Luboš Náhlík

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.

Life Time Estimation of the Multilayer Plastic Pipes

In the contribution solution of special fracture mechanics problems connected with multi-layer plastic pipes was investigated. The assumptions of linear elastic fracture mechanics were accepted. A complex three-dimensional numerical model of multi-layer pipe system consisting of main (functional) pipe and protective layers has been suggested and numerically solved by finite element method. Two basic problems connected with lifetime expectation of multilayer pipe system have been considered and discussed, namely: question of fracture mechanics description of multilayered pipe system and corresponding measurements of the material properties. The suggested approach can help for more accurate estimation of the multilayer pipe damage.

The Effect of the Singularity Induced by the Free Surface on Fatigue Crack Growth in Thin Structures

In many industrial applications is necessary to predict fatigue lifetime of thin structures, where the stress field near the crack front have a real three-dimensional nature. Due to the existence of vertex singularity in the point where the crack front touching free surface, crack propagation in 3D structures cannot be reduced to a series of plane strain or plane stress problems along the crack front edge. The paper describes the influence of vertex singularity on the distribution of the stresses around the crack front for three-dimensional body. The distribution of the stress singularity through the thickness of the specimen gives us indication of the crack behavior in thin structures. The estimation of the thickness of the specimen where the change of singularity plays an important role on fatigue crack growth rate (in dependence on Poisson’s ratio) is carried out. The results contribute to a better understanding of the crack behavior in thin structures, and can help to more reliable estimates of their residual fatigue life.

Fatigue Crack Propagation Rate in EUROFER 97 Estimated Using Small Specimens

The proposed paper describes fatigue damage evolution in Eurofer 97 reduced activation ferritic-martensitic steel. The short crack growth study was performed on small cylindrical specimens using an MTS 880 servohydraulic machine at constant strain amplitude. Based on the fatigue crack growth data obtained and corresponding 3D finite element analysis Paris law region of the fatigue curve were estimated. The results obtained were compared with standard determination of the fatigue crack growth rate according to ASTM using CT specimens. The presented results can help to transfer experimental data measured on small specimens to large structures and vice versa.

Numerical lifetime prediction of polymer pipes taking into account residual stress

In this paper a methodology for assessment of residual stress effects on crack behaviour in the polymer pipe is developed. For simplicitys sake, a linear distribution of residual stresses across the pipe wall is assumed. Linear elastic fracture mechanics is used for the fracture mechanics analysis of the cracked pipe. An approximate relation for the stress intensity factor estimation for a crack in a polymer pipe, with residual stress taken into account is suggested and discussed. The methodology presented can be helpful for a rapid lifetime estimation of polyolefin pipelines.

Crack propagation in a welded polyolefin pipe

Purpose – The purpose of this paper is to study the effect of the material inhomogeneity on crack behavior initiated both axially and circumferentially in or near the butt weld and to discuss consequences on residual lifetime of the welded structure.Design/methodology/approach – A three‐dimensional numerical model of pipe weld with smooth and continuous change of material properties has been used to study the fracture behavior of the cracked pipe structure. The stress intensity factor was considered as a parameter controlling the fracture behavior. The semi‐elliptical shape of the crack front was estimated under assumption of constant stress intensity factor along the crack front.Findings – According to the results obtained in the paper the following conclusions were deduced. First, the most critical location of the crack is in the middle of the inhomogeneous region (weld center) regardless of the crack orientation. The stress intensity factor is substantially higher than in the case of a crack located in...

Influence of Initial Inclined Surface Crack on Estimated Residual Fatigue Lifetime of Railway Axle

Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect on railway axle surface is often required. The contribution deals with an estimation of residual fatigue lifetime of railway axle with initial inclined surface crack. 3D numerical model of inclined semi-elliptical surface crack in railway axle was developed and its curved propagation through the axle was simulated by finite element method. Presence of press-fitted wheel in the vicinity of initial crack was taken into account. A typical loading spectrum of railway axle was considered and residual fatigue lifetime was estimated by NASGRO approach. Material properties of typical axle steel EA4T were considered in numerical calculations and lifetime estimation.

Influence of Different Crack Propagation Rate Descriptions on the Residual Fatigue Lifetime of Railway Axles

The paper deals with an estimation of the residual fatigue lifetime of the railway axles. The railway axles can include some cracks either from manufacturing process or from previous loading operation. Because of cyclic loading of the railway axles there is a risk of fatigue failure of the railway axles with unacceptable consequences. Based on this fact, for conservative establishment of the residual fatigue lifetime of the railway axle is necessary to consider an existing crack in the railway axle during design process. The fatigue lifetime estimation of railway axles is very sensitive to used crack propagation rate description (e.g. v-K curve). Typical bending of this curve (knee) can be found in the vicinity of the threshold value in fatigue crack propagation rate dependence (typically v-K curve expressed in log-log coordinates). For accurate estimation of residual fatigue lifetime of the railway axle is necessary to use approximation of v-K curve that takes into account existence of the knee close to the threshold value of the stress intensity factor. The paper shows important differences between different crack propagation rate descriptions on the residual fatigue lifetime estimation of the railway axles. Results obtained can be used for safer design and operation of the railway axles.

Influence of Crack Retardation on Fatigue Crack Propagation in Steels for Railway Axles

The railway axles are subjected to cyclic loading, therefore there is a risk of fatigue failure. For reason that possible crack could not be detected by non-destructive testing method an existing crack in the railway axle must be considered. This is conservative approach commonly used in applications where potential fatigue failure has unaccepted consequences. This paper deals with retardation effect caused by overload cycles and compares results obtained by no retardation approach and results obtained by generalized Willenborg model, which takes into account the retardation effects due to plastic zone around the crack tip. Results obtained can contribute to the better understanding of fatigue crack behavior in railway axles.

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.