Experimental Mechanics最新文献

Background

Excessive levels of residual stresses (RS) in metallic components of flexible pipes can lead to failure by fatigue or stress-corrosion cracking (SCC) mechanisms. Accurate measurements of residual stresses are essential in order to support comprehensive integrity assessments of these structures.

Objective

This paper aims at proposing a methodology for in-field evaluation of the residual stresses in the tensile armour of flexible pipes. This methodology is evaluated by comparing results from the contour method (CM), a portable (PXRD) and a benchtop (BXRD) X-ray diffractometer.

Methods

A set of samples with a controlled RS state was used to evaluate PXRD depth-profiles by comparison with BXRD measurements. CM was used to investigate stress values elsewhere in the cross-section of the samples. PXRD was then used to measure the total stress on a full-scale flexible riser.

Results

Strong texture was seen in XRD measurements due to the preferential orientation in the samples, but a method to overcome this is proposed. RS measurements with PXRD were in close agreement with BXRD values in all samples. Total stress profiles obtained by PXRD on a tensile armour of flexible pipe are given. CM did not provide reliable results in the near-surface region of shot-peened samples, but reasonable agreement was found with BXRD after manual adjustment of a smoothing parameter when depths from 0.3 mm to 1.5 mm were analyzed.

Conclusion

A methodology has been established for non-destructive in-loco profiling of the total stress state in tensile armours of flexible pipes with PXRD; in a real application scenario it could provide valuable information that could help to understand damage initiation mechanisms.

Background

Deep learning-based digital image correlation (DL-based DIC) has gained increasing attention in the last two years. However, existing DL-based DIC algorithms are impractical because their application scenarios are mostly limited to small deformations.

Objective

To enable the use of DL-based DIC in real-world general experimental mechanics scenarios that would involve large deformations and rotations, we propose to improve DL-based DIC with the domain decomposition method (DDM).

Methods

In the improved method, the region of interest is divided into subimages, and subimages are pre-aligned using the preregistered control points to effectively eliminate the large deformation components. The residual deformations in each subimage are small and limited, which can be well extracted using existing DL-based DIC methods.

Results

Through synthesized and real-world experiments, the improved DL-based DIC method can achieve high-accuracy pixelwise matching in practical applications with strong robustness and high computational efficiency.

Conclusions

The improved DL-based DIC combines the advantages of traditional and DL-based DIC methods but overcomes the limitations, greatly improving the robustness and applicability of existing DL-based methods.

Background

DIC is a widely used optical method that uses cameras to track the motion of an applied random surface pattern to measure the full-field deformation. Due to its non-contacting nature, DIC is very preferable to be used in the areas of high temperature experimental mechanics. One of the biggest challenges of DIC at extreme temperatures is the blackbody radiation emitted from the glowing surface of the specimen. This glow from the blackbody radiation of the specimen is relatively higher at longer wavelengths and lower at shorter wavelengths.    

Objective

Previously, studies have shown the usefulness of using shorter wavelength of lights such as blue filtered light (450 nm) and UV-A filtered light (365 nm) for high temperature measurements. By contrast, this study uses UV-C filtered technique which utilizes even shorter wavelength of filtered light (UV-C, 254 nm) to demonstrate its effectiveness at elevated temperatures.

Methods

Four different DIC techniques using an unfiltered blue light (200–1000 nm), a blue filtered light (450 nm), a UV-A filtered light (365 nm), and a UV-C (254 nm) filtered light have been performed at extreme temperatures in this study. 

Results

It was found that the techniques using unfiltered blue, blue filtered, and UV-A filtered lights could only go up to a temperature of 900 °C, 1200 °C, and 1600 °C respectively before showing significant saturations in the images.

Conclusions

The new UV-C DIC showed no sign of saturation even up to a temperature of 1600 °C while providing comparable axial displacement and coefficient of thermal expansion (CTE) data and therefore demonstrating the usefulness of this method in higher temperatures. We also include helpful recommendations for how to produce speckle patterns having sufficient contrast at UV-C wavelengths.

Background

Main desired features of biaxial tests are: uniformity of stresses and strains; high strain levels in gauge areas; reliable constitutive parameters identification. Despite cruciform specimen suitability to modern tensile devices, standard testing techniques are still debated because of difficulties in matching these demands.

Objective

This work aims at providing rational performance objectives and efficient cruciform specimens shapes in view of constitutive characterization.

Methods

Objective performance is evaluated along particular lines lying on principal directions in equibiaxial tensile tests. A rich specimen profile geometry is purposely optimized via finite elements analysis by varying cost function and material compressibility. Experimental tests, monitored via digital image correlation, are carried out for validation.

Results

New shapes are designed and tested in a biaxial tensile apparatus and show to perform better than existing ones. Elastic parameter identification is efficiently performed by only exploiting full field strain measurements along statically significant lines.

Conclusions

Small gauge areas and small fillet radii cruciform specimens approach the ideal deformation behaviour. For the constitutive parameters identification in planar tensile experiments, it suffices to monitor strains along the gauge lines.

Background

For the static loading test in the aerospace field, conventional strain field reconstruction methods relying on finite element analysis (FEA) or test data are difficult to meet the accuracy requirements of test monitoring.

Objective

This study aims to construct a high-accuracy strain field for real-time test monitoring.

Methods

An improved strain field reconstruction method based on digital twin (DT) named as DT-SFRM is proposed. The DT is built by data fusion of FEA results and test data, which combines the benefits of these data. The FEA conducted before formal test provides approximate strain field distribution, and the strain gauges data with high accuracy are used to modify FEA strain fields in real time. After that, the real-time DT is used to determine the possible risk regions of test articles. Finally, a large opening cylindrical shell (LOCS) buckling test is conducted to validate the advantages of DT-SFRM.

Results

Results show that the accuracy of DT-SFRM is much higher and less affected by the nonlinearity of test data than that of conventional methods. Compared with the time cost by conventional real-time FEA (about 50 min), the DT method only takes 9s to reconstruct strain field, and the possible risk regions predicted by DT-SFRM are more consistent with test buckling regions of LOCS than conventional methods.

Conclusions

The DT-SFRM is validated to have a higher accuracy and better monitoring effect, and it is more suitable for test monitoring of complex structures.

Background

Hole drilling is a measurement technique used to determine near surface residual stresses and has been codified in ASTM E837-20. In ASTM E837-20, the minimum allowable distance to a free edge is prescribed as 1.5 times the gauge circle diameter.

Objective

This work examines the effect arising from the distance from a free edge on a hole drilling measurement and provides an approach to determine residual stress for measurements where the edge distance is closer than that currently permitted by ASTM E837-20.

Methods

Numerical experiments were performed to understand how the compliance matrices change when the distance from a hole drilling measurement to a free edge varies. In addition, a series of hole drilling measurements were performed at various distances from a free edge using a shot peened aluminum plate with a nominally equibiaxial stress state to demonstrate the approach.

Results

The numerical experiments determined that the use of corrected compliance matrices is appropriate when the edge distance is as small as 0.35 times the gauge circle diameter. Physical measurements supported the use of custom compliance matrices for a given free edge distance and specimen thicknesses.

Background

Incremental hole-drilling (IHD) has shown its importance in the measurement of the residual stress distribution within the layers of composite laminates. However, validation of these results is still an open issue, especially near the interfaces between plies.

Objectives

In this context, this study is focused on experimentally verifying its applicability to fibre metal laminates.

Methods

Tensile loads are applied to cross-ply GFRP-steel [0/90/steel]s samples. Due to the difference in the mechanical properties of each ply, Classical Lamination Theory (CLT) predicts a distribution of the uniform stress within each layer, with pulse gradients between them. The interfaces act as discontinuous regions between the plies. The experimental determination of such stress variation is challenging and is the focus of this research. A horizontal tensile test device was designed and built for this purpose. A differential method is used to eliminate the effect of the existing residual stresses in the samples, providing a procedure to evaluate the ability of the IHD technique to determine the distribution of stress due to the applied tensile loads only. The experimentally measured strain-depth relaxation curves are compared with those determined numerically using the finite element method (FEM) to simulate the hole-drilling. Both are used as input for the IHD stress calculation method (unit pulse integral method). The distribution of stress through the composite laminate, determined by classical lamination theory (CLT), is used as a reference.

Results

Unit pulse integral method results, using the experimental and numerical strain-depth relaxation curves, compare reasonably well with those predicted by CLT, provided that there is no material damage due to high applied loads.

Conclusions

IHD seems to be an important measurement technique to determine the distribution of residual stresses in fibre metal laminates and should be further developed for a better assessment of the residual stresses at the interfaces between plies.

Background

Hydrogels are one of the most ubiquitous polymeric materials. Among them gelatin, agarose and polyacrylamide-based formulations have been effectively utilized in a variety of biomedical and defense-related applications including ultrasound-based therapies and soft tissue injury investigations stemming from ballistic and blast exposures. Interestingly, while in most cases accurate prediction of the mechanical response of these surrogate gels requires knowledge of the underlying finite deformation, high-strain rate material properties, it is these properties that have remained scarce in the literature.

Objective

Building on our prior works using Inertial Microcavitation Rheometry (IMR), here we present a comprehensive list of the high-strain rate (> 10(^3) 1/s) mechanical properties of these three popular classes of hydrogel materials characterized via laser-based IMR, further showing that the choice in finite-deformation, rate-dependent constitutive model can be informed directly by the type of crosslinking mechanism and resultant network structure of the hydrogel, thus providing a chemophysical basis of the the choice of phenomenological constitutive model.

Methods

We analyze existing experimental gelatin IMR datasets and compare the results with prior data on polyacrylamide.

Results

We show that a Neo-Hookean Kelvin-Voigt (NHKV) model can suitably simulate the high-rate material response of dynamic, physically crosslinked hydrogels like gelatin, while the introduction of a strain-stiffening parameter through the use of the quadratic Kelvin-Voigt (qKV) model was necessary to appropriately model chemically crosslinked hydrogels such as polyacrylamide due to the nature of the static,covalent bonds that comprise their structure.

Conclusions

In this brief we show that knowledge of the type of underlying polymer structure, including its bond mobility, can directly inform the appropriate finite deformation, time-dependent viscoelastic material model for commonly employed tissue surrogate hydrogels undergoing high strain rate loading within the ballistic and blast regimes.

Background

Nanoindentation experiments require the calibration of the tip area function, which involves up to 9 fitting coefficients following classical method. These coefficients are determined from indentation tests on a reference material. However, their values may vary from one test batch to another. Consequently, these coefficients cannot describe the amplitude of the indenter tip defect.

Objective

The main objective of this study is to propose a contact area function that uses only one fitting coefficient to represent the indenter tip defect. This coefficient corresponds to the distance between the blunt and ideal indenter tip.

Methodology

To demonstrate the efficiency of the proposed contact area function, we reanalyzed nearly 40 calibration procedures, while keeping the same experimental protocol, performed between 2014 and today. A novel two-step calibration methodology is advanced. We compared the results of the proposed method to those obtained with the classic methodology.

Results

This two-step methodology was applied to a fused silica calibration sample. The values of the Young's modulus and instrumented hardness are equals to 71 and 10 GPa, respectively. The length of the indenter tip defect increases gradually from 5 to 30 nm accordingly to the frequency of use of the indenter. The values of the mechanical properties calculated by this methodology are in good agreement with those obtained using the classical contact area function.

Conclusion

The methodology presented in this paper demonstrates its ability to accurately calibrate the tip area function. This new calibration procedure considers both the Young’s modulus and the tip defect parameter as free parameters. Furthermore, the calibration parameters have a clear physical meaning and their values remain stables from one batch to another.