Experimental Techniques最新文献

For the structural health monitoring of in-service structural components, an important topic is to accurately assess the changes in their microstructure and performance through effective methods. Conventionally, magnetic non-destructive testing technology is very sensitive to changes in microstructure, making it highly suitable for online monitoring of product quality. Based on hysteresis loop and X-ray diffraction techniques, the relationships between microstructure, residual stress and magnetic properties of the full-scale 35CrMo steel components during the heat treatment were studied. Compared to the original state, the quenching and tempering heat treatment processes resulted in significant variations in the magnetic coercive force and the surface residual stresses, which were effectively characterized by the X-ray diffraction cosα method with the Debye–Scherer ring. It was found that the increment in the coercive field has occurred due to the domain wall pinning caused by grain refinement, precipitated carbides and compressive residual stress, while the hardness was increased mainly by precipitation hardening. In particular, the coercivity exhibited a good fit with the hardness and strength, and the correlation coefficients were 0.96 and 0.98, respectively. The current state of the arts allows to forecast the possibility to apply magnetic coercivity measurements for monitoring the microstructure and properties of steel products. However, in order to ensure the reliability of the measurement, it is recommended to establish a standardized procedure for magnetic measurement to reduce the influence of external interference.

Accurate uncertainty quantification (UQ) in digital image correlation (DIC) deformations is essential for quantitative DIC-based finite element (FE) model validation. DIC UQ is well-studied in the current literature, both from a theoretical as well as experimental point-of-view, but rarely from the model validation perspective. Moreover, the DIC uncertainties are usually considered as spatial averages over the whole field of view while local contrast variations generally lead to spatially-varying noise floors. This paper investigates how DIC UQ should be performed when validating FE models. UQ was performed using experimental stationary images of a test sample. Spatial maps of point-wise temporal standard deviation (noise) and mean (bias) were constructed to be used in the model validation process. The effectiveness of reference image averaging at reducing bias and noise was also studied. Specular reflection (‘hotspots’) was given special attention, an important additional source of uncertainty not simulated by the Digital Twin (DT) used to perform the validation. As expected, image noise was found to be the most dominant source of DIC uncertainty. The spatially-random noise on the reference stationary image was found to be responsible for the temporal bias of the displacement distribution, as the copy of noise from that initial image affects all displacement maps since this image is used for all displacement maps. Spatially-random noise on the deformed stationary images was found to be responsible for the temporal standard deviation (noise). Both temporal noise and bias were found to be comparable in magnitude, highlighting the necessity for a spatially heterogeneous model validation criterion that accounts for both. The impact of specular reflection was difficult to quantify and exhibits potential for significantly increasing DIC uncertainties. The use of polarized lights and polarizing filters can mitigate this issue but more work is needed to allow for a realistic error budget to be established for this. Heat haze (refraction from warm air flow between camera and object) and camera heating are additional effects that are difficult to error-budget for. Finally, the effect of stereo-DIC calibration errors needs to be studied further.

Accurate measurement of dynamic stresses in concrete is important for mechanical analysis and theoretical innovation of concrete materials. In dynamic stress measurements of concrete, the matching error between the transducer and the concrete is the controlling error in the total error. To improve the measurement accuracy, in this study, the dynamic stress test method was investigated by considering the matching error. The propagation law of the time- and frequency-domain characteristics of the concrete stress under an explosion load was obtained through simulation, and the measurability of the stress was analyzed. The installation range and the minimum installation spacing of the transducers was analyzed based on the control of the matching error, and a matching error with a practical value was obtained. A stress transducer installation method was designed to meet the measurement requirements, and explosion tests were carried out. The results show that the peak value and bandwidth of the stress signal decayed rapidly with increasing propagation distance. The peak value decays exponentially, while the bandwidth decreases rapidly in the near field of the explosion to near a stable value and then rises gradually in the region close to the free boundary as the propagation distance increases. The minimum distance of the transducer from the explosives is 25 cm. The minimum distance of the transducer from the concrete boundary is 20 cm. The minimum installation distance of the transducers is 18 times the transducer thickness.

This study addresses the growing need for precision static low-force measurement in industries and metrology. Traditional force measurement systems rely on contact-based methods, involving the attachment of deflection sensors to the spring element, which can lead to electronic complexity and limited robustness, especially for low-force measurement. Therefore, this study demonstrates and presents a novel and robust force sensing approach for static low-force measurement by introducing a simple and easily implementable non-contact force sensing method. The research begins with the designing and modelling of cross beam spring element, followed by virtual testing using Ansys Finite Element Analysis (FEA) software to determine the maximum induced stress for validating the design for a 5 N load capacity. Additionally, the FEA study explores the optimal detectable deflection to assess the feasibility of utilizing speckle pattern imaging techniques for non-contact force measurement. Experimental simulated results reveal a linear correlation between angular deflection and exerted force with a calibration constant of approximately 0.0008 radians per Newton. This approach has offered a promising and efficient solution for precision static low force measurement and could also be used in other fields of metrology.

Impedance Matched Multi-Axis Tests (IMMATs) can replicate in-service vibration induced stress more accurately than single axis shaker table tests as they can better match a part’s operational boundary conditions and excite it in multiple degrees of freedom simultaneously. The shakers used in IMMATs are less powerful than shaker tables, so shaker force limits can be exceeded during tests if they are not placed adequately for the desired environment. The ability to predict shaker voltage and force before performing a test is, therefore, helpful in selecting shaker locations so that their limits are not exceeded. In this study, electrodynamic shakers were modeled as discrete electromechanical systems, and the shaker parameters were chosen to match experimentally obtained acceleration/voltage frequency response functions (FRFs). These models were coupled to a finite element model of the device under test (DUT) via dynamic substructuring, and the substructured model was demonstrated to accurately predict shaker voltage as well as the error in reproducing the environment at multiple accelerometer locations. A simple method called the FRF Multiplication method, in which the FRF of the substructured system is approximated as the product of two separate FRFs of the shaker and DUT respectively, was proposed and applied to the same system, yielding similar voltage and error predictions to those obtained using substructuring. Simple case studies were presented to explore the applicability of the proposed method, and it was demonstrated to have similar accuracy to the substructuring method in a range of cases. Additionally, we showed that while it was not possible to derive a unique model of the shakers from acceleration/voltage FRFs alone, the models that could be obtained were sufficient to predict test error almost perfectly and shaker voltage with less than 40 percent error.

The article proposes a method to calculate the main direction strain inside the carbon fiber laminate based on the measured strain on the surface. The method is to paste strain gauges on the surface of the laminate, and the main direction strain inside the laminate is deduced through the selection of strain gauges, the determination of the paste method, the acquisition and processing of data, and the combination of the relevant theories of mechanics of materials and mechanics of composite materials. This method effectively solves the difficult problem of strain measurement inside carbon fiber laminates. The article validates the method with different layers of the laminate as test objects. The accuracy of the strain measurement of the surface layer and the validity of the calculation of the main directional strain of the internal layers based on the measured strain are verified by means of tests and finite element software calculations.

The fatigue damage spectrum (FDS) model characterizes how the damage potential is distributed over the excitation frequency range, similarly to how the power spectral density (PSD) characterizes the distribution of the excitation level. However, reproducing the in-service PSD during vibration testing on shakers does not necessarily mean that the FDS would be also reproduced. This is because some peaks, higher than those in a signal generated from the PSD, occur in vibrations of automobiles, trucks, and railway vehicles. Presence of these peaks in real in-service vibrations and their absence in the PSD-based random testing is the reason why the FDS obtained by the ordinary PSD control of the shaker is different in shape and usually lies below the in-service FDS. It is shown in the paper that the FDS shape as a function of frequency can be controlled by manipulating some of the IFFT phases instead of prescribing all of them randomly. Since the phase manipulation does not affect the excitation PSD, the FDS can be controlled with the PSD preserved. It was demonstrated in the paper for an example of automobile vibration that, when the ordinary PSD imitation failed in the numerical simulations, the developed PSD + FDS imitation was able to match not only the FDS of in-service vibration with non-Gaussian high peaks but also the accelerated FDS profile, which was 3 times higher than the in-service FDS. Numerical results have also shown that the FDS precision is maintained if the testing continues longer than the duration of the measured data record.

A Gleeble thermal–mechanical simulator equipped with an induction heating system was employed to perform single pass compression test to study the dynamic recrystallization behavior of a high-strength low-alloy steel in a wide range of temperatures ( 900℃-1050℃) and strain rates (0.1 s⁻¹-10 s⁻¹). Based on the flow behavior of the tested steel, a new method has been proposed to determine the stress or strain for the onset of dynamic recrystallization. The resultant stresses or strains are compared with the ones determined by using the previous method to confirm the validity of the new method. Moreover, the stress or strain for the onset of dynamic recrystallization is modeled using Zener-Hollomon parameter. With the assistant of the process parameters, temperature, strain and stress, the activation energy has been determined. Finally, the dynamic recrystallization kinetics model is constructed and the validity is tested.

As the demand for aluminum alloys in various industries continues to grow, the development of robust welded joints has become an essential consideration. Traditional welding techniques struggle to meet the rigorous demands of modern applications. The aim of this study is to explore alternative welding methods, focusing on friction stir welding (FSW) and its underwater variant, underwater friction stir welding (UFSW), using Mediterranean seawater cooling, to join 2017AA aluminum alloy. From a methodological point of view, the study includes experimental measurements of thermal cycling and a comparative analysis of the resulting microstructures in weld joints. The unique cooling environment provided by Mediterranean seawater is a key variable in this analysis. The results reveal important information, particularly for the UFSW technique, where the introduction of Mediterranean seawater cooling leads to a substantial reduction in maximum temperatures and a decrease in plastic deformation level confirmed through fracture analysis in mechanical study. In addition, this cooling environment promotes the formation of a finer grain structure in the weld zone, exceeding the characteristics observed in conventional FSW joints. The potential benefits observed, including improved microstructure, corrosion resistance and wear resistance.