Influence of Interface Scattering on Shock Waves in Heterogeneous Solids

In heterogeneous media, the scattering due to interfaces between dissimilar materials play an important role in shock wave dissipation and dispersion. In this work the influence of interface scattering effect on shock waves was studied by impacting flyer plates onto periodically layered polycarbonate/6061 aluminum, polycarbonate/304 stainless steel and polycarbonate/glass composites. The experimental results (using VISAR and stress gauges) indicate that the rise time of the shock front decreases with increasing shock strength, and increases with increasing mechanical impedance mismatch between layers; the strain rate at the shock front increases by about the square of the shock stress. Experimental and numerical results also show that due to interface scattering effect the shock wave velocity in periodically layered composites decreases. In some cases the shock velocity of a layered heterogeneous composite can be lower than that of either of its components

2018 Timoshenko Medal Acceptance Lecture: Academic Family

Dedicated to my immediate academic family, my wonderful graduate students and postdocs and Stefan Timoshenko's academic great, great, great, great, great, great grandchildren. Dear friends, I was brought up in Greece to believe in the power of families. As a result families are very important to me and so are all of you, whom I consider to be my extended academic family. This is exactly the reason for which I feel so excited and honored to receive the Timoshenko Medal in front of you tonight since I truly consider all of you, working in the general area of mechanics at all length and time scales, as my cherished academic brothers and sisters, parents and grandparents. So please allow me to make “Academic Family” my theme for tonight, because I truly feel that in addition to inspiration and creativity, collegiality and mentoring are the two most important corner stones of our profession

Full-field optical measurement of curvatures in ultra-thin-film–substrate systems in the range of geometrically nonlinear deformations

This article describes coherent gradient sensing (CGS) as an optical, full-field, real-time, nonintrusive, and noncontact technique for the measurement of curvatures and nonuniform curvature changes in film-substrate systems. The technique is applied to the study of curvature fields in thin Al films (6 mum) deposited on thin circular silicon wafers (105 mum) of "large" in-plane dimensions (50.8 mm in diameter) subjected to thermal loading histories. The loading and geometry is such that the system experiences deformations that are clearly within the nonlinear range. The discussion is focused on investigating the limits of the range of the linear relationship between the thermally induced mismatch strain and the substrate curvature, on the degree to which the substrate curvature becomes spatially nonuniform in the range of geometrically nonlinear deformation, and finally, on the bifurcation of deformation mode from axial symmetry to asymmetry with increasing mismatch strain. Results obtained on the basis of both simple models and more-detailed finite-element simulations are compared with the full-field CGS measurements with the purpose of validating the analytical and numerical models

Interpretation of optical caustic patterns obtained during unsteady crack growth: an analysis based on a higher-order transient expansion

The optical caustic method is re-examined considering the presence of transient effects. Based on the higher-order asymptotic expansion provided by Freund and Rosakis, regarding the stress field near a non-uniformly propagating crack tip, the caustic mapping and the initial curve equations are derived. The dynamic stress intensity factor, K^d_I(t), is related to experimentally measurable quantities of the caustic pattern by an explicit expression. It is shown that the classical analysis of caustics is a special case of the new interpretation method. The Broberg problem is used as an example problem to check the feasibility of analysing caustics in the presence of higher-order transient terms. It is shown that the caustic patterns are sensitive to transient effects, and that use of the classical analysis of caustics in the interpretation of the optical patterns for this problem may result in large errors in the value of the stress intensity factor, especially at short times after initiation

On the influence of fault bends on the growth of sub-Rayleigh and intersonic dynamic shear ruptures

Earthquake ruptures are modeled as dynamically propagating shear cracks with the aim of gaining insight into the physical mechanisms governing their arrest or, otherwise, the often-observed variations in rupture speeds. Fault bends have been proposed as being the main cause for these variations. Following this line of reasoning, the existence of deviations from fault planarity is chosen as the main focus of this study. Asymmetric impact is used to generate shear loading and to propagate dynamic mode-II cracks along the bonded interfaces of two otherwise identical homogeneous constituents. Secondary paths inclined at various angles are also introduced to represent fault bends or kinks. The experiments show that certain fault bend inclinations are favored as alternate paths for rupture continuation, whereas others suppress further motion of the incoming rupture. The asymptotic elastodynamic stress fields at the tip of the growing rupture are used to develop two criteria (one energetic and one stress based) for rupture propagation or arrest at the kinked interfaces. These criteria correlate very well with the experimental results. Since most field evidence suggests that the average rupture speeds during crustal earthquakes are sub-Rayleigh, this work first focuses on incoming rupture speeds that are just below the Rayleigh wave speed. Reports of intersonic crustal fault rupture speeds having surfaced recently, experiments and analyses are also performed within that speed regime

2018 Timoshenko Medal Acceptance Lecture: Academic Family

Dedicated to my immediate academic family, my wonderful graduate students and postdocs and Stefan Timoshenko's academic great, great, great, great, great, great grandchildren. Dear friends, I was brought up in Greece to believe in the power of families. As a result families are very important to me and so are all of you, whom I consider to be my extended academic family. This is exactly the reason for which I feel so excited and honored to receive the Timoshenko Medal in front of you tonight since I truly consider all of you, working in the general area of mechanics at all length and time scales, as my cherished academic brothers and sisters, parents and grandparents. So please allow me to make “Academic Family” my theme for tonight, because I truly feel that in addition to inspiration and creativity, collegiality and mentoring are the two most important corner stones of our profession

Analysis of supershear transition regimes in rupture experiments : the effect of nucleation conditions and friction parameters

We consider the effect of the rupture initiation procedure on supershear transition of Mode II ruptures on interfaces governed by linear slip-weakening friction. Our study is motivated by recent experiments, which demonstrated the transition of spontaneous ruptures from sub-Rayleigh to supershear speeds in the laboratory. In these works the experiments were analysed using the Burridge–Andrews model of supershear transition, in which a supershear daughter crack is nucleated in front of the main mother rupture. It was concluded that the critical slip of the linear slip-weakening formulation needs to be pressure-dependent for a good match with experiments. However, the dynamic rupture initiation mechanism in the experiments was conceptually different from the quasi-static one adopted in the numerical work used for comparison. Here, our goal is to determine the effect of the nucleation by numerically modelling the experiments using a rupture initiation procedure that captures the dynamic nature of the wire explosion mechanism used in the experiments. We find parameter regimes that match the experimentally observed transition distances for the entire range of experimental conditions. Our simulations show that the dynamic rupture initiation procedure significantly affects the resulting transition distances, shortening them by about 30–50 per cent compared to those predicted through the quasi-static rupture initiation process. Moreover, for some cases, the dynamic initiation procedure changes the very mode of transition, causing a direct supershear transition at the tip of the main rupture instead of the mother–daughter mechanism. We find reasonable parameter regimes which match experimentally determined transition distances with both direct supershear transition at the rupture tip and the Burridge–Andrews (mother–daughter) mechanism, using both pressure-independent and pressure-dependent critical slip. The results show that there are trade-offs between the parameters of the rupture initiation procedure and the properties of interface friction. This underscores the importance of quantifying experimental parameters for proper interpretation of the experiments and highlights the importance of the rupture initiation procedure, in simulations of both experiments and real-life earthquake events

Million frames per second infrared imaging system

An infrared imaging system has been developed for measuring the temperature increase during the dynamic deformation of materials. The system consists of an 8×8 HgCdTe focal plane array, each with its own preamplifier. Outputs from the 64 detector/preamplifiers are digitized using a row-parallel scheme. In this approach, all 64 signals are simultaneously acquired and held using a bank of track and hold amplifiers. An array of eight 8:1 multiplexers then routes the signals to eight 10 MHz digitizers, acquiring data from each row of detectors in parallel. The maximum rate is one million frames per second. A fully reflective lens system was developed, consisting of two Schwarszchild objectives operating at infinite conjugation ratio. The ratio of the focal lengths of the objectives determines the lens magnification. The system has been used to image the distribution of temperature rise near the tip of a notch in a high strength steel sample (C-300) subjected to impact loading by a drop weight testing machine. The results show temperature rises at the crack tip up to around 70 K. Localization of temperature, and hence, of deformation into "U" shaped zones emanating from the notch tip is clearly seen, as is the onset of crack propagation

Rupture modes in laboratory earthquakes: Effect of fault prestress and nucleation conditions

Seismic inversions show that earthquake risetimes may be much shorter than the overall rupture duration, indicating that earthquakes may propagate as self-healing, pulse-like ruptures. Several mechanisms for producing pulse-like ruptures have been proposed, including velocity-weakening friction, interaction of dynamic rupture with fault geometry and local heterogeneity, and effect of bimaterial contrast. We present experimental results on rupture mode selection in laboratory earthquakes occurring on frictional interfaces, which were prestressed both in compression and in shear. Our experiments demonstrate that pulse-like ruptures can exist in the absence of a bimaterial effect or of local heterogeneities. We find a systematic variation from crack-like to pulse-like rupture modes with both (1) decreasing nondimensional shear prestress and (2) decreasing absolute levels of shear and normal prestress for the same value of nondimensional shear prestress. Both pulse-like and crack-like ruptures can propagate with either sub-Rayleigh or supershear rupture speeds. Our experimental results are consistent with theories of ruptures on velocity-weakening interfaces, implying that velocity-weakening friction plays an important role in governing the dynamic behavior of earthquake ruptures. We show that there is no measurable fault-normal stress decrease on the fault plane due to the nucleation procedure employed in experiments, and hence, this is not a factor in the rupture mode selection. We find that pulse-like ruptures correspond to the levels of nondimensional shear prestress significantly lower than the static friction coefficient, suggesting that faults hosting pulse-like ruptures may operate at low levels of prestress compared to their static strength

Crack-tip deformation field measurements using coherent gradient sensing

A real time, full field, lateral shearing interferometry - coherent gradient sensing (CGS) - has recently been developed for investigating fracture in transparent and opaque solids. The resulting interference patterns are related to the mechanical fields by means of a first order diffraction analysis. The method has been successfully applied to quasi-static and dynamic crack tip deformation field mapping in homogeneous and bimaterial fracture specimens