Laser shock adhesion test numerical optimization for composite bonding assessment

TLDR: In this article, the authors present the latest development of laser shock adhesion test (LASAT) technology, targeting the weak bond detection in bonded aeronautic structures.

About: This article is published in Composite Structures.The article was published on 2020-09-01 and is currently open access. It has received 16 citations till now. The article focuses on the topics: Shock (mechanics).

Figures

Fig. 11. Symmetrical laser shock investigations for first and highest stresses positioning on the interface (0.15 GPa – 4 mm diameter), time/position/pressure diagram with maximum stresses display for a) a symmetric assembly, Dt = 0 ns and b) a non-symmetric assembly, Dt = 400 ns.

Fig. 4. Sketch of the 2D effect phenomenon in a homogeneous material submitted to a laser shock – a) Time/position/pressure diagram for 0.3 GPa single shot with a 2 mm diameter focal spot, b) synthesis chart of the whole parametric study showing stress magnitude and stress location as a function of focal spot diameter.

Fig. 12. First experimental demonstration of the symmetrical laser shock configuration – experimental setup and its schematic description - Cross section observation of shocked composite assemblies (red lines for laser beams, white lines for damage).

Fig. 5. Numerical results on the bonded composite assembly in case of a single 0.3 GPa shot with various focal spot diameters when the laser beam hits the thick composite plate (a) or the thin composite plate (b) – Evidencing different locations of the maximum of tensile stresses.

Fig. 6. Synthesis charts of the 2D effect numerical investigations for bonding assemblies showing stress magnitude and stress location as a function of focal spot diameter, in case of thick plate (a) or thin plate (b) loading.

Fig. 10. Double pulse numerical investigations for bonded assembly (0.3 GPa–4 mm diameter – [50–500 ns]), a) Synthesis chart showing stress magnitude and stress location as a function of the delay between the two pulses, b) Optimal stress distribution for 250 ns delay between the pulses shown as a time/position/pressure diagram.

Frequently asked questions

Q1. What is the way to test a composite interlaminar?

Only the symmetrical laser shock gives the possibility to test a bond interface with a bond strength up to 100% of the composite interlaminar strength value, without damaging the composite laminates. 

Q2. How much of the composite interlaminar strength can be detected without damaging the composite skins?

It was also demonstrated that, using the conventional mono pulse configuration, only a bond with an adhesion strength value up to 40% of the composite interlaminar strength is possible to detect without damaging the composite skins. 

Q3. What is the importance of the position of the bondline with respect to the hit face?

The position of the bondline with respect to the hit face is important because it can have an influence on the stress distribution. 

Q4. What is the main reason why the model is considered as validated?

Although the model is considered as validated, it’s weakness lay in the attenuation modeling, leading to stress over-estimation, also estimated about 10% [25]. 

Q5. How much pressure is used to bond aluminum sacrificial layers?

With an aluminum sacrificial layer, and in a water confinement regime, the input pressure is about 1 GPa, for a 50 ns pulse width [27]. 

Q6. What can be done to quantify the level of stresses withstand by the interface?

Numerical results can then be used to quantify the level of stresses withstand by the interface, and to optimize the laser shock parameters in relation to applications. 

Q7. What is the effect of a shock coming back from the bondline?

A shock, coming back from the bondline, is reflected on the front face into a tensile wave which reloads the main tensile loading. 

Q8. What is the way to prevent disbonds?

Of course, this is a utopian perspective and mechanical fasteners will be used for the long term at least to prevent disbond initiation at tips, but a mass reduction of 5% is already significant in aeronautics [5]. 

Q9. What is the effect of a bending like loading on the ply?

for a single shot, after the spallation initiation, the damage propagation is helped by a three-point bending like loading. 

Q10. What is the effect of a second laser shock?

With a second laser shock from the opposite surface, this bending effect might be balanced, thus leading to a sharper delamination between the plies and less transvers cracks. 

Q11. What is the common reason why a weak bond is not detected?

This weakness can come from a surface contamination prior to bonding, or a curing process deviation, and thus cannot be detected using conventional NDT. 

Q12. How can the edge effects be tracked?

From this point, they can be tracked along the wave propagation because they are desynchronized compared to the main shock propagation.