E. Roos

A procedure for the experimental assessment of the J-integral by means of specimens of different geometries

Abstract It is necessary to determine the elastic-plastic fracture mechanics parameter J on specimens of different loading mode and size in order to further clarify the dependence of the J- integral on these parameters. Using the fundamental equations of linear-elastic fracture mechanics and fully plastic material behaviour, the factors η el and η pl for the experimental evaluation of the J- integral for different specimen geometries are determined. The basic equations used are summarised, the relations obtained for η el and η pl are plotted. The application of these relations in the testing of large scale specimens of different specimen type and size shows the variation of the J- integral over the stable crack growth.

An experimental and numerical investigation of fracture resistance behaviour of a dissimilar metal welded joint

Abstract Dissimilar welds impose a challenge to the engineers concerned with the structural integrity assessment of these joints. This is because of the highly inhomogeneous nature of these joints in terms of their microstructure, mechanical, thermal, and fracture properties. Fracture mechanics-based concepts cannot be directly used because of the problems associated with the definition of a suitable crack-tip loading parameter such as J-integral crack tip opening displacement (CTOD), etc. Again, depending upon the location of initial crack (i.e. base, weld, buttering, different interfaces, etc.), further crack propagation can occur in any material. The objective of the current work is to use micro-mechanical models of ductile fracture for initiation and propagation of cracks in the bimetallic welds. The authors have developed a finite element formulation that incorporates the porous plasticity yield function due to Gurson—Tvergaard—Needleman and utilized it here for the analysis. Experiments have been conducted at MPA Stuttgart using single edge-notched bend (SEB) specimens with cracks at different locations of the joint. The micro-mechanical (Gurson) parameters of four different materials (i.e. ferrite, austenite, buttering, and weld) have been determined individually by simulation of fracture resistance behaviour of SEB specimens and comparing the simulated results with those of the experiment. In order to demonstrate the effectiveness of the damage model in predicting the crack growth in the actual bimetallic-welded specimen, simulation of two SEB specimens (with initial crack at ferrite—buttering and buttering—weld interface) has been carried out. The simulated fracture resistance behaviour compares well with those of the experiment.

Evaluation of different fracture-mechanical J-integral initiation values with regard to their usability in the safety assessment of components

Abstract Determining fracture-mechanical material characteristic values on the basis of the J -integral is described and stipulated in a variety of standards and guidelines. The individual specifications differ in terms of procedure when determining the characteristic values and, therefore, also in terms of the meaningfulness of the results. This paper presents the different procedures, suggested in the course of the development of test methods in the field of elastic—plastic fracture mechanics, used to characterize crack initiation behaviour with regard to their features as material characteristic values and their usability in the safety assessment of components.

Assessment of large scale pipe tests by fracture mechanics approximation procedures with regard to leak-before-break

Abstract On the basis of numerous tests which were performed to demonstrate the “Leak-Before-Break” (LBB) behaviour predictions based on analyses could be further developed and substantiated. The methods applied (local flow stress concept, plastic limit load concept and two criteria approach) for the calculation of failure loads in pipes with circumferential cracks subjected to bending moment and internal pressure loading are evaluated and compared to the results gained by experiments. LBB predictions on the basis of the methods described show that reliable statements can be made. This has been confirmed by the experiments performed.

The influence of the material toughness and the state of stress on fracture of large scale specimens

Abstract The influence of the material toughness on fracture behaviour of large scale tensile specimens is being described in this report. Selected large scale specimens show that the amount of stable crack growth prior to instability is affected by the constraint in the specimen. The constraint depends on the specimen geometry, the specimen size, and in the case of elastic plastic material behaviour on the ductility of the material. A possibility of estimating these influences is presented by quantifying the constraint with the multi-axiality quotient q .

Application and evaluation of different numerical methods for determining crack resistance curves

Abstract The engineering assessment of precracked components is frequently carried out with the help of crack resistance curves on the basis of the J- integral . Nevertheless, there are severe uncertainties regarding the validity of the J- integral in the case of large plastic deformations and unloading processes due to crack growth. Numerical and theoretical derivations are used to examine the influence of large plastic deformations and stable crack growth on the calculation of the J- integral . Numerical investigations were carried out on the example of a CT 25-specimen made of 20 MnMoNi 5 5 by means of the finite-element method. The following methods of calculations were selected: • - Calculation of the specimen behaviour without consideration of the stable crack growth. • - Calculation of the specimen behaviour using a J-Δ a -curve as crack crack growth law. • - Calculation of the specimen behaviour using a damage model (“ocal approach”) to compute the crack growth. The results of the calculation carried out on the basis of the damage model makes it possible to carry out an assessment of the various methods for the experimental determination of the J- value .

(Cr-Al) bi-layer coatings obtained by ion assisted EB PVD on C/C-SiC composites and Ni-based alloys

Abstract Cr and (Cr–Al) bi-layers were deposited on carbon reinforced composites (C/C–SiC) and Ni-based alloy substrates by ion assisted EB PVD. The ion current in the substrate region was measured using a special ion detector. Surface morphology of the films was studied using optical, scanning electron, and atomic force microscopy. The variation of the deposited elements was determined by means of a line-scan electron probe on the cross-sections of the samples. Interdiffusion of the deposited elements at the interface was observed. It could have been caused by ‘ion mixing’ during the deposition process. X-ray diffraction (XRD) investigations show the formation of intermetallic compounds Cr3Si andCrAl. Comparative measurements proved that the microhardness of Cr layers on Ni-base alloys, obtained by ion assisted deposition was higher than that of Cr layers deposited without the acceleration of ions.

Behaviour of Ni-Based Alloys for Fossil-Fired Power Plant Components in the Long-Term Creep Regime

High efficiency steam power plants are planned to operate at temperatures higher than 700°C and at a pressure of up to 350 bar. Due to this increase of the steam parameters, Ni-based alloys are required for constructing these plants. Materials testing - based on appropriate manufacturing and design criteria - is necessary in order to have a reliable data base of the relevant design characteristics. Additionally, a better understanding of the specific material behaviour under service like loading conditions for the evaluation of possible damage mechanisms is essential. This paper describes research on the behaviour of thick-walled power plant components made of Alloy 617 mod. and Alloy 263. Results from basic qualification programs with standard specimens including welded joints show the applicability of the materials. Results from creep rupture tests of base material and welded joints and microstructural investigations to obtain information on precipitations and dislocations in the virgin and aged conditions are presented. Information on the influence of chemical composition on creep rupture was obtained by analysing creep rupture data sets.

Numerical and experimental investigations into life assessment of blade–disc connections of gas turbines

The positively engaged connection between blade and disc of a gas turbine is highly stressed by fatigue and creep fatigue loadings. For this purpose, a new calculating method based on inelastic finite element analyses considering the main influences on damage was developed at MPA Stuttgart. Low cycle fatigue (LCF) tests with component-like specimens have been conducted for verification. Experimental data and life assessment results based on the Smith, Watson and Topper parameters were compared well.

Some conclusions with regard to the safety assessment of cracked components drawn from the research programme “integrity of components” (FKS II) atthe present state

Abstract The continuation of the research program “Integrity of Components”, Phase II, mainly deals with further evaluation and assessment of material properties and the application of data from small standard specimens to large scale specimens and components. This includes the use of advanced numerical methods to check the transferability of fracture mechanics parameters with regard to the type of load and degree of multiaxiality on the failure behaviour of fracture mechanics specimens with component-like dimensions. Further points of interest are the relationship between upper shelf toughness and load-bearing capacity, the influence of neutron irradiation on the properties, and the effect of corrosion on cyclic crack growth.