Experimental Techniques最新文献

During the practical service condition, the strain-controlled fatigue is an important reason for component failure, which cannot be demonstrated by the traditional fatigue crack growth test under stress-controlled mode in accordance to ASTM E647. Therefore, this study is devoted to investigating the fatigue crack propagation behaviour of U-notched specimen under strain-controlled mode. The numerical study is firstly performed to clarify the stress/strain concentration effect and crack tip mechanical behaviour, which is then validated by strain-controlled fatigue tests. The compliance method is also adopted to calibrate the crack length obtained by optical measurement. A modified geometry factor for stress intensity factor (SIF) K considering short crack is proposed for comparison with a J-integral solution based on Electric Power Research Institute (EPRI) method. Digital image correlation (DIC) technology is also adopted to capture the strain field at crack tip to validate the numerical strain distribution. Moreover, different fracture parameters, including ΔK and ΔJ, are applied to characterize the crack driving force. It is shown that the strain concentration phenomenon at the notch root can reflect the accumulation of fatigue damage. The ΔK under small scale yielding (SSY) situation is not applicable due to the large plastic deformation occurring at the crack tip. Whereas, agreement is found between crack propagation rate and the fracture parameter ΔJ based on the elastic-plastic fracture mechanics (EPFM).

The low wave velocity of stress waves in viscoelastic polymer materials makes it difficult for viscoelastic materials to maintain dynamic equilibrium in dynamic loading experiments. There is a scarcity of research findings on dynamic experiments of viscoelastic materials, particularly in the realm of dynamic tensile tests. To investigate the dynamic tensile mechanical behavior of viscoelastic materials, polyurethane (PU) was chosen as the subject of study. Electronic universal testing machines were used to conduct quasi-static tensile experiments at strain rates of 0.001(:{s}^{-1}), 0.005(:{s}^{-1}), and 0.025(:{s}^{-1}), verifying the tensile mechanical behavior of viscoelastic PU under quasi-static experimental conditions. Using the split Hopkinson tensile bar (SHTB) as the dynamic loading apparatus, we simulated and verified the propagation patterns of dynamic tensile waves under various experimental scenarios. The most suitable connection scheme was then selected for experimental validation. Based on the macro and micro experimental analysis results obtained under the incident tensile wave, we determined the dynamic tensile experimental scheme for viscoelastic materials that fulfills the stress equilibrium condition. Additionally, we obtained the design results for the incident wave shape and the dimensions of the dynamic tensile samples.

In oil and gas production operations, complex and variable cyclic external loads can trigger bolt loosening, leading to flange leakage, blowout, and other severe accidents. In-service flange leak monitoring is challenging owing to the limited working conditions and equipment integration costs. In addition, most current seal evaluation methods are not applicable to verification in vibrational working conditions. This study proposes a method for predicting the leakage rate of bolted flange joints based on the degree of bolt loosening under dynamic load conditions, which can directly monitor and evaluate the sealing status of flanges in service in real time. This method employs a low-cost, highly stress-sensitive, and stable magnetic measurement method to determine the change in the loosening index of the bolt, analyse its influence on the contact stress of the gasket by finite-element simulation, and propose a leakage prediction model based on critical contact mechanical parameters by simulating contact with a rough surface. The method was verified by a bolt loosening and flange leakage monitoring test under axial cyclic loadings, and provides a new method for evaluating the sealing performance of in-service flanges.

This research explores the application of Python-driven automated machine vision algorithms for inspection in sheet metal forming, a critical manufacturing process. The study addresses the need for advanced, reliable, and efficient inspection techniques to enhance quality control, thereby improving product performance and manufacturing efficiency. The methodology used in this research involves inspecting formed sheet metal products using Python-based methods, namely the Structural Similarity Index Measure (SSIM) and Normalized Cross Correlation (NCC), along with MATLAB for image correlation, are applied directly for contour inspection. In addition to contour inspection, feature detection, which includes dimensional measurement, is also carried out as a critical part of assessing the quality and performance of the formed sheet metal products. This research integrates machine vision algorithms with Python, offering a comprehensive inspection of sheet metal products. The use of Python-based methods and the Hough Transform (HT) algorithm for inspecting sheet metal formed components introduces a novel approach with immense potential for enhancing efficiency in the quality control of the sheet metal inspection process. This signifies a notable breakthrough in automated inspection within the sheet metal forming industry, allowing comprehensive inspection of both features and dimensional measurements. By adopting the most effective method, manufacturers in the sheet metal fabrication field can enhance inspection efficiency and accuracy, thereby improving product quality and operational performance.

High quality measurements are paramount to a successful application of experimental techniques in structural dynamics. The presence of noise and disturbances can significantly distort the information stored in the data and, if not adequately treated, may result in erroneous findings and misleading predictions. A common technique to filter out noise relies on decomposing the dataset into singular components sorted by their degree of significance. Discarding low-value contributions helps to clean the data and remove spuriousness. This paper presents PRANK, a novel singular value-based reconstruction approach for multiple-response vibration datasets. PRANK integrates the effect of Principal Response Functions and Hankel filtering actions, resulting in an improved data reconstruction for both system poles and zeros. The proposed formulation is tested on both analytical and numerical examples, showcasing its robustness, efficiency and versatility. PRANK operates with both time- and frequency-based data. Applied to noisy full-field camera measurements, the filter delivered excellent performance, indicating its potential for various identification tasks and applications in vibration analysis.

We present a novel experimental setup called miniBLAST, which enables systematic and repeatable laboratory tests of structures subjected to blast loads. The explosive source is based on the discharge of high electrical loads on a thin conductor, producing repeatable blast-type shock waves of controlled intensity. Conducting blast experiments under safe laboratory conditions offers significant advantages over large-scale experiments, which are expensive, require specialized personnel, and face repeatability issues. In this work, we provide a comprehensive description of the setup’s design rationale and technical characteristics. We place particular emphasis on the installation phases, safety, and metrology. The explosive source is analyzed and the signature of blast waves is retrieved. Finally, we present an example of a masonry wall subjected to blast loads of varying intensity. We then report its dynamic response in three dimensions and at different time instances. This new experimental setup offers a cost-effective, safe, and repeatable method to study structural dynamics under blast loads, with results that can be upscaled to real structures. It also aids in evaluating numerical models and lays the groundwork for further investigations into blast effects and mitigation.

Dynamic substructuring, especially the frequency-based variant (FBS) using frequency response functions (FRF), is gaining in popularity and importance, with countless successful applications, both numerically and experimentally. One drawback, however, is found when the responses to shocks are determined. Numerically, this might be especially expensive when a huge number of high-frequency modes have to be accounted for to correctly predict response amplitudes to shocks. In all cases, the initial response predicted using frequency-based substructuring might be erroneous, due to the forced periodization of the Fourier transform. This drawback can be eliminated by completely avoiding the frequency domain and remaining in the time domain, using the impulse-based substructuring method (IBS), which utilizes impulse response functions (IRF). While this method has already been utilized successfully for numerical test cases, none of the attempted experimental applications were successful. In this paper, an experimental application of IBS to rods considered as one-dimensional is tested in the context of shock analysis, with the goal of correctly predicting the maximum driving point response peak. The challenges related to experimental IBS applications are discussed and an improvement attempt is made by limiting the frequency content considered through low-pass filtering and downsampling. The combination of a purely time domain based estimation procedure for the IRFs and the application of low-pass filtering with downsampling to the measured responses enabled a correct prediction of the initial shock responses of the rods with IBS experimentally, using displacements, velocities and accelerations.

Carbon fiber-reinforced polymer (CFRP) composites are expected to be increasingly adopted in automotive structures to achieve car weight reductions that yield an effective reduction of carbon dioxide emissions. Fatigue damage evaluation of CFRP composites is indispensable to guarantee their long-term use. In a thermography-based approach, a suitable temperature component to evaluate the fatigue damage of discontinuous CFRP composites—CFRP composites of discontinuous fibers—should be elucidated. In this study, the non-dimensional thermoelastic temperature amplitude, obtained through thermoelastic temperature variation measurements, is employed. This amplitude can be used to evaluate the stress state of a material subjected to cyclic loading. Moreover, the relationship between the non-dimensional thermoelastic temperature amplitude and fatigue damage is investigated under tension–tension cyclic loading for carbon fiber sheet molding compound (C-SMC), which is a discontinuous CFRP composite produced via sheet molding compound methods. Experimental results reveal that a decrease in the non-dimensional thermoelastic temperature amplitude is associated with the fatigue damage. This decrease is attributed to two factors. One is a change in the stress state applied in the longitudinal and transverse directions of the fiber caused by a shift in the dominant fiber orientation as fatigue damage progresses. The other is the difference in the thermal expansion coefficients in the longitudinal and transverse directions of the fiber. Therefore, the possibility of monitoring the fatigue damage evolution using the non-dimensional thermoelastic temperature amplitude is confirmed. Future studies should assess the remaining fatigue life of CFRP composites using thermoelastic temperature variations.

Earthquakes pose a significant threat to cities worldwide, necessitating the development of effective and affordable methods to evaluate structural performance under dynamic loading. Although large-scale shake table testing of full structures offers the most accurate approach, the high costs involved make it impractical for many, especially research institutions in developing countries. This study addresses this limitation by developing an affordable, single-degree-of-freedom shake table. The developed shake table is displacement-controlled with a payload capacity of 200 kg, offering a practical solution for testing scaled-down models of real structures. It is powered by a brushed DC motor, which is widely available in developing regions, and utilizes a modified proportional-derivative (PD) control algorithm to simulate both harmonic and real earthquake motions. Using this design, displacements up to ±150 mm and accelerations up to 1g were achieved with acceptable error margins. This paper provides a comprehensive explanation of the shake table’s mechanical design, control algorithm, hardware components, and user interface. It also presents comparisons between theoretical and experimental displacements and accelerations for sine wave and earthquake simulations under various payloads. The results highlight the effectiveness of this low-cost shake table as a viable tool for seismic testing, expanding access to critical structural evaluation in resource-limited settings.