Multiaxial Strain Fields Research Articles

Tunable thermoelectric transport in nanomeshes via elastic strain engineering

Recent experimental explorations of silicon nanomeshes have shown that the unique metastructures exhibit reduced thermal conductivity while preserving bulk electrical conductivity via feature sizes between relevant phonon and electron mean free paths, aiding in the continued promise that nanometer-scale engineering may further enhance thermoelectric behavior. Here, we introduce a strategy for tuning thermoelectric transport phenomena in semiconductor nanomeshes via heterogeneous elastic strain engineering, using silicon as a model material for demonstration of the concept. By combining analytical models for electron mobility in uniformly stressed silicon with finite element analysis of strained silicon nanomeshes in a lumped physical model, we show that the nonuniform and multiaxial strain fields defined by the nanomesh geometry give rise to spatially varying band shifts and warping, which in aggregate accelerate electron transport along directions of applied stress. This allows for global electrical conductivity and Seebeck enhancements beyond those of homogenous samples under equivalent far-field stresses, ultimately increasing thermoelectric power factor nearly 50% over unstrained samples. The proposed concept and structures—generic to a wide class of materials with large dynamic ranges of elastic strain in nanoscale volumes—may enable a new pathway for active and tunable control of transport properties relevant to waste heat scavenging and thermal management.

Anisotropy and tension–compression asymmetry modeling of the room temperature plastic response of Ti–6Al–4V

The mechanical behavior of the alloy Ti–6Al–4V is characterized using uniaxial tension, uniaxial compression, simple shear and plane strain tests in three orthogonal material directions. The experimental results reveal tension/compression asymmetry, anisotropic yielding and anisotropic strain-hardening. These features are incorporated into an elasto-plastic constitutive law based on the macroscopic orthotropic yield criterion “CPB06” adapted to hexagonal metals. A new identification method for the yield criterion parameters is proposed by inverse modeling of the axial strain field of compression specimens in the three orthogonal directions of the material. The sensitivity of different sets of material parameters to the identification method is also analyzed and the capacity of the model to accurately predict the forces and displacement field is discussed. A validation of the best set of identified CPB06 material parameters is performed by comparing the load–displacement curves in different loading directions for tensile tests on notched round bars with different levels of stress triaxiality and for compression tests on elliptical cross-section specimens, both tests involving multiaxial strain fields and large deformations.

Residual strain-induced birefringent FBGs for multi-axial strain monitoring of CFRP composite laminates

Several solutions to measure multi-axial strain in composite materials using fibre Bragg gratings have already been presented in the literature. Most of them use Bragg sensors written in highly birefringent fibres. Unless they exhibit a higher transverse strain sensitivity, a major drawback of those highly birefringent Bragg sensors is the necessity to orient them before embedding them into the composite material, because otherwise an underestimation of the present strain field can occur. Therefore, in this paper we describe how the residual strain, which is built up during the cure cycle of a composite laminate, can in-situ create a highly birefringent fibre Bragg sensor making the need of extra manipulations before embedding redundant. Consequently, in the paper this Bragg sensor is used to quantitatively measure multi-axial strain fields of cross-ply laminates loaded along their three principal mechanical axes. Good similarities were found for the measured strains and the reference strains.

Temperature-insensitive photoluminescence above 300 K in strained GaxIn1 − x As multiple quantum wire heterostructures

Stable peak wavelength photoluminescence (PL) spectra has been observed in Ga x In 1 − x As strained multiple quantum wire (MQWR) heterostructures over a temperature range between 250 and 380 K. The MQWR samples were grown in situ by molecular beam epitaxy using the strain-induced lateral-layer ordering (SILO) process to create the quantum wires. The formation of the strained Ga x In 1 − x As quantum wires and the amount of the strain associated with the wires depends on the growth conditions and structural configuration of the MQWR active region. Samples grown near a substrate temperature of 500°C show a negligible net PL peak wavelength shift between 77 and 380 K. Red shifts and blue shifts of PL peak wavelengths between 77 and 380 K are observed in samples grown at temperatures less than and greater than 500°C, respectively. Due to the multi-axial strain fields induced by the SILO process in the MQWR active regions, the PL peak wavelength of all the MQWR samples stabilize at ∼ 1.61 μ m for temperatures above 250 K. The temperature dependent shift of peak PL wavelength between room temperature and 72°C for these MQWR heterostructures is as low as 0.2 Å/°C.

Three-dimensional strain fields in a uniform osteotomy gap.

Stable internal fixation usually results in a unique histological healing pattern which involves direct cortical reconstruction and an absence of periosteal bridging callus. While it has been suggested that longitudinal interfragmentary strain levels control this healing pattern, the complex, multiaxial strain fields in the interfragmentary region are not well understood. Based on an in-vivo study of gap healing in the sheep tibia by Mansmann et al., we used several finite element models of simplified geometry to: explore modeling assumptions on material linearity and deformation kinematics, and examine the strain distribution in a healing fracture gap subjected to known levels of interfragmentary strain. We found that a general nonlinear material, nonlinear geometric analysis is necessary to model an osteotomy gap subjected to a maximum longitudinal strain of 100 percent. The large displacement, large strain conditions which were used in the in-vivo study result in complex, multiaxial strain fields in the gap. Restricting the maximum longitudinal strain to 10 percent allows use of a linear geometric formulation without compromising the numerical results. At this reduced strain level a linear material model can be used to examine the extent of material yielding within a homogeneous osteotomy gap. Severe local strain variations occurred both through the thickness of the gap and radially from the endosteal to periosteal gap surfaces. The bone/gap interface represented a critical plane of high distortional and volumetric change and principal strain magnitudes exceeded the maximum longitudinal strains.