P.P. Milella

On the dependence of the Weibull exponent on geometry and loading conditions and its implications on the fracture toughness probability curve using a local approach criterion

In a previous paper the authors assessed the probability of failure of a three point bend specimen, SE(B), using a local approach criterion. In that paper the Weibull exponent, m, was derived from tests performed on round notch bars in traction, RNB(T), following the procedure suggested by Mudry. In the present study, it is addressed the issue of the dependence of the Weibull exponent m on geometry and loading conditions. It is shown that the amplitude and shape of the notch tip stress field and, in particular, the triaxiality characterising the stress state determines the value of the exponent m. Tests performed on RNB(T) specimens of carbon steel 22NiMoCr37, type A 508 Cl 3, at temperatures ranging from −18xa0°C to −196xa0°C actually indicate that m varies from ∼6 to 40, depending on the notch depth and root radius while for specimens carrying sharp cracks its value drops down to ∼4. This last result seems to be consistent with the Wallin hypothesis of a theoretical value equal to 4 for fracture mechanics specimens with high constraint, such as C(T) or SE(B), with positive values of the Q-stress or T-stress and triaxiality factor, TF, approaching 2.5. Temperature, in as long as it does not modify the stress state from plane strain to plane stress and the TF, has no effect on the value of m which is independent of the material as well.

Outline of nuclear piping research conducted in Italy

Abstract Following an early phase of limited activity at the University of Pisa on small stainless steel pipes containing axial cracks, in 1981 ENEA, the Italian Committee for Research and Development of Nuclear Energy and Alternative Energies, has started a massive research campaign on fracture of carbon and stainless steel piping containing through and part-through cracks loaded either under pressure or in bending. The purpose of the program was to develop a better understanding on pipes fracture behaviour in order to set new design criteria more realistic, yet conservative, than the guillotine break and prepare acceptance criteria for in-service flaws particularly under the growing pressure of IGSCC that has merciless affected worldwide practically any BWR piping system. The analysis of more than 100 tests carried out at CISE research centre, in Milano, on 4 inch, 6 inch, 8 inch and 10 inch pipes has indicated that unstable fracture requires at least 150° through wall crack under ASME maximum design stresses. The leak area, before instability takes place, is always less than 10% of the net cross section area of the pipe. This has led ENEA to consider a 10% break area as a reasonable value to calculate jet forces. Further, it was found that the net section collapse load criterion by far underestimates the actual collapse load and that 360° part-through cracks tend to switch from ductile to brittle failure mode of a pipe loaded in bending. Further work is planned for the next 3 years including high temperature tests, stainless steel weldments and HAZ tests, high compliance tests and eventually burst tests. Besides the ENEAs research program, Ansaldo AMN, the Italian Nuclear Architect Engineer, is developing theoretical studies and codes to treat the problem of pipe fracture.

Fracture behaviour of carbon steel pipes containing circumferential cracks at room temperature and 300°c

Abstract The results of a six-years experimental program on carbon steel pipe fracture behaviour are presented in this study. Pipes containing circumferential through-wall cracks were loaded under pure bending at room temperature and 300°C. Tests show that the Net Section Collapse Load criterion (NSCL) underestimates the actual fracture resistance of pipes with large cracks while pipes with small cracks, or no crack at all, behave as predicted by NSCL. A modified version of the NSCL criterion is proposed. An important parameter to be considered is the radius to thickness ratio, R/t . Increasing R/t ratios reduce the fracture resistance of a pipe. The GE-EPRI estimation scheme accurately predicts collapse loads, particularly at a high temperature. At 300°C, a dynamic strain aging phenomenon can occur which reduces fracture resistance. Leak areas are also presented as a function of the crack angle.

Prediction of leak areas and experimental verification on carbon and stainless steel pipes

Abstract This paper presents a study on the pipe crack opening area which is a task in the frame of the six-years Pipe Test Program that ENEA (Comitato Nazionale per la Ricerca e lo Sviluppo dell E nergia N ucleare e delle E nergie A lternative) started aiming to address some of the uncertainties related to Leak-Before-Break (LBB) concept. The study focuses on the opening generated in a pipe containing a through wall circumferential crack loaded in pure bending, trying to assess the relative leak area of paramount importance in any evaluation of dynamic effects, i.e. thrust forces and jet impingement, and leak detection systems capability as well. The purpose of this report is to check some available analytical tools against the experimental results obtained on stainless and carbon steel pipes. In particular two different methods are considered, that is: 1. (a) Tada/Paris formula, 2. (b) General Electric Engineering Approach. Calculaion are compared with experimental results from pure bending pipe tests, run at room temperature.

Theoretical forecasting and experimental validation of damage tolerance and accumulation in glass/epoxy laminates

Damage and damage accumulation mechanisms under cyclic loads were studied in orthotropic cross-ply composite laminates realized in S-glass fibers with epoxy resin. Five different basic mechanisms of damage were observed and evaluated making use of x-ray and dye-penetrant nondestructive techniques. They are: transverse matrix cracks, fiber failure, fiber debonding, intralaminar delamination and random disperse longitudinal cracks. The effects of these different damage mechanisms are revealed through the reduction of the mechanical properties of the composite material. A comprehensive experimental analysis of GFRP and CFRP, with two different lay-ups for each one, was made. Variations of the elastic moduli in two different directions and Poissons ratio were measured as a function of the number of cycles, for different stress levels, under alternate loads. The basic relations, developed by Talreja et al., which permit the prediction of the reduction of the elastic moduli for a given damage state, were directly related to the number of cycles N and to the stress levels employed in the fatigue tests. Tests to evaluate an eventual frequency effect on damage accumulation processes in composite material were made. Cumulative damage tests were also conducted to analyse material response under different and alternate load spectra. A damage index based on residual normalized strain energy ratio, which can describe a global damage state, is also proposed.

Fracture behaviour of stainless steel pipes containing circumferential cracks at room temperature and 280°C

Abstract The paper presents the experimental results of a research programme on fracture behaviour of austenitic stainless steel and TIG welds in pipes containing circumferential through-wall cracks at room temperature and 280°C. Pipes were loaded in pure bending using a four-point bend test method. The diameter of the pipes under investigation was 168 mm and 324 mm, with a thickness varying from 10 to 17 mm. As opposed to the behaviour of carbon steel pipes, it is found that the Net Section Collapse (NSC) criterion predicts the moment of instability. Crack mouth opening displacements (COD) and collapse moments calculated using the GE-EPRI engineering approach show a rather high scatter with respect to experimental results.

Failure Mode of Pipes Containing Circumferential Cracks Under Bending and Its Consequence on the Application of NSCM Criterion

The results of some 60 tests performed in Italy on 2”, 4”, 6” and 8” pipes of A 106 B and 304 stainless steel, carrying circumferential through-wall cracks of various size under four point bending conditions (FPB), at room temperature and 300° C, have been analyzed using the Net Section Collapse Moment Criterion (NSCM) and the dimensionless plastic zone parameter (DPZP). Most of the test results have shown that the NSCM applies even though the DPZP is lower than unity. This apparent inconsistency is due to the fact that cracked pipes under bending fail by plastic hinge formation of the type occurring in FPB specimens carrying notches, like the Charpy VN ones, as predicted by the slip line theory. Under these conditions, two half circle plastic zones develop at both sides of the notch while the plastic zone straight ahead the notch tip is almost negligible. FE Calculations have confirmed this behavior: the plastic zone underneath the crack tip has not yet reached the neutral axis when the plastic hinge is formed on the sides of the piping, making the NSCM applicable. This, actually, implies that the DPZP as presently used in the screening criteria is not precisely the proper parameter to adopt in the assessment of the NSCM criterion applicability.Copyright

Cleavage fracture prediction and KIc assessment of a nuclear pressure vessel carbon steel using local approach criteria

Abstract This paper presents a theoretical and experimental work carried out by the Aerospace Department of the University of Rome and ENEA-DISP on local approach. The main goal was to apply the local approach criteria to cleavage fracture prediction in a structural material. The material considered was a low alloy steel used in the nuclear industry, type 22NiMoCr37, of German production provided by KWU in the framework of an international round robin on local approach to fracture in steels, promoted by the European Structural Integrity Society (ESIS). Twelve tension tests on round notched bars were performed at low temperature (−90°C) in order to get the experimental data needed to calculate the theoretical Weibull stress and exponent and infer the probability of failure by a finite element analysis. Results have been applied to a three point bend specimen of the same material to predict cleavage fracture.

Structural response of a nuclear power plant steel containment under H2 detonation

Abstract At present studies are in progress at ENEA-DISP to assess the performance of a steel containment under hydrogen detonation. In this framework a number of topics needs to be investigated and they are considered in the present paper. To get a better understanding of the containment wall behaviour under a detonation a simple but complete model is analysed in order to study the fluid-structure interaction during the explosion. The structure is represented by a single degree of freedom (SDOF) elastic-plastic system. This system is coupled to a monodimensional model of the containment atmosphere excited by hydrogen bursting. The atmosphere modeling allows to represent the shock propagation and the reflected wave effects. In the model a cylindrical geometry is used as reference. The obtained results are compared with data adopted in Italy to assess the structural integrity of the Alto Lazio NPP steel containment in the case of a severe accident. The limits of the model as well as the possible extensions are discussed in the paper together with a possible application in an experimental program directed to the assessment of failure criteria under severe accident conditions.

Ultimate strength and rupture modes of the metal containment of a BWR NPP in case of severe accident

Abstract The purpose of this paper is to present an assessment of the BWR MARK III steel containment under severe accident conditions. The accident follows a station black-out with the unavailability of the emergency cooling system, core melting and RPV rupture. The performance of the containment vessel has been evaluated with and without a possible flaw in the most stressed regions of the shell. The accident conditions assumed in the analysis derive from the rise of the March 3 model. The response of the containment to those conditions has been evaluated by means of the elastic-plastic finite-element computer code MENTAT-MARC with large displacements. The flow-stress is locally achieved under a differential pressure of about two times the calculated severe accident maximum pressure and about eight times the design LOCA pressure. It appears that the BWR steel-containment vessel of the MARK III type can withstand high loads deriving from a severe accident even if the presence of a flaw is assumed. Penetrations represent the most limiting areas which may lead to a leakage well before instability.