Paulo J. Tavares

Crack Closure Effects on Fatigue Crack Propagation Rates: Application of a Proposed Theoretical Model

Structural design taking into account fatigue damage requires a thorough knowledge of the behaviour of materials. In addition to the monotonic behaviour of the materials, it is also important to assess their cyclic response and fatigue crack propagation behaviour under constant and variable amplitude loading. Materials whenever subjected to fatigue cracking may exhibit mean stress effects as well as crack closure effects. In this paper, a theoretical model based on the same initial assumptions of the analytical models proposed by Hudak and Davidson and Ellyin is proposed to estimate the influence of the crack closure effects. This proposal based further on Walker’s propagation law was applied to the P355NL1 steel using an inverse analysis (back-extrapolation) of experimental fatigue crack propagation results. Based on this proposed model it is possible to estimate the crack opening stress intensity factor, , the relationship between quantity and the stress intensity factor, the crack length, and the stress ratio. This allows the evaluation of the influence of the crack closure effects for different stress ratio levels, in the fatigue crack propagation rates. Finally, a good agreement is found between the proposed theoretical model and the analytical models presented in the literature.

SIF determination with digital image correlation

Purpose – Hybrid methods, wherefore numerical and experimental data are used to calculate a critical parameter, have been used for several years with great success in Experimental Mechanics and, in particular, in fracture mechanics. The purpose of this paper is to report on the comparison of the strain field from numerical modelling forecasts against the experimental data obtained with the digital image correlation method under Mode II loading in fatigue testing. The numerical dual boundary element method has been established in the past as a very reliable method near singular regions where stresses tend to grow abruptly. The results obtained from the strain data near the crack tip were used in Williams expansion and agree fairly well with both the numerical results and the analytical solution proposed for pure Mode II testing. Design/methodology/approach – The work presented in this note is experimental. The proposed methodology is of an hybrid experimental/numerical nature in that a numerical stress int...

Accurate subpixel corner detection on planar camera calibration targets

Feature detectors have long been one of the touchstones of image processing. Most vision tasks are entirely dependent on the accurate determination of fiducial marks on images, which ultimately led to a quest for methods able to detect feature locations with high resolution. We report the development of an intensity-based subpixel corner detector based on the two-dimensional (2-D) Hilbert transform. Extensive testing of both accuracy and precision with live images finds the method adequate for subpixel detection at better than 10 -1 pixel accuracy. The subpixel corner detectors and the evaluation proposals to date are briefly reviewed, and the proposed method is described. The results are shown and discussed.

Compact tension fracture specimen: Experimental and computational implementations on stress intensity factor

This work concentrates on the characterization of the stress intensity factor range for a compact tension specimen tested under a uniaxial tensile fatigue loading condition. The experimental solution is obtained using a three-dimensional full-field optical technique, digital image correlation. The deformation field is measured and documented for distinct crack lengths. As a relevant fracture parameter, stress intensity factor is thus experimentally measured combined with a computational overdeterministic algorithm for different crack lengths. Moreover, to verify the performance of the proposed fracture model, the cracked compact tension specimen is elasto-statically resolved using advanced discretization techniques, such as the finite element method, the meshless radial point interpolation method and the meshless natural neighbour radial point interpolation method. The finite element method model is thereby analysed with ABAQUS© to enable computation of mode I stress intensity factor results based on strain energy release rate criterion for different crack measurements in addition to strain contours. Likewise, the resolution pattern is repeated for meshless methods, and analogous numerical solutions are thus obtained. Overall, the experimental and numerical stress intensity factor results are compared with an available solution (ASTM E647) exhibiting a reasonable agreement. The novelty of this investigation is the amalgamation of an experimental digital image correlation procedure with a computational overdeterministic algorithm and, most importantly, the meshless formulation performance in the linear elastic fracture mechanics.

Advanced image based methods for structural integrity monitoring: Review and prospects

There is a growing trend in engineering to develop methods for structural integrity monitoring and characterization of in-service mechanical behaviour of components. The fast growth in recent years of image processing techniques and image-based sensing for experimental mechanics, brought about a paradigm change in phenomena sensing. Hence, several widely applicable optical approaches are playing a significant role in support of experiment. The current review manuscript describes advanced image based methods for structural integrity monitoring, and focuses on methods such as Digital Image Correlation (DIC), Thermoelastic Stress Analysis (TSA), Electronic Speckle Pattern Interferometry (ESPI) and Speckle Pattern Shearing Interferometry (Shearography). These non-contact full-field techniques rely on intensive image processing methods to measure mechanical behaviour, and evolve even as reviews such as this are being written, which justifies a special effort to keep abreast of this progress.There is a growing trend in engineering to develop methods for structural integrity monitoring and characterization of in-service mechanical behaviour of components. The fast growth in recent years of image processing techniques and image-based sensing for experimental mechanics, brought about a paradigm change in phenomena sensing. Hence, several widely applicable optical approaches are playing a significant role in support of experiment. The current review manuscript describes advanced image based methods for structural integrity monitoring, and focuses on methods such as Digital Image Correlation (DIC), Thermoelastic Stress Analysis (TSA), Electronic Speckle Pattern Interferometry (ESPI) and Speckle Pattern Shearing Interferometry (Shearography). These non-contact full-field techniques rely on intensive image processing methods to measure mechanical behaviour, and evolve even as reviews such as this are being written, which justifies a special effort to keep abreast of this progress.

Three dimensional geometry characterization using structured light

In this paper I review several studies related with the development of the doctoral thesis of Paulo J. Tavares. The thesis was directed at implementing different three-dimensional surface acquisition methods with structured light fields and essentially describes these implementations, and the difficulties experienced thereof. Several innovative works have been done in the areas of 3D imaging with active image processing, Fourier transform phase analysis, coherent light fringe projection, phase-to-height calibration in phase measurement profilometry, feature detection in planar calibration targets for camera calibration and; fringe distortion correction resourcing to the Optical Transfer Function. This paper presents the main results obtained in those studies which were all performed at LOME - the Optics and Experimental Mechanics Laboratory of the Mechanical Engineering department of Porto University.

Digital image correlation through image registration in the frequency domain

The present work analyses displacement field computations for several digital image correlation test cases obtained through the application of image registration in the Fourier domain. The applied method relies on the properties of the discrete Fourier transform with regard to the cross-correlation function to find the rigid displacement between two subsets. The computed displacement fields were compared to their analogous values obtained through a commercial digital image correlation code. The test cases cover both two-dimensional and three-dimensional digital image correlation and make use of the ability of the developed method to calculate the translation and rotation between subsets from images at different loading states, as well as its capacity to match subsets from two cameras of a stereo rig. The presented analysis aims to contribute to the investigation of the use of correlation in the frequency domain as the basis for an alternative to conventional digital image correlation which could provide advantages in terms of robustness and efficiency, particularly in field measurements, where the lack of control over lighting and other factors impacting image quality requires a system which is optimised towards low dependence on the optimal quality of these conditions.

A nonlinear simulation of a bi-failure specimen through improved discretisation methods: A validation study

A robust and efficient scheme is rendered to elastoplastically study the material nonlinearity of structural components. In this investigation, a specimen manufactured from the aluminium alloy AA6061-T6 is considered. It is mechanically loaded under a uniaxial tensile state and the experimental strain datum is analysed by three-dimensional digital image correlation. Due to specific specimen geometry, complex stress states will occur. However, the specimen yields due to an approximated uniaxial stress state. The obtained remote stress/strain from experimental data is used to validate the computational solutions using advanced discretisation approaches. Therefore, as a preliminary numerical study, the model is simulated through the finite element method formulation. Afterwards, another numerical strategy is adopted – the radial point interpolation method. The Newton–Raphson initial stiffness method is thereby adapted to complete the nonlinear solutions algorithm. Furthermore, the elastoplastic demeanour of aluminium alloys is determined with the von Mises yield criterion, an isotropic hardening rule and an associative flow rule. Obtained computational results fit the experimental digital image correlation solution, which allow to conclude that the proposed meshless methodology is efficient and reliable.

An Automated Method for the Measurement of Fatigue Crack Progression

This letter describes the work conducted at our laboratory for the implementation of an automated vision system for fatigue crack growth measurement. The system relies on a dedicated illumination system with grazing incidence and optimized feature extraction by morphological image processing and continuous calculation of the crack growth, for adjustment of the optimal time interval for image registration.

Single frame method for resolution enhancement of the Fourier fringe analysis method

Fourier Transform Profilometry (FTP) is one of the most popular fringe analysis methods in Optical Metrology for a wealth of applications in Mechanical Engineering, such as shape profilometry or defect detection with shearography or holographic interferometry methods. Gradient range and spatial resolution in the FTP method depend on the size of the filter window in reciprocal space. The authors have previously reported on a method that uses a single crossed fringe pattern for the elimination of the fundamental frequency and enlargement of the carrier window, which is therefore inherently able to cope with dynamic situations. This article describes an improved version of the technique that no longer resources to bit-shifting operations, greatly improving the flexibility of the previously reported technique, whilst retaining its main advantages.