## 1 Introduction  
The application of composite materials in performance components used daily such as bicycle frames has increased due to its beneficial mechanical properties with respect to metals. Such materials are strong, lightweight, and fatigue- and corrosion-resistant when combined with the proper resins [1]. Composite materials are technically complex structures due to their anisotropic behavior, which makes quality control particularly challenging. Thus, structural health monitoring is essential in the production and lifetime stages of such composite structures. This work is concerned with carbon bicycle frames [2]. To detect and analyze defects and guarantee an extended lifetime, composite materials have been the subject of a tremendous amount of non-destructive inspection (NDI) research [3,4]. Non-destructive evaluation (NDE) techniques are broadly used to evaluate material properties and to inspect the integrity of structures without damaging the structure [5]. Infrared (IR) thermography has remarkable advantages with respect to conventional NDE techniques, such as ultrasound C-scanning,X-ray radiography, tap-testing, and visual testing [6]. The technology is contactless, safe, fast, and reliable [7]. The study of active thermography has become an important NDE technique for damage detection and material updating in metallic structural components [6; 8; 9] and in carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) composites [10]. As the application of composite laminates such as CFRPs and GFRPs increases, the thermal conductance inside the components becomes anisotropic, and quantitative prediction becomes complex. The common form of active thermography makes use of heat pulses emitted to the surfaces and is called pulse thermography (PT) [11]. PT is used not only to identify defects but also to determine the thermal diffusivity of the material, inspect the structural integrity, and monitor the production process of composite structures. High-performance carbon fiber bicycle frames are strong and lightweight. However, they are susceptible to damage caused by low-energy impact loading, which have many possible causes: small stones can be projected by nearby cars or the front wheel of the bicycle, the bicycle may fall and hit the ground, and defects can occur during manufacturing [12; 13]. In addition, material thickness inconsistency can occur during manufacturing and cause false positive defect hits. Finite element (FE) modeling is common in the design of large structures but generally remains in the design stage. Two of the largest drawbacks of the use of FE beyond the design stage are the approximated material property parameters and the high computational cost of building an accurate FE model of the full structure. Finite element model updating (FEMU) can deliver a solution to these two problems, whereby the model is updated with non-destructive testing data, mainly done with ultrasound vibration C-scanning [14; 15; 16]. The foundation of the technique has been described by [PERSON] and [PERSON] in 1995 [14]. The main idea is to solve a set of initial numerical models with variable parameter values for the uncertain properties. By comparing the resulting response values (mostly done on vibration modes and/or eigenfrequencies) with experimental measurements, the variable parameters are modified iteratively until the response values correlate with the experimental verification data. Especially for complex-shaped objects and non-linear behavior, the methodology delivers faster and more accurate results with the inclusion of higher-order terms in the equations [14]. Unfortunately, the computational cost of an FEMU routine is even more expensive than that of a regular numerical simulation, as for each iteration at least one new numerical model needs to be computed. Therefore, a methodology based on the technology developed by [PERSON] has been used to replace the FE model with an approximative regression model in order to reduce the calculation times [17].