30 episodios

During the course of their editorial work, it has become apparent to Professor M Neil James (Co-Editor of the International Journal of Fatigue, published by Elsevier) and Professor John R Yates (Chair of the Editors of Fatigue & Fracture of Engineering Materials & Structures, published by Wiley-Blackwell), that an increasing number of authors and research groups, particularly in Europe, are working on the characterising crack tip stresses using more than one fracture mechanics parameter.
Single parameter characterisation of crack and notch tip fields using the fracture mechanics parameters K, J and CTOD has been very successful in advancing predictive tools for critical and sub-critical crack growth. It has also become clear over the last 40 years that single parameter approaches have limitations particularly in dealing with crack growth phenomena arising from crack tip shielding, often resulting from the plastic enclave surrounding a crack. Influences of this enclave on the crack tip stress field ahead of the crack are maximised during cyclic loading. In the case of parameters, such as stress intensity factor K, which characterise the crack tip field via an elastic approximation, it is not surprising that any set of plasticity-induced circumstances which perturb the size of the plastic enclave and its associated strain field lead to predictive difficulties. Over the last 30 years, notable areas of activity related to such difficulties include short cracks, plasticity-induced closure, variable amplitude and multiaxial loading and notch effects.
One approach to extending elastic analyses has been to incorporate the T-stress into life prediction methods. The T-stress is the second term in a Williams-type expansion of the crack tip stresses and it affects the extent and shape of crack tip plasticity. It would therefore be expected to be influential in plasticity-related crack growth phenomena and a number of publications have demonstrated this to be true. The situation is further complicated where a crack experiences multiaxial loading and Modes II and III fracture mechanics parameters are also necessary. Alongside this, other groups have focused attention on incorporating additional elastic fracture mechanics parameters into crack/notch tip characterisation, which describe the effects of an Eshelby-type ‘plastic inclusion’ on an elastic stress field.
The organisers of this unique research event therefore extend an invitation to the fatigue and fracture community in Europe to join us in a workshop to share ideas, data and analytical methods relevant to the characterisation of crack and notch tip stress fields in an informal, interactive format at a conference venue in a beautiful scenic area of Northern Italy.

M. N. James, University of Plymouth, UK
J. R. Yates, University of Manchester, UK
L. Susmel, University of Ferrara, Italy (Workshop Secretariat)
F. Iacoviello, University of Cassino, Italy (IGF President)

2011 Forni di Sopra - First IJFatigue & FFEMS Joint Workshop Gruppo Italiano Frattura (IGF)

    • Tecnología

During the course of their editorial work, it has become apparent to Professor M Neil James (Co-Editor of the International Journal of Fatigue, published by Elsevier) and Professor John R Yates (Chair of the Editors of Fatigue & Fracture of Engineering Materials & Structures, published by Wiley-Blackwell), that an increasing number of authors and research groups, particularly in Europe, are working on the characterising crack tip stresses using more than one fracture mechanics parameter.
Single parameter characterisation of crack and notch tip fields using the fracture mechanics parameters K, J and CTOD has been very successful in advancing predictive tools for critical and sub-critical crack growth. It has also become clear over the last 40 years that single parameter approaches have limitations particularly in dealing with crack growth phenomena arising from crack tip shielding, often resulting from the plastic enclave surrounding a crack. Influences of this enclave on the crack tip stress field ahead of the crack are maximised during cyclic loading. In the case of parameters, such as stress intensity factor K, which characterise the crack tip field via an elastic approximation, it is not surprising that any set of plasticity-induced circumstances which perturb the size of the plastic enclave and its associated strain field lead to predictive difficulties. Over the last 30 years, notable areas of activity related to such difficulties include short cracks, plasticity-induced closure, variable amplitude and multiaxial loading and notch effects.
One approach to extending elastic analyses has been to incorporate the T-stress into life prediction methods. The T-stress is the second term in a Williams-type expansion of the crack tip stresses and it affects the extent and shape of crack tip plasticity. It would therefore be expected to be influential in plasticity-related crack growth phenomena and a number of publications have demonstrated this to be true. The situation is further complicated where a crack experiences multiaxial loading and Modes II and III fracture mechanics parameters are also necessary. Alongside this, other groups have focused attention on incorporating additional elastic fracture mechanics parameters into crack/notch tip characterisation, which describe the effects of an Eshelby-type ‘plastic inclusion’ on an elastic stress field.
The organisers of this unique research event therefore extend an invitation to the fatigue and fracture community in Europe to join us in a workshop to share ideas, data and analytical methods relevant to the characterisation of crack and notch tip stress fields in an informal, interactive format at a conference venue in a beautiful scenic area of Northern Italy.

M. N. James, University of Plymouth, UK
J. R. Yates, University of Manchester, UK
L. Susmel, University of Ferrara, Italy (Workshop Secretariat)
F. Iacoviello, University of Cassino, Italy (IGF President)

    • video
    Measurements of crack tip fields with DIC

    Measurements of crack tip fields with DIC

    We have started from the premise that fatigue crack growth is simply the permanent displacement of atoms from the tip of a crack. From there it naturally follows that we must observe and measure displacements in the vicinity of a crack at a scale and resolution that is appropriate to the phenomena that we are dealing with and the analytical models that we wish to use. In this paper we describe our use of digital image correlation to measure crack tip displacement fields of a growing fatigue crack, and provide a range of experimental data for the use of other researchers.

    • 11 min
    • video
    Measurement and modelling of near-tip displacement fields for fatigue cracks in 6082 T6 aluminium

    Measurement and modelling of near-tip displacement fields for fatigue cracks in 6082 T6 aluminium

    Recent work by de Matos and co-workers has used digital image correlation to examine the surface displacements in the near-tip region of a propagating fatigue crack. The primary purpose of the experiments was to investigate crack closure, and successful measurements were made. However, the data collected can also be used to investigate the more general in-plane displacement field in the neighbourhood of the crack tip and to compare this with alternative models for near-tip displacements. Experiments were undertaken using CCT specimens manufactured from 6082 T6 aluminium alloy. Three different specimen thicknesses were examined (3, 10, and 25mm), in order to examine the effect of this parameter on the measured displacements, closure levels, and crack propagation rates. The current paper re-analyses the experimental measurements by evaluating crack face displacements as a function of loading for a region close (i.e. within 500 μm) to the crack tip. These provide an excellent means of validating proposed models of crack tip stresses, strains, and displacements. The measurements will be compared to the predictions of classical crack tip models (such as the Westergaard solution, together with a recent elastic-plastic model, proposed by Pommier and co-workers, which partitions the field into elastic and plastic components. The results of these comparisons are discussed and recommendations made for future experimental work.

    • 19 min
    • video
    Overload effects on local fatigue crack-tip strain fields in plane stress samples

    Overload effects on local fatigue crack-tip strain fields in plane stress samples

    Conventional linear elastic fracture mechanics (LEFM) provides a rigorous basis for analysing sub-critical crack growth in terms of parameters (e.g. stress intensity factor (K), CTOD, J) that capture the local conditions at the crack-tip, yet can be determined solely in terms of external, macroscopic loading and geometrical parameters. However in many cases (e.g. for variable loading) the stress at the crack-tip cannot easily be inferred solely from a global approach and local information is required. The advent of 3rd generation synchrotron facilities has opened up the possibility of measuring local crack-tip strains under both plane strain and plane stress. In this work, the behaviour of the crack-tip before, during, immediately after an overload event is examined for fatigue cracks in thin (plane stress) stainless steel CT specimens. X-ray diffraction has revealed large compressive residual stresses ahead of the crack, but no evidence for plasticity induced closure behind the crack-tip.

    • 21 min
    • video
    The plastic ‘inclusion’ as a bridge between local crack plasticity and the global elastic field

    The plastic ‘inclusion’ as a bridge between local crack plasticity and the global elastic field

    This paper presents a novel ‘plastic inclusion’ concept for dealing with the local plasticity which occurs at the tip of a growing fatigue crack. Such localised plasticity essentially blunts the crack and induces stresses which act on the applied elastic stress field at the boundary of the elastic-plastic enclave surrounding the crack. The paper outlines a model of crack tip stresses that explicitly incorporates these stresses and also those that might arise from crack wake contact. This offers a way of reconciling local crack tip plasticity-induced stress field perturbations with the driving force of the overall elastic stress field. The outcome is the identification of a modified crack tip stress intensity factor which should be able to explicitly predict the magnitude of plasticity-induced crack tip shielding and resolve the many controversies associated with plasticity-induced shielding. A full-field approach is developed for stress using photoelasticity and also for displacement using digital image correlation.

    • 20 min
    • video
    Crack tip shielding effects, Part 1: direct measurement of the plastic enclave

    Crack tip shielding effects, Part 1: direct measurement of the plastic enclave

    The direct observation or measurement of the size and shape of plastic zones associated with a propagating fatigue crack has been difficult. In earlier work the authors used the growth of fatigue cracks in polycarbonate specimens combined with transmission photoelasticity to observe qualitatively the plastic enclave ahead of a crack and the development of a plastic wake along the crack flanks. Recently this approach has been extended to studying the effect of overloads on the size of the plastic enclave and the correlation with crack growth rate. In more recent work thermoelastic stress analysis has been employed to map the size and shape of the plastic zone ahead of cracks in aluminum compact tension specimens. The phase difference between the measured temperature signal and the applied loading permitted the identification of the extent of plastic behavior, or the limit of thermoelastic behavior. Experiments were performed with overloads for single cycles and ten cycles and the change in size of the plastic enclave was defined and correlated to both stress intensity factor and growth rate. The coefficients in Wheeler’s model were evaluated from direct measurements and used to reliably predict crack behavior. Work is in progress to obtain the strain field around the crack tip, both within and around the plastic zone using digital image correlation. The data from photoelasticity, thermoelastic stress analysis and digital image correlation confirm the influence on the crack growth rate of the plastic enclave associated with the crack tip and the plastic wake behind the tip. This influence occurs as a result of the plastic enclave shielding the crack tip from the full effect of the applied loading and the shear stresses established between the elastic and plastic regions in the wake. The traditional descriptions of the singularity-dominated, elastic stress field do not appear to be capable of describing this complex situation. The experimental data obtained with the above techniques has inspired the development of a new multi-parameter model in which the stress field can be characterized by a new set of intensity factors which quantify crack driving and crack retarding effects separately. As will be described in part II [1], the results from the new model correlate well with experimental measurements.

    • 17 min
    • video
    Experimental and numerical analysis of elastic-plastic strains and stresses ahead of a growing fatigue crack

    Experimental and numerical analysis of elastic-plastic strains and stresses ahead of a growing fatigue crack

    Reliability and fatigue life estimation of structural and mechanical components subjected to cyclic loads require predictions of stresses and strains at critical locations such as notches and cracks. Such analyses are concerned with solving complex elastic-plastic boundary problems involving varying geometrical boundaries and cyclic loading.
    The objective of the paper is an analysis of elastic-plastic strains and stresses arising in the plastic zone adjacent to the tip of a propagating fatigue crack. The analyzed below boundary value problem concerns Compact Tension specimen with growing fatigue crack subjected to non-symmetric constant amplitude cyclic loading history. A special finite element code has been developed to calculate the elastic-plastic stress-strain response ahead of a growing Mode I fatigue crack. The code consists of the cyclic plasticity Mroz-Garud model, contact elements and a module simulating the growth of the fatigue crack. The simulation was carried out for the SAE4140 steel material. The aim of the analysis was to study both the crack opening displacement field behind the moving crack tip and fluctuations of the elastic-plastic strains and stresses ahead of the crack. It has been found that both the crack opening displacement and the crack tip stress-strain fields are related to each other and occasional contacts between crack surfaces do not have as strong effect on the crack tip stress-strain affairs as it is often claimed. It was also found that the crack region adjacent to the crack tip often stays open regardless of optically visible presence of the contact between crack surfaces away from the crack tip. The effect of the plasticity induced residual stress field on the crack opening displacement field at the maximum and minimum load levels has been illustrated as well. The studies indicate that the simulation of the load history effect based only on the contact between crack surfaces, known as the crack tip closure, and manipulation of the minimum stress intensity factor is insufficient for appropriate modeling of the spectrum loading effects on the stress-strain history ahead of the crack tip and subsequently on the fatigue crack propagation process.
    Limited experimental measurements of near the crack tip strains influenced by an overload were also presented and discussed in terms of its effect on the crack tip strain field at the maximum and minimum load level.

    • 21 min

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