Abstract
The spall strength of polydimethylsiloxane silicone oils has been studied using the planar impact of thin flyers to generate large transient negative pressures near the free surface of target samples. The liquids were contained within sealed capsules in which a 4-µm-thick aluminized Mylar diaphragm formed a free surface at the back of the sample. The liquid targets were impacted by PMMA flyers at velocities ranging from 130 to 700 m/s using a 64-mm-bore gas-gun, thus allowing for large variations in the strain rate and incident shock pressure. The peak tension in the liquid was determined by monitoring the free surface velocity using a photonic Doppler velocimetry (PDV) system. The paper focuses on the study of a system of silicone oils having vastly different viscosities (5 cSt to 1000 cSt), but otherwise similar liquid properties. The effect of viscosity on spall strength is compared to previously published data and models.
Highlights
Liquids can exist in a metastable state of significant hydrostatic tension under certain conditions [1]
The experiments performed on the silicone oils indicate that the spall strength of the low and high viscosity oils are equivalent within measurement error (3.4 MPa)
The lack of correlation between viscosity and spall strength as well as the one order of magnitude difference in spall strength between the work of Couzens and Trevena and this study indicate a possible transition in the mechanism of cavitation nucleation as strain rate is increased from 102 s−1 in the bullet-piston studies to 104 s−1 for spall experiments
Summary
Liquids can exist in a metastable state of significant hydrostatic tension under certain conditions [1]. The reflection of a shock wave from a free surface can momentarily place liquids in a state of tension due to the interaction of the resulting reflected expansion front with an expansion front behind the shock This highly dynamic phenomenon causes the liquid to be stretched until sufficient tension is reached to cause the liquid to rupture through the formation and growth of voids. Surface tension creates an energy barrier which must be overcome to grow cavitation bubbles beyond a critical radius, allowing liquids to exist in a metastable state [5] equivalent to that of a superheated fluid, until sufficiently large cavities form and grow to relieve the tension. At low strain rates or in contaminated liquids, heterogeneous nucleation is likely to dominate void formation and growth, for pure liquids undergoing dynamic stretching within the bulk liquid, homogeneous nucleation may play a significant role in the cavitation process [8]
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