critical value. This process was interated until the stress of all nodes did not exceed the critical value. Thus, the mesh re-division technology was realized for the interface debonding progressively. The proposed VCFEM is validated by results from MARC. The results show that the proposed VCFEM was more efficient. Based on the developed model, the influence of interphase properties on stress-strain cure was studied during the debonding process. This showed that the interphase modulus and thickness had great influence on the overall mechanical behavior of composites.

Modelling interfacial debonds in unidirectional fibre-reinforced composites under biaxial transverse loads

A numerical study of the onset and growth of debonds at fibre–matrix interfaces, for a fibre bundle under far field biaxial transverse loads, is presented. The behaviour of the fibre–matrix interface cracks was studied by means of a Linear Elastic–Brittle Interface Model (LEBIM). The simplified, but representative, model of an actual unidirectional lamina considered included ten fibres embedded in a matrix cell whose external dimensions were much larger than the fibre radius. The results presented extend the results obtained by the authors in previous studies using a single fibre model. The aim was, firstly to predict the failure loads (critical loads) originating the first debond onset in a small bundle of fibres and, secondly, to verify the unstable character of subsequent debond growth under transverse loads. The failure curves obtained in this simplified problem help to elucidate some aspects of the failure mechanisms of an actual composite material subjected to these kinds of loads.

Micro-mechanical analysis of the effect of ply thickness on the transverse compressive strength of polymer composites

A micro-mechanical model is used to study the effect of ply thickness on constrained 90° plies subjected to transverse compressive loading (in situ effect). For cross-ply sublaminates with conventional, standard-thickness 90° plies, failure is dominated by fibre–matrix interface cracking and large localised plastic deformation of the matrix, forming a localised band in a plane that is not aligned with the loading direction. Ultra-thin plies show a dispersed damage mechanism, combining wedge cracking with ply fragmentation/separation. Moreover, a transverse crack suppression effect is clearly observed. To the authors’ knowledge, it is the first time an in situ effect in transverse compression has been identified. When comparing the results of the micro-mechanical model with the predictions from analytical models for the in situ effect, the same trends are obtained. These results also show that, for realistic ply thicknesses, these analytical models can be considered fairly accurate.

Influence of dwell times on the thermomechanical fatigue behavior of a directionally solidified Ni-base superalloy

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests with T=100–750°C and optional dwells of 20min at 750°C were carried out on directionally solidified Ni-base Alloy 247 LC DS. Introducing dwells reduced the lifetime for both phase angles to about one sixth. Specific damage mechanisms were internal carbide and carbide–matrix interface cracks in IP tests and crack propagation along {111}-microtwin planes in OP tests. Introducing dwells intensified both effects, thus contributing to the lifetime reduction. During dwells, the gauge length may exhibit transversal creep because of extensometer forces distorting the strain measurement.

Numerical characterisation of the fibre–matrix interface crack growth in composites under transverse compression

Inter-fibre failure under compression transverse to the fibres is studied at micromechanical level. Interfacial fracture mechanics concepts, associated to both the open model and the contact model, are applied. A numerical study is performed using the boundary element method aimed at explaining the origin and evolution of the damage at micromechanical level, considered as fibre–matrix interface cracks. Assuming that the damage starts as small debonds originated by shear stresses at the position where their maximum values are reached, it has been found that the crack shows different morphologies at both tips: an open one and a closed one with a large contact zone. Then the interface crack grows unstably in mixed mode only on the open tip side until this growth changes to stable, once the crack closes at this tip, with the generation of a contact zone.

Damaging micromechanisms in ferritic–pearlitic ductile cast irons

Microstructure influences on damaging mechanisms in ferritic–pearlitic ductile cast irons (DCIs) were investigated. Four different ferrite/pearlite volume fractions were considered, and in situ scanning electron microscope observations were performed during step by step tensile tests in a microtensile holder. SEM observations allowed to identify an evident microstructure influence on ferritic–pearlitic DCIs damaging mechanisms and the key role played by graphite constituents. Phase volume fractions and distribution affect both crack initiation and crack propagation. The importance of the damage mechanism based on graphite nodules–matrix interface cracking, usually indicated as the most relevant, is only partially confirmed, depending on the DCI microstructure.

Mechanical behaviour of graphite in fracture of austempered ductile iron

An investigation was carried out to examine the mechanical behaviour of graphite in the fracture of austempered ductile iron (ADI) by in situ tensile testing with an SEM. The results indicate that the graphite in ADI cannot be regarded as voids with no strength because graphite–matrix (G–M) interface cracking and the internal fracture of graphite were observed. Under tensile testing, microcracks always initiated at and propagated along the G–M interface first. Graphite nodules do not cause micronotch stress concentration thermselves in advance of the G–M interface cracking. The propagation of interfacial cracks along the G–M interface resulted in crack deflection. When the main crack propagated to a graphite nodule whose G–M interface had cracked and formed a void, it was obviously blunted. Graphite–matrix interface cracking occurred ahead of a propagating crack. The G–M interfaces and graphite nodules have a certain strength.