# Possibilities of Detecting Damage Due to Osmosis of GFRP Composites Used in Marine Applications
## 2 Vibrothermography  
In infrared thermography, the distribution of the temperature field on the surface of the test object is used to assess the condition of the test object. Modern thermal imaging cameras can record changes in electromagnetic energy in the infrared band radiated from the surface of the tested object with high frequency and temperature resolution (approx. 15 mK). The whole process, after transforming the radiated energy into an electronic signal, is recorded in the form of a sequence of images [13].  
Figure 2: Osmosis foci on the yacht’s hull [5].  
Non-destructive testing with infrared thermography can be performed with both passive and active techniques [14; 15; 16; 17; 18; 19; 20; 21; 22]. When using an active technique, it is necessary to use the source of thermal excitation of the tested object. For this purpose, either the heating or the cooling source of the test object can be used. Material defects which, before the test begins, have a homogeneous temperature equal to the ambient temperature do not generate \"useful\" temperature signals and, for this purpose, require heating or cooling the whole object or its part. During such a test a dynamic temperature field is created, and the results of the test of temperature distribution depending on the observation time.  
The name vibrothermography itself can be slightly misleading because this method does not rely on a direct combination of the physical phenomena used in the vibration method and thermography. The vibration method uses information on the change of direction and time of passage of waves inside the tested structure, which are affected by the elasticity and homogeneity of this structure. The detection of defects is influenced by, among other factors, the length of the mechanical wavelength and the ratio of the wavelength to the size of the defect. Only the source-generating mechanical waves were adopted from the vibration method to vibrothermography.  
In vibrothermography, the evaluation of latent structural heterogeneities of materials is based on changes in the surface temperature field under cyclic mechanical loads. The reason for the increase in temperature is the internal friction of the cavity walls when they are stimulated by mechanical waves. If the cyclic loads do not exceed the elasticity of the material and the rate of their changes is high, then heat losses due to thermal conductivity are small. After removing the load, the tested object returns to its original shape and temperature. Thermographic methods are usually non-contact methods. Vibrothermography differs from this principle, and the generator of mechanical waves must be in contact with the surface of the tested object.  
In order to effectively detect defects using vibrothermography, the most favorable values of the frequency and amplitude of the mechanical waves for thermal excitation of the tested object should be determined. The ThermoSon software was developed for this purpose.  
In vibrothermography, ultrasounds are usually used to excite the material under test, the frequency range of which is from 20 kHz to 1 GHz. Occasionally, sound waves with a frequency range of 20 Hz to 20 kHz are used.