Leonardo Bertini

Fatigue crack propagation through residual stress fields with closure phenomena

Abstract The paper deals with the evaluation of the effects produced by residual stress fields on fatigue crack propagation. After a short discussion of general phenomenological aspects, the results of fatigue crack growth tests on compact tension specimens containing residual stresses due to welding are summarized. Two numerical methods, i.e. the finite element method and the weight function method, are then applied to model the effects produced by the residual stress field, allowing a detailed analysis of the experimental data to be obtained.

Numerical analysis of plasticity effects in the hole-drilling residual stress measurement

The aim of this work was to analyze the effect of plasticity on residual stress measurement when the through thickness center-hole technique is used. The study investigated the effect of the most important loading, measuring, and material parameters, i.e., the residual stress intensity, the ratio between the principal residual stresses, the orientation of the strain-gage rusette with reference to the residual stress principal directions, the yield strength, and the strain hardening characteristics of the material. By means of a finite element simulation of the measurements, the errors that are usually produced by the direct use of ASTM E 837 (Test Method for Determining Residual Stresses by the Hole-Drilling Strain Gage Method) for the elaboration of the rosette strain gage readings was firstly determined by considering large enough ranges of the above-mentioned parameters to represent many conditions of practical concern. Afterwards it was shown that, at least within the limit of validity of the model, a considerable reduction of those errors can be obtained by using the proposed analytical procedure for elaborating the readings of a classical these elements strain-gage rosette. Moreover, the use of a new type of rosette with four radially oriented strain gages was proposed which could lead to further improvement of the measurement accuracy in any considered condition.

Recent advances in the hole drilling method for residual stress measurement

The main activities in the hole drilling residual stress measurement technique recently developed at the University of Pisa are reviewed and presented. Particular attention was paid to developing tools for increasing the limits indicated by the presently applied standard procedures for residual stress evaluation. For residual stresses that were assumed to be uniform through-thickness, the effect of plasticity was numerically analyzed and results formed the basis for a procedure that allows an increase in the maximum measurable residual stress up to 0.9 of the material yield strength. For nonuniform through-thickness residual stress, accurate analytical influence functions are proposed by which arbitrary interpolation of the influence coefficients is avoided and all the experimentally obtained strains, with no regard to their number, can be used as input for residual stress evaluation.

Weight function for an inclined edge crack in a semiplane

The matrix-like structure of the Weight Function (WF) for determining the Stress Intensity Factors (SIFs) in a nonsymmetric plane body is obtained from the general properties of the elastic field. General asymptotic and symmetric properties of the WF are discussed. By extending a previously proposed methodology, an analytical approximate WF is determined for an edge crack in a semiplane within the range of inclination (-75^–75^). Finite Element evaluations considering the minimum number of loading conditions were performed to this purpose. The accuracy of the SIFs obtained by the WF is found in the order of a few tenths of percent. The solution of a typical problem illustrates the practical usefulness of the WF.

Influence of post weld treatments on the fatigue behaviour of Al-alloy welded joints

In this paper the influence of different post welding treatments, such as ageing or shot peening, on the fatigue behaviour of Al-alloy welded joints was investigated. The analysed joints were candidates for car structural applications. Several four point bending fatigue tests were conducted on GMAW specimens subjected to different post weld treatments. The residual stress field acting on specimens was measured by the X-ray diffraction (XRD) technique. The results of tests were discussed with the aid of a finite element model of the specimen aimed to calculate the actual fatigue cycle, also taking account of residual stresses and of their redistribution during the test. This allowed to characterize the fatigue resistance of the joints, taking account of the effective stress acting in the region of crack initiation.

A procedure for evaluating high residual stresses using the blind hole drilling method, including the effect of plasticity

When the blind hole drilling method is used to evaluate high residual stresses in a metallic component, plastic relaxed strain can be produced in the hole region because of the stress concentration that causes the local stresses to reach yielding. By assuming a linear–elastic behaviour of the material, a significant error can result. The present paper analyses the phenomenon of the plasticity locally induced by the introduction of the hole and proposes a procedure to take into account its effects on the residual stress evaluation. The correcting procedure has been developed by elaborating a large database of elastic–plastic finite element analyses performed considering a wide range of material properties and testing parameters, including all the strain gauge rosettes commonly used. As plasticity induces non-linearity in the relationship between residual stress and relaxed strain, the superposition principle cannot be applied, so the correction is limited to uniform in-depth residual stress fields. However, four hole depths were considered and the related correcting procedures were provided. When variable through thickness residual stress is expected, and high residual stress is confined near the surface region, the correction procedure can be applied to an initial limited depth.

Fatigue behaviour of induction hardened notched components

Abstract In this paper the results of a numerical and experimental study concerning the effect of induction hardening on the fatigue behaviour of mechanical parts is presented. The study was performed on the UNI50CrV4 steel. Rotating bending fatigue tests were carried out in order to obtain the basic fatigue properties, while three point bending tests were carried out on notched specimens in the “as tempered” and in the “induction hardened” condition. Residual stresses produced by the induction hardening were evaluated by means of a numerical-experimental technique, making use of XRD measurements. A finite element model of the notched specimen was set up for the analysis of the stress field acting during fatigue loading; this allowed a prediction of its fatigue behaviour, which appeared to be in satisfactory agreement with experimental observations.

Measurement of coatings' elastic properties by mechanical methods: Part 2. Application to thermal barrier coatings

Elastic properties of a thermal barrier ceramic coating composed of an NiCoCrAIY bond coat and a ZrO2(Y2O3) top coat were measured by a four-point bending rig in the temperature range 20°C–900°C. Different types of specimens (i.e., with bond coat only or with bond coat and top coat, on one side or on both sides) were employed. Test procedures were based on the theory discussed in Part 1 to enhance accuracy and to estimate confidence intervals. In particular, the method employed at high temperature was calibrated at room temperature by comparing the results with those obtained by methods with low sensitivity to layer thicknesses. For the bond coat, Youngs modulus was found to be temperature independent up to about 500°C; a decreasing trend was observed above this temperature. For the top coat, a slightly temperature range examined. A possible explanation is given on the basis of phase transformation and the microstructure of the two layers. At room temperature, Poissons ratio for the bond coat was found to be near 0.3, whereas a near zero value was measured for the top coat.

Strain gradients in plasma-sprayed zirconia thermal barrier coatings

Abstract Neutron diffraction was used to measure the residual strain field in plasma-sprayed zirconia thermal barrier coatings (TBCs). Data were collected at the British neutron spallation source of ISIS (Didcot), on ENGIN, a recently installed TOF (time-of-flight) instrument designed for residual strain depth profiling. Its particular geometry permitted a direct measurement of the interplanar distances of crystallographic planes lying parallel to the component surface as well as the measurement of zero-strain reference samples. The latter were annealed samples of the three present phases: zirconia (top coat), NiCoCrAlY (bondcoat), and copper (substrate). In this way ϵ 33 , the strain component perpendicular to the sample surface, was determined at several positions inside the component, for all the present phases. The results of this analysis, consisting of a strain profile throughout the entire cross-section of the coated component, were integrated by those obtained by a destructive testing, performed after TOF data collection, consisting in the measurement of curvature change of the ceramic after substrate removal by chemical attack.

Residual stress in partially-stabilised-zirconia TBCs: experimental measurement and modelling

Abstract The residual stress field in 7wt% yttria partially stabilised zirconia (Y-PSZ) plasma sprayed (300-μm thick) coatings on Al substrates was studied by means of two different techniques: X-ray residual stress analysis (XRSA), and elongation and curvature measurement after substrate removal (CD). Results were considerably improved with respect to conventional XRSA, by using a synchrotron radiation source and specifically designed high resolution optics: an extended beam penetration, achieved by using different wavelengths down to 0.94 A allowed the determination of the strain field in the outer 50 μm of the ceramic. XRSA (surface) and CD (volume averaged) strain data were analysed by a mechanical model of the coated component capable of integrating the results of both techniques. The procedure, designed to appropriately consider the different nature of the information coming from the two measurement techniques, permitted us to study the residual stress distribution through the entire Y-PSZ coating thickness. In particular, it pointed out the presence of a considerable stress gradient in the surface region of the ceramic.