Off-axis Directions Research Articles

A periodic micromechanical model for the rate- and temperature-dependent behavior of unidirectional carbon fiber-reinforced PVDF

The rate- and temperature-dependent mechanical behavior of unidirectional carbon fiber-reinforced polyvinylidene fluoride (PVDF) is investigated through uniaxial tension and compression experiments under various off-axis loading conditions. To improve the understanding of the behavior of the composite, a 3D micromechanical model is developed. Microscopic analyses are used to characterize the geometrical properties of the UD composite at the fiber length-scale. These properties are used to construct a periodic 3D representative volume element (RVE). In combination with periodic boundary conditions, uniaxial macroscopic deformation (in any possible direction) is applied to the RVE to accurately and efficiently model off-axis loading. The rate- and temperature-dependent behavior of the PVDF matrix is accurately described using an elasto-viscoplastic constitutive model. The finite element simulations of uniaxial tension and compression tests are compared to the experimental data and the micromechanical response is analyzed. The micromechanical model accurately describes the rate-dependent macroscopic behavior of unidirectional carbon fiber-reinforced PVDF for various off-axis loading directions at different temperatures. Analysis of the local matrix response in the RVE reveals the influence of the matrix on the macroscopic behavior of the composite.

High precision wavefront correction method in interferometer testing

A wavefront correction method is proposed for high-precision optic surfacing, addressing the discrepancy between wavefront and real surface errors in Fizeau interferometer testing. We believe this to be a proposed novel method that encompasses optical surface function parameters fitting, lateral distortion correction, misalignment error removal, and sag surface error calculation. The method's error has been thoroughly analyzed, including aspects of function parameters fitting, ray tracing, and interpolation. The effectiveness of the method was demonstrated by correcting the wavefront of an off-axis parabolic mirror in null testing configurations, significantly reducing artificially created annular errors and improving off-axis direction errors from 0.23λ to 0.05λ (λ=632.8 nm), with the PV of aspheric departures exceeding 8.5 mm.

Regional structure-function relationships of lumbar cartilage endplates

Cartilage endplates (CEPs) act as protective mechanical barriers for intervertebral discs (IVDs), yet their heterogeneous structure–function relationships are poorly understood. This study addressed this gap by characterizing and correlating the regional biphasic mechanical properties and biochemical composition of human lumbar CEPs. Samples from central, lateral, anterior, and posterior portions of the disc (n = 8/region) were mechanically tested under confined compression to quantify swelling pressure, equilibrium aggregate modulus, and hydraulic permeability. These properties were correlated with CEP porosity and glycosaminoglycan (s-GAG) content, which were obtained by biochemical assays of the same specimens. Both swelling pressure (142.79 ± 85.89 kPa) and aggregate modulus (1864.10 ± 1240.99 kPa) were found to be regionally dependent (p = 0.0001 and p = 0.0067, respectively) in the CEP and trended lowest in the central location. No significant regional dependence was observed for CEP permeability (1.35 ± 0.97 * 10−16 m4/Ns). Porosity measurements correlated significantly with swelling pressure (r = −0.40, p = 0.0227), aggregate modulus (r = −0.49, p = 0.0046), and permeability (r = 0.36, p = 0.0421), and appeared to be the primary indicator of CEP biphasic mechanical properties. Second harmonic generation microscopy also revealed regional patterns of collagen fiber anchoring, with fibers inserting the CEP perpendicularly in the central region and at off-axial directions in peripheral regions. These results suggest that CEP tissue has regionally dependent mechanical properties which are likely due to the regional variation in porosity and matrix structure. This work advances our understanding of healthy baseline endplate biomechanics and lays a groundwork for further understanding the role of CEPs in IVD degeneration.

The Effect of Load Steps and Initial Crack Length on Stress Distribution of Fiber Metal Laminates

Fiber Metal Laminates (FMLs) are advanced composite materials composed of alternating layers of fiber-reinforced composites and metal sheets, offering a unique blend of high strength, stiffness, and fatigue resistance. Understanding the behavior of FMLs under different loading conditions, particularly in the presence of initial cracks, is crucial for predicting their structural integrity and performance. This study investigates the effect of initial crack length and load steps on the stress distribution within FML structures through numerical simulation. A glass fiber-reinforced epoxy (GFRE) composite is constituted by glass fibers in off-axis directions of 0o/90o. The crack length of 3 mm, 6 mm, 9 mm, and 12 mm is initially defined on the side of fiber metal laminates. The tensile load was gradually applied to the laminates for five different ratios. The ratios are defined by dividing the tensile load with the area of the laminates which is then called stress ratio. The results show that the greater the initial crack length and load step ratio, the greater the stress around the crack tip. The stress distribution in each layer of the laminates shows different characteristics between tensile and compressive. This will affect the crack propagation behavior of the laminates. Moreover, the findings also provide valuable insights for optimizing FML design, improving damage tolerance, and extending service life in diverse engineering applications.

Effects of DC bias on evolutions of repetitively pulsed streamer discharge in humid air

Modulation efficiency and mechanisms of repetitively pulsed streamer discharge in humid air are ambiguous with dramatic variations in free electron availability, residual ion mobility, enhanced heat release, etc, caused by water molecules intentionally supplemented or existing in the surrounding environment. The inception and propagation patterns of repetitively pulsed streamer discharge modulated by superimposed DC bias are experimentally investigated in the needle-plane electrode configuration. The inception voltage decreases due to negative ion drift under positive DC bias. The secondary streamer with a bright glowing cloud prolongs towards the plane electrode and the diameter decreases under positive DC bias. The primary streamer tends to propagate along the off-axis direction under negative DC bias. The number of applied pulses before breakdown decreases with the increase in positive DC bias and illustrates an insignificant dependence on the negative DC bias. The effect of air humidity is more pronounced than the DC bias. The streamer inception, propagation, and morphological transition are explained by residual space charge distributions and drift velocity.

Off-axis limiting photometric distance of Lambertians and narrow beams

The near-field - far-field disparity of light sources leads to discussion within the lighting community. In the far-field, light sources are approximated by a point source with a luminous intensity distribution from which other photometric quantities can be computed. In the near-field, light sources must be considered as extended sources and no closed-loop analytical solutions can be found for the illuminance in off-axis directions. The illuminance allows to compute the apparent intensity used for the assessment of the limiting photometric distance (LPD), that is, the threshold between near- and far-field regions. Numerically, the illuminance can be determined through a discretisation of the luminous surface. This approach is verified for on- and off-axis directions through direct comparison with ray tracing simulations based on near-field goniophotometric measurements. A good match is observed for Lambertian and narrow beam light sources. Using the numerical approach, the LPD in all directions is assessed. For light sources of which the luminous intensity strictly decreases while moving away from the optical axis, the LPD decreases as well until a minimal value is reached. After this, the LPD increases again, in certain scenarios up to values resulting in the invalidity of the inverse square law.

Microstructural modeling on electrical conductivities of 3-D carbon/epoxy woven composites along different directions

Carbon fiber composites have widely been used in aircrafts and high speed vehicles. Electrical conductivity of the carbon fiber composites is critical to electrical safety for the aircrafts and vehicles under high voltage applications. Here we report microstructural modeling on electrical conductivities of three-dimensional (3-D) carbon/epoxy angle-interlock woven composites along on-axis and off-axis directions. A 3-D resistor network model and finite element analysis (FEA) model based on fabric microstructure have been developed to reveal the electric conductivities along different directions. We have found that the conductivity of the woven structure exhibits full anisotropy. The difference of electrical conductivity between in-plane and through-thickness direction decreased due to the existence of through-the-thickness carbon fibers. The electrical conductivity through-thickness direction increases with warp directional length. With increasing off-axis directional length, the conductivity perpendicular to the off-axis direction increases, while the conductivity parallel to the off-axis direction remains basically unchanged. Electric potential distribution and electrical current density are highly related with fabric structure and current directions. Owing to the dependence of mechanical properties are also related with fabric structure and directions, it is expected that both the electrical and mechanical properties of the 3-D woven carbon/epoxy composites could be optimized simultaneously from the composite microstructure designs.

Optimal audio beam pattern synthesis for an enhanced parametric array loudspeaker

A parametric array loudspeaker (PAL) generates highly directional audible sound in air with a small aperture size compared to a conventional loudspeaker. But in indoor applications, the long propagation distance of a PAL causes reflections, which disturbs the reproduction of narrow audio beams. Moreover, sound distortion appears along the off-axis direction due to the frequency dependence of the beam width. This study proposed an optimal audio beam pattern synthesis for a PAL-based convex optimization, which can design the audio beam of a PAL with an optimal solution. The proposed method overcame the mentioned limitations by applying it to a length-limited PAL for audio spot control and a multichannel PAL array for a constant beam width audio beam. In a length-limited PAL, the proposed method restricts the audio spot to a smaller region and weakens the sound leakage along the off-axis direction. Whereas in a multichannel PAL array, the proposed method also achieves a constant beam width near the radiator axis. Simulations and experiments verify the effectiveness of the proposed method, which will enhance the performance of a PAL in scenarios where control of the audio beam is required.

Thermal response of an induction-heated fabric reinforced thermoplastic composite with anisotropic electrical conductivity: An experimental study

In this study, the electrical conductivity of PolyEtherKetoneKetone reinforced with five harness satin weave carbon fabric was measured. Induction heating experiments using a hairpin shaped coil were conducted on the same material. The study points out that the thermal response depends on the direction in which the eddy currents are generated in the laminate. The thermal response showed a linear relation with the measured in-plane anisotropic electrical conductivity, pointing towards Ohmic heating mechanisms being dominant during induction heating of the used material. This relation enables the accurate and efficient simulation of the induction heating of woven fabric reinforced composites, using only the electrical conductivity measured in one of the fabric’s weave directions and in the fabric’s 45° off-axis direction.

Ultrashort and high-collimation X/γ-rays generated by nonlinear inverse Thomson scattering between off-axis electrons and circularly polarized intense laser pulses.

The properties of nonlinear inverse Thomson scattering (NITS) are investigated in the collision between a circularly polarized tightly focused intense laser pulse and a relativistic off-axis electron with numerical simulations. Due to the asymmetric effect of the laser field on the off-axis electrons, the electron trajectory is torqued to the off-axis direction, and the symmetry of the spatial radiation is also destroyed, which causes the concentrations of the radiation in the off-axis direction. With the increase of laser intensity, the torsion effect is more obvious, the radiation collimation improves, the direction turns to sideways. With the increase of electron's initial energy, the direction turns back to backwards and the degree of off-axis effect decreases. In both cases, the power exponentially enhances, the pulse width shortens, the spectrum broadens and super-continuity appears. With the laser intensity, the duration of sideways X-ray pulse from the low-energy (2.61MeV) electron is only 0.2 as, and the normalized intensity reaches 109. While using ultra-high-energy (100MeV) electrons, the duration of backwards γ-ray pulse reaches 1.22 zs, and the normalized intensity reaches 1017. These results help the understanding of nonlinear Thomson scattering and provide important numerical references for the research of NITS as high-quality X-ray and γ-ray sources.

Photometry and spectroscopy of the Type Icn supernova 2021ckj

We present photometric and spectroscopic observations of the Type Icn supernova (SN) 2021ckj. This rare type of SNe is characterized by a rapid evolution and high peak luminosity as well as narrow lines of highly ionized carbon at early phases, implying an interaction with hydrogen- and helium-poor circumstellar matter (CSM). SN 2021ckj reached a peak brightness of ∼ − 20 mag in the optical bands, with a rise time and a time above half maximum of ∼4 and ∼10 days, respectively, in the g and cyan bands. These features are reminiscent of those of other Type Icn SNe (SNe 2019hgp, 2021csp, and 2019jc), with the photometric properties of SN 2021ckj being almost identical to those of SN 2021csp. Spectral modeling of SN 2021ckj reveals that its composition is dominated by oxygen, carbon, and iron group elements, and the photospheric velocity at peak is ∼10 000 km s−1. Modeling the spectral time series of SN 2021ckj suggests aspherical SN ejecta. From the light curve (LC) modeling applied to SNe 2021ckj, 2019hgp, and 2021csp, we find that the ejecta and CSM properties of Type Icn SNe are diverse. SNe 2021ckj and 2021csp likely have two ejecta components (an aspherical high-energy component and a spherical standard-energy component) with a roughly spherical CSM, while SN 2019hgp can be explained by a spherical ejecta-CSM interaction alone. The ejecta of SNe 2021ckj and 2021csp have larger energy per ejecta mass than the ejecta of SN 2019hgp. The density distribution of the CSM is similar in these three SNe, and is comparable to those of Type Ibn SNe. This may imply that the mass-loss mechanism is common between Type Icn (and also Type Ibn) SNe. The CSM masses of SN 2021ckj and SN 2021csp are higher than that of SN 2019hgp, although all these values are within those seen in Type Ibn SNe. The early spectrum of SN 2021ckj shows narrow emission lines from C II and C III, without a clear absorption component, in contrast with that observed in SN 2021csp. The similarity of the emission components of these lines implies that the emitting regions of SNe 2021ckj and 2021csp have similar ionization states, and thus suggests that they have similar properties as the ejecta and CSM, which is also inferred from the LC modeling. Taking the difference in the strength of the absorption features into account, this heterogeneity may be attributed to viewing angle effects in otherwise common aspherical ejecta. In particular, in this scenario SN 2021ckj is observed from the polar direction, while SN 2021csp is seen from an off-axis direction. This is also supported by the fact that the late-time spectra of SNe 2021ckj and 2021csp show similar features but with different line velocities.

A compact and highly collimated atomic/molecular beam source

We describe the design, characterization, and application of a simple, highly collimated, and compact atomic/molecular beam source. This source is based on a segmented capillary design, constructed using a syringe needle. Angular width measurements and free molecular flow simulations show that the segmented structure effectively suppresses atoms traveling in off-axis directions, resulting in a narrow beam of Helium atoms having a width of 7 mrad (full width half maximum). We demonstrate an application of this source by using it for monitoring real-time changes in surface coverage on a clean Cu(110) surface exposed to oxygen by measuring the specular reflectivity of the Helium beam generated using this source.

The Off-Axis Embedment Performance of Bamboo Panels Used for Glubam Structures

The off-axis embedment behaviors of unidirectional and multidirectional laminated engineered bamboo panels used for glued-laminated bamboo (glubam) structures under dowel-type bolts with three different diameters, 12, 14, and 16 mm, were experimentally studied in this research. The half-hole loading method given by current standards was adopted to perform the embedment tests. Seven different off-axis loading directions from 0° to 90° related to the main bamboo fiber direction at an interval of 15° in the panel plane were considered. The different failure patterns, stress-displacement curves, and the corresponding full-field surface strain measured through the digital image correlation (DIC) method are reported. The loading angles significantly influenced the embedment performance of unidirectional bamboo panel specimens, and the strength values decreased with the increasing off-axis loading angles. A difference of 35% in the strength values between 0° and 90° loading specimens was observed. However, quasi-isotropic embedment properties were noticed for the multidirectional laminated engineered bamboo panels. The capacity functions were provided to predict the mean off-axis embedment strength and characteristic strength values by inputting the characteristic density values into the functions. Compared with the characteristic strength values estimated through the statistical method, with a 95% probability of exceedance and 75% significant level, the conventional capacity-function method underestimated the off-axis embedment strength values, and thus may leading to an overconservative design. The statistical characteristic embedment strength values are suggested to be used to calibrate the safety factor of capacity equations given by wood standards. Results from this research can be used to predict the ultimate strength of glubam joints under complex loading conditions and obtain reliable design values for engineering applications.

Negative Poisson’s ratio: A ubiquitous feature of wood

Most materials contract laterally when stretched axially i.e. they have a positive Poisson’s ratio. Negative Poisson’s ratios (NPR, also auxetic) are largely limited to single crystals or to artificial meta-materials such as honeycombs, foams and composites, which does limit their applications. This meta-study shows that NPR is abundantly present in an extremely common and useful category of natural materials, woods. This effect is so ubiquitous that 87 out of 123 measured hardwood samples and 58 of 62 softwood samples exhibit the property. In wood, NPR occurs predominantly in quite narrow off-axis directions, with values as low as − 3.32. This effect is chiefly attributable to the tubular structure of the wood cells. This suggests that low-cost, large-scale auxetic structural parts can be obtained by cutting low to medium density timber in specific off-axis directions, with potential benefits in a wide range of structural and construction applications.

Spatial radiation features of the off-axis collision between a relativistic electron and a tightly focused linearly polarized laser

The collision between relativistic electrons and a tightly focused linearly polarized laser pulse produced nonlinear inverse Thomson scattering (NITS), which generates backward x-rays. The effects of the initial transverse position of electrons with varied initial energy on the angular distribution of radiation power and spectrum are studied through numerical simulation. For the electrons with low initial energy (i.e. ), the off-axis collision breaks the spatial symmetry of x-ray radiation compared with the axial collision, the radiation direction is also torqued towards the same off-axis direction. There exists an optimal initial transverse position of electrons, which means the electron emitted from this position obtains greater radiation power. For the high-energy () electrons, the effect of torsion is not significant. The impact of off-axial is mainly manifested in the high-order harmonic spectrum. These findings help obtain high quality x-rays and modulate NITS radiation symmetry by electron parameters.

Cylindrical ionization chamber response in static and dynamic 6 and 15 MV photon beams

Purpose. To investigate the response of the CC13 ionization chamber under non-reference photon beam conditions, focusing on penumbra and build-up regions of static fields and on dynamic intensity-modulated beams. Methods. Measurements were performed in 6 MV 100 × 100, 20 × 100, and 20 × 20 mm2 static fields. Monte Carlo calculations were performed for the static fields and for 6 and 15 MV dynamic beam sequences using a Varian multi-leaf collimator. The chamber was modelled using EGSnrc egs_chamber software. Conversion factors were calculated by relating the absorbed dose to air in the chamber air cavity to the absorbed dose to water. Correction and point-dose correction factors were calculated to quantify the conversion factor variations. Results. The correction factors for positions on the beam central axis and at the penumbra centre were 0.98–1.02 for all static fields and depths investigated. The largest corrections were obtained for chamber positions beyond penumbra centre in the off-axis direction. Point-dose correction factors were 0.54–0.71 at 100 mm depth and their magnitude increased with decreasing field size and measurement depth. Factors of 0.99–1.03 were obtained inside and near the integrated penumbra of the dynamic field at 100 mm depth, and of 0.92–0.94 beyond the integrated penumbra centre. The variations in the ionization chamber response across the integrated dynamic penumbra qualitatively followed the behaviour across penumbra of static fields. Conclusions. Without corrections, the CC13 chamber was of limited usefulness for profile measurements in 20-mm-wide fields. However, measurements in dynamic small irregular beam openings resembling the conditions of pre-treatment patient quality assurance were feasible. Uncorrected ionization chamber response could be applied for dose verification at 100 mm depth inside and close to large gradients of dynamically accumulating high- and low-dose regions assuming 3% tolerance between measured and calculated doses.

Multiscale modelling of material degradation and failure in plain woven composites: A novel approach for reliable predictions enabled by meta-models

In this paper a multiscale method for modelling damage evolution at meso and macro scales with application to plain woven composites is presented for the first time. The method couples the macro and micro-scale simulations using an averaging theorem in which the computations in the two scales are performed simultaneously. Unlike the local–global approach, the developed multiscale analysis is carried out over the entire macro domain rather than a predefined segment. To address the computational challenges in multiscale damage analysis, the proposed method a) employs a non-local approach in constructing the macro–micro interface to avoid the pathological localisation effect introduced by microdamage at the macro continuum; b) reduces the considerable computing effort incurred by the non-linear behaviours in multiscale damage analysis by the introduction of machine learning meta-models. Experimental tensile tests conducted are shown to be in good agreement with the proposed method for both on-axis and off-axis load directions. The biaxial loading cases are also investigated with the results illustrated by the failure envelopes and compared with the existing damage criteria. Finally, an example of open-hole tension test is presented to demonstrate an application of the proposed method to model progressive damage and crack propagation in composite plates.

A Torque-Controlled Actuator With High Rigidity and Low Aspect Ratio

To ensure safe and accurate environment-robot interaction, a torque sensor is usually mounted at the output of an actuator to detect and control the output torque. Deformation-based sensors at the actuator output would significantly increase the actuator size and complexity while decreasing the output rigidity in off-axis directions. To reduce the extra complexity, space, and output compliance required for torque sensing, this article proposes a torque-controlled actuator that does not rely on deformation-based sensing at the actuator output. Instead, a torsion spring is mounted at the input of the harmonic drive. Encoders are used to sense the spring deformation to control the output torque with low sensing noise. Hence, the size of the actuator can be minimized while the output rigidity can be maintained. Using the torsion spring can also significantly reduce the reflected output inertia of the actuator. Experiments are provided to demonstrate torque control accuracy and bandwidth. Position control results also show a comparable response to actuators that do not have torque sensors. This new torque-controlled actuator can provide a more competitive solution for collaborative robots to interact with humans or the environment.

Failure assessment of 3D woven composites under compression after low-velocity impact

The failure mechanism of 3D woven composites subjected to compression loading along principal/off-axis direction after low-velocity impact (LVI) was assessed by experimental and numerical methods. The low-velocity impacts under 26.8 J and 80 J energies were applied to the specimens with off-axis angles of 0° and 45°. It can be observed that the impact damages are direction-dependent, which is determined by the weft and warp orientations. By performing the compression-after-impact (CAI) tests, it is found that the CAI strength along principal direction is more sensitive to the low-velocity impact than that along off-axis direction. A finite element dynamic analytical method was established, considering four off-axis angles (0°, 30°, 45° and 60°). The results show that the extension direction of the impact damage changes regularly with the off-axis angle. During the compression, the small off-axis angle can make the specimen prone to produce a sudden crushing failure determined by the fiber failure due to the high axial stress. As the off-axis angle increases, the matrix damage gradually holds the dominant position due to the growing shear effect, which makes the specimen produce a ductile failure governed by the accumulated matrix failure.

Flexural behavior of wood in the transverse direction investigated using novel computer vision and machine learning approach

Abstract A deep-learning-based semantic segmentation approach (U-Net) was used to partition the anatomical features in the cross-section of hinoki (Chamaecyparis obtusa) wood during a micro three-point bending test. Using the Crocker–Grier linking algorithm, thousands of cells were successfully extracted, and several parameters (area, eccentricity, fitted ellipse aspect ratio, bounding box aspect ratio) were used to evaluate the intensity of the cells’ deformation. Thus, the 2D map of the deformation intensity distribution was constructed. By analyzing flat-sawn, quarter-sawn, and rift-sawn specimens, it was confirmed that the annual ring orientation affects the flexural behavior of wood in the transverse direction. The quarter-sawn specimens exhibited the largest modulus of elasticity (MOE) and modulus of rupture (MOR). The ray tissue aligned against the load may have contributed to the restriction of cell deformation. The rift-sawn specimens exhibited the smallest MOE and MOR, possibly owing to the loading of the specimen in the in-plane off-axial direction, which induced the shear deformation of the cell wall. For all three specimen types, the fracture had high occurrence probability in the tension part of the specimen, which exhibited large cell deformation. Therefore, the proposed method can be adapted to the prediction of wood specimen fractures. With different test wood species, this approach can be of great help in elucidating the relationship between the anatomical features and the mechanical behavior of wood to improve the effective utilization of wood resources.