D. Sturm

Leak-before-break behaviour of austenitic and ferritic pipes containing circumferential defects

In recent years several research projects have been carried out at MPA Stuttgart to investigate the leak-before-break (LBB) behaviour of pressure-bearing components which are relevant to plant safety. In these investigations the test pipes have for the most part been made of ferritic material. International research programmes such as, for example, the Degraded Piping Programme (Wilkowski et al., 1986, 1989. Degraded Piping Program, Phase II. Report NUREG/CR-4082, vol. 4, Sept. 1986, and vol. 8, March 1989, Battelle, Columbus, Ohio, USA) or the IPIRG-Program (Schmidt et al., 1991. The International Piping Integrity Research Group (IPIRG), Program-An Overview. SMiRT 11 Proceedings, Paper G23/1, Tokyo, Japan, August 1991) have also dealt with pipes made of austenitic materials. However, they were fabricated of not stabilized quality. To take into account the material of comparable components of German nuclear power plants, the experiments reported in the following are focussed on pipes made of Ti- and Nb-stabilized austenitic material. The results presented below relate to pipes containing circumferential defects subjected to internal pressure and external bending loading. As regards the ferritic components an overview of the experimentally determined results is presented. The predictive capability of engineering calculational methods are presented by way of example. The current programme of investigations is presented together with the testing techniques and the initial results.

Resonant excitation of a pipe section

Abstract During the recent phase of the project “vessel failure” tests have been carried out on pipe sections under fast external cyclic bending. In connection to former examinations with slowly alternating bending loading now the resonance effect with high accelerations of masses together with energy dissipation due to material plastification was taken into account. In this contribution the design calculations and their results are described as well for the pretests with small dimensions as for the main tests with dimensions of outer pipe diameter 250 mm × 32 mm wall thickness. The main reason for performing pretests was to find out the right regulating technique for the load controlled excitation mechanism and to have a verification for the calculational model. A comparison between the predicted calculated system response and the measured one is pointed out. A good agreement between calculation and measurement was observed. An outlook to the kind of the main tests is given by calculation.

The behaviour of dynamically loaded pipes with circumferential flaws under internal pressure and external bending loads

Abstract For the determination of the load bearing capacity, the deformation, leak-before break- and fracture behaviour, bending tests on vessels out of ferritic and austenitic materials containing a circumferential defect were performed. The vessels were loaded over a temperature range extending from 20°C to 580°C by internal pressure and additionally by an externally applied quasi-static or dynamic bending moment. For the test a 4-point bending test rig providing a bending moment up to 3 MNm was used. Parallel to the tests the failure curves as well as the leak-before-break curves were calculated with the aid of engineering type calculational methods. No significant difference as far as load bearing capacity is concerned, can be established between quasi-static and dynamic loading. In most cases fracture occurred after considerable plastic deformation in the test cross section. The engineering type methods applied to the theoretical calculation of the leak-before-break curve on the corresponding defect curve proved to be conservative with respect to safety.

Analysis of the failure behaviour of longitudinally flawed pipes and vessels

Abstract A post-calculation by means of four engineering approaches, based on toughness, yield stress, plastic instability and ligament stress criteria, was made of the failure pressure on 134 pipes and vessels. The calculation was assessed by comparing the calculated with the experimentally obtained results. A statistical-based evaluation was made, since the results from the calculation and the experiment are affected by natural scattering of characteristic values, such as material properties and geometrical dimensions, among others. It was possible to find for each equation an individual weighting factor, which helped to improve considerably the approximation of the calculation to the experimentally determined failure pressure.

Exclusion of rupture for welded piping systems of power stations by component tests and failure approaches

High temperature seamless pipes of dimensions O.D. × T × L = 457 × 15 × 5500 mm3 made from 15 NiCuMoNb 5 material were loaded by internal pressure and a superimposed external bending moment. 27 tests with different geometries and positions of circumferential flaws in the base metal or in circumferential welds were performed at room temperature. Depending on the geometric conditions, the load bearing and failure behaviour of the investigated pipes could clearly be represented in a common failure diagram. Significant differences in maximum loads due to the position of the flaws in the structure, or whether inside or outside, could not be detected. Maximum loads were compared with calculations using applied engineering failure approaches, based on concepts of local flow stress and plastic limit load. Elastic-plastic fracture mechanics was applied to some of the component tests and the initiation and instability criteria verified.

Die Bruchgefahr bei Reaktordruckbehaeltern und Rohrleitungen

Zusammenfassung By proper material selection, design and analysis, together with the applicable material testing-methods, bursting of pressure-vessels and pipings may be excluded, even if the components contain material defects to a certain degree. The methods of strength calculation applied are based on the ability of the material to plastic deformation, which therefore must be guaranteed under all service conditions and possible states of the material. This paper deals with unanswered questions in the field of safety-analysis. Some of these concern embrittlement phenomena caused by unfavourable states of stress — i.e. with multi-axial residual tensile stresses, so far not considered in fracture mechanics —, furthermore the effective material properties in the finished structural component (basic material, welded joints with local embrittlement and/or stress-relief embrittlement) including the influence of service conditions and the question of the allowable maximum nominal stress. In the case of seamless and normalized or quenched and tempered uncomplicated structure members, there exist relatively simple relationships between specimens or models and the real component. This is generally not true for thick-walled and/or welded structures even if the latter are stress relief heat-treated. Such structures may collapse at nominal stresses far below the yield stress, as soon as a critical value of the residual stresses is exceeded if simultaneously unfavourable material conditions exist. These statements are based on tests with unnotched and notched plates, pipes and pressure vessels with liquid and gaseous pressure media under static or repeated loading, furthermore on the analysis of damaged components.

Crack behaviour of pipes under internal pressure and simultaneous external bending moment

Abstract To investigate the strength behaviour, the influence of toughness to crack initiation and instability, the strain behaviour, the crack opening as well as the leak-before-break behaviour of artificially cracked piping, pipe bend tests were carried out. The dimensions of the pipes were 800 mm O.D. and 47 mm wall thickness, similar to the dimensions of the main cooling piping system of a 1300 MWe German PWR plant. Two types of nuclear grade materials similar to A 533 Cl 2 were used with high upper shelf impact energy of 50 and 150 J. The pipes were weakened by circumferential oriented surface notches respectively by through-wall flaws having a length of 60 degrees and various depths. The 5 m-long pipe, closed on both ends, was mounted into a newly developed 10 MNm 4-point bending device for loading. After heating up to 300°C and pressurizing up to 15 MPa, the bending moment was increased until failure occurred.

Load bearing and deformation behaviour of dynamically loaded wide plate specimens

Abstract For the testing of large-scale specimens, a 12 MN-High Loading Rate Tensile Testing Machine was designed and built at MPA Stuttgart. The aim was to determine the influence of high loading rates on the stress and strain behaviour of unwelded and welded components of ferritic and austenitic materials. This new generation of testing machines is driven by a propellant charge, and generates a maximum tensile force of 12 MN with a piston velocity of 25 m/s after a stroke of 20 mm, or a maximum velocity of 60 m/s after a stroke of 400 mm. In a first test programme, welded and unwelded wide plate specimens made of material X 6 CrNi 18 11 were tested at room temperature with different strain rates from 10 −3 /s to 63/s. In addition to a description of the 12 MN-High Loading Rate Tensile Testing Machine, the results of the high loading rate tensile tests performed will be presented and compared with quasistatically tested wide plate specimens.

Crack initiation, crack growth and fracture behaviour of large diameter pipes with circumferential defects under internal pressure and superimposed alternating bending load

Pipes made of steel 20 MnMoNi 5 5 and MnMoNiV-special melt having an external diameter of 800 mm, wall thickness of 47 mm, and length of up to 5500 mm were provided with circumferential defects of defined length and depth. They were loaded by internal pressure and a superimposed alternating bending moment. During the tests deformation and crack growth were determined in the wall thickness and circumferential direction. Pipes with an outer diameter of 226 mm and a wall thickness of 20 mm were used to investigate the leak-before-break behaviour in the dynamic sphere. These pipes were also made of steel 20 MnMoNi 5 5 and a MnMoNiV-special melt and were loaded with internal pressure and an alternating bending moment. The excitation took place at the resonance frequency of the pipes. The pipes also contained circumferential defects of defined length and depth.

Crack growth and fracture behaviour of pipes with circumferential defects under internal pressure and superimposed alternating bending load

Abstract Pipes made of 20 MnMoNi 5 5 steel and an MnMoNiV special melt, with an external diameter of 800 mm, wall thickness of 47 mm and length of up to 5500 mm, were provided with circumferential defects of defined length and depth. The pipes were loaded by internal pressure and a superimposed alternating bending moment. During the tests, deformation and crack growth were determined in the wall thickness and circumferential direction, and these were compared with calculated values. Pipes with an outer diameter of 226 mm and a wall thickness of 20 mm were used to investigate the leak-before-break behaviour in the dynamic sphere. These pipes also were made of 20 MnMoNi 5 5 steel and an MnMoNiV special melt, and were loaded with internal pressure and an alternating bending moment. The excitation took place at the resonance frequency of the pipes. The pipes also contained circumferential defects of defined length and depth.