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Sample records for homalite

  1. Dynamic fracture toughness

    NASA Technical Reports Server (NTRS)

    Kobayashi, A. S.; Ramulu, M.; Dadkhah, M. S.; Yang, K.-H.; Kang, B. S. J.

    1986-01-01

    Dynamic fracture toughness versus crack velocity relations of Homalite-100, polycarbonate, hardened 4340 steel and reaction bonded silicon nitride are reviewed and discrepancies with published data and their probable causes are discussed. Data scatter in published data are attributed in part to the observed fluctuations in crack velocities. The results reaffirmed our previous conclusion that the dynamic fracture toughness versus crack velocity relation is specimen dependent and that the dynamic arrest stress intensity factor is not a unique material property.

  2. Evaluation of static and dynamic contact stresses in simulated granular particles using strain gages

    SciTech Connect

    Xu, Y.; Shukla, A. )

    1993-01-01

    The application of strain gages for the determination of static and dynamic contact loads in granular particles is demonstrated. For experimental convenience, the granular particles are simulated by circular disks fabricated from Homalite-100, a brittle polyester material. Stress field equations in the vicinity of the contact points are carefully evaluated to optimize the relative position of strain gages. The results obtained from strain gages were compared with those obtained using the optical technique of photoelasticity for both static and dynamic problems. Finally, as an example, strain gages are used to study wave propagation in a single chain assembly of disks.

  3. Constraining friction laws by experimental observations and numerical simulations of various rupture modes

    NASA Astrophysics Data System (ADS)

    Lu, X.; Lapusta, N.; Rosakis, A. J.

    2006-12-01

    Several different types of friction laws, such as linear slip-weakening law and variants of rate- and state- dependent friction laws, are widely used in earthquake modeling. It is important to understand how much complexity one needs to include in a friction law to properly capture the dynamics of frictional rupture. Observations suggest that earthquake ruptures propagate as slip pulses (Heaton, 1990). In the absence of local heterogeneities and bimaterial effect, only one mechanism, namely strong rate-weakening friction, is shown, theoretically and numerically, to be capable of generating pulses on homogeneous interfaces separating two identical materials. We have observed pulses in our recent experiments designed to reproduce such a setting (Rosakis, Lu, Lapusta, AGU, 2006). By exploring experimental parameter space, we have identified different dynamic rupture modes including pulse-like, crack-like, and mixed modes. This suggests that rate weakening may play an important role in rupture dynamics. The systematic transition between rupture modes in the experiments is consistent with the theoretical and numerical study of Zheng and Rice (1998), who studied the behavior of rate-weakening interfaces. They concluded that whether strong rate weakening results in a pulse-like or crack-like behavior depends on the combination of two parameters: the level of prestress before rupture propagation and the amount of rate weakening on the fault. If we use Dieterich-Ruina rate-and-state friction laws with enhanced rate weakening at high slip rates, as appropriate for flash heating, to describe frictional properties of Homalite, use reasonable friction parameters motivated by previous studies, and apply Zheng and Rice analysis, we can qualitatively explain the rupture modes observed in experiments. Our current work is focused on modeling the experimental setup numerically to confirm that one indeed requires rate dependence of friction to reproduce experimental results. This

  4. Dynamic crack curving and branching in line-pipe

    SciTech Connect

    Ramulu, M.; Kang, B.S.J.; Kobayashi, A.S.

    1982-11-01

    The newly derived dynamic crack curving and crack branching criteria are briefly reviewed. The two criteria are justified by a micro-mechanics model and are used to predict crack curving and crack branching of dynamic photoelastic experiments involving Homalite-100 fracture specimens. The latter crack branching criterion requires as a necessary condition a critical dynamic stress intensity factor, K /SUB lb/ , which is accompanied by a sufficient condition involving the former crack curving criterion. This criteria is further verified by a dynamic finite element analysis of a bursting steel pipe of Congleton (26) and Almond, et al. (27), where the numerically computed branching stress intensity factor and branching angle are in good agreement with experimental results.

  5. On the role of microcracks in the dynamic fracture of brittle materials

    NASA Astrophysics Data System (ADS)

    Ravi-Chandar, K.; Yang, B.

    1997-04-01

    The dynamic fracture behavior of brittle materials is investigated. The morphology of the fracture surface is examined in detail in four polymers: polymethylmethacrylate, Solithane-113, Homalite-100 and poly-carbonate. The fracture surface markings are examined to determine the micromechanisms of fracture. This examination reveals clearly that the operative micromechanism that governs dynamic fracture in brittle materials is the nucleation, growth and coalescence of microcracks. Following a quantitative characterization of the microcracking patterns, a very simple nucleation and growth model is then put forward. Imposing nucleation and growth criteria based on the experimental observations, the simulation recreates the experimental observations, not only of the microscope surface features, but also of the macroscopic behavior such as the constancy of the crack speed.

  6. Laboratory Experiments and Theoretical Studies of Rupture Modes and Supershear Transition

    NASA Astrophysics Data System (ADS)

    Lu, X.; Lapusta, N.; Rosakis, A.

    2007-12-01

    Theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws, stress conditions on faults, energy partition and heat generation during earthquakes, scaling laws, and spatio-temporal complexity of fault slip. Early theoretical models often treated earthquakes as crack-like ruptures, but seismic inversions indicate that earthquake ruptures may propagate in a self-healing pulse-like mode. A number of explanations for the existence of slip pulses have been proposed, including strong weakening of the interface with sliding rate, interaction of rupture with local heterogeneities, and normal stress variation due to a bimaterial effect. We observe pulse-like and crack-like rupture modes in the experimental configuration of a Homalite plate with inclined interface prestressed both in compression and in shear, similarly to faults in the Earth's crust. Dynamic rupture is initiated by exploding a 0.1 mm nickel wire. Digital high-speed cameras are used to record photoelastic images. Two interferometry-based velocimeters are used to determine the history of relative sliding velocity at one location along the interface. Our results indicate that pulse-like ruptures can exist on such interfaces in the absence of a bimaterial effect or local heterogeneities. For a set of experiments with increasing ratio of shear to normal prestress, which is achieved by increasing the inclination angle of the interface, we observe a change in rupture modes from pulse-like to crack-like. This systematic variation is consistent with the theoretical study of velocity-weakening interfaces by Zheng and Rice (1998). We also establish experimentally, for the first time, that both pulse-like and crack-like rupture modes can transition to supershear speeds. After the supershear transition, both modes have speeds within the open interval \\sqrt{2} Cs to Cp, where Cs and Cp are the S- and P-wave speeds of Homalite, respectively. However, the rupture

  7. Identifying the unique ground motion signatures of supershear earthquakes: Theory and experiments

    NASA Astrophysics Data System (ADS)

    Mello, M.; Bhat, H. S.; Rosakis, A. J.; Kanamori, H.

    2010-10-01

    The near field ground motion signatures associated with sub-Rayleigh and supershear ruptures are investigated using the laboratory earthquake experiment originally developed by Rosakis and coworkers (Xia et al., 2004, 2005a; Lu et al., 2007; Rosakis et al., 2007). Heterodyne laser interferometers enable continuous, high bandwidth measurements of fault-normal (FN) and fault-parallel (FP) particle velocity "ground motion" records at discrete locations on the surface of a Homalite test specimen as a sub-Rayleigh or a supershear rupture sweeps along the frictional fault. Photoelastic interference fringes, acquired using high-speed digital photography, provide a synchronized, spatially resolved, whole field view of the advancing rupture tip and surrounding maximum shear stress field. Experimental results confirm that near field ground motion records associated with the passage of a sub-Rayleigh rupture are characterized by a FN velocity swing which dominates over the FP velocity swing. The situation is shown to reverse in the supershear rupture speed regime whereby the motion along the shear Mach front is characterized by a FP particle velocity swing which dominates over the FN velocity swing. Additional distinguishing particle velocity signatures, consistent with theoretical and numerical predictions, and repeatedly observed in experimental records are (1) a pronounced peak in the FP velocity record, induced by the leading dilatational field, which sweeps the measurement station just prior to the arrival of the shear Mach front, and (2) a pronounced velocity swing in the FN record associated with the arrival of a "trailing Rayleigh disturbance", which sweeps the measurement station following passage of the shear Mach front. Each of these features are addressed in detail. We conclude by reexamining the 2002, Mw7.9 Denali fault earthquake and the remarkable set of ground motion records obtained at Pump Station 10 (PS10), located approximately 85 km east of the epicenter

  8. Finite element simulations of dynamic shear rupture experiments and dynamic path selection along kinked and branched faults

    NASA Astrophysics Data System (ADS)

    Templeton, Elizabeth L.; Baudet, AuréLie; Bhat, Harsha S.; Dmowska, Renata; Rice, James R.; Rosakis, Ares J.; Rousseau, Carl-Ernst

    2009-08-01

    We analyze the nucleation and propagation of shear cracks along nonplanar, kinked, and branched fault paths corresponding to the configurations used in recent laboratory fracture studies by Rousseau and Rosakis (2003, 2009). The aim is to reproduce numerically those shear rupture experiments and from that provide an insight into processes which are active when a crack, initially propagating in mode II along a straight path, interacts with a bend in the fault or a branching junction. The experiments involved impact loading of thin Homalite-100 (a photoelastic polymer) plates, which had been cut along bent or branched paths and weakly glued back together everywhere except along a starter notch near the impact site. Strain gage recordings and high-speed photography of isochromatic lines provided characterization of the transient deformation fields associated with the impact and fracture propagation. We found that dynamic explicit 2-D plane-stress finite element analyses with a simple linear slip-weakening description of cohesive and frictional strength of the bonded interfaces can reproduce the qualitative rupture behavior past the bend and branch junctions in most cases and reproduce the principal features revealed by the photographs of dynamic isochromatic line patterns. The presence of a kink or branch can cause an abrupt change in rupture propagation velocity. Additionally, the finite element results allow comparison between total slip accumulated along the main and inclined fault segments. We found that slip along inclined faults can be substantially less than slip along the main fault, and the amount depends on the branch angle and kink or branch configuration.

  9. Off-fault tensile cracks: A link between geological fault observations, experiments and earthquake rupture models

    NASA Astrophysics Data System (ADS)

    Ngo, D.; Huang, Y.; Rosakis, A.; Griffith, W. A.; Pollard, D. D.

    2009-12-01

    Motivated by the occurrence of high-angle pseudotachylite injection veins along exhumed faults, we use optical experiments and high-speed photography to interpret the origins of tensile fractures that form during dynamic shear rupture in laboratory experiments. Sub-Rayleigh (slower than the Rayleigh wave speed) shear ruptures in Homalite-100 produce damage zones consisting of a periodic array of tensile cracks. These cracks nucleate and grow within cohesive zones behind the tips of shear ruptures that propagate dynamically along interfaces with frictional and cohesive strength. The tensile cracks are produced only along one side of the interface where transient, fault-parallel, tensile stress perturbations are associated with the growing shear rupture tip. We use an analytical, linear velocity weakening, rupture model to examine the local nature of the dynamic stress field in the vicinity of the tip of the main shear rupture which grows along a weak plane (fault) with sub-Rayleigh speed. It is this stress field which is responsible for driving the off-fault mode-I microcracks that grow during the experiments. We show that (1) the orientation of the cracks can be explained by this analytical model; and (2) the cracks can be used to simultaneously constrain the constitutive behavior of the shear rupture tip. In addition, we propose an extension of this model to explain damage structures observed along exhumed faults. Results of this study represent an important bridge between geological observations of structures preserved along exhumed faults, laboratory experiments and theoretical models of earthquake propagation, potentially leading to diagnostic criteria for interpreting velocity, directivity, and static pre-stress state associated with past earthquakes on exhumed faults.

  10. Dynamic Optical Investigations of Hypervelocity Impact Damage

    NASA Astrophysics Data System (ADS)

    Lamberson, Leslie Elise

    One of the prominent threats in the endeavor to develop next-generation space assets is the risk of space debris impact in earth's orbit and micrometeoroid impact damage in near-earth orbit and deep space. To date, there is no study available which concentrates on the analysis of dynamic crack growth from hypervelocity impacts on such structures, resulting in their eventual catastrophic degradation. Experiments conducted using a unique two-stage light-gas gun facility have examined the in situ dynamic fracture of brittle polymers subjected to this high-energy-density event. Optical techniques of caustics and photoelasticity, combined with high-speed photography up to 100 million frames per second, analyze crack growth behavior of Mylar and Homalite 100 thin plates after impact by a 1.8 mm diameter nylon 6-6 right cylindrical slug at velocities ranging from 3 to 7 km/s (7000--15500 mph). Crack speeds in both polymers averaged between 0.2 and 0.47 cR, the Rayleigh wave speed (450--1000 mph). Shadow spots and surrounding caustics reveal time histories of the dynamic stress intensity factor, as well as the energy release rate ahead of the mode-I, or opening, crack tips. Results indicate that even under extreme impact conditions of out of-plane loading, highly localized heating, and energetic impact phenomena involving plasma formation and ejecta, the dynamic fracture process occurs during a deformation regime dominated by in-plane loading. These findings imply that the reliability of impacted, thin-walled, plate and shell space structures, idealized by the experimental configuration investigated, can be predicted by the well defined principles of classical dynamic fracture mechanics.

  11. Finite Element Modeling of Dynamic Shear Rupture Experiments Along Non-Planar Faults

    NASA Astrophysics Data System (ADS)

    Templeton, E. L.; Baudet, A.; Bhat, H. S.; Rice, J. R.

    2004-12-01

    The study of dynamically propagating shear cracks along weak paths like faults is of great interest for the study of earthquakes. We adapted the ABAQUS/Explicit dynamic finite element program to analyze the nucleation and propagation of shear cracks along a non-planar, kinked, weak path corresponding to the one that was used in recent laboratory fracture studies by Rousseau and Rosakis [JGR, 2003]. Their experiments involved impact loading of thin plates of Homalite-100, a photoelastically sensitive brittle polymer, which had been cut along a kinked path and then weakly glued back together everywhere except along a starter notch near the impact site. Under different conditions, propagation speeds were observed in both the sub-Rayleigh and intersonic (supershear) regimes. Strain gage recordings and high speed photography of isochromatic lines (lines of constant difference between the in-plane principal strains) provided characterization of the transient deformation fields associated with the impact and fracture propagation. For the finite element analyses, we implemented a slip-weakening failure model through an option in the ABAQUS program allowing user defined constitutive relations. The analyses of impact loading and of rupture nucleation and propagation were then carried out in the 2D framework of plane stress. In a first set of studies of nucleation and propagation of rupture along a straight fault, we determined after some trial and error an appropriate CFL number, and examined different element types and layouts, finding that the most acceptable results were obtained for low order elements. We used constant strain triangles, arrayed in groups of four to effectively form four-sided elements with corner nodes and one internal node. The studies also showed that to obtain representations of slip rate and shear stress near the propagating rupture tip that were relatively free from numerical oscillations, it was necessary to have element side lengths of order Ro/50

  12. Identifying the Unique Ground Motion Signatures of Supershear Earthquakes: Theory and Experiments

    NASA Astrophysics Data System (ADS)

    Mello, Michael

    The near-field ground motion signatures associated with sub-Rayleigh and supershear ruptures are investigated using the laboratory earthquake experiment originally developed by Rosakis and co-workers (Xia et al., 2004, 2005; Lu et al., 2007; Rosakis et al., 2007). Heterodyne laser interferometers enable continuous, high-bandwidth measurements of fault-normal (FN), fault-parallel (FP), and vertical (V) particle velocity ``ground motion" records at discrete locations on the surface of a Homalite-100 test specimen as a sub-Rayleigh or a supershear rupture sweeps along the frictional fault. Photoelastic interference fringes, acquired using high-speed digital photography, provide a synchronized, spatially resolved, whole field view of the advancing rupture tip and surrounding maximum shear stress field. The first phase of experimental investigations examine and verify the ground motion signatures of supershear ruptures. Experimental results demonstrate that a shear Mach front produced by a stable supershear rupture is characterized by a dominant FP velocity component. The situation is shown to reverse in the sub-Rayleigh rupture speed regime whereby the FN particle velocity component dominates the ground motion record. Additional distinguishing particle velocity signatures, consistent with theoretical and numerical predictions, and repeatedly observed in experimental records are, (1) a pronounced peak in the FP velocity record induced by the leading dilatational field, which sweeps the measurement station in advance of the shear Mach front, and (2) a pronounced velocity swing in the FN record associated with the arrival of a trailing Rayleigh sub-Rayleigh (secondary) rupture, which follows the arrival of the shear Mach front. Analysis of the particle velocity records also confirms 2D steady-state theoretical predictions pertaining to the separation, attenuation, and radiation partitioning of the shear and dilatational portions of the rupture velocity field components

  13. Testing Friction Laws by Comparing Simulation Results With Experiments of Spontaneous Dynamic Rupture

    NASA Astrophysics Data System (ADS)

    Lu, X.; Lapusta, N.; Rosakis, A. J.

    2005-12-01

    Friction laws are typically introduced either based on theoretic ideas or by fitting laboratory experiments that reproduce only a small subset of possible behaviors. Hence it is important to validate the resulting laws by modeling experiments that produce spontaneous frictional behavior. Here we simulate experiments of spontaneous rupture transition from sub-Rayleigh to supershear done by Xia et al. (Science, 2004). In the experiments, two thin Homalite plates are pressed together along an inclined interface. Compressive load P is applied to the edges of the plates and the rupture is triggered by an explosion of a small wire. Xia et al. (2004) link the transition in their experiments to the Burridge-Andrews mechanism (Andrews, JGR, 1976) which involves initiation of a daughter crack in front of the main rupture. Xia et al. have measured transition lengths for different values of the load P and compared their results with numerical simulations of Andrews who used linear slip-weakening friction. They conclude that to obtain a good fit they need to assume that the critical slip of the slip-weakening law scales as P-1/2, as proposed by Ohnaka (JGR, 2003). Hence our first goal is to verify whether the dependence of the critical slip on the compressive load P is indeed necessary for a good fit to experimental measurements. To test that, we conducted simulations of the experiments by using boundary integral methodology in its spectral formulation (Perrin et al., 1995; Geubelle and Rice, 1995). We approximately model the wire explosion by temporary normal stress decrease in the region of the interface comparable to the size of the exploding wire. The simulations show good agreement of the transition length with the experimental results for different values of the load P, even though we keep the critical slip constant. Hence the dependence of the critical slip on P is not necessary to fit the experimental measurements. The inconsistency between Andrews' numerical results

  14. Finite Element Simulations of Dynamic Shear Rupture Experiments and Path Selection Along Branched Faults

    NASA Astrophysics Data System (ADS)

    Templeton, E. L.; Baudet, A.; Bhat, H. S.; Dmowska, R.; Rice, J. R.; Rosakis, A. J.; Rousseau, C. E.

    2005-12-01

    The study of dynamically propagating shear cracks along geometrically complex paths is important to understanding the mechanics of earthquakes. Recent laboratory fracture studies of Rousseau and Rosakis examined a branched configuration, analogous to their study of rupture along a bent fault path [Rousseau and Rosakis, JGR, 2003], to enhance understanding of the behavior of a shear rupture approaching the intersection of two paths. Whereas crack motion along a simple bent path is readily explained by means of the energy available to sustain the propagating crack, or through a crack tip stress field criterion, the behavior of multiple paths displays more intricate variations featuring the inability of the crack to extend along secondary paths situated at shallow angles with respect to the initial direction of propagation. Secondary paths located at larger angles, on the extensional side, generally promote simultaneous extension along both paths beyond the junction, in contrast to preferred motion along the straight path, which is favored when secondary paths are situated on the compressional side. The experiments involve impact loading of thin plates of Homalite-100, a photoelastic polymer, which are cut along branched paths and weakly glued back together everywhere except along a starter notch near the impact site. High-speed photography of isochromatic fringe patterns (lines of constant difference between in-plane principal stresses) characterized the transient deformation field associated with the impact and rupture propagation. We adapted the ABAQUS/Explicit dynamic finite element program to analyze the propagation of shear cracks along such branched weakened paths. Two configurations for weakened paths, branches at 35° to the compressional side and the extensional side, were analyzed. We implemented a linear slip-weakening failure model as a user-defined constitutive relation within the ABAQUS program, where weakening could be included in either or both of (1