D. Sturm’s research while affiliated with University of Stuttgart and other places

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.

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.

Since 1993 MPA Stuttgart is engaged in two new research projects in the field of Leak-Before-Break (LBB) behaviour of austenitic piping. The pipes are fabricated in the dimensions O.D. × t = 219 × 14 mm resp. 331 × 32 mm and prepared with fatigue precracked, circumferential flaws of various depth and up to a crack length equivalent to 2α = 270 deg. The flaw location is in the base material or in sitemade girth welds. The pipes are loaded with internal pressure and external bending moment at room temperature. In this paper the results of the tested pipes with 219 mm diameter containing through-wall flaws are represented as a comparison with the results of applied analytical methods. On these pipes the crack initiation clearly occured before maximum bending moment, which was attained under full yielding conditions even in case of the 180 deg through-wall flaw. With the applied analytical methods crack initiation as well as maximum load could be assessed in a realistic manner.

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.

Over the last 35 years, researchers worldwide have conducted hundreds—if not thousands—of pipe fracture experiments. In the early years, researchers focused their attention on studying the failure pressure and crack propagation behavior of axially cracked pipe loaded by internal pressure. The earliest work was sponsored by the oil and gas industry and, as such, involved relatively thin-walled, low toughness carbon steel pipes. This work was eventually followed up by efforts in the USA and Germany on nuclear piping with axial cracks. In recent years, attention has turned to understanding the behavior of circumferentially cracked nuclear piping subjected to both pressure and bending loads. The loading histories for these experiments range from the relatively simple case of quasi-static, monotonic displacement control to the more complex cases of dynamic cyclic loading, and pipe system experiments. In this paper, two of the leaders in this research, i.e. Battelle in the USA and MPA Stuttgart in Germany, have collaborated to develop a database of pipe fracture experiments. The database includes data from other organizations as well as the data from Battelle and MPA. In addition, as part of this paper, an example of how the database was used to assess the failure pressure of axially cracked pipe is given.

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.

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.

Pipes with an outer diameter of 406 mm, a wall thickness of 12.5 mm and a length of 1000 mm were tested under tensile loading. They were prepared with part-circumferential flaws of uniform length but different depth. A fine grained constructional steel having a low upper shelf ISO-notch impact energy ≤ 50 J at test-/room temperature was chosen as test material. With the aid of the test results, the predictions of established approximation methods for the calculation of maximum load and, as far as possible, the load at crack initiation were checked. The applied methods which are based on modelling of the pipe by means of flat tensile specimens proved to be unsuitable for estimation of the maximum load. The methods for maximum load evaluation which had been specially developed for pipe geometries showed mostly a good agreement with the experiments. The calculations based on elastic-plastic fracture mechanics gave results which underestimated maximum load, whilst the crack initiation loads were to some extent overestimated.

Pipes made of steel 20 MnMoNi 5 5 having an external diameter of 800 mm, wall thickness of 47 mm and length of 5 m were provided with circumferential flaws in defined length and depth. They were loaded simultaneously by internal pressure, alternating or pulsating external bending moment. Deformation and the crack growth were determined in the wall thickness and circumferential direction and compared with calculated values.

For the testing of large scale specimens, a 12 MN high-speed 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.