Z. Knésl

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...

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.

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...

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.

Calculation of the Critical Stress in two-Phase Materials

Publisher Summary Concrete is considered as a two-phase material and the problem of cracking of coarse aggregate in cement paste is analyzed in this chapter. The boundary between the fractured matrix and the aggregate particle to be fractured is considered as an interface between two different materials and characterized by the corresponding elastic mismatch of both materials. A fracture model that deals with the influence of aggregate particle dimension and elastic mismatch between matrix and aggregate is suggested and discussed in the chapter. The crack is postulated to grow and to fracture the particle if the stress averaged over the particle exceeds its critical value. The procedure has been applied to concrete, where the matrix consists of cement paste and the particles correspond to fine and coarse aggregates. Calculations are performed by a combination of analytical and numerical approaches. A parametric study, showing the influence of particles dimension and elastic mismatch between the paste and aggregate is also discussed.