Numerical Study on Dimensions and Orientation Effect of Semi-Elliptical Cracks in PE100 Pipelines

The through-thickness crack or surface crack in PE100 pipes subjected to internal pressure represents a serious risk to the structural integrity of HDPE pipes, which has attracted wide attention in modern industry. Although experimental research offers reliable predictions of surface crack influence on pipes, the relatively high cost hinders its application. The numerical simulation, as a cost-effective alternative, has been widely applied to assess stress displacement and strain to the entire pipe structure. This is the initial approach adopted in recent decades. This article provides simulations tests of an uncracked pipe and cracked PE100 pipe under different internal pressure values, with varying each time the dimensions of the crack with 1 mm rate for minor and major radius and 0.5mm rates for the largest contour radius, using ANSYS MECHANICAL STRUCTURAL STATIC for simulation.

CRACK INFLUENCE ON A PIPE WITH DOUBLE SLOPE UNDER INTERNAL PRESSURE: NUMERICAL SIMULATION WITH XFEM

This work analyses the effect of elliptical cracks on a pipe with double slope thickness transition, using the extended finite element method (XFEM), level sets were defined to describe the three-dimensional (3D) cracks. The Computation of the stress intensity factors (SIFs) of cracks is performed. The values of SIFs are compared between straight pipes and pipes with a double (single) slope thickness transition. The results show that the XFEM is an effective tool for modelling cracks in pipes. A pressurized pipe with double slope thickness transition is more sensitive to the defect in comparison with another type of pipe. Parameters of the transition zone have an effect on stress intensity factors, precisely, the parameters of the first thickness transition are more influential on the gravity of the defect compared to the second thickness transition. ABSTRAK: Kajian ini menganalisa kesan retakan elips pada paip dengan peralihan ketebalan cerun berganda, menggunakan kaedah elemen terhingga dipanjangkan (XFEM), set tahap ditentukan bagi menentukan keretakan tiga dimensi (3D). Pengiraan faktor intensiti tekanan (SIF) retakan dilakukan. Nilai SIF dibandingkan antara paip lurus dan paip peralihan ketebalan cerun berganda (tunggal). Hasil kajian menunjukkan bahawa XFEM adalah alat yang berkesan bagi memodel keretakan paip. Paip bertekanan mengikut peralihan ketebalan cerun berganda, lebih sensitif terhadap kecacatan berbanding paip lain. Parameter zon peralihan mempunyai pengaruh terhadap faktor intensiti tegangan, tepatnya, parameter peralihan ketebalan pertama lebih mempengaruhi pada graviti kecacatan berbanding dengan peralihan ketebalan kedua.

Concentration of dynamic stresses in an elastic space with twoperiodic array of elliptical cracks

Normal incidence of the plane time-harmonic longitudinal wave on double-periodic array of coplanar elliptical cracks, which are located in 3D infinite elastic space is considered. Corresponding symmetric wave scattering problem is reduced to a boundary integral equation for the displacement jump across the crack surfaces in a unit cell by means of periodic Green’s function, which is presented in the form of Fourier integrals. A regularization technique for this Green’s function involving special lattice sums in closed forms is adopted, which allows its effective calculation in a wide range of wave numbers. The boundary integral equation is correctly solved by using the mapping method. The frequency dependencies of mode-I stress intensity factor in the vicinity of the crack front points for periodic distances in the system of elliptical cracks are revealed.

Definition of acceptable parameters of defects in basic details of forging and press machines

In the Soviet Union, a unique worldwide industry was created for the serial production of forging equipment. Currently, even these machines that have served their design life are in working or maintainable condition. This has led to the fact that there is now a large market for the secondary sale of such machines, including in the countries of Southeast Asia and India. However, with the secondary sale of such equipment, some inevitable damage builds up in the basic parts. In addition, the first copies of such models, put up for secondary sale, were produced at a time when non-destructive testing methods were not developed. Some defects could be undetected. When buying a machine dismantle, transport, install and test. Cases were recorded when defects were strained during installation and testing in the base parts, which made it difficult or impossible to use such equipment. Sometimes this type of equipment (during modernization) is used to carry out technological operations that require more power than previously declared. Therefore, when buying/selling (or upgrading), it is necessary to draw up cards of permissible defects, which, when superimposed on the cards of the detected defects, will allow to reject/determine the remaining life/limit the technological strength of the crank machine so that defects during further operation are not affectted and do not lead to emergency situation. The methodology for determining the permissible parameters of subsurface elliptical cracks is shown on the example of a press bed KD2130 (BZMP).

Retardation of Fatigue Crack Growth in Rotating Bending Specimens with Semi-Elliptical Cracks

This work investigates overload-induced retardation effects for semi-elliptically cracked steel round bars. The specimen geometry equals the shaft area of a 1:3 down-scaled railway axle and the material is extracted from railway axle blanks made of EA4T steel. Rotating bending tests under constant amplitude loading as well as overload tests considering overload ratios of ROL = 2.0 and ROL = 2.5 are conducted. The experimental results are compared to a crack growth assessment based on a modified NASGRO equation as well as the retardation model by Willenborg, Gallagher, and Hughes. The evaluated delay cycle number due to the overload by the experiments and the model shows a sound agreement validating the applicability of the presented approach.