Dynamic Strain Response Research Articles

Experimental validation of the spectral criterion for the diagnosis of periodic rod structures using ferroelectric strain gauges

Successive impact excitation and construction of Fourier transforms of linear and nonlinear strain responses and phase portraits of undamaged or damaged vertically suspended models of a periodic welded rod structure were used to establish the dependence of the area ratio of the phase portraits of the nonlinear and linear domains of the strain response, which characterizes the stress (damage) of the model, on the impact momentum. The dynamic strain responses were recorded using ultra-wideband (10−2–108 Hz) ferroelectric microgauges based on thin Pb(Zr0.53Ti0.47)O3 films 2 µm in thickness with a 0.8 mm2 contact area.

Advanced imaging for early prediction and characterization of zone of Lüders band nucleation associated with pre-yield microstrain

The present study involves experimental investigation of thermal and strain fields associated with generation of pre-yield plastic microstrain during tensile loading of mild steel using imaging NDE techniques such as infrared thermography (IRT) and digital image correlation (DIC). A fast array based infrared detector is used to map the temperature distribution and a DIC system is used to map the strain distribution in local zones of the material. Studies reveal that dynamic thermal and strain response of the material observed before macroscopic yield point and their spatial distributions are found to have good correlation with phenomenon of pre-yield microstrain and related experimental findings. Techniques used not only offered the possibility of visualizing the thermal and strain distributions associated with pre-yield microstrain but also to predict and visualize the zone of nucleation of Lüders band much ahead of upper yield point.

Experimental characterization and diagnostics of the early‐age behavior of a semi‐integral abutment FRP deck bridge

PurposeThe purpose of this paper is to further validate a wireless sensor system developed at Clarkson University for structural monitoring of highway bridges. The particular bridge monitored employs a fiber reinforced polymer (FRP) panel system which is fairly innovative in the field of civil engineering design. The superstructure was monitored on two separate occasions to determine a change in structural response and see how the structural system performs over time.Design/methodology/approachA series of wireless sensor units was deployed at various locations of the steel superstructure, to measure both the modal response from acceleration measurements as well as quasi‐static and dynamic strain response. Ambient and forced loading conditions were applied to measure the response. Data results were compared over two separate periods approximately nine months apart.FindingsThe first eight mode shapes were produced from output‐only system identification providing natural frequencies ranging from approximately 6 to 42 Hz. The strain response was monitored over two different testing periods to measure various performance characteristics. Neutral axis, distribution factor, impact factor and end fixity were determined. Results appeared to be different over the two testing periods. They indicate that the load rating of the superstructure decreased over the nine month period, possibly due to deterioration of the materials or composite action.Research limitations/implicationsThe results from the two testing periods indicate that further testing needs to be completed to validate the change in response. It is difficult to say with certainty that the significant change in response is due to bridge deterioration and not other factors such as temperature effects on load rating. The sensor system, however, proved to provide high quality data and responses indicating its successful deployment for load testing and monitoring of highway infrastructure.Originality/valueThe paper provides a depiction of the change in structural behavior of a bridge superstructure using a wireless sensor system. The wireless system provided high‐rate data transmission in real time. Load testing at two different points in time, eight months apart, showed a significant change in bridge behavior. The paper provides a practical and actual physical load test and rating during these two periods for quantifiable change in response. It is shown that the wireless system is capable of infrastructure monitoring and that possible deterioration is expected with this particular bridge design. Additionally, the load testing occurred during different seasons, which could create cause for temperature effects in load rating. This can provide a basis for future performance monitoring techniques and structural health monitoring.

Statistical analysis of stress spectra for fatigue life assessment of steel bridges with structural health monitoring data

This paper aims at developing a monitoring-based method for fatigue life assessment of steel bridges with use of long-term monitoring data of dynamic strain. A standard daily stress spectrum is derived by statistically analyzing the stress spectra accounting for highway traffic, railway traffic, and typhoon effects. The optimal number of daily strain data for derivation of the standard daily stress spectrum is determined by examining the predominant factors which affect the prediction of fatigue life. With the continuously measured dynamic strain responses from the instrumented Tsing Ma Bridge carrying both highway and railway traffic, the proposed method is exemplified to evaluate the fatigue life of fatigue-critical welded details on the bridge.

PZT 소자의 정압전 응답을 이용한 보 구조물의 모드 변형에너지기반 손상 모니터링

본 연구에서는 PZT 소자의 정압전 효과에 의한 동적 응답신호를 이용하는 보 구조물 손상 모니터링 기법을 제안하였다. 특히, 모드 변형에너지기반 보 구조물 손상 모니터링에 PZT 정압전 응답신호를 입력자료로 활용하는 방안에 대한 연구에 주안점이 있다. 먼저, PZT 소자의 정압전 효과 및 동적 변형률 응답의 이론적 배경을 요약하였다. 다음으로, 모드 변형에너지기반 보 구조물 손상위치 모니터링 기법을 제시하였다. 제시된 기법의 적합성을 검증하기 위해, 캔틸레버 보 모형을 대상으로 강제진동 실험을 수행하였으며, 세 종류의 센서(가속도계, PZT 센서, 변형률계)를 통해 동적 응답신호가 계측되었다. 손상 전후에 계측된 이들 진동신호들을 사용하여 모드 변형에너지기반의 손상위치 모니터링이 수행되었다. The main objective of this study is to examine the feasibility of using lead zirconate titanate (PZT)'s direct piezoelectric response as vibrational feature for damage monitoring in beam structures. For the purpose, modal strain energy (MSE)-based damage monitoring in beam structures using dynamic strain response based on the direct piezoelectric effect of PZT sensor is proposed in this paper. The following approaches are used to achieve the objective. First, the theoretical background of PZT's direct piezoelectric effect for dynamic strain response is presented. Next, the damage monitoring method that utilizes the change in MSE to locate of damage in beam structures is outlined. For validation, forced vibration tests are carried out on lab-scale cantilever beam. For several damage scenarios, dynamic responses are measured by three different sensor types (accelerometer, PZT sensor and electrical strain gage) and damage monitoring tasks are performed thereafter. The performance of PZT's direct piezoelectric response for MSE-based damage monitoring is evaluated by comparing the damage localization results from the three sensor types.

가속도 데이터를 활용한 선형 시스템의 변형률 예측

This paper investigates the prediction of the dynamic strain response using acceleration response only. Two methods are proposed for the strain prediction; one is based on beam theory and the other is calculated by the frequency response function between acceleration and strain. First, it is estimated the dynamics of the simple notched beam, including the non-linearity, through the uni-axial vibration testing. Then, the dynamic strain response is predicted under two different methods using acceleration response. The validation of proposed methods is conducted by the comparison between measured strain and predicted values. The comparison reveals that the proposed method based on the FRF between acceleration and strain is more reliable one than that stemmed from beam theory and the maximum relative error is less than 8 %.

Hyaluronic acid–collagen network interactions during the dynamic compression and recovery of cartilage

A compression cell designed to fit inside an NMR spectrometer was used to investigate (i) the in situ dynamic strain response and structural changes of the internal pore network, and (ii) the diffusion and flow of interstitial water, in full thickness cartilage samples as they were mechanically deformed under a constant compressive load (pressure) and then allowed to recover (swell again) when the load was removed. Selective enzymatic digestion of the collagen fibril network and the glycopolysaccharide hyaluronic acid (HA) was performed to mimic some of the structural and compositional changes associated with osteoarthritis. Digestion of collagen gave rise to mechanical ‘dynamic softening’ and—perhaps more importantly—nearly complete loss in the ability to recover through swelling, both effects due to the disruption of the hierarchical structure and fibril interconnectivity in the collagen network which adversely affects its ability to deform reversibly and to properly regulate the pressurization and resulting rate and direction of interstitial fluid flow. In contrast, digestion of HA inside the collagen pore network caused the cartilage to ‘dynamically stiffen’ which is attributed to the decrease in the osmotic (entropic) pressure of the digested HA molecules confined in the cartilage pores that causes the network to contract and thereby become less permeable to flow. These digestion-induced changes in cartilage's properties reveal a complex relationship between the molecular weight and concentration of the HA in the interstitial fluid, and the structure and properties of the collagen fibril pore network, and provide new insights into how changes in either could influence the onset and progression of osteoarthritis.

Real-Time Strain Monitoring of a Navy Vessel During Open Water Transit

In this work we describe the installation of a real-time, fiber-optic strain monitoring system aboard a US Navy vessel undergoing sea trials. The system is lightweight, unintrusive, corrosion resistant, and provides dynamic strain response measurements from direct current (DC) up to 360 Hz. Also described are the data acquisition and subsequent analysis. The goals of the study were to demonstrate a fiber-optic strain monitoring system under real world conditions and to better understand the source of deck cracking above an area housing mission-critical components. The results of the two-day trial indicate that there exist large stresses in the deck plate during normal ship operation. Based on the results, it is clear that ship maneuvers, slewing of the radar located on the deck plate, and insolation-related effects are not the primary source of the observed stress.

Experimental validation of finite element–boundary element model dynamic strain model under diffuse acoustic field loading.

Structural finite element (FE) models naturally output displacement or acceleration response data. However, they can also be used to compute stress, internal forces, and strain response. When coupled with a boundary element model (BEM) of the fluid surrounding the structure, a fully coupled analysis can be performed. Modeling a diffuse acoustic field in the BEM fluid provides an excitation like that found when the structure is placed in a reverberation chamber. Fully coupling the structural FE model to the acoustic BEM model provides a means to predict not only the acceleration response of the panel to diffuse field loading but also the ability to predict the dynamic stress and strain response. This type of model has been available with current predictive tools, but experimental validation of the prediction of dynamic stress or strain is difficult to find. An aluminum panel was instrumented with accelerometers and strain gauges and hung in a reverberation room and subjected to a diffuse acoustic field. Th...

Characterization of FBG sensor interrogation based on a FDML wavelength swept laser

In this study, we develop an ultra-fast fiber Bragg grating sensor system that is based on the Fourier domain mode-locked (FDML) swept laser. A FDML wavelength swept laser has many advantages compared to the conventional wavelength swept laser source, such as high-speed interrogation, narrow spectral sensitivity, and high phase stability. The newly developed FDML wavelength swept laser shows a superior performance of a high scan rate of 31.3 kHz and a broad scan range of over 70 nm simultaneously. The performance of the grating sensor interrogating system using a FDML wavelength swept laser is characterized in both static and dynamic strain responses.

Using ROC curves to assess the efficacy of several detectors of damage-induced nonlinearities in a bolted composite structure

We offer a comparison of several different detectors of damage-induced nonlinearities in assessing the connectivity of a composite-to-metal bolted joint. Each detector compares the structure's measured vibrational response to surrogate data, conforming to a general model for the healthy structure. The strength of this approach to detection is that it works in the presence of certain types of varying ambient conditions and is valid for structures excited with any stationary process. Here we employ several such detectors using dynamic strain response data collected near the joint as the structure was driven using simulated wave forcing (taken from the Pierson–Moskowitz frequency distribution for wave height). In an effort to simulate in situ monitoring conditions the experiments were carried out in the presence of strongly varying temperatures. The performance of the detectors was assessed using receiver operating characteristic (ROC) curves, a well known method for displaying detection characteristics. The ROC curve is well suited to the problem of vibration-based structural health monitoring applications where quantifying false positive and false negative errors is essential. The results of this work indicate that using the estimated auto-bicoherence of the systems response produced the best overall detection performance when compared to features based on a nonlinear prediction scheme and another based on information theory. For roughly 10% false alarms, the bicoherence detector gives nearly 90% probability of detection (POD). Conversely, for several of the other detectors 5–10% false alarms leads to ∼ 70 % POD. While the bicoherence (and bispectrum) have been used previously in the context of damage detection, this work represents the first attempt at using them in a surrogate-based detection scheme.

Structural Damage Detection in the Frequency Domain using Neural Networks

A bi-level damage detection algorithm that utilizes dynamic responses of the structure as input and neural network (NN) as a pattern classifier is presented. The signal anomaly index (SAI) is proposed to express the amount of changes in the shape of frequency response functions (FRFs) or strain frequency response function (SFRF). SAI is calculated by using the acceleration and dynamic strain responses acquired from intact and damaged states of the structure. In a bi-level damage identification algorithm, first the presence of damage is identified from the magnitude of the SAI value. Then the location of the damage is identified using the pattern recognition capability of the NN. The proposed algorithm is applied to an experimental model bridge to demonstrate the feasibility of the algorithm. Numerically simulated signals are used for training the NN, and experimentally acquired signals are used to test the NN. The results of this example application suggest that the SAI based pattern recognition approach may be applied to the structural health monitoring system for a real bridge.

Dynamic response measurement of offshore platform model by FBG sensors

The dynamic response of offshore platform under seismic excitation is a coupled vibration of liquid and solid interaction. In recent years, the computation of dynamic responses and design of offshore platform have attracted the attention of many researchers. This paper presents a shaking table test of offshore platform model scaled down an actual one. Fiber Bragg grating (FBG) is a promising measurement technology for its superior ability of explosion proof, immunity to electromagnetic interference and high accuracy. Some FBG sensors and strain gauges were used to monitor the dynamic response of offshore platform model. The maximum working frequency of FBG was calculated according to its dynamic strain response. Based on the theoretical and experimental results, it can be concluded that FBG sensor is superior to strain gauge and satisfies the demand of dynamic strain measurement.

Transverse strain of silicone dielectric elastomer actuators

Some dielectric elastomers produce large electric-field-induced strains that can be used for electromechanical actuation. When an electric field E is applied on an electroactive polymer film electroded on both sides, the film is subjected to a stress T due to the electrostatic force (Maxwell stress) and this causes it to deform, producing a strain in the plane perpendicular to the applied field. We have measured the transverse strain responses of silicone (Dow Corning HS III) Maxwell stress actuator samples of different sizes over a wide displacement range and a frequency range from DC up to 100 Hz. The static and dynamic strain responses of the materials to a variety of driving electric flelds such as step fields, AC fields and DC bias fields have been measured as functions of amplitude and frequency. The effect of a mechanical tensile pre-load on the transverse strain has also been investigated. A pre-load initially causes an increase in the strain but the strain decreases when larger values of pre-load are applied. Numerical finite element simulations of the material using the commercial software package ANSYS and suitable models of hyperelasticity provide good agreement with most of our observations on the electric field and pre-load dependencies of the transverse strain.

Short-term and long-term health monitoring experience of a short highway bridge: case study

Processing and interpreting a large amount of data represents a great challenge for infrastructure health monitoring. This study demonstrates how to apply some available algorithms to the data measured by a practical structural health monitoring system and how to evaluate the results. Short-term and long-term field tests were conducted on a steel highway bridge to monitor the structural behavior, including strain, displacement and temperature. The short-term test was carried out with a special vehicle crossing the bridge at different speeds. Measured dynamic strain responses are used to extract the dynamic characteristics of the structure by Eigensystem Realization Algorithm and to analyze the vehicle – bridge interaction properties by Wavelet Transform. In the long-term monitoring, strain time history and displacement versus temperature were recorded at several points. Cumulative fatigue damage model, extreme value distribution and linear regression model are, respectively, applied to the data and severa...

Seasonal Load Response Behavior of a Thin Portland Cement Concrete Pavement

The mechanistic-empirical pavement design methods currently under development have demonstrated a need for seasonal material and load response behavior characterization. The seasonal dynamic load strain response of a thin [5-in. (127-mm)], low-cost, portland cement concrete pavement at the Minnesota Road Research Project was examined. Environmental and load-related factors to be considered in this type of study are described. For the truck speeds used, analysis found minimal effects on measured dynamic strain. Nonlinear temperature profiles in the slabs prompted the use of the temperature-moment concept in the analysis. For the approach side of the joint in the slabs, during periods with unfrozen base and subgrade layers, there is only a small increase in the dynamic strain response with decreasing temperature-moment. Average dynamic strain responses range from 50 to 80 microstrain, with little difference in magnitude between the 80,000-lb (355-kN) and 102,000-lb (453-kN) loadings. For the leave side of the joints in the slabs, there is a larger increase in dynamic strain response with decreasing temperature-moment. In addition, the 102,000-lb load response is nearly 60% larger than the 80,000-lb load response for large negative temperature-moments. Recommendations for improving dynamic load testing of pavements are given.

Effects of piezoactuator delamination on the transfer functions of vibration control systems

The purpose of this theoretical work is to present a general bending–extensional model of the response of a simply supported laminated beam to excitation by a nonsymmetric actuator made using piezoelectric elements. The edge delamination is modeled by changing the effective length of debonded actuator. Dynamic equations, joint conditions between sections with and without active layers as well as the boundary conditions at the two ends of the beam form a boundary value problem. The dynamic strain response to the excitation by the applied voltage term is determined from the solution of this boundary value problem. The dynamic extensional strain on the beam surface is calculated by including the free stress conditions at the piezoelectric actuator boundaries, by considering the dynamic coupling between the actuator and the beam, and by taking into account a finite bonding layer with the finite stiffness. The analysis indicates that the edge delamination has a harmful effect on the performance of piezoactuators, but the significant decrease of natural frequencies with an increase in delamination length is not observed. The influence of the delamination length on the system transfer functions (the beam surface strain, the beam transverse displacement and the shear stresses in bonding layers) is shown.

Pultruded fiber optic ribbon sensor for applications in severe environments

A new multichannel optical sensor for monitoring large structures, consisting in an eight-optical-fiber ribbon pultruded in composite material, has been realized and completely characterized. Both strain and temperature sensitivity, and both dynamic and thermal strain response, as well as birefringence tests and mechanical characterization are reported. The experimental results confirm the capabilities of pultru- sion to realize multichannel optical sensors for large structures that are able to operate in a severe environment. The sensor can fulfill mechanical, chemical, and electrical custom requirements with quality consistency at low cost. The sensors have been used as strain sensors for an 85-m-long oil-rig riser.

Dynamic strain response of an infinite beam and inverse calculation of impact force by numerical Laplace transform

A general method for determining the dynamic strain response of an infinite beam to impact force is presented. The method consists of formulating and solving the dynamic problem in the Laplace transform domain and obtaining the strain response by numerical inversion of the transformed solution. Also, an inverse method for estimating the impact force on an infinite beam is investigated. Once the strain is known, it is shown how the impact force can be reconstructed.

Soft Body Impact Damage on CF/PEEK Laminates using Gelatin Projectile

In order to use composite materials in aeronautical turbo engines, their resistance to impact damage caused by soft bodies (birds) must be understood. In this work the subperforation flat wise soft body impact resistance of two kinds of carbon fiber/PEEK systems were evaluated using a gelatin projectile. Tested systems were AS4/PEEK (APC2/AS4) and AS4/PEEK + IL, which consists of APC 2 prepreg and PEEK film inserted between layers as an interleave. To investigate the effects of stacking sequence on resistance, three lay ups were tested. A gas gun system was used for the impact tests, where the velocity range was between 90 and 190 m/s. Mass of the projectile was about 3 g. The projected damage area was measured with an ultrasonic C Scan. The dynamic strain response was also measured for some of the impact tests. The impact resistance to soft body impact is compared with that of hard body impact investigated in a previous study by the first author.