Increase Of Off-axis Angle Research Articles

Optimizing tensile strength and failure prediction in hemp fiber composites: Coupling effects of microfibril angle and fiber orientation under off-axis loading

In response to environmental concerns associated with synthetic fiber-reinforced materials, plant fiber-reinforced composites are increasingly recognized as a more sustainable alternative. However, predicting the mechanical properties of these composites remains challenging due to the unique microstructure of plant fibers. This study proposes an optimized Tsai-Hill failure criterion that incorporates the microfibril angle (MFA) and fiber orientation to enhance failure predictions. The MFA of hemp fibers was measured using X-ray diffraction (XRD), and the failure mechanisms of unidirectional hemp fiber composites under off-axis tensile stresses were thoroughly analyzed. Experimental results reveal a 10.69 % increase in tensile strength at a 5° off-axis angle compared to the fiber direction. As the off-axis angle increases, the failure mode transitions from fiber fracture and pull-out to matrix tearing and interlayer shear failure. For angles above 10°, the failure stress aligns with both the maximum stress and Tsai-Hill criteria. For angles below 10°, integrating MFA into the Tsai-Hill criterion significantly improves prediction accuracy. These findings offer critical insights for optimizing the tensile performance of hemp fiber composites and predicting their behavior under off-axis loading conditions. Consequently, this study can support the use of hemp fiber composites in a broader range of applications.

Characterization on fibre kinking fracture of laminated composites under combined compression and shear at high loading rate

This paper presented a novel method to characterize the fracture toughness and cohesive law of laminated composites under combined compression and shear at high loading rate. Compact compression specimens with off-axis fibres which introduce the in-plane shear stresses were conducted on the uniaxial bidirectional electromagnetic Hopkinson bar system and the displacement field and strain field were recorded by the high-speed camera for the digital image correlation analysis and J-integral calculation. The results reveal that the in-plane shear stresses bring the increase of fracture toughness at crack initiation and propagation and advance the damage initiation of the specimens. The fracture surfaces indicate that the shear stresses cause the fibre bundles' shear failure during the formation of the kink band, accompanied by more energy dissipation with the increase of off-axis angle.

On asymmetric failure in additively manufactured continuous carbon fiber reinforced composites

Due to a filament-by-filament and layer-by-layer structure produced by an additive manufacturing (or namely 3D printing) technique, mechanical properties of 3D printed materials diverge considerably from those of carbon fiber reinforced polymer (CFRP) composites made by traditional processes. This study aims to experimentally characterize their mechanical properties through the off-axis tensile, compressive and V-notch shear tests. Advanced imaging techniques, such as digital image correlation (DIC), scanning electron microscopy (SEM) and computed tomography (CT), are used to investigate anisotropic nature of materials and failure mechanisms of the 3D printed CFRP composites. The applicability of conventional failure criteria to the 3D printed CFRP composites, including Tsai-Wu, Tsai-Hill, Hoffman, Maximum Stress, Hashin, Puck and LaRC05, is assessed systematically. The experimental results divulge that the filament-by-filament and layer-by-layer features intrinsic to 3D printed CFRP composites lead to an uneven yet organized distribution of voids. This characteristic contributes on the development of claw mark strain patterns, parallel track inter-fiber failure patterns, and distinct compression failure modes such as delamination and interlayer crack under loading perpendicular to the fiber direction. The voids in the 3D printed CFRP materials are partially responsible for the significant asymmetry when off-axis angle increases. Notably, the conventional failure criteria exhibit limited capability for predicting the off-axis tensile strength accurately. This phenomenon can be attributed to the redistribution of inherent fiber waviness as the fiber encounters tensile loads. Based on the experimental results, the inter-fiber failure as per the LaRC05 is modified to obtain an enhanced failure criterion for predicting the off-axis tensile strength. This study is expected to provide fundamental understanding of structural characteristics and mechanical properties for 3D printed CFRP composites.

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.

Mechanical response and damage mechanism of 2D woven SiCf/SiC ceramic matrix composites under tension-shear coupling load

The macroscopic mechanical behavior and coupling damage mechanism of 2D woven SiCf/SiC ceramic matrix composites (CMC) prepared by chemical vapor infiltration (CVI) process under tension-shear coupling load were experimentally analyzed. At macroscale, the uniaxial tension, uniaxial shear and off-axis tension tests were completed and the results were compared. The tensile modulus and proportional limit have no significant difference with the increase of off-axis angle, while failure strength decreases by 22.3% compared the 45° tensile (165.4 MPa) with the 0° tensile (212.8 MPa). Based on the strain-difference analysis, the coupling damage effect of SiCf/SiC-CMC was characterized. At microscale, the main damage types of SiCf/SiC-CMC during loading were analyzed by scanning electron microscope (SEM), and the corresponding laws between micro damage development and macro mechanical behavior under different loading conditions were revealed by cluster analysis of acoustic emission (AE) signals. The test results show that the micro damage caused by tensile and shear stress components promote each other. The macro mechanical behavior of SiCf/SiC-CMC under complex loading is controlled by basic micro damage forms such as matrix cracking, interface debonding and fiber fracture. The tension-shear coupling load affects the evolution process of the basic damage forms, and then change the macro properties of SiCf/SiC-CMC.

Prediction of strength and fatigue life for 2D plain-woven high-alumina fiber reinforced alumina matrix composites under a complex in-plane stress state

This paper presents a strength criterion and fatigue life prediction method for 2D braided alumina matrix composites under a complex in-plane stress state. The static strength of the material was obtained by in-plane tensile, compression, and pure shear tests. Considering the difference between tensile and compressive properties of materials and the influence mechanism of in-plane tensile and shear coupling on material strength, a revised Hoffman strength theory was proposed. The predicted off-axis tensile strength is consistent with the test results, and the deviation is not more than 10%. Tensile fatigue tests were carried out with the off-axis angle θ=0°, 15°, 30°, 45°, the stress ratio R=0.1, and frequency f=10 Hz. The test results show that the fatigue life decreases with the increase of off-axis angle. Due to the in-plane shear stress component, the fatigue failure is gradually changed from fiber-dominated to fiber-matrix dominated mode. Based on a combination of the uniaxial tensile fatigue life curve, the Broutman-Sahu residual strength model, which is used to characterize the variation of the residual strength with the fatigue cycles, and the modified Hoffman strength theory, the paper proposes a fatigue life prediction model under complex in-plane loading conditions. The fatigue shear damage factor is defined to characterize the effect of the normal and shear stress interaction on fatigue life. The fatigue life prediction model is used to predict the fatigue life of specimens in the off-axis tensile fatigue tests. The predicted result agrees with the test result, and the deviation is within the 1-time life span. The results indicate that the proposed fatigue life prediction model can be used to predict the fatigue life of 2D braided alumina matrix composites under the complex in-plane stress condition with the given stress ratio, temperature, and fatigue load frequency.

Progressive Damage Analysis of Carbon Fabric-reinforced Polymer Composites under Three-point Bending

Damage evolution of carbon fabric-reinforced polymer composite with various off-axis angles during the progressive bending tests was monitored by acoustic emission and Micro-CT. Meanwhile, the acoustic emission signals can be post-processed by k-means clustering methods. The results indicate that the maximum load and stiffness of laminates decrease with the increase of off-axis angle. Three key points (linear growth point, maximum load point and fail point) are selected to research the progressive damage. The existence of the Kaiser effect is observed. With the increase of off-axis angle, the damage degree and the load shared by the elastic-plastic matrix increase. The bending of matrix can enhance the fracture toughness and restrain the damage. The complementary technology can provide a basis for health monitoring of CFRP laminates.

Effect of elliptical notches on mechanical response and progressive damage of FMLs under tensile loading

Abstract This paper aims to explore the residual strength and elucidate the failure mechanisms of elliptical notched FMLs with different design variables through experimental investigations and numerical simulations. Quasi-static tensile experiments are carried out to investigate the effects of off-axis angle, inclination angle and aspect ratio on the tension behaviors with the aid of digital image correlation technique (DIC). Subsequently, a progressive damage model integrated with a VUMAT subroutine is employed to study the progressive damage evolution and failure mechanism, considering thermal residual stress. Meanwhile, damage initiation and evolution as well as the finial damage patterns are explored systemically combining the strain distributions from DIC technique, equivalent plastic strain distributions of aluminum layers and final damage morphologies of specimens. Results show that the notch strength is more sensitive to off-axis angle and inclination angle as compared with the aspect ratio. The failure morphology exhibits a neat and straight crack for on-axis cases, whereas it presents a fracture along the fiber direction of adjacent layer with apparent pull out of fiber for off-axis cases. With the increase of off-axis angle, the ultimate failure mode gradually transfers from fiber-driven to aluminum-driven accompanied by the corresponding critical failure mechanism varying from tension to tension-shear.

Micromechanics-based characterization of mechanical properties of fuzzy fiber-reinforced composites containing carbon nanotubes

General off-axis mechanical behavior of fuzzy fiber-reinforced composites (FFRCs) is investigated using a 3-dimensional unit cell micromechanics model. In the FFRCs studied herein, uniformly aligned carbon nanotubes (CNTs) are radially grown on the circumferential surfaces of carbon fibers. The effects of orientation and volume fraction of carbon fiber, volume fraction of CNT, stiffness and thickness of the interphase region created due to non-boned interaction between the CNT and polymer matrix on the elastic response of the FFRCs are studied. The results reveal that the mechanical properties of the FFRCs are strongly dependent on the off-axis angle of carbon fiber. With the increase of off-axis angle from 0° up to 90°, the elastic modulus of FFRC sharply decreases up to 45° and then its value increases, whereas without considering CNTs on the surface of carbon fiber, the elastic modulus continuously decreases. It is shown that the growth of CNTs on the surface of carbon fiber can lead to the highest enhancement for the elastic modulus of 90° on-axis coupon. Poisson's ratio of the FFRC rises with the increase of off-axis angle from 0° up to about 35° and then its value decreases. Also, the increase of CNT volume fraction yields a significant increase for the elastic modulus of 90° on-axis coupon, while rising carbon fiber volume fraction can substantially enhance the elastic modulus of 0° coupon. According to the obtained results, increasing both stiffness and thickness of the interphase can improve the elastic modulus of FFRC over the range of 0–90°, especially for 90° on-axis coupon. The results predicted by the present model are much closer to the experiment than those predicted by the numerical simulations available in the literature.

Orientation-dependent tensile deformation and damage of a T700 carbon fiber/epoxy composite: A synchrotron-based study

Uniaxial tensile experiments are conducted on a T700 carbon fiber/epoxy composite along various off-axis angles. Stress–strain curves are measured along with strain fields mapped via synchrotron x-ray digital image correlation, as well as computerized tomography. Elastic modulus and tensile strength decrease with increasing off-axis angles, while fracture strain exhibits a nonmonotonic trend as a combined result of tensile strength decrease and fracture mode transition. At high off-axis angles, strain field mapping demonstrates distinct tensile and shear strain localizations and deformation bands approximately along the fiber directions, while deformation is mainly achieved via continuous growth of tensile strain at low off-axis angles. Roughness of fracture planes decreases exponentially as the off-axis angle increases. The stress–strain curves, strain fields, tomography and fractographs show consistent features, and reveal a fracture mode transition from mainly tension (fiber fracture) to in-plane shear (interface debonding).

Paraxial models for the surface plasmon self- interference at off-axis excitation.

The surface plasmon self-interference excited by a strongly focused, linearly polarized vortex beam at off-axis illumination in a paraxial regime is analytically studied. The off-axis excitation is investigated using a geometrical model. The combination of an angular spectrum representation and homogeneous transformation is applied to derive the integral expressions of the surface plasmon polariton fields for off-axis directions both parallel and perpendicular to polarization plane, and an off-axis convergence angle is used to compute the integral. The surface plasmon excitation is represented by the relative peak intensity of the longitudinal field, while its standing wave is characterized by the full width at half-maximum of the transmitted field intensity distribution profile. Both models consistently show that even in ideal Gaussian microscopic imaging systems, self-interference degradation exists. When the off-axis angle increases, the surface plasmon interference disappears and the fields detune out of surface plasmon resonance.

Analysis on Visual Imaging Quality Affected by Human Eye’s Higher Order Aberration Measurement via Off-axis Wavefront

This paper mainly discusses the system design and establishes the experimental system which conducts realtime off-axis measurement of the human eye’s aberration. The target is installed in the optical path of human eye objective measuring system. When observing the target, the human eye adjusts itself to a relaxing state so as to conduct measurement. The target being observed conducts movement vertical to optical axis within its field range, and then realized the off-axis aberration measurement. Research data show that only when normal incidence of the light occurs can changes the human eye’s aberration which caused by measuring errors be minimized. Data from all the objects reveal that second order aberration has the largest proportion among all the aberrations and does not show significant change within the increased off-axis angle in the vision field. Third order aberration increases with the increase of off-axis angle in the vision field. Fourth order aberration shows no obvious increase except the first item. High order aberrations for fifth order and above do not change much with the increase in off-axis angle in the field range. Meanwhile, the proportion of fifth order or above aberrations is the smallest among all the objects’ aberrations. The study results demonstrate that it is the aberrations of the former four orders that have a significant affect on human eye. With regard to off-axis aberration, third order and fourth order affect human eye imaging quality significantly. On the contrary, the high order aberrations of fifth order and above have a minor influence on human eye imaging quality.

Performances of Non-Line-of-Sight Ultraviolet Multi-Scatter Propagation for Noncoplanar Geometries

A multi-scatter propagation model based on Monte Carlo method is presented. This model can be applied to all the geometries, including coplanar or noncoplanar scenario. The mathematical description of this model is deduced. We obtain the spatial positions of photon with three Cartesian coordinates after each propagation step and the received judgment conditions. Employing a photon tracing technique, Monte Carlo simulation is performed to investigate the signal impulse response and the path loss. The results indicate that, when the off-axis angle increases, the amplitude of the impulse response decreases, while the path loss increases. In addition, it is observed that the pulse width increases with the off-axis angle.

Micromechanical analysis of off-axis loading of fiber reinforced composites with imperfect interface bonding

A micromechanical method based on generalized method of cells for investigating elastic and plastic response of composites subjected to off-axis loading is presented. Through implementing the formulas of interfacial displacement into average displacement continuity, the relationship between sub-cell strain rate and macro-strain rate is constructed in the condition of composites with wake interface bonding. On this basis, microstructure parameters, such as fiber cross-section shape, fiber volume fraction, are considered. Numerical results indicate that increasing fiber volume fraction will decrease the flow stress of composites in the condition of weak interfacial bonding. While the tendency is independent on fiber cross-section shape. Also, the effect of off-axis tensile strength tends to be weakened with the increase of off-axis angle in the condition of weak interfacial bonding. However, square fiber always provides more flow stress than circular fiber in both perfect interfacial binding and weak interfacial bonding.

The Adaptive Blade Design and Evaluation of Wind Turbine

This article explored the design method of the wind turbine blade being of flapping-twist adaptive performance and how to evaluate its feasibility and reliability according to the comprehensive factors. The results indicate that both spar cap and skin with off-axis carbon fiber can achieve the efficient flapping-twist coupling effect. Through overall investigation, the results show that the maximum fiber strains of tensile and compressive go up with increase of the off-axis angle, and the peak inter-laminar shear stress increase more rapidly. While, all of these evaluating indicators should be kept in the reference range for used materials. Moreover, when the off-axis angle increases, the peak Von Mises stress declines. In addition, the impact of natural frequencies on the blade design is proved to be insignificant. Finally, utilizing the medial axis laminates in the blade decoupled area is helpful to strengthen the blade fatigue resistivity.

Characterization of Schottky Diodes on 4H-SiC with Various Off-Axis Angles Grown by Sublimation Epitaxy

The effects of basal-plane defects on the performance of 4H-SiC Schottky diodes using a Ni electrode are demonstrated. Systematic characterization was performed using 4H-SiC epitaxial layers grown by sublimation epitaxy on substrates with various off-axis angles. As the off-axis angle increases, the ideality factor of the current-voltage characteristics increases, and the Schottky barrier height decreases, corresponding to an increase in the number of basal-plane defects. The reverse-bias current degrades for high off-axis samples. These results indicate that basal-plane defects degrade the device performance. Schottky diodes that possesses good characteristics were obtained for samples with low off-axis angles (2o- and 4o-off samples).

Off-axis crushing of GFRP tubes

Four types of round composite tubes, [±75] 3 glass/polyester, [0/±75] 2 glass/epoxy, [±15] 3 glass/polyester and [±15] 3 glass/epoxy, were compressed under off-axis loadings to examine how the ply pattern and off-axis angle affect the energy absorption performances. The off-axis angle, i.e., the inclination angle of loading direction with respect to the tube axis, varied from 5 to 25° with an increment of every 5°. At a small off-axis angle, the specimen demonstrated similar crushing behavior to the axial crushing. At a moderate off-axis angle, three characteristic crushing stages were identified as triggering stage, sustained crushing stage and toppling stage. At a relative large off-axis angle, e.g. 20 and 25°, the toppling stage was soon arrived and the specimen absorbed little energy. Toppling character depends upon the composite ply pattern. [±75] 3 glass/polyester specimen shows the strongest toppling tendency. For [0/±75] 2 glass/epoxy specimen, 0 ply is beneficial in preventing toppling, but the crushing mode changes significantly while off-axis angle increases. Under off-axis loadings, [±15] 3 specimens are likely to experience catastrophic failure. The transition characters of load-displacement history, tendency of toppling, fracture pattern and material utilization efficiency with respect to off-axis angle were discussed.

Off-axis fatigue behaviour and its damage mechanics modelling for unidirectional fibre–metal hybrid composite: GLARE 2

The fatigue behaviour of a unidirectional fibre–metal laminate GLARE 2 has been studied under various off-axis loading conditions. Tension–tension fatigue tests were first performed at room temperature on nine kinds of plain coupon specimen with a different off-axis angle. A non-dimensional effective stress defined on the basis of the classical static failure theory was applied as an off-axis fatigue strength parameter. A macroscopic fatigue damage mechanics model was then developed using the non-dimensional effective stress, and it was compared with the classical fatigue failure models for composites. The absolute off-axis fatigue strength decreases as the off-axis angle increases. The longitudinal fatigue strength of GLARE 2 is about two times as high as that of the high-strength aluminium alloy , while the transverse fatigue strength is almost one-half. The S – N relationships are almost linear for all off-axis angles in the intermediate range of fatigue life 10 3 < N f <10 5 , and they are followed by fatigue limits. The off-axis fatigue data plotted using the strength ratio (i.e. the maximum fatigue stress normalized by the static strength) are approximately represented by a single master S – N curve. The non-dimensional effective stress succeeds in describing this characteristic of the off-axis fatigue behaviour. The damage mechanics model developed using the non-dimensional effective stress can favourably reproduce the directional nature of the constant amplitude off-axis fatigue behaviour of GLARE 2. This model has an advantage over the classical fatigue failure models for composites with respect to the numerical procedure for fatigue life analysis.