International Journal of Fracture最新文献

This paper presents the results of axi-symmetric extension tests on a Rock Analogue Material that showed a continuous transition from extension fracture to shear fracture with an increase in compressive stress. The analysis used non destructive full-field experimental methods—digital image correlation (DIC), as well as the post-mortem specimens observation. When the mean stress was small, the fractures formed through the mode I cracking at tensile equal to the material tensile strength with smooth surfaces. These surfaces became rougher or delicate plumose patterns as the mean stress increased. Fracture angles also increased progressively from extension fractures to shear fractures. Hybrid fractures formed under mixed tensile and compressive stress states and presented plumose patterns on the rupture surface. DIC results showed the localisation of tensile deformation and the acceleration of deformation at the zone that induced the fracture. The fracture caused a reduction of deformation in the surrounding areas, which showed a release of elastic energy stored in the material during the propagation of fracture.

Fracture prognosis and characterization efforts require knowledge of crack tip position and the Stress Intensity Factors (SIFs) acting in the vicinity of the crack. Here, we present an efficient numerical approach to infer both of these characteristics under a consistent theoretical framework from noisy, unstructured displacement data. The novel approach utilizes the separability of the asymptotic linear elastic fracture mechanics fields to expedite the search for crack tip position and is particularly useful for noisy displacement data. The manuscript begins with an assessment of the importance of accurately locating crack tip position when quantifying the SIFs from displacement data. Next, the proposed separability approach for quickly inferring crack tip position is introduced. Comparing to the widely used displacement correlation approach, the performance of the separability approach is assessed. Cases involving both noisy data and systematic deviation from the asymptotic linear elastic fracture mechanics model are considered, e.g. inelastic material behavior and finite geometries. An open source python implementation of the proposed approach is available for use by those doing field and laboratory work involving digital image correlation and simulations, e.g. finite element, discrete element, molecular dynamics and peridynamics, where the crack tip position is not explicitly defined.

This article presents the results of an investigation of crack nucleation and propagation in a transparent polydimenthylsiloxane (PDMS) elastomer. The main objective of the investigation is to characterize quantitatively the evolution of crack nucleation and propagation behavior not just through the usual macroscopic load and displacement data, but with synchronized optical images at high spatial and adequate temporal resolution that will resolve the evolution of the failure processes. This is augmented with X-ray computed tomography (CT) scans to characterize the three-dimensional geometry of the cracks nucleated in the interior of the elastomer. Towards this goal, we reproduce the classical poker-chip experiment of Gent and Lindley (Proc R Soc Lond A 249(1257):195–205, 1959) in which the specimen’s diameter-to-thickness ratio is varied over a broad range to cover crack nucleation, propagation, and their coalescence. These experiments are performed on transparent PDMS with different compositions, first in a specially built loading machine that is fitted with a high magnification microscopic camera that permits the measurement of the load while simultaneously providing images of the specimen configuration and subsequently in an apparatus built for in situ observations using an X-ray CT scanning system. These experiments reveal that nucleation of multiple microcracks dominates when the diameter-to-thickness aspect ratio (alpha ) is sufficiently large, because the incompressibility of the material induces substantial, nearly uniform hydrostatic tension in the specimen. In contrast, specimens with smaller aspect ratio tend to nucleate fewer cracks, and are dominated by the growth of these cracks. At even smaller (alpha ), the hydrostatic stress is significantly lowered and failure is dominated by surface flaws. The three-dimensional geometry, and the spatial distribution of the nucleated cracks were evaluated using optical microscopy and X-ray CT scans. This revealed cracks of three different shapes, one of which was confined in a layer near to the upper or bottom boundary of the poker-chip, another was across the thickness, but with a tilt relative to the axis of the specimen, and the last was propagating along the radial direction.

In this work, the essential work of fracture (EWF) method is introduced for a peridynamic (PD) material model to characterize fracture toughness of ductile materials. First, an analytical derivation for the path-independence of the PD J-integral is provided. Thereafter, the classical J-integral and PD J-integral are computed on a number of analytical crack problems, for subsequent investigation on how it performs under large scale yielding of thin sheets. To represent a highly nonlinear elastic behavior, a new adaptive bond stiffness calibration and a modified bond-damage model with gradual softening are proposed. The model is employed for two different materials: a lower-ductility bainitic-martensitic steel and a higher-ductility bainitic steel. Up to the start of the softening phase, the PD model recovers the experimentally obtained stress–strain response of both materials. Due to the high failure sensitivity on the presence of defects for the lower-ductility material, the PD model could not recover the experimentally obtained EWF values. For the higher-ductility bainitic material, the PD model was able to match very well the experimentally obtained EWF values. Moreover, the J-integral value obtained from the PD model, at the absolute maximum specimen load, matched the corresponding EWF value.

The alternate (stick-slip) cracking phenomenon in Poly(methyl methacrylate) (PMMA) was investigated using high-speed imaging and digital image correlation (DIC). PMMA is known to show a great variety of fracture behaviors by even small changes in loading conditions. With TDCB-shaped samples and under a range of constant extension rates, the phenomenon of alternate cracking is observed. Here, loops of successive quasi-static and dynamic crack propagation are found within a single fracture experiment suggesting a ‘forbidden’ velocity regime. For the first time, such material/structural cyclic fracture behavior is examined through the lens of linear elastic fracture mechanics (LEFM) by using in-situ High-Speed (HS) DIC. Energy release rates and crack velocities during fracture experiments are derived from full-field measurements using Williams’ series expansion. Fracture surfaces of post-mortem samples have been systematically analyzed using optical microscopy. The investigation of the actual limits of the ‘forbidden’ velocity regime in terms of critical velocity and energy release rate in relation to post-mortem crack length features is achieved by holistic experimental data on alternate cracking. This work provides key experimental data regarding the improved understanding of a unified theoretical framework of crack instabilities.