JEC COMPOSITES MAGAZINE - Issue #116 - October 2017 - 62

TECHNOLOGY NDT
lates around 0 m/s. A negative FSV might be induced by two-dimensional effects. The corresponding period for this type of event is about
3 μs, which is coherent with the longitudinal shock velocity of the constitutive materials. Conversely, disbonding implies the creation of a free
surface at the TA6V/epoxy interface, meaning this layer is not unloaded by release waves like the rest of the sample and, therefore, its FSV
profile exhibits a quasi-constant velocity just after the main peak. Back
and forth travels of the shock waves in the TA6V layer will generate
FSV oscillations after disbonding (Figure 5 (a)). However, the temporal resolution of the PDV analysis used here (50 ns) will prevent these
oscillations from being detected. It also seems that FSV curves might
be impacted by two-dimensional effects as negative values are visible
in some cases. These edge effects are hardly understandable without an
appropriate simulation.
Non-destructive testing
Finally, post-shock micrographs of the samples subjected to the highest
intensities did not reveal any failure in the composite or the TA6V layer.
A separate characterization of the TA6V and the 3D composite layer
under laser shock in the same conditions (thickness and laser intensity)
had already resulted in the same observation.
These observations show that in this case, laser-induced shocks do not
seem to have affected the material. The LASAT configuration used
here does not seem to induce damage in the substrates. Regarding the
nature of the substrates and the respective thickness of the materials, it
seems that the LASAT configurations used in this study are non-destructive.
Deeper investigations are required to assess LASAT as a non-destructive method. Mechanical testing after laser shocks is planned to confirm that the substrates and the joint are not degraded below their respective damaging threshold.

Conclusion
The LASAT configuration used can discriminate a weak bond from a
healthy one with the help of disbonding diagnostics. In addition, the
healthy bond's disbonding threshold was clearly identified. This means
that a weak bond just beyond the tolerance (+/- 20% of the healthy
strength) should also be discriminated using the same configuration.
Some further experiments should be conducted with such a bond
quality.
In-situ free surface velocimetry provided encouraging results for detecting disbonding in a given configuration. Deeper investigations are
necessary to confirm this technique's potential to detect disbonding
coupled with laser-induced shock waves.
Post-shock diagnostics confirmed the potential of LASAT as a non-destructive method. However, some modifications of the material due to
the shock might be non-detectable, even with micrographs. Mechanical
testing of samples subjected to laser shocks is necessary to demonstrate
the non-destructive nature of such a technology.
1D simulation was used to build the time-position diagrams required
to understand the shock wave propagation patterns involved. Based on

jec composites magazine / N°116 October 2017

Fig. 7: Evolution of the free surface velocity with time for composite illumination
shots on a weak bond - Temporal resolution = 50 ns - Signals in dotted lines
correspond to identified disbonding configurations

these tools, an interpretation of the disbonding threshold dependence
on the illumination side was proposed.
2D or 3D simulations are needed to simulate attenuation phenomena and edge effects more precisely. A quantitative simulation could
provide more information on the evolution of stresses within the substrates and, if needed, help to define a more optimized configuration
to lower stresses in the substrates, while still covering weak bond detection. In addition, the mechanical resistance of bonds under shock
loading could be determined using such a simulation.
More information:
Ducousso Mathieu (Safran), mathieu.ducousso@safrangroup.com
Bergara Tomas (Rescoll), tomas.bergara@rescoll.fr
Laurent Berthe (PIMM laboratory), laurent.berthe@ensam.eu
Acknowledgement
The authors would like to acknowledge the consortium of the Compochoc project composed by the following companies: Alphanove, Airbus, CEA, Cilas, Idil,
Kuka Aerospace,Laser Metrologie, PIMM, PSA Peugeot Citroen, Rescoll, Safran,
Simchoc. This project received funding from the FUI french program and
Nouvelle-Aquitaine and Ile de France regions with the support of Aerospace
Valley, Astech and Route des Lasers.

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Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #116 - October 2017