JEC COMPOSITES MAGAZINE - Issue #116 - October 2017 - 61

Non-destructive evaluation of the mechanical strength of a TA6V4/composite bond using LASAT

mismatch between the TA6V and the epoxy.
This propagation pattern generates successive release waves in the
epoxy layer between 1100 and 2000 nanoseconds, progressively inducing tensile stresses in the bond by recombination with the incident
release waves. If the initial loading (laser ablation pressure) is sufficiently high, the resulting tensile stresses can exceed the mechanical
strength of the bond and/or the composite/epoxy interface and hence
generate a disbonding. The maximum tensile stresses in the bond are
reached following shock waves through the thickness of the sample
(t=1500ns).

maximum tensile stresses exceed the mechanical strength of the bond
and/or the composite/epoxy interface.
However, in this case, the shock waves propagate twice through the
composite before generating maximum tensile stresses at the interface
(t=2500 ns).
The shock wave propagation patterns shown here suggest that the
composite illumination side is an optimum configuration for LASAT.
Shock waves propagate on a shorter acoustic path in that configuration
and, hence, are less subjected to attenuation through the composite.
This is believed to account for the dependence of disbonding thresholds on the illumination side. However, 2D or 3D simulations with a
developed composite model, taking anisotropy into account, are required. This should enable a quantitative simulation of shockwave attenuation and thus provide a more precise stress evaluation within the
stacking. A quantitative evaluation of the bond mechanical strength
would be possible with the set of experimental data exposed here.

In the case of TA6V irradiation (Figure 6), the incident shock wave is
generated in the TA6V layer and then partially transmitted as a shock
wave at the TA6V/epoxy interface (X=-0.4mm). The wave reflected at
this interface is a release wave (ZTA6V > ZEpoxy) that goes back to the
front face and reflects again as a shock wave in the TA6V layer. The shock
wave pattern in the TA6V layer is very comparable to the one exposed
in the composite illumination configuration, except that it generates
successive shock waves in the rest of the sample. These shock waves travel
in the epoxy layer and are transmitted in the composite as a shock wave.
The reflection of these shock waves at the epoxy/composite interface
(X=-0.55mm) is neglected here due to their low
amplitude. The first incident shock wave in the composite
reaches the composite free surface of the sample
(X=-4.35mm - t=1100ns), where it is fully reflected as a release wave.
The recombination of this reflected release wave with incident ones
creates a tensile stress zone. This tensile zone then goes backward and
reaches the epoxy layer. As described before, disbonding occurs if the

In fact, the FSVs recorded during the experiments leading to bond failure exhibit a spallation- type curve [9] [10] . In Figure 7, for example,
the laser shot achieved at 0.97GW/cm² (black curve) shows a clear
back and forth travel of shock waves in the whole stacking. FSV oscil-

Fig. 5: (a) Time-position (X-t) stress diagram; and (b)
stress evolution at the epoxy/composite interface in the
composite illumination case

Fig. 6: (a) Time-position (X-t) stress diagram; and (b)
stress evolution at the epoxy/composite interface in the
TA6V illumination case

Free surface velocity measurement
Free surface velocities (FSV) on the back face of the sample also
showed some interesting results. In the configurations with laser illumination on the composite side, a strong correlation between FSVs
and ultrasonic diagnostics can be identified.

N°116 October 2017

/ jec composites magazine

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