Porosity Distribution in Composite Structures with Infrared Thermography

Abstract

Composite structures are increasingly used in the transport industry especially in the aeronautical sector thanks to their favorable strength-to-weight ratio with respect to metals. However, this is true if the final part is defects free and complies with quality requirements. A main weakness in composites is porosity, which is likely to be introduced during manufacturing processes and which may knock down the material characteristics affecting its performance in service. Porosity plays a key role in sandwich structures, which involve novel metal foams as core, since the foam performance strongly depends on size and distribution of pores. The determination of porosity is mostly attained by destructive methods, which supply only a general indication linked to the production part number. Conversely, composites may entail local significant variation of porosity, which may be discovered only with effective nondestructive techniques. The attention of the present work is focused on the possibility to use infrared thermography to get information about the amount and distribution of porosity. In particular, two techniques: flash thermography and lock-in thermography are used to comply with requirements of both monolithic composites and metal foams.

Flash Thermography to Evaluate Porosity in Carbon Fiber Reinforced Polymer (CFRPs)

“…In particular, a pack-threads effect of fibers is observed mainly for Pu specimens cured at P c = 50% and 25%. On the other side, this effect was already visualized [ 34 ] within unidirectional fibers as mainly pronounced for the shorter fibers oriented at 90° (along the shorter side). For the plots in Figures 8 and 9 , the error bars are visualized to account for the variability of data with respect to the correlation curve.…”

“…A more extensive investigation, involving a large number of CFRP samples of several different stacking sequences and with different percentages of porosity, was recently carried out by Toscano et al [ 32 , 33 ]. Some samples similar to those used by Toscano et al [ 32 – 34 ] are herein considered to better investigate the influence of the fibers’ orientation coupled with a variation of porosity percentage. Such specimens are tested with both flash thermography and ultrasonics with the purpose of a data comparison.…”

“…In this section, several specimens of two stacking sequences, which were fabricated following the procedure already used by Toscano et al [ 32 – 34 ], are considered. Data obtained with flash thermography are compared to those obtained with destructive methods and with ultrasonic testing.…”

“…As already pointed out, β is a good marker of porosity. Therefore, from each attenuation c-scan, the average attenuation β value was measured over an area of almost 20,000 points [ 5 , 34 ] (excluding the area within the Kapton insert). For both coupon types, results are presented in terms of attenuation c-scans (with decibel units) in Figure 4a,b and of ToF c-scans (with microsecond units) in Figure 5a,b ; an arrow indicates the position of the Kapton disk in each map.…”

A convection–diffusion porous media model for moisture transport in polymer composites: Model development and validation

“…The pore network in polymer composites may be viewed as an assortment of imperfections or cracks in the material. These may be the result of debonds between the fibers/fillers and the resin matrix, even at the initial stage of material …”