Maher Y. A. Younan

3-D Finite Element Modeling of the Welding Process Using Element Birth and Element Movement Techniques

The modeling and simulation of the welding process has been of main concern for different fields of applications. Most of the modeling of such a problem has been mainly in 2-D forms that may also include many sorts of approximation and assumptions. This is due to limitations in the computational facilities as the analysis of 3-D problems consumes a lot of time. With the evolution of new finite element tools and fast computer systems, the analysis of such problems is becoming in hand. In this research, a simulation of the welding process with and without metal deposition is developed. A new technique for metal deposition using element movement is introduced. It helps in performing full 3-D analysis in a shorter time than other previously developed techniques such as the element birth.

Limit-load analysis of pipe bends under out-of-plane moment loading and internal pressure

The purpose of this work is to study the load-carrying capacity of pipe bends, with different pipe bend factor (h) values, under out-of-plane moment loading; and to investigate the effect of internal pressure on the limit moments in this loading mode. The finite element method is used to model and analyze a standalone, long-radius pipe bend with a 16-in, nominal diameter; and a 24-in, bend radius. A parametric study is performed in which the bend factor takes ten different values between 0.0632 and 0.4417 Internal pressure is incremented by 100 psi for each model, until the limit pressure of the model is reached. The limit moments were found to increase when the internal pressure is incremented. However, beyond a certain value of pressure, the effect of pressure is reversed due to the additional stresses it engenders. Expectedly, increasing the bend factor leads to an increase in the value of the limit loads. The results are compared to those, available in the literature, of a similar analysis that treats the in-plane loading mode. Pipe bends are found to have the lowest load-carrying capacity when loaded in their own plane, in the closing direction. They can sustain slightly higher loads when loaded in the out-of-plane direction, and considerably higher loads under in-plane bending in the opening direction.

Determination of Shakedown Limit Load for a 90-Degree Pipe Bend Using a Simplified Technique

In this paper a simplified technique is presented to determine the shakedown limit load of a 90-degree pipe bend subjected to constant internal pressure and cyclic in-plane closing bending moment using the finite element method. The simplified technique determines the shakedown limit load without performing time consuming full elastic-plastic cyclic loading simulations or conventional iterative elastic techniques. Instead, the shakedown limit load is determined by performing two finite element analyses namely; an elastic analysis and an elastic-plastic analysis. By extracting the results of the two analyses, the shakedown limit load is determined through the calculation of the residual stresses developed in the pipe bend. In order to gain confidence in the simplified technique, the output shakedown limit moments are used to perform full elastic-plastic cyclic loading simulations to check for shakedown behavior of the pipe bend. The shakedown limit moments output by the simplified technique are used to generate the shakedown diagram of the pipe bend for a range of constant internal pressure magnitudes. The maximum moment carrying capacity (limit moment) the pipe bend can withstand and the elastic limit are also determined and imposed on the shakedown diagram of the pipe bend. In order to get acquainted with the simplified technique, it is applied beforehand to a bench mark shakedown problem namely, the Bree cylinder (Bree, J., 1967, J. Strain Anal., 3, pp. 226-238) problem. The Bree cylinder is subjected to constant internal pressure and cyclic high heat fluxes across its wall. The results of the simplified technique showed very good correlation with the analytically determined Bree diagram of the cylinder.

Nonlinear Analysis of Pipe Bends Subjected to Out-of-Plane Moment Loading and Internal Pressure

The behavior of a pipe bend, with bend factor h=0.1615 (D=16 in., R=24 in, and t =0.41 in. ), subjected to out-of-plane bending and internal pressure is studied, taking geometric and material nonlinearity into account, using the finite element code ABAQUS. Material behavior is taken as elastic-perfectly plastic. The distribution of stress and strain along the axial direction and across the thickness of the bend is studied, with and without internal pressure, at the onset of yielding and at instability. Before instability is reached, through-the-thickness yielding appears at many points. The loaded end of the bend is found to be the most severely strained cross section. The circumferential distribution of stress and strain, and its variation with increased moment loading are then investigated for that section, at internal pressure values of zero and 1200 psi.

Nonlinear analysis and plastic deformation of pipe elbows subjected to in-plane bending

The purpose of this study is to investigate the large strain and stress analysis for pipe elbows subjected to in-plane bending moments. A finite element model for the bend was constructed and loaded taking geometric and material nonlinearities into account using (ABAQUS) nonlinear finite element code. The initiation of yielding for the opening and closing cases appears at the inside surface of the elbow crown. However, further loading causes a significant difference in strain distribution and deformed shapes. The limit moment for the opening cases is higher than that for closing due to the geometric stiffening effects.

The Effect of Modeling Parameters on the Predicted Limit Loads for Pipe Bends Subjected to Out-of-Plane Moment Loading and Internal Pressure

The purpose of this study is to investigate the effect of modeling parameters on the determination of limit loads for standalone pipe bends, subjected to an out-of-plane end moment and internal pressure. A pipe bend, with bend factor h =0.1615, is modeled and analyzed using the nonlinear finite element code ABAQUS. Small and large-displacement analyses are performed with elastic-perfectly plastic and strain-hardening material models. Small-displacement analyses fail to predict the stiffening effect of pressure and give a continuously decaying limit load with increased pressure. Material strain hardening gives a higher limit load than perfectly plastic materials. In the large-displacement analysis with a strain-hardening material, the limit moment levels off as the pressure increases, and does not decrease as in the case of a perfectly plastic material.

Comparison of Pipe Bend Ratchetting/Shakedown Test Results With the Shakedown Boundary Determined via a Simplified Technique

Scarce experimental verification exits in the open literature concerning determination of the shakedown boundary for pipe bends subjected to steady internal pressure and cyclic bending loading. The objective of the present paper is to test the capability of a simplified technique presented by the authors in recent ASME JPVT publications and PVP conferences [1–4] in adequately predicting the shakedown boundary obtained through experimental testing. Recently, Chen et al. [5] published experimental and finite element (FE) simulation results on ratchetting of low-carbon steel pressurized 90-degree pipe bend specimens subjected to cyclic reversed in-plane bending forces. Chen et al. [5] performed experimental testing on a pipe bend specimen subjected to a steady internal pressure magnitude of 20.0 MPa. Through FE simulations employing a modified form of the Ohno-Wang non-linear kinematic hardening (KH) rule, Chen et al. [5] predicted a shakedown boundary for a steady internal pressure spectrum ranging from 10.0 to 25.0 MPa. Chen et al. [5] experimental and FE outcomes are utilized for comparison with the simplified technique outcomes. The simplified technique outcomes showed very good correlation with Chen et al. [5] shakedown boundary predictions for the 18.0 – 25.0 MPa steady internal pressure spectrum. On the contrary, noticeable disagreement was found for the lower magnitudes of steady internal pressure. Reasons behind the discrepancy are discussed.Copyright

Shakedown Limits of a 90-Degree Pipe Bend Using Small and Large Displacement Formulations

In this paper the shakedown limit load is determined for a long radius 90-deg pipe bend using two different techniques. The first technique is a simplified technique which utilizes small displacement formulation and elastic-perfectly plastic material model. The second technique is an iterative based technique which uses the same elastic-perfectly plastic material model, but incorporates large displacement effects accounting for geometric nonlinearity. Both techniques use the finite element method for analysis. The pipe bend is subjected to constant internal pressure magnitudes and cyclic bending moments. The cyclic bending loading includes three different loading patterns, namely, in-plane closing, in-plane opening, and out-of-plane bending. The simplified technique determines the shakedown limit load (moment) without the need to perform full cyclic loading simulations or conventional iterative elastic techniques. Instead, the shakedown limit moment is determined by performing two analyses, namely, an elastic analysis and an elastic-plastic analysis. By extracting the results of the two analyses, the shakedown limit moment is determined through the calculation of the residual stresses developed in the pipe bend. The iterative large displacement technique determines the shakedown limit moment in an iterative manner by performing a series of full elastic-plastic cyclic loading simulations. The shakedown limit moment output by the simplified technique (small displacement) is used by the iterative large displacement technique as an initial iterative value. The iterations proceed until an applied moment guarantees a structure developed residual stress, at load removal, equal to or slightly less than the material yield strength. The shakedown limit moments output by both techniques are used to generate shakedown diagrams of the pipe bend for a spectrum of constant internal pressure magnitudes for the three loading patterns stated earlier. The maximum moment carrying capacity (limit moment) the pipe bend can withstand and the elastic limit are also determined and imposed on the shakedown diagram of the pipe bend. Comparison between the shakedown diagrams generated by the two techniques, for the three loading patterns, is presented.

Study of the Effect of Boundary Conditions on Residual Stresses in Welding Using Element Birth and Element Movement Techniques

The structure in which the welding process is performed highly affects the residual stresses generated in the welding. This effect is simulated by choosing the appropriate boundary conditions in modeling the welding process. The major parameters of the boundary conditions are the method by which the base metal is being fixed and the amount of heat being applied through the torch. Other parameters may include the coefficients of thermal heat loss from the plate which may simulate the media in which the welding is taking place. In modeling the welding process, two-dimensional forms of approximation were developed in analyzing most of the models of such problem. Three-dimensional models analyzing the welding process were developed in limited applications due to its high computation time and cost. With the development of new finite element tools, namely the element movement technique developed by the authors, full three-dimensional analysis of the welding process is becoming in hand. In the present work, three different boundary conditions shall be modeled comparing their effect on the welding. These boundary conditions shall be applied to two models of the welding process: one using the element birth technique and the other using the element movement technique showing the similarity in their responses verifying the effectiveness of the latter being accomplished in a shorter time.

DETERMINATION OF SHAKEDOWN LIMIT LOADS FOR A CYLINDRICAL VESSEL-NOZZLE INTERSECTION VIA A SIMPLIFIED TECHNIQUE

In the current research, the shakedown limit loads for a cylindrical vessel–nozzle intersection is determined via a simplified technique. The cylindrical vessel–nozzle intersection is subjected to a spectrum of steady internal pressure magnitudes and cyclic in–plane bending moments on the nozzle. The determined shakedown limit loads are utilized to generate the Bree diagram of the cylindrical vessel–nozzle intersection. In addition, the maximum moment carrying capacity (limit moments) and the elastic limit loads are determined and imposed on the Bree diagram of the structure. The simplified technique outcomes showed excellent correlation with the results of full elastic–plastic cyclic loading finite element simulations.Copyright