Fracture behavior of carbon steel pipe with local wall thinning subjected to bending load

To evaluate the structural integrity of power plant piping, monotonic bending tests are conducted on 4- and 3.5-in. diameter full-scale carbon steel pipe specimens with local wall thinning. The local wall thinning is simulated as erosion/corrosion metal loss. The eroded area of the wall thinning is subjected to tensile or compressive stress by applied bending moment. The deformations or fracture behaviors at maximum moments are found to be classified into three types. When the eroded area is subjected to tensile stress, ovalization or crack initiation/growth occurs at the maximum moment. When an eroded area is subjected to compressive stress, ovalization or local buckling occurs. The occurrence of ovalization, crack initiation/growth, or local buckling depends on the initial size of local wall thinning. From the relationships among ovalization, crack growth and local buckling, allowable sizes for local wall thinning are proposed.     

Fracture strength and behavior of carbon steel pipes with local wall thinning subjected to cyclic bending load

The power plant piping is designed to withstand seismic events using the design fatigue curve. However, the fatigue strength of a pipe with local wall thinning caused by erosion/corrosion is not clear. To evaluate the fatigue strength of pipes with local wall thinning, low cycle fatigue tests were conducted on 100A carbon steel pipes with local wall thinning. In load controlled tests on these pipes, ratcheting deformation was observed, and the fatigue strength became lower than that of cracked pipes. In displacement controlled tests, the fatigue strength of eroded pipes with 100 mm in eroded axial length, 0.5 in normalized eroded depth and 90° in eroded angle was almost equal to that given by the design fatigue curve in ASME B&PV Code Sec. III. To evaluate the local strain range in the maximum wall thinning area, the finite element analysis was conducted on the eroded pipes in the displacement controlled tests. It is concluded that the Mises strain range in the maximum wall thinning area and the low cycle fatigue curve can be used to conservatively estimate the low cycle fatigue life of an eroded pipe and the validity of estimated results can be confirmed experimentally.     

Effect of local wall thinning on the collapse behavior of pipe elbows subjected to a combined internal pressure and in-plane bending load

The objective of this study was to investigate the effect of local wall thinning on the collapse behavior of pipe elbows subjected to a combined internal pressure and in-plane bending load. This study evaluated the global deformation behavior and collapse moment of the elbows, which contained various types of local wall-thinning defects at their intrados or extrados, using three-dimensional elastic–plastic finite element analysis. The analysis results showed that the global deformation behavior of locally wall-thinned elbows was largely governed by the mode of the bending and the elbow geometry rather than the wall-thinning parameters, except for elbows with considerably large and deep wall thinning that showed plastic instabilities induced by local buckling and plastic collapsing in the thinned area. The reduction in the collapse moment with wall-thinning depth was considerable when local buckling occurred in the thinned areas, whereas the effect of the thinning depth was small when ovalization occurred. The effects of the circumferential thinning angle and thinning length on the collapse moment of elbows were not major for shallow wall-thinning cases. For deeper wall-thinning cases, however, their effects were significant and the dependence of collapse moment on the axial thinning length was governed by the stress type applied to the wall-thinned area. Typically, the reduction in the collapse moment due to local wall thinning was clearer when the thinning defect was located at the intrados rather than the extrados, and it was apparent for elbows with larger bend radius.     

The ratchetting behavior of pressurized plain pipework subjected to cyclic bending moment with the combined hardening model

A series of six tests have been conducted using carbon steel and stainless steel cylindrical specimens having mean diameter/thickness ratios in the range 8 ≤ D_m/t ≤ 28. Each cylinder is pressurized up to its calculated design pressure and is loaded with an alternative bending moment at frequencies typical of seismic events simultaneously. Ratchetting of the cylinder wall has been observed and recorded in the hoop direction. A finite element analysis with the nonlinear isotropic/kinematic (combined) hardening model has been used to evaluate ratchetting behavior of the cylinder under mentioned loading condition. Stress-strain data and material parameters have been obtained from several stabilized cycles of specimens that are subjected to symmetric strain cycles. The finite element results are compared with those obtained from experimental set-up. The results show that initial the rate of ratchetting is large and then it decreases with the increasing cycles. The FE model predicts the hoop strain ratchetting rate to be near that found experimentally in all cases that M/M_P0.2 ≤ 1. Also, M/M_y ratios for the onset of ratchetting in stainless steel specimens are less than carbon steel specimens with same D_m/t ratios.     

Comparative behavior of a nonlinear system subjected to impulsive load

Analytical and experimental studies of the dynamic response of a system with geometric and material nonlinearity are described. The dynamic excitation consists of an impulsive base acceleration. The dynamic system consists of a cantilever beam with a lumped mass and a gapped support beam at the free end. The material nonlinearity considers both the effect of yielding and the effect of strain rate on the inital yield level.Two analytical models are considered. The first is a generalized single degree-of-freedom system with a bilinear hysteresis. The second is a multiple degree-of-freedom system which uses beam elements with concentrated plastic hinges to represent the system. Numerical methods are used to solve the resulting equations of dynamic equilibrium.Experimental studies are performed and accelerations and residual plastic deformations are measured. Critical comparisons are made between the calculated and measured responses. The results show that the single degree of freedom model may overestimate the displacement response.     

Crack opening displacement of a through-wall crack in a plate subjected to bending load

This study was performed in order to clarify crack opening displacement (COD) of through-wall cracks in a plate subjected to bending load. The former COD evaluation methods were mainly developed corresponding to tensile load, but there has been nothing that has been developed corresponding to bending load. Therefore, the authors evaluated CODs of the through-wall cracks in plates which were subjected to a bending load using finite element method (FEM) analyses, and proposed a simplified COD evaluation method accounting for both tensile and bending loads. The proposed method is useful for leakage evaluation at a crack opening of an elbow crown or in the vicinity of the coolant surface of a vessel in which the bending stress is relatively large.     

Approximate evaluation method for ductile fracture analysis of a circumferentially through-wall-cracked pipe subjected to combined bending and tension

To estimate the structural integrity of the Light Water Reactor piping, combined loading consists of a tensile load due to internal pressure and a bending load under seismic conditions should be considered as a basic loading mode. However, theoretical investigation on the methodology to evaluate ductile fracture behavior is not adequate to date. In this study, an approximate evaluation method, ‘LBB.ENGC’, for ductile fracture analysis of a circumferentially through-wall-cracked pipe subjected to combined bending and tension was newly developed. This method can explicitly incorporate the contribution of both tension and bending. The effect of growing crack is also considered in the method. The LBB.ENGC was then applied to the full-scale pipe fracture tests. Based on the comparison with experimental results as well as finite element calculations, it could be ascertained that the LBB.ENGC could well predict ductile fracture behavior under combined loading. The effect of combined loading on ductile fracture was sensitivity-studied using the LBB.ENGC. As a result, it was quantitatively found that the superposition of longitudinal stress reduced the maximum bending load of cracked pipe.     

Plastic collapse and LBB behavior of statically indeterminate piping system subjected to a static load

The plastic collapse and LBB behavior of statically indeterminate piping system were investigated in this study, compared with those of the statically determinate piping system. Special attention was paid to evaluate the crack opening displacement after a crack penetrated wall thickness. The main results obtained were as follows:

1. The reduction of ultimate strength caused by a crack was relatively small in the statically indeterminate piping system. The main reason is thought to be that a sufficient redistribution of the bending moment occurs in this system.

2. A method to evaluate the crack opening displacement after crack penetration in a pipe with a non-penetrating crack was proposed. From this method, it was known that the crack opening displacement could be evaluated by using the incremental plastic rotation angle.

3. The acceptable defect size considering the deformation of a pipe was estimated by comparing the plastic moment at the defective part and the gross yielding moment at the non-defective part.

Ductile fracture behavior of circumferentially cracked type 304 stainless steel piping under bending load

A ductile pipe fracture test program has been conducted in Japan Atomic Energy Research Institute (JAERI) to investigate the ductile fracture behavior of circumferentially cracked pipes and to demonstrate the validity of the leak before break concept in LWR pipings.In the paper are described the scope of the pipe test program and current test results for 6-inch diameter type 304 stainless steel pipes. Test pipes with a through-wall or a part-through crack in the circumferential direction were bent under low or high compliance condition, and stable or unstable pipe fracture behavior was investigated. J based tearing instability criterion and the net section collapse criterion are compared with the pipe test results, and the validity of these fracture criteria is discussed. Furthermore, geometries of acceptable flaws in pipes are evaluated considering the leak before break condition.     

Short circumferential through-wall cracked pipes subjected to stretch-bending load

The methods for assessment of elastic–plastic fracture behaviour of cracked components include the net section plastic collapse concept, the J-integral approach, and the two-parameter R-6 failure assessment diagram, Revision 3. These failure assessment methods are usually used to obtain fracture behaviour prediction with a reasonable degree of accuracy without carrying out complicated full-length numerical fracture analysis. In the current work, fracture experiments on stainless steel pipes with short circumferential through-wall cracks under stretch-bending load were conducted. Stretch-bending load refers to the loading situation where axial load is generated that is proportional or related to the applied bending load. The J-integral values derived from the experimental load-point load–displacement data under stretch-bending and pure bending conditions are compared to investigate the effect of axial load on the J–resistance curves. The results show clear dependence of crack resistance force on axial load for short circumferential cracks. Crack resistance force decreased noticeably for increased stretch-bending loading compared to pure bending loading.     

The bending and pressurizing of pipe bends with flanged tangents

Pipe bends with flanged tangents are analyzed, by linear thin shell theory, for the stresses and flexibilities by pure inplane bending and pressurization, assuming that they are not interactive. Results are presented for flexibilities, decay constants and stresses. Zero tangent lengths, that is, flanges directly on the bend, substantially reduce the stresses and bend deflections due to applied moments, but tangents longer than one pipe circumference effectively allow 90° bends to act as if they are unterminated. With pressurization, flanges on 90° bends only marginally reduce stresses and bend deflections compared to those for an unterminated bend.     

Crack opening displacement of circumferential through-wall cracked cylinders subjected to tensile and through-wall bending loads

This study is concerned with crack opening displacements (CODs) of cylinders with a circumferential through-wall crack which is subjected to tensile and through-wall bending loads. A series of FEM analyses were performed in various scaled cylinders, and then the present results on the CODs were compared with the previous studies. Especially, the crack opening behaviors of the large scaled cylinders under a membrane stress and a through-wall bending stress were characterized evidently in this study. The present results are expected to be valid for the leakage evaluation of structures which is subjected to internal pressure and thermal distribution.     

Evaluation of the lifespan indicators of parts subjected to multidimensional cyclic actions

Damage in the form of fatigue cracks can arise and corrosion processes can be initiated in the material of a structural element under prolonged simultaneous exposure to variable stresses of different nature. In this connection, it is necessary to construct an adequate mathematical model of the reliability that would make it possible to determine the lifespan of the structure during operation. Accurate values of the lifespan indicators of a part are impossible to obtain in the case of multidimensional loads. For this reason, it is of practical interest to obtain estimates of these indicators, which is the subject of discussion in this article. __________ Translated from Atomnaya énergiya, Vol. 102, No. 6, pp. 358–362, June, 2007.     

Reinforced concrete membrane elements subjected to reversed cyclic in-plane shear stress

A macroscopic model has become noticeable instead of a microscopic model by FEM analysis for concrete structures. Among them the model proposed by Vecchio and Collins is considered to be one of the powerful tools because of its rationalility and simplicity. The essential point of this model is a modified compression theory of cracked concrete and the stress-strain relationship for cracked concrete.This paper follows basically Collins' theory but is revised so as to be able to predict the response of reinforced concrete elements subjected to alternate reversed cyclic in-plane shear simulating earthquake forces. The overall characteristics of the new model exist in the tensile residual strains accumulated by cycles in cracked concrete, which is different from the monotonic loading.     

Dynamic behaviour of thin cylindrical shells subjected to impulsive bending moments along their tips

The dynamic behaviour of thin cylindrical shells with concentrated masses at their tips is investigated when they are subjected to impulsive bending moments at their tips. The relationships between the dimensions of a cylinder, concentrated mass and maximum values of dynamic stresses are obtained. The fundamental equation of motion is solved by the Laplace transformation method. From the results of the theoretical analysis, it became evident that impulsive stresses become much larger than static ones. For the case where bending moments are applied statically, the fundamental equations are also derived, taking into account the effect of shearing force.     

Creep of a straight pipe under combined bending and internal pressure

Several analyses are presented for the isothermal creep behaviour of a thin, constant thickness, circular crosssection cylindrical shell under the action of conbined bending and internal pressure loading. Results are presented in a way that should be helpful for general pipework design. Previous analyses are summarised and discussed.     

Prediction of semi-elliptical defect form, case of a pipe subjected to internal pressure

The fatigue propagation of a semi-elliptical surface crack in a pipe subjected to internal pressure is always an open question in spite of a number of numerical works. The crack front development is generally predicted by empirical relationships or parameters c and m of the crack propagation law. This fatigue propagation in the case of a pipe subjected to internal pressure have been obtained using a special device allowed to produce an internal pressure up to 700 bars. The crack aspect ratio development is similar to that of a crack in a plate submitted to tension.     

Creep-fatigue failure test and analysis of a vessel-type structure subjected to cyclic thermal transients

A SUS 304 vessel type test model, designed to include several typical structural parts of FBR components, was subjected to 1,300 cycles of severe thermal transient loadings using a large scale sodium loop. After th test, the model was inspected by a dye penetrant test. Crackings were observed at the intended portions of the model, and the fracture surfaces were nearly perfect rock-candy patterns (IG crackings) which are usually observed in creep rupture crackings. A-Heat transfer analysis using the measured temperature data, and elastic and inelastic thermal stress analyses were carried out to evaluate the creep-fatigue damage and the crack initiation. Strain concentration factors due to geometrical and metallurgical discontinuities or to inelastic behavior of the material are discussed. Finally fracture mechanics analyses were performed to explain the crack growth behavior.     

Shear transfer constitutive model for pre-cracked RC plate subjected to combined axial and shear stress

This paper proposes the constitutive model for the shear transfer through the cracks of pre-cracked reinforced concrete (RC) plate subjected to combined axial and shear stress. The plate is a scale model of a shear wall of a nuclear power plant (NPP) building. Twelve plate specimens were initially cracked and then loaded to the failure point by increasing cyclic shear under constant axial stress. Tangential shear modulus, G_cr, values are estimated from the v–γ relationships observed in the test results and formulated to the constitutive model as the correlation function of the normal strain perpendicular to the crack plane, _cr, and shear strain, γ_cr, based on the smeared crack model concept. By incorporating the proposed model to a nonlinear FEM analysis program and comparing the analysis results with the test results, it is apparent that the program could be improved in its analytical accuracy. The proposed model will be useful for the nonlinear analysis of RC shear walls when the walls are exposed to simultaneous multi-directional load.     

Study on steel plate reinforced concrete panels subjected to cyclic in-plane shear

This paper describes the derivation of the equation for evaluating the strength of steel plate reinforced concrete structure (SC) and the experimental results of SC panels subjected to in-plane shear.Two experimental research programs were carried out. One was the experimental study in which the influence of the axial force and the partitioning web were investigated, another was that in which the influence of the opening was investigated.In the former program, nine specimens were loaded in cyclic in-plane shear. The test parameters were the thickness of the surface steel plate, the effects of the partitioning web and the axial force. The experimental results were compared with the calculated results, and good agreement between the calculated results and the experimental results was shown.In the later programs, six specimens having an opening were loaded in cyclic in-plane shear, and were compared with the results of the specimen without opening. FEM analysis was used to supplement experimental data. Finally, we proposed the equation to calculate the reduction ratio from the opening for design.