NUCLEATION AND GROWTH OF CRACKS IN ELASTOMERS [Keynote]

We explore fracture nucleation and propagation within a transparent polydimenthylsiloxane elastomer using the “poker-chip” specimen. Global measurements are correlated with optical visualization at high spatial and adequate temporal resolution to identify the sequence of events; this is augmented with interrupted tests and x-ray computed tomography scans to probe the three dimensional geometry of the nucleation and growth of cracks. The experimental results are used to identify the different types of response, ranging from growth of surface cracks, to interior nucleation and growth of a single crack, to a completely nucleation dominated response. The dependence of the response on the specimen constraint, characterized by the specimen thickness, is explored through simulations within a finite deformation framework; a preliminary criterion for nucleation of cracks under multiaxial loading is proposed.
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THEORETICAL, EXPERIMENTAL AND COMPUTATIONAL STUDY THE OFF-AXIS ELASTIC CONSTANTS, FRACTURE AND STRENGTH OF UNIDIRECTIONAL FIBER COMPOSITE [Keynote]

In this work a theoretical/analytical, computational and experimental study of unidirectional glass-fiber reinforced epoxy composites is carried out. The concept of boundary interphase is used in order to determine the elastic constants of the composite. A finite element analysis is adopted in order to correlate with the derived theoretical values of the elastic constants. Finally, these results are compared with experimental findings obtained from tensile experiments performed on composites of the material used in order to predict the fracture of composites.
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NON-FOURIER HEAT CONDUCTION AND NONLOCAL THEORY, RECENT PROGRESS AND APPLICATION IN THERMAL FRACTURE ANALYSIS [Keynote]

Non-Fourier heat conduction theories have recently been introduced to thermal stress analysis to account for the wave-like behavior of heat conduction under extreme thermal environments, such as high temperature gradient, extremely low temperature, or heat transport in heterogenous microstructures. When considering the highly localized heating process in laser manufacturing, nonlocal heat conduction needs to be included in the heat conduction equation. Combined non-Fourier, nonlocal thermoelastic theories revealed new phenomena in thermal stress analysis of cracked structures. This presentation summarizes some recent progress in thermal fracture analysis using nonlocal, non-Fourier thermoelastic theories.
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A PHASE FIELD FATIGUE MODEL FOR COMPLEX LOADING SITUATIONS [Keynote]

The phase field method for fracture mechanics has drawn a lot of attention in the past decade because of its simple formulation and easy implementation. Recently, the phase field model is also applied for fatigue fracture for a uniform loading. However, there is still a lack of studies on how to consider complex loading cases in the phase field fatigue model. In this work, we extend the phase field model for non-uniform loading situations by combing it with the rainflow counting algorithm.
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3D FINITE FRACTURE MECHANICS UNDER MODE I LOADING: THE FLAT ELLIPTICAL CRACK [Keynote]

In recent years, the Finite Fracture Mechanics approach, originally proposed by Leguillon in 2002, has been applied successfully to several material and geometrical configurations. However, up to now, most of the applications were restricted to two-dimensional geometries. In the present paper, we provide an insight to a simple yet challenging three-dimensional case, namely the flat elliptical crack. Results are provided in analytical form.
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ON THE DIFFICULTY OF IMPLEMENTING THE COUPLED CRITERION TO PREDICT GLASS FRACTURE [Keynote]

Glass is an extremely brittle material that behaves almost perfectly linear elastic until it fractures. The linear-elastic fracture mechanics (LEFM) approach described by Griffith’s energy criterion is typically used to explain failure from a pre-existing crack like defect. However, LEFM reaches its limits in explaining failure processes at general stress concentration points and implementing the Coupled Criterion (CC) to take over is a tricky task. This mainly because it requires the knowledge of the tensile strength of the material which is a parameter not easy to characterize in glass. It is in general defined through a statistical law and relies strongly with surface flaws. The general aim of this work is to give an overview of the current understanding of glass tensile strength.
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ON FFM/PFM FAILURE CRITERIA FOR METALS UNDERGOING SSY – NEW INSIGHTS AT V-NOTCHED TIPS [Keynote]

Structures made of steel alloys may fracture at V-notch tips at which a small plastic zone usually evolves. Failure criteria for predicting fracture loads for such quasi-brittle alloys, as a function of the V-notch opening angle are very scarce and have not been validated, to the best of our knowledge, by a set of experimental observations. Neither the FFM coupled criterion (FFMCC) for brittle fracture, nor two phase-field models (the classical AT1 for brittle materials and a ductile version) could predict the increase of the crack nucleation force observed in the four-point bend (4PB) experiments performed on AISI 4340 specimens as the opening angle increased. Extension of the FFMCC to account for the small plastic zone and further 4PB experiments on a new steel alloy (H13) are being considered to improve the failure criterion.
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CRACK DEFLECTION AT CURVED INTERFACES. A FINITE FRACTURE MECHANICS ANALYSIS [Keynote]

Curved weak interfaces present promising advantages to be implemented as crack arrestors in structures designed under the tolerant-design principles. Among other advantages, they neither add extra weight nor affect significantly to the global stiffness of the structural element, in contrast with other crack arrestors. To be employed as crack arrestor, it is key that the interface can deviate the crack. If the crack penetrates across the interface, the effect of the weak interface as crack arrestor is canceled. In view of this, this work studies how to set the interface parameters to promote the crack deviation along the interface. In particular, following the dimensional analysis of the problem, the effect of three significant dimensionless parameters is studied: interface to bulk fracture toughness, interface to bulk tensile strength and the interface curvature radius normalized with the material characteristic length. The study is carried out using the Coupled Criterion of the Finite Fracture Mechanics.
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THE THEORY OF CRITICAL DISTANCES TO MODEL THE STATIC STRENGTH OF ADDITIVELY MANUFACTURED CONCRETE/POLYMERS CONTAINING MANUFACTURING DEFECTS/VOIDS [Keynote]

The present paper deals with the use of the Theory of Critical Distances to model the detrimental effect of manufacturing defects and voids in 3D-printed concrete/polymers subjected to static loading. The validity and robustness of the proposed approach is assessed against a large number of experimental results that were generated by testing 3D-printed specimens of both concrete and polylactide (PLA) containing manufacturing defects/voids. The sound agreement between experiments and predictive model makes it evident that the Theory of Critical Distances is not only a reliable design approach, but also a powerful tool suitable for guiding and informing effectively the additive manufacturing process.
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SINGULAR ELASTIC SOLUTIONS IN CORNERS AND CRACKS WITH SPRING BOUNDARY CONDITIONS WITH VARYING STIFFNESS [Keynote]

Singular elastic solutions in corners and cracks with spring boundary conditions with varying spring stiffness are studied. First, a novel analytic procedure is developed for the antiplane strain case. Then, some general observations obtained are checked for the plane strain case by using a FEM code. Finally, applications of these observations in a suitable computational implementation of the Coupled Criterion of Finite Fracture Mechanics are discussed.
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