*2.1. Experimental Composite Wing Specimen*

The composite wing being tested came from a two-seater aircraft designed for the LSA category. The maximum take-off mass of the aircraft is 600 kg, with a wingspan of about 7.2 m and a length of 7.5 m. The aircraft is powered by a piston engine. This category of aircraft is characterized by the use of extremely thin-walled structures. Commonly used NDE methods are not suitable for this task because of their lower sensitivity.

#### 2.1.1. Composite Wing Description

The test sample was the right half of the wing of a small composite aircraft demonstrator. The wing was used for the development of static structural strength testing. Its structure consisted of spars and ribs made from CFRP. The wing skins were made of a sandwich structure consisting of a carbon sheet and a foam core. The outer and inner skins were made from one layer of CFRP fabric, using TeXtreme® 100 material (Oxeon AB, Boras, Sweden), with a nominal layer thickness of 0.1 mm. Wing skins were glued to the load-bearing structure using the HexBond® EA9394 epoxy adhesive (Hexcel Corporation, Stamford, CT, USA). Bonding was done by applying a thin layer of adhesive to the wing skin and a thicker layer of glue in the shape of a "snake" on the wing spar caps and ribs. Subsequent curing was done in the assembly jig. The maximum thickness of the adhesive layer should be up to 1.5 mm. Figure 1 shows a system drawing of the upper half of the wing (the lower half is similar), with the areas of interest marked for testing. The scheme of the configuration of the bonded joints in these areas is pictured in Figure 2.

**Figure 1.** Composite wing configuration with the area of interest.

**Figure 2.** Bonded joint configuration.

#### 2.1.2. Material Properties

Within this work, a determination of the basic thermal properties of both HexBond® EA9394 and TeXtreme® 100, in the direction perpendicular to their surfaces, was performed. The density *ρ*, thermal diffusivity *α* and specific heat capacity *cp* were determined

experimentally. From the data obtained, the values of thermal conductivity λ and thermal effusivity *e* were subsequently determined by calculation.

Experimental specimens from the examined materials were made with an outer diameter of *D* = 48 mm. The thicknesses of the specimens were *z* = 1.745 mm for the EA9394 material and *z* = 2.510 mm for the TeXtreme® 100 material. The composite lay-up of the second specimen was as follows: [0◦, 90◦]6S. The thickness of the reference specimen, which was made of aluminum alloy 6061 T6, was *z* = 1.240 mm.

The densities of the materials being examined were determined by measuring the difference between the weights of the specimens in air and the weights of the same specimens when immersed in distilled water according to Archimedes' law. The configuration of the test is shown in Figure 3. The resulting densities were calculated according to Equation (1)

$$
\rho = \frac{m}{\Delta m\_w} (\rho\_w - \rho\_d) + \rho\_d \tag{1}
$$

where *m* is the weight of the sample weighed in air, Δ*mw* is the difference between the weights of the sample measured in air and the samples immersed in water, *ρ<sup>w</sup>* is the density of water (*ρ<sup>w</sup>* = 998 kg·m–3 at 20 ◦C) and *<sup>ρ</sup><sup>a</sup>* is the density of air (*ρ<sup>a</sup>* = 1.2 kg·m–3). The resulting density values for the individual materials are given in Table 1.

**Figure 3.** Density measurement configuration.

**Table 1.** Used materials' densities.


The thermal diffusivity, *α*, of these materials was measured on the basis of the thermal curve of the surface of a thin specimen, after excitation by a short thermal pulse. Excitation was performed from the other side of the specimen using a flash lamp. This measuring technique is based on the ASTM E 1461 standard [10]. The value of thermal diffusivity is calculated from the thickness of the sample *z* and the time *t* (measured from the excitation moment), when the temperature increase, Δ*T*, on the measured surface reaches a certain percentage of the maximum surface temperature increase Δ*Tmax*. The surface temperature of the specimen was measured using an Infrared (IR) camera and determined as the average value from a circular area in the center of the specimen, which had a diameter of 10 mm. To ensure the same surface emissivity for all specimens, a thin layer of paint with a defined emissivity *ε* (Therma Spray 800 with *ε* = 0.96) was applied to both sides of the specimen. For each specimen, 5 measurements were performed and evaluated, and the resulting value of the thermal diffusivity *α* is their average.

Equation (2) [25] was used to determine the thermal diffusivity *α*0.5 in time *t*0.5:

$$
\alpha = 0.13879 \frac{z^2}{t\_{0.5}} \tag{2}
$$

where *z* is the specimen thickness, and *t*0.5 is the time taken for the temperature to rise to 50% Δ*Tmax*.

Since the above equation is based on the simplified assumption that no heat losses occur during the test, a correction for these losses needs to be done. One possibility is to use the correction reported by Clark and Taylor [25,26], which is based on the ratio of times *t*0.25 and *t*0.75, when the temperature rise reaches 25% and 75% of the maximum temperature rise, respectively. The correction factor *KR* is then calculated according to Equation (3):

$$K\_R = -0.3461467 + 0.361578 \frac{t\_{0.75}}{t\_{0.25}} - 0.06520543 \left(\frac{t\_{0.75}}{t\_{0.25}}\right)^2 \tag{3}$$

The corrected value of the thermal diffusivity *αcorr* is then (4):

$$
\alpha\_{corr} = \frac{\alpha\_{0.5} \,\, K\_R}{0.13885} \tag{4}
$$

A measuring device was assembled for the purpose of this experimental work. Its scheme can be seen in Figure 4. The device consisted of a flash lamp with a reflector, a specimen holder and an IR camera to record the temperatures.

**Figure 4.** Thermal diffusivity and specific heat capacity measurement configuration.

The resulting values of thermal diffusivity are given in Tables 2 and 3. Figure 5a,b shows the cooling curves for the EA9394 material specimen (left) and the TeXtreme® 100 material specimen (right).




**Table 3.** Thermal diffusivity—TeXtreme® 100.

**Figure 5.** Measurement of thermal diffusivity—heating curves: (**a**) EA9394; (**b**) TeXtreme® 100.

The standard deviation value given in Tables 2 and 3 is the sample standard deviation from the measured (calculated) values defined by Equation (5):

$$s = \sqrt{\frac{\sum (x\_i - x\_a)^2}{n - 1}}\tag{5}$$

where *xi* is the measured (calculated) value from the *i*-th measurement, *xa* is the arithmetic mean of the values from the measured (calculated) data and *n* is the number of measurements.

The determination of the specific heat capacity of the material is based on the assumption that the supplied heat *Q* per unit area *A* is manifested by an increase in temperature depending on the density of the material *ρ*, the specific heat capacity *cp* and the specimen thickness *z*, as shown in Equation (6).

$$\frac{\mathcal{Q}}{A} = \Delta T \times c\_p \times z \times \rho \tag{6}$$

If we perform these measurements under the same conditions (temperature, heat input) for different specimens, where for one reference specimen (index R), all parameters are known, and for the others, the only unknown value is the specific heat capacity, it is possible to determine unknown specific heat capacity *cp* according to Equation (7) [27]:

$$\varepsilon\_{p} = \frac{\Delta T\_{R}}{\Delta T} \frac{z\_{R} \times \rho\_{R}}{z \times \rho} c\_{pR} \tag{7}$$

The specimen was made of the aluminum alloy 6061 T6 with a thickness of *zR* = 1.240 mm, density of *<sup>ρ</sup><sup>R</sup>* = 2687 kg·m−<sup>3</sup> and specific heat capacity of *cp* = 896 J·kg−1·K−<sup>1</sup> was used as a reference standard. This measurement was performed on the same equipment, with the

same specimens and under the same conditions as the thermal diffusivity measurement. The individual values of the temperature increases are given in Table 4. The specific heat capacity values were subsequently determined from the average values of the temperature increase parameter.


**Table 4.** Values of the temperature increase.

The values of thermal conductivity *λ* and thermal effusivity *e* were calculated from the determined thermal properties according to Equations (8) and (9):

$$
\lambda = \ a \times \rho \times c\_p \tag{8}
$$

$$
\mathfrak{e} = \sqrt{\lambda \times \mathfrak{p} \times \mathfrak{e}\_p} \tag{9}
$$

A summary of the determined and calculated thermal properties is given in Table 5.



#### *2.2. PT Experimental Method Description*

#### 2.2.1. PT Theory

The PT method is based on the principle of heating a sample from one side with a short thermal pulse (for example, a halogen light or a flash lamp) and the subsequent monitoring of the cooling curve at each point of the surface using an IR thermal camera. By sending a pulse, the heat wave begins to propagate through the material. The surface cools due to heat wave propagation (conduction) into the depth of the material as well as due to convection and radiation losses. If beneath the surface there is a defect with a different thermal effusivity to that of the base material (delamination, cavity or void in an adhesive joint), the heat wave will be reflected back to the surface and the cooling process will change at this point. This behavior of the surface cooling curves is demonstrated in Figure 6. Defects that occur at a greater depth will appear on the thermogram with a time delay [12,16]. The time *t* required to manifest the temperature deviation is a function of the depth of the defect *z* and the thermal diffusivity *α* according to the relation (10) [12]

$$t \propto \frac{z^2}{a} \tag{10}$$

**Figure 6.** Surface cooling curves.

Figure 7 shows an example of the time evolution of thermograms for the different moments after the excitation pulse. The figure shows a time-sequential drawing of the deeper layers of the adhesive joint. At time *t* = 5 s, the poor quality of the joint can be seen as resulting from inadequate technology (the adhesive bead was not compressed and spread sufficiently). At the same time, it can be seen that due to lateral diffusion, thermograms lose their sharpness with increasing time.

**Figure 7.** Sequence of the thermograms of the bonded joint.

The disadvantage of this method is a sensitivity to the unevenness of the heat source and the distribution of emissivity on the surface. This can be partially eliminated by subsequent postprocessing.

#### 2.2.2. PT Method Verification

For the purpose of verifying the adhesive joints testing method, and for the setting up of the measuring device, a reference gauge was produced. This gauge corresponds in its composition to the point of the adhesive joints on the wing (Layers 1, 3, 4 in Figure 2). The individual thicknesses represent the depths of the occurrence of the defects of the adhesive joint formed within the adhesive-air interface. The total thickness of the gauge is

cut in a range of *z* = 0.25 mm (skin alone) to *z* = 2.3 mm (skin + adhesive). With the assumed maximum thickness of the adhesive layer in the adhesive joint of approx. 1.5 mm, this larger gauge thickness should represent a correctly glued joint. Due to the homogenization of the emissivity of the surface, the gauge was sprayed with paint with a defined emissivity of *ε* = 0.96. The dimensions of the reference gauge are shown in Figure 8.


**Figure 8.** Dimensions of the reference gauge.

Figure 9 shows the sequence of thermograms at five different time points *t* after excitation. It represents the temperature distribution on the surface of the gauge. The warmest spot is represented by a white color; the coldest by black, with each image in the sequence being normalized to achieve the maximum dynamic range. This figure clearly shows the gradual delineation of individual thicknesses and the gradual blurring of the boundaries between the individual steps of the gauge due to lateral diffusion. Local deviations in temperature distribution within the individual steps of the gauge are caused by imperfections in its production (deviations from the optimal thickness and the occurrence of air voids in the adhesive).

**Figure 9.** Thermogram sequence for reference gauge.

The following graph in Figure 10 shows the course of temperature distribution along the longitudinal axis of the gauge for the same time steps as in the previous thermograms. The temperature curves are again normalized separately for each curve. From the above thermograms and temperature distribution curves, it is clear that the performed measurement confirms an ability to detect defects in the adhesive joint within the entire range of expected depths.

Figure 11a,b shows the time course of the cooling curves measured at the centers of the individual steps of the reference gauge. The first image has a linear timeline, while the second image is logarithmic. The second figure clearly shows a turning point

in the temperature decrease at time *t* = 0.05–0.10 s after excitation, which is caused by the different thermal diffusivity of the skin and of the adhesive material. Both graphs show a time-varying deviation of the individual cooling curves from the curve, representing a correctly made joint (*z* = 2.3 mm).

**Figure 10.** Course of temperature distribution along the longitudinal axis of the gauge.

**Figure 11.** Cooling curves for the reference gauge: (**a**) linear time axis; (**b**) logarithmic time axis.

Since the resulting thermograms are normalized to the temperature range for a given time, the following graph in Figure 12 shows the temperature profile normalized to the temperature range from the highest temperature (curve for *z* = 0.25 mm) to the lowest temperature (curve for *z* = 2.3 mm) at each measurement point. This graph shows the dimensionless contrasts within the data obtained at a given time.

**Figure 12.** Image contrast curves for the reference gauge.
