Zdeněk Majer

Determination of the Effect of Interphase on the Fracture Toughness and Stiffness of a Particulate Polymer Composite

The main objective of this paper is the numerical investigation of the fracture behavior of particulate polymer composites. The composite was modeled as a three-phase continuum consisting of a matrix, particles, and an interphase. Its mechanical response and the interaction between a microcrack propagating in the matrix and rigid particles covered with the interphase. Computations were performed with various material properties of the matrix and interphase by using the commercial finite-element code ANSYS. All calculations were based on the linear elastic fracture mechanics.

Crack Behaviour in Polymeric Composites: The Influence of Particle Shape

In this paper polymeric particulate composites are studied (especially polypropylene (PP) matrix stuffed by rigid mineral fillers). Presently, polymeric particulate composites are frequently used in many engineering applications. The composite was modeled as a three-phase continuum – matrix, interphase and particle. The properties of the particles (size, shape) have a significant effect on the global behaviour of the composite. On the basis of fracture mechanics methodology the interaction of micro-crack propagation in the matrix filled by rigid particles covered by the interphase was analyzed. The effect of the composite structure on their mechanical properties is studied here from the theoretical point of view.

Load Influence on the Behavior of Micro-Crack in the Particulate Composite

Particulate composite with soft polymer matrix and rigid mineral fillers are one of most frequently used construction and engineering materials. The main focus of a present paper is an estimation of the load influence on behavior of micro-crack placed in close proximity to the particle with interphase in soft matrix. The particulate composite with polymer matrix filled by magnesium-based mineral fillers is investigated by means of the finite element method. A non-linear material behavior of the matrix was considered. Numerical model on the base of representative plane element (RPE) was developed. The conclusions of this paper can contribute to a better understanding of the behavior of micro-crack in particulate composites with soft polymer matrix.

The Effect of an Interphase on Micro-Crack Behaviour in Polymer Composites

The present chapter is focused mainly on polypropylene based composites filled by mineral fillers, but some results can be generalized for other particle composites. The stiffness and toughness of the composite is modelled using a three-phase continuum consisting of the polymer matrix, mineral particles and an interphase between them. It is shown that the effect of the interphase on the macroscopic characteristics of the composite is decisive. Generally, the addition of the mineral filler to the polymer matrix leads to ebrittlement of the composite. The computational methodology presented quantifies the effect of the microstructure properties and morphology on the macroscopic material response. It is shown that properties of the interphase control both the stiffness and embrittlement of the particulate composite. Primarily, the interaction of micro-cracks with coated particles is studied. It is concluded that in some cases of the microscopic particles, size and specific interphase properties the addition of mineral fillers can lead to a good balance between fracture toughness and stiffness. Linear elastic fracture mechanics is used for calculations.

Pressure Pipe Damage: Numerical Estimation of Point Load Effect II

It has been shown that the service lifetime of buried pipes can be reduced especially in relation to stress concentration caused by external point loads. If the pipe is loaded locally the stress is concentrated there and a crack can initiate at this position or the existing crack can be affected by corresponding stress redistribution. In the paper, the effect of a hard indenter, material properties, and the hoop stress level on the stress intensity factor of the axial inner crack are evaluated. A simple relation for estimation of the stress intensity factor value without FEM simulations is proposed.

Stress Strain Analysis of High Porous Ceramics

The presented paper describes the creation of a computational model of highly porous materials, and a stress-strain analysis is performed. The computational model is created using micro-CT by the finite element method in the ANSYS 12.0 software. The micro-CT slices are converted into a 3D model using image processing. The local equivalent stress (HMH criterion) and struts deformation are analyzed. Commercially available ceramic foam, 85%Al2O3-14%SiO2- 1%MgO, was used in the experiment part of the paper.

Computational Analysis of Crack-Like Defects Influence on the Open Cell Ceramic Foam Tensile Strength

This work deals with a computational analysis and quantification of the influence of processing (primarily crack-like) defects of various amount on the (tensile) strength of open cell ceramic foam structures. This information is essential e.g. for application of these materials in the mechanically loaded application, where a design with certain reliability to operating conditions is required. The analysed ceramic foam structures are composed of both regular and irregular cells and crack-like defects (pre-cracked struts) are simulated inside them. The foam structure is modelled using a 3D FE beam element based model created by utilization of the Voronoi tessellation technique. The tensile strength upon presence of various amount of pre-cracked struts is analysed based upon an iterative FE simulation on whose base the critical failure force leading to specimen fracture is determined. The performed parametric study relates the tensile strength of the foam structure to the amount of initial defects. With increasing amount of these defects, the foam strength decreases by approximately 30% with every 10% of broken struts. This information can be directly used for a fast estimation of the foam tensile strength if the fraction of broken struts to the intact ones is known (e.g. from a microscopic analysis).

Optimization of Design Parameters of Fracture Resistant Piezoelectric Vibration Energy Harvester

This paper is focused on an analysis of a multilayer ceramic-based piezoelectric vibration energy harvester, which could be excited by ambient vibrations or external forces and thus provide a useful source of electricity for modern electronics. The proposed multilayer concept of the energy harvester enables introduction of tensile / compressive residual stresses inside particular layers. These stresses are intended to be used for enhancement of the harvester ́s fracture resistance and simultaneously for the improvement of the energy gain upon its operation. A crack arrest, by means of compressive residual stresses (in the outer “non-piezo” layer), will be utilized to this end. Primarily, the extended classical laminate theory (taking into account the piezoelectric characteristics of selected layers) will be used to define various designs of particular layers with various levels of residual stresses inside them. The weight function method is subsequently employed to select a design, which is most resistant to propagation of preexisting cracks. Selected laminate configurations are verified by means of FE simulations. Such analysis is essential for development of new energy harvesting systems formed of new smart materials and structures, which could be integrated in future development processes.

Fracture-Mechanics Behaviour of Ceramic Foam with Macroscopic Stress Concentrator upon the Tensile Test

The work investigates an influence of the macroscopic stress concentrator inside the ceramic open cell foam structure on the fracture-mechanics response of the foam upon the tensile test. As the concentrator, the central crack/rectangular notch was taken into account. The influence of the crack/notch length and width on the stress concentration ahead the concentrator tip was assessed using the simplified FE beam element based model with irregular cells simulating the real ceramic foam structure. Average principal stresses calculated on set of struts ahead the crack/notch tip were compared with average stresses in the intact structure. It was found that the ratio of these stresses increases linearly with the crack length. The stress concentration ratio is slightly lower in case of thick rectangular notch than in case of a thin crack. Furthermore, the failure load leading to complete fracture of the studied specimens, subjected to the tensile loading, were calculated using the same model. It is shown that the difference factor between the critical fracture force in case of structure without concentrator and with concentrator is very close to the concentration factor calculated from the average stresses on particular struts in the region in front of the concentrator tip.

Lifetime Assessment of Particulate Ceramic Composite with Residual Stresses

A micro-crack propagation in particulate ceramic based composite was studied using finite element method (FEM). Subcritical crack growth (SCG) was numerically simulated under complex load conditions (mechanical loading and loading by internal residual stresses). The effect of residual stresses on the crack propagation was studied. Two-dimensional computational model of particulate ceramic composite with material properties corresponding to low temperature co-fired ceramics (LTCC) was developed. The results indicate that the presence of residual stresses significantly reduces values of stress intensity factor in the vicinity of composite surface and the direction of residual stresses around the particles contributes to the micro-crack deflection from the particles. The time to failure of the composite under mechanical loading was determined. Results obtained contribute to a better understanding of the role of residual stresses during micro-crack propagation in ceramic particulate composites.