Effective vibration test planning method for equipment with high slenderness ratio

The main objective of this paper is to evaluate the capabilities of the non-destructive impact-response method in detecting the existence of defects in piles with high slenderness ratio. The impact-echo technique was developed many years ago for non-destructive detection of defects in piles and structures. However, it is recognised that this method could be applied confidently for piles with length/diameter relationship up to 20. In the last years, with the development of hardware in computers and sensors, it is now possible to use this method for piles with high slenderness ratio. In this paper, piles of length/diameter relationship of about 40 are studied. A numerical-experimental study was carried out. It was shown that the results of the experiment were in agreement with those of the numerical studies and the accuracy of the impact-echo method was influenced by the type, size and location of the defects. Taking into account the uncertainties related to the studied problem, the obtained results preliminarily allow consider this NDT methodology as a promissory alternative to detect defects in piles with high slenderness ratio.

An automated method has been developed for identifying mode survey test measurement degrees-of-freedom (DOFs). The method has been shown to be an effective tool for identifying efficient sets of test measurement locations. The automated approach is based upon two established methods for measurement DOF identification, Residual Kinetic Energy (RKE) and Effective Independence (EI). The RKE method is here extended to an iterative application, called Iterative Residual Kinetic Energy (IRKE), that can generate a candidate measurement set with little input beyond the analytical structural model. The IRKE method requires an initial measurement set, which can be as small as a single DOF. By requiring only a single DOF to seed the IRKE method, the effort to create an initial set based on engineering judgment, or other methods, is eliminated. This paper proposes the use of the IRKE algorithm for the development of relatively large candidate measurement sets of sizes, which are themselves impractical for test instrumentation purposes, but can then be reduced to a manageable number by utilizing a variation of the EI method designated the MassWeighted Effective Independence (MWEI) method. MWEI improves upon the EI method by application of mass-weighting to the modal matrix and using the EI method on the product of the reduced mass matrix and the modal matrix. The use of the IRKE and MWEI methods jointly affords analysts an automated tool for identification of efficient mode survey test measurement DOF sets.

The stress and strain dependent response of THF hydrate

As environmentally friendly pile foundations with small diameters and higher slenderness ratios, micropiles are widely used in fields such as transmission line engineering and building reinforcement. However, the available research has primarily focused on their bearing performance under compressive and horizontal loads, and there is insufficient research on predicting the uplift capacity of micropiles. This study investigated the load transfer mechanism and the behavior of the surrounding soil using model tests and finite element simulations. The ultimate uplift capacities and load distributions of micropiles with different slenderness ratios were analyzed. The results show that as the slenderness ratio increases, the ultimate uplift capacity of a pile gradually increases. However, this rate of increase diminishes gradually. Additionally, the restraining effect and range of the surrounding soil at the lower part of the pile are enhanced. The critical embedment depth of the micropiles shifts further away from the pile tip as the slenderness ratio increases. Finally, this study proposed a novel modification to Shanker’s model of incorporating variations in the critical embedment depth based on the slenderness ratios. Subsequently, a modified model for the ultimate uplift capacity of micropiles was proposed and validated using a model test. The proposed model effectively predicts the uplift bearing capacity of micropiles with high slenderness ratios, which is practical for engineering applications.

Lateral buckling of longitudinal rebars is a crucial phenomenon in the response of reinforced concrete structures since it often anticipates crushing of the core concrete and triggers failure of the member. Owing to this, lateral buckling has been simulated in some uniaxial steel models available in the literature. However, most of these models concentrate their novelties on the compression backbone curve of rebars characterised by low-to-medium slenderness ratios and inaccurately simulate the response of bars characterised by high slenderness ratio or the transition from the compression backbone curve to tension. Moreover, all these models neglect the effects of the concrete cover on the lateral buckling of the rebar. This paper presents a new uniaxial material model of reinforcing steel bars with the aim of achieving a more accurate evaluation of the response of rebars including the effects of lateral buckling. The proposed model is calibrated on the results of numerical analyses conducted on refined models of steel rebars characterised by low-to-high slenderness ratios. The proposed model incorporates the possibility to simulate the behaviour of bare and embedded rebars. The response is path-dependent and described by means of smooth curves so as to guarantee numerical stability in nonlinear finite element simulations. The accuracy of the model has been validated by comparison with the response of laboratory tests performed by other researchers on steel rebars. Finally, applications are shown on six reinforced concrete columns in order to highlight the effects of the simulation of buckling of the steel rebars on the global and local responses.

During a seismic event, first-story end columns in steel Special Moment Frames (SMFs) experience ‎lateral drift demands combined with high levels of axial load due to gravity loading and dynamic ‎overturning moment effects. Deep slender wide-flange sections are typically assigned for the ‎seismic design of perimeter SMFs in North America. The flange and web of these sections have ‎high slenderness ratios, just below the compactness limits specified by AISC 360-10. This makes ‎these sections vulnerable to local and lateral torsional buckling failure modes. ‎ In this paper, the cyclic behavior of deep slender wide-flange steel beam-columns, under combined ‎lateral drift and axial load, is investigated through detailed finite element analysis (FEA). The main ‎design variables that are considered as part of the FEA investigation are (a) the section slenderness; ‎‎(b) the lateral loading protocol; and (c) the axial load level. Emphasis is placed on the effect of ‎these variables on (a) the cyclic deterioration in flexural strength; (b) the column axial shortening; ‎and (c) the lateral torsional buckling of the steel beam-columns. To this end, a range of deep ‎slender wide-flange sections, commonly found in low to mid-rise SMFs, is selected. Two lateral ‎loading protocols are used: 1) a symmetric protocol and 2) a near fault protocol. The axial load is ‎applied in two forms: 1) constant compressive load; and 2) cyclic axial load varying from ‎compression to ‎tension. The latter represents the variation in the axial load experienced by an end ‎column due to the dynamic overturning ‎moments during an earthquake. Using a validated FEA ‎modeling approach, typical first-story end columns of prototype steel SMFs are modeled and ‎analyzed under the aforementioned-coupled lateral and axial loading protocols. The boundary ‎conditions at the base of the columns are fixed. At the top of the columns the flexibility associated ‎with the corresponding exterior beam-to-column connection is considered. Lateral braces are also ‎provided to the column, as per the design code requirements.‎ The FEA simulations demonstrated that higher rates of cyclic ‎deterioration in flexural strength are ‎associated with sections with larger web slenderness, due to severe web local buckling. Sections ‎with large web and flange slenderness ratios also suffer from higher axial shortening levels at lower ‎story drifts compared to more compact sections. It is also concluded that deep slender wide-flange ‎sections with lower resistance to out-of-plane and torsional force demands, experience higher ‎twisting and are likely to be subjected to lateral torsional buckling. The study also shows that axial ‎force demands on the column’s lateral braces exceed their code design capacity at relatively low ‎story drift ratios. Furthermore, the location of the column inflection point and the associated ‎column moment gradient is monitored during the finite element analysis in order to assess the ‎potential of plastic hinge formation at the top of the column once plastification and cyclic ‎deterioration in flexural strength of the column occurs at its base. This is a critical issue since it ‎could potentially lead to weak-story collapse mechanisms.‎

No AccessTechnical NoteClustering of Sensor Locations Using the Effective Independence MethodMichael I. Friswell and Rafael Castro-TrigueroMichael I. FriswellCollege of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom*Professor of Aerospace Structures, College of Engineering; .Search for more papers by this author and Rafael Castro-TrigueroDepartment of Mechanics, University of Cordoba, 14071 Cordoba, Spain†Department of Mechanics, Campus de Rabanales.Search for more papers by this authorPublished Online:12 Mar 2015https://doi.org/10.2514/1.J053503SectionsView Full TextPDFPDF Plus ToolsAdd to favoritesDownload citationTrack citations ShareShare onFacebookTwitterLinked InRedditEmail About References [1] Pickrel C. R., “A Practical Approach to Modal Pretest Design,” Mechanical Systems and Signal Processing, Vol. 13, No. 2, 1999, pp. 271–295. doi:https://doi.org/10.1006/mssp.1998.1212 0888-3270 CrossrefGoogle Scholar[2] Penny J. E. T., Friswell M. I. and Garvey S. 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Infra-red and vibration tests of hybrid ablative/ceramic matrix technological breadboards for earth re-entry thermal protection systems

The Hamburg wheel tracking test (HWTT) has been extensively used to identify asphalt mixtures prone to rutting or moisture damage. AASHTO T 324, “Hamburg Wheel-Track Testing of Compacted Hot Mix Asphalt (HMA),” describes the procedure for testing asphalt mixture samples using the HWTT device. The method specifies the testing of submerged, compacted asphalt mixture in a reciprocating rolling-wheel device. The test results provide information about the rate of permanent deformation from a moving concentrated load. However, there is no information on the precision of the test method, including the allowable differences between two replicate measurements in one laboratory or measurements in two laboratories. In addition, important aspects of the test are not sufficiently specified in the test method; these include position of the wheel with respect to specimen, verification of the location of the measurements, specimen preparation and assembly, and analysis and reporting of test data. Because these factors could significantly affect HWTT measurements and performance verification of asphalt mixtures, it is important to identify the factors causing variability of measurements and further specify their limits in the test method. The objective of this study was to determine precision estimates for AASHTO T 324. To accomplish this objective, the research: determined the variability of the deformation measurements after specified number of load passes and the creep slope for well-performing mixtures; determined the variability of the number of passes to threshold deformation, creep slope, stripping slope, and number of passes to the stripping inflection point for poorly performing mixtures; compared the mean and variance of the measured properties of gyratory and slab specimens; compared the mean and variance of properties measured using all measurement locations with those measured using all except the three middle measurement locations and all except two measurement locations at each end; identified causes of variability of the test results; and proposed modifications to the test method for optimum use of the deformation measurements, improvement to the specimen preparation and assembly, and necessary adjustments to the machine components.

In modal testing, the quality of vibration signals and clarity of peak points to extract the natural frequencies, corresponding mode shapes and damping ratio depend on the number and location of the sensors. High measurement costs are required for structural identification and long-term structural health monitoring in large structures, which require a high number of sensors. Therefore, the minimum number of sensors should be placed at appropriate locations on the system during experimental measurements to ensure both information of sufficient quality and cost reductions. The aim of this study is to perform cost-efficient non-destructive modal tests for a laminated composite and steel cantilever beams using an optimal sensor placement approach. Finite element models of the sample beams are constituted in ANSYS® software to determine the initial candidate set of sensor locations. Then, ambient vibration tests are conducted. Based on the experimental modal amplitudes and mode shapes, optimal sensor locations are determined using the effective independence method, and measurements are repeated. The study shows that there is good agreement between the natural frequencies and mode shapes obtained from the initial measurements and those obtained using the limited number of sensors.

The majority of precast concrete electric pole structures (EPSs) consist of prestressed concrete and reinforced concrete hybrid members with high slenderness ratios. Currently, large prestressing (PS) forces are being applied to EPSs to carry heavier weights of electric transformer machineries and to reduce deflection by increasing structural stiffness. Moreover, when EPSs are exposed to an outdoor environment year-round, their durability decreases. Therefore, the objectives of this study are to transform the failure behavior of EPSs from brittle to ductile by varying the number of PS tendons and steel rebars, and to improve the ductility and durability of the EPS by using short polypropylene fiber-reinforced concrete to control crack formation and propagation. Six different types of EPS specimens were manufactured and tested to evaluate their maximum flexure capacity and to verify their ductility behavior. The results showed that the modified EPSs displayed ductile failure behavior while either maintaining their original flexural strength capacity or improving it compared with the current EPSs sold on the market.

Lightning rod structures are susceptible to wind loads due to their high slenderness ratio, high flexibility, and light weight. The wind-induced dynamic response of a lightning rod is critical for structural safety and reliability. The traditional methods for this response, including observation and simulation, focus on structural health monitoring (SHM), wind tunnel tests (WTTs), or fluid–structure interaction (FSI) simulations. However, all these approaches require considerable financial or computational investment. Additionally, problems such as data loss or data anomalies in the sensor monitoring process often occur during SHM or WTTs. This paper proposes an algorithm based on a long short-term memory (LSTM) network to predict the wind-induced dynamic response and to solve the problem of data link fracture caused by abnormal sensor data transmission or wind-induced damage to lightning rod structures under different wind speeds. The effectiveness and applicability of the proposed framework are demonstrated using actual monitoring data. Root-mean-squared error (RMSE), determination of coefficient (R2), variance accounted for (VAF), and the refined Willmott index (RWI) are employed as performance assessment indices for the proposed network model. At the same time, the random forest algorithm is adopted to analyze the correlation between the data of the different measurement points on the lightning rod structure. The results show that the LSTM method proposed in this paper has a high accuracy for the prediction of “missing” strain data during lightning rod strain monitoring under wind speeds of 15.81~31.62 m/s. Even under the extreme wind speed of 31.62 m/s, the values of RMSE, MAE, R2, RWI and VAF are 0.24053, 0.18213, 0.94539, 0.88172 and 0.94444, respectively, which are within the acceptable range. Using the data feature importance analysis function, it is found that the predicted strain data of the measurement point on the top part of the lightning rod structure are closely related to the test strain data of the two adjacent sections of the structure, and the effect of the test strain data of the measurement points that are far from the predicted measurement point can be ignored.

Stiffness variation is useful in soft robots, especially for continuum manipulators which have a high slenderness ratio. Softness intrinsically gives robots high flexibility of movement, but stiffness gives the robots enough carrying capacity. Many types of continuum manipulators can change their stiffness; however, the both large-range and fast-response stiffness variation has not been realized. In this work, an omnidirectional steering flexible manipulator with large-range and fast-response stiffness variation was designed and fabricated. It was composed of a nested structure with a stiffness variation layer, a tendon-driven layer, and a transport channel. The stiffness variation was achieved by the phase transition of LMPA (Low-Melting Point Alloy) with a melting point of 50 °C. Heating and cooling to the LMPA were performed by a tubular spring-like copper tube embedding in the LMPA. The tube not only acts as a skeleton of the manipulator, but also the material and structure of it bring the fast heat exchange. The motion of the manipulator was driven by tendons going through the holes of a threaded spring-like elastic skeleton which is flexible still can appropriately balance the self-weight of the manipulator. Continuum manipulators with one and two omnidirectional segments were fabricated to show the performance of movement and stiffness variation. Each segment is of 15 cm length and with diameter of 21 mm. The test shows that the stiffness of the manipulator has a factor of 45 variations in 15 seconds, and the stiffness variation is adequate to complete some challenging tasks like screw tightening and loosing.

Bowing of marble slabs: Evolution and correlation with mechanical decay

The Bending-Gradient theory for flexural wave propagation in composite plates