Alexander Čapka

Manufacturing and Mechanical Characterization of Bio-Based Laminates and Sandwich Structures

Jute fabric is well-known reinforcing material in composite science, however, there is a necessity to treat these fabrics to reduce moisture uptake and improve properties. Nevertheless, every modification increases the cost and reduces the possible applications. Presented research deals with an investigation of possibility to use untreated jute in various fabric weight as a reinforcing material in sandwich structures facings. Untreated jute reinforcements and two types of cork cores were saturated in one step during vacuum infusion creating a lightweight sandwich composite. All samples were mechanically tested in three-point bending test. Experimental results showed the most appropriate material combination and produced sandwich structure are proposed for design applications.

Fatigue Life of Al-Honeycomb Core Composites Construction

The composite materials (CM) are increasingly replacing conventional structural and building materials. A special type of composite material is the polymer composite sandwich panel with a honeycomb core (PCHSP). PCHSPs have been used in aviation since the 1960s. First on tertiary structures, later on secondary and nowadays on primary structures. At present, PCHSPs are also used in the construction of land vehicles (cars and buses, railway trains, etc.). The design of the aircraft and land vehicles is stressed by flight and ground loads (surface loads: aerodynamic, volume loads: own weight, etc.). These are always random processes. This work deals with statistical analysis of PCHSP testing. The substance of evaluation is in the determination of PCHSP ́s lifetime. The issue of fatigue loading of PCHSPs is not fully explored and general conclusions have not been formed yet. Unlike metallic materials, where two standards [1] and [2] for metal fatigue testing used in the Czech Republic, however, there is no specialized standard describing testing and statistical evaluation of PCHSP ́s fatigue lifetime. There is only ISO 13003:2003 standard [3], which is applicable to composite materials (laminates) and the military standards; MIL-HDBK-23A [4]. There is a relatively small number of scientific publications dealing with the life fatigue test of PCHSPs. Only static tests on sandwich panels are included in the book [5] and fatigue tests of composite structures are described in [6] (laminates only). There is also a limited number of scientific articles dealing with PCHSPs fatigue tests [7, 8, 9, and 10]. The issue of fatigue testing of sandwich panels was solved in Czechoslovakia already in 1972 [11]. A PCHSP has specific fatigue life properties. The problem is complicated by the different properties of each of sandwich components. The PCHSPs tests are always tests of the structural element. Therefore, the complications resulting from the demands of adhesive bonding technology are increasing due to the known factors affecting the number of the achieved cycles, ie. stress values, structure of the individual components, geometric shape and dimensions. This also explains non-existent standard for the PCHSP ́s fatigue test. The fatigue testing methodology have to always include a statistical approach for the evaluation of the test results and with respect to the loading and conducted cycles itself we differ between low and high-cycle fatigue [12-16]. The statistical evaluation of the PCHSPs fatigue tests results is in a graphical representation using the classic S-N (Wöhler) curve and it determinines the reliability of the estimates of statistical variables. As a result, median of lifetime, the shortest life and the lower limit of fatigue life were investiquded in this work. During fatigue testing, it is advisable to monitor the formation and propagation of the sample ́s damage by infrared thermography. This non-destructive testing method is very well suited in CM and PCHSP testing and it has been validated in other experiments, eg. in [17].

Preparation and Dynamic Mechanical Analysis of Glass or carbon Fiber/Polymer Composites

Nowadays, glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) composites are broadly used so it is essential their properties to be explored in depth. In this study, GFRPs and CFRPs were produced by vacuum bag oven method and their viscoelastic behaviour at elevated temperatures was investigated. In particular, by Dynamic Mechanical Analysis experiments properties such as storage modulus, loss modulus, tanδ and glass transition temperature were determined. The results showed that, apart from a very small increment in the low temperatures in some of the compounds, as the temperature increases the storage modulus of the composites decrease while the composite containing unidirectional glass fibers in longitudinal direction and 34% percentage of matrix achieved the higher storage and loss modulus. Furthermore, as the unidirectional fibers change direction from transverse to longitudinal the GFRPs exhibit much higher storage and loss modulus.

Detecting Water in Composite Sandwich Panels by Using Infrared Thermography

Honeycomb composite structures widely used in aviation are sturdy and light-weight but they may accumulate water from the atmosphere during aircraft operation. The presence of water in honeycomb cells leads to a higher airplane mass and excessive corrosion of aluminum cores, while the frozen water endangers panel integrity. This work describes the use of infrared thermography for detecting water trapped in aviation honeycomb cells.

Modelling, detecting and evaluating water ingress in aviation honeycomb panels

Abstract The use of infrared thermography for quantitative evaluation of water ingress in aviation honeycomb cells is discussed. Numerical modelling has been performed by analysing a 3D panel model where water fully or partially occupies honeycomb cells. Calculation of several test cases has allowed better understanding of how the thickness of the water layer affects surface temperature anomalies and times of their appearance in active one-sided thermal tests. Experimental results have been obtained on both reference samples and real honeycomb panels.

Analysis of Mechanical and Impact Properties of Prepreg Composites under Elevated Temperature

This paper presents an experimental investigation of mechanical and impact properties of carbon and Kevlar-glass composites prepared from pre-impregnated materials. Namely, flexural performance in three-point bending at different temperatures is evaluated. Moreover, Charpy impact test and low-velocity impact test are also conducted for classification of impact properties and character of rupture. These all properties are important for material design of sport bike rims and many sport and other applications.