Real-time Strain Measurement Research Articles

Experimental and numerical studies on indentation and perforation characteristics of honeycomb sandwich panels

Aluminum sandwich panels with honeycomb core have been widely used as energy absorption structure in lightweight design. This study aimed to characterize the indentation and perforation behaviors of sandwich structures with different geometric configurations. The specimens with four characteristic geometric variables, namely, facesheet thickness, core height, honeycomb core thickness and side length of hexagon cell were tested experimentally. Photographs of cross-sectional view near the loading area and failure modes in the tests were investigated in detail. For the first time, digital image correlation (DIC) technique through an ARAMIS™ real-time optical strain measurement system was adopted for capturing the deformation process of lower skin by acquiring the displacement-time data. Three typical damage modes were identified from the force-displacement curves with different geometric parameters and configurations. It was found that the thickness of facesheet has the most significant effects on both force-displacement curves and energy absorption capacity. Changes in the core parameters have relatively small influences in total energy absorption but sizeable effects on the force-displacement curve and failure modes. A finite element model for predicting damage evolution was also developed and validated through the force-displacement relation and deformation process on the bottom skin. The damage mechanisms of the sandwich panel subject to quasi-static indentation and perforation were analyzed through the numerical models. The present study contributed on understanding how the geometric parameters affect the characteristics of indentation and perforation, thereby providing useful guidelines for its potential applications in impact engineering.

Self-calibration single-lens 3D video extensometer for high-accuracy and real-time strain measurement.

The accuracy of strain measurement using a common optical extensometer with two-dimensional (2D) digital image correlation (DIC) is not sufficient for experimental applications due to the effect of out-of-plane motion. Although three-dimensional (3D) DIC can measure all three components of displacement without introducing in-plane displacement errors, 3D-DIC requires the stringent synchronization between two digital cameras and requires complicated system calibration of binocular stereovision, which makes the measurement rather inconvenient. To solve the problems described above, this paper proposes a self-calibration single-lens 3D video extensometer for non-contact, non-destructive and high-accuracy strain measurement. In the established video extensometer, a single-lens 3D imaging system with a prism and two mirrors is constructed to acquire stereo images of the test sample surface, so the problems of synchronization and out-of-plane displacement can be solved easily. Moreover, a speckle-based self-calibration method which calibrates the single-lens stereo system using the reference speckle image of the specimen instead of the calibration targets is proposed, which will make the system more convenient to be used without complicated calibration. Furthermore, an efficient and robust inverse compositional Gauss-Newton algorithm combined with a robust stereo matching stage is employed to achieve high-accuracy and real-time subset-based stereo matching. Tensile tests of an Al-alloy specimen were performed to demonstrate the feasibility and effectiveness of the proposed self-calibration single-lens 3D video extensometer.

Experimental Study Regarding of Bending Behaviour of Stabilizator Link

Abstract This paper presents an experimental study of the behavior of anti-roll power link subjected to bending, power link coming from an Opel Astra G. The power link is made of PA66 GF30 polyamide. For this study, there were used a universal testing machine (Instron 5587) and a real-time strain measurement optical system (Aramis). The results showed are those obtained in the case of a compression force of 1,000 [N] namely: major Strain ε1, minor strain ε2, equivalent von Mises strain, displacement on X axis, displacement on Y axis (compression force direction), displacement on Z axis.

The Effect of Fiber Orientation on the Formability and Failure Behavior of a Woven Self-Reinforced Composite

This article investigates the effect of fiber orientation on forming and failure behaviors of a preconsolidated woven self-reinforced polypropylene (SRPP) composite during the stamp-forming process. Specimens with different aspect ratios were employed to study their formability during forming through a hemispherical punch in an open die configuration. The strain evolution in specimens was captured using a real-time strain measurement system (the aramis). A forming limit diagram (FLD), inspired from metal forming, was constructed to investigate the failure onset in the woven composite. The FLD revealed dominant failure mechanisms in [0 deg,90 deg] specimens were yarn splitting and fiber fracture as depicted by optical microscopy. A modified FLD was proposed to investigate failure mechanisms in [45 deg,−45 deg] samples. It was shown that delamination and intralaminar shear are the main causes of failure in this set of specimens. The outcomes of this study suggest that by changing forming parameters, such as fiber orientation, boundary condition, and aspect ratio, the formability of a preconsolidated SRPP can be improved. These results show the possibility of producing cost-effective, flawless, and fully recyclable products from consolidated woven composites. The proposed criterion can accurately predict failure in a woven composite by considering the combined strain interactions. This is reflected in the implementation of induced deformation modes and strain history into the failure criterion, making it a practical measure for rapid manufacturing techniques, such as stamping. The novel path-dependent failure criterion, introduced in this study, attempts to fill the gap for a reliable and accurate failure measure for woven composites.

High Performance Time Domain FBG Dynamic Interrogation Scheme Based on Pulse Coding

A novel method to improve the measurement range, resolution, and multiplexing capability of time division multiplexed (TDM) fiber Bragg grating (FBM) sensors, based on advanced cyclic pulse coding, is proposed and experimentally demonstrated. The mechanism of noise reduction by quasi-periodic cyclic coding is quantitatively demonstrated, pointing out significant improvement in dynamic strain resolution with respect to single pulse TDM FBG interrogation. The use of cycling pulse coding allows for enhanced dynamic performance of real time strain measurement with respect to other coding techniques. The proposed technique can also enhance the sensing range of hybrid optical fiber sensor systems in which continuous static temperature/strain distributions and discrete dynamic strain in specific critical points are simultaneously measured over the same sensing fiber.

The Fiber Bragg Grating Sensor and its Interrogator for Dynamic Strain Measurement of Concrete

The fiber Bragg grating (FBG) sensor is the most appropriate to measure strain of concrete. Employing a specifically designed FRP package, a FBG sensor for concrete dynamic strain measurement was designed. To interrogate the dynamic response of the FBG sensor, the corresponding interrogator was developed by using a novel reflect wavelength measurement method. A series of tests such as measuring dynamic strain of concrete specimens impacted with drop hammer and split Hopkinson pressure bar (SHPB) were conducted. The results show that the sensor and its interrogator respond rapidly to impact and have a stable performance. The system can be used for a real-time and rapid dynamic strain measurement in concrete dynamic performance tests or weapons effects tests.

Hybrid Fibre Optic Sensor Network for Real-time High Temperature Performance Monitoring of Steel Structures

In and post a fire event, an accurate and real-time evaluation and monitoring of a structure’s performance can assist firefighters for efficient survivor rescuing, which significantly improve the fire rescuing safety both for fire fighters and the trapped survivors. However, due to the lack of durable sensors, the structural performance of steel structures in fire conditions is challenging to be evaluated in real time, especially when the associated civil structures are in a large scale. In this paper, a fiber optic sensor network is developed and used to monitor the structural performance of the steel structures in high temperature environments. The fiber optic sensor network has the capacity of real-time large strain measurement up to 12% and temperature of up to 700oC simultaneously. The capability of large strain measurement up to 12% enables the sensor system to monitor the strain distribution of steel structures during fire events in real time. An one-story one-bay steel frame (A36 steel) is used as an example in this paper to perform the structural performance assessment of steel structures in high temperature using the developed sensor network. The simulated fire tests were performed using high temperature furnace through gradual temperature increase from room temperature to 800 °C at a rate of 10 °C/min. The evaluated fiber optic sensor network consists of two long period fiber grating (LPFG) temperature sensors, five movable extrinsic Fabry-Perot interferometric (EFPI) large strain sensors, and two hybrid EFPI/LPFG sensors, which were distributed along the steel frame inside and around the heating zone of the frame. Experimental results demonstrated that the developed sensor network effectively monitored the plastic hinge formation and failure mode of the steel frame, approving the feasibility of the sensor network for steel structure evaluation in high temperature environments.

Fiber-Optic Strain Sensor System With Temperature Compensation for Arch Bridge Condition Monitoring

This paper presents an innovative sensor system, created specifically for new civil engineering structural monitoring applications, allowing specially packaged fiber grating-based sensors to be used in harsh, in-the-field measurement conditions for accurate strain measurement with full temperature compensation. The sensor consists of two fiber Bragg gratings that are protected within a polypropylene package, with one of the fiber gratings isolated from the influence of strain and thus responding only to temperature variations, while the other is sensitive to both strain and temperature. To achieve this, the temperature-monitoring fiber grating is slightly bent and enclosed in a metal envelope to isolate it effectively from the strain. Through an appropriate calibration process, both the strain and temperature coefficients of each individual grating component when incorporated in the sensor system can be thus obtained. By using these calibrated coefficients in the operation of the sensor, both strain and temperature can be accurately determined. The specific application for which these sensors have been designed is seen when installed on an innovative small-scale flexi-arch bridge where they are used for real-time strain measurements during the critical installation stage (lifting) and loading. These sensors have demonstrated enhanced resilience when embedded in or surface-mounted on such concrete structures, providing accurate and consistent strain measurements not only during installation but subsequently during use. This offers an inexpensive and highly effective monitoring system tailored for the new, rapid method of the installation of small-scale bridges for a variety of civil engineering applications.

Dynamic Strain Mapping and Real-Time Damage-State Estimation Under Random Fatigue Loading

This paper presents a passive sensing technique for real-time structural health monitoring. In the proposed technique, two reference or healthy state dynamic models are estimated using a nonlinear statistical pattern recognition technique known as the Gaussian process. The nonlinear dynamic models are estimated by mapping real-time dynamic strain measurements at two different locations with corresponding load histories. The strain measurements are performed using strain gauge rosettes placed on opposite sides of a probable damage path. Using the estimated reference model with the new loading information at any given instant of time, the corresponding dynamic strains are predicted. The predicted strains are compared with the actual strain measurements and used to estimate the corresponding damage states via correlation analysis. The state estimation approach is demonstrated on an aluminum-2024 cruciform specimen fatigued under random biaxial loading. The numerical study shows that the proposed approach can estimate real-time damage states caused by both stage II and stage III fatigue cracks.

Arch-bridge Lift Process Monitoring by Using Packaged Optical Fibre Strain Sensors with Temperature Compensation

This paper presents a novel sensor design and packaging, specifically developed to allow fibre grating-based sensors to be used in harsh, in-the-field measurement conditions for accurate strain measurement, with full temperature compensation. After these sensors are carefully packaged and calibrated in the laboratory, they are installed onto the paragrid of a set of flat-packed concrete units, created specifically for forming a small-scale, lightweight and inexpensive flexi-arch bridge. During the arch-bridge lifting process, the sensors are used for real-time strain measurements to ensure the quality of the construction. During the work done, the sensors have demonstrated enhanced resilience when embedded in concrete structures, providing accurate and consistent strain measurements during the whole installation process and beyond into monitoring the integrity and use of the structure.

Wireless strain measurement for structural testing and health monitoring of carbon fiber composites

A novel device patented by Intel and called WISP (Wireless Identification and Sensing Platform) is modified to interface with a conventional foil resistance strain gage. The wireless, battery-free, digital device communicates with and is powered by an Ultra-High Frequency (UHF) RFID (Radio Frequency Identification) reader. The standard Intel WISP has been modified with the addition of a PCB (printed circuit board), which act as the analog interface with the strain gage. The so-called WISPs/g (for strain gage) has been utilized during uniaxial tension tests on carbon fiber composite specimens to compare accuracy and repeatability with conventional wired strain gage and extensometer. Validation of the technology is performed with a structural test, whereby four independent WISPs/g devices are positioned on the surface of a carbon fiber composite flat panel subjected to quasi-static indentation. Measurements are compared to the predictions of a NASTRAN finite element model (FEM) and show excellent agreement. Applications of this technology include strain measurement during static airframe tests, as well as on-board real-time strain measurements during test flight and certification of new aircraft.

Application of Reverse Engineering to Product Development

This 3D optical metrology has become a standard tool in industrial process chains by providing fast and precise full-field 3D data for quality control, rapid manufacturing and reverse engineering applications. In this report, the trend that 3D optical metrology has widely used by the product development is described. Concretely, it is described to inspect shape on the free-form surface, to digitize the design model, to manufacture solid shape without CAD model, to define CAE model from real shape, to make analysis for sheet metal components, and to make real-time strain measurement for tensile tests.

5 kWh 복합재 플라이휠 로터의 회전 시험

본 논문은 에서는 5 kWh 복합재 플라이휠 로터를 설계/제작 하고, 회전 시 복합재 로터에 발생되는 변형률 분포와 로터의 파손 속도를 측정하기 위해 회전 시험을 수행하였다. 회전 시험 시 블루투스 방식의 무선 통신시스템을 이용하여 복합재 로터의 반경방향과 원주방향의 변형률을 실시간으로 측정하였다. 측정된 로터 변형률과 미리 예측된 로터 변형률을 비교하여 초기 로터 설계를 검증하였다. 복합재 로터는 파손속도인 22,000 rpm보다 11 %낮은 19,499 rpm에서 파손 되었다. 파손은 축과 복합재 로터를 연결하는 허브에서 발생하였다. 본 논문은 일반적인 관점에서 로터의 성능을 검증하였고, 고속 회전하는 복합재 로터의 예상치 않은 파손 위험을 입증하였다. 플라이휠 에너지 저장 시스템 설계 시 복합재 로터 뿐 아니라 허브 설계에도 특별한 주의가 요구된다. 플라이휠 에너지 저장 시스템을 실시간으로 관찰하기 위해서, 특히 높은 원심력을 받는 동안에도 작동 될 수 있는 무선 통신 시스템 개발이 앞으로 선행 되어야 한다. A 5 kWh composite flywheel rotor was designed and manufactured, and its spin test was performed to monitor strain distribution and burst speed. Strain distribution in radial and circumferential directions of the rotor were measured using a wireless telemetry system based on bluetooth technology for real-time strain measurement. The strains was compared with pre-calculated design values to verify the initial rotor design. We noticed the rotor failed at 19,499 rpm in the spin test, 11 % lower than the predicted burst speed of 22,000 rpm. Failure occurred at the hub which connects the shaft and the composite rotor. The performance of the composite rotor was confirmed in a general sense, and the danger of unexpected failure of composite rotor during high-speed spinning was also demonstrated in this paper. Special attention should be paid to not only composite rotor but also hub when designing a flywheel energy storage system. The telemetry system needs to be further developed, especially enduring the high centrifugal forces, and can be used in a real time monitoring system for the flywheel energy storage system.

Multidimensional Dynamic Displacement and Strain Measurement Using an Intensity Demodulation-Based Fiber Bragg Grating Sensing System

We set up a multidimensional dynamic fiber Bragg grating (FBG) displacement and strain sensing system. The proposed sensing system allows the FBGs to have the ability to measure transient point-wise displacement (in-plane and out-of-plane) and strain responses simultaneously with high sensitivity. The sensing system employs a long-period fiber grating filter and two FBG filters to dynamically demodulate the responses of the FBG sensors. To demonstrate the dynamic sensing ability and the application of the proposed system, the transient elastic wave propagation in a solid subjected to vertical and lateral impact loadings are investigated. A laser Doppler vibrometer is employed in our experiments simultaneously to verify the transient responses of out-of-plane particle transient motions measured by the out-of-plane FBG displacement sensor. The in-plane displacement measurement ability is demonstrated by the responses obtained by two geometrically orthogonal in-plane FBG displacement sensors. Dynamic strain sensing ability of our all-fiber sensing system is also demonstrated. The experimental results both in time domain and frequency domain show that the proposed FBG sensing system is capable of performing real-time displacement (out-of-plane and in-plane) and strain measurements.

Fundamental Experiment in Real-time Strain Measurement of Multipoint Fiber Bragg Grating using Optical Frequency Sweeping

With the objective of real-time monitoring of the structural integrity of man-made structures such as bridges, high buildings, and dams and areas of bedrock collapse and other such hazardous areas, we are attempting to use fiber Bragg grating (FBG) to construct a structural health monitoring system. To construct a multipoint strain measurement system with FBG, which is an optical fiber type sensor, we investigated ways to use FBGs that have the same reflection characteristics and FBGs that have different reflection wavelengths in multipoint measurement. The constructed system combines optical frequency sweeping with the multipoint FBG to accomplish measurement of the reflection wavelengths of multipoint FBG and real-time graphic display. This paper describes fundamental experiment on multipoint strain measurement, demonstrating that the constructed real-time measurement system can measure the reflection wavelengths of FBGs that have the same reflection characteristics and FBGs that have different reflection wavelengths every four seconds.

Biaxial fatigue behavior of an epoxy polymer with mean stress effect

Biaxial fatigue behavior of an epoxy polymer was investigated under cyclic shear and proportional axial-shear combined loadings with mean strains. Axial and shear strains were simultaneously measured by a non-contact real-time strain measurement system during fatigue process. Then the mechanical parameters (stress/strain components, mean stress/strain, strain energy densities, etc.) of the specimens during entire fatigue life were able to be quantitatively retrieved from the recorded stress-strain data. Multiaxial fatigue life prediction models were established upon stress-, strain- and energy-based approaches with consideration of mean stress/strain effect, where better agreement was achieved in stress-based and energy-based approaches.

Profile production in multi-veneer sheets by continuous roll forming

Abstract Roll forming is a rapid operation that employs consecutive roll stations to progressively deform a strip into a desired profile. The continuous nature of the process coupled with its versatility and speed makes it extremely attractive for producing lightweight, structurally efficient components from sheet materials. This paper describes the roll forming of plywood by heated rolls, which is an innovative concept that can increase the range of usage and add value to the end products. Deformation length measurements from static roll bend tests were carried out to determine the roll spacing and the results agree well with the values predicted by a modified theory based on the minimisation of the total deformation energy for a rigid-plastic material. The deformation lengths, such as in metals and composite sheets, depend essentially on the geometric parameters of the profiles produced and are practically independent of the mechanical properties of the work material. Further, the real-time strain measurements by strain gauges bonded on the roll formed specimens show that the strain profiles compare reasonably well with those obtained for metallic and continuous fibre-reinforced thermoplastic sheets. Finally, the macroscopic deformation of veneer sheets due to roll forming has been analysed kinematically by means of grid strain analysis and the geometric conformance of the product has been determined.

Fatigue behavior of an epoxy polymer subjected to cyclic shear loading

Strain-range-controlled fully reversed cyclic shearing fatigue tests have been performed on thin-walled tubular specimens made of an epoxy polymer. The engineering shear strain was measured by a non-contact real-time strain measurement and control system recently developed in our lab. The synchronized stress–strain data were recorded by the system for full lives of the specimens. Therefore evolutions of mechanical properties of the epoxy polymer during fatigue process could be analyzed based on the retrieved stress–strain data. It was found that life data of the shearing fatigue tests were well correlated with the uniaxial fatigue test data of the material by using the suggested stress-, strain- or strain-energy-density-based multiaxial fatigue criteria.

Mean stress/strain effect on fatigue behavior of an epoxy resin

Mean stress/strain effect on the fatigue life of an Epon 826/Epi-Cure Curing Agent 9551 epoxy system has been investigated by carrying out series uniaxial fatigue tests with four different mean strain ratios under strain-range-controlled mode. Quantitative analysis of the mechanical properties, such as mean stress relaxation, evolution of stress range and strain energy density, have been performed based on the stress–strain data recorded by a non-contact real-time strain measurement and control system. Mean stress/strain functions have been introduced into the equivalent damage parameters to include the mean stress/strain effect on the fatigue life of the epoxy polymer material. The procedure to characterize the mean stress/strain functions has been demonstrated and material constants were calibrated in the case of either stress, strain or energy approach, respectively. Good agreement can be seen between the predictions and the test data.

Ratcheting behavior of an epoxy polymer and its effect on fatigue life

Ratcheting behavior of an epoxy resin and its effect on fatigue life has been experimentally investigated. All the fatigue tests were carried out under stress-controlled mode, and stress–strain data were recorded during the entire fatigue life period by a newly established non-contact real-time strain measurement and control system. Quantitative results of evolutions of ratcheting strain, strain range and strain energy density in tests with various mean stresses and stress amplitudes have been retrieved from the stress–strain data. The amount of ratcheting strain and accumulation rate was sensitive to the stress amplitude and mean stress. Fatigue lives of these tests were compared with previous test results under strain-range-controlled mode and no obvious ratcheting strain effect on fatigue life was observed for this epoxy polymer. Additional fatigue tests with resting periods were further performed to investigate the mechanism of the experimental observations on this epoxy polymer.