Marco Beghini

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.

A Sensor-Integrated Tool for Cutting Force Monitoring

Abstract This paper describes the development of a new concept of cutting tools using strain gages for the measurement of forces in turning operations. The basic idea is the integration of the sensor within the tool shank, in order to obtain a system which is easy to use, easy to install and capable of transmitting data to the CNC through wireless equipment. In particular, the output signal of the measurement bridge is amplified and sent to an external data acquisition system by infra-red transmission. The present paper reports the design principles and the results of some machining tests illustrating the behaviour of the tool in different cutting conditions.

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.

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.

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.

Stress intensity factors for an inclined edge crack in a semiplane

Abstract Mode I and II Stress Intensity Factors under uniform general biaxial loadings were derived for an inclined edge crack in a semiplane. By interpolating Finite Element results in the angular range [0°÷80°], analytical expressions were obtained for both KI and KII with an accuracy better than 1%. Influence coefficients were defined in the crack reference frame thus highlighting the coupling effects between Modes I and II due to the loss of symmetry when the crack is not normal to the surface.

Effective stress intensity factor and contact stress for a partially closed Griffith crack in bending

Abstract A method based on the weight function for solving the problem of partially closed cracks is proposed. An explicit exact expression for the effective stress intensity factor and an approximate analytical expression for the contact stress was determined for the Griffith crack in bending. The solution was obtained without the evaluation of displacement. The method requires modest computational resources and it can be employed also in the study of more complex problems having practical relevance. By means of the obtained results the effect of closure on the effective stress intensity factor was accurately determined and it was shown that, in many circumstances, a simplified linear approach (not including the effect of contact) is not adequate.

Residual stress modelling by experimental measurements and finite element analysis

Abstract The paper describes an experimental-numerical technique for modelling plane residual stress fields with application to steel slabs containing laser welds of different geometry. This technique is based on the analysis of the strain measured by several small-size strain gauges as a function of the depth of a narrow slot which was cut in the slab itself in small increments. This information was combined with sensitivity coefficients obtained via finite element models in order to derive initial strain distributions for the welded plates. These distributions allowed a prediction to be made of the residual stresses acting in generic bodies extracted from the original plates. The stress fields predicted for square specimens cut from the slabs were then compared with the measured values, showing a fairly good agreement.