V. Bhasin

Crack initiation and growth behaviour of circumferentially cracked pipes under cyclic and monotonic loading

Abstract Studies have been carried out on carbon steel pipes to demonstrate the leak before break design criterion and validate the analytical procedures. Fatigue crack initiation, fatigue crack growth rate and fracture resistance behaviour of the pipes have been experimentally and analytically evaluated. The tests have been carried out on pipes of outer diameter 219 and 15.1 mm wall thickness having a part through notch in the circumferential direction. The aspect ratios (2 c / a ) of the notches were 56, 28 and 18. Comparing the analytical and experimental results has validated analytical procedures. It has been observed that the analytical and experimental results compare well. The fatigue crack growth curve (d a /d N ∼Δ K ) obtained from three point bend specimens and pipe tests have been compared with the fatigue crack growth curve in ASME Section XI. The comparison shows that use of the ASME curve in analysis of components will give a conservative result in comparison to the curves obtained from the actual pipe tests. Fracture resistance behaviour of the pipe has been observed to be strongly dependent on the load histories to which the pipe has been subjected.

Effect of cyclic loading on elastic-plastic fracture resistance of PHT system piping material of PHWR

The objective of the paper is to study the effect of cyclic loading on the fracture resistance of the SA 333 Gr.6 carbon steel material of the primary heat transport (PHT) system piping of Indian pressurized heavy water reactors (PHWR). Tests have been carried out on compact tension specimens machined from the pipe used in the PHT system. The effect of load ratio, incremental plastic displacement and loading displacement rate on fracture resistance properties of the material has been studied. The cyclic fracture resistance curve has been compared with the monotonic fracture resistance curve and has been found to vary significantly. A negative load ratio with very low incremental plastic displacement between two consecutive loading cycles, significantly affects the fracture resistance of the material.

Probabilistic fracture analysis of a straight pipe with through-wall circumferential crack using R6 method

Abstract Deterministic fracture analysis of cracked structures has been successful in demonstrating the behavior of structures under observed or postulated flaws. The key ingredients in a deterministic fracture mechanics analysis are the initial crack size, crack-driving force, applied stress and material properties. However, crack size, postulated accidental loading, mechanical and fracture parameters are often subjected to considerable scatter or uncertainty. Hence, the result of fracture mechanics analysis must be viewed with some skepticism. Quite often, conservative bounds on inputs are employed on these by providing a conservative estimate to fracture. However, it may lead to predict the overly conservative and unrealistic results. These uncertainties encourage us to adopt a statistical or probabilistic approach to the structural integrity. R6 is a well-known engineering assessment procedure for the evaluation of the structural integrity of the flawed structure. This paper presents the determination of failure probabilities of a straight pipe with a through-wall crack in the circumferential direction, using the R6 method. The pipe is subjected to an in-plane bending moment. The scatter in the crack size, mechanical and fracture properties are considered. The probability of failure is studied for variation in applied loads. Monte Carlo simulation is used to find the failure probability. The study is performed by constructing failure assessment diagram (FAD) using all the three options of R6. The results are compared with the approximate analytical formulations and with those available in the literature.

Numerical Simulations of 3-D Cracks Using Coupled EFGM and FEM

This article presents a numerical solution of three-dimensional cracks using coupled EFGM and FEM. The FEM adequately handles problems without discontinuities but faces severe limitations in solving crack growth problems, while EFGM does fairly well in handling such problems. Hence, EFGM has been used in a region near the crack whereas FEM has been utilized in the rest of the region. In the interface/transition region, a coupling scheme known as mixed-mode interpolation is used to obtain the shape functions. These simulations show that the results obtained by the coupled scheme are found to be in good agreement with analytical solutions.

Assessment of integrity of components in piping of 500 MWe PHWR: using R-6 method

Abstract The primary heat transport system of 500 MWe Indian PHWR comprises of straight pipes, elbows and headers. A study was conducted to qualify piping system for leak before break. R-6 method was used to assess integrity of system for leakage size crack (LSC), the margins on crack initiation load and unstable crack growth loads. Option 2 (material specific failure assessment diagram), Category 3 (ductile tearing) analysis was used for straight pipes, elbows and header. In order to enhance the confidence in the analytical results, detailed sensitivity analysis was also performed. For sensitivity analysis, variation in material properties, LSC was considered. The effect of variation in temperature on material properties was also considered. Tensile and fracture properties used for base and weld material data were generated from pipe material obtained from 220 MWe Nuclear Power Plant, under construction.

Effects of Simulated Seismic Loading on LBB Assessment of High Energy Piping

The current Leak Before Break (LBB) assessment is based primarily on the monotonic fracture tearing instability. In it the maximum design accident load is compared with the fracture-tearing resistance load. The effect of cyclic loading has not been generally considered in the fracture assessment of nuclear power plant piping. It is a well-known fact that reversible cyclic loading decreases the fracture resistance of the material, which leads to increased crack growth. Indian nuclear power reactors consider Operational-Basis-Earthquake (OBE) and Safe-Shutdown-Earthquake (SSE) events in the design of various structures, systems, and components. Keeping this in view a series of cyclic tearing tests have been conducted on straight pipes, made of ASTM SA333 Gr.6 carbon steel. This is the material of primary heat transport (PHT) piping material of Indian Pressurized Heavy Water Reactors (PHWR). In this series 13 tests have been carried out circumferentially through wall cracked seamless and circumferential seam welded straight pipes under reversible cyclic bending loading. All the tests have been conducted under quasistatic, i.e., slow loading rates and dynamic inertia effects are not considered. The cyclic test results have been compared with the corresponding monotonic pipe fracture test results. These test results and its comparison with corresponding monotonic tearing clearly illustrate the need of addressing the reduction in apparent fracture toughness of material under reversible cyclic loading and the safe number of load cycles in the LBB assessment.

Application of Ball Indentation Technique for Mechanical Properties Estimation of Bi-Metallic Weld

This work studied the ball indentation test at room temperature to characterize the local tensile properties of bi-metallic weld joints. The weld specimens used were fabricated by joining between SA 508 Gr. 3 ferritic steel and Type 304LN stainless steel with Alloy 82 buttering on the ferritic steel side and Alloy 82/182 weld metal. The test results showed that yield stress (YS) of weld metal was slightly higher than that of Type 304 and smaller than that of SA508 Gr.3, and ultimate tensile stress (UTS) of weld metal was similar as those of Type 304 and SA508 Gr.3 base metals. Also, the values of YS and UTS of buttering layer (Alloy 82) were nearly same as those of weld metal. Heat-affected-zones (HAZs) showed higher YS and UTS values compared to their base metals.

Experimental Investigations on Effects of Simulated Seismic Loading on LBB Assessment of High Energy Piping

The current Leak Before Break (LBB) assessment is based primarily on the monotonic fracture tearing instability. In it the maximum design accident load is compared with the fracture-tearing resistance load. The effect of cyclic loading has generally not been considered in the fracture assessment of nuclear power plant piping. It is a well-known fact that the reversible cyclic loading decreases the fracture resistance of the material, which leads to increased crack growth. Indian nuclear power reactors consider Operational-Basis-Earthquake (OBE) and Safe-Shutdown-Earthquake (SSE) event in the design of various structures, systems and components. Keeping this in view a series of cyclic tearing test have been conducted on straight pipes, made of ASTM SA333 Gr.6 carbon steel. This is the material of primary heat transport (PHT) piping material of Indian Pressurised Heavy Water Reactor (PHWR). In this series 13 tests have been carried out on circumferentially through wall cracked seamless and circumferential seam welded straight pipes under reversible cyclic bending loading. All the tests have been conducted under quasi-static i.e. slow loading rates and the dynamic effect is not considered. The cyclic test results have been compared with the corresponding monotonic pipe fracture test results. These test results and its comparison with corresponding monotonic tearing clearly illustrates the need of addressing the reduction in apparent fracture toughness of material under reversible cyclic loading and safe number of load cycles in the LBB assessment.Copyright

PARAMETER AND INTERACTION STUDY OF EDGE CRACK PROBLEM USING MESHFREE METHOD

In this work, element free Galerkin method (EFGM) has been used to obtain the solution of edge crack problem under mechanical loads as it provides a versatile technique to model static as well as moving crack problems without any requirement of remeshing. At first, some techniques are presented for enriching the EFG approximations near the crack tip such as extrinsic and intrinsic enrichment. Extrinsic enrichment involves the addition of solution form to the trial function, whereas the EFG basis is expanded to include few terms from crack tip solution in intrinsic enrichment. Apart from enrichment techniques, four basic techniques of smoothing meshless approximations near nonconvex boundaries are also presented such as diffraction, transparency, see through and wedge model. These techniques are then used for the parameteric analysis of an edge crack problem under mode-1 loading and results obtained using different approaches are compared with each other as well as with exact solution. Among these techniques, the extrinsic PU enrichment technique was found to be more accurate as compared to other approaches. Extrinsic PU enrichment technique has also been used for the analysis of a shear edge crack problem. In all these techniques, the value of mode-1 stress intensity factor and mode-2 stress intensity factor has been evaluated by interaction integral approach. Effect of crack orientation is also studied for different cases.

Improvement in Welding of Austenitic Stainless Steel Thick Pipe: Metallurgical, Mechanical & Full Scale Pipe Weld Tests

The paper aims at demonstration of reduction in residual stress, distortion and sensitisation in austenitic stainless steel pipe welds. The welding processes considered are Gas Tungsten Arc Welding (GTAW) and Shielded Metal Arc welding (SMAW) along with Hot wire GTAW with narrow gap welding technique. It was shown that residual stress is significantly reduced in case of hot wire GTAW. The reduction in residual stress is due to the low heat input and high deposition rate. Lower heat input leads to higher cooling rate giving more margins on sensitization in heat affected zone (HAZ). This has been confirmed by measuring temperature and cooling rate during welding in HAZ of the weld joints. Susceptibility to sensitization of the welds has been compared by carrying out ASTM E262 Practice A and E along with Electo-Potentiokinetic Reactivation (EPR) method. The results of the tests show that values are within acceptable limit for both the cases. However, hot wire GTAW has marginally better sensitization resistance. Fatigue crack growth rate and fracture resistance behavior of the above mentioned weld joints have been compared by carrying out tests on the specimens (Compact Tension) and full scale pipe welds with crack. Fatigue crack growth rate and fracture toughness of the weld joints (hot wire GATW) is superior to conventional GTAW and SMAW. The paper presents the details of the above mentioned studies.