Experimental Mechanics最新文献

Abstract

Background

Advanced ceramic matrix composites (CMC) are aimed at highly demanding thermo-mechanical environments, such as space or hypersonic applications. Their design and manufacturing require a reliable, cost-effective method for estimating and quantifying CMC mechanical properties.

Objective

CMC composite materials present orthotropic mechanical properties. Different tests are often conducted to measure all independent elastic properties. These variety of tests are expensive and demand significant time and effort. Therefore, it is highly desirable to have one mechanical test that can generate all or most of the elastic properties.

Method

This study proposes a single Brazilian disc (BD) test setup. It investigates an efficient inverse problem to evaluate the Young’s and shear moduli of an orthotropic Liquid Silicon Infiltrated (LSI) C/C–SiC CMC. A combined BD test and Digital Image Correlation (DIC) technique were used for this purpose. The BD test employs a circular disc compressed diametrically in varying orientations of the tested multi-layered CMC 0°/90 ^o woven carbon fibers. The BD introduces a multi-axial stress and strain field highly affected by these primary elastic moduli. DIC enables full-field multi-axial strain calculations in large spatial variations.

Results

A numerical study resulted in a unique iterative inverse-mechanics test procedure based on BD tests conducted on orthotropic materials at two material orientations. This approach was applied to measure C/C-SiC material properties, showing an efficient and reliable appraisal of major elastic moduli.

Conclusions

The presented approach may drive down both the duration and cost of the mechanical evaluation, thereby potentially improving future process designs.

Background

This review discusses high-speed thermal measurements and their significance in understanding solid materials' behavior, focusing on rapid loading conditions.

Objective

While high-speed thermal measurements are challenging in some cases, these measurements provide unique insights into material response at high rates, by shedding light on failure modes, thermomechanical coupling, and thermal dissipation phenomena that are otherwise overlooked.

Methods

The review presents various direct measurement techniques (contact and non-contact) relevant to high-speed loading, with emphasis on the frequently used ones in mechanics of materials applications: thermocouples, infrared detectors, and high-speed infrared cameras.

Results

Pros and cons of each technique, alongside with typical applications are discussed. Understanding the interplay between thermal effects and mechanical responses opens new avenues for enhancing material performance and energy efficiency.

Conclusions

This review is expected to serve as a valuable resource for researchers and practitioners seeking to leverage high-speed thermal measurements to drive innovation and advance materials science in various applications.

Background

Stress measurement for thin films is crucial in a variety of fields such as in semiconductor manufacturing, the optoelectronics industry, and biomedical science, among others. However, most measurement methods require surface treatment of the thin film.

Objective

A label-free measurement method for plane stress states in optical isotropic thin films based on spectroscopic ellipsometry analysis is proposed and verified in this paper.

Methods

The proposed method is based on the modulation of the stress-optic effect on reflected spectroscopic ellipsometry. A theoretical model is established to describe the relation between all components of the plane-stress state and the classic ellipsometric parameters (Ψ, Δ). An algorithm is developed to determine all components of a plane-stress state by fitting the model to the experiment data.

Results

In the verification experiment, we determined the plane stress state of a Cu film coated on a PI (polyimide) substrate. The results show a reasonable agreement between the experimental measurements from spectroscopic ellipsometry and the theoretical analysis based on the applied loading.

Conclusion

The results prove that our method can effectively measure the plane stress state of optical isotropic thin films.

Background

The service quality and life of photopolymerized materials are dramatically impaired by shrinkage stress generated during the polymerization process. Several soft-start photocuring protocols including two-step, ramp, and pulse delay have been proposed to reduce the shrinkage stress. However, the mechanism for the shrinkage stress reduction by soft-start photocuring remains largely elusive.

Objective

This study aims to explore the mechanism for shrinkage stress reduction in soft-start photocuring protocols and then propose a universal strategy to maximize the stress reduction.

Method

A theory-experiment-combined method was developed to investigate the effect of soft-start photocuring protocols on the shrinkage stress evolution. Shrinkage stresses under different protocols were measured by a standardized cantilever beam-based instrument. An improved theoretical model incorporating the evolutions of the reaction kinetics and material properties was developed to predict the shrinkage stress evolution under different curing protocols.

Results

Compared to the standard protocol with a constant photo-irradiation, all the soft-start photocuring protocols could effectively reduce the shrinkage stress and the two-step protocol achieved a maximum reduction of 25% among all experimental conditions. The elastic modulus of photopolymers coincided under the same radiant exposure and irradiation intensity. Unlike previous studies focusing on the mechanical properties of the photopolymers, we found that the shrinkage stress reduction by soft-start photocuring protocols could be attributed to a delayed gelation and a reduction in the peak temperature change after gelation. Based on these mechanisms, adding a delay time before the gelation was proposed as an effective strategy to reduce the shrinkage stress, leading to a reduction of up to more than 40% according to the theoretical predictions. Additionally, the timing for introducing the delay and its duration can be effectively and conveniently determined by monitoring the real-time evolution of shrinkage stress in the standard photocuring protocol.

Conclusions

This theory-experiment-combined study not only uncovers that the shrinkage stress reduction by soft-start photocuring protocol is attributed to the delay in the gelation and the reduction of the peak temperature change after the gelation but also proposes an effective approach to mitigate shrinkage stress by adding a delay time before the gelation. Such a strategy for maximizing the shrinkage stress reduction while maintaining the mechanical and curing properties is to guide the practical applications of photopolymers.

Background

Active infrared thermography is proved to be viable and attractive for non-destructive evaluation of interfacial defects like delaminations in a coating-substrate system. But it is a challenging task to accurately quantify small and deeply buried defects from thermal images, due to the inevitable effects of lateral heat diffusion and measurement noise.

Objective

The aim of this work is to estimate the size and pattern of defects at the interface of a two-layer system with high accuracy and high reliability based on step heating thermography.

Methods

To characterize the effect of defect on the heat flow, a virtual heat flux is assumed at the interface, which is reconstructed from measured surface temperature by solving a three-dimensional inverse problem. The inverse solution is obtained using the Green’s function and regularization techniques, and then used for estimating the defect pattern by threshold segmentation. An improvement on computational efficiency is achieved by an iteratively substitution of nodal temperature.

Results

Simulations with synthetic data generated by a finite element model validate the feasibility of this approach. Results obtained from experiments for an Aluminum oxide/steel system show the robustness of this approach, when temperatures are contaminated with measurement noise. Both the performance on estimation of various defect shapes and the effects of regularization are discussed.

Conclusion

This study show that the present approach brings an improvement in accuracy and reliability for the estimation of size and pattern of defects with various diameter-to-depth ratios, in comparison with conventional techniques.

Background

Subsea power cable failures in offshore wind farms result in significant financial losses. One common failure mode is submarine power cable bending.

Objective

The primary objective of this study is to validate two analytical models using strain readings obtained from a novel 3-point bending setup designed for power cable specimens. The setup incorporates two types of optical fiber sensors for simultaneous strain measurement.

Methods

A 3-point bending setup is constructed, integrating optical fiber sensors installed on the embedded fiber optic cable within the submarine power cable. One set of sensors is fixed to the fiber optic cable sheath, while a second set consists of loose tube fibers that are inside the fiber optic cable. The strain readings of the fixed sensors are compared to two analytical models. The first analytical model assumes a constant power cable curvature, while the second model considers variable curvature.

Results

The analytical models both predict nearly flat strain profiles and are in line with each other. The strain data, however, approaches zero strain away from the cable center. Model assumptions such as perfect sensor positioning and zero slip of the fiber optic cable cause this discrepancy. The results of the constant curvature model agree well with strain averages of the fixed sensors around the central region of the power cable, and both scale linearly with amplitude. Finally, the strain readings from the loose tube fibers demonstrate high reproducibility, facilitating the development of a calibration curve for estimating power cable curvature.

Conclusions

The analytical models surpass existing models by providing good agreement with the measured strain around the cable center. Moreover, the highly reproducible strain readings from the loose tube fibers allow estimating power cable curvature.

Background

Helmet systems most commonly experience oblique blunt impacts which cause simultaneous linear and rotational accelerations. The ability to attenuate both linear and rotational accelerations by absorbing the normal shock while accommodating large shear deformations with energy dissipation is critical to developing superior helmet liners that prevent traumatic brain injury (TBI).

Objective

To investigate the quasistatic compression-shear response of vertically aligned carbon nanotube (VACNT) foams—which are known for their exceptional specific energy absorption in compression—and explore their potential of accommodating large shear strains at lower shear stress levels, under large compression-shear loadings.

Methodology

We investigate the quasistatic compression shear response of freestanding vertically aligned carbon nanotube foams subjected to varied initial precompressions. We use in situ high speed microscopy to visualize the microscale deformations during shear.

Results

Vertically aligned carbon nanotube foams exhibit a nonlinear hysteric shear stress–strain response that varies as a function of initial normal precompression. At a given precompression, initial linear response at very low shear strains leads to a behavior showing increasing compliance leading to a plateau like regime at moderate shear strains and then transitions into a stiffening behavior at high shear strains. The shear stress–strain response softens with the increase in initial precompression demonstrating the vertically aligned carbon nanotube foam’s potential to accommodate large shear strains more effectively at severe compression-shear loads unlike other solids that typically jam. In situ high-speed microscopy reveals the unraveling of carbon nanotubes that collectively buckled during precompression, allowing them to accommodate large shear strains at low shear stress levels.

Conclusion

We demonstrate the ability of vertically aligned carbon nanotube to accommodate large shear strains at lower shear stress levels under large compression-shear loadings. We propose a model to predict the compression-shear response at different precompressive strains and use this model to develop a deformation modality diagram that categorizes the dominant deformation mechanisms at different loads along different loading angles.