Vibration Modes Of Structure Research Articles

Tunable Imaging Distance Focusing Module Directly Driven by a Thin‐plate Piezoelectric Actuator

AbstractWith the advancement of imaging technology, the performance of optical focusing modules becomes crucial for determining imaging quality. Conventional focusing modules frequently face challenges of excessive bulk and weight, complicating the integrated design and limiting application fields. Herein, this study presents a piezo‐actuated optical focusing module (POFM) and the construction strategy to alleviate these issues. A novel radial‐bending resonant‐type piezoelectric actuator (RB‐RPA) is developed to directly drive the POFM. The RB‐RPA, which features a hollow structure (27 × 27 × 7 mm3), utilizes both radial and bending vibration modes of a thin metal plate. It achieves a speed of 122.93 mm s−1, a thrust force of 0.14 N, and a displacement resolution of 0.52 µm. Employing the proposed construction strategy, RB‐RPA is integrated with imaging components to fabricate a POFM prototype. Furthermore, the micrometer‐level displacement resolution of POFM facilitates optical focusing on imaging targets, allowing the acquisition of sharp images within 70 ms. It achieves an image resolution of 119.57 lp mm−1 within a 42 mm segment of the captured image. Finally, a QT‐based interface for the POFM, enables real‐time image capture and sharpness evaluation, thereby enhancing imaging efficiency and integration. This work provides a potential candidate for next‐generation imaging devices.

Design of a mixed robotic machining system and its application in support removal from metal additive manufactured thin-wall parts

Robotic machining could provide a solution for removing supports from metal additive manufactured workpieces, replacing labor-intensive work. However, the robot’s intrinsic weaknesses of low positioning accuracy and structural rigidity primarily restrict its applications. Improving the accuracy of robotic machining remains an unresolved issue. A mixed solution is proposed, in which a portable CNC machine with the capability of visual feature recognition is equipped with a universal industrial robot. The robot implements positioning motions in a large space, while the portable CNC fulfills accurate machining motions on a local feature of the workpiece. A sizeable weight of the portable CNC exerts a moderate load on the industrial robot’s joints, increasing joint stiffness. The mixed machining system exhibits high accuracy and stiffness when milling a steel/titanium alloy workpiece, achieving tolerances up to ±0.04 mm on a 60×80 mm U-shaped path without exciting any structural vibration modes. When the dimension of the workpiece exceeds the machining range of the portable CNC, a combined algorithm of coarse-fine registration based visual identification and robot error compensation is designed to align the spatial coordinates of the machining motion with that of the positioning motion, thereby extending the machining range with high accuracy. Through the proposed mixed robot machining method, experiments of doubling the machining range have been done to verify that the mixed machining robotic system is able to slot a 550 mm-long path with accuracy of ±0.1 mm. Furthermore, the mixed robotic machining system is applied to recognize and remove multiple supports of lattices, grids and ribs from a titanium-alloy additive manufactured thin-wall workpiece with high accuracy and high efficiency.

Facile synthesis of novel cobalt molybdenum sulfide-graphitic carbon nitride (CoMoS4/g-C3N4) nanocomposite electrode for improved supercapacitor and photocatalytic applications

Supercapacitors have gained significant attention from scientists due to their wide range of applications in energy storage devices. However, searching for abundant and eco-friendly materials on earth is still necessary while preserving their outstanding performance. Herein, cobalt molybdenum sulfide-graphitic carbon nitride (CoMoS4/g-C3N4) nanocomposite was synthesized to explore an exceptional electrode material for energy and photocatalytic applications. The structural analysis, vibrational modes, and porosity within the CoMoS4/g-C3N4 nanocomposite were studied by XRD, Raman, and SEM. The direct energy band gap decreased from 2.54 to 2.44 eV along with the increased ratio of CoMoS4 in the nanocomposite. The electrochemical measurements of CoMoS4/g-C3N4 nanocomposite exhibited an excellent specific capacitance (Cp) of 3675 F/g at 1 mV/s and 3150 F/g at 2.5 A/g with noticeable specific energy density (Eg) of 109.4 Wh/kg, high power density (Pd) of 625 W/kg and retain high capacitance (88 % of its initial value) even after 4000 cycles. Furthermore, the photocatalytic results revealed that high photodegradation efficiency (97.85 %) was attained against methyl orange (MO) dye under UV light irradiation. Prepared electrode materials are promising materials for next-generation energy storage and environmental applications.

Bridge damage identification based on synchronous statistical moment theory of vehicle–bridge interaction

AbstractConsidering the weak noise resistance and low identification efficiency of traditional bridge damage identification methods, a data‐driven approach based on synchronous statistical moment theory and vehicle–bridge interaction vibration theory is proposed. This method involves two main steps. First, a two‐axle test vehicle is used to collect acceleration response signals synchronously from adjacent designated measurement points while stationary. This operation is repeated to calculate the second‐order statistical moment curvature (SOSMC) difference of entire bridge points corresponding signals in different states. By comparing with the reference value, the preliminary damage location of the bridge can be obtained. Second, the first‐order modal shape curve is constructed using the second‐order statistical moment (SOSM). The refined identification of bridge damage is then based on an improved direct stiffness back calculation of the bridge's stiffness. This article proposes the synchronization theory for the first time and combines it with the statistical moment clustering method, forming an innovative approach to obtaining structural vibration modes. The effectiveness of this method has been well validated through numerical simulations with different parameters and on‐site bridge tests. The research results indicate that SOSMC indicators have better noise resistance and higher recognition efficiency in identifying damage locations, compared to modal curvature and flexibility curvature indicators. Additionally, compared to transfer rate and random subspace methods, the SOSM method results in smaller error and higher identification efficiency.

Optical, physical, mechanical, structural, and radiation shielding investigations of B2O3–TeO2-GeO2-MgO-PbO for ionizing protection and optical transmission application

This study aims to investigate a new heavy glass system's potential use in the radiation shielding and optical transmission applications. Glass former like borate is replaced in this work by heavy metal oxides like lead oxide in the following glass system: (35-x) B2O3–20TeO2-10GeO2-35MgO-xPbO (where x = 2.5, 5, 7.5, and 10 mol%). Through the melt-annealing process, four transparent glasses were synthesized. A number of physical properties were reported, namely the boron–boron separation x 10−8 m, field strength x 1018 (cm2), inter nuclear distance x 10−8 m, ion concentration (N) x 1023 (ion cm3), molar volume (cm3/mol), molecular weight (g/mol), and polaron radius x 10−9 m. Moreover, the following mechanical and structural properties of the synthesized glasses were explored: bulk modulus (B, GPa), dissociation energy (kcal/cm3), fractal bond connectivity (d), longitudinal modulus (L, GPa), hardness (H, GPa), oxygen packing density, packing density (Vt), oxygen molar volume, packing factor (Vi), shear modulus (S, GPa), Poisson radius (σ), and Young modulus (Y, GPa). To study the bulk glasses' nature, the X-ray diffraction was examined. Optical absorption along with optical transmission profile have been recorded to extract various optical properties, as well as the direct and indirect band gap. The transmittance spectra of the prepared glass samples' FTIR were investigated within the 400–2000 cm−1 range. This allowed for the examination of molecular structural groups and vibrational modes. Radiation shielding parameters have been calculated and studied using XCOM software and MCNP-5 algorithm.

Structural optimization of PVDF cellular resonators for energy-harvesting enhancement based on backpropagation neural network and NSGA-II algorithm

Metamaterials with honeycomb structures have garnered significant attention in energy-harvesting applications. In this study, we present three conventional cantilever resonators with honeycomb substrates featuring positive, zero, and negative Poisson’s ratios (PPR, ZPR, and NPR). Polyvinylidene fluoride consisting electrode layers is attached to the substrate surfaces. A process parameter optimization method of honeycomb structure design was proposed, in which finite element simulation for data generation, backpropagation neural network for data prediction, and nondominated sorting genetic algorithm II (NSGA-II) for data optimization. This method aims to determine the optimum geometric parameters of the honeycomb structures, which addresses general cantilever resonators, where the first structural vibration modes are typically of interest and have to match specific target eigenfrequencies for engineering applications. Finite element simulations show that the peak voltage of PPR, ZPR, and NPR resonators at 75 Hz is increased by 12 %, 22 %, and 32 %, respectively, due to optimization. The optimized structures are fabricated and measured to validate the numerical model. The performance of the resonators with cellular materials featuring a full characteristic range of Poisson’s ratios is then compared systematically to explore their energy-harvesting potential. The results revealed that the NPR structure exhibits superior performance for energy-harvesting. Moreover, stress distribution and displacement have also been highlighted in this paper.

Vibration attenuation and wave propagation analysis of 3D star-shaped resonant plate structures and their derivatives with ultra-wide band gap

Based on the local resonance effect of elastic waves, three single-phase acoustic metamaterials are proposed in this paper. Based on the finite element method and Bloch's theorem, the energy band structure diagrams and vibration modes are plotted, and the band gap properties and band gap opening mechanism of these structures are explored. New structures possessing lower frequency band gaps are obtained by topological optimization. The transmission curves verify the accuracy of the band gap and the vibration attenuation ability of the structure. Finally, the structural parameters were adjusted and the effect of each parameter change on the band gap characteristics was analyzed. The results show that the proposed structure has a maximum band gap coverage of 72.4 % and a strongest attenuation peak of less than −400 (dB) due to the occurrence of a local resonance. This paper provides a methodology for analyzing the vibration and noise reduction performance of single-phase material phononic crystals, as well as a three-dimensional phononic crystal with potential for practical applications.

EVALUATION OF THE SCATTER OF LIQUID LAUNCH VEHICLE POGO OSCILLATION AMPLITUDES DUE TO THE INFLUENCE OF THE SCATTER OF INTERNAL FACTORS

Almost all liquid launch vehicle developers faced the problem of ensuring stability in relation to POGO oscillations. The level of POGO amplitudes oscillations of the launch vehicle can be significantly affected by the scatter of internal factors. The study aims to create a mathematical model that can determine the range of POGO amplitudes in liquid launch vehicles. This will be demonstrated through the example of the Dnipro launch vehicle, which is affected by a variety of internal factors that cause its POGO amplitudes to vary. We developed the non-linear non-stationary mathematical model of POGO oscillations of the prototype of the Dnipro space launch vehicle. The model is built by taking into account the two lower vibration modes of the LV structure, two lower oscillation modes of the oxidizer feedline, and the first oscillation mode of the fuel feedline of the propulsion system. Modeling of dynamic processes was conducted in a combination of four liquid rocket engines based on the schematic of the staged rocket engine. The simulation takes into account cavitation phenomena in the engine pumps and delay times in the gas generators’ chambers. We have developed a method for determining the scatter of the POGO oscillations caused by the action of internal factors, which is based on the use of the LP uniformly distributed sequences. As internal factors, the frequencies, decrements, and shapes of LV structural oscillation modes, the values of pressurization of the propellant tanks, and the engines’ specific thrust impulses were considered. Based on the results of the calculations, the dependence of the POGO amplitudes in two regions of LV instability was determined, and the lower and upper enveloping curves for the POGO amplitudes were constructed. It is shown that the maximum POGO amplitudes oscillations in the first region of instability lie in the range from 0.23 g to 0.72 g and in the second region of instability — from 0 g to 0.60 g. Variants of combinations of internal factors, which provided the largest and smallest values of POGO amplitudes, were analyzed. This made it possible to determine the internal factors, the scatter of which has the greatest effect on the POGO amplitudes scatter: frequency, decrement, shape coefficients of oscillations of the oxidizer feedlines and the LV 1st mode structural longitudinal oscillations in the payload cross-section.

Structural properties and vibrational modes of different polytypes of gallium selenide crystals (ε, β, γ, δ): insights from a DFT study

Structural properties and vibrational modes of different polytypes of gallium selenide crystals (ε, β, γ, δ): insights from a DFT study

Effect of B2O3 content on the structural and optical properties of (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 glass

This study explores the structural and optical properties of (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 system mixed with B2O3. The samples with the composition xB2O3 + (100-x) (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 (where x = 5, 10, 20, 30 and 40 mol. %) were prepared using the melting quenching technique to form borate-based glass containing BaO, CaO, ZrO2 and TiO2 (BBCZT). X-ray diffraction analysis confirmed the amorphous nature of the prepared samples. The macroscopic structure of the glasses was assessed from the density measurements determined through Archimedes' principle, while information regarding the atomic structural units and vibrational modes were obtained from Infrared measurements. Increasing the B2O3 content led to variation of the relative intensity of these modes together with a decrease (increase) in density (molar volume), indicating a less dense and open-structure for the glass network. The absorption spectra showed absorption bands associated with the optical transitions B22g→B21g and B22g→A21g of Ti3+ ions occupying distortion octahedral sites (TiOh3+) in the glass matrix. The ligand field strength (10Dq), optical energy gap (Eopt), Urbach's energy (ΔE), and nonlinear optical properties (NLO) were calculated in the study.

Effect of heavy Dy3+ doping on the magnetic, structural, morphological, and optical characteristics of CuDyxFe2-xO4 nanoparticles

The present study explored the impacts of dysprosium (Dy³⁺) doping on the structure, optical, and magnetic characteristics of CuDyxFe2-xO4 nanospinel ferrites synthesized via a hydrothermal method. X-ray diffraction revealed a tetragonal spinel matrix with increasing Dy³⁺ content, accompanied by a secondary Dy2O3 phase at higher dopant levels for x ≥ 0.1. Crystallite size and microstrain increased with doping, while infrared spectroscopy confirmed the spinel structure and distinct vibrational modes. Interestingly, saturation magnetization displayed a non-monotonic trend, first rising due to spin polarization and cation redistribution, then decreasing with higher Dy³⁺ content attributable to the Dy2O3 phase and larger grains. Coercivity exhibited a maximum at x = 0.2, reflecting the interplay between magnetocrystalline anisotropy, canting angles, and domain wall pinning. These findings highlight the complex interplay between Dy³⁺ doping and the resulting properties, paving the way for tailored CuDyxFe2-xO4 nanospinel ferrites functionalities. . Further research involving advanced techniques and targeted dopant levels holds promise for optimizing properties for diverse technological applications such as magnetic storage, microwave devices, sensors, and hyperthermia treatment.

Research of the accuracy and efficiency of typical structures using wave shell finite element models

Research of the accuracy of wave shell finite element models was carried out. The results were compared with the results of analytical calculations. According to the research results, for all considered vibration modes of standard structures, the discrepancy in the values of natural frequencies does not exceed 5%.

Modal analysis of honeycomb panel satellite structure based on MSC Apex

Modal analysis plays an important role in the process of Satellite structure development. In this paper, the honeycomb sandwich plate and the satellite structure are taken as the research objects, and the corresponding finite element models are established. The analysis results of the actual modeling of honeycomb plate and the equivalent modeling of sandwich sandwich plate are obtained, and the fundamental frequency deviation of the whole satellite structure is discussed when the equivalent correction coefficient γ of honeycomb plate is different. The results show that the maximum relative error of the modal frequency between the equivalent method of sandwich sandwich plate and the actual modeling is 2.8%. Therefore, the equivalent method can be applied to large-scale finite element analysis and calculation, and the calculation efficiency can be improved. When the value of γ is 0.4∼0.6, the vibration modes of the whole satellite structure are basically consistent, and the maximum fundamental frequency difference is 10%. The results can evaluate the accuracy of the sandwich sandwich plate equivalent method for satellite mechanical analysis.

Energy Convergence in a Structure Optimization and Vibrational Modes of Some Acene Molecules

Acene molecules, exhibit interesting electronic and structural properties due to the presence of alternating single and double bonds. There has been a recent surge in interest in studying small acenes, such as anthracene, tetracene, and pentacene. Geometry optimization and energy convergence for acene molecules are essential steps in understanding their properties and behavior. The modellings of MJs was done using Jmol computer code and the geometry of these structures was relaxed towards its minimum energy using a two-step procedures which are; pre-relax the structure with light settings and post-relaxation run using tight settings down to 10-2 eV/Å stopping criteria. The results showed that total energy uncorrected and the maximum force converged. The converged energies for anthracene, tetracene and pentacene molecules are, 0.0057 eV/Å, 0.0056 eV/Å, and 0.0060 eV/Å respectively. For the vibrational analysis, some of the results indicated that the input geometry is indeed a local minimum and not a saddle point. The results also showed a low IR absorption, with maximum intensity of 1.83 D2Å-2, 2.00 D2Å-2, and 1.91 D2Å-2 for anthracene, tetracene, and pentacene respectively.

Aperiodic Two-Dimensional Acoustic Black Holes for Broadband Vibration Attenuation in a Strip

Abstract Purpose Acoustic black holes (ABHs) are promising for vibration control in lightweight structures as proven for one- or two-dimensional periodic arrangements. Here, we explored the effects of spatial disorder and heterogeneous designs of ABHs to broaden an intrinsically limited attenuation bandwidth of periodic counterparts. Method We proposed several strategies to introduce non-periodic arrangements and/or different ABH profiles by solving a maximization problem for the attenuation bandwidth of a plate strip decorated by five ABHs. These strategies allow for finding appropriate dimensions and positions of the ABHs by analyzing a small design subset and are verified experimentally. Results The identified periodic heterogeneous ABHs enable greatly extending the attenuation bandwidth, while disordered identical ABHs allow for increasing the attenuation intensity as compared to the corresponding periodic configurations. The mechanisms underlying the wave attenuation enhancement were clarified by tracing the evolution of the wave transmission and structural vibration modes at each design step. We have found that the broadened wave attenuation attributes to the activation of strongly localized modes at broadband frequencies in aperiodic scenarios. These abundant modes are multi-frequency local resonances in ABHs that are sensitive to both the ABH profile and their spatial arrangement. Conclusion We prove that relaxing the periodicity requirement on multiple two-dimensional ABHs can extend the vibration attenuation to broadband regimes below the ABH characteristic frequency, numerically and experimentally. Aperiodic designs of ABHs thus enlarge the design space by enabling a broadband wave mitigation with attenuation intensity comparable to that of periodic counterparts without increasing the structural mass.

Experimental Investigation of a Flexible Airframe Taxiing Over an Uneven Runway for Aircraft Vibration Testing

<div>The ground vibration test (GVT) is an important phase in a new aircraft development program, or the structural modification of a certified aircraft, to experimentally determine the structural vibrational modes of the aircraft and their modal parameters. These modal parameters are used to validate and correlate the dynamic finite element model of the aircraft to predict potential structural instabilities (such as flutter), assessing the significance of modifications to research vehicles by comparing the modal data before and after the modification and helping to resolve in-flight anomalies. Due to the high cost and the extensive preparations of such tests, a new method of vibration testing called the taxi vibration test (TVT) rooted in operational modal analysis (OMA) was recently proposed and investigated as an alternative method to conventional GVT. In this investigation, an experimental setup was constructed to further investigate the applicability of the TVT to flexible airframes encountered in fixed-wing autonomous aerial systems with oleo-pneumatic shock absorber landing gears in a tricycle configuration. The influence of the taxiing speed and the landing gear–shock absorber damping setting on the outcome of the TVT is also investigated. The taxiing speed was found to strongly influence the success of the test with an optimal taxiing speed existing for the assembled airframe that allows for the best TVT outcome. The shock absorber damping setting was found to increase the level of the airframe excitation during the TVT; however, it did not impact its outcome as compared to the taxiing speed. Certain modes were not identified during the TVT tests, which was attributed to the way the assembled aircraft is secured to the moving belt and to the lack of sufficient excitation through taxiing. The experimental vibrational modes were successfully matched against the modes obtained from the normal modal analysis of an uncorrelated flexible multibody dynamics model. Further investigations are suggested before the TVT method can be deemed suitable for all classes of fixed-wing aerial systems.</div>

Vibration modes of three-dimensional spiral cochlea covering the organ of Corti

So far, explaining the mechanism on active phonosensitive amplification in the cochlea is a major and difficult medical question. Among them, one of the key problems is that the motion pattern of the organ of Corti (OC) is still unknown. To this end, a multi-scale cochlear model including a three-dimensional spiral OC was established based on CT data and light source imaging experimental data, which complete combined the macroscopic and microscopic structure. On the basis of verifying the reliability of the model, acoustic-solid coupling calculation and modal analysis were performed on the model, and the vibration modes of basilar membrane (BM) and structures of the OC at different characteristic frequencies were discussed. The results show that tectorial membrane (TM) exhibits completely different vibration modes from BM at low frequencies, while the two movements gradually synchronize as the frequency increases. The amplitude position of OC’s motion moves laterally with increasing frequency from Deiters’ cells to Hensen’s cells and then back to Deiters’ cells. The OC exhibits longitudinal vibrations following BM when BM’s displacement is large, while it manifests more as lateral movement of Deiters’ cells when BM’s displacement is small. This model can well simulate the motion process of BM and OC in the lymphatic fluid, which provides theoretical support and a numerical simulation computational platform to explore the interaction between macroscopic and microscopic tissue structures of the overall cochlea.

Passive mitigation of low-frequency vibration modes in lightweight structures based on a new generation of acoustic black holes

Topology optimization and generative design are methods used to lighten structures under specific loads. The resulting lightweight components are usually softer in the directions perpendicular to the applied efforts, such as in the case of cranes. This results in the possible excitation of structural modes at very low frequencies, which can lead to premature aging of lightweight structures. In this paper, we present a new design of acoustic black hole (ABH), able to mitigate vibration modes in a test 3D-printed lightweight structure, from 20 Hz upwards. The novel ABH weights less than 10 grams and can be readily screwed on the structure. Its design is based on that of a classic ABH, with additional winglets and/or rods, to increase the vibrating mass without significantly changing the stiffness of the ABH. This results in a strongly reduced ABH cut-on frequency. The winglets and/or rods also result in a greatly increased contact area between the ABH and the viscoelastic dissipative material, with positive effects on the ABH damping efficiency. The new ABHs could successfully be used, first to damp the fundamental vibration mode of aluminum cylinders, and then to reduce the amplitude of the first three structural modes of the test lightweight structure. The new ABH manages to damp modes at frequencies several times lower than the ones found in the literature, which are promising results.

THz‐Wave Absorption Properties of Organic–Inorganic Hybrid Perovskite Materials: A New Candidate for THz Sensors

Over the past two decades, organic–inorganic hybrid perovskite materials (OHP) have been extensively explored across various scientific disciplines, including physics, chemistry, and materials science, with a primary focus on solar cells. Building on numerous studies, the development of OHP‐based solar cells has transitioned into practical product realization, instilling the anticipation of novel solar cell advancements. Notably, OHP demonstrates versatility beyond its conventional application in solar cell materials. The physical properties of OHP materials exhibit a unique signature, thereby underscoring their potential utility as innovative functional materials, encompassing light‐emitting diodes, lasers, and photodetectors. Recent reports on terahertz (THz)‐wave absorption properties of OHP materials indicate a high possibility of their potential application as THz sensors. From the viewpoint of medical devices, which hold the most promising application potential, the exploration of optical phonon vibrational modes in the 0.5–3 THz frequency range is important. Moreover, understanding the correlations between atomic structure and lattice vibration modes is indispensable. In this concise review, the THz‐wave absorption properties exhibited by 3D OHP materials are meticulously explored. Furthermore, future research directions for THz sensors using OHP materials are suggested.

Experimental investigation of flow-induced vibration and flow field characteristics of a flexible triangular cylinder

Flow-induced vibration (FIV) of a flexible cylinder with a triangular cross-section, allowed to oscillate in the cross-flow, inline and torsional direction, is studied experimentally through water tunnel tests. The dynamic response of the cylinder was studied for three different angles of attack ( $0^\circ, 30^\circ, 60^\circ$ ), at reduced velocities of 0.9–16.27, corresponding to Reynolds numbers of 364–3600. At the angle of attack of $0^\circ$ , vortex-induced vibration at low reduced velocity was observed, which transitioned to galloping at higher reduced velocities. At the angles of attack of $30^\circ$ and $60^\circ$ , galloping-type response was observed over the range of the reduced velocities tested. Our results show that the cylinder's torsional oscillation breaks the system's symmetry and affects the structural response at higher reduced velocities regardless of the angle of attack. The FIV response of the flexible triangular cylinder is distinct from that of a rigid flexibly mounted triangular cylinder due to torsional oscillation, spanwise flexibility and the two fixed boundary conditions limiting the amplitude of oscillation. Flow field analysis in the wake of the cylinder was done qualitatively and quantitatively using hydrogen bubble flow visualisation and time-resolved volumetric particle tracking velocimetry techniques, respectively. Our results show the existence of highly three-dimensional vortex structures in the wake of the cylinder. We studied the coupling between the vortex shedding modes in the wake of the cylinder and the structural vibration modes through the spatiotemporal mode analysis using the proper orthogonal decomposition technique to distinguish between different types of the FIV response observed.