Z. Azari

Burst Test and J-Integral Crack Growth Criterion in High Density Poly-Ethylene Pipe Subjected to Internal Pressure

In the present work we are interested on the analysis of the severity of crack defects created by a disc cutter and to study the behavior of a high density polyethylene pipe (HDPE pipe) when subjected to an internal pressure, either in the absence or presence of a pre-crack. In order to do this, experimental tests was performed to measure the toughness and to determine the mechanical behavior of HDPE. These features were used to perform numerical simulations using ABAQUS on pipe solicited by an increase in internal pressure. This allows to compare with burst tests of cracked pipes and to determine the fracture energy that will be compared to the toughness.

HDPE Pipe Failure Analysis Under Overpressure in Presence of Defect

Pipe failure and leaks are frequent phenomena in urban areas. In order to minimize the risk of long-term leakage, nearly 60% of drinking water systems are renewed with third-generation polyethylene pipes, PE100. Due to its characteristics, it is a material of choice for water supply networks. However, the presence of a defect can lead the pipe failure under the effect of transient flow. In order to examine this problem, the cracking behavior of PE100 pipes with a defect has been studied. Using burst tests and finite element modeling, we have demonstrated that the concept of elastic-plastic fracture mechanics, the J-integral, can define with acceptable precision the crack initiation and the failure behavior of PE100 pipe. The J-integral value at the time of appearance of the damage zone, plastic strain-hardening, is considered numerically to be the value of the toughness. This allowed us to define the pressure leading to the crack initiation. The results are compared with the experimental burst pressures.

Collapse Analysis of Longitudinally Cracked HDPE Pipes

High-density polyethylene (HDPE) is one of the most widely used materials in fluid transport networks due to its good resistance to wear and corrosion, ease of installation, and low cost. However, HDPE is a flexible material and therefore more vulnerable to scratches and other types of damage during transport and installation. Therefore, accurate prediction of crack initiation pressure in damaged pipes is a very important point in the safety analysis of HDPE piping systems. In this study, a new semi-empirical formulae, which predicts this critical pressure, is developed. The cracking pressure depends on the mechanical characteristics of the material and the geometric parameters (pipe geometry and defect size). A parametric study based on numerical simulations was established in order to quantify the influence of each parameter on the cracking pressure. The pressures calculated by the proposed formulae in a HDPE pipe having a superficial defect are in good agreement with the burst pressure determined experimentally for the same geometry.

Effect of Water Hammer on Bent Pipes in the Absence or Presence of a Pre-crack

In this study, transient pressure in piped liquid due to waterhammer is a function of structural restraint at elbows. To study the effect of this phenomenon on polyethylene elbowed pipe networks, experimental tests were performed on polyethylene bent pipes, either in the absence or presence of a pre-crack by measuring the toughness and determining the mechanical behavior of HDPE. These characteristics were been used to perform simulations using ABAQUS software, the internal pressure increase, to investigate the safety of bent pipes using an angle of 90 ° bend with 11.4 mm thickness and a right portion of 150 mm as length.

Effect of Water Hammer in Relatively Long Inclined Pipelines

Consideration is given in this paper to the numerical solution of transient flows in relatively long pipeline provoked by the water hammer phenomenon. Equations which describe these one-dimensional flows in a cylindrical pipe, of linear elastic behavior according to the Hooke’s law, are solved in the time domain by the method of characteristics using linear integration. The obtained results due to a rapid closure valve at the downstream end of a long pipeline show that the gravity lift may have an important effect on the maximum pressures, which may become very important near the valve and provoke the failure of the pipe especially in presence of defect. The safety factor, computed at equidistant sections of the pipe, determines the distance between the supplying reservoir and the defect from which the failure may happen.

Statistical Study of Temperature Effect on Fatigue Life of Thin Welded Plates

Martensitic stainless steel sheets with 12% Cr are used as protective envelopes for hot and vibrating structures such as aircraft engines and gas turbines. Since the envelope shape may be complex, butt welding is chosen to assemble the parts which can undergo fatigue failure especially in the heat-affected zone (HAZ). The aim of this study is to present experimental fatigue life results showing the effect of temperature and to present a reliable statistical approach in order to correctly describe a given loading level, in relation to its fatigue life, which has not yet been tested using the only available experimental data. This situation is dictated by optimizing the costly and lengthy fatigue experiments for modeling. Normal and Weibull statistical models are used to predict fatigue lifetime based on S-N curves in welded and seamless plates. Both models provide comparable results with experimental data at 293 K and 690 K for failure probability limits of 5% and 95%. Based on Weibull model, untested levels are assessed using a cumulative fatigue life function. Two new dimensionless parameters (X: square stress ratio and H: thermal life ratio) are used to calculate, for a given stress level, the corresponding fatigue life in accordance to the sought lower and upper confidence limits. The proposed modified Weibull approach delivers results within less than 5% error.