Shock Wave Propagation Characteristics of Cylindrical Charge and Its Aspect Ratio Effects on the Damage of RC Slabs

Antiknock research of reinforced concrete (RC) slabs is often carried out with spherical or nearly spherical explosives, although many explosives used in engineering and military are cylinder shaped. It is known that the shock wave caused by cylindrical explosives varies in different directions, which is quite different from the spherical charge. In this paper, the shock wave propagation characteristics of spherical and cylindrical explosives with different aspect ratios are compared and analyzed. The 2D numerical results show the peak overpressure from the cylindrical explosive is significantly affected by the L/D (length/diameter) ratio. Subsequently, the damage features of RC slabs under spherical and cylindrical explosives with a certain L/D ratio are investigated through an explosion experiment. Finally, the influence of the L/D ratio on the dynamic response of RC slabs under cylindrical explosives is studied by the fully coupled Euler–Lagrange method. The accuracy and reliability of the coupled model are verified by comparing the numerical with experimental results. Based on the experimental and numerical studies, it can be concluded that the explosive shape directly determines the shape of upper surface crater damage, and the spall damage area of RC slabs becomes larger as the L/D increases. For the L/D increases to a certain value, the cylindrical explosive will induce larger spall damage than that induced by spherical charge with the same amount of explosives. Hence, the effect of the cylindrical charge should be considered in the antiknock design of the RC structure.

Spall Behaviors of Metaconcrete: 3D Meso-Scale Modelling

Spalling is a typical tensile fracture phenomenon due to insufficient tensile strength of concrete. Concrete structure might experience severe spall damage at the rear surface of the structure owing to reflected tensile stress wave induced by impulsive load. In recent years, metaconcrete consisting of engineered aggregates has attracted attentions as metaconcrete exhibits extraordinary wave-filtering characteristics. Metaconcrete can be used to attenuate stress wave generated by impulsive load and hence possibly mitigate the spall damage. In this study, engineered aggregate is designed via the software COMSOL to have the frequency bandgap coincide with the dominant frequency band of stress wave propagating in the normal concrete (NC) specimen to reduce the stress wave propagation and hence spall damage. The wave propagation behaviors in metaconcrete specimen with periodically distributed engineered aggregates have been investigated in a previous study. This study establishes 3D meso-scale model of metaconcrete including mortar, randomly distributed natural aggregates and engineered aggregates to simulate spall behaviors of metaconcrete via the software LS-DYNA. The responses of metaconcrete composed of engineered aggregates with single bandgap and multiple bandgaps are studied. The results show that stress wave can be more effectively attenuated by using engineered aggregates with multiple bandgaps. It is found that although engineered aggregates mitigate stress wave propagation, the soft coating of the engineered aggregates reduces the concrete material strength, therefore spall damage of metaconcrete specimen is not necessarily less severe than the normal concrete, but has different damage mode. In addition, the influences of loading intensity and duration on stress wave, as well as the spall behaviors of metaconcrete specimen are also studied.

Физико-химические процессы, сопровождающие локализацию пластической деформации при импульсном нагружении

Investigation of the strains development under pulsed loading showed the decisive role of powerful ultrasonic sample vibrations in the standing wave mode. The effect of mass transfer of atoms and ultrafine hardening phase particles from the matrix material to the spall damage zone on the strain bands microstructure is determined. Rapid cooling of the metal inside the localization bands, the size of which does not exceed several tens of microns, indicates that changes in the phase composition occur as a result of cold deformation. Rapid the metal cooling inside the localization bands, the size of which does not exceed several tens of microns, indicates that changes in the phase composition occur as a result of cold deformation.

FEATURES STRAIN LOCALISATION UNDER IMPACT LOADS

The localization of plastic strain under impulse loads is determined by the sample geometry and is practically independent on the material properties. Deformation bands are a consequence of ultrasonic oscillations, which occurs when waves are reflected (compression and unloading) on the sample faces, and when the reflected waves interact with each other, and flows in the form of standing waves. Localized strain bands originate and develop at the nodes of standing waves under conditions of alternating deformation and are accompanied by mass transfer of particles from the matrix material (interstitial and substitutional atoms, impurity atoms, ultrafine particles of the hardening phase) to the sites of spall damage. The absence of energy transfer through the nodal points of the standing wave increases the duration of the sample deformation after the passage of the shock wave.

Damage characteristic of aluminum nanorod under hypervelocity impact

Understanding the dynamic behavior of materials under hypervelocity impact is of great importance to develop new materials or structures for protective applications. The present work gives insight into the damage characteristic of aluminum nanorod under hypervelocity impact based on atomistic simulations. First of all, the propagation of impact wave is found to experience a rapid decaying because of its release from the side surface, which leads to a complex three-dimensional stress wave and two tension regions inside the nanorod. The damage mode under this tension state is found to be very different from the classical spallation. Due to the interaction of two release waves from the side and end surfaces, a temporary spall damage is observed and its initial tensile strength is close to that of bulk material. However, that early spall damage does not develop into a complete spall fracture. More importantly, all generated voids are found to be closed eventually after their coalescence. Furthermore, the mass continues expanding outward from the impact plane and finally causes a radial annular fragmentation. The annular fragmentation shows a clear crystalline direction dependence for low impact velocities. The number and the size of final fragments are found to follow a power law relationship for all impact velocities.