###### Abstract  
Recently, thermographic testing has been applied as a non-destructive evaluation technology for composite materials due to its remarkable advantages, such as its non-contact, non-destructive, and high-speed properties, it is harmless to the human body and easy to use, as well as having both qualitative and quantitative inspection performance. In this paper, multiple lock-in frequency modulated thermography is applied to investigate defects on the subsurface of polypropylene/glass fiber composites, and experimental investigations were conducted on various defect conditions for qualitative and quantitative estimation. A thin rectangle-shaped PP-GF plate of dimension 100 \(\times\) 180 mm and a depth of 3 mm with various 24 defects conditions was considered as a test sample. The sample was stimulated at multiple modulation frequencies, viz., 1 Hz, 0.2 Hz, 0.1 Hz, 0.05 Hz, 0.03 Hz, and 0.01 Hz. For qualitative and quantitative evaluation, two representatives, multiple lock-in frequency modulated algorithms of lock-in thermography, HA and FFT, were applied to the raw data. Moreover, their detectability performance was compared in terms of phase contrast and signal-to-noise ratio. polypropylene/glass fiber composites; lock-in thermography; multi frequency approach; phase contrast; signal-to-noise ratio +
Footnote †: journal: Applied Sciences  
radiographic inspection (RT), and acoustic emission (AE) techniques have been used as non-destructive testing techniques for polypropylene composites. However, each technique has problems and needs improvement [6; 7]. UT is the most widely used technology for control and non-destructive inspection in various industrial fields, such as internal defect detection of composite materials, power generation turbine plates, nuclear power plant components, ships, autonomous aircraft structures, automobile parts, etc. In previous research, studies on the shape, size, location, and depth quantification of various types of defects were conducted using A-scan and C-scan UT techniques [8; 9]. However, there are limitations in the narrow inspection area, long inspection time, and difficulty in accessing complex structures. RT is widely used mainly for the detection of discontinuities and defects in materials. However, special considerations are required when a specific material, such as a polymer with low X-ray attenuation coefficients, is scanned. Through many previous studies, X-ray CT was used to detect various types of defects and to characterize composite materials and structures [10; 11]. VI is only effective at detecting macroscopic defects, such as poor surface finish and poor bonding. AE takes a long time to process and analyze data and requires specific skills and experience [12; 13]. In this context, infrared thermography (IRT) can be used as an alternative technology and has the advantage of being able to inspect a large area quickly without contact and destructiveness. Recently, IRT has been of the promising technologies and has been applied in various fields of application [14; 15; 16]. Moreover, thermographic testing has proved to be a valuable approach for non-destructive testing and evaluation of the structural stability of materials. Figure 1 shows the classification of approaches of IRT non-destructive testing methods. IRT is classified into two approaches: passive thermography and active thermography, as shown in Figure 1. In passive thermography, no external energy is used to stimulate the thermal gradients at the target's surface being tested [17]. The principle is that a temperature gradient is naturally generated by self-heating. Active thermography maximizes the temperature difference between defects and sound areas by heating the sample surface through an external excitation source [18]. A variety of external energy sources can be used for active thermography, for example optical excitation (halogen lamp, flash lamp, laser), eddy current, ultrasound, and others [19].