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Damage in Transparent Materials

#3391


Experimental, Theoretical, and Numerical Studies of Damage Nucleation and Growth in Transparent Materials

Tech Area / Field

  • PHY-SSP/Solid State Physics/Physics
  • MAT-ELE/Organic and Electronics Materials/Materials

Status
3 Approved without Funding

Registration date
09.11.2005

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Collaborators

  • The Boeing Company, USA, CA, Huntington Beach\nLos Alamos National Laboratory, USA, NM, Los-Alamos

Project summary

An ability to predict the behavior of engineering materials subjected to an arbitrary loading environment is of fundamental interest to industry. Difficult enough in the case of quasistatic loading, the problem poses an even greater challenge when the material is subjected to a loading environment wherein the dominant gradients in applied load take place over micro-second or even sub-microsecond time scales. Complicating matters further, is the necessity of predicting the initiation, evolution, and effects of damage in cases where the applied loads are of sufficient magnitude to cause damage that is of critical importance in some engineering applications.

A tool, which researchers have applied with some success to the problem of assessing the behavior of engineering materials to “large” impulsive loads, is the so-called spall test. In this test, the specimen is loaded dynamically in such a way as to produce one or more compressive shock waves within the specimen. With careful design of the experiment, it is possible to tailor the interaction of subsequent rarefaction waves (produced when a shock wave interacts with a boundary), in such a way as to create a region of near hydrostatic tension in the interior of the specimen. Some of the first papers reporting on the use of this technique date to the 1950s. Despite approximately sixty years of investigation, however, many questions remain unanswered, particularly with respect to the mechanisms of damage initiation, the evolutionary process of damage growth, and the behavior of damaged material.

Current understanding of the damage process (at least as it applies to ductile materials), involves the initiation of localized damage sites (often with the morphology of pores or voids), which over time grow (during which time new sites of localized damage may form) and ultimately coalesce in such a way as to produce macroscopic damage, such as a crack. One difficulty in gaining a full understanding of the process of damage formation and evolution is that it occurs in the interior of the test sample, making direct optical observations impossible (for most materials), and making meaningful observations using other diagnostic techniques impractical because of a issues with resolution. Moreover, since most studies of the spall process have been directed at metals, most studies have occurred without the benefit of direct optical measurement. Lacking such observations, what most researchers have done is to conduct multiple experiments, loading a number of specimens with a variety of loading magnitudes in order to produce recovered specimens with varying degrees of damage (from incipient to macroscopic). These specimens would then be examined using metallographic techniques to assess the damage morphology. The observed morphology would be compared to that predicted by numerical models. If the model was capable of predicting the resultant damage state, it was hoped that its predictions of the process involved in arriving at that state would at least be qualitatively reliable. Accurately assessing the kinetics of damage formation in this scenario (using opaque materials) is beyond the reach of existing diagnostics. It would be extremely valuable to conduct experiments with “transparent” materials so that real-time observations of the damage process could be made.

The results of numerous experiments show that the mechanisms involved in spall fracture (micro-pore formation followed by growth and coalescence) is common for both metals and at least some polymeric materials /1-3/. We therefore propose, the use of a “transparent material,” for example PMMA, in studies aimed at obtaining real-time direct observation of the damage initiation and evolution process. Taking this approach affords the opportunity of gaining a better understanding of the process of damage formation and evolution, and an opportunity to then apply that understanding to the improvement of our predictive capabilities. It is expected, that at a minimum, aspects of what is learned in this study will prove to have applicability in predicting the behavior of materials other than the particular subject of this study (the experimental evidence alluded to above would seem to support this expectation). The timeliness of this study, and its value to industry, are evidenced by the many publications that have appeared in the literature during recent years.

VNIIEF possesses a wealth of experience (and is a recognized world leader), in conducting the types of experiments (e.g., spall experiments involving discs or wedges) called for in this proposal. The experience of the well-respected VNIIEF theoretical-mathematical pision will also be brought to bear for purposes of numerical and theoretical analysis in the performed study. Numerical simulations using available 2D and 3D codes, which are the part of the Lagrangian DMK code /5/, shall be used in the selection of optimal test conditions. Experimental results will be used to guide potential improvements to a VNIIEF-developed two-stage micro-statistical kinetic model of spall fracture (NAG type model) /1,4/. This model, which has been under development since the late 1980s at VNIIEF, has proven to be effective in predicting some aspects of spallation in metals (uranium, copper, Armco-iron, nickel, titanium, etc.), and has been shown to have advantages over some of the other models evaluated /6-9/. The model has been effectively applied to problems of both planar and cylindrical geometry /9/.

The following activities are planned in support of the proposal objectives:

  • analyze and generalize the available experimental data on spall fracture of PMMA for the purpose of determining unexplored area of stress state characteristics;
  • analyze possible schemes of the test set-up by using numerical simulation;
  • work out the experimental complex, including the systems for recording of fast-passing processes;
  • perform a series of experiments to study the growth of damage in PMMA;
  • where necessary modify the existing model of spall fracture to describe the transient processes more adequately.

In executing the experimental components of the project, it is proposed to use samples in the form of discs and wedges. Loading of the samples is to be accomplished using a thin layer of high explosive (HE). Hence, the scope of activities under the project may be pided along two branches: experimental and theoretical.

Execution of the test series will involve the following:

  • methods for loading the samples by plane and sliding detonation waves, including initiation systems;
  • method of optical recording of fast-passing processes by using of high-speed cameras;
  • methodology of light dispersion on micro-defects, arising during the fracture process, which includes method of light dispersion indicatrix recording;
  • method of post-shot study of the sample state.

The tests will be performed at internal tests sites of VNIIEF using available and purchased equipment (after modification where necessary).

2D and 3D numerical capabilities, present in the context of the DMK code are available at the VNIIEF Institute of Theoretical and Mathematical Physics (ITMF), and will be used. These capabilities have been developed over the last three decades. In order to improve the VNIIEF-developed model of spall fracture, the following will be done:

  • modification (where necessary) of the numerical algorithm;
  • verification of spall fracture model parameters in accordance with available information;
  • numerical simulation of the performed experiments (validation of the model).

In summary, the execution of this project will provide new information pertaining to the fundamental nature of the processes of nucleation and growth of damage in solid “transparent” materials under different conditions of loading, including one- and two-dimensional stress-strain states. This will contribute to an improved understanding of the nature of these processes in the material under study. There is also reason to believe that the knowledge gained through this study will find application to other engineering materials as well.

In accordance with the scope of activities in the project framework, the following lines of cooperation with foreign collaborators are supposed:

  • participation in working out of project proposal, working plan, and research program;
  • interchange of information during the project execution; holding of joint workshops, meetings and consultations;
  • joint use of test materials (samples and obtained results);
  • participation in on-site monitoring and audit of all activities of the project;
  • continuing scientific and technical cooperation based on the experience gained during project execution, which will contribute to finding commercial applications for the results.

Execution of the proposed ISTC project will provide scientists and experts involved in weapon design and development, an opportunity for scientific and research activities in peaceful areas, namely, in the area of fundamental study of the behavior of engineering material under impulse loadings. The reputation that VNIIEF has for doing the highest quality work, combined with a high level of interest within industry research and development organizations, will enable the project participants to become more closely integrated into the international scientific research and development community by means of making presentations at international conferences and symposiums and through joint publications in refereed journals.

References

  1. Barbee T., Seaman L., Crewdson R., Curran D. J.Materials. 1972. V. 7. # 3. P. 393.
  2. Fracture of non-uniformly scaled objects under explosion // edited by A.G.Ivanov. RFNC-VNIIEF. Sarov. 2001. 482 p. (in Russian).
  3. V.K.Golubev, S.A.Novikov, Yu.S.Sobolev PMTF. 1982. № 1. P. 143-150. (in Russian).
  4. Seaman L., Curran D., Shockey D. J.Appl.Phys. 1976. V. 47. # 11. P. 4814-4826.
  5. V.V.Rasskazova, V.N.Motlokhov, A.N.Shaporenko, et al VANT, Ser. Math. Modelling of Physical Processes, 1999, No.4, pp.51-56.
  6. B.L.Glushak, I.R.Trunin, O.A.Uvarova Chem.Phys.Reports. 1998. Vol. 17 (1-2). P.307-316.
  7. B.L.Glushak, I.R.Trunin, O.A.Uvarova Chem.Phys.Reports. 2000. Vol. 19 (2). P.317-325.
  8. B.L.Glushak, I.R.Trunin, O.A.Uvarova, S.V.Koritskaya J. Phys. IV, France, 2000, N 10, p. 439-444.
  9. A.G.Ioilev, B.L.Glushak, A.A.Sadovoy et al. Intern. Journal of Impact Engineering. 2003. Vol. 29. P. 369-375.


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