Discrete Wavelet Transformation Approach for Surface Defects Detection in Friction Stir Welded Joints

Friction Stir Welding joint quality depends on input parameters such as tool rotational speed, tool traverse speed, tool tilt angle and an axial force. Surface defects formation occurs when these input parameters are not selected properly. The main objective of the recent paper is to develop Discrete Wavelet Transform algorithm by using Python programming and further subject it to the Friction Stir Welded samples for the identification of various external surface defects present.

Modular Test Stand for Fatigue Testing of Aeronautical Structures – Verification of Assumptions

The Modular Test Stand was developed and manufactured to decrease the cost of fatigue testing and reduce the time of its completion as well as to enable testing specimens under more complex load conditions. The stand consists of three connected sections, similar to a wing box, all being loaded in the same way. Thanks to that, several specimens can be tested simultaneously. This configuration requires that stress and strain distribution should be reasonably uniform, as assumed in the design stage. The structure can be loaded with bending or torsion. A whole section, selected structural node or a specimen mounted in the structure as well as a repair or a sensor can be a test object. Two stands, one for bending and one for torsion were prepared. This paper presents the verification of the assumed strain and stress distributions on the skin panels. The measurements were performed with the use of Digital Image Correlation (DIC) as well as strain gauges. DIC measurements were performed on one skin panel of the central section. Five strain gauge rosettes were installed on both panels of the one section. In addition, one rosette was applied to one skin panel in each of two other sections. Measurements were performed on the stand for torsion as well as on the stand for bending. The results of DIC analysis and strain gauge measurement during torsion show uniform shearing strain distributions on the panels. During bending, on the tensioned side, the strains obtained indicate quite uniform strain distributions. On the compressed side, local buckling of the skin panels results in high strain gradients. Strain levels obtained with the use of a DIC analysis and strain gauge measurements were similar. Moreover, horizontal displacements of markers in the spar axis during bending was determined based on a series of photographic. The deflection line obtained in this way has a shape similar to arc, which is characteristic of the constant bending moment. The stand was tested with torsional and bending loads in order to verify the design assumptions. The results of strain distributions on the skin panels with the use of DIC and strain gauges as well as the deflection line of the spar axis indicate that the Modular Test Stand performs as assumed and can be used for tests.

Analysis of the Corrosion Resistance of Aircraft Structure Joints with Double-Sided Rivets and Single-Sided Rivets

An important factor having a negative impact on the technical condition of aircraft structure elements is the adverse effect of the atmosphere, which causes formation of corrosion in aircraft structures, especially in riveted lap joints. The electric potential difference between the sheet material and the rivet, in the presence of humid air, may cause electrochemical corrosion. The paper presents specimens that imitate the repair on the Mi-24 helicopter with the use of blind rivets in places where solid double-sided rivets could not be used. The aim of the research was to assess the corrosion resistance of lap joints with the use of single-sided and double-sided rivets. The analysis of corrosion resistance was carried out based on accelerated aging tests in a salt spray chamber. The salt chamber tests were aimed at determining the changes taking place in the specimens exposed to the marine environment. In the course of periodic observations changes in the mass of the specimens and in the form of corrosion losses were recorded. These activities were aimed at determining whether the exposure of specimens in the salt chamber causes electrochemical corrosion or pillowing. In addition, the specimens were subjected to static strength tests to assess the effect of corrosion on the strength properties of riveted joints.

Service Life Extension of Parachutes with Use of Non-Desctructive and Partially Destructive Testing Methods of Textile Materials

The specificity of personal rescue and reserve parachutes is the fact that they are practically never used for jumping during their service life as they are intended for use only in emergency situations. Therefore, these parachutes throughout the entire period of use are only periodically aired and repacked every 6-12 months. Airing and repacking is necessary even if the parachute is only stored. Rescue and reserve parachutes’ components wear unevenly because the canopy with the suspension lines is inside the container and the cover, while the external components of the harness and the container undergo typical operational wear. Therefore, the service life of rescue parachutes can even reach 20 years (this refers to the canopy with the suspension lines alone). During normal exploitation, parachutes are subjected to non-destructive visual and tactile inspection in preparation for packing. When a parachute reaches its maximum service life, extension of its service life can be calculated based on its technical condition. The procedure for extending parachute’s service life involves non-destructive tests at a fabric air permeability test stand and partially destructive tests at the strength test stand. In the paper, both methods are described and their advantages and disadvantages are discussed. Also, observations some regarding the packers’ work and the desired new properties of raw materials that could be introduced to the parachute industry are presented.

Some Comments on Fatigue Life Tests of Aircraft Cable Control Systems

Cable control systems are widely used in aircraft and gliders. This paper deals with the problem of collecting real loads acting cable control systems and cable tests preparation (load spectrum) and performance. The author proposes a method for defining real loads acting on control systems, preparing and carrying out fatigue tests of cables revealing symptoms of fretting. The fatigue tests results can be used to predict service life, to plan and prepare periodic and details inspections. This method could be used to increase service life of aircraft control cables and could help to replace the commonly used Time-Based Maintenance (TBM) strategy with the Damage Tolerance (DT).

Impact of Manufacturing Tolerances on Stress in a Turbine Blade Fir-Tree Root

Low Cycle Fatigue (LCF) is one of most common mechanisms behind turbine blade failures. The reason is high stress concentration in notch areas, like fir-tree root groves, which can cause cyclic stress beyond the safe threshold. The stress levels strictly depend on the manufacturing accuracy of the fir-tree lock (for both fitted together: blade root and disk groove). The probabilistic study aimed at determination of stress was performed using Finite Element Method (FEM) simulation on a population of 1000 turbine models (disk + blades +friction dampers), where fir-tree lock dimensions were sampled according to the normal distribution, within limits specified in the documentation. The studies were performed for different manufacturing quality levels: 3-Sigma, 6-Sigma and 3-Sigma with tolerance ranges reduced twice. Based on the results, the probabilistic distributions, probabilities and expected ranges of values could be determined for: material plastification, stress, strain, LCF lifetime, etc. The study has shown how each tooth of the root is loaded and how wide a stress range should be expected in each groove. That gives information on how the definition of tolerances should be modified to make the construction more optimal, more robust, with lower likelihood of damage, taking into account the cost-quality balance. It also shows how the Six Sigma philosophy can improve the safety of the construction, its repeatability and predictability. Additionally, the presented numerical study is a few orders of magnitude more cost- and time-effective than experiment.

Dynamic Fatigue Tests Of Landing Gears

Landing gears are one of the main components of an aircraft. The landing gear is used not only during take-off and landing but also, in most cases, during ground manoeuvres. Due to its function, the landing gear is also one of the key safety components of the aircraft due to dissipating landing loads acting on the aircraft. The mentioned loads come from both the vertical and horizontal speeds during touchdown and by the aircraft’s losing the speed by braking. The landing gear is then loaded with constantly changing forces acting in various directions during every landing, with the only difference coming from their magnitude. The repeatable loading conditions cause significant wear of the landing gear. This wear can be divided into two categories, one is the wear of consumable parts such as the brake linings and the other is the fatigue wear of the structural components. The latter type of wear is much more dangerous due to its slow, and in many cases, unnoticeable progression. Fatigue wear can be estimated by numerical analyses – this method works with a great degree of probability on single components but due to the complexity of the landing gear as a whole it is not precise enough to be applied to the full structure. In order to evaluate the fatigue of the whole landing gear the best method accepted by regulations is the laboratory testing method. It involves a series of various drop tests resembling the real landing condition distribution. The aim of the tests is to check the fatigue wear of the landing gear and to prove its reliability for certification and/or operational purposes. In this paper the author describes the basics of the landing gear fatigue wear, possibilities of its evaluation and presents laboratory dynamic method used for extensive tests in life-like operation conditions.

Digital Twin For Fatigue Analysis

The main design parameters that impact the fatigue of components are geometry, material and loading. Simulation with Finite Element Analysis (FEA) and tests on a vibrating table are often used to understand the dynamic behaviour of components and thus validate those items. Accelerated tests are used for the mission profile and test definition, as described in GAM-EG-13, MIL-STD-810F and RTCA DO-160E. The shock response spectrum (SRS) and the extreme response spectrum (ERS) allow for a comparison of the power spectrum density (PSD) and the acceleration factor applied in terms of fatigue severity through the fatigue damage spectrum (FDS). In addition, the hypothesis of linear damage accumulation enables the combination of several events for specifying a mission profile. Ultimately, the mission profile, which represents a usage that might span over several years, can be reduced to a shorter duration with a damage extraction technique. This is particularly useful for the definition of vibrating table specifications. An advantage of the virtual vibrating table is the reduction of the number of prototypes and the understanding of failure modes. To achieve this objective, finite element analysis in the frequency domain (harmonic analysis) is used and the structural stress response is evaluated with a PSD loading. A statistical model of rainflow allows assessing the damage on the components. The presentation also shows the effects of the damping factor on damage results. To achieve accurate results and define a Digital Twin, the correlation between test results and the finite element analysis is fundamental. Experimental modal analysis, based on the measured acceleration responses, helps to validate calculated modal frequencies and to assess the damping for each mode. This study shows the importance and the sensitivity on damping of the structural response, and in turn on fatigue.

Mechanical Tests Applied to Structural Health Monitoring: An Overview of Previous Experience

Laboratory for Materials Strength Testing (LMST) has been conducting accredited mechanical research for aviation from 2003. Among accredited procedures are e.g. low and high cycle fatigue tests, fracture toughness tests and fatigue crack growth rate tests. The main goal of them is obtaining materials constants and characteristics. However knowledge how to conduct these tests could be used also in other applications, for instance in the work on development of Structural Health Monitoring systems (SHM). When cracks propagate in a controlled way in laboratory conditions, it allows verifying the operation of a single sensor or a network of sensors. In this paper, an overview of mechanical tests carried out at the Laboratory for Materials Strength Testing within Air Force Institute of Technology (AFIT) work on research and development of SHM systems is presented. Specimens prepared from materials such as aluminum alloys (among other withdrawn PZL-130 Orlik TC-II aircraft) and CFRP composite were tested under different mechanical loads, i.e., cycle and impact loads. In the presented research, both constant amplitude and spectrum loads were applied.

Microstructural and Mechanical Properties of Selective Laser Melted Inconel 718 for Different Specimen Sizes

Selective laser melting (SLM) falls into the category of additive manufacturing technologies that are being increasingly used in the aerospace industry. This study presents the results of the examination of the microstructure and mechanical properties of selective laser melted Inconel 718. The tests were carried out for samples of different geometry (thickness, shape). The investigation showed the effect of the specimen’s size and the printing direction on the microstructure and mechanical properties. In the microstructural investigation, light and scanning electron microscopes were used. The microstructure investigation included measurements of the grain size and the carbides’ content. In order to estimate porosity computer tomography was used. Tensile tests were carried out at room temperature. The results showed differences in mechanical and microstructural properties of different size specimens.