Local Stiffness Change Research Articles

Effect of Laser Shock Strengthening on Modal Frequency

The phenomenon of laser shock strengthening (LSP) changing the resonance frequency of a structure can be used to adjust the frequency of aero-engine blades, and has a broad application prospect. In this paper, through the combination of mechanical analysis, numerical simulation and vibration test, the effect of LSP technology on structural vibration characteristics is studied. The influence mechanism of local stiffness changes on the modal frequency of the structure is analyzed from the perspective of vibration and energy. It is found that when the laser shock is strengthened in a region with large bending deformation, the greater the contribution to the overall modal stiffness and elastic potential energy, the greater the modal frequency. The impact is also greater. Established a mapping relationship among laser shock strengthening, local stiffness and modal frequency, and finally proved that LSP technology can significantly change the local elastic modulus of TC4 titanium alloy, which provides a reliable application of the technology in the production department Theoretical support.

Piezoelectric Transducers for Structural Health Monitoring of Joint Structures in Cylinders: A Wave-Based Design Approach.

Joint structures, such as riveting, hinges, and flanges, are widely used in complex mechanical systems. A small unexpected change of a joint can lead to complicated wave-scattering in its connected waveguides. The conversion between wave modes can be used to quantify the variation of the connection status of joints. This gives rise to the challenge of exciting and sensing only one specific wave mode in practice. In this paper, transmitted wave amplitudes of a flange joint are first calculated by the wave finite element method (WFEM) to study the quantitative relationship between the local stiffness changes of the damaged site and the wave-mode conversion. Wave-mode piezoelectric transducers are subsequently designed for torsional, longitudinal, and flexural waves in cylindrical waveguides. The idea is to use the distribution and interconnection of the piezoelectric materials to cancel the charge contributed from the non-targeting waves. We conducted numerical simulations to demonstrate the selective coupling features of the designed wave transducers and found difference of several orders of magnitude in voltages between targeting wave mode and other wave modes. Four selected wave transducers were then extended to monitor the connection status of the flange. The wave-scattering features in the simulation and WFEM were verified to be in good agreement.

Towards predicting the buckling and crushing strength of graded SA pine with high slenderness ratios

There is an apparent increase in failure of timber truss web members or compression chords. This paper seeks to reassess buckling of the compression chords with specific focus on web members. There are two possibilities for the seemingly lower buckling strength of web members, namely that the mean modulus of elasticity is a poor predictor of buckling strength where knots and other local changes in stiffness are concerned, and secondly that the actual crushing strength of the timber is lower than the design values. This paper will investigate both of these possibilities in order to achieve a safer approach to the design of timber compression members.

Vibration‐based structural state identification by a 1‐dimensional convolutional neural network

AbstractDeep learning has ushered in many breakthroughs in vision‐based detection via convolutional neural networks (CNNs), but the vibration‐based structural damage detection by CNN remains being refined. Thus, this study proposes a simple one‐dimensional CNN that detects tiny local structural stiffness and mass changes, and validates the proposed CNN on actual structures. Three independent acceleration databases are established based on a T‐shaped steel beam, a short steel girder bridge (in test field), and a long steel girder bridge (in service). The raw acceleration data are not pre‐processed and are directly used as the training and validation data. The well‐trained CNN almost perfectly identifies the locations of small local changes in the structural mass and stiffness, demonstrating the high sensitivity of the proposed simple CNN to tiny structural state changes in actual structures. The convolutional kernels and outputs of the convolutional and max pooling layers are visualized and discussed as well.

External Loading Effects on Guided Wave Magnetostrictive Sensor Using a Surface-Bonded Nickel Patch

This paper presents the external loading effects on an ultrasonic guided wave (GW) magnetostrictive sensor using a nickel disk patch permanently attached to a thin metallic waveguide plate. The circular magnetostrictive patch transducer (CMPT) consists of the surface-bonded nickel patch and a detachable magnetic circuit device enclosing a ring-shaped sensing coil and cylindrical biasing magnets. Various compressive and tensile stresses were applied to the nickel patch on account of the buckling response of the plate structure. Strain gauges attached to the nickel patch, a Hall effect sensing device, and a laser displacement sensor were utilized to verify the in-plane deformation and the corresponding magnetic flux density change of the nickel patch and the plate's central deflection, respectively, under various external loading conditions of the plate. Several PZT wafers also mounted to the plate were individually used to generate a 30 kHz toneburst GW, which is less sensitive to local stiffness changes of the plate due to the buckling mode. The external loading effects on the CMPT were determined by evaluating the peak-to-peak amplitude variation of the direct arrival waveform of the GW signals acquired by the PZT and CMPT sensor networks. The experimental results demonstrate that the external load applied to the waveguide structure induces compressive and tensile stresses in the nickel patch, leading to distorted GW sensing performance and the directional pattern of the CMPT. Although the presented CMPT has an omnidirectional sensing profile due to its design configuration, the planar-stressed CMPT exhibits a specific directional sensing performance as a result of the stress-induced anisotropic magnetostriction of the nickel disk patch.

Rapid changes in tissue mechanics regulate cell behaviour in the developing embryonic brain.

Tissue mechanics is important for development; however, the spatio-temporal dynamics of in vivo tissue stiffness is still poorly understood. We here developed tiv-AFM, combining time-lapse in vivo atomic force microscopy with upright fluorescence imaging of embryonic tissue, to show that during development local tissue stiffness changes significantly within tens of minutes. Within this time frame, a stiffness gradient arose in the developing Xenopus brain, and retinal ganglion cell axons turned to follow this gradient. Changes in local tissue stiffness were largely governed by cell proliferation, as perturbation of mitosis diminished both the stiffness gradient and the caudal turn of axons found in control brains. Hence, we identified a close relationship between the dynamics of tissue mechanics and developmental processes, underpinning the importance of time-resolved stiffness measurements.

Influence of Gradual Damage on the Structural Dynamic Behaviour of Composite Rotors: Experimental Investigations.

Fibre-reinforced composite structures subjected to complex loads exhibit gradual damage behaviour with the degradation of the effective mechanical properties and changes in their structural dynamic behaviour. Damage manifests itself as a spatial increase in inter-fibre failure and delamination growth, resulting in local changes in stiffness. These changes affect not only the residual strength but, more importantly, the structural dynamic behaviour. In the case of composite rotors, this can lead to catastrophic failure if an eigenfrequency coincides with the rotational speed. The description and analysis of the gradual damage behaviour of composite rotors, therefore, provide the fundamentals for a better understanding of unpredicted structural phenomena. The gradual damage behaviour of the example composite rotors and the resulting damage-dependent dynamic behaviour were experimentally investigated under propagating damage caused by a combination of out-of-plane and in-plane loads. A novel observation is the finding that a monotonic increase in damage results in a non-monotonic frequency shift of a significant number of eigenfrequencies.

Extracellular Matrix Components HAPLN1, Lumican, and Collagen I Cause Hyaluronic Acid-Dependent Folding of the Developing Human Neocortex

Neocortical expansion, thought to underlie the cognitive traits unique to humans, is accompanied by cortical folding. This folding starts around gestational week (GW) 20, but what causes it remains largely unknown. Extracellular matrix (ECM) has been previously implicated in neocortical expansion and here we investigate the potential role of ECM in the formation of neocortical folds. We focus on three specific ECM components localized in the human fetal cortical plate (CP): hyaluronan and proteoglycan link protein 1 (HAPLN1), lumican and collagen I (collectively, HLC). Addition of HLC to cultures of human fetal neocortex (11-22 GW) caused local changes in tissue stiffness, induced CP folding, increased CP hyaluronic acid (HA), and required the HA-receptor CD168 and downstream ERK signaling. Importantly, loss of HA reduced HLC-induced and 22 GW physiological nascent folds. This was altered in samples with neurodevelopmental disorders, indicating it may be a useful system to study such disorders.

Application of energy measures in detection of local deviations in mechanical properties of structural elements

The identification of local damages in structural elements is considered. In the proposed formulation, the damages are represented as local changes in structural stiffness and they are defined by a set of parameters. The main effort of this work is directed to the use of global measures of energy to indicate the local changes of stiffness. An idea of use of additional design parameters in order to optimize the experiment and to enlarge the sensitivity of objective functional with respect to damage parameters is applied. In order to accomplish this goal, the distribution of energy within the structural domain is optimized. Two cases, namely the eigenproblem and the structure under the static external load, are discussed. Simple illustrative example of identification of damage is discussed, and numerical results are presented.

Free vibration analysis of bi-directional functionally graded single/multi-cracked beams

Cracks are the major cause for failure and dynamic fracture in different mechanical systems. For this reason, in this study, a comprehensive study on dynamic behavior of bi-directional functionally graded (BDFG) single/multi cracked beams is presented. Material variation is presented using different material gradation laws such as power-law, sigmoidal and exponential varying functions in both thickness and longitudinal directions to present a comprehensive model of BDFG beams. Euler–Bernoulli beam theory and a novel finite element approach is presented to model the beam with single or multiple cracks and accordingly, stiffness and mass matrices for BDFG cracked beams are derived. Further, a general analysis on BDFG beams with different types of cracks is presented and influence of different parameters such as position of the cracks, depth of the cracks, material variation type and number of cracks on natural frequencies and mode shapes of such structures is discussed. It is shown that incorporating BDFG material in structures with proper varying functions for both axial and thickness directions could lead to an optimized beam with better performance while being cracked in different positions. Also it is concluded that cracks in BDFG beams could cause noticeable variation in dynamic behavior due to change in local stiffness depending on the position and the depth of the cracks.

Experimental validation of numerical structural dynamic models for metal plate joining techniques

Joined structures are of great industrial relevance. The dynamic effects of joints are, however, often practically difficult to accurately account for in numerical models, as they often lead to local changes in stiffness and damping. This paper discusses the comparison between measurements and simulations of joined panels considering four different joining techniques: adhesive bonding, metal inert gas welding, resistance spot welding and flow drill screwing. An experimental modal analysis is performed on the different systems and the power injection method is applied to determine the loss factors of single plate systems and their joined counterparts. The joined panels are modeled in a holistic simulation environment with particular focus on the joining region, by the application of predefined and generic joint models. A very good agreement is obtained between the simulated dynamic behavior and the experimental results, showing that an accurate representation of the joints has been obtained.

Arterial structure and function in patients with acute coronary syndrome after 1‑year treatment.

INTRODUCTION The modification of arterial stiffness and intima-media thickness (IMT) is controversial in patients with clinically significant atherosclerosis. OBJECTIVES We evaluated the effects of 1‑year pharmacological therapy on arterial stiffness and IMT in survivors of non‑ST‑segment elevation myocardial infarction (NSTEMI) who were treated according to current clinical guidelines. PATIENTS AND METHODS A total of 298 patients with NSTEMI (median age, 64 years; 85 women) were enrolled to this study. Local (carotid) arterial stiffness and IMT were measured noninvasively before discharge and after 12 months of contemporary pharmacological treatment according to current clinical guidelines. The study group was divided into patients with normal systolic blood pressure (BP) (<140 mm Hg) and those with increased systolic BP (≥140 mm Hg) at 12 months. The results were presented as median (25th-75th percentile). RESULTS There were no significant changes in local arterial stiffness between patients with normal and those with increased systolic BP (8.9 m/s [7.9-10.9 m/s] vs 8.7 m/s [7.8-10.1 m/s] at baseline and 9.6 m/s [8.3-11.0 m/s] vs 10.4 m/s [9.1-12.4 m/s] at 12 months, P = 0.67 and P = 0.05, respectively); however, a significant reduction in IMT was found in both groups (777 μm [664-896 μm] vs 715 μm [619-841 μm] at baseline and 818 μm [720-962 μm] vs 760 μm [674-897 μm] at 12 months, P = 0.0003 and P = 0.001, respectively). Arterial stiffness and IMT were affected by age and mean BP; however, adjustment for these variables did not affect the obtained results in multivariate models. CONCLUSIONS The 1‑year pharmacological treatment of patients after NSTEMI was associated with a significant reduction in IMT but had no effect on the properties of the arterial structure.

Investigating Actin Mechanics during Phagocytic Uptake and Transport

Phagocytosis, the internalization and consecutive digestion of biological material by macrophages is a major part of the innate mammalian immune response. Fc gamma receptor-mediated uptake is driven by actin recruitment to the internalization site and consecutive formation of membrane protrusions around the target object. Molecular motors and cytoskeletal elements contribute to the subsequent transport of phagosomes inside the cells. Although a large number of molecules that are involved in the phagocytic uptake and the phagosomal transport are identified by now, the mechanics of these processes are largely unknown. We therefore investigated the mechanics of phagocytic uptake and phagosomal transport by using live cell microscopy in combination with optical and magnetic tweezers with a focus on the role of actin filaments. We hypothesize that the actin recruitment during phagocytic uptake leads to a transient and localized increase of the stiffness of the uptake region.To test this hypothesis we induced Fc gamma receptor-mediated phagocytosis in J774A.1 mouse macrophages by offering the cells target microbeads coated with immunoglobulin-G. During the uptake we measured local cell stiffness changes with a ‘blinking optical traps’ technique executed via periodic intensity modulation of the optical force. Preliminary data indicates that that the stiffness of the uptake region increases temporarily during the uptake. In addition, we investigated the influence of actin filaments on the phagosomal transport by magnetic tweezers-based bead displacement assays and cytochalasin treatment of the cells.Our measurements are expected to identify characteristic length- and time-scales for the variation of cell mechanical properties during phagocytosis and to contribute to a more comprehensive understanding of this medically relevant process.

Spatial-dependent mechanical properties of the heel pad by shear wave elastography

The heel pad plays an important role in gait, and its biomechanical behavior and functionality are determined by its specialized architecture and mechanical properties. The purpose of this study was to apply supersonic shear wave elastography, an ultrasound-based noninvasive modality that can quantitatively estimate the shear stiffness of the tissue, to investigate the spatial-dependent mechanical properties of the heel pad. Measurements were conducted in 40 heel pads of 20 normal participants aged between 20 and 30 years by shear wave elastography. The continuous change in local shear stiffness of the heel pad along the depth direction of the heel pad was measured. The result showed that the mechanical properties of the heel pad were highly heterogeneous but followed a simple and specific pattern that local heel pad shear stiffness was highest beneath the plantar skin and decreased continuously with increasing depth. This finding provides a better understanding of the heel pad biomechanics and basis for further investigation of the heterogeneous properties of the heel pad.

Spatial-Dependent Mechanical Properties of the Heel Pad by Shear Wave Elastography

The heel pad plays an importante role in gait, and its biomechanical behavior and functionality are determined by its specialized architecture and mechanical properties. The purpose of this study was to apply supersonic shear wave elastography, an ultrasound-based noninvasive modality that can quantitatively estimate the shear stiffness of the tissue, to investigate the spatial-dependent mechanical properties of the heel pad. Measurements were conducted in 40 heel pads of 20 normal participants aged between 20 and 30 years by shear wave elastography. The continuous change in local shear stiffness of the heel pad along the depth direction of the heel pad was measured. The result showed that the mechanical properties of heel pad were highly heterogeneous but followed a simple and specific pattern that local heel pad shear stiffness was highest beneath the plantar skin and decreased continuously with increasing depth. This finding provides a better understanding of the heel pad biomechanics and basis for further investigation of the heterogeneous properties of the heel pad.

Using water hammer to enhance the detection of stiffness changes on an out-of-round pipe with distributed optical-fibre sensing

Summary Over the last few decades, distributed optical fibre sensor (DOFS) has been introduced to monitor the structural health of water pipelines. Most of the previous studies show that DOFS is very effective as a static measurement and monitoring platform. However, there is still a lack of research being done using DOFS to monitor the dynamic response of the pipeline. This paper will first demonstrate the dynamic capability of optical frequency domain reflectometry-based DOFS on a pipe. To be specific, the primary monitoring work is conducted on an out-of-round plastic pipe subjected to water hammer. It is important to monitor the dynamic response of the pipe as it is well known that water hammer can occur in any pressurised pipeline system due to changes in the operating conditions. The ability to detect local stiffness irregularity on the noncircular pipe subjected to water hammer is also demonstrated. The result shows that the presence of the local stiffness change is accentuated when the pipe is subjected to water hammer. The dynamic capability of DOFS facilitates the application of water hammer as a stimulus and hence shows the potential to enhance pipeline health monitoring.

Parametrically Induced Damping in a Cracked Rotor

A transverse shaft crack in a rotor is usually modeled as a local change in shaft stiffness. This local stiffness change is not constant and varies as a result of a so-called breathing mechanism, explained with periodical opening and closing of crack faces under the load of external forces applied to the rotor. The rotor with a periodically varied stiffness can be modeled as a parametrically excited linear system. In the presence of a parametric excitation, the vibrations of the system can be amplified or damped at specific excitation frequencies. Usually, the frequencies at which the vibrations are amplified are important, since they can affect stability of the system. However, the increased damping at specific frequencies is a significant feature of a parametrically excited system that can have some potentially useful applications. One of such applications can be an early detection of a shaft crack. This paper presents results of numerical analysis of the influence of Rayleigh's damping and gyroscopic effects on the increase in damping in a parametrically excited rotor with a cracked shaft. It is shown that the increase in damping in a parametrically excited system is rather a rare phenomenon that can be observed only at properly selected values of the excitation frequency and Rayleigh's damping. Furthermore, gyroscopic effects influence the exact values of antiresonance frequencies at which the phenomenon appears.

Location sensitivity of fundamental and higher mode shapes in localization of damage within a building

It has been recently demonstrated through mathematical derivation as well as numerical studies that the fundamental mode shape and its derivatives show excellent performance in localizing damage of buildings. Although it is understood from the literature that higher mode shapes are very sensitive to local changes in stiffness, no convincing theory is available in the literature about the efficiency of higher mode shapes in localizing damage. The present work focuses on this aspect. To achieve this goal, a mathematical formulation has been derived for change in a mode shape due to damage with damage parameters. This expression shows that the efficiency of the mode shape-based approach is dependent on the location of damage within a building. To illustrate the concept, numerical studies have been performed considering shear building models, where damage conditions have been simulated by reducing the stiffness at the desired locations. Further, an experimental study has been performed considering a three-dimensional steel moment-resisting frame model. It has been found that although the higher mode shapes and their derivatives are effective in localizing damage, their efficiency may significantly get reduced depending on the location of damage, particularly for shorter buildings. Hence, this location sensitivity of fundamental and higher mode shapes may be kept in mind for effective damage localization in buildings.

Study on an auto-correlation-function-based damage index: Sensitivity analysis and structural damage detection

The damage index based on the auto correlation function to detect the damage of the structure under white noise excitation is studied in detail in this paper. The maximum values of the auto correlation function of the vibration response signals (displacement, velocity and acceleration) from different measurement points of the structure are collected and formulated as a vector called Auto Correlation Function at Maximum Point Value Vector (AMV), which is expressed as a weighted combination of the Hadamard product of two mode shapes. AMV is normalized by its root mean square value so that the influence of the excitation can be eliminated. Sensitivity analysis for the different parts of the normalized AMV shows that the sensitivity of the normalized AMV to the local stiffness is dependent most on the sensitivity of the Hadamard product of the two lower order mode shapes to the local stiffness, which has a sudden change of the value around the local stiffness change position. The sensitivity of the normalized AMV has the similar shape and same trend that shows it is a very good damage indicator even for the very small damage. The relative change of the normalized AMV before and after damage occurs in the structure is adopted as the damage index to show the damage location. Several examples of the stiffness reduction detection of a 12-story shear frame structure are utilized to validate the results in sensitivity analysis, illustrate the effectiveness and anti-noise ability of the AMV-based damage detection method and compare the effect of the response type on the detectability of the normalized AMV.

Experimental Verification of Substructure Identification for Damage Detection in Shear Buildings

AbstractDamage detection for civil structures is limited by several factors including poor signal-to-noise ratios, a large number of unknown parameters, and a limited set of measured responses. Global vibration techniques that track modal parameters often remain insensitive to common forms of structural damage. Moreover, large sets of identified parameters make inverse problems ill-conditioned. To overcome some of these limitations, researchers have advanced substructure identification as a methodology to directly detect local stiffness changes using measured responses to improve damage detection and scalability in civil structures. This paper develops a substructure identification estimator that identifies the story stiffness of a shear building. Concurrent with the estimator derivation, identified parameter confidence intervals are developed and identification performance is predicted. Using the developed estimator, experimental testing is performed on a 3.66-m (12-ft) four-story steel structure subject...