P. Lopez-Crespo

The stress intensity of mixed mode cracks determined by digital image correlation

A generalized approach for determining the stress intensity factors (SIFs) K I and K II for any mode mixity directly from displacement fields obtained by digital image correlation is presented using a centre fatigue cracked aluminium plate as an example problem. It was found that the crack-tip position could be determined on average to within 50 per cent of the displacement vector spacing (60 μm). The approach has been shown to be fairly robust, both in terms of the stability of the SIFs thus obtained and their sensitivity (less than 0.07 MPa

Evolution of crack-bridging and crack-tip driving force during the growth of a fatigue crack in a Ti/SiC composite

High spatial resolution diffraction and imaging using synchrotron X-rays are combined to monitor the incremental growth of a fatigue crack through the matrix of a Ti-6Al-4V/SCS-6 SiC monofilament metal matrix composite. X-ray tomography is used to quantify the crack opening displacement (COD) and diffraction to measure the crack-tip stress field in each phase, the wear degraded interfacial strengths, as well as the crack face tractions applied by the bridging fibres, at maximum () and minimum () loading as a function of crack length. In this way, it has been possible to quantify the crack-tip driving force (the stress intensity range effective at the crack-tip) in three ways: from the COD, the bridging stresses and the crack-tip stress field. The fibre stresses act to prop open the crack at and shield the crack at such that the change in COD is small over the fatigue cycle. Consequently, the effective stress intensity range at the crack tip remains around 10 MPa√m as the crack lengthens, as more and more fibres bridge the crack despite the normally applied stress intensity rising to 60 MPa√m. The implications of the derived fracture mechanics parameters are assessed and the wider potential of X-ray diffraction and imaging for crack-tip microscopy is discussed.

Mixed Mode (KI+KII) Stress Intensity Factor Measurement by Electronic Speckle Pattern Interferometry and Image Correlation

The surface displacement fields of a fatigue precracked compact tension sample under tensile load were registered by electronic speckle pattern interferometry and image correlation. The in-plane elastic strain fields calculated from the displacement data were used to obtain the first stress invariant, which for the case of plane stress is proportional to the real part of the first complex potential in Muskhelishvilis approach. Solutions for the stress fields around the crack tip, KI and KII were sought in the form of the Fourier series using Muskhelishvilis complex stress functions. The Fourier series coefficients were calculated from the displacement data using multiple point overdeterministic method (MPODM). The nominal and inferred KI values differ by around 10%; this is probably due in part to mixed mode (KII) loading introduced by some degree of misalignment during the experiment.

Numerical and Experimental Analysis of Crack Closure

The fatigue life of metallic materials is strongly influenced by crack closure effects. Finite element (FE) methods allow the study of crack closure with great detail and can provide valuable information about phenomena occurring in the bulk of the material. In this work the distribution of stresses through the thickness of a cracked specimen has been studied using 3D FE simulations. It was found that the transition between the interior of the specimen (plane strain) and the surface (plane stress) differs from that predicted by 2D plane stress models. In addition, an attempt is presented to experimentally validate the results at the surface level. For this purpose full-field image correlation technique was utilized. This allowed direct comparison between the displacement field predicted by the numerical simulations and the experimental results measured by digital image correlation.

2D mapping of plane stress crack-tip fields following an overload

The evolution of crack-tip strain fields in a thin (plane stress) compact tension sample following an overload (OL) event has been studied using two different experimental techniques. Surface behaviour has been characterised by Digital Image Correlation (DIC), while the bulk behaviour has been characterised by means of synchrotron X-ray diffraction (XRD). The combination of both surface and bulk information allowed us to visualise the through-thickness evolution of the strain fields before the OL event, during the overload event, just after OL and at various stages after it. Unlike previous work, complete 2D maps of strains around the crack-tip were acquired at 60m spatial resolution by XRD. The DIC shows less crack opening after overload and the XRD a lower crack-tip peak stress after OL until the crack has grown past the compressive crack-tip residual stress introduced by the overload after which the behaviour returned to that for the baseline fatigue response. While the peak crack-tip stress is supressed by the compressive residual stress, the crack-tip stress field changes over each cycle are nevertheless the same for all Kmax cycles except at OL.

High magnification crack-tip field characterisation under biaxial conditions

This work presents a novel methodology for characterising fatigue cracks under biaxial conditions. The methodology uses high magnification Digital Image Correlation (DIC) technique for measuring displacement and strain crack-tip fields. By applying micro-speckle pattern on the metal surface it is possible to achieve high magnification for DIC technique. The speckles were created by electro-spray technique. The validity of this novel technique is demonstrated by direct comparison with standard extensometer measurements, under tension-compression and torsion conditions. In order to image the correct region, the notch effect on the fatigue life was also evaluated.

Prediction of Fatigue Life of Structural Steel S355-J2G3 with SK Critical Plane Model

The present work assesses the fatigue life prediction capability of a recently proposed critical plane model. For this study, multiaxial fatigue data of S355-J2G3 steel were used; in-phase and 90o out-of-phase sinusoidal axial-torsional straining from 103 to 106 cycles, so it was possible to evaluate the model at low and high cycle fatigue, as well as the hardening effect. The damage parameters considered in this paper include the effect of hardening, mean shear stress effect and the effect due to interaction of shear and normal stress on the critical plane. A comparative evaluation of well accepted models (Wang-Brown, Fatemi-Socie and Liu 1 and 2) with the new recently proposed model (Suman-Kallmeyer) is done. The ability of the different models to predict the fatigue life for large and diverse load data set are discussed.

Investigation of the Biaxial Behaviour of 316 Stainless Steel Based on Critical Plane Method

In this work the biaxial behavior of 316 stainless steel is studied under the lens of critical plane approach. A series of ten experiments were developed on dog bone shape hollow cylindrical specimens made of type 316 stainless steel. Five different loading conditions were assessed, with (i) only axial stress, (ii) only hoop stress, (iii) proportional combination of axial and hoop stresses, (iv) non-proportional combination of axial and hoop stresses with square shape and (v) non-proportional combination of axial and hoop stresses with L-shape. The fatigue analysis is performed following four different critical plane theories, namely Wang-Brown, Fatemi-Socie, Liu I and Liu II. The efficiency of all four theories is studied in terms of the accuracy of their life predictions.

On the perceptions of students and professors in the implementation of an inter-university engineering PBL experience

ABSTRACT The new educational paradigm has led to a change in the teaching methodologies toward those more focused on the student, among these, project-based learning (PBL) is postulated as one of the most promising. This work is focused on the description of the experience of using PBL methodology in Materials Science courses, conducted by four different Spanish universities on different engineering degrees. The other main objective is to analyse and evaluate how the PBL was perceived by the students and the lecturers that took part in the PBL process. This investigation was an embedded, sequential mixed-methods study, which began by administering a survey to one hundred and four students and then a focus group with six students and six lecturers in four different engineering degrees of four Universities. Results generally show a good degree of acceptance of this approach by all parties involved.

Mid-thickness studies of the stress intensity factor in the bulk of bainitic steel

The current work aims at estimating the stress intensity factor deep inside the bulk from elastic strain data measured by synchrotron X-ray diffraction. Key features affecting the evaluation of the stress intensity factor are the number of terms in the analytical model describing the crack tip field, the extension and position of the area of interest of the experimental data, the effect of the experimental data collected within the plastic zone and the number of elastic strain data points used. Here a parametric study of these features is presented in terms of their influence for the stress intensity factor determination. It was found that 3 or 4 terms in Williams’ expansion is often sufficient; the data should be collected from across the full range of angles around the crack tip; and the number of points/number of terms should be greater than 40.