Modal analysis is done periodically to ensure blades performance for non-occurrence of resonance obtained under various operating conditions. Since metal or composite helicopter blades have a certain flexibility at different speeds in different planes can lead to deformation of the structure that can have a great effect on the aerodynamic performance of the helicopter. In this paper, Modal analysis of helicopter blade has been carried out by the lumped mass methods (myklestad and finite element method) and Modal test on the rotor structure. The finite element analysis has been developed by two approach of analytical and software method. Modal testing was also performed in both static and dynamic states, with constant speed, to validate the lumped mass methods. Finally, the results of both methods are compared with each other. After ensuring there correspondence between lumped mass methods and experimental results, an analysis was done according to the operating conditions of the rotor and finally Campbell diagram has been generated. According to this diagram became clear there was not any problem with the resonance of the rotor blades, while the numerical result of case has an acceptable match with the experimental results.

In this paper, an alternative approach in operational Modal analysis is presented, utilizing image processing technique and transmissibility functions. Imaging sensors do not impose additional mass on the structure due to their non-contact nature, while transmissibility functions, independent of excitation type, can directly extract mode shapes. The innovation of this research lies in combining these two techniques to record dynamic responses and identify Modal properties. To capture the temporal response history from video signals, the block-matching method with sub-pixel accuracy was employed. Validation was conducted by recording the response of the tip of a cantilevered steel beam subjected to impact excitation, using a high-speed camera and a laser vibrometer, simultaneously. The RMSE plots in the time domain and the PSD in the frequency domain indicate high accuracy of this method. Using this approach, the displacement time histories of various points on the structure were extracted from the video signals, and the Modal properties, including natural frequencies, damping ratios, and mode shapes, were identified using the transmissibility matrix method. The results obtained from the proposed method were compared with the stochastic subspace identification (SSI) method and analytical solutions. The findings reveal the accuracy of the Modal identification approach introduced in this article. The highest relative error in estimating the natural frequencies of the first and second modes, compared to the values from the laser method, are 0.19% and 0.13%, respectively, and in comparison to the analytical values, they are 0.34% and 1.5%, respectively.

In this research, the Modal parameters of a beam in free-free condition are extracted by performing different experiments in the laboratory. For this purpose, two different techniques are employed. The first methodology is considered as a time domain method in Operational Modal Analysis. The other one is frequency domain impact hammer test which is categorized as an Experimental Modal Analysis method and can be regarded as the most common method in Modal analysis. Checking the results obtained by the two methods, one can notice a distinct inconsistency in Modal damping ratios extracted by each method. However, based on recent publications on the subject, it can be inferred that the time domain methods have better accuracy in identifying damping ratios of structures. In order to confirm the findings, the effect of excitation is examined for each method by altering the excitation tool. For the operational method, it is concluded that changing the excitation tool will not have a noticeable influence on the identified damping ratios, whilst for the Experimental Modal Analysis method changing the hammer tip leads to inconsistent results for damping ratios. This study exemplifies the deficiency of Experimental Modal Analysis methods in their dependency on excitation techniques.

In the present study three composite sandwich structures with grid stiffened core (SSGSC) samples made of different materials and thicknesses are fabricated with hand lay-up method using a silicon rubber mold and epoxy resin. Also, two metallic samples of the same dimensions as the composite ones, including a monolithic and a SSGSC samples made of aluminum are fabricated. In order to study their behavior subjected to the quasi-static transverse loads, the samples undergo three-point bending tests. Results of the practical tests on the composite samples showed that beyond the failure of the face sheets, the grid stiffened core will tolerate load, also there are no delamination between the face sheet layers due to good curing process. The experimental Modal testing is achieved on all the samples. The frequency response, mode shape as well as damping coefficients are obtained from each experiment. Finally, numerical Modal analysis is done and the results are compared with the experiments.

The natural frequencies and mode shapes of bone are widely used in the diagnosis of bone fracture healing, bone remodeling, osteoporosis diagnosis, and implant-bone interaction. These characteristics can be estimated using numerical analysis and finite element modeling and measured and validated by Modal testing of laboratory samples. Bone heterogeneity in finite element models affects its natural frequency. In this research, the effect of bone heterogeneity and density-elasticity relationships on the natural frequency of bone and the accuracy of their estimation have been investigated. A cow tibia bone was prepared and subjected to Modal test. The experimental results of the Modal test were compared with the results of the 3D finite element model created from the bone CT scan images. Inhomogeneity based on the proposed density-elasticity relationships was applied element by element (continuous) and regionally based on a tolerance (discrete) in the model. Comparing the results of heterogeneous models with experimental results shows that there is very little difference between the natural frequencies of discrete and continuous heterogeneous models. So, for the first five frequencies, a discrete heterogeneous model with 20 regions can predict the natural frequencies with an error of less than 2% with less cost and shorter time.

Background: Pedicle screw fixation devices are the predominant stabilization systems adopted for a wide variety of spinal defects. Accordingly, both pedicle screw design and bone quality are known as the main parameters affecting the fixation strength as measured by the pull-out force and insertion torque. The pull-out test method, which is recommended by the standards of the American Society for testing and Materials (ASTM), is destructive. A non-destructive test method was proposed to evaluate the mechanical stability of the pedicle screw using Modal analysis. Natural frequency (ω n ) extracted from the Modal analysis was then correlated with peak pull-out force (PPF) and peak insertion torque (PIT). Methods: Cylindrical pedicle screws with a conical core were inserted into two different polyurethane (PU) foams with densities of 0. 16 and 0. 32 g/cm3. The PIT and PPF were measured according to the well-established ASTM-F543 standard at three different insertion depths of 10, 20, and 30 mm. Modal analysis was carried out through recording time response of an accelerometer attached to the head of the screw impacted by a shock hammer. The effect of the insertion depth and foam density on the insertion torque, natural frequency, and pull-out force were quantified. Results: The maximum values of ω n, PIT, and PPT were obtained at 2, 186 Hz, 123. 75 N. cm, and 981. 50 N, respectively, when the screw was inserted into the high-density foam at the depth of 30 mm. The minimum values were estimated at 332 Hz, 16 N. cm, and 127 N, respectively, within the low-density PU at the depth of 10 mm. The higher value of ω n was originated from higher bone screw stability and thus more fixation strength. According to the regression analysis outcomes, the natural frequency (ω n ) was linearly dependent on the PIT (ω n =14 PIT) and also on the PPF (ω n =1. 7 PPF). Coefficients of variation as the results of the Modal analysis were significantly less than those in conventional methods (i. e. pull-out and insertion torque). Conclusion: The Modal analysis was found to be a reliable, repeatable, and non-destructive method, which could be considered a prospective alternative to the destructive pull-out test that is limited to the in-vitro application only. The Modal analysis could be applied to assess the stability of implantable screws, such as orthopedic and spinal screws.

Back pain is a common medical problem. There is no clear cause for the back pain problem so far, but in most cases, spinal instability can be noted. Lumbar spine fixation is performed to treat the problems of low back pain. Spinal fixation can be done with or without surgery. One of the surgical methods is the use of spinal screws in which the strength and stability of the screw are of great importance. The strength and stability of the screw in the bone reduces the time and cost of treatment, reduces the amount of bleeding and accelerates the patient's treatment. In this study, screws were inserted using a digital torque meter. An impact was applied using an impact hammer and resonated sound was recorded using a microphone. The vibration mode of the screw was obtained by processing the signal generated by MATLAB R2017 software and plotting the fast Fourier transform. Finally, tensile test was performed to obtain the ultimate pull-out force. The innovation of this study was to use Modal analysis method and to correlate its results with that of the ultimate pull-out force and peak insertion torque. In this study, five screws with different screw depth, and screw thread crest thickness were examined. Also, the effect of self-tapping was investigated. The peak insertion torque, ultimate pull-out strength and natural frequency occurred at 182 Nm, 992 N and 1916 Hz, respectively, for the cylindrical pedicle screw. By comparing the obtained data, results showed a linear relationship between insertion torque and pull-out force of the screws. Due to the lack of significant difference between natural frequency and pull-out force of the self-drilling and non-self-drilling tip screws (comparing between screws number 3 and 4 and between 1 and 5), the use of self-tapping screws can be advantageous. The trend of the dependent parameters in all three methods i. e. insertion torque, pull-out force and natural frequency are the same, indicating the non-destructive advantage of Modal analysis in in-vivo surgical application.

There is a famous idea in Modal epistemology according to which conceivability of a proposition is a good guide for its possibility. Yablo (1993) persents a model for justification of Modal beliefs, based on which Conceivability of a proposition is evidence for its possibility. Van Inwagen (1998) believes that if we accept Yablo’s model, we have to accept Modal skepticism. To argue for this, Van Inwagen examines the Modal status of the proposition that transparent iron exists on Yablo’s model. Van Inwagen claims that this proposition is undecidable on Yablo’s model. So we cannot have a justified belief that it is possible that transparent iron exists. If van Inwagen’s claim about the Modal status of the proposition that transparent iron exists is correct, Yablo’s model, one might think, faces a serious problem. For if we generalize van Inwagen’s analysis of the proposition that transparent iron exists, we have to count intuitively possible propositions, propositions the Modal status of which can be intuitively known as possible, as undecidable. But it is quite plausible that our beliefs about the possibility of some intuitively possible propositions are justified, so these propositions are not undecidable. I will, however, argue that van Inwagen’s analysis of Modal status of the proposition that transparent iron exists cannot be generalized to all (or most) intuitively possible propositions. And therefore it is possible to accept at the same time both Yablo’s justification model and van Inwagen’s analysis about the Modal status of propositions like transparent iron exist.

Contact dermatitis is an inflammatory skin disease due to contact with different substances and caused by two mechanisms: allergy and irritation. Contact dermatitis is a common skin disease and the most common occupational dermatosis. Differentiating between allergic and irritant contact dermatitis is essential in the identification of etiologic factor(s) and hence, its control and prognosis. This differentiation can be done only by patch test. Unfortunately, this simple, safe and useful test is always neglected. We review the basics of patch test, the method of performance, reading and interpretation of its results, possible side effects and results of studies done with patch test in different countries.