Effects Of Microvoids Research Articles

Tensile performance prediction of CFRPs with voids using multiscale analysis and neural networks

The overall work provides a novel multiscale modeling strategy applicable to the mechanical response prediction of arbitrarily laid porous laminates under uniaxial tension by establishing representative defective volumes (RDVs) for the inter-ply and intra-ply regions. A representative defective volume modeling method based on void characterization results using X-ray computed tomography for the interlaminar region was proposed. Elastic properties, tensile strength, and fracture energies of porous and poreless microscale models were obtained and used in the macroscale model. Defective properties were assigned to partial elements in the macroscale model according to the preset distribution mode. The Extended Finite Element Method was taken to simulate the failure process. Tensile properties of porous laminates with stacking sequences of [90]8, [90/0/90/0]2S, and [45/90/-45/0]2S were experimentally tested and numerically simulated. The predicted tensile properties and crack propagation behavior coincide well with the test results. Besides, the effects of micro-voids and macroscopic void distribution were discussed in detail. Further, a back-propagation neural network was used to achieve a fast prediction of the tensile properties and save computational costs. The computation time (<1 s) greatly improved compared with multiscale modeling (>38 h). This work provides an efficient tool for composite performance prediction, design, and optimization.

Effects of micro voids on the early stage of transverse crack formation in unidirectional composites

Effects of micro voids on the early stage of transverse crack formation in unidirectional composites

Research on the effect of micro-voids on the deformation behavior and crack initiation lifetime of titanium alloy under cyclic loading by crystal plasticity finite element method

Micro-voids defects associated with materials formed by diffusion bonding and super-plastic connection limit their usage in the aerospace industry. A crystal plasticity finite element (CPFE) model was established in the current work to investigate the relationship between local stress concentration and the micro-void defects, and predict the fatigue crack initiation lifetime. Based on the concept of the curvature of the crack tip, the shape and size of the voids are strictly separated, so that the influence of the single parameter of voids on the microplasticity deformation behavior is better obtained. It's found that the micro-voids lead to the stress concentration phenomenon in the material, the effect of the curvature of the void tip surface is greater than that of the void size. Stress concentration tends to occur in the hard grains. When L/d is greater than 2.0, stress concentration factor reaches a constant small value. The simulated results based on the Manson-Collin law and Fatemi-Socie parameter show that when the void tip curvature is greater than 0.4 μm−1 or L/d is less than 2.0, the fatigue crack initiation lifetime is less than 106 cycles, indicating that the influence of such micro-voids on fatigue crack initiation is small. This work has a good guiding significance for properties and lifetime prediction of the material with defects.

Effect of microvoids on microplasticity behavior of dual-phase titanium alloy under high cyclic loading (I): Crystal plasticity analysis

A crystal plasticity finite element (CPFE) model was established and 2D simulations were carried out to study the relationship between microvoids and the microplasticity deformation behavior of the dual-phase titanium alloy under high cyclic loading. Results show that geometrically necessary dislocations (GND) tend to accumulate around the microvoids, leading to an increment of average GND density. The influence of curvature in the tip plastic zone (TPZ) on GND density is greater than that of the size of the microvoid. As the curvature in TPZ and the size of the microvoid increase, the cumulative shear strain (CSS) in the primary α, secondary α, and β phases increases. Shear deformation in the prismatic slip system is dominant in the primary α phase. As the distance between the microvoids increases, the interactive influence of the microvoids on the cumulative shear strain decreases.

Influence of Pressure and Temperature on the Strength Properties of a Laminate Produced by the RTM Method

Abstract This paper discusses the effect of pressure on the content of microvoids and defects inside laminates fabricated under different pressures, by vacuum methods. Two basic vacuum methods resin transfer molding (RTM) and vacuum bag method were used in this paper. A glass mat with an alignment angle of (0□/90□) and a mass of 450 g/cm2 was used to produce the laminate, and a polymer resin was used as the matrix. Special attention was paid to the technological parameters of both processes. A mathematical analysis of the most important parameters which include flow rate, permeability, and gelation temperature has been carried out. In addition, the resin temperature is used to reduce the viscosity of the resin to facilitate its flow through the reinforcement, and in the final stage of production to control the chemical reactions occurring in the mold. The pressure is chosen so that the resin flow is continuous. The synchronization of these two parameters and the measurement of the time in which they occur are called the “cure cycle”. In the final step of the study, the composite was subjected to a static tensile test, using specimens of two different dimensions (scale effect) to evaluate the effect of microvoids and microcracks created by the processes on the strength of the material.

Validation of a finite element method for simulation of components produced by continuous carbon fiber reinforced additive manufacturing

In this research, a method is examined by which the behavior of continuous carbon fiber reinforced additive manufacturing may be simulated using Finite Element Analysis. This technique is used in a simulated tensile test experiment in which the findings are compared to results determined from theoretical calculations according to the Rule of Mixtures method and from existing mechanical testing results. Four different fiber reinforcement configurations are examined with fiber volume fractions ranging from 4% to 32%. It was found that for fiber volume fractions of 11%, the simulation results closely match those predicted theoretically by the Rule of Mixtures as well as the mechanical testing results published in existing research. Lower fiber volume fractions near 4% yield less accurate results, with a 20% error due to the fact that the anisotropic behavior of the polymer matrix is the dominant material trait. Simulation of higher volume fractions near 32% closely approximate theoretical predictions, however neither the theoretical results nor the simulation results accurately reflect real world mechanical testing, indicating that nonideal condition factors such as the effect of micro-voids between the start and end of the fiber reinforcements play a significant role in the overall strength of the material. Thus, for fiber volume fractions near 11%, this simulation method can accurately be used to predict the behavior of end-use components, but more study must be done to increase simulation accuracy in low and high fiber volume fractions.

Nonlinear forced vibrations of functionally graded piezoelectric cylindrical shells under electric-thermo-mechanical loads

This paper develops a new solution approach to solve nonlinear forced vibrations of functionally graded (FG) piezoelectric shells in multi-physics fields. The FG piezoelectric shells are subjected to electric-thermo-mechanical loads, and the effect of micro-voids is considered here. Motion equations are obtained by using Hamilton's principle, and combining with the Donnell nonlinear shallow shell theory. Afterwards, a new method combining multi-mode Galerkin scheme and Pseudo-arclength continuation method is used to solve the nonlinear multiple internal resonances and bifurcations of the multi-degree-of-freedom systems. The novel feature of this approach is that it can efficiently obtain the unstable solution and tackle the difficult problems in mathematics encountered during formulation. The results show that the external applied voltage, temperature change, external excitation, power-law exponent, and porosity volume fraction play important roles on nonlinear vibration response and bifurcation analysis of FG piezoelectric shells with micro-voids.

A comparative analysis of polyacrylonitrile-based carbon fibers: (II) Relationship between the microstructures and properties

The relationships between the mechanical properties and the microstructures of typical polyacrylonitrile-based carbon fibers were studied on the basis of the elastic wrinkle model and Griffith microcrack theory. The effects of microcrystallite structure and its preferred orientation on tensile modulus, the effects of microvoids and mass density changes on the tensile strength, and the effects of internal residual compressive stress on the strain to failure, were analyzed, based on which the structural factors played a key role in determining the mechanical properties of the fibers.

Effects of microvoids on strength of unidirectional fiber-reinforced composite materials

Microvoids are common defects generated during manufacturing or prolonged services of composites. Depending on the local physical environment, microvoids may form as two types, namely, matrix voids and interfiber voids. These voids are known to have detrimental effects on the load bearing capacity of composites. However, it is relatively unknown the quantitative influence of these voids on the mechanical strength of composites. Without a direct mapping between defects and ply-level constitutive behavior, high-fidelity progressive failure predictions of composite structures is challenging. To bridge the gap, a micromechanics-based finite element framework is developed to investigate composites failure in the presence of three interfiber voids and the matrix void under transverse compression, tension, shearing, and longitudinal shearing. Mechanical strengths under these four loading modes are correlated with microstructural parameters including the void volume fraction, fiber-to-void number ratio, and void orientation. In general, at the same void volume fraction, the pentagonal interfiber void lowers strength the most, followed by the square interfiber void, the triangular interfiber void, and the circular matrix void. The weakening effect is strongly associated with stress concentration in the vicinity of voids, making the fiber-to-void number ratio an important parameter as it determines the local geometry. While void orientation shows limited influence, void size is found to play an important role. Results are expected to guide design and manufacturing of composite materials to achieve less critical flaws and higher strengths. Quantitative understanding of the criticality of voids of various configurations will also assist in-situ evaluation of composite materials.

Effect of micro-void on surface integrity after machining of Ti-6Al-4V workpieces prepared by HIP and forging

In order to study the effects of residual micro-voids on surface integrity of machined Ti-6Al-4V hot isostatic pressing (HIP) material, the HIP workpieces with high density was prepared and Ti-6Al-4V forging workpiece also was machined to compare the progressions of surface integrities of machined Ti-6Al-4V HIP and forging workpieces versus cutting parameters, cutting temperature, and cutting forces in the machining process also were analyzed to study in detail the micro-void influence. The results showed that surface defects of the machined forging workpiece were feed mark, debris, and tearing, while the micro-voids led to the formation of micro-void defect in the HIP workpiece surface. The micro-void of the HIP material had a relief on plastic deformation of the workpiece surface layer, making the tensile residual stress of the HIP workpiece surface less than that of the forging workpiece surface. Besides, surface roughness of the machined HIP workpiece was less than that of the forging workpiece.

코일 그라인딩 처리에 따른 린 듀플렉스 스테인리스강의 미소 기공 형성과 황의 깊이별 분포 변화가 내식성에 미치는 영향

코일 그라인딩 처리에 따른 린 듀플렉스 스테인리스강의 미소 기공 형성과 황의 깊이별 분포 변화가 내식성에 미치는 영향

Multiscale approach with RSM for stress–strain behaviour prediction of micro-void-considered metal alloy

Abstract This paper presents the concept of using a representative volume element (RVE) in a multiscale approach to predict the macroscopic stress–strain behaviour of a cast SS316L specimen under tension up to the point prior to necking. RVE models with various micro-void spatial configurations were built, and the effects of micro-voids and strain rate on the material properties (e.g., yield strength, ultimate tensile strength (UTS), ultimate tensile strain and strain hardening coefficient) were analysed. The spatial configuration of the micro-voids inside the cast SS316L specimen was acquired by the X-ray CT scanning system and each micro-void in the gauge length part was converted into a matching RVE model in the finite element (FE) analysis. Response surface methodology (RSM) was employed to investigate the effect of RVE configurations, i.e., the size of the RVE and the shape and spatial location of the micro-voids, on the material properties (yield strength and UTS) of the cast SS316L specimen at the macroscopic level, and then the optimal levels of the RVE configuration were determined. The stress–strain curve from the simulation did show a good agreement with the experimental results and hence the proposed concept was verified.

A micromechanical study of porous composites under longitudinal shear and transverse normal loading

The mechanical response of porous unidirectional composites under transverse normal and longitudinal shear loading is studied using the finite element analysis. The 3D model includes discrete and random distribution of fibers and voids. The micromechanical failure mechanisms are taken into account by considering the mixed-mode interfacial debonding and pressure-dependent yielding of the matrix using the modified Drucker–Prager plasticity model. The effect of the micromechanical features on the overall response of composite is discussed with a focus on the effect of microvoids and interfacial toughness. Finally, the computational prediction of the porous composite in the transverse normal-longitudinal shear stress space is obtained and compared with Puck’s model. The results show that both interfaces with low fracture toughness and microvoids with even small void volume fraction can significantly reduce the macroscopic strength of composite. The size and shape of microvoids can also microscopically lead to different crack paths.

Effect of Micro-Voids on Crack Initiation and Propagation in Bending Deformation of Al-Mg-Si Alloy Sheet

The crack initiation and propagation during bending have been considered to be affected by second phase particles, micro-voids and shear-bands. However, the effects of the second phase particles and the micro-voids on the crack initiation and propagation during bending have not been fully investigation. In this study, the effect of the second phase particle distribution on the formation of micro-voids, and the effect of the micro-voids on the crack initiation and propagation during bending were investigated using the largest synchrotron radiation facility “SPring-8” and FE-SEM/EBSD. With the bending ratio increasing, the micro-voids increased around the coarse particles near the outer surface. In particular, coarse micro-voids were formed around coarse particles with a high aspect ratio on the shear-bands. At a large cracked part, coarse micro-void was observed at the outmost layer section as a crack initiation site, and coarse micro-voids and asheared fracture surface were observed at the crack propagation site. At the small cracked part with no propagation, cube orientation grains were located under the small crack. It was considered that these cube orientation grains inhibited the formation of shear-bands, therefore, propagation of the cracks did not occur at the small cracked area.

Relationship between performance and microvoids of aramid fibers revealed by two-dimensional small-angle X-ray scattering

Although the crystal structure in aramid fibers and the relationship between the size and orientation of crystallites and the performance of a material have been explored in detail, the effect of microvoids in an aramid fiber on its performance is still not clear. However, it is known that the mechanical properties depend strongly on the fiber morphology. In the present research, two-dimensional small-angle X-ray scattering is applied to characterize the microvoids in aramid fibers. Pauw's two-dimensional full pattern fitting method and scattering model have been enhanced by introducing orientation parameters, such as zenith angle distribution and azimuthal angle distribution, and instrumental parameters like point spread function and beam profile function. A series of aramid fibers with different strengths were studied using the new two-dimensional full pattern fitting method to extract the microvoid parameters from the scattering patterns. The results show that the microvoids in the aramid fiber affect the fiber strength directly. The greater the number of spherical microvoids and the larger the ellipsoidal microvoids, the weaker the aramid fiber.

Micro void coalescence of ductile fracture in mild steel during tensile straining

The ductile fracture occurs mainly in three stages i.e. void nucleation, void growth and the voidcoalescence. The present work focuses on the study the coalescence of existing micro void in a ductile material,mild steel. The specimen with holes in square array at various angle to load axis have been tested. The holeswere machined in the specimen and assuming those hole as the voids. The growth and coalescence behavioursduring tensile straining were observed both in macro and micro levels. Since the existing facility is not adequateto make hole size in micron, this work has been carried out by making hole upto 500 micron. The results arecompared with other specimen with bigger size hole and without any hole. Also the effects of micro voids(present in the material) on the progress of crack have been studied. It is found that with same amount of voids,present in different positions, the mechanical properties of the material are altered.

Effect of micro voids on stress triaxiality-plastic strain states of notched steels

Brittle fracture of structural steel for building use, in general, is triggered by a ductile crack initiated at notched surface after undergoing a noticeable amount of plastic strain. The initiation of the ductile crack correlates to the stress triaxiality-plastic strain states in the notched section. As stress triaxiality and plastic strain are usually derived from finite element analysis, two material models with voids (Gurson-Tvergaard-Needleman model) and without voids (Mises model) are used to do the analyses to study the stress triaxiality-plastic strain states in notched steels. The results show that the effect of micro voids on stress triaxiality-plastic strain states of notched steels is small until stability limit and become significant gradually after stability limit.

The correlation between microstructure and mechanical properties of high-pressure die-cast AM50 alloy

Scanning acoustic microscopy was used to characterize the micro-voids distribution for specimens in non-destructive mode. In addition, the in-situ scanning electron microscopy observation was performed during tensile deformation of high-pressure die-cast (HPDC) of AM50 alloy to obtain the mechanism of fracture induced by micro-voids. The effects of micro-voids on the mechanical properties were discussed. The results obtained from the examination suggest that fracture tends to occur at bigger micro-voids or in the cluster micro-voids area.

Microstructure evolution and interfacial failure mechanism in 316LSS diffusion-bonded joints

Microscopic tensile test was conducted on 316LSS vacuum diffusion-bonded joints to investigate the microstructure evolution and effect of micro-voids on interfacial failure mechanism. In situ observation of the interfacial crack initiation and propagation was carried out during the whole testing. The results showed that the most likely sites for cracks initiation are grain boundaries. The favorable grain boundaries containing cracks are oriented at 0–20° to the loading axis. Intergranular cracks play a dominant role in interfacial failure. Micro-voids do not link up each other until the load is increased to 352 MPa (63% σ b). For 316LSS diffusion-bonded joints, interfacial failure depends mainly on microstructure of joints.

EFFECT OF MICRO VOIDS ON ELASTIC-PLASTIC STATES OF NOTCHED STEELS

Brittle fracture of structural steel for building use, in general, is triggered by a ductile crack initiated at notched surface after undergoing a noticeable amount of plastic strain. The initiation of the ductile crack correlates to the triaxial states of stress and plastic strain in the notched section. As stress triaxiality and plastic strain are usually derived from finite element analysis, two material models with voids (Gurson-Tvergaard-Needleman model) and without voids are compared in this study. The results show that the effect of micro voids on elastic-plastic states of notched steels is small until ultimate strength. Furthermore, it is clarified that prediction of ductile crack can also be done well without consideration of voids.