Depth penetration in composites using pulsed thermography: the influence of pulse energy

Depth penetration in composites using pulsed thermography: the influence of pulse energy Jacques Lewandowski1, Clemente Ibarra-Castanedo2, Benoit Masson3 1. Centre technologique en aérospatiale, 5555 place de la Savane,... [ view full abstract ]

Depth penetration in composites using pulsed thermography:
the influence of pulse energy
Jacques Lewandowski1, Clemente Ibarra-Castanedo2, Benoit Masson3
1. Centre technologique en aérospatiale, 5555 place de la Savane, St-Hubert (QC) J3Y 8Y9 Canada
2. Visiooimage inc.,2560, rue Lapointe, Québec (QC) G1W 1A8, Canada
3. Bombardier Aérostructures et services d’ingénierie, 2351 Boul. Alfred Nobel, Saint-Laurent, QC, H4S 2A9, Canada
Corresponding author : jacques.lewandowski@cegepmontpetit.ca and (450) 678-2001, ext.4659
Keywords : Pulsed thermography, Composite materials, Depth penetration, Pulse energy, Small radius

Abstract

Ultrasound testing (UT) is the standard non-destructive testing (NDT) technique for the inspection of aerospace composites. UT however suffers from some drawbacks. More specifically, UT becomes more complicated to perform when complex structures exhibiting small radius are to be inspected. Active thermography is an alternative NDT technique that, even though is generally limited to lesser depth penetration than UT, allows large field analysis and has demonstrated to be adequate for the assessment of several complex structures.
This paper will show experimental results using pulsed thermography (PT) for the detection of inserts located inside the small radius of a T-Stringer (Figure 1). Several Teflon artificial defects (diameter 3 and 5 mm) were inserted at three different depths (shallow = 0.14 mm, deep = 0.50 mm and deepest = 1 mm). The impact of the pulse energy (6 kJ and 12 kJ) on insert detection is reported and experiments show that depth penetration is not proportional to pulse energy. However, doubling the pulse energy has the advantage of improving detection and contrast of any insert; although, as it will be shown, the energy increase is not enough to clearly reveal the deepest inserts.
To analyze the effect of pulse energy on depth penetration, without the effect of radius, a planar reference composite plate containing 17 inserts (0,25 mm ≤ depth ≤ 4,25 mm) was inspected. The pulse energy was varied from 94 J to 3000 J. Raw images together with Principal Component Thermography (PCT) and Fourier Transform (FT) phase were used to determine depth penetration of PT.
Experimental results show that some defects are already detected with the lowest pulse energy (as low as 94 J) and that the increase in depth penetration decreases rapidly with pulse energy. Experimental results showing depth penetration vs. pulse energy will be illustrated both with PCT and FT phase analysis (see figure 2). These experimental results are in agreement with simulations found in other scientific papers.