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Dynamic fracture under plane wave loading

Abstract

A new plate impact experiment is presented for studying dynamic fracture processes that occur under sub-microsecond loading. The experiment is designed to provide comparatively straightforward interpretation within the framework of fracture mechanics. A disc containing a mid-plane, pre-fatigued, edge crack that has been propagated halfway across the diameter is impacted by a thin flyer plate of the same material. A compressive pulse propagates through the specimen and reflects from the rear surface as a step, tensile pulse with a duration of 1μs. This plane wave loads the crack and causes initiation and propagation of the crack. The motion of the rear surface is monitored during this event using a laser interferometer system. The location of the crack front is mapped before and after the experiment using a focussed ultrasonic transducer.

Experiments have been conducted on a hardened 4340 VAR steel at temperatures ranging from room temperature to — 100°C. Crack advance increases monotonically with increasing impact velocity and with decreasing temperature. Critical values of the stress intensity factorK Ic are inferred from known elastodynamic solutions and the assumption that the measured crack advance occurs at a constant energy release rate. Fracture modes are characterized by means of scanning electron microscopy of the fracture surfaces.

A finite difference method is used for numerical simulation of the experiments. The loading is modelled as that of a plane, square, tensile pulse impinging at normal incidence on a semi-infinite crack. Crack advance is assumed to initiate when the crack-tip stress intensity factor reaches the critical valueK Ic. Crack velocities are prescribed corresponding to various fracture models. The predicted motion of the rear surface is found to be in good agreement with the measured motion when the crack velocity is taken to be a constant.

Résumé

On présente un nouvel essai de choc sur plaque pour l'étude du processus de rupture dynamique qui se produit sous des charges inférieures à la micro-seconde. L'essai est conçu de manière à fournir une interprétation comparativement directe dans le cas de las mécanique de la rupture. L'essai consiste à soumettre à un choc un disque comportant une fissure du bord suivi un plan médian, préfatiguée et propagée sur la moitié du diamètre du disque, à l'aide d'une plaque mince mobile du même matériau. Une impulsion de compression se propage au travers de l'echantillon, se réfléchit sur la surface arrière comme sur un seuil, entraînant une impulsion de traction avec une durée d'une micro-seconde. Cette onde plane soumet la fissure à la sollicitation et provoque l'amorçage de la propagation de la fissure. On enregistre le mouvement de la surface arrière au cours du phénomène en utilisant un système d'interférométrie à laser. La localisation du front de fissure est tracée avant et après l'essai, en utilisant un transducteur ultrasonique focalisé.

Des essais ont été conduits sur un acier 4340 Var durci, à des températures comprises entre la température ambiante et — 100°C. L'avancement de la fissure augmente de manière monotone avec les vitesses croissantes de choc et avec l'abaissement de la température. Des valeurs critiques des facteurs d'intensité de contrainteK lc, sont déduites à partir des solutions élasto-dynamiques connues et de l'hypothése que l'avancement de la fissure mesurée se produit suivant une vitesse de relaxation en énergie constante. Les modes de rupture sont caractérisés au moyen de la microscopie électronique à balayage des surfaces de rupture.

Une méthode finie différentielle est utilisée pour l'assimiliation numérique des essais. La mise en charge est modélisée comme celle d'une impulsion de traction plane et carrée agissant suivant une incidence normale sur une fissure semi-infinie. L'avancement de la fissure est supposé commencer lorsque le facteur d'intensité de contrainte à son extrémité atteint la valeur critiqueK Ic. Les vitesses de fissuration sont établies en correspondance avec différents modèles de rupture. On trouve que le mouvement prévu de la surface arrière est en bon accord avec les mouvements mesurés, lorsque la vitesse de fissuration est considérée comme constante.

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Ravichandran, G., Clifton, R.J. Dynamic fracture under plane wave loading. Int J Fract 40, 157–201 (1989). https://doi.org/10.1007/BF00960599

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  • DOI: https://doi.org/10.1007/BF00960599

Keywords

  • Stress Intensity Factor
  • Rear Surface
  • Crack Velocity
  • Crack Advance
  • Flyer Plate