Structural Integrity of Bi-Metallic Welds in Piping Fracture Testing and Analysis

2008 ◽  
Author(s):  
Claude Faidy
Keyword(s):  
Stainless Steel ◽  
Crack Growth ◽  
Structural Integrity ◽  
Safety Issue ◽  
Crack Size ◽  
Evaluation Procedure ◽  
Critical Crack ◽  
Surface Corrosion ◽  
Fabrication Defects ◽  
Critical Crack Size

The French field experience in stainless steel bi-metallic welds (BMW) has shown different degradations like external surface corrosion cracks close to the low alloy steel/stainless steel interface or fabrication defects in different other locations. In many countries, some degradation has been encountered in different type of bi-metallic welds: stainless steel BMW or Ni-based alloy BMW through different degradation mechanisms (corrosions). The critical crack size in different location of a BMW is a key safety issue. To-day, there is no flaw evaluation procedure for this type of components in existing operation codes, like ASME XI [7], RSE-M [6] or R6 rule [5]. Consequently a fracture mechanic procedures is under preparation in the French RSE-M operation Code [6] in order to evaluate the critical crack sizes of defects in different area of a bi-metallic weld. The procedure validation is based on 2 specific experimental projects that have been performed on 6" and 16" bi-metallic welds at room temperature and 300°C. Detailed residual stress measurements and simulation have been done, in order to check their influence on the critical crack size. The major results of these projects are: • no instable crack growth for cracks closed to the fusion line, • crack ductile tearing growth take place in the lower strength material and not in the lower toughness material, • the residual stresses have a negligible effect on the critical crack size, • the crack initiation take a place close to the maximum limit loads, • the ductile crack growth is mainly at the deepest point and negligible on the outer surface (for the cracks considered in these test programs), • no existing engineering methods are really available for this type of cracked components, • the crack distance to the interface is a key parameter in term of toughness, • comparison of notched and pre-crack specimen has been done and confirm a small increase of toughness in the case of electro-eroded crack. After a brief summary of the validation programs, the paper ends with a proposed procedure to analyse the critical crack size in a bi-metallic weld through two methods: an engineering method (1) with large safety factor and an elasto-plastic FEM (Finite Element Method) (2) with lower safety factors. They will be included soon in the RCC-M [8] for design consideration of BMW and the RSE-M [6] codes for service behaviour of BMW. Similar method can be used for different type of BMW (VVER, Ni-based alloy...).

Comparison of Leak-Before-Break Assessment of Main Loop Piping Lines Fabricated of Different Materials

2016 ◽  
Author(s):  
Linwei Ma ◽  
Xiaotao Zheng ◽  
Yan Wang ◽  
Jiasheng He ◽  
Anqing Shu
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Material Properties ◽  
Plastic Instability ◽  
Crack Size ◽  
Critical Crack ◽  
Main Loop ◽  
Critical Crack Size ◽  
Leak Before Break

Leak-Before-Break (LBB) assessment is used for the design of nuclear reactor coolant system main loop piping to lower the cost of construction and operation in China. In these applications, the materials of main loop piping lines are cast austenitic stainless steel (CASS) or wrought stainless steel (WSS) due to the different type of reactor design. According to US.NRC SRP3.6.3, LBB assessment includes two major calculations, such as critical crack size calculation and leakage flaw size calculation. The elastic-plastic instability analysis or plastic instability analysis is chosen to perform critical size calculation depending on material properties, especially fracture toughness. In this paper, LBB assessment in the guidance of SRP 3.6.3 was performed to evaluate main loop piping lines of CASS and WSS. The JR curve tests and the adjustment due to thermal aging are performed to achieve reasonable material properties. J integral/tearing modulus approach is used to determine critical crack size of CASS pipe and net section collapse (NSC) approach is used to determine critical crack size of WSS pipe. Leakage flaw size under 1gpm leakage detection capability is determined based on Henry’s homogeneous nonequilibrium critical flow model. In order to demonstrate that fatigue crack growth is not a potential source of pipe rupture for the evaluated piping lines, the fatigue crack growth of a postulated circumferential part-through-wall crack under nuclear power plant full life time operating transients and the fatigue crack growth of a circumferential through-wall crack under one time safe shutdown seismic are analyzed. And the LBB assessment procedure and results of CASS pipe and WSS pipe are compared.

Effect of In Plane and Out of Plane Bending on Weld Residual Stresses

2014 ◽  
Author(s):  
Etienne Bonnaud ◽  
Jens Gunnars
Keyword(s):  
Stainless Steel ◽  
Residual Stresses ◽  
316L Stainless Steel ◽  
Crack Size ◽  
Cylindrical Vessel ◽  
Critical Crack ◽  
Metal Plates ◽  
Mechanics Analysis ◽  
Out Of Plane ◽  
Critical Crack Size

When manufacturing cylindrical or conical structures from metal plates, residual stresses originate from both welding and bending. The sequence in which these fabrication steps are carried out is essential as it can radically change the final distribution of residual stresses. To study this effect, detailed welding and bending simulations have been performed on both axial and circumferential X-welds of a 316L stainless steel cylindrical vessel. Bending after welding is shown to reduce residual stresses markedly more than bending before welding and the benefit on critical crack size is illustrated by a Fracture Mechanics analysis.

Ligament Crack Growth in a Main Steam Superheater Outlet Header

2006 ◽  
Author(s):  
Annette Karstensen ◽  
Jorge Guerra ◽  
David Knowles ◽  
Ohgeon Kwon
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Structural Integrity ◽  
Pressure Fluctuations ◽  
Crack Size ◽  
Steam Superheater ◽  
Critical Crack ◽  
Main Steam ◽  
Crack Growth Model

Ligament cracks were discovered during a cold survey in a main steam outlet header in a 250MW coal/gas fired unit following over 100,000 hours of service. Subsequent inspection revealed similar cracking in the same headers of two other units in the plant which had seen similar service. To ensure the safety of the headers a structural integrity programme was initiated by the operator, Genesis Energy, in order to determine their fitness for service. The maximum stress and stress range due to the temperature and pressure fluctuations during operation and starts was established by carrying out a detailed thermo-mechanical FE analysis, validated against detailed thermocouple measurements. The critical crack size was calculated and the time for the flaw to reach the critical flaw size was established by considering creep and fatigue crack growth mechanisms. Mechanical testing on material obtained during a repair of one of the headers was used to refine estimates of toughness and tensile properties. Moreover examination of the fracture face removed during the repair indicated that the crack growth was primarily due to fatigue. This was found to be due to frequent thermal cycling of the unit during nominally “steady state” operation. The crack growth rate established from the fracture surface analysis was compared with NDT measurements for validation of the fatigue crack growth model. The demonstrated predictability for the rate at which the ligament cracks are growing as a function of operation has provided Genesis with the opportunity develop a long term strategy for inspection, repair or replacement of their superheater headers.

Effect of Rotational Stiffness Evolution at Crack Part on Critical Crack Size in SFR

2012 ◽  
Author(s):  
Takashi Wakai ◽  
Hideo Machida ◽  
Shinji Yoshida
Keyword(s):  
Evaluation Method ◽  
Large Diameter ◽  
Crack Size ◽  
Evolution Model ◽  
Piping System ◽  
Rotational Stiffness ◽  
Critical Crack ◽  
Rotational Spring Model ◽  
Size Evaluation ◽  
Critical Crack Size

This paper describes the efficiency of the deployment of rotational stiffness evolution model in the critical crack size evaluation for Leak Before Break (LBB) assessment of Sodium cooled Fast Reactor (SFR) pipes. The authors have developed a critical crack size evaluation method for the thin-walled large diameter pipe made of modified 9Cr-1Mo steel. In this method, since the SFR pipe is mainly subjected to displacement controlled load caused by thermal expansion, the stress at the crack part is estimated taking stiffness evolution due to crack into account. The stiffness evolution is evaluated by using the rotational spring model. In this study, critical crack sizes for several pipes having some elbows were evaluated and discuss about the effect of the deployment of the stiffness evolution model at the crack part on critical crack size. If there were few elbows in pipe, thermal stress at the crack part was remarkably reduced by considering the stiffness evolution. In contrast, in the case where the compliance of the piping system was small, the critical crack size could be estimated under displacement controlled condition. As a result, the critical crack size increases by employing the model and LBB range may be expected to be enlarged.

Determination of Critical Crack Size Utilizing a Fracture Mechanics Test in Equipment Under Fatigue

2009 ◽  
Author(s):  
Irene Garcia Garcia ◽  
Radoslav Stefanovic
Keyword(s):  
Heat Treatment ◽  
Fracture Mechanics ◽  
Crack Size ◽  
Operational Experience ◽  
Element Analysis ◽  
Critical Crack ◽  
Circumferential Welds ◽  
Fea Model ◽  
Critical Crack Size

Equipment that is exposed to severe operational pressure and thermal cycling, like coke drums, usually suffer fatigue. As a result, equipment of this sort develop defects such as cracking in the circumferential welds. Operating companies are faced with the challenges of deciding what is the best way to prevent these defects, as well as determining how long they could operate if a defect is discovered. This paper discusses a methodology for fracture mechanics testing of coke drum welds, and calculations of the critical crack size. Representative samples are taken from production materials, and are welded employing production welding procedures. The material of construction is 1.25Cr-0.5Mo low alloy steel conforming to ASME SA-387 Gr 11 Class 2 in the normalized and tempered condition (N&T). Samples from three welding procedures (WPS) are tested: one for production, one for a repair with heat treatment, and one for repair without heat treatment. The position and orientation of test specimen are chosen based on previous surveys and operational experience on similar vessels that exhibited cracks during service. Fracture mechanics toughness testing is performed. Crack finite element analysis (FEA) model is used to determine the path-independed JI-integral driving force. Methodology for the determination of critical crack size is developed.

Critical crack size in the fracture of metals

2007 ◽  
Vol 52 (7) ◽  
pp. 937-939
Author(s):  
V. A. Ivanskoĭ
Keyword(s):  
Crack Size ◽  
Critical Crack ◽  
Critical Crack Size

scholarly journals On the fracture toughness of snow

2004 ◽  
Vol 38 ◽  
pp. 1-8 ◽  
Author(s):  
Jürg Schweizer ◽  
Gerard Michot ◽  
Helmut O.K. Kirchner
Keyword(s):  
Fracture Toughness ◽  
Crack Size ◽  
Laboratory Measurements ◽  
Critical Crack ◽  
Arrhenius Relation ◽  
Linear Fracture ◽  
Different Temperatures ◽  
Increasing Temperature ◽  
Critical Crack Size ◽  
Key Parameter

AbstractThe release of a dry-snow slab avalanche involves brittle fracture. It is therefore essentially a non-linear fracture mechanics problem. Traditional snow-stability evaluation has mainly focused on snow strength measurements. Fracture toughness describes how well a material can withstand failure. The fracture toughness of snow is therefore a key parameter to assess fracture propagation propensity, and hence snows lope stability. Fracture toughness in tension KIc and shear KIIc was determined with notched cantilever-beam experiments in a cold laboratory. Measurements were performed at different temperatures and with different snow types of density ρ = 100–300 kgm–3, corresponding to typical dry-snow slab properties. The fracture toughness in tension KIc was found to be larger (by about a factor of 1.4) than in shear KIIc. Typical values of the fracture toughness were 500–1000 Pam1/2 for the snow types tested. This suggests that snow is one of the most brittle materials known to man. A power-law relation of toughness KIc on relative density was found with an exponent of about 2. The fracture toughness in tension KIc decreased with increasing temperature following an Arrhenius relation below about –8°C with an apparent activation energy of about 0.16 eV. Above –6°C the fracture toughness increased with increasing temperature towards the melting point, i.e. the Arrhenius relation broke down. The key property in dry-snow slab avalanche release, the critical crack size under shear at failure, was estimated to be about 1 m.

Fracture Resistance in GFRP-BMC Composites Induced by Sub-Millimeter Length Fiber Dispersion

2012 ◽  
Vol 706-709 ◽  
pp. 907-913
Author(s):  
Michael C. Faudree ◽  
Yoshitake Nishi
Keyword(s):  
Fracture Resistance ◽  
Unsaturated Polyester ◽  
Glass Fibers ◽  
Acoustic Emissions ◽  
Crack Size ◽  
Critical Crack ◽  
Fiber Dispersion ◽  
Static Tensile ◽  
Styrene Butadiene ◽  
Critical Crack Size

Based on previous results of both an increase of nearly 40% in static tensile strain by shortening fiber length from commercial 6.4 mm to 0.44 mm in an unsaturated polyester/styrene-butadiene GFRP-BMC composite containing 20 mass% short E-glass fibers and their acoustic emissions (AE), the fracture resistance mechanics of sub-mm length fiber dispersion reinforcement is proposed. Since the 40% strain increase acts to improve strength and toughness, the mechanics is useful. This paper aims to present the mechanism of strain-driven improvement where microcracks are prevented from propagating beyond the critical crack size (2ac) for thermoset polymers, resulting in an increased and more dispersed total microcrack surface area as recorded by AE raising fracture strain.

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