Michal Zouhar

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.

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.

Constraint effect on the slow crack growth in polyethylene

Purpose – From the practical point of view, most relevant damage to high density polyethylene (HDPE) structures is caused by slow crack growth. Therefore, detailed information about this type of damage is necessary. Experimental results transfer from specimens to real structure can be influenced by structure geometry (constraint). Therefore, the purpose of this paper is to investigate and discuss the effect of the constraint and relation between crack mouth opening displacement (CMOD) and crack length.Design/methodology/approach – The constraint effect is mainly effect of the structure geometry and can be quantified by T‐stress. Two different test specimens with different constraint level (T‐stress) were prepared: single edge notched specimen and modified single edge notch (SEN) specimen. The crack mouth opening displacement, crack tip opening displacement and crack length was measured.Findings – The main conclusions of this work can be summarized as: the slow crack growth rate in HDPE materials correspon...

Basic Modes of Crack Propagation through the Interface in a Polymer Layered Structure

The multi-layered structure components of dissimilar materials are used in many engineering applications to protect the base structure from outer damage. Typical examples of coated structures are pipes with the dimensional addition of protective layers (multi-layer pipes). The purpose of this development is to protect the main (functional) part of the pipes from damages (e.g. surface scratches, internal crack propagation). In the contribution the attention is paid to cracks existing and propagating in inner protective layer. In many cases the cracks are stopped at the interface between protective and functional layers. The important task is to decide if they penetrate further through the interface in the bulk material and thus cause the failure of the system. The critical stress for the crack propagation through the interface depends on the relation between bulk and coating materials as well as on the crack geometry. All these important topics are taken into account and their effect on crack propagation is broadly discussed.

Assessment of the Stability of a Surface Crack in Laminates

The critical configuration of a crack arrested at a bimaterial interface is estimated. The stability of the crack in the soft material, whose front is situated at the interface between soft and stiff materials and who grows continuously sideways, is analyzed. A laminate with an ABAB structure containing a surface crack is considered in this study. The effective value of the stress intensity factor for the central segment of the crack front lying at the material interface is calculated for various geometrical and material configurations. Comparing the effective value of the factor with the corresponding threshold values, the stability of the configuration studied can be assessed. It is found that a 3D solution for estimating the effective stress intensity factor is necessary to describe the crack behavior at the interface even for a relatively shallow crack. The results presented can be helpful for the inspection of lamellar structures.

Damage of Multilayer Polymer Materials under Creep Loading

Failure of layered materials is frequently caused by the existence of interfaces between single layers. In structures loaded by long-term constant stress the damage can be described on micro-scale by material changes due to active creep mechanism and at the macro-level by a corresponding change of the stress and strain field. In the contribution the behavior of a crack propagating through the interface is analyzed. Primary attention is devoted to a crack with its tip at the interface under creep exposition. The step change of material properties at the bi-material interface means that standard fracture mechanics access is not applicable and a modified approach based on generalization of the classical concept has to be used. The approach is illustrated on the damage of a plastic pipe with protective layer.