Unidirectional Samples Research Articles

Cracking mechanisms in additively manufactured pure tungsten from printing single tracks, thin walls and cubes

Cracking is a critical issue in the additive manufacturing of pure tungsten (W). To eliminate crack formation, it is imperative to gain an in-depth understanding of the underlying mechanisms behind this process. In this study, we systematically investigated the crack behaviors of single tracks, thin walls, and cubes fabricated using powder bed fusion–laser beam (PBF–LB) technology with nonrotational parallel-hatching scanning. The energy framework was employed to elucidate the mechanism of crack formation. The longitudinal cracks appearing in the microstructures of single tracks and the through cracks existing in thin walls and cubes were characterized. Notably, periodic through cracks extended upward across the sample, appearing at every single hatch in unidirectional samples and at every other hatch in bidirectional samples. The horizontal, longitudinal, and transverse cross sections of cubes were studied to clarify the correlation between through crack arrangement and solidification microstructure. Based on a comprehensive analysis of grain boundaries, we proposed a deformation-cracking competition mechanism in PBF–LB tungsten. Geometric effects in the crack and microstructure were also revealed. This study could provide valuable insights into the formation of cracks in PBF–LB tungsten and serve as a foundation for future investigations aimed at eliminating cracks.

Mode I interlaminar fracture characteristics of CNTs doped woven and unidirectional CFRP via acoustic emission

This paper investigates the delamination behavior of two composite structures with carbon nanotubes (CNTs). Double cantilever beam (DCB) samples with CNTs of different areal densities are adopted to analyze the toughening and damage mechanism via acoustic emission (AE) method. The result demonstrates that the CNT layer improves the fracture toughness of unidirectional (UD) sample interfaces effectively, but for the plain woven (PW) samples, only works on initial fracture toughness with low CNTs areal density. In addition, more AE events are recorded in the CNT modified samples than in the reference sample. Combined with the observation of the fracture surface and crack propagation path, the agglomeration with high content CNTs weakens the interlayer strength in PW samples, whereas the fracture toughness is significantly enhanced in UD composites due to the increase of the crack propagation path.

Effects of inter-ply mismatch angle on interlaminar properties and their influence in numerical simulations

In this study, the interlaminar shear strength and mode II fracture toughness of carbon-epoxy laminates with respect to different inter-ply mismatch angles are determined and shown to generally decrease as inter-ply mismatch angle increases. Despite this dependency, most numerical simulations of multi-directional laminates involving delamination adopt interlaminar properties measured from unidirectional laminates only. Using the derived angle-dependent shear strength and fracture toughness, computational simulations of short-beam shear and out-of-plane loading of multi-directional laminate plates were performed and compared with physical experiments. It is shown that delamination tends to form intensively at interfaces with larger mismatch angles where delamination resistance is lower. This characteristic is captured by assigning interlaminar properties corresponding to the inter-ply mismatch angles in the numerical simulation. The practice of applying interlaminar properties obtained from tests on unidirectional samples may lead to erroneous predictions of varying severity for the simulation of multi-directional laminates.

Orthogonal sampling moiré method and its application in microscale deformation field measurement

Sampling moiré (SM) method is one of the modern deformation measurement methods in photomechanics. This method realizes the visualization measurement of deformation field by a sparse sampling of grating images, showing the advantages of high efficiency and strong robustness. However, in the traditional sampling moiré method, the unidirectional sampling mode is used with the result that the two-dimensional deformation cannot be acquired at the same time. In this paper, by combining the orthogonal sampling and Fourier phase analysis techniques, an orthogonal sampling moiré (OSM) method is proposed with which the two-directional moiré phase of specimen grating can be separated synchronously. The accuracy of the OSM method for the deformation measurement is verified by numerical experiments and PMMA uniaxial tensile tests. As an application, in a scanning electron microscope (SEM), the deformation field near the crack tip of small fatigue cracks is measured, based on which the crack opening displacement (COD) and the stress intensity factors (K) are obtained. Furthermore, the crack closure effects at different temperatures are characterized. The experimental results show that the crack closure significantly affects the process of small crack growth at both room temperature and high temperature, and the crack opening load at high temperature is greater than that at room temperature, which indicates that small cracks are easier to close at high temperature.

Texture and Microstructure Evolution of Ultra-High Purity Cu-0.1Al Alloy under Different Rolling Methods

The microstructure and texture distribution of ultra-high purity Cu-0.1Al alloy target play a key role in the quality of the sputtering film. The Cu-0.1Al alloy sheets were processed by unidirectional (UR) and cross rolling (CR), and X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) technologies were adopted to observe the texture and microstructure evolution. XRD results reveal that the texture types vary greatly in UR and CR due to the change of strain path. As the strain increases to 90%, S texture occupies the most, followed by copper texture in the UR sample, while brass texture dominates the most in the CR sample. Additionally, the orientation density of texture does not increase significantly with the increase of strain but shows a downward trend both in UR and CR modes. EBSD analysis demonstrates that compared with UR, the deformation microstructure in CR is more uniform, and the layer spacing between the deformation bands is smaller, which can reduce the local-region stress concentration. After the completion of recrystallization, the difference in average grain size between the UR and CR-annealed samples is not significant, and the recrystallized grains become much finer with the increase of strain, while more equiaxed grains can be observed in CR-annealed samples.

Effect of scan pattern on Hastelloy-X wall structures built by laser-directed energy deposition-based additive manufacturing

This article systematically analyzes the effect of scan pattern on the geometry and material properties of wall structures built using laser-directed energy deposition (LDED)-based additive manufacturing. Hastelloy-X (Hast-X), a nickel superalloy, is deposited using an indigenously developed 2-kW fiber laser–based LDED system. The wall structures are built using unidirectional and bidirectional scan patterns with the same LDED process parameters and effect of scan pattern on the geometry, microstructural and mechanical characteristics of Hast-X wall structures built using LDED. The wall width is higher for samples deposited with the bidirectional pattern at the starting and ending points as compared to walls built with the unidirectional pattern. Further, the range of width value is higher for walls built with bidirectional strategy as compared to walls built with unidirectional strategy. Wall height is more uniform with unidirectional deposition at the central region, with the range and standard deviation for walls built using bidirectional deposition at 3 and 2.5 times more than unidirectional deposition, respectively. The deposition rate for bidirectional deposition is two times that of unidirectional deposition. The microstructure of the built walls is cellular/dendritic, with bidirectional deposition showing a finer grain structure. Elemental mapping shows the presence of elemental segregation of Mo, C and Si, confirming the formation of Mo-rich carbides. Micro-hardness and ball indentation studies reveal higher mechanical strength for samples built using the bidirectional pattern, with unidirectional samples showing strength lower than the conventional wrought Hast-X samples (197 HV). This study paves a way to understand the effect of scan pattern on LDED built wall structures for building intricate thin-walled components.

Effect of Non-uniformity on Time-of-Flight Measurement of Critically Refracted Longitudinal Waves for Stress Evaluations of Carbon Composite Materials

Acoustoelasticity is an inexpensive and reliable technique for measuring stresses in mechanical structures, provided the influencing factors are controlled. However, ultrasonic waves propagating inside solid bodies are affected by temperature, roughness, differences in microstructure, defects, and others. Non-uniformity is one of the most influential characteristics as it is difficult to evaluate. In this study, experiments were performed to assess the time-of-flight (TOF) of critically refracted longitudinal (Lcr) waves traveling in different regions of a unidirectional carbon-epoxy sample with allowable manufacturing non-uniformities using transducers of 1 MHz and 3.5 MHz to vary the depth of propagation. A phased array system (PAS) with transducers of 5 and 10 MHz was used to identify the regions with non-uniformities in the structure through the signal-to-noise ratio (SNR) and B-Scan images. The material under test is a unidirectional composite bar made of carbon fibers (HexTow® AS4) pre-impregnated with epoxy matrix (HexPly® 8552). The investigation shows that the TOF of the Lcr wave and the SNR results can be linearly related, for an optimized configuration. Therefore, the influence of non-uniformities can be removed from TOF using SNR values. Based on the results, a coupled PAS-Lcr ultrasonic system is proposed to measure stress in composites, including those with non-uniformities.

EFFECT OF DIFFERENT STRAIN ROUTES ON MECHANICAL PROPERTIES AND MICROSTRUCTURE OF COPPER

In the current paper, the role of change in strain routes was investigated, along with the cold rolling of copper metal. Four different strain routes including, (a) unidirectional rolling, (b) reverse rolling, (C) two-stage cross-rolling, and (d) multi-stage cross-rolling, were utilized to investigate the effect of strain routes change on microstructure, texture evolution, and anisotropy. Tensile strength in the unidirectional rolling sample compared to the cross-rolling sample decreased in the direction of initial rolling from 364 Mpa to 340 Mpa, in the direction of 45˚ to the initial rolling from 359 Mpa to 347 Mpa, and in the direction of perpendicular to the initial rolling from 371 Mpa to 360 Mpa. Texture intensity also decreased from 1413 in the unidirectional rolling sample to 992 in the cross-rolled sample. The results demonstrated that by rolling in different routes, the cross-rolling has led to a more homogeneous microstructure, less anisotropy, and weaker texture.

Influences of Printing Pattern on Mechanical Performance of Three-Dimensional-Printed Fiber-Reinforced Concrete.

Underperformed interfacial bond and anisotropic properties are often observed in three-dimensional-printed concrete, where the printing pattern is unidirectional. Such issues could be potentially alleviated by replicating microstructures of natural materials or applying different architectures, where printed layers are arranged into unique and unconventional patterns. Furthermore, the quest to develop printing methods for highly complex or self-support concrete architecture could benefit from these nature-inspired patterns. In this work, the influences of different architectural arrangements of layers on mechanical properties of hardened concrete on compressive and flexural strengths are investigated. Specifically, unidirectional (0°), cross-ply (0°/90°), quasi-isotropic (0°/ ± 45°/90°), and helicoidal patterns (with pitch angles of 10°, 20°, and 30°) are used to create unidirectional, bidirectional, and multidirectional layers in printed objects without and with 0.75% by volume of 6 mm-long steel fibers. The experimental results demonstrate considerable improvements in the flexural strengths of nontraditional specimens without steel fibers over the unidirectional control with a few exceptions. Among investigated patterns, the quasi-isotropic demonstrates significant influences in both compressive and flexural responses of printed concrete samples without steel fibers. The addition of steel fibers leads to noticeable improvement on both compressive and flexural strengths of samples in any pattern compared with their counterparts without fibers. Besides, the inclusion of steel fibers into unconventional layups (cross-ply, quasi-isotropic, and helicoidal patterns) leads to the alleviation of directional dependence of mechanical properties, which is a limitation of the unidirectional samples with fibers. Of all helicoidal patterns, the one with a 10°-angle layup is shown to be more beneficial to the flexural strength enhancement and damage resistance in bending. X-ray microcomputed tomography measurements are performed to visualize the direction and distribution of fibers.

Reduce the Planar Anisotropy of AA6016 Aluminum Sheets by Texture and Microstructure Control

In order to produce a more isotropic Al-Mg-Si alloy sheet in the T4 temper, pre-annealed AA6016 hot rolled strips were cold rolled by conventional rolling, transverse rolling and an alternative rolling schedule. In this work, a weak and random texture was obtained in the alternative rolling sample, which was treated by solution treatment, as well as a fine and homogeneous recrystallized grain structure that reduced the earing percentage from 8.2% to 2.7%. An earing test was used to characterize the planar anisotropy of the AA6016 alloy. The crystallographic texture and microstructure of the AA6016 aluminum sheet were analyzed by X-ray diffraction and electron back scatter diffraction (EBSD) techniques. The earing percentage was reduced and eight-fold ears were produced in unidirectional rolled samples with strong β-fiber textures. Conversely, a higher earing percentage and four-fold ears were obtained for the alternative rolled sample, which was characterized by an appropriate combination of the Cube {001}<100> and stabilized BND {011}<3`22> [(φ1,¢,φ2) = (42°,45°,0°)] orientations. The main reason for the distinct earing profiles was the grain spatial distribution of textures formed during the cold rolling process. The dynamic recovery of the AA6016 alloy was promoted and both the nucleation and subsequent growth of the Cube orientation were suppressed by alternative rolling.

Exploration of irreversible residual stresses in a carbon/epoxy composite

Residual stresses in composite material may rise from several sources; difference in thermal straining between fibre and matrix, chemical shrinkage of the matrix material. The presented paper explores an irreversible Eigen-strain in the material which is shown to change and be removed by a thermal treatment above the Tg of the matrix material. This has been observed though TMA measurements of a carbon/epoxy unidirectional sample. The local Eigen-strain is translated into stresses using a visco-elastic constitutive relation. Further exploration indicates that cooling rate influence the generation of residual stresses in the material.

Effect of residual stress on the bending response of L-shaped composite laminates

Residual stresses after manufacturing are critical in fiber-reinforced composite materials, since they may contribute significantly premature failure. In this study, the effect of residual stress on the out-of plane strength of corner sections of L-shaped laminates was investigated. The composite laminates with different thicknesses, and stacking sequences were manufactured in an autoclave and four-point bending tests were conducted on the samples. In order to investigate the internal damage of the samples, optical microscopy technique was used. A combined four step model was introduced, which includes the evaluation of residual stress and four-point bending analysis of the laminates. Residual stresses were calculated in the first three steps and quasi-static tests were simulated in the last step. In the last step, Cohesive Zone Model and Hashin’s failure criteria were used for interlaminar and intralaminar failure respectively. According to the results, residual stresses do not change the region and the type of initial failure mode. However, residual stresses decrease the initial failure load for cross-ply (XP) samples. Also, they change the magnitude of the hoop stress values for unidirectional (UD) samples. The models predicted stiffness and the location of delamination well for UD samples and the location of matrix tensile failure for XP samples.

Determination of in-plane and through-the-thickness coefficients of thermal expansion in composite angle brackets using digital image correlation

A procedure for the simultaneous determination of in-plane and through-the-thickness coefficients of thermal expansion (CTEs) in composite angle brackets is presented. Carbon fibre/epoxy samples are heated and digital image correlation is used to determine the in-plane strains and out-of-plane displacements in one side of each sample. In-plane CTEs are obtained from the average in-plane strains measured at different temperatures. Through-the-thickness CTE is obtained from the angular distortion of the samples at different temperatures, which is determined from the out-of-plane displacements. One quasi-isotropic and two unidirectional samples are used. Unidirectional samples allow the material’s in-plane CTEs in orthotropic directions to be determined. Material’s in-plane CTEs permit predicting an in-plane CTE value for the quasi-isotropic sample that agrees with the experimental measurements. Conversely, results obtained for the through-the-thickness CTE in all samples show a significant dispersion. This fact is attributed to the manufacturing distortions present in the curved part of the samples.

The effect of loading rate on the compression properties of carbon fibre-reinforced epoxy honeycomb structures

The effect of varying strain rate on the compression strength and energy absorption characteristics of a carbon fibre-reinforced plastic honeycomb core has been investigated over a wide range of loading rates. The honeycombs were manufactured by infusing an epoxy resin through a carbon fibre fabric positioned in a dismountable honeycomb mould. The vacuum-assisted resin transfer moulding technique yielded honeycomb cores of a high quality with few defects. Compression tests were undertaken on single and multiple cells and representative volumes removed from the cores in order to assess how the compression strength and specific energy absorption vary with test rate. Crushing tests over the range of strain rates considered highlighted the impressive strength and energy-absorbing response of the honeycomb cores. At quasi-static rates of loading, the compression strength and specific energy absorption characteristics of the unidirectional samples exceeded those of the multidirectional cores. Here, extensive longitudinal splitting and fibre fracture were the predominant failure mechanisms in the cores. For all three stacking sequences, the single-cell samples offer higher compression strength than their five-cell counterparts. In contrast, the specific energy absorption values were found to be slightly higher in the five-cell cores. The experiments highlighted a trend of increased compression strength with loading rate in the multidirectional samples, whereas the strength of the [0°]4 samples was relatively insensitive to strain rate. Finally, the energy absorbing capacity of all structures studied was found to be reasonably constant at increasing rates of strain.

Прогнозирование и расчет анизотропии механических свойств однонаправленного углепластика при скоростном нагружении

A model based on the relations of a hereditarily elastic medium, an anisotropic theory of elasticity, and the Volterra correspondence principle has been developed, which allows one to calculate and predict the anisotropy of the rheological properties of unidirectional carbon fiber in a linear area. Based on the proposed model, expressions are obtained for the stress strain curves of unidirectional samples, which were tested on samples made of carbon fiber reinforced plastic IM7/8552 and carbon fiber plastic AS4/3501-6, loaded at different angles to the direction of reinforcement and strain rates corresponding to quasistatic and dynamic loading. It was assumed that the time dependent properties of unidirectional carbon fiber reinforced plastics are manifested in the direction perpendicular to the reinforcement and in-plane shear of the layer. The implementation of the calculation algorithm turned out to be possible due to the use of relations of the algebra of the resolvent operators. To obtain numerical values of the model parameters, the experimental curves of the deformation of carbon fiber samples were processed.

Recrystallization Study of the Al4.5wt%Cu Alloy Conventionally and Unidirectionally Solidified, Deformed and Heat Treated

Abstract The Al4.5wt%Cu is an aeronautical and automobile alloy with extensive use in industry for structural purposes. The aim of this work was to evaluate two different solidification processes of the Al4.5wt%Cu alloy, conventional and unidirectional, as well as its recrystallization process. Firstly, the Al4.5wt%Cu alloy was deformed by cold rotary forging and then heat treated at temperatures that varied from 250 to 450 °C. The samples for analysis were obtained after 54, 76 and 91% of reductions in area. Tests of optical microscopy, scanning electron microscopy and Vickers microhardness were performed to evaluate the recrystallization process. The results indicated that the recrystallization started at 350 oC, being that the conventional samples presented full recrystallization after 5 minutes, while the unidirectional samples presented only partial recrystallization. In general, both solidification processes presented similar results for all of the analysis performed.

Measurement of 3D thermal diffusivity distribution with lock-in thermography and application for high thermal conductivity CFRPs

This paper presents a new technique for measuring detailed three-dimensional thermal diffusivity by the laser-spot-periodic-heating-method using lock-in thermography. Using lock-in thermography, the in-plane thermal diffusivity distribution and out-of-plane thermal diffusivity distribution could be obtained. First, the validity of the measurement method and principle were confirmed using pure metal, isotropic graphite, and a ceramic as reference materials. As a result, the in- and out-of-plane thermal diffusivities coincided with the recommended value, respectively. Next, the three-dimensional thermal diffusivity measurement was applied for high thermal conductivity carbon-fiber-reinforced plastic (CFRP) samples. The two types of CFRP sample, unidirectional (UD) with unidirectional carbon fibers and cross-ply (CP) with orthogonal two-directional carbon fibers, were measured. As a result, it was revealed that UD sample had extremely high thermal diffusivity in the fiber direction only. On the other hand, it was inferred by measurement and by thermal analysis that the result of CP sample had a high, isotropic, in-plane thermal diffusivity distribution.

Analysis of Heat Treatment Response for Cryorolled AA2219 Alloy

AA2219 sheets used in space applications need a combination of properties as strength and ductility for its enhanced usage. For this purpose, the heat treatment parameters were optimized to bring out the better strength in these alloys. AA2219 sheets were rolled at a cryogenic liquid nitrogen temperature at − 196 °C and room temperature for 50% and 75% reduction. DSC analysis was done to relate the effect of the precipitation kinetics to the strength of the material. The rolled samples were subjected to annealing at different temperatures for shorter periods. The annealing parameter was optimized by using mean results from the full factorial design based on the microhardness values obtained. Using the optimized annealing parameter, artificial ageing was performed at temperatures ranging from 75 to 125 °C for 30 h. The uni-directional rolled samples showed maximum strength after ageing at 125 °C for 24 h and cross-rolled sample at 100 °C for 18 h. A predictive model using regression and ANFIS were designed to determine the responses for the various input parameter settings for both the annealing and ageing and was validated. Analysis of variance was used to determine the significance of the ageing process parameters proving reduction percentage and ageing time having more effect on the heat treatment process.

<italic>X</italic>-Band Circuit-Analog Absorbers Using Unidirectional Carbon Fiber

This letter explores single-ply pre-impregnated (pre-preg) unidirectional carbon-fiber-reinforced polymer (CFRP) laminas for use in X -band (8.2–12.4 GHz) absorbers. Specifically, the unidirectional CFRP sheets were used as lossy frequency selective surfaces in single- and two-layer circuit-analog absorbers (CAAs). The aim of this letter was to assess the feasibility of using only the fiber orientation and areal weight of the material as a tuning aid. A free-space materials characterization system was used to characterize the anisotropic complex permittivity and anisotropic surface impedance of two different areal weight unidirectional CFRP samples. The absorbers were prototyped in a mechanically reconfigurable fixture with the aid of a vacuum-bagging system. The reconfigurable approach allowed reflectivity measurement performance to be measured without the need to fabricate cured sandwich constructions. Reflectivity measurements were compared to transmission-line models as well as full-wave models developed using commercial simulation software. Regions of validity for the transmission line models were identified. The results show that single-ply pre-preg unidirectional CFRP laminas have the potential to be used in single- and two-layer CAA configurations for use in X -band absorbers.

Effect of Strain Path on Texture Evolution in Cold Rolled Pure Titanium

The cold rolling texture evolution as a function of strain path in pure titanium with initial typical recrystallized texture has been studied using viscoplastic self-consistent simulations. Three different strain paths, namely unidirectional rolling, two-step cross rolling and multi-step cross rolling have been employed to investigate the effect of strain path change on the evolution of deformation texture. The simulation results indicate that the activation of predominant prismatic slip in unidirectional rolling sample results in the formation of commonly cold rolling fiber texture RD//<10-10> in pure titanium, whereas the increased activity of basal slip over that of prismatic slip is responsible for the strong ND//<hkil> fiber texture in the two cross rolled samples.