Experimental Mechanics最新文献

Background

Needle-induced cavitation (NIC) is a commonly used technique for probing the mechanical properties of soft matter. However, its measurements often exhibit substantial variability, limiting reproducibility and interpretability. Such inconsistencies obscure meaningful trends in material behavior and hinder quantitative comparisons across studies.

Objective

This study aims to identify and quantify key experimental factors contributing to variability in NIC measurements and to develop refined protocols that enhance measurement reproducibility for soft polymer gels.

Methods

NIC experiments were systematically conducted on poly(methyl methacrylate)–poly(n-butyl acrylate)–poly(methyl methacrylate) triblock copolymer gels, focusing on the effects of sample preparation, thermal equilibration, and syringe pump configuration on needle-tip pressurization dynamics and cavitation pressure. A predictive framework for pressurization dynamics was developed.

Results

The refined sample preparation protocol, which minimized solvent evaporation and improved temperature control, reduced batch-to-batch variability by up to 70% and improved repeatability in mechanical property extraction. Insufficient thermal equilibration was found to substantially reduce cavitation pressure, with values decreasing by nearly 50% in response to sample temperature increases of only 3(^circ )C prior to testing. Controlled needle retraction had little to no meaningful effect on cavitation pressure values. Model predictions agreed well with experimental results, highlighting the importance of controlling the pressurization rate. Radial position experiments confirmed that changing the sample geometry from cylindrical to annular did not introduce measurable boundary effects.

Conclusions

These findings establish practical guidelines for improving the reproducibility of NIC experiments and provide a validated framework for more robust and quantitative characterization of soft matter systems.

Background

The integral computational method has been used extensively for residual stress measurement with incremental hole-drilling and makes use of a single top surface strain gauge rosette. The method is depth limited due to the loss of strain gauge sensitivity as the hole depth increases.

Objective

The objective of this study is to develop an experimental and computational approach to properly incorporate, into an integral solution, the data from a strain gauge rosette on the bottom surface in addition to those of the existing rosette on the top surface and hence increase the maximum achievable measurement depth.

Methods

Top and bottom surface strain gauge rosettes are bonded coaxially on a thin aluminium AA6082-T6 specimen and the IHD procedure is performed through the thickness of the specimen. The unit pulse integral method is applied to generate calibration coefficients for the top and bottom rosettes through FE analysis. All the strain measurements and calibration coefficients are used in a single least-squares solution to determine the through-thickness residual stress distribution. Tikhonov regularization is additionally employed to smooth the stress solution.

Results

The proposed approach, which combines the top and bottom rosette data, demonstrates reduced uncertainty through the full thickness of a specimen thinner than 50% of the mean rosette diameter. The maximum hole depth can be increased to 245% of that of a single rosette IHD test.

Conclusions

Combining the top and bottom rosette data improves the residual stress solution in a thin specimen compared to a single rosette solution through reduced uncertainty, especially in the mid-depth of the specimen, and significantly increases the achievable measurement depth.

Background

Chemo-mechanical degradation of layered oxide electrodes is strongly influenced by crystallographic anisotropy, local stress evolution, and ion insertion, yet the intrinsic mechanical response of layered materials remains incompletely understood. Indeed, most prior studies have focused on polycrystalline materials but single crystals enable direct observation of coupling between anisotropic ion diffusion and mechanical response.

Objective

This study aims to determine how crystallographic anisotropy and lithiation affect deformation, fracture, and mechanical properties in single-crystal V₂O₅, and compares this behavior with polycrystalline counterparts.

Methods

Polycrystalline V₂O₅ thin films and single-crystal α-V₂O₅ were studied using nanoindentation, scanning electron microscopy, focused ion beam cross-sectioning, and Raman spectroscopy. Single crystals were tested in pristine and chemically lithiated states, including experiments in which crystals were first plastically deformed via nanoindentation and subsequently lithiated.

Results

Polycrystalline films exhibited significantly higher hardness and elastic modulus than single crystals. Single crystals indented normal to the exposed (001) basal plane exhibited pronounced anisotropic deformation, including directional slip, crystallographically-guided cracking, anisotropic crack propagation, interlayer separation, and shear localization. Lithiation caused substantial softening, reduced hardness and modulus, and suppressed displacement bursts during nanoindentation, while previously indented regions showed crack formation and growth upon lithiation.

Conclusions

Mechanical behavior of α-V2O5 is strongly governed by crystallographic anisotropy and further altered by lithiation, with pre-existing deformation serving as a strong driver of fracture during ion insertion. These findings illuminate the coupling among ion insertion, deformation, and fracture in layered oxides and provide a basis for understanding and mitigating mechanical failure in electrochemical energy-storage materials.

Background

When subjected to tensile stress at elevated temperatures, precipitation-strengthened Ni-based single crystals exhibit the elongation in shape, coarsening in size, and more intriguingly rafting of the L1₂-structured γ' precipitates, accompanied by the formation of face-centered-cubic γ matrix bands oriented perpendicular or parallel to the loading direction.

Objective

Existing modeling efforts predominantly emphasize Eshelby-type configurational forces, i.e., the reduction of elastic potential energy arising from elastic interactions and interfacial dislocations. However, these analyses are mainly for the precipitate elongation, while detailed kinetic analyses remain lacking and the Herring-type self-diffusional mechanisms are generally not incorporated.

Methods

In this work, we establish a comprehensive chemomechanical framework that integrates both Eshelby and Herring formalisms. Guided by extensive experimental parametric studies spanning a wide range of temperatures and loading rates, a mechanistic phase diagram is identified that distinctly separates elongation and rafting regimes.

Conclusions

Energetic and kinetic comparisons demonstrate that rafting is governed by a stress-gradient-controlled self-diffusional process, rather than by the Eshelby-type species-diffusional mechanism. This unified framework not only rationalizes the experimentally observed mechanistic phase diagram but also clarifies the distinct physical origins of elongation and rafting in Ni-based single-crystal superalloys.

Background

Silicon anodes possess an exceptionally high theoretical capacity, rendering them promising candidates for next-generation lithium-ion batteries, while their practical deployment is severely hindered by pronounced mechanical degradation during cycling. Polymer binders are essential for maintaining electrode integrity, yet their mechanical roles are typically assessed using limited descriptors, obscuring the complex coupling between binder mechanics and electrochemical performance.

Objective

This study aims to establish a multidimensional mechanical-electrochemical framework to quantitatively elucidate how binder mechanical properties govern the cycling stability of silicon electrodes.

Methods

Eight representative binders with diverse molecular architectures were systematically characterized in terms of elastic modulus, ultimate strength, fracture elongation, fracture toughness, adhesive strength, and fatigue resistance. These properties were then correlated with the electrochemical performance of electrodes using Sobol sensitivity analysis.

Results

The results identify adhesive strength as the dominant mechanical factor controlling the cycling performance of silicon electrodes. Additionally, Pearson correlation analysis reveals an inherent trade-off between stiffness/adhesion and deformability/toughness, leading to two fundamental binder archetypes: structural-constraint and deformation-adaptive binders.

Conclusions

This study provides a quantitative, mechanics-based understanding of binder functionality and offers design principles for advanced binders that enable durable, high-performance silicon electrodes.

Background

Structural members such as welded rails are subjected to bending and localized contact stresses during service and may additionally experience axial stresses arising from restricted strains leading to combined loading scenarios that influence crack behaviour.

Objective

This study examines variations in fracture parameters under the combined action of contact loading, bending, shear and axial tension in a beam to understand the crack-tip response.

Methods

Photoelastic experiments are conducted on a cracked beam model subjected to axial tension combined with contact loads at different positions using a specially fabricated loading jig. The isolated influence of contact loading and axial restraint is examined. Whole-field isochromatics representing constant principal stress difference (σ₁-σ₂) are processed using a multiparameter formulation. The stress intensity factors (K_I, K_II) and T-stress are extracted from the leading coefficients of the converged solution.

Results

For practical cracks, the response under combined loading is governed by interacting stress fields and requires consideration of crack face displacements. For a given axial tension, K_I increases more than K_II with increasing contact load. With increasing axial tension and constant contact load, K_I increases while K_II remains largely unaffected. The sign and magnitude of T-stress show strong dependence on load position, attaining high positive values when the contact load is directly above the crack.

Conclusion

Contact loading in the presence of high axial tension leads to an increase in crack-tip constraint. These findings are useful in assessing crack behaviour in components such as welded rails subjected to combined axial and contact loading during service.

Background

Single-axis hole-drilling ESPI measurements give direct indications of the residual stresses in the sensitivity direction, but can identify the transverse residual stresses only through Poisson’s ratio. This gives a significant directional bias to the measurements.

Objective

It is desired to develop a method to make hole-drilling ESPI residual stress measurements without directional bias and also to enhance the measurement robustness.

Methods

A two-axis hole-drilling ESPI residual stress measurement system was proposed and was made by adding a second measurement axis to a commercial single-axis ESPI residual stress measurement device. Hole-drilling measurements were made in a calibration standard specimen and the results compared with expected values.

Results

Good correspondence with the expected residual stresses was achieved. The expected directional bias in stress evaluation quality was observed when each axis was considered individually, with the stress evaluation noise in the direction perpendicular to the sensitivity direction ~ 1.5 to 2.0 times greater than in the parallel direction. The two-axis approach was successful in achieving low noise in both axial stresses. The stress evaluations also showed a good ability to identify modest sized residual stresses, even within much larger surface stresses.

Conclusions

The use of two-axis measurements successfully achieved good stress sensitivity in both axes. The redundancy of the data from the two measurement axes increases measurement robustness and measurement security. Measurement time is not significantly increased over the single-axis technique.

Background

Poly(lactic-co-glycolic acid) (PLGA) is widely employed in tissue engineering and drug-delivery systems due to its biodegradability and biocompatibility. Under physiological conditions, PLGA experiences concurrent hydrolysis, thermolysis, mechanical stress, and fluid sorption, generating acidic byproducts that affect mass loss, mechanical integrity, and drug-release safety. Although hydrolytic and thermal degradation have been studied extensively, the role of mechanical loading in altering degradation kinetics and composition remains unclear.

Objective

This study investigates how static mechanical loading at physiological temperature influences the surface composition, degradation product evolution, and molecular structure of PLGA 85/15 during hydrolysis.

Methods

PLGA 85/15 specimens were subjected to static loading at 37 °C in aqueous conditions. Fourier-transform infrared spectroscopy (FTIR) was employed to monitor degradation product evolution and changes in surface composition. The results were further correlated with thermal behavior, molecular weight changes, and water uptake behavior.

Results

FTIR analysis revealed that static loading promoted earlier diffusion and neutralization of carboxylic end groups, delaying carboxylate ion accumulation and slowing the autocatalytic degradation. In comparison, non-loaded samples showed premature carboxylate ion formation, enhanced swelling, and faster autocatalysis. FTIR peak-fitting of methyl and methylene bending regions indicated an increased relative lactide content at the sample surfaces under both loaded and non-loaded conditions.

Conclusions

Static mechanical loading facilitates the diffusion of acidic byproducts in PLGA, slowing the autocatalytic degradation. In contrast, non-loaded samples retain the degradation products, enhancing the autocatalysis process and degradation rate.