Strain Measurement System Research Articles

Influence of Position on Optoelectronic Strain Measurement Systems for Flywheels

Influence of Position on Optoelectronic Strain Measurement Systems for Flywheels

Experimental Study on the Mechanical Properties and Influencing Factors of Glass Fiber-Reinforced Permeable Concrete.

In order to improve the mechanical properties and deformation characteristics of permeable concrete, glass fiber was added to this type of concrete. Based on an unconfined compressive strength test, non-contact full-field strain measurement system, and scanning electron microscopy test, the effects of aggregate particle composition, shaking time, fly ash content, fiber length, and fiber content on the strength and permeability of permeable concrete were studied. The results show that the strength and water permeability of permeable concrete are negatively correlated with an increase in shaking time. When the aggregate particle size is 5-10 mm, the permeable concrete has both good strength and permeability. Proper incorporation of fly ash improves the compactness inside the structure. The influence of different lengths of glass fiber on the strength of permeable concrete first increases and then decreases, and the permeable property decreases. With the same fiber length, the strength increases first and then decreases with an increase in the content, while the porosity and water permeability coefficient decrease. Under the test conditions, when the length of glass fiber is 6 mm, and the dosage is 2 kg/m3, the strength performance of permeable concrete is the best, and the permeability effect is good at the same time.

A Multi-Channel Borehole Strain Measurement and Acquisition System Based on FPGA.

In this study, an FPGA(Field Programmable Gate Array)-based borehole strain measurement system was designed that makes extensive use of digital signal processing operations to replace analog circuits. Through the formidable operational capability of FPGA, the sampled data were filtered and denoised to improve the signal-to-noise ratios. Then, with the goal of not reducing observational accuracy, the signal amplification circuit was removed, the excitation voltage was reduced, and the dynamic range of the primary adjustments was expanded to 130 dB. The system's online compilation function made it more flexible to changes in measurement parameters, allowing it to adapt to various needs. In addition, the efficiency of the equipment use was enhanced. The actual observational results showed that this study's FPGA-based borehole strain measurement system had a voltage resolution higher than 1 μV. Clear solid tides were successfully recorded in low-frequency bands, and seismic wave strain was accurately recorded in high-frequency bands. The arrival times and seismic phases of the seismic waves S and P were clearly recorded, which met the requirements for geophysical field deformation observations. Therefore, the system proposed in this study is of major significance for future analyses of geophysical and crust deformation observations.

Elevated-temperature mechanical property and constitutive model of Galfan-coated and full-locked steel cables

Steel cables are an essential stress-bearing member in pre-tensioned steel structures. Their exposure to fire may lead to prestress loss, which may cause structural failure or even overall collapse. This study used a non-contact strain measurement system to investigate the mechanical properties of 1670 MPa Galfan-coated steel cable and 1570 MPa full-locked steel cable at elevated temperatures. The test results showed that the mechanical properties of the two types of cable decayed significantly with the increase in temperature. At 700 °C, the ultimate strength of Galfan-coated steel cable was only 2.36% of that at room temperature, and that of full-locked steel cable was 3.56%. By analyzing the stress-strain relationships between the yielding and hardening stages at 9 test temperature levels, it was found that the high-temperature nominal yield strength could be taken as the stress at the strain level of 1.25% for both types of steel cables. Based on the full stress-strain curves at each temperature level, the calculation formulae for high-temperature reduction factors of elastic modulus, proportional limit, nominal yield strength, and ultimate strength of Galfan-coated and full-locked steel cables were proposed. Besides, the high-temperature constitutive models of the two cable types were also proposed based on the experimental data. The findings of this study can improve the fire safety of pre-tensioned steel structures.

Influence of hole diameter on mechanical properties and stability of granite rock surrounding tunnels

Nowadays, the development and utilization of more and more engineering construction are closely related to granite. However, many granite rock masses in Qingdao contain natural hole defects, which have a great impact on the mechanical properties of granite. It may even cause instability of surrounding rock and safety accidents. Therefore, in this paper, we discuss the influence of the hole diameter on the mechanical properties and stability of granite rock surrounding tunnels. Uniaxial compression experiments were conducted on granite with different hole diameters, and monitoring was carried out using the acoustic emission system and the XTDIC (Xintuo 3D Digital Image Correlation) three-dimensional–full-field strain-measurement systems. The relationship between the strength, deformation, and hole size of granite was investigated. In addition, using the Yangkou tunnel as the prototype and the PFC2D (Particle Flow Code of 2D) particle-flow–numerical-simulation program, a working tunnel model with different hole sizes was established to simulate the influence of natural hole defect sizes on the stability of rock. The results show that: (1) with an increase in hole diameter, the uniaxial compressive strength and elastic modulus of the granite sample gradually decreased. The brittleness of the granite samples gradually decreased, and the ductility gradually increased. (2) Under the action of axial stress and with an increase in the hole diameter, the sample was more likely to produce a stress concentration around the hole defect, which increased the deformation localization band, development, and expansion, as well as the intersection degree. As a result, granite samples are more likely to develop new cracks. These cracks increase in number and size, reducing the compressive strength of the granite sample. (3) The size of the hole defects significantly affected the damage and mechanical properties of the model surrounding rock. When increasing the hole diameter, the defect area increased and the tensile stress concentration near the hole in the localized rock became more evident. In addition, the stability of the rock surrounding the tunnel was significantly reduced, and its bearing capacity was weakened, leading to easier crack initiation and rock damage.

The influence of the heat generation during deformation on the mechanical properties and microstructure of the selected TWIP steels

The TWIP (Twinning Induced Plasticity) steels are one of the most promising materials in reducing the weight of vehicles. Despite a lot of research on TWIP steel, there are some issues that are not explained enough. Due to the future use of TWIP steel and the manufacturing of the final part by metal forming, three issues still need to be clarified. The first one, which is the most important, is the increase of the temperature due to the conversion of the deformation work into heat. TWIP steel has a high limit strain, strength and lower conductivity than conventional steel, therefore the heat generation of TWIP steel is greater than for other materials. The second and third issues are combined. They concern the influence of V microadditions on the stress–strain curves, the strain hardening coefficient n and the strain rate sensitivity coefficient m under cold deformation conditions. These properties determine the cold formability of TWIP steels. In the research, two TWIP steels were used with and without V microadditions (MnAl and MnAl-V steel). The special methodology using strain and temperature measurement systems as well as light and scanning electron microscopy (SEM) were applied. Research shows a significant increase of the temperature in the material due to high plastic deformations as well as a high level of yield stress. In the neck area, for the highest strain rate of 0,1 s -1, at the moment of rupture, the temperature reaches more than 200 °C. The difference between the average temperature in the rupture area and the maximum temperature is equal to 100° C. Its high increase can lead e.g. to changes in the deformation mechanism from twinning to dislocation gliding, which is also connected with a worsened workability, and thus also energy consumption of the bodywork elements. MnAl-V steel has better or similar ductility for the deep drawing in comparison to MnAl steel at low strain rates for almost isothermal conditions (constant temperature during deformation). However the MnAl steel has better ductility for the larger strain rates over 0.1 s−1 then there is large heat concentration in a very narrow area for MnAl-V steel. The obtained results are very important from an application point of view. The strain rate sensitivity coefficient m of the steel MnAl has very low, and even negative, values, which can make the production of complicated drawpieces difficult. Higher values of the strain rate sensitivity coefficient are exhibited by steel MnAl-V, i.e. at the level of 0,05, which is almost constant in the whole range of the obtained deformations.

Study on 3D printing process of continuous polyglycolic acid fiber-reinforced polylactic acid degradable composites.

A continuous polyglycolic acid (PGA) fiber-reinforced polylactic acid (PLA) degradable composite was proposed for application in biodegradable load-bearing bone implant. The fused deposition modeling (FDM) process was used to fabricate composite specimens. The influences of the printing process parameters, such as layer thickness, printing spacing, printing speed, and filament feeding speed on the mechanical properties of the PGA fiber-reinforced PLA composites, were studied. The thermal properties of the PGA fiber and PLA matrix were investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The internal defects of the as-fabricated specimens were characterized by the micro-X- ray 3D imaging system. During the tensile experiment, a full-field strain measurement system was used to detect the strain map and analysis the fracture mode of the specimens. A digital microscope and field emission electron scanning microscopy were used to observe the interface bonding between fiber and matrix and fracture morphologies of the specimens. The experimental results showed that the tensile strength of specimens was related to their fiber content and porosity. The printing layer thickness and printing spacing had significant impacts on the fiber content. The printing speed did not affect the fiber content but had a slight effect on the tensile strength. Reducing the printing spacing and layer thickness could increase the fiber content. The tensile strength (along the fiber direction) of the specimen with 77.8% fiber content and 1.82% porosity was the highest, reaching 209.32 ± 8.37 MPa, which is higher than the tensile strength of the cortical bone and polyether ether ketone (PEEK), indicating that the continuous PGA fiber-reinforced PLA composite has great potential in the manufacture of biodegradable load-bearing bone implants.

Laser projection system for polymer transverse strain measurements in tensile testing

A self-built laser projection system for optical transverse strain measurements in tensile testing is presented. The setup based on laser diode modules, charge-coupled device array detectors, and standard optomechanical components is proposed to be a low-cost system for testing polymer samples. The optical setup is mobile and can easily be mounted on a tensile test machine. Circular cross-sectional changes in polymer diameter were detected with 10-μm resolution and stress-strain curves obtained by a tensile test machine. The article describes signal processing using the open-source programming language R, and the transverse deformation of the polymer sample is evaluated in two perpendicular planes. The results obtained by the optical system may be used to accurately describe the models of energy-based mechanical properties of polymers for complex load conditions.

The study of mechanism of localized shear fracture in AlMg6 alloy under dynamic loading on a split Hopkinson pressure bar

This work is devoted to the substantiation of one of the mechanisms of plastic strain localization under conditions of high-rate dynamic loading associated with jump-like processes in the defect structure of materials. The specimen behavior under dynamic loading was investigated using a split Hopkinson pressure bar. In order to identify the characteristic stages of strain localization, we performed in-situ thermodynamical studies of the deformation process by recording temperature fields with the CEDIP Silver 450M high-speed infrared camera. The temperature measurements made in the zone of strain localization do not support the wide-spread idea that the strain localization mechanism is caused by thermoplastic instability. Dynamic tests were carried out for specimens, which were specially designed to study plastic strain localization in AlMg6 alloy under conditions of dynamic loading on the split Hopkinson pressure bar using the StrainMaster strain measurement system. The specimens after the experiment were subjected to microstructural analysis using an optical interferometer-profile meter and a scanning electron microscope. The structural analysis revealed the correlated behavior of defect ensembles, which can be classified as a structural transition facilitating strain localization. Experimental data, examination of deformed specimen structure and data of numerical simulation with consideration for kinetics of microdefect accumulation in the material allow us to infer that one of the mechanisms of plastic strain localization at high loading rates is caused by jump-like processes in the defect ensembles.

Shear Strength and Re-Failure Characteristics of Intact Red Sandstone and Grouting-Reinforced Body of Fractured Red Sandstone under Different Shear Angles

To reveal the strengthening mechanism and re-failure mechanism of grouting on fractured rock, the variable-angle shear tests, real-time acoustic emission (AE) tests and VIC-3D (non-contact full field strain measurement system) tests were carried out on intact red sandstone (IRS) and grouting-reinforced body of fractured red sandstone (GRBFRS). The results show that the peak shear strength of IRS and GRBFRS decreases with the increase of shear angle. Grouting reinforcement mainly increases the cohesion of GRBFRS to improve its shear strength, but its reconstructability decreases with the increase of shear angle. In the whole process of shear deformation, the shear micro damage and shear fracture of GRBFRS are more than those of IRS. Although the shear re-failure before and after the peak stress stage is the most notable, its intensity or degree is reduced. When the shear angle is 45°, both IRS and GRBFRS undergo shear-splitting failure. However, when the shear angle is large, the IRS and GRBFRS only occurs shear failure. Moreover, the larger the shear angle, the more likely IRS and GRBFRS is to produce secondary shear cracks. The low strength of the rock–grout interface in GRBFRS is the main inducer of shear re-failure.

Polar effective plastic strain - forming limit diagram of DP 440 steel sheet with application to two-step forming test

In this work, experimental forming limit curve (FLC) and polar effective plastic strain -forming limit diagram (PEPS-FLD) are developed in order to predict the formability of dual phase (DP) steel grade 440. The Forming Limit Diagrams (FLD) of the examined steel are first obtained using the Nakajima stretch-forming test with a decreased hemispherical punch diameter of 50 mm in combination with Auto-Grid Vialux strain measurement system. To achieve a PEPS-FLD, an experimental FLC data must be completely projected onto the polar space using the results of FLC, Hill's 48 anisotropic yield criterion, and PEPS algorithm through plasticity calculation. Finally, a two-step forming test is conducted both experiments and simulation to demonstrate the practical applicability of the PEPS-FLD. It really would appear that the PEPS-FLD could predict the formability in non-linear strain path and can be utilized to enhance formability by creating non-linear strain path of DP440 more accurately than FLD.

Assessment of laser cutting parameters and heat-affected zone on microstructure and fatigue behaviour of carbon fibre-reinforced epoxy

In this study, the effects of laser cutting on the fatigue properties of carbon fibre-reinforced polymers are investigated. For this purpose, unidirectional carbon fibre-reinforced epoxy laminate is cut by laser. The material to be cut evaporates directly in the laser beam and the laser beam can produce thermally induced damage, which is referred to as heat-affected zones. Specimens are cut with two different parameters (varying cutting velocity and break time between passes) to widths up to 15 mm for investigation of the heat-affected zones effects on fatigue behaviour. Prior to fatigue tests microstructure is evaluated using light and scanning electron microscopy with regard to cutting geometry and heat-affected zones. Fatigue tests are performed using multiple amplitude tests with a stress ratio of 0.1, instrumented with a video extensometer system and thermocouples for strain and temperature measurements to determine the influence on mechanical properties and evaluate the sensitivity of the used testing method and measurement instrumentation for this use case. Lower cutting velocities and break time passes lead to a larger edge angle and thus in different specimen widths on front (laser inlet) and bottom (laser outlet) of the carbon fibre-reinforced epoxy laminate. Therefore, the resulting true geometry was considered using statistical approaches to eliminate possible cross-sectional errors regarding mechanical stress application. The results show that the specimen widths and process parameters impact the fatigue properties, highlighting the importance of knowledge about heat induction and the detection of inflicted laminate damage.

Differential temperature compensation method for edge filtering strain sensors

The edge filtering system is a common optical fiber strain measurement system. However, temperature changes in the measurement will interfere with the results. A differential temperature compensation method for edge filtering strain sensors is proposed. An additional temperature compensation FBG is added to the traditional edge filtering interrogation system. In the experiment, we analyze the influence of temperature on the sensor and filter. Temperature cross-sensitivity can be eliminated by differencing two FBGs, and the influence of temperature on the filter can also be eliminated. Experiments show that the system's temperature compensation ability is ideal, the influence of temperature on the strain test is reduced by about 80 %, and the sensitivity of the system is 0.003 mV/με. This method can reduce the influence of temperature on the edge filter measuring system.

A portable device for single point strain analysis in sheet metal forming processes

Strain measurement is one of the basic techniques in sheet metal forming for quantifying the deformation of a material under the effects of various loading conditions. The main components in the automatic strain measurement system are the image acquisition hardware and image processing software. This paper presents an automatic single point strain analysis system for sheet metal forming applications. The proposed system is very portable and consists of a USB microscope with different end caps for image acquisition. The software system was developed using Python – OpenCV library and provided with a graphical user interface (GUI).

A plantar surface shear strain methodology utilising Digital Image Correlation

The increase in the global diabetic population is leading to an increase in associated complications such as diabetic foot ulceration (DFU), associated amputations, morbidity, which substantial treatment costs. Early identification of DFU risk is therefore of great benefit. International guidelines recommend off-loading is the most important intervention for healing and prevention of DFU, with current research focused on pressure measurement techniques. The contribution of strain to DFU formation is not well understood due to challenges in measurement. The limited data available in the literature suggest that plantar strain is involved in ulcer formation. As a consequence, there is a need for plantar strain measurement systems to advance understanding and inform clinical treatment. A method was developed to determine plantar strain based on a Digital Image Correlation (DIC) approach. A speckle pattern is applied to the plantar aspect of the foot using a low ink transference method. A raised walkway with transparent panels is combined with a calibrated camera to capture images of the plantar aspect throughout a single stance phase. Plantar strain is then determined using 2D DIC and custom analysis summarises these data into clinically relevant metrics. A feasibility study involving six healthy participants was used to assess the efficacy of this new technique. The feasibility study successfully captured plantar surface strain characteristics continuously throughout the stance phase for all participants. Peak mean and averaged mean strains varied in location between participants when mapped into anatomical regions of plantar interest, ranging from the calcaneus to the metatarsal heads and hallux. This method provides the ability to measure plantar skin strain for use in both research and clinical environments. It has the potential to inform improved understanding of the role of strain in DFU formation. Further studies using this technique can support these ambitions and help differentiate between healthy and abnormal plantar strain regimes.

Functionality Study of an Optical Measurement Concept for Local Force Signal Determination in High Strain Rate Tensile Tests

Many mechanical material properties show a dependence on the strain rate, e.g. yield stress or elongation at fracture. The quantitative description of the material behavior under dynamic loading is of major importance for the evaluation of crash safety. This is carried out using numerical methods and requires characteristic values for the materials used. For the standardized determination of dynamic characteristic values in sheet metal materials, tensile tests performed according to the guideline from [1]. A particular challenge in dynamic tensile tests is the force measurement during the test. For this purpose, strain gauges are attached on each specimen, wired to the measuring equipment and calibrated. This is a common way to determine a force signal that is as low in vibration and as free of bending moments as possible. The preparation effort for the used strain gauges are enormous. For these reasons, an optical method to determine the force by strain measurement using DIC is presented. The experiments are carried out on a high speed tensile testing system. In combioantion with a 3D DIC high speed system for optical strain measurement. The elastic deformation of the specimen in the dynamometric section is measured using strain gauges and the optical method. The measured signals are then compared to validate the presented method. The investigations are conducted using the dual phase steel material HCT590X and the aluminum material EN AW-6014 T4. Strain rates of up to 240 s-1 are investigated.

Measurement and Analysis of Deformation Behavior of Polyethylene Tube Subjected to Linear Loading and Unloading Stress Paths with Large Strain

A material testing apparatus for measuring the biaxial deformation behavior of a polymer tube has been developed to quantitatively evaluate the deformation behavior of polymeric materials. The testing apparatus can apply axial force and internal pressure to a tubular specimen. A noncontact strain measurement system was also developed, and the biaxial strain components and the radius of curvature in the axial direction of the bulging specimen are continuously measured to control the stress path applied to the specimen. Polyethylene tube with an outer diameter of 17 mm and a thickness of 2 mm are used as a test sample. The tubular specimens were subjected to linear stress paths with stress ratios of σΦ:σθ =1:0, 4:1, 2:1, 4:3, 1:1, 3:4, 1:2, 1:4, and 0:1, where σΦ and σθ are the axial and circumferential stress components, respectively, applied to the central area of the bulging specimen. Loading and unloading tests were performed to determine the biaxial true stress-logarithmic plastic strain curves. The strain rate was 1×10-3s-1. From these test results, contours of plastic work and the directions of the plastic strain rates were measured to identify a proper material model for the test sample using the Yld2000-2d yield function (Barlat et al., 2003).

A comparative study on the effect of grid printing methods and camera hardware in automatic strain measurement through CGA

Strain analysis is an important step in sheet metal forming operations to identify the potential locations of failure. Circle grid analysis is a widely used method to evaluate the strains in sheet metal forming, where deformed circles on a formed component are measured to evaluate the strains at different locations. In this paper, two different USB cameras are used to capture the images of the deformed grids. The images are processed using the in-house software developed in MATLAB for strain measurement. The effect of image acquisition hardware and grid printing methods on strain measurement accuracy has been analyzed. For this, the sheets are printed with circular grids by screen printing, laser engraving, and electrochemical etching. The sheets are then deformed in incremental forming to produce different levels of strain in the formed components. The deformed grids on the formed components are captured using proposed USB cameras with appropriate attachments and processed by in-house strain measurement software. The strain measured using proposed hardware and software is also compared with the commercial strain measurement system. The mean absolute percentage error in strain measurement with the proposed USB cameras was found to be 0.671 and 0.761. The statistical results show that there is no evidence to support the hypothesis that the two hardware perform differently on average. Hence, the proposed hardware and software can be used in industry and research laboratories for strain analysis in sheet metal forming operations.

Effects of combination mode on mechanical properties of bi-material samples consisting of rock and coal

We performed uniaxial compression tests on coal–rock, rock–coal, and rock–coal–rock bi-material samples. We studied their mechanical behavior using an acoustic emission monitoring and digital speckle strain measurement systems. The average uniaxial compressive strengths of the rock–coal–rock, coal–rock, and rock–coal bi-material samples were 20.54 MPa, 14.28 MPa, and 14.03 MPa, respectively. The strength of the coal determines the overall strength of the bi-material sample. However, the overall strengths of the coal–rock, rock–coal, and rock–coal–rock bi-material samples were greater than that of coal by 11.76%, 10.19%, and 31.84%, respectively. A crack in the coal part of a bi-material sample can develop and spread to the rock part, thus resulting in its splitting failure. The rock–coal–rock bi-material samples exhibited plastic failure, while the coal–rock and rock–coal bi-material samples exhibited brittle failure. Progressive failure of the coal sample induced rebound deformation of the rock sample. The rebound deformation rates of the rock–coal–rock, coal–rock, and rock–coal samples were 0.57%, 0.09%, and 0.12%, respectively. In the pre-peak stage, the average elastic energy of the rock–coal–rock bi-material sample was 92.57%, indicating a strong ability to store external energy. In the post-peak stage, the average post-peak release energy of the rock–coal–rock bi-material sample was the largest, but the average surplus energy was the smallest. Therefore, the rock–coal–rock bi-material exhibited the highest macro-cracking but the dynamic behavior was the least prominent. The coal–rock and rock–coal bi-material samples exhibited less prominent macro-cracking after failure but a more pronounced dynamic behavior.

Bonding performance of the grooved interface between ultrahigh performance concrete and normal concrete

• The shear test was conducted to study the bond behavior between UHPC prefabricated and NC post-cast. • Effects of NC strength and groove density on UHPC-NC bond behavior were investigated. • The deformation and shear strain were monitored using a non-contact 3D speckle strain measurement and analysis system. • Grooving methods of interface preparation develops a good interface bond. • Mathematical models to estimate grooved interface cohesion following Mohr-Coulomb theory are built. When a diseased tunnel is reinforced by a UHPC-NC composite structure, the grooved interface has good bonding performance to ensure that the UHPC precast pipe co-works well with postcast NC. In this paper, the bonding properties of the UHPC-NC grooved interface were investigated by both slant shear tests and double-sided shear tests. Two factors, i.e., NC strength and groove density, are mainly considered. The results showed that the slant shear sample failed in the form of interface failure + NC failure or NC failure. The failure mode of the double-sided shear test was that the NC groove at the interface was cut off. The higher the NC strength is, the greater the shear strength of the interface. The increasing groove density leads to a high bond strength of the double-sided shear specimen, but the strength of the slanted shear specimen is basically unchanged. According to the failure mode of two-sided shear test, the shear bearing mechanism of UHPC-NC grooved interface is obtained, and the cohesion calculation formula of UHPC-NC slot interface is established. The results might provide a reference for design and construction of the interface of the UHPC-NC composite structure for tunnel reinforcement.