Journal Articles by Gokhan Serhat
Integrating discrete-variable anisotropic topology optimization with lamination parameter interpolation-based stiffness tailoring
Computer Methods in Applied Mechanics and Engineering, 2025
This paper introduces a novel computational design framework for topology and fiber path optimiza... more This paper introduces a novel computational design framework for topology and fiber path optimization of variable stiffness laminated composites. Based on the finite element discretization of the design domain, the topology and material stiffness are represented using elemental densities and lamination parameters (LPs), respectively. The density distribution is optimized via the discrete-variable topology optimization to obtain a precise structural layout. The recently proposed Topological Derivative-based Sensitivity Analysis is extended to calculate discrete-variable sensitivities for anisotropic materials, where the formulation accuracy is verified by finite difference computations. Elemental stiffness properties are described through the LPs, which are used with discrete-variable topology optimization for the first time. Specifically, the lamination parameter interpolation method (LPIM) is employed to significantly reduce the number of design variables while ensuring smooth variation of fiber angles throughout the laminate. In addition, unlike the previous LPIM-based works involving only master points for LP interpolation, the concept of master lines is introduced to enlarge the design space for stiffness distribution. Elemental densities and LPs are iteratively optimized to avoid solving the mixed integer programming problem directly. The effectiveness of the developed methodology is demonstrated through various case studies where designs providing minimum compliance or maximum directed displacement at specific locations are determined. The results show that the proposed approach can efficiently provide optimal variable stiffness laminate designs with clear density distributions and manufacturable fiber paths.
Process optimization to maximize bonding performance of injection-moulded short-fibre composites
Composites Part A: Applied Science and Manufacturing, 2025
Adhesive bonding has the major advantage of preserving structural integrity when joining laminate... more Adhesive bonding has the major advantage of preserving structural integrity when joining laminated composites, whose stacking sequence influences the joint strength. For short fibre reinforced thermoplastics (SFRT), injection moulding process parameters such as the flow rate, wall temperature, and packing pressure directly affect the resulting fibre orientations. This study explores the fundamental effects of the fibre architecture in SFRT substrates on the stress distribution in single lap joints. Considering the observed influences, a metamodel-based optimization approach is proposed to improve the joint bonding performance by modifying the production process, where the maximization of the bending stiffness is selected as the primary performance indicator. Experimental results validate the introduced methodology as they revealed approximately 30 % average difference of between the lap shear strength levels of the samples with maximized and minimized bending stiffness. These findings highlight the importance of a well-controlled production procedure respecting the bonding area of SFRT composites.
Materials & Design, 2025
Fiber-reinforced composites have gained worldwide popularity due to their superb properties inclu... more Fiber-reinforced composites have gained worldwide popularity due to their superb properties including high specific stiffness and strength. In addition to their favorable physical attributes, the mechanical characteristics of these materials can be tailored to maximize their performance for distinct applications. Recent advances in additive manufacturing have enabled the fabrication of curvilinear fibers, which can further improve internal load allocation. Composite structures become even more effective when stiffness tailoring is combined with topology optimization, which concerns determining the ideal material distribution for a specific structural design problem. However, collective optimization of material anisotropy and geometry poses inherent challenges that prompted the development of diverse methodologies. This review article summarizes the state-of-the-art composite design techniques developed to optimize fiber paths and structural topology sequentially or simultaneously. The available approaches are categorized according to their scope and intrinsic principles unlike many existing works employing classification based on optimization or material parametrization schemes. The paper also covers experimental results as another rare feature. The advantages and shortcomings of the investigated methods are discussed considering various aspects including effectiveness, ease of use, computational cost, versatility, robustness, and suitability for manufacturing. The review concludes with remarks on relevant open problems and potential future research directions.
Biomechanics and Modeling in Mechanobiology, 2024
Predicting how the fingertip will mechanically respond to different stimuli can help explain huma... more Predicting how the fingertip will mechanically respond to different stimuli can help explain human haptic perception and enable improvements to actuation approaches such as ultrasonic mid-air haptics. This study addresses this goal using high-fidelity 3D finite element analyses. We compute the deformation profiles and amplitudes caused by harmonic forces applied in the normal direction at four locations: the center of the finger pad, the side of the finger, the tip of the finger, and the oblique midpoint of these three sites. The excitation frequency is swept from 2.5 to 260 Hz. The simulated frequency response functions (FRFs) obtained for displacement demonstrate that the relative magnitudes of the deformations elicited by stimulating at each of these four locations greatly depend on whether only the excitation point or the entire finger is considered. The point force that induces the smallest local deformation can even cause the largest overall deformation at certain frequency intervals. Above 225 Hz, oblique excitation produces larger mean displacement amplitudes than the other three forces due to excitation of multiple modes involving diagonal deformation. These simulation results give novel insights into the combined influence of excitation location and frequency on the fingertip dynamic response, potentially facilitating the design of future vibration feedback devices.
Eigenfrequency optimization of variable stiffness manufacturable laminates using spectral Chebyshev approach and lamination parameters
Mechanics of Advanced Materials and Structures, 2024
This study presents a meshless modeling approach to design variable-stiffness laminates consideri... more This study presents a meshless modeling approach to design variable-stiffness laminates considering manufacturing constraints. The governing equations are derived using lamination parameters and first-order shear deformation theory. The solution approach uses Chebyshev polynomials and Galerkin’s method to obtain the discretized equations of motion. The developed framework was used to maximize the fundamental frequency of composite plates. The variable-stiffness designs provided up to 28.4% higher frequencies compared to optimum constant-stiffness laminates, although the actual level of improvement depends on the number of layers. Finally, manufacturable fiber paths were obtained considering the allowed fiber curvature, which can also reduce the frequency values.
Advanced lamination parameter interpolation and extrapolation methods for designing manufacturable variable stiffness laminates
Composite Structures, 2023
The design of variable stiffness laminates requires efficient methodologies due to the increased ... more The design of variable stiffness laminates requires efficient methodologies due to the increased number of optimization variables associated with curvilinear fiber paths. Here, this need is addressed by the development of two novel approaches: the lamination parameter extrapolation method (LPEM) and the relaxed lamination parameter interpolation method (RLPIM). These techniques build on the previously proposed lamination parameter interpolation method (LPIM), and collectively they form a spectrum of approaches that differ in optimization capacity, conservativeness regarding fiber curvature constraints, and computational cost. The resulting governing equations are solved using the spectral Chebyshev method to further improve the efficiency of the optimization process. The case studies demonstrate the effectiveness and unique properties of the developed algorithms.
IEEE Access, 2023
Thermal feedback has been proven to enhance user experience in human-machine interactions. Yet st... more Thermal feedback has been proven to enhance user experience in human-machine interactions. Yet state-of-the-art thermal technology has focused on the single finger or palm in static contact, overlooking dynamic and multi-finger interactions. The underlying challenges include incompatible designs of conventional interfaces for providing salient thermal stimuli for such interactions and, thereby, a lack of knowledge on human thermal perception for relevant conditions. Here we present the ThermoSurf, a new thermal display technology that can deliver temperature patterns on a large interface suitable for dynamic and multi-finger interactions. We also investigate how user exploration affects the perception of the generated temperature distributions. Twenty-three human participants interacted with the device following three exploration conditions: static-single finger, dynamic-single finger, and static-multi finger. In these experiments, the individuals evaluated 15 temperature differences ranging from-7.5 • C to +1.5 • C with an initial temperature of 38 • C. Our results showed that human sensitivity against thermal stimuli is significantly greater for static-single finger contact compared to the other tested conditions. In addition, this interaction type resulted in higher thermal discrimination thresholds than the ones reported in the literature. Our findings offer new perspectives on providing salient and consistent thermal feedback for future tactile interfaces. INDEX TERMS Human-machine interaction, human thermal perception, thermal display, thermal feedback.
Soft Robotics, 2022
Haptic displays act on the user's body to stimulate the sense of touch and enrich applications fr... more Haptic displays act on the user's body to stimulate the sense of touch and enrich applications from gaming and computer-aided design to rehabilitation and remote surgery. However, when crafted from typical rigid robotic components, they tend to be heavy, bulky, and expensive, while sleeker designs often struggle to create clear haptic cues. This article introduces a lightweight wearable silicone finger sheath that can deliver salient and rich vibrotactile cues using electromagnetic actuation. We fabricate the sheath on a ferromagnetic mandrel with a process based on dip molding, a robust fabrication method that is rarely used in soft robotics but is suitable for commercial production. A miniature rare-earth magnet embedded within the silicone layers at the center of the finger pad is driven to vibrate by the application of alternating current to a nearby air-coil. Experiments are conducted to determine the amplitude of the magnetic force and the frequency response function for the displacement amplitude of the magnet perpendicular to the skin. In addition, high-fidelity finite element analyses of the finger wearing the device are performed to investigate the trends observed in the measurements. The experimental and simulated results show consistent dynamic behavior from 10 to 1000 Hz, with the displacement decreasing after about 300 Hz. These results match the detection threshold profile obtained in a psychophysical study performed by 17 users, where more current was needed only at the highest frequency. A cue identification experiment and a demonstration in virtual reality validate the feasibility of this approach to fingertip haptics.
PLOS One, 2022
Pressing the fingertips into surfaces causes skin deformations that enable humans to grip objects... more Pressing the fingertips into surfaces causes skin deformations that enable humans to grip objects and sense their physical properties. This process involves intricate finger geometry, non-uniform tissue properties, and moisture, complicating the underlying contact mechanics. Here we explore the initial contact evolution of dry and hydrated fingers to isolate the roles of governing physical factors. Two participants gradually pressed an index finger on a glass surface under three moisture conditions: dry, water-hydrated, and glycerin-hydrated. Gross and real contact area were optically measured over time, revealing that glycerin hydration produced strikingly higher real contact area, while gross contact area was similar for all conditions. To elucidate the causes for this phenomenon, we investigated the combined effects of tissue elasticity, skin-surface friction, and fingerprint ridges on contact area using simulation. Our analyses show the dominant influence of elastic modulus over friction and an unusual contact phenomenon, which we call friction-induced hinging.
Maximizing buckling load of elliptical composite cylinders using lamination parameters
Engineering Structures, 2022
Structural members made of fiber-reinforced polymers (FRP) attract increasing attention in the de... more Structural members made of fiber-reinforced polymers (FRP) attract increasing attention in the development of novel architectural systems that challenge the standard design methodologies. Cylindrical surfaces constitute one of the typical geometric sets obtained with the FRP component fabrication. This paper explores the influences of two geometric parameters on the buckling performance of elliptical cylinders: inverse slenderness (ratio of minimum diameter to height) and eccentricity (ratio of radii along semi-axes). The overall stiffness properties are defined using lamination parameters. This analysis method eliminates the dependency of optimal solutions on the initial assumptions regarding the laminate configuration, which needs to be explicitly described in multi-layer modeling. Finite element analyses are utilized to compute buckling loads of the cylinders under axial compression force and bi-axial bending moments. The optimal lamination parameters and buckling stresses are determined for various parameters, and the lamination parameters corresponding to the optimal and simple [±45°] angle-ply design points are presented in the lamination parameter plane via Miki's diagram. The results reveal the level of performance that can be achieved by a specific geometry and provide guidelines for the optimal design of elliptical laminated cylinders against buckling.
Predicting the Force Map of an ERT-Based Tactile Sensor Using Simulation and Deep Networks
IEEE Transactions on Automation Science and Engineering, 2022
Electrical resistance tomography (ERT) can be used to create large-scale soft tactile sensors tha... more Electrical resistance tomography (ERT) can be used to create large-scale soft tactile sensors that are flexible and robust. Good performance requires a fast and accurate mapping from the sensor's sequential voltage measurements to the distribution of force across its surface. However, particularly with multiple contacts, this task is challenging for both previously developed approaches: physics-based modeling and end-to-end data-driven learning. Some promising results were recently achieved using sim-to-real transfer learning, but estimating multiple contact locations and accurate contact forces remains difficult because simulations tend to be less accurate with a high number of contact locations and/or high force. This paper introduces a modular hybrid method that combines simulation data synthesized from an electromechanical finite element model with real measurements collected from a new ERT-based tactile sensor. We use about 290,000 simulated and 90,000 real measurements to train two deep neural networks: the first (Transfer-Net) captures the inevitable gap between simulation and reality, and the second (Recon-Net) reconstructs contact forces from voltage measurements. The number of contacts, contact locations, force magnitudes, and contact diameters are evaluated for a manually collected multi-contact dataset of 150 measurements. Our modular pipeline's results outperform predictions by both a physics-based model and end-to-end learning.
Multi-objective optimization of composite sandwich panels using lamination parameters and spectral Chebyshev method
Composite Structures, 2022
Composite materials are widely used in various industries because of their distinct properties. H... more Composite materials are widely used in various industries because of their distinct properties. Hybridization is an efficient way of designing composite panels to decrease the cost and/or weight while maintaining stiffness properties. In this study, an accurate and efficient framework is developed to optimize laminated sandwich panels composed of high-stiffness face sheets and low-stiffness core. The stiffness properties of face sheets and core are represented using lamination parameters. The governing equations are derived following first-order shear deformation theory and solved using the spectral Chebyshev approach. In multi-objective optimization problems, genetic algorithm is used to determine Pareto-optimal solutions for fundamental frequency, frequency gap, buckling load, and cost metrics. In these analyses, optimal lamination parameters and thickness are found for face-sheets and core of sandwich panels, and the results are presented as 2D and 3D Pareto-optimal design points. When the individual performance metrics lead to different optimum points, a scattering behavior is observed in the 3D Pareto sets whose boundaries are defined by the 2-objective Pareto fronts. The results provide insights into the design requirements for improving the dynamic and load-carrying behavior of sandwich laminates while minimizing the cost that presents the usability of the presented approach in the multi-objective optimization.
A design methodology for fiber layup optimization of filament wound structural components
Structures, 2022
The applications of fiber-reinforced polymer (FRP) composites extend rapidly along with the devel... more The applications of fiber-reinforced polymer (FRP) composites extend rapidly along with the development of new manufacturing techniques. However, due to the complexities introduced by the material and fabrication processes, the application of conventional structural design methods for construction members has been significantly challenging. This paper presents an alternative methodology to find optimum fiber layups for a given tube-shape geometry via a graphical optimization strategy based on structural performance requirements. The proposed technique employs simplified shell element models based on classical lamination theory (CLT) to avoid explicit fiber modeling in the FEA simulations. Lamination parameters are utilized to generate the reduced stiffness matrices for continuous multi-layer FRP lamination. The fiber layup of the component is retrieved from the optimal lamination parameters that maximize the structural performance. The case study results demonstrate that the developed method provides compact solutions, linking the structural design requirements with optimal fiber orientations and volumetric proportions. In addition, the determined solutions can be interpreted directly by the winding fabrication settings.
Applied Sciences, 2021
Computational analysis of free and forced vibration responses provides crucial information on the... more Computational analysis of free and forced vibration responses provides crucial information on the dynamic characteristics of deformable bodies. Although such numerical techniques are prevalently used in many disciplines, they have been underutilized in the quest to understand the form and function of human fingers. We addressed this opportunity by building DigiTip, a detailed three-dimensional finite element model of a representative human fingertip that is based on prior anatomical and biomechanical studies. Using the developed model, we first performed modal analyses to determine the free vibration modes with associated frequencies up to about 250 Hz, the frequency at which humans are most sensitive to vibratory stimuli on the fingertip. The modal analysis results reveal that this typical human fingertip exhibits seven characteristic vibration patterns in the considered frequency range. Subsequently, we applied distributed harmonic forces at the fingerprint centroid in three principal directions to predict forced vibration responses through frequency-response analyses; these simulations demonstrate that certain vibration modes are excited significantly more efficiently than the others under the investigated conditions. The results illuminate the dynamic behavior of the human fingertip in haptic interactions involving oscillating stimuli, such as textures and vibratory alerts, and they show how the modal information can predict the forced vibration responses of the soft tissue.
Materials, 2021
This study concerns optimizing the eigenfrequencies of circular cylindrical laminates. The stiffn... more This study concerns optimizing the eigenfrequencies of circular cylindrical laminates. The stiffness properties are described by lamination parameters to avoid potential solution dependency on the initial assumptions of the laminate configurations. In the lamination parameter plane, novel response contours are obtained for the first and second natural frequencies as well as their difference. The influence of cylinder length, radius, thickness, and boundary conditions on the responses is investigated. The lamination parameters yielding the maximum response values are determined, and the first two mode shapes are shown for the optimum points. The results demonstrate that the maximum fundamental frequency points of the laminated cylinders mostly lie at the inner lamination parameter domain, unlike the singly curved composite panels. In addition, the second eigenfrequency shows a nonconvex response surface containing multiple local maxima for several cases. Moreover, the frequency difference contours appear as highly irregular, which is unconventional for free vibration responses.
Applied Sciences, 2021
Despite their versatility in treating irregular geometries, the raster methods have received limi... more Despite their versatility in treating irregular geometries, the raster methods have received limited attention in solving packing problems involving rotatable objects. In addition, raster approximation allows the use of unique performance metrics and indirect consideration of constraints, which have not been exploited in the literature. This study presents the Concurrent or Ordered Matrix-based Packing Arrangement Computation Technique (COMPACT). The method allows the objects to be rotated by arbitrary angles, unlike the right-angled rotation restrictions imposed in many existing packing optimization studies based on raster methods. The raster approximations are obtained through loop-free operations that improve efficiency. Additionally, a novel performance metric is introduced, which favors efficient filling of the available space by maximizing the overall contact within the domain. Moreover, the objective functions are exploited to discard the overlap and overflow constraints and enable the use of unconstrained optimization methods. The results of the case studies demonstrate the effectiveness of the proposed technique.
Materials, 2021
The operational performance of cantilever composite structures can benefit from both stiffness ta... more The operational performance of cantilever composite structures can benefit from both stiffness tailoring and geometric design, yet, this potential has not been fully utilized in existing studies. The present study addresses this problem by simultaneously optimizing layer and taper angles of cantilever laminates. The design objective is selected as minimizing the average deflection of the tip edge subjected to shear loads while keeping the length and total volume constant. The plate stiffness properties are described by lamination parameters to eliminate the possible solution dependency on the initial assumptions regarding laminate configuration. The responses are computed via finite element analyses, while optimal design variables are determined using genetic algorithms. The results demonstrate that the plate aspect ratio significantly influences the effectiveness of stiffness tailoring and tapering as well as the optimal layer and taper angles. In addition, concurrent exploitation of the lamination characteristics and plate geometry is shown to be essential for achieving maximum performance. Moreover, individual and simultaneous optimization of layer and taper angles produce different optimal results, indicating the possible drawback of using sequential approaches in similar composite design problems.
Dynamic analysis of doubly curved composite panels using lamination parameters and spectral-Tchebychev method
Computers & Structures, 2020
Efficient modeling and optimization techniques are required to overcome the high design complexit... more Efficient modeling and optimization techniques are required to overcome the high design complexity and computational costs concerning the engineering of composite structures. In this paper, a modeling framework for the dynamic analysis of doubly curved composite panels is developed. Lamination parameters are used to characterize the stiffness properties of the laminate, and the responses are calculated through the two-dimensional spectral-Tchebychev method. The proposed framework combines the computational efficiency advantages of both lamination parameters formulation and spectral-Tchebychev method which is extended for dynamic analysis of curved composite laminates. Compared to the finite element method, the developed model significantly decreases the computation duration, thereby leading to analysis speed-ups up to 40 folds. In the case studies, fundamental frequency contours for the doubly curved composite panels are obtained in lamination parameters space for the first time. The results show that, unlike flat or singly curved laminates, the maximum frequency design points for doubly curved panels can be inside the feasible region of lamination parameters requiring multiple layer angles. The fundamental mode shapes for the maximum frequency designs are also computed to investigate the influence of panel curvatures on the vibration patterns, which can exhibit mode switching phenomenon.
Unifying lamination parameters with spectral-Tchebychev method for variable-stiffness composite plate design
Composite Structures, 2020
This paper describes an efficient framework for the design and optimization of the variable-stiff... more This paper describes an efficient framework for the design and optimization of the variable-stiffness composite plates. Equations of motion are solved using a Tchebychev polynomials-based spectral modeling approach that is extended for the classical laminated plate theory. This approach provides highly significant analysis speed-ups with respect to the conventional finite element method. The proposed framework builds on a variable-stiffness laminate design methodology that utilizes lamination parameters for representing the stiffness properties compactly and master node variables for modeling the stiffness variation through distance-based interpolation. The current study improves the existing method by optimizing the locations of the master nodes in addition to their lamination parameter values. The optimization process is promoted by the computationally efficient spectral-Tchebychev solution method. Case studies are performed for maximizing the fundamental frequencies of the plates with different boundary conditions and aspect ratios. The results show that significant improvements can be rapidly achieved compared to optimal constant-stiffness designs by utilizing the developed framework. In addition, the optimization of master node locations resulted in additional improvements in the optimal response values highlighting the importance of including the node positions within the design variables.
Materials & Design, 2019
Laminated composite plates are extensively used in various industries due to their high stiffness... more Laminated composite plates are extensively used in various industries due to their high stiffness-to-weight ratio and directional properties that allow optimization of the stiffness characteristics for specific applications. In multi-objective optimization problems, optimal designs for individual performance metrics may be conflicting, necessitating knowledge on the design requirements for different metrics and potential trade-offs. In this paper, a multi-objective design methodology for laminated composite plates with dynamic and load-carrying requirements is presented. Lamination parameters are used to characterize laminate stiffness matrices in a compact form resulting in a convex design space. Single and multi-objective optimization studies are carried out to determine the optimal stiffness properties. For improving the dynamic performance, maximization of the fundamental frequency metric is aimed. For enhancing the load-carrying capability, buckling load and equivalent stiffness metrics are maximized. Conforming and conflicting behavior of multiple objective functions for different plate geometries, boundary conditions and load cases are presented by determining Pareto-optimal solutions. The results provide a valuable insight for multi-objective optimization of laminated composite plates and show that presented methodology can be used in the design of such structures for improving the dynamic and load-carrying performance.
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Journal Articles by Gokhan Serhat