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thickness Sp and the computed distance between the rear
surface and the first fracture predicted by the model. The
influences of the laser intensity, target thickness and pulse
duration are rather well reproduced by the simulation. That
overall coherence indicates the brittle fracture behaviour of
polycarbonate, characterized by a sudden failure with no sig-
nificant effect of a progressive damage growth. Quantitative
agreement is correct for thin targets. For thick targets, the
spall thickness is systematically over-estimated in the com-
putations. In other words, the first cracks appear before the
Fig. 10a,b. Measured (solid line) and computed (dotted lines) responses
model predicts that the failure criterion is met. That may be
of a piezoelectric transducer stuck at the back of a 620 µm-thick poly-
due to an inaccurate description of the pulse decay, more
carbonate target submitted to a laser shock of duration 2.3 ns and
critical for thick samples. Furthermore, the model is obvi-
intensity 511 GW/cm2 a or a laser shock of duration 0.6 ns and intensity
968 GW/cm2 b ously too simple to provide a satisfying prediction of incip-
ient damage, observed at low laser intensities. Indeed, the
324
computed spall thickness is too high in those conditions. In duced from the laser irradiation conditions. Then, the model
fact, the measured values must be discussed too, when the has been applied to determine the approximate stress histo-
nucleation of incipient cracks can be conditioned by two- ries experienced by the specimens in the spall experiments.
dimensional effects such as a slightly inhomogeneous en- Those stress histories have been correlated to the post-test
ergy deposition in the laser beam, or some tiny inclusion in observations, and a high threshold tensile stress for spall
the material. Such local effects may favor fracture while the damage has been inferred, confirming the good dynamic re-
overall tensile stress predicted by the macroscopic model re- sistance of polycarbonate. The complete model provides a
mains lower than the spall strength. Taking such effects into fair estimate of the location of the first cracks in the samples,
account would necessitate a much more sophisticated spall depending on the test parameters. However, it cannot pre-
criterion, as well as a very accurate description of the con- dict the whole fracture process, especially near the threshold
stitutive relations of the material in both compression and conditions leading to incipient spall. More experimental data
tension. The experimental data of the present study is too is needed to develop a more sophisticated fracture model.
limited and scattered to support an attempt to develop such
a model.
Acknowledgements. This work was supported by the GdR DRET / CNRS
#972. We thank all the staff in L.U.L.I. where the laser experiments have
been made.
5 Conclusion
References
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