Appl. Mech., Volume 3, Issue 1 (March 2022) – 20 articles

The present work is framed inside a broader activity aimed at improving the accuracy of numerical models in predicting the crashworthiness behavior of flexible fuel tanks. This paper describes a comprehensive experimental and numerical study aimed at estimating the impact force of a test article, consisting of a soft nylon bag filled with water, subjected to crash impact tests. In order to understand and improve response predictions, the test article drops freely from different heights, and then strikes onto a rigid plate which is instrumented with different types of sensors. Strain gauges, piezoceramic sensors, and fiber optics are used to measure the strain induced by the impact force during the experiments. To tune the test matrix and the measurement chain parameters, numerical computations are carried out to predict the dynamics of drop impact through FE explicit analyses. Through analysis and comparison with experimental results, a relationship between strain and impact energy correlated with the drop height is established, and the overall accuracy of the entire measurement chain is assessed to determine the effectiveness of such a methodology in a full-scale test on a flexible fuel tank structure. Full article

This work presents an innovative honeycomb cell geometry design with enhanced in-plane energy absorption under quasi-static lateral loads. Numerical and experimental compression tests results under axial and lateral loads are analyzed. The proposed cell geometry was designed to overcome the limitations posed by standard hexagonal honeycombs, which show relatively low stiffness and energy absorption under loads that have a significant lateral component. To achieve this, the new cell geometry was designed with internal diagonal walls to support the external walls, increasing its stiffness and impact energy absorption in comparison with the hexagonal cell. 3D-printed unit-cell specimens made from ABS thermoplastic material were subjected to experimental quasi-static compression tests, in both lateral and axial directions. Energy absorption was compared to that of the standard hexagonal cell, with the same mass and height. Finite element models were developed and validated using experimental data. Results show that the innovative geometry absorbs approximately 15% more energy under lateral compression, while maintaining the same level of energy absorption of the standard hexagonal cell in the axial direction. The present study demonstrates that the proposed cell geometry has the potential to substitute the standard hexagonal honeycomb in applications where significant lateral loads are present. Full article

The approaches used to calculate the fatigue life of components must inevitably consider multiaxial stresses. Compared to proportional loading, the calculation of nonproportional loading is particularly challenging, especially since different materials exhibit the effects of nonproportional hardening and shifts in fatigue life. In this paper, the critical plane approach of scaled normal stresses, first proposed by Gaier and Dannbauer and later published in a modified version by Riess et al., is investigated in detail. It is shown that, on the one hand, compatibilities exist or can be established with known proportional strength criteria that can account for the varying ductility of different materials. Furthermore, it is demonstrated that the scaled normal stress approach can be formulated in such a way that different strength criteria can be used therein. As an example, the generally formulated approach for scaled normal stresses is applied to test results from ductile cast iron material EN-GJS-500-14. Different correction factors accounting for nonproportional loading are investigated. Through appropriate parameterization of one of the studied corrections, proportional and nonproportional test results were observed to fall within one common scatter band. Full article

Modal parameter identification can be a valuable tool in mechanical engineering to predict vibrational behaviour and avoid machine damage during operation. Operational modal analysis is an output-only identification tool motivated by the structural identification of civil engineering structures, which are excited by ambient conditions. This technique is increasingly applied in mechanical engineering in order to characterise the system behaviour during operation as modal parameters can vary under operating conditions. The following study investigates the application of operational modal analysis on an axial compressor under operating conditions. Since the modal parameters of the system change depending on the life history and during the operation of the system, a corresponding data analysis might allow us to identify the present status of the system. Eigenfrequencies and eigenvectors are studied for the use of structural health monitoring approaches. According to the analysis, eigenfrequencies represent robust parameters for the studied purpose. Eigenvectors are sensitive to damages but need further investigation, especially for rotating machinery. This study will help the user to set up a virtual model, which describes the system behaviour for different boundary conditions. This in turn, will provide an accurate prediction of the vibrational behaviour in order to assure a safe operation. Full article

A wide variety of hyperelastic rubber-like materials, exhibiting strong nonlinear stress–strain relations under large deformations, is applied in various industrial mechanical systems and engineering applications involving shock and vibration absorbers. An optimal design procedure of an elevator chassis crashing on a hyperelastic shock absorber in a fail scenario, applicable in large-scale mechanical systems or industrial structures of high importance under strong nonlinear dynamic excitation, is presented in this work. For the characterization of the hyperelastic absorber, a Mooney–Rivlin material model was adopted, and a series of in-lab compression quasi-static tests were conducted. Applying a fully parallelizable state-of-the-art stochastic model updating methodology, coupled with robust, accurate and efficient Finite Element Analysis (FEA) software, the hyperelastic behavior of the shock absorber was validated under uniaxial large deformation, in order to tune all material parameters and develop a high-fidelity FE model of the shock absorber system. Next, a series of in situ full-scale experimental trials were carried out using a test-case elevator chassis, representing the crash scenario on the buffer absorber system, after a controlled free fall. A limited number of sensors, i.e., triaxial accelerometers and strain gauges, were placed at characteristic points of the real structure of the elevator chassis recording experimental data. A discrete Finite Element (FE) model of the experimentally tested arrangement involving the elevator chassis and updated buffer absorber system along with all boundary conditions was developed and used in explicit nonlinear analysis of the crash scenario. Steel material properties and the characterized updated Mooney–Rivlin material model were assigned to the elevator chassis and buffer, respectively. A direct comparison of the numerical and experimental data validated the reliability and accuracy of the methodology applied, whereas results of the analysis were used in order to redesign and optimize a new-design elevator chassis, achieving minimum design stresses and satisfying serviceability limit states. Full article

This work presents the bending–torsion coupled free vibration analysis of prestressed, layered composite beams subjected to axial force and end moment using the traditional finite element method (FEM) and dynamic finite element (DFE) techniques. Current trends in the literature, in terms of different types of modeling techniques and constraints, were briefly examined. The Galerkin-type weighted residual method was applied to convert the coupled differential equations of motion into a discrete problem using a polynomial interpolation function in the finite element method. In the dynamic finite element method, trigonometric shape functions were implemented to describe the equations in terms of nodal displacements. The eigenvalue problem resulting from the discretization along the length of the beam was solved in order to determine the system’s natural frequencies and modes. The results, showing the effects of axial load, end moment, and combined loading on natural frequencies, are discussed and are followed by some concluding remarks. Full article

The design of flexible and efficient aircraft engines and propulsion systems plays a crucial role in the development of future low-emission aircraft. Implementing shape-variable blades to compressor front stage rotors presents a high potential for increasing efficiency, since through adaptation, the blades are capable of optimizing their shape for different flight phases and aerodynamic conditions. Modifying the shape of the blades by using structurally integrated actuators allows this adaptation and therefore helps enhance their aerodynamic behavior for different flight regimes. Since up to now no morphing compressor or any other aircraft engine blades exist, here a multidisciplinary method for their design is introduced. This new method brings together existing structural and aerodynamic design methodologies, couples them together already at the earliest stages of the design process, while addressing the challenges that arise with a tightly coupled multidisciplinary design. As a result, first performance gain evaluations applied to the NASA 67 rotor test case are presented, showing the potential of morphing compressor blades and the potential of the introduced design methodology. Full article

Foot progression angle (FPA) is a gait-related clinical measurement commonly used for assessing the rotational profile of the lower extremity. This study examined the accuracy of two methods based on force-plate data for estimating FPA during walking by comparing them with a reference method using a motion capture system. Ten healthy adults performed a series of overground walking trials at three different speeds: slow, preferred and fast. FPA was estimated from two methods using data on center of pressure—one method previously reported in the literature, and a novel method proposed here. The FPA estimated by each of these two force-plate methods were compared with the reference FPA determined from kinematic data. Results showed that the novel force-plate method was more accurate and precise when measuring the FPA in the three speed conditions than the force-plate method previously reported in the literature. The mean absolute error obtained with this novel method was 3.3° ± 2.1° at slow speed, 2.0° ± 1.2° at preferred speed and 2.0° ± 1.2° at fast speed, with no significant effect of gait speed (p > 0.05). These findings suggest that the novel force-plate method proposed here is valid for determining FPA during walking at various speeds. In the absence of kinematic data, this method constitutes an attractive alternative for measuring FPA. Full article

The sensory acquisition of in situ data in technical systems is one of the key requirements set by ongoing digitalization. The sensory utilization of mechanical design elements is a step towards the accomplishment of this requirement. To set a common ground for further research in the context of sensory utilizable design elements, this paper reviews the current state of research in this topic. First, the aim, potentials and classification of sensory utilizable design elements are introduced. Next, examples of sensory utilizable design elements are presented. These examples are used to demonstrate the technical and methodical challenges that have to be addressed in order to establish sensory utilizable design elements as a solution for the requirements of digitalization. Full article

Traditional neuromuscular tests (e.g., jumping and sprinting tasks) are useful to assess athletic performance, but the basic outcomes (e.g., jump height, sprint time) offer only a limited amount of information, warranting a more detailed approach to performance testing. With a more analytical approach and biomechanical testing, neuromuscular function can be assessed in-depth. In this article, we review the utility of selected biomechanical variables (eccentric utilization ratio, force–velocity relationship, reactive strength index, and bilateral deficit) for monitoring sport performance and training optimization. These variables still represent a macroscopic level of analysis, but provide a more detailed insight into an individual’s neuromuscular capabilities, which can be overlooked in conventional testing. Although the aforementioned “alternative” variables are more complex in biomechanical terms, they are relatively simple to examine, with no need for additional technology other than what is already necessary for performing the conventional tests (for example, even smartphones can be used in many cases). In this review, we conclude that, with the exception of the eccentric utilization ratio, all of the selected variables have some potential for evaluating sport performance. Full article

Stiffened panels constitute structural assemblies of the entire ship hull, i.e., double bottom, side shell, deck areas, etc. Prescriptive dimensioning of the stiffeners (web thickness and height and flange thickness and breadth) is solely based on the application of beam bending theories. This work is divided into two parts. The first part involves the assessment of the structural response of one-way (single-bay) stiffened panels under uniform pressure. The objective is to evaluate the effectiveness of alternative approaches in obtaining accurate secondary stress fields. Both state-of-the-art analytical solutions (Paik, Schade, CSR, Miller) and numerical calculation tools (finite element analysis (FEA)) are employed and compared for this purpose. When it comes to cross-stiffened panels, numerical methods are usually used within the design process which is time demanding. The second part of this work focuses on the development of a fast, yet effective, prescriptive approach. This approach will allow the dimensioning of the longitudinal stiffeners by considering the secondary stress field. Combining finite element analysis and the Euler–Bernoulli bending theory, the effect of the transverse stiffeners to the longitudinal stiffeners is examined in order to estimate the type of support on the boundaries of the transverse stiffeners. Determining the type of support, will make it possible to apply the classical formula of bending stress instead of using finite element analysis, thus limiting the computational cost. Preliminary calculations show that most of the examined cases may be treated as fully clamped beams subjected to uniform pressure. Full article

Free vibration analysis of prestressed, homogenous, Fiber-Metal Laminated (FML) and composite beams subjected to axial force and end moment is revisited. Finite Element Method (FEM) and frequency-dependent Dynamic Finite Element (DFE) models are developed and presented. The frequency results are compared with those obtained from the conventional FEM (ANSYS, Canonsburg, PA, USA) as well as the Homogenization Method (HM). Unlike the FEM, the application of the DFE formulation leads to a nonlinear eigenvalue problem, which is solved to determine the system’s natural frequencies and modes. The governing differential equations of coupled flexural–torsional vibrations, resulting from the end moment, are developed using Euler–Bernoulli bending and St. Venant torsion beam theories and assuming linear harmonic motion and linearly elastic materials. Illustrative examples of prestressed layered, FML, and unidirectional composite beam configurations, exhibiting geometric bending-torsion coupling, are studied. The presented DFE and FEM results show excellent agreement with the homogenization method and ANSYS modeling results, with the DFE’s rates of convergence surpassing all. An investigation is also carried out to examine the effects of various combined axial loads and end moments on the stiffness and fundamental frequencies of the structure. An illustrative example, demonstrating the application of the presented methods to the buckling analysis of layered beams is also presented. Full article

This article investigates the potential detrimental effects of cyclic load during the installation of externally bonded (EB) carbon fiber-reinforced polymer (CFRP) on a damaged reinforced concrete (RC) structure. Four RC specimens were tested in three point bending to study the consequences of crack cyclic opening-closure during epoxy-curing period. A first RC specimen (without bonded CFRP) was loaded monotonically up to failure to serve as undamaged control sample. The three other specimens were pre-cracked before being subjected to a fatigue loading procedure to simulate service condition of a damaged RC structure. Two of the three pre-cracked specimens were strengthened by EB CFRP. One specimen was repaired before the fatigue test while the other one was repaired during the fatigue test. Finally, remaining capacities of all three pre-cracked specimens were measured through monotonic bending tests until failure. It was found that, although bonding of CFRP reinforcement during cyclic load can induce some interesting features with regard to serviceability, cyclic crack opening and closing alters the cure process of epoxy located below the initial crack and decreases the effectiveness of the strengthening at ultimate state. Extended experimental studies are then needed to assess reliable safety factor for the design of repairing operations in which the bridge has to be maintained in service during CFRP installation. Full article

Applying a constant electric field on a suspended film of liquid that carries an electric current, either by the transport of ions or surface charges, induces a rotation in the film. This system is known as “liquid film motor”. So far, the effect of permittivity of the liquid on its rotation has been ignored. We showed that the permittivity of the liquid can significantly affect the dynamics of rotation. Using an experimental approach, we studied the liquid film rotation for a broad range of pure liquids with diverse permittivities and surface tensions. We observed two different regimes of rotation depending on the permittivity of the liquids. We also found that there is no correlation between the surface tension of the liquid and the angular velocity of the rotation. We considered a theoretical framework and suggested scenarios to explain our experimental observations. These results help in better understanding the physics of liquid film motors and suggest opportunities for new flow manipulation techniques at small scales. Full article

This paper presents issues related to the determination of the selected mechanical properties of adhesive joints made of polymeric pipes and the evaluation of the leak-tightness of the adhesive joints. The article attempts to demonstrate that the type of adhesive may affect the quality of adhesive joints in terms of both tightness and strength of joints. Five types of the polymer pipes differing in a polypropylene and a polyvinyl chloride, diameter and a wall thickness were used in the experiments. Two types of the adhesives were used to make the adhesive joints: Loctite 3430 A&B Hysol, a two-component epoxy adhesive, and Loctite 406, a one-component cyanoacrylate adhesive. Based on the leak-tightness tests results, it was possible to determine the quality of their adhesive joints without damaging the samples, while their tensile strength was determined through the strength tests. The tests performed allowed for the conclusion that the use of the polyvinyl chloride pipes and Loctite 406 one-component adhesive is recommended for this type of adhesive joints. Full article

Pin joints are widely used mechanisms in different industrial machineries such as aircrafts, cranes, ships, and offshore drilling equipment providing a joint with possibility of relative rotation about one single axis. The rigidity of the joint and its service lifetime depend on the clamping force in the contact region that is provided by the applied torque. However, due to the tolerance needed for insertion of a pin in the equipment support bore, the pin is prone to relative displacement inside the bore. The amplitude of this relative displacement usually increases as time passes and since the material of the support often has lower quality grade than the pin, it leads to creation of slack in the equipment and malfunctioning of the machine. An Expanding Pin System (EPS) can be a solution to this problem where the split sleeve expands to remove the gap while the joint is torqued. Therefore, slack in the joint system disappears and 360° contact area could be achieved, providing a better stress distribution and preventing the stress localization. Determining the EPS preload and the resulting contact pressure and stresses in the joint parts are important to avoid damaging to the contact surfaces of the joints and making the dismantling of the EPS difficult. Therefore, finding the amount of the required torque is a compromise between preventing slack in the EPS and prohibiting damage to the joint parts. Stress analysis in this study is performed based on the industrially recommended torque for the EPS type under study. This article reports the study conducted on the stress distribution and the magnitude of stresses exerted to the equipment support when EPS is installed on the machine. To achieve this purpose and to investigate the stress distribution in the joint, both experimental and finite element (FE) methods were used. The experimental results show how much of the applied energy to the EPS in the form of torque is spent to expand the split sleeve and test boss and also to overcome friction. The finite element analysis provides magnitude and distribution of stresses in the EPS components. Full article

The Nerang Broadbeach Roadway (NBR) embankment in Australia is founded on soft clay deposits. The embankment sections were preloaded and surcharged-preloaded to limit the post-construction deformation and to avoid stability failure. In this paper, we discuss the NBR embankment’s geology, geotechnical properties of the subsurface, and long-term field monitoring data from settlement plates and piezometers. We demonstrate a comparison of cone penetration test (CPT) and piezo cone dissipation test (CPT-u) interpreted geotechnical properties and the NBR embankment’s foundation stratification with laboratory and field measured data. We also developed two elasto-viscoplastic (EVP) models for long-term performance prediction of the NBR embankment. In this regard, we considered both the associated and the non-associated flow rule in the EVP model formulation to assess the flow rule effect of soft clay. We also compared EVP model predictions with the Modified Cam Clay (MCC) model to evaluate the effect of viscous behavior of natural Estuarine clay. Both EVP models require six parameters, and five of them are similar to the MCC model. We used the secondary compression index of clay in the EVP model formulations to include the viscous response of clay. We obtained numerical models’ parameters from laboratory tests and interpretation of CPT and CPTu data. We observed that the EVP models predicted well compared with the MCC model because of the inclusion of soft clay’s viscosity in the EVP models. Moreover, the flow rule effect in the embankment’s performance predictions was noticeable. The non-associated flow rule EVP model predicted the field monitoring settlement and pore pressure better compared to the MCC model and the associated flow EVP model. Full article

Stochastic eigenvalue problems are nonlinear and multiparametric. They require their own solution methods and remain one of the challenge problems in computational mechanics. For the simplest possible reference problems, the key is to have a cluster of at the low end of the spectrum. If the inputs, domain or material, are perturbed, the cluster breaks and tracing of the eigenpairs become difficult due to possible crossing of the modes. In this paper we have shown that the eigenvalue crossing can occur within clusters not only by perturbations of the domain, but also of material parameters. What is new is that in this setting, the crossing can be controlled; that is, the effect of the perturbations can actually be predicted. Moreover, the basis of the subspace is shown to be a well-defined concept and can be used for instance in low-rank approximation of solutions of problems with static loading. In our industrial model problem, the reduction in solution times is significant. Full article