Showing papers on "Stress concentration published in 2018"

Crack nucleation in variational phase-field models of brittle fracture

Abstract: Phase-field models, sometimes referred to as gradient damage or smeared crack models, are widely used methods for the numerical simulation of crack propagation in brittle materials. Theoretical results and numerical evidences show that they can predict the propagation of a pre-existing crack according to Griffith’ criterion. For a one-dimensional problem, it has been shown that they can predict nucleation upon a critical stress, provided that the regularization parameter be identified with the material’s internal or characteristic length. In this article, we draw on numerical simulations to study crack nucleation in commonly encountered geometries for which closed-form solutions are not available. We use U- and V-notches to show that the nucleation load varies smoothly from that predicted by a strength criterion to that of a toughness criterion when the strength of the stress concentration or singularity varies. We present validation and verification numerical simulations for both types of geometries. We consider the problem of an elliptic cavity in an infinite or elongated domain to show that variational phase field models properly account for structural and material size effects. Our main claim, supported by validation and verification in a broad range of materials and geometries, is that crack nucleation can be accurately predicted by minimization of a nonlinear energy in variational phase field models, and does not require the introduction of ad-hoc criteria.

Laser powder bed fusion of Hastelloy X: Effects of hot isostatic pressing and the hot cracking mechanism

Abstract: Hastelloy X is the trademark for a nickel-based, high-temperature superalloy that is increasingly applied in gas turbine engines because of its exceptional combination of oxidation resistance and high-temperature strength The superalloy suffers from hot cracking susceptibility, however, particularly when processed using additive manufacturing and laser powder bed fusion (LPBF) This paper systematically studies for the first time the effect of post-treatment hot isostatic processing (HIP) on the microstructure and mechanical properties of LPBF-fabricated Hastelloy X, with an emphasis on fatigue performance The experimental results demonstrate that despite the very small number of remaining gas-filled micropores due to pressure counteraction, the high temperature and high pressure during the HIP process promote recrystallisation and closing of the internal microcracks and gas-free pores The HIP-processed specimens are shown to be roughly 130 MPa and 60 MPa weaker than the non-processed specimens in yield strength and ultimate tensile strength, respectively The HIP-processed Hastelloy X exhibits significant improvements in fatigue life, however: the effect of the HIP processing is apparent once the applied stress decreases This improvement in fatigue performance is attributable to the reduction in stress concentration and residual stress release caused by the HIP process The paper also studies the hot cracking mechanism and finds that intergranular microcracks generally occur along high angle grain boundaries; the interdendritic liquid pressure drop between dendrite tip and root is found to be a significant factor in the hot crack mechanism The significance of this research is in developing a comprehensive understanding of HIP processing on the fatigue behaviour of the LPBF-fabricated Hastelloy X The insights on the cracking mechanism, which presents a significant step towards using additive manufacturing to produce complex crack-free parts from this superalloy

Mechanics Of Fatigue

Abstract: Mechanics of Fatigue addresses the range of topics concerning damage, fatigue, and fracture of engineering materials and structures. The core of this resource builds upon the synthesis of micro- and macro-mechanics of fracture. In micromechanics, both the modeling of mechanical phenomena on the level of material structure and the continuous approach are based on the use of certain internal field parameters characterizing the dispersed micro-damage. This is referred to as continuum damage mechanics.The author develops his own theory for macromechanics, called analytical fracture mechanics. This term means the system cracked body - loading or loading device - is considered as a mechanical system and the tools of analytical (rational) mechanics are applied thoroughly to describe crack propagation until the final failure.Chapter discuss:preliminary information on fatigue and engineering methods for design of machines and structures against failures caused by fatiguefatigue crack nucleation, including microstructural and continuous modelstheory of fatigue crack propagationfatigue crack growth in linear elastic materials subject to dispersed damagefatigue cracks in elasto-plastic material, including crack growth retardation due to overloading as well as quasistationary approximationfatigue and related phenomena in hereditary solidsapplication of the theory fatigue crack growth considering environmental factorsunidirectional fiber composites with ductile matrix and brittle, initially continuous fiberslaminate compositesMechanics of Fatigue serves students dealing with mechanical aspects of fatigue, conducting research in fracture mechanics, structural safety, mechanics of composites, as well as modern branches of mechanics of solids and structures.

Fracture Evolution Around a Cavity in Brittle Rock Under Uniaxial Compression and Coupled Static–Dynamic Loads

Abstract: To experimentally investigate the stability of underground excavations under high in situ stress conditions, several rock samples with a mini-tunnel were prepared and subjected to monotonic axial and coupled static–dynamic loading until failure. Mini-tunnels were generated by drilling circular or cubic cavities in the centre of granite rock blocks. Strain gauges were used to monitor the deformation of the mini-tunnels at different locations, and a high-speed camera system was used to capture the cracking and failure process. We found that the dynamic crack initiation stress, failure mode and dynamic crack velocity of the specimen all depend on the pre-stress level when the sample is under otherwise similar dynamic disturbance conditions. The crack initiation stress threshold first increased slightly and then decreased dramatically with the increase in the pre-stress value. The specimens were mainly fractured by tensile cracks parallel to the compression line under lower pre-stress, while they were severely damaged with additional shear cracks under higher pre-stress. Furthermore, the propagation velocity of the primary crack was significantly larger than that of the subsequent cracks. The effect of applying different amounts of static pre-stresses on the velocity of the primary tensile crack was similar to that observed for the crack initiation stress threshold; however, it did not affect the velocity of the secondary and subsequent tensile cracks.

Anisotropy of fatigue crack growth in wire arc additive manufactured Ti-6Al-4V

Abstract: The effects of crack growth direction on the fatigue crack growth of wire arc additive manufactured Ti-6Al-4V were studied. The fatigue crack growth rate (FCGR) of horizontal and vertical samples has different stress intensity factor transition point, ΔKT, which is 11.3 MPa m1/2 and 10.3 MPa m1/2, respectively. The FCGR of vertical sample is 5% higher than that of horizontal sample when the stress intensity factor, ΔK is smaller than ΔKT. The difference in FCGR is resulted from the microstructure characterization and the fatigue crack growth direction.

A micromechanical study of stress concentrations in composites

Abstract: Random and periodic representations of composite microstructures are inherently different both in terms of the resultant range of stresses that each phase carries as well as the total load over the entire volume comprising both matrix and fiber phases. In this study, an algorithm was developed to generate random representative volume elements (RVE) with varying volume fractions and minimum distances between fibers. The random microstructures were analyzed using finite element models (FEM) and the results compared to those for periodic microstructured RVEs in terms of the range of stress values, maximum stress, and homogenized stiffness values. Using a large number of random RVE analyses, a meaningful estimation for range and average maximum stress in the matrix phase was achieved. Results show that random microstructures exhibit a much larger range of stress values than periodic microstructures, resulting in an uneven distribution of load and distinct areas of high and low stress concentration in the matrix. It is shown that the maximum stress in the matrix phase, often responsible for failure initiation, is largely dependent on the random morphology, minimum distances between fibers, and volume fraction. Moreover, it is shown that the predicted overall load-carrying capacity of the matrix changes depending on the use of random or periodic microstructures.

Assessment of the effect of isolated porosity defects on the fatigue performance of additive manufactured titanium alloy

Abstract: Studies on additive manufactured (AM) materials have shown that porosity reduces the fatigue strength. However, the quantitative impact is not well understood. This paper presents a mechanistic approach to quantify the influence of size, location and shape of gas pores on the fatigue strength of AM Ti-6Al-4V. Ideal spherical and oblate spherical pore geometries were used in the finite element (FE) analysis. The FE results showed a stress concentration factor of 2.08 for an internal spherical pore, 2.1 for a surface hemispherical pore and 2.5 for an internal oblate spherical pore. Subsurface pores within a distance of the pore diameter from the free surface were found to be most critical. The material’s constitutive relation under the cyclic load was modelled by a mixed non-linear hardening rule that was calibrated with published literature on selective laser melted Ti-6Al-4V. The cyclic plasticity effect caused a local mean stress relaxation, which was found to be dependent on the pore geometry, the applied stress amplitude and the stress ratio. Fatigue life was predicted by using the FE calculated local strain amplitude and maximum stress in the strain-life relationship proposed by Smith-Watson-Topper. The methodology was validated by published literature with crack initiation at gas pores of known size, location, and shape. Parametric study showed that for internal pores, fatigue performance is more sensitive to the shape and location of the pore than the size. An S-N curve was proposed by the parametric study to account for the fatigue strength reduction due to internal gas pores.

The Temperature Effect on the Compressive Behavior of Closed-Cell Aluminum-Alloy Foams

Abstract: In this research, the mechanical behavior of closed-cell aluminum (Al)-alloy foams was investigated at different temperatures in the range of 25-450 °C. The main mechanical properties of porous Al-alloy foams are affected by the testing temperature, and they decrease with the increase in the temperature during uniaxial compression. From both the constant/serrated character of stress–strain curves and macro/microstructural morphology of deformed cellular structure, it was found that Al foams present a transition temperature from brittle to ductile behavior around 192 °C. Due to the softening of the cellular structure at higher temperatures, linear correlations of the stress amplitude and that of the absorbed energy with the temperature were proposed. Also, it was observed that the presence of inherent defects like micropores in the foam cell walls induced further local stress concentration which weakens the cellular structure’s strength and crack propagation and cell-wall plastic deformation are the dominant collapse mechanisms. Finally, an energy absorption study was performed and an optimum temperature was proposed.

The influence of particles size and its distribution on the degree of stress concentration in particulate reinforced metal matrix composites

Abstract: In this work, the degree of stress concentration in particulate reinforced metal matrix composites (PRMMCs) is investigated by finite element analysis method. It is found that, besides the particle morphology, particle size and its distribution can also remarkably influence the stress distribution of the composites, especially that in the matrix. The results indicate that the particles will bear more loads in the composites reinforced by irregular particles than those reinforced by spherical ones and the stress concentration in the matrix near the particle angularity is more serious. As to the particle size, the degree of stress concentration in the matrix will be reduced with the decrease of the size, while the size distribution has greater influence on that than the size itself. The calculated standard deviation of the stress indicates that the degree of stress concentration in matrix in multi-size particles reinforced models is obviously weaker than that in single-size particles reinforced models. Therefore, changing the size distribution is more effective to reduce the degree of stress concentration in the matrix and can be used to improve the fracture toughness of PRMMCs. It is considered that the results could be helpful for design better performance PRMMCs by controlling the ratio of particles with different sizes.

Influence of ceramic material, thickness of restoration and cement layer on stress distribution of occlusal veneers.

Abstract: The aim of this study was to evaluate stress distribution in an occlusal veneer according to the restorative material, restoration thickness, and cement layer thickness. A tridimensional model of a human maxillary first molar with an occlusal veneer preparation was constructed using a modeling software of finite element analysis. The model was replicated 9 times to evaluate the factors: restoration thickness (0.6, 1.2, and 1.8 mm) and cement layer thickness (100, 200, and 300 μm). Then, each model received different restorative materials (High Translucency Zirconia - [YZHT], Lithium Disilicate - [LD], Zirconia Reinforced Lithium Silicate - [ZLS], Feldspathic - [F], and Hybrid Ceramic - [HC]), totaling forty-five groups. An axial load (600 N) was applied on the occlusal face for static structural analysis. Solids were considered isotropic, homogeneous, and linearly elastic. Contacts were considered perfectly bonded. Fixation occurred in the dental root and a mechanical static structural analysis was performed. Descriptive statistical analysis and one-way ANOVA (α =10%) were performed for tensile stress peak values in the restoration and cement layer. The difference between groups was compared using the Tukey's test with 10% significance to match the percentage of the mesh convergence test. According to the results, the cement layer thickness did not influence stress distribution in the restoration (p ≥ 0.10). The thicker the restoration, the higher the tensile stress concentration in the restoration. The graphs showed higher stress concentration in the YZHT, followed by LD, F, ZLS, and HC. Also, the restorative material influenced stress concentration on the cement layer, which decreased according to the sequence HC>YZHT>ZLS>LD>F. HC stood out for causing the least stress concentration in the restoration. Cement layer thickness did not interfere in the mechanical performance of the restorations.

Very High Cycle Fatigue of Ni-Based Single-Crystal Superalloys at High Temperature

Abstract: Very high cycle fatigue (VHCF) properties at high temperature of Ni-based single-crystal (SX) superalloys and of a directionally solidified (DS) superalloy have been investigated at 20 kHz and a temperature of 1000 °C. Under fully reversed conditions (R = − 1), no noticeable difference in VHCF lifetimes between all investigated alloys has been observed. Internal casting pores size is the main VHCF lifetime-controlling factor whatever the chemical composition of the alloys. Other types of microstructural defects (eutectics, carbides), if present, may act as stress concentration sites when the number of cycles exceed 109 cycles or when porosity is absent by applying a prior hot isostatic pressing treatment. For longer tests (> 30 hours), oxidation also controls the main crack initiation sites leading to a mode I crack initiation from oxidized layer. Under such conditions, alloy’s resistance to oxidation has a prominent role in controlling the VHCF. When creep damage is present at high ratios (R ≥ 0.8), creep resistance of SX/DS alloys governs VHCF lifetime. Under such high mean stress conditions, SX alloys developed to retard the initiation and creep propagation of mode I micro-cracks from pores have better VHCF lifetimes.

Quasi-static fracturing in double-flawed specimens under uniaxial loading: the role of strain rate

Abstract: Mechanical properties and cracking behaviors of flaw-weakened rock mass are distinguishably influenced by the quasi-static strain rate. Such advanced knowledge is mainly obtained through the numerical simulations, which are rarely verified experimentally. This paper carries out a coupled experimental-numerical investigation on double-flawed rock-like specimens at different quasi-static strain rates, with consideration given to the macro-mechanical properties, the quasi-static cracking behaviors and the underlying fracture mechanism. Three ordinary arrays of double pre-existing flaws, namely, the coplanar array, the vertical non-overlapping array and the vertical aligning array, are analyzed experimentally. The obtained results show that the crack initiation stress and the peak stress of double flaw-contained specimens generally increase with increasing the strain rate. The extension length of the first macroscopic crack, the crack initiation mode, the amount of far-field cracks, and the degree of ultimate fragmentation in double flaw-contained specimens are closely related to the strain rate. Furthermore, the vertical non-overlapping array of double pre-existing flaws is studied numerically by using peridynamics. The quasi-static fracture behaviors obtained by peridynamics are in good agreement with the experimental observations. The stress concentrates around the tips of flaws when the strain rate is relatively low, while apart from around the tips of flaws, the stress concentration also occurs in the interior of specimens when the strain rate is relatively high. This study provides the better understanding of the quasi-static strain rate effects on mechanical properties and fracture behaviors of rock mass, in particular those containing natural flaws that appear in sets or groups with similar orientation and characteristics.