Low Velocity Surface Fracture Patterns in Brittle Material: A Newly Evidenced Mechanical Instability (original) (raw)

Instability in the propagation of fast cracks

Physical Review B, 1992

We report on experimental investigations of the propagation of cracks in the brittle plastic polymethyl methacrylate (PMMA). Velocity measurements with resolution an order of magnitude better than previous experiments reveal the existence of a critical velocity (330+30 m/s) at which the velocity of the crack tip begins to oscillate, the dynamics of the crack abruptly change, and a periodic pattern is formed on the crack surface. Beyond the critical point the amplitude of the oscillations depends linearly on the mean velocity of the crack. The existence of this instability may explain the failure of theoretical predictions of crack dynamics and provides a mechanism for the enhanced dissipation observed experimentally in the fracture of brittle materials.

Damage mechanisms in the dynamic fracture of nominally brittle polymers

International Journal of Fracture, 2013

Linear Elastic Fracture Mechanics (LEFM) provides a consistent framework to evaluate quantitatively the energy flux released to the tip of a growing crack. Still, the way in which the crack selects its velocity in response to this energy flux remains far from completely understood. To uncover the underlying mechanisms, we experimentally studied damage and dissipation processes that develop during the dynamic failure of polymethylmethacrylate (PMMA), classically considered as the archetype of brittle amorphous materials. We evidenced a well-defined critical velocity along which failure switches from nominally-brittle to quasibrittle, where crack propagation goes hand in hand with the nucleation and growth of microcracks. Via postmortem analysis of the fracture surfaces, we were able to reconstruct the complete spatiotemporal microcracking dynamics with micrometer/nanosecond resolution. We demonstrated that the true local propagation speed of individual crack fronts is limited to a fairly low value, which can be much smaller than the apparent speed measured at the continuum-level scale. By coalescing with the main front, microcracks boost the macroscale velocity through an acceleration factor of geometrical origin. We discuss the key role of damage-related internal variables in the selection of macroscale fracture dynamics.

Universal Aspects of Dynamic Fracture in Brittle Materials

Experimental Chaos, 2004

We present an experimental study of the dynamics of rapid tensile fracture in brittle amorphous materials. We first compare the dynamic behavior of "standard" brittle materials (e.g. glass) with the corresponding features observed in "model" materials, polyacrylamide gels, in which the relevant sound speeds can be reduced by 2-3 orders of magnitude. The results of this comparison indicate universality in many aspects of dynamic fracture in which these highly different types of materials exhibit identical behavior. Observed characteristic features include the existence of a critical velocity beyond which frustrated crack branching occurs 1, 2 and the profile of the micro-branches formed. We then go on to examine the behavior of the leading edge of the propagating crack, when this 1D "crack front" is locally perturbed by either an externally introduced inclusion or, dynamically, by the generation of a micro-branch. Comparison of the behavior of the excited fronts in both gels and in soda-lime glass reveals that, once again, many aspects of the dynamics of these excited fronts in both materials are identical. These include both the appearance and character of crack front inertia and the generation of "Front Waves", which are coherent localized waves 3-6 which propagate along the crack front. Crack front inertia is embodied by the appearance of a "memory" of the crack front 7,8 , which is absent in standard 2D descriptions of fracture. The universality of these unexpected inertial effects suggests that a qualitatively new 3D description of the fracture process is needed, when the translational invariance of an unperturbed crack front is broken.

Straight cracks in dynamic brittle fracture

We study the dynamics of cracks in brittle materials when the velocity of the crack is comparable to the sound velocity by means of lattice simulations. Inertial and damped dynamics are analyzed. It is shown that dissipation strongly influences the shape of the crack. While inertial cracks are highly unstable, dissipation can stabilize straight cracks. Our results can help to explain recent experiments on PMMA.

An experimental investigation of dynamic crack propagation

Engineering Fracture Mechanics, 1983

The dynamic fracture behavior of polymethylmethacrylate (PMMA) has been investigated. The specimens were in the form of rectangular sheets with sharp notches. The elastodynamic crack tip stress field and the crack velocity were determined by the use of resistance strain gauges. An analytic expression for the dynamic crack tip stress field was used to evaluate the dynamic stress intensity factors, and the dynamic arrest toughness was also determined. The dynamic response of the stresses at the notch tip at varying loading rates was considered and some "hysteresis" fracture phenomena were observed.

Brittle-Quasibrittle Transition in Dynamic Fracture: An Energetic Signature

Physical Review Letters, 2010

Dynamic fracture experiments were performed in PMMA over a wide range of velocities and reveal that the fracture energy exhibits an abrupt 3-folds increase from its value at crack initiation at a well-defined critical velocity, below the one associated to the onset of micro-branching instability. This transition is associated with the appearance of conics patterns on fracture surfaces that, in many materials, are the signature of damage spreading through the nucleation and growth of microcracks. A simple model allows to relate both the energetic and fractographic measurements. These results suggest that dynamic fracture at low velocities in amorphous materials is controlled by the brittle/quasi-brittle transition studied here.

Morphological instabilities of dynamic fractures in brittle solids

Physical Review E, 1996

We present a study of the stress fields in the neighborhood of a moving crack tip in the framework of linear elastic fracture mechanics. This approach is found to be physically relevant for a large range of the crack speeds. We show that the stability analyses based on conditions of attainment of a critical tensile stress on some plane are inadequate to describe the instabilities of the crack path. A study of the largest principal stress in the neighborhood of the crack surface is reported. We show that at ''low'' crack velocities the path of the crack extension is an opening mode. However, this property disappears when the crack speed exceeds a critical velocity V c and reappears again beyond a faster speed V B , but at a different orientation from that of pure opening mode. These variations have been interpreted as the onset of roughening and branching instabilities.

Understanding fast macroscale fracture from microcrack post mortem patterns

2011

Dynamic crack propagation drives catastrophic solid failures. In many amorphous brittle materials, sufficiently fast crack growth involves small-scale, high-frequency microcracking damage localized near the crack tip. The ultra-fast dynamics of microcrack nucleation, growth and coalescence is inaccessible experimentally and fast crack propagation was therefore studied only as a macroscale average. Here, we overcome this limitation in polymethylmethacrylate, the archetype of brittle amorphous materials: We reconstruct the complete spatio-temporal microcracking dynamics, with micrometer / nanosecond resolution, through post mortem analysis of the fracture surfaces. We find that all individual microcracks propagate at the same low, load-independent, velocity. Collectively, the main effect of microcracks is not to slow down fracture by increasing the energy required for crack propagation, as commonly believed, but on the contrary to boost the macroscale velocity through an acceleration factor selected on geometric grounds. Our results emphasize the key role of damage-related internal variables in the selection of macroscale fracture dynamics.