Experimental Mechanics最新文献

Background

In a previous work, the problem of identifying residual stresses through relaxation methods was demonstrated to be mathematically ill-posed. In practice, it means that the solution process is affected by a bias-variance tradeoff, where some theoretically uncomputable bias has to be introduced in order to obtain a solution with a manageable signal-to-noise ratio.

Objective

As a consequence, an important question arises: how can the solution uncertainty be quantified if a part of it is inaccessible? Additional physical knowledge could—in theory—provide a characterization of bias, but this process is practically impossible with presently available techniques.

Methods

A brief review of biases in established methods is provided, showing that ruling them out would require a piece of knowledge that is never available in practice. Then, the concept of average stresses over a distance is introduced, and it is shown that finding them generates a well-posed problem. A numerical example illustrates the theoretical discussion

Results

Since finding average stresses is a well-posed problem, the bias-variance tradeoff disappears. The uncertainties of the results can be estimated with the usual methods, and exact confidence intervals can be obtained.

Conclusions

On a broader scope, we argue that residual stresses and relaxation methods expose the limits of the concept of point-wise stress values, which instead works almost flawlessly when a natural unstressed state can be assumed, as in classical continuum mechanics (for instance, in the theory of elasticity). As a consequence, we are forced to focus on the effects of stress rather than on its point-wise evaluation.

Background

Ultra high molecular weight polyethylene composites are fiber based composites used in armor applications. While some characterization has been conducted experimentally, this study varies multiple parameters simultaneously to investigate material response under a wide range of conditions.

Objective

This work focuses on characterizing the response of Dyneema^® HB26 hard laminate composites under high-speed impacts to examine the influence of plate diameter, clamping pressure, and plate spacing on target performance. Additionally, micro Computer Tomography scans are used to nondestructively evaluate the damage evolution in the targets.

Methods

These scan results are used in concert with more traditional armor performance metrics to evaluate the effect of various parameters using the method of orthogonal array analysis. This technique allows for multiple variables to be investigated in the same test series, saving time and budget while still providing quality results across a range of variables and variable values.

Results

We conclude that of the parameters investigated, the plate spacing parameter has the largest effect on performance, followed by the plate diameter. Bolt torque was found to not have a significant impact on results, indicating that an edge clamping pressure is not critical to material response. Additionally, by examining the high resolution scans, we can quantify the damage with an effective damage angle and that this angle is a good predictor of performance.

Conclusion

Finally a damage theory involving the effective bending strength of the plates is discussed as an explanation for all of the results observed in this test series.

Background

Digital image correlation (DIC) has been widely used for motion tracking and estimation, however, the process is often sensitive to the initial guess, especially under large translations and rotations.

Objective

To provide novel and effective solutions for the DIC in measuring large translations and rotations.

Methods

A ring-projection-based two-scale approach is proposed. In the integer-pixel scale, a novel ring projection scheme, including amplitude and phase correlations of the rings, is developed to quickly get the integer-pixel initial estimation of the translations and rotation. In the sub-pixel scale, the gradient-based inverse compositional Gauss-Newton (IC-GN) algorithm, which is free from repeat computation of Hessian matrix, is adopted to efficiently get the optimal motion parameters.

Results

The numerical example show that the absolute error is no more than 0.05 pixel for measured large translations and no more than 0.05(^circ) for measured large rotations. While test experiments on a rotated blade and a flexible arch demonstrate the effectiveness, accuracy and applicability of the proposed approach in measuring the rotating motion, flexible large deformation and vibrational modal parameters of structures.

Conclusions

The ability and effectiveness of the proposed approach for large translations and rotations measurement have been verified. Since large deformations and rotations are frequently encountered in rotating and flexible structures, the proposed approach is believed to constitute a feasible and powerful tool for static and dynamic deformation measurement of these structures.

Composite pressure vessels are seeing increasing demand in the oil and gas sector due to their excellent corrosion resistance. However, the assessment of the fatigue state of those structures still an open question. The goal of this work is use elastic wave data to access the fatigue damage (exudation). The Dynamic Time Warping method is proposed as a means of extracting features from guided wave ultrasound data that can describe the on-going fatigue induced damage of glass-fibre reinforced plastic pipes under fatigue-cycle loading. To test its efficiency, three pipe samples were fatigue tested to failure under internal pressure cycles with maximum values of 45 bar, 55 bar and 65 bar, and minimum pressures equal to 10% of the maximum, at a frequency of 0.8 Hz. A Guided Wave monitoring system consisting of a set of permanently attached piezoelectric sensors produced signals which were processed to obtain the Dynamic Time Warping distance, that was then used to obtain a Damage Index that expresses the cumulative fatigue damage suffered by the samples for each loading level. These results were comparable to data obtained from surface-mounted strain-gauges, even though temperature variations of up to 20 °C occurred during the tests and no direct temperature compensation was applied to the GW signals. The Dynamic Time Warping distance presents smaller influence of temperature and was able to better access the exudation of the samples.

Background

Traditional fatigue testing methods can be expensive due to the need of specialised equipment for engineering materials and structures. Thus, a new fatigue testing approach utilising machining cutting forces to induce cyclic stresses, enabling fatigue life assessment of engineering materials and structures, has been developed.

Objective

This research aims to develop and verify a new testing approach using machining processes to enable the fatigue life assessment of engineering materials and structures. This is achieved by the utilisation of machining-induced cutting forces to generate cyclic stresses into welded samples used in applications of wind turbine monopile structures.

Methods

The methodology employes the development of a fixture encompassed with strain gauges and purposefully designed machining operations to mimic the cyclic stresses experienced in real applications. The machining-based fatigue testing approach was demonstrated on welded samples by replicating cyclic stresses of offshore wind turbine monopiles subject to in-service loads.

Results

The results show that rapid fatigue testing of engineering materials and structures is possible by utilising existing machine tools and centres, which are widely accessible to industry. Cyclic stresses were induced in welded structural steel samples proving the concept of this method.

Conclusion

This novel fatigue testing method showed that cyclic stresses can be induced by machining cutting forces to address real application needs. The key advantages are that this method can be quickly set up in industry, enabling fast fatigue testing that can lead to reduction of lead times for product and process development of industrial components.

Background

Digital Image Correlation (DIC) is an advanced measurement technique capable of capturing full-field surface displacements in a non-invasive manner. However, the application of such measurements in the identification of bimodular materials remains insufficiently exploited.

Objective

Recalibration with Analytic Solution Updating (RAU) has been proposed for the identification of mechanical elastic parameters of asymmetric constitutive law behavior using the Brazilian test. This method accomplishes identification by minimizing the gap between the measurements and the semi-closed form solution.

Methods

Two types of data are employed: the first derived from the semi-closed form solution and the second measured on a 42-day aged mortar specimen using DIC. In the RAU method, three distinct cases are implemented to identify mechanical elastic parameters. These cases are determined by the nature of the data utilized, which can be categorized into axial displacement field, strains at the center, and full-field surface displacement measured on a given specimen area.

Results

The RAU method successfully identified the compressive, tensile Young’s modulus, and the compressive Poisson’s ratio from the surface data provided. The identification with full-field surface displacement presented the highest level of accuracy in the RAU method using the identified results of synthetic data.

Conclusion

The RAU method demonstrates significant accuracy and practicality in identifying the mechanical elastic parameters of bimodular materials.

Background

The interaction of stress fields between cracks or cracks with discontinuities like holes, etc., has been widely studied. Another crucial class of problems include cracks interacting with contact stresses but there has been no work to study them systematically.

Objective

This study aims to understand the role of contact stress in influencing the crack-tip stress field which is essential for reliable estimation of stress intensity factors (SIFs) experimentally.

Method

The contact stress influence on crack-tip isochromatic features is initially discussed using an experimental result for a moderately-deep beam with a small crack. SIFs are evaluated using the over-deterministic nonlinear least squares method. The crack-contact stress interaction is then studied by a superposed crack-contact analytical solution. Photoelastic experiments are conducted for a cracked moderately-deep beam subjected to three-point bending. The SIFs evaluated using the multiparameter solution compare well with finite element predictions. Subsequently, multiple interaction configurations are experimentally examined in a cracked moderately-slender beam by varying the magnitude and position of the contact load relative to the crack.

Results

Even a small crack shows a noticeable change in isochromatics due to influence of contact stress and a two-parameter solution is inadequate here. A multiparameter crack-tip solution is observed to capture the isochromatic fringe field very effectively towards SIF evaluation.

Conclusion

The changes in isochromatics at a crack-tip due to contact stresses are significant. A systematic analysis shows that with appropriate data collection, the multiparameter solution provides SIFs with very little uncertainty in the presence of contact stresses with varying complexities.

Background

The elastic–plastic fracture toughness (J_c) is an important mechanical parameter for studying the failure behavior of air plasma-sprayed (APS) thermal barrier coatings (TBC) at high temperatures.

Objective

This study aims to: (1) develop an effective test method to characterize the J_c of TBC at high temperatures; (2) acquire accurate J_c data for TBC at high temperatures; (3) analyze the influence of plasticity of top-coat on the J_c characterization.

Methods

The elastic–plastic Ramberg–Osgood equation of the ceramic top-coat and the deformation fields of single edge notched tension (SENT) specimens were measured by high-temperature in-situ tension with digital image correlation (DIC) system. The J_c of TBC was calculated by the numerical J-integral with DIC-measured (DIC-J) deformation fields by adopting Ramberg–Osgood equation of the top-coat. The finite element analysis (FEA) method was adopted to analyze the influence of plasticity of top-coat on the J_c characterization.

Results

The curves of J_c varying with crack propagation length (Δa) of TBC were obtained and were expressed as J_R = 24.47 × [ 1 + 1.0446 × ((widetilde{Delta a}))^0.7624] J/m² and J_R = 16.52 × [ 1 + 1.4806 × ((widetilde{Delta a}))^0.6742] J/m² at 800 and 1000 ℃, respectively.

Conclusions

A high-temperature in-situ tensile test of SENT specimens combined with the DIC-J method was developed to characterize J_c of TBC. The J_c of TBC displays a rising resistance curve behavior, and FEA results indicated that J_c would be underestimated without considering the plasticity of the top-coat at 800 and 1000 ℃.

Background

To evaluate the stability of jointed rock masses subjected to dynamic disturbance, laboratory dynamic shear test on rock joint is necessary. Developing dynamic shear test equipment for rock joint is currently a pressing issue.

Objective

To address this issue, a new apparatus is developed to reproduce the shear-slip process of rock joint subjected to dynamic disturbance under various initial stress state.

Methods

The disturbance load, which has a dominant frequency close to that of seismic waves, is generated by an electromagnetic-driven disturbance generator, and its amplitude and duration can be accurately controlled in a stable manner. The initial normal and shear stresses can be applied in the shear test under dynamic disturbance using servo-controlled loading unit, which facilitates the simulation of the real stress state of rock joint.

Results

The shear tests under dynamic disturbance show that when an initial shear stress is applied to rock joint, an additional deformation stage of stress recovering can also trigger a slip displacement, which contributes to the destabilization of jointed rock masses. With increasing initial shear stress, the dynamic slip displacement, stress drop and post-disturbing deformation increase. The feasibility of the apparatus to conduct quasi-static direct shear tests with both the constant normal loading (CNL) and constant normal stiffness (CNS) boundaries is also verified.

Conclusions

Test results demonstrate that using the new apparatus, shear-slip properties of rock joint subjected to dynamic disturbance can be tested in various initial stress states.