Gyula Nagy

Assessment of Methods in Girth Welds of Steel Pipelines

This paper presents two basic methods for the assessment of failed girth welds of steel hydrocarbon transporting pipelines. One of them is based on the principles of linear elastic fracture mechanics (LEFM) and stress intensity factor conception for planar material discontinuities, and the other can be used for the complex assessment of all kinds of occurring defects. The results of the presented methods are compared to the results of burst test of pipeline sections containing a failed girth weld and cut from a Hungarian gas pipeline.

The Role of the External and Internal Reinforcing on the Structural Integrity of Industrial and Transporting Steel Pipelines

The lifetime management of different engineering structures and structural elements is one of the important technical-economic problems nowadays. On the one hand, the aim of our research work is to develop an integrity management plan for pipelines and pipeline systems, and afterwards a Pipeline Integrity Management System. Material databases play important role both on the integrity management and on the engineering critical assessment of the pipeline systems. On the other hand, the aim of our research work is to establish the Pipeline Integrity Management System with different data, frequently with experimental data. The direct purpose of the paper is to present the role of the external and internal reinforcing on the structural integrity of industrial and transporting steel pipelines, based on own examinations. External and internal reinforcement was developed using carbon fibre and glass fibre polymer matrix composites, respectively. Fatigue and burst tests were performed on pipeline sections containing natural and artificial metal loss defects, and girth welds including weld defects. Both unreinforced and reinforced pipeline sections were examined. The burst pressures belonging to the unreinforced and the reinforced pipelines, and belonging to the passed and not passed girth welds were compared.

Connection among the Characteristics of the Low Cycle Fatigue, High Cycle Fatigue and Fatigue Crack Growth

The material quality, the deformation rate, the temperature and the stress state influence mechanical behaviour and properties of different materials. Due to this great variety of the influencing factors we do not have one model of general validity describing the behaviour of materials, but we have to use a great number of material constants in order to characterize the properties. The exponents of the Manson-Coffin, the Basquin and the Paris-Erdogan laws were applied for the verification of the connection among the fatigue fracture types. Own measured values and test results can be found in the literature were used for the illustration of the connections. “Fracture surface”-s were determined for characterizing of different steel grades and their welded joints. It can be concluded that “fracture surface”-s are suitable for the describing of the fracture behaviour and the conversion of different fracture parameters of steels.

Connection between the Results of the Low-Cycle Fatigue and the Fatigue Crack Growth Tests

Material characteristics determined for low cycle fatigue (LC F), high cycle fatigue (HCF) or fatigue crack growth (FCG) are different. The question aris es, whether common mechanical and microstructural features can be found or not in this field of loadin g types, based on which relationship among the materials constants can be assumed. The paper introduces the similarity of the stress and strain state, furthermore the dislocation structure having been developed during low cycle fatigue and fatigue crack growth. This can form a basis f or the establishment of a connection between the material characteristics, the exponents of the Mans on-Coffin equation and the ParisErdogan law. The validity of the hypothesis has been illustrated he re by comparing the test results determined for different material grades, steels, one superalloy and a luminium alloys.