Papers by Keyword: Waviness

Abstract: Composite sandwich panels are extensively used in aerospace, automotive, and construction applications due to their exceptional strength-to-weight ratio and structural efficiency. However, local surface deviations, such as waviness and dents, often develop during manufacturing and operation, potentially leading to adhesion failures and delamination between the composite skin and the core. This study aims to establish acceptable defect size limits that can be corrected through technological pressing, ensuring structural integrity of composite material while minimizing the negative impact on load-bearing capacity of sandwich panels. An analytical approach was adopted to assess the stress behavior of composite skins with waviness and elliptical dent defects. The analysis was based on beam and plate theory, incorporating the effects of flexural rigidity, material anisotropy, and applied technological pressure. The Hill strength criterion was applied to define permissible defect limits, considering variations in structural criticality levels. The study determined the maximum allowable sizes for waviness and dents in composite sandwich panels, factoring in the responsibility level of the panel, expressed as the maximum stress intensity coefficient. The results show that the acceptable defect size decreases with increasing structural criticality. It was also found that forced compression of dents induces pre-stress zones within the composite skin, potentially altering its stress distribution and reducing its long-term load-bearing capacity. The proposed methodology provides a quantitative framework for evaluating acceptable defect limits, supporting manufacturing quality control and repair optimization. The results offer practical insights for enhancing the reliability and durability of composite structures, ensuring that local surface deviations remain within permissible limits without compromising structural performance.

Abstract: The discovery of Carbon Nanotubes (CNT) has opened the doors for revolutionary applications in the mechanical, aerospace, and electrical sectors. However, to fully utilize the potential of carbon nanotubes, there is a persisting need to identify all sorts of structural modifications that can be observed in any type of manufacturing procedure for CNTs. Thus, the presented study investigates the mechanical properties of CNTs with variable waviness and defect density. Furthermore, the study is performed using classical Molecular Dynamics simulations (MD). The structures are then characterized with single or multiple vacancy defects along the axis of the nanotube structure, which is modeled as wavy structures to replicate their natural structure. After the simulation results were analyzed, it was observed that the increase in the surrounding temperature from 300K to 1500K reduces the overall tensile strength of the CNT sample from 89-47 GPa. However, introducing a single vacancy defect to the same structures was shown to reduce the tensile strength to 41 GPa at 1500K and 62 GPa at 300K.

Abstract: WAAM (Wire-Arc-Additive-Manufacturing) is an additive manufacturing process which uses arc welding to produce metal parts. This process is prone to heat accumulation, i.e. a progressive increase of the interlayer temperature and molten pool size, having detrimental consequences on the material properties and on the workpiece integrity. This paper investigates the effect of air jet impingement, an active cooling technique, to prevent heat accumulation, on the surfaces of WAAM workpieces. A reference test case was manufactured using traditional free convection cooling and air jet impingement. The workpiece temperature was measured using Ktype thermocouples. The manufactured surfaces were measured using a coordinate measuring machine and compared in terms of deposition efficiency, deposit height and average arithmetical deviation. The temperature results highlight that air jet impingement is effective in preventing the occurrence of heat accumulation. The surface data highlight that air jet impingement increase the deposited height and the surface waviness with a consequent decrease of the deposition efficiency.

Abstract: An experimental research on the reduction of vibration of ball bearings is provided in this paper. The waviness of inner race and outer race are decreased by superfinishing process, and their excitation frequencies are obtained. After the bearings’ vibration is tested, the powers of vibration distributed in different frequency regions are computed. The results reveal that the waviness excitations have a considerable influence on vibration of ball bearings in medium-frequency ranges, and the vibration can be reduced greatly by superfinishing process.

Abstract: Various defects on the CNT wall have been reported, which are formed during the synthesizing process. CNTs have superior properties compared to the traditional engineering materials. However, these properties hold only for the ideal case of carbon nanotubes, where these are made of perfect hexagonal graphite honeycomb lattice of mono-atomic layer thickness. The advantages or disadvantages of the presence of defects in carbon nanotubes depend on their applications. Structural defects may increase the adhesion of other atoms and molecules to carbon nanotubes. It has also been found that the defects in CNT do cause a change in its resonant frequency as compared to that of a non-defective CNT. The defects that have been considered for the purpose of analysis in this research includes defects in the carbon nanotubes likewise Waviness, Vacancy Defect, Pinhole Defect, Fracture and Stone Wales Defect. It has been observed that with the increase in the number of defects in CNT, a reduction in the fundamental frequency is observed.

Abstract: The explosive welding is a non-conventional technique gaining popularity due to its ability to join dissimilar metals. The technique is very successful in achieving area joining by using the controlled energy of explosives which creates a metallurgical bond between two similar or dissimilar materials. This paper explains the technique of explosive welding for joining SS304 and AA6061 using pure aluminum (2 mm) as an interlayer. The joining was done in two stages. The explosive used is a mixture of Trimonite and salt having velocity of detonation (VOD) in the range of 1500-1600 m/sec. Ultrasonic testing showed good bonding over more than 80%. Micro-hardness variations as compared to parent materials have been evaluated along with microstructure study done to analyze the interface characteristics. SEM/EDS also have been used to check the presence of any possible brittle phases. Both the interfaces are found to be laminar, continuous, uniform and free from micro-cracks.

Abstract: The paper states a problem of providing quality of geometrical characteristics of machined surfaces, which makes it necessary to forecast the occurrence and amount of oscillations appearing in the course of mechanical treatment. Objectives and tasks of the research are formulated. Sources of oscillation onset are defined: these are coordinate connections and nonlinear dependence of cutting force on the cutting velocity. A mathematical model of forecasting steady-state self-oscillations is investigated. The equation of the cutter tip motion is a system of two second-order nonlinear differential equations. The paper shows an algorithm describing a harmonic linearization method which allows for a significant reduction of the calculation time. In order to do that it is necessary to determine the amplitude of oscillations, frequency and a steady component of the first harmonic. Software which allows obtaining data on surface waviness parameters is described. The paper studies an example of the use of the developed model in semi-finished lathe machining of the shaft made from steel 40H which is a part of the BelAZ wheel electric actuator unit. Recommendations on eliminating self-oscillations in the process of shaft cutting and defect correction of the surface waviness are given.

Abstract: Experimental results of forming the geometry of a single imprint during the process of ultrasonic surface hardening are shown. A single imprint is formed on the machined surface as a result of interaction with an instrument vibrating at the ultrasonic frequency. Regression equations for calculating geometric parameters of an imprint depending on the machining conditions (static load and tool diameter) are derived. Contrary to the existing models the emergence of a hill formed around the tool in the process of its indentation into the surface layer of a workpiece is taken into account. The equations obtained make it possible to determine the height of this hill as well as the depth and diameter of the imprint with the hill. The calculation of these parameters is necessary to identify the boundary conditions of the beginning of initiation of a plastic flow wave in ultrasonic surface hardening of metallic materials.These conditions determine the criteria for the formation of waviness and represent restrictions on critical values of feed and speed in the machining of cylindrical surfaces.

Abstract: The purpose of present study is to present experimental results and a mathematical model for the evolution of surface waviness parameters with plastic strain of Interstitial Free - IF steel sheet under uniaxial and biaxial stretching tests. Roughness and waviness are very important quality parameters to be evaluated in sheet metal forming. Various waviness profile parameters such as the arithmetic average waviness Wa, the total height peak-valley waviness Wt, maximum peak height Pp and maximum valley depth Pv were measured during uniaxial and biaxial tests. Tensile test specimens at 0º, 45º and 90º to the direction of rolling and Nakazima type specimens of IF steel were fabricated. After preparing the test specimens, incremental simple tensile and Nakazima biaxial tests with flat punch were performed to characterize the negative and positive quadrant of the Map of Principal Surface Limit Strains, MPLS, of IF steel sheet. Measurements of waviness parameters of the specimen surface at incremental plastic strain stages were performed at the same surface site. Also, during the uniaxial and biaxial tests, the following plastic strains were calculated from printed circular mesh at each incremental step: ε₁ longitudinal major strain and ε₂ transverse minor strain. From these data, curves of waviness parameters versus equivalent strain were plotted to obtain a phenomenological equation of 4th or 3rd degree polynomial type. Furthermore, the growth rates of Wa and Wt parameters with the equivalent plastic strain were assessed. From the growth rate curves, it was possible to verify how the sheet thickness imperfections evolves during straining, being possible to predict the influence of plastic strain on the waviness values of IF steel sheets. From the analysis of Wa and Wt growth rates during straining, it was possible to proposed a criteria for the onset of local necking or limit strains in the MPLS. The waviness parameters Wt is the best for characterizing the onset of local necking in sheet metal forming.

Abstract: To describe the shaping process of the part profile the transfer functions device is used. These functions are obtained with the help of the operation frequency characteristics. The results show the dependences of the transfer function coefficients on the cutting conditions parameters.