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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.

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.

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 homogenous pipe. Second, with regard to crack shapes, the circumferentially oriented cracks are practically identical regardless to the crack location if compared with the axial cracks. Third, the stress intensity factors of axially‐oriented cracks are approximately twice higher than in the case of circumferential cracks. This implies that the cracks are more likely to grow in an axial direction.

The results described in the paper can be used for estimation of critical crack length or for estimation of the critical applied inner pressure of medium transported in the pipe and are of paramount importance for service life estimations of polymer welded pipes in actual use.

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.

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.

The main conclusions of this work can be summarized as: the slow crack growth rate in HDPE materials corresponds to velocity of CMOD; the influence of the presented specimen geometry on slow crack growth rate can be considered as negligible; and for transfer of the experimental results from specimens to real structure the influence of the structure geometry (constraint) is not critical.

Experimental results obtained from different specimens with different constraint level are rare and can lead to better data transfer from experimental specimens to the real structures.

The main aim of this study is to determine the fracture toughness and accordingly to predict the fracture initiation, crack propagation and mode of crack extension accurately in polypropylene subsea pipes subjected to internal pressure.

Tensile test was performed following the ISO 527–1 standard. An elastic-plastic constitutive model was developed based on the tensile test results, and it is implemented in the FEA model to describe the constitutive behaviour of the polypropylene material. Three-point bend tests with linear-elastic fracture mechanics (LEFM) approach were conducted following ISO-13586 standard, from which the average fracture toughness of the polypropylene pipe material in crack-opening mode was found as KIc = 3.3 MPa√m. A numerical model of the experiments is developed based on the extended finite element method (XFEM), which showed markedly good agreement with the experimental results.

The validated XFEM modelling approach is utilised to illustrate its capabilities in predicting fracture initiation and crack propagation in a polypropylene subsea pipe subjected to an internal pressure containing a semi-elliptical surface crack, which agrees well with existing analytical solutions. The XFEM model is capable of predicting the crack initiation and propagation in the polypropylene pipe up to the event of leakage.

The methodology proposed herein can be utilised to assess the structural integrity and resistance to fracture of subsea plastic pipes subjected to operational loads (e.g. internal pressure).

The service environment of urban polyethylene (PE) pipes has a crucial influence on their long-term safety and performance. Based on the application and structural performance analysis of PE pipe failure cases, this study aims to investigate the impact of organic substances in the soil on the aging behavior of PE pipes by designing organic solutions with different concentrations, which are based on the composition of organic substances in the soil environment, and periodic immersion tests.

Soil samples in the vicinity of the failed pipes were analyzed by gas chromatography-mass spectrometry, sensitive organic substances were screened and soaking solutions of different concentrations were designed. After the soaking test, the PE pipe samples were analyzed using differential scanning calorimetry, Fourier-transform infrared spectroscopy and other testing methods.

The performance difference between the outer surface and the middle of the cross section of PE pipes highlights the influence of the soil service environment on their aging. Different organic solutions can have varying impacts on the aging behavior of PE pipes when immersed. For instance, when exposed to amine organic solutions, PE pipes may have an increased weight and decreased material yield strength, although there is no reduction in their thermal or oxygen stability. On the contrary, when subjected to ether organic solutions, the surface of PE pipe specimens may be affected, leading to a reduction in material fracture elongation and a decrease in their thermal and oxygen stability. Furthermore, immersion in either amine or ether organic solutions may result in the production of hydroxyl and other aging groups on the surface of the material.

Understanding the potential impact of organic substances in the soil environment on the aging of PE pipe ensures the long-term performance and safety of urban PE pipe. This research approach will provide valuable insights into improving the durability and reliability of urban PE pipes in soil environments.