Showing papers on "Stress concentration published in 2016"

A study on the residual stress during selective laser melting (SLM) of metallic powder

Abstract: The complex thermal history of the parts manufactured by selective laser melting (SLM) leads to complex residual stress, having a significant impact on the quality of SLM part. The origin of residual stress was investigated in terms of temperature gradient mechanism. Then, stresses along the height and horizontal directions were measured by X-ray diffraction, and effects of processing parameters on the stress distribution were studied. Results showed that residual stress distribution and evolution along the height direction are affected by the subsequent thermal cycling (STC) significantly. In the horizontal direction, higher energy input and longer track length induce larger residual stress. The stress parallel to the scanning direction is much larger than that perpendicular to the scanning direction, and the peak values of residual stress always occurs at the onset of scanning tracks. Based on this study, corresponding measures can be taken to reduce the residual stress or avoid stress concentration, thereby improving the process stability of SLM.

Stress and Deformation Evaluations of Scanning Strategy Effect in Selective Laser Melting

Abstract: Selective laser melting (SLM) has emerged as one of the primary metal additive manufacturing technologies used for many applications in various industries such as medical and aerospace sectors. However, defects such as part distortion and delamination resulted from process-induced residual stresses are still one of the key challenges that hinder widespread adoptions of SLM. For process parameters, the laser beam scanning path will affect the thermomechanical behaviors of the build part, and thus, altering the scanning pattern may be a possible strategy to reduce residual stresses and deformations through influencing the heat intensity input distributions. In this study, a 3D sequentially coupled finite element (FE) model was developed to investigate the thermomechanical responses in the SLM process. The model was applied to test different scanning strategies and evaluate their effects on part temperature, stress and deformation. The major results are summarized as follows. (1) Among all cases tested, the out-in scanning pattern has the maximum stresses along the X and Y directions; while the 45° inclined line scanning may reduce residual stresses in both directions. (2) Large directional stress differences can be generated by the horizontal line scanning strategy. (3) X and Y directional stress concentrations are shown around the edge of the deposited layers and the interface between the deposited layers and the substrate for all cases. (4) The 45° inclined line scanning case also has a smaller build direction deformation than other cases.

Full length articleStress and deformation evaluations of scanning strategy effect in selective laser melting

Abstract: Selective laser melting (SLM) has emerged as one of the primary metal additive manufacturing technologies used for many applications in various industries such as medical and aerospace sectors. However, defects such as part distortion and delamination resulted from process-induced residual stresses are still one of the key challenges that hinder widespread adoptions of SLM. For process parameters, the laser beam scanning path will affect the thermomechanical behaviors of the build part, and thus, altering the scanning pattern may be a possible strategy to reduce residual stresses and deformations through influencing the heat intensity input distributions. In this study, a 3D sequentially coupled finite element (FE) model was developed to investigate the thermomechanical responses in the SLM process. The model was applied to test different scanning strategies and evaluate their effects on part temperature, stress and deformation. The major results are summarized as follows. (1) Among all cases tested, the out-in scanning pattern has the maximum stresses along the X and Y directions; while the 45° inclined line scanning may reduce residual stresses in both directions. (2) Large directional stress differences can be generated by the horizontal line scanning strategy. (3) X and Y directional stress concentrations are shown around the edge of the deposited layers and the interface between the deposited layers and the substrate for all cases. (4) The 45° inclined line scanning case also has a smaller build direction deformation than other cases.

A review of Finite Fracture Mechanics: crack initiation at singular and non-singular stress raisers

Abstract: Crack initiation in brittle materials is not covered by classical fracture mechanics that deals only with the growth of pre-existing cracks. In order to overcome this deficiency, the Finite Fracture Mechanics concept assumes the instantaneous formation of cracks of finite size at initiation. Within this framework, a coupled criterion was proposed at the beginning of the 2000’s requiring two necessary conditions to be fulfilled simultaneously. The first one compares the tensile stress to the tensile strength, while the other uses an energy balance and the material toughness. The present analysis is restricted to the 2D case, and, through a wide list of references, it is shown that this criterion gives predictions in agreement with experiments in various cases of stress concentration, which can be classified in two categories: the singularities, i.e. indefinitely growing stresses at a point, and the non-singular stress raisers. It is applied to different materials and structures: notched specimens, laminates, adhesive joints or embedded inclusions. Of course, a lot of work remains to do in these domains but also in domains that are almost not explored such as fatigue loadings and dynamic loadings as well as a sound 3D extension. Some ideas in these directions are issued before concluding that FFM and the coupled criterion have filled a gap in fracture mechanics.

Effect of hydrogen-induced plasticity on the stress corrosion cracking of X70 pipeline steel in simulated soil environments

Abstract: The susceptibility to stress corrosion cracking (SCC) of X70 pipeline steel under cathodic protection in near-neutral pH and acidic solutions was investigated by slow-strain-rate tensile test, circumferential-notch tensile (CNT) test, and three-point-bending (TPB) test. Results confirmed the existence of a hydrogen-induced plasticity (HIP) effect within a particular range of cathodic potentials. HIP effect lowered the SCC risk of X70 steel by releasing stress concentration at crack-initiation spots and then decreasing the stress intensity. Crack-growth behavior examined by CNT and TPB tests proved the existence of an HIP effect.

Phase field simulations of plastic strain-induced phase transformations under high pressure and large shear

Abstract: Pressure and shear strain-induced phase transformations (PTs) in a nanograined bicrystal at the evolving dislocations pile-up have been studied utilizing a phase field approach (PFA). The complete system of PFA equations for coupled martensitic PT, dislocation evolution, and mechanics at large strains is presented and solved using the finite element method (FEM). The nucleation pressure for the high-pressure phase (HPP) under hydrostatic conditions near a single dislocation was determined to be 15.9 GPa. Under shear, a dislocation pile-up that appears in the left grain creates strong stress concentration near its tip and significantly increases the local thermodynamic driving force for PT, which causes nucleation of HPP even at zero pressure. At pressures of 1.59 and 5 GPa and shear, a major part of a grain transforms to HPP. When dislocations are considered in the transforming grain as well, they relax stresses and lead to a slightly smaller stationary HPP region than without dislocations. However, they strongly suppress nucleation of HPP and require larger shear. Unexpectedly, the stationary HPP morphology is governed by the simplest thermodynamic equilibrium conditions, which do not contain contributions from plasticity and surface energy. These equilibrium conditions are fulfilled either for the majority of points of phase interfaces or (approximately) in terms of stresses averaged over the HPP region or for the entire grain, despite the strong heterogeneity of stress fields. The major part of the driving force for PT in the stationary state is due to deviatoric stresses rather than pressure. While the least number of dislocations in a pile-up to nucleate HPP linearly decreases with increasing applied pressure, the least corresponding shear strain depends on pressure nonmonotonously. Surprisingly, the ratio of kinetic coefficients for PT and dislocations affect the stationary solution and the nanostructure. Consequently, there are multiple stationary solutions under the same applied load and PT, and deformation processes are path dependent. With an increase in the size of the sample by a factor of two, no effect was found on the average pressure and shear stress and HPP nanostructure, despite the different number of dislocations in a pile-up. The obtained results represent a nanoscale basis for understanding and description of PTs under compression and shear in a rotational diamond anvil cell and high-pressure torsion.

Effects of Thermal Damage and Confining Pressure on the Mechanical Properties of Coarse Marble

Abstract: Heating treatment generally causes thermal damage inside rocks, and the influence of thermal damage on mechanical properties of rocks is an important topic in rock mechanics. The coarse marble specimens drilled out from a rock block were first heated to a specific temperature level of 200, 400 and 600 °C except the control group left at 20 °C. A series of triaxial compression tests subjected to the confining pressure of 0, 5, 10, 15, 20, 25, 30, 35 and 40 MPa were conducted. Coupling effects of thermal damage and confining pressure on the mechanical properties of marbles including post-peak behaviors and failure modes, strength and deformation parameters, characteristic stresses in the progressive failure process had been investigated. Meanwhile, accompanied tests of physical properties were carried out to study the effect of thermal damage on microstructure, porosity and P-wave velocity. Finally, the degradation parameter was defined and a strength-degradation model to describe the peak strength was proposed. Physical investigations show that porosity increases slowly and P-wave velocity reduces dramatically, which could be re-demonstrated by the microscopy results. As for the post-peak behaviors and the failure modes, there is a brittle to ductile transition trend with increasing confining pressure and thermal effect reinforces the ductility to some degree. The comparative study on strength and deformation parameters concludes that heating causes damage and confining pressure inhibits the damage to develop. Furthermore, crack damage stress and crack initiation stress increase, while the ratios of crack damage stress to peak strength and crack initiation stress to peak strength show a decreasing trend with the increase of confining pressure; the magnitude of crack damage stress or crack initiation stress shows a tendency of decrease with the increasing heating temperature and the tendency vanishes subjected to high confinement.

Extreme Mechanics of Probing the Ultimate Strength of Nanotwinned Diamond.

Abstract: Recently synthesized nanotwinned diamond (NTD) exhibits unprecedented Vickers hardness exceeding 200 GPa [Q. Huang et al., Nature (London) 510, 250 (2014)]. This extraordinary finding challenges the prevailing understanding of material deformation and stress response under extreme loading conditions. Here we unveil by first-principles calculations a novel indenter-deformation generated stress confinement mechanism that suppresses the graphitization or bond collapse failure modes commonly known in strong covalent solids, leading to greatly enhanced peak stress and strain range in the indented diamond lattice. Moreover, the twin boundaries in NTD promote a strong stress concentration that drives preferential bond realignments, producing a giant indentation strain stiffening. These results explain the exceptional indentation strength of NTD and offer insights into the extreme mechanics of the intricate interplay of the indenter and indented crystal in probing ultrahard materials.

Generalized approach to estimation of strains and stresses at blunt V-notches under non-localized creep

Abstract: Geometrical discontinuities such as notches need to be carefully analysed by engineers because of the stress concentration generated by them. Notches become even more important when the component is subjected, in service, to very severe conditions, such as high-temperature fatigue and imposed viscoplastic behaviour such as creep. The knowledge of strains and stresses in such stress concentration zones is essential for an efficient and safe design process. The aim of the paper is to present an improvement and extension of the existing notch-tip creep stress–strain analysis method developed by Nunez and Glinka, validated for U-notches only, to a wide variety of blunt V-notches. The key in obtaining the extension to blunt V-notches is the substitution of the Creager–Paris equations with the more generalized Lazzarin–Tovo solution, allowing a unified approach to the evaluation of linear elastic stress fields in the neighbourhood of both cracks and notches. Numerous examples have been analysed to date, and the stress fields obtained according to the proposed method were compared with appropriate finite element data, resulting in a very good agreement. In view of the promising results discussed in the paper, authors are considering possible further extension to sharp V-notches and cracks introducing the concept of the strain energy density.

Atomic vacancies significantly degrade the mechanical properties of phosphorene

Abstract: Due to low formation energies, it is very easy to create atomic defects in phosphorene during its fabrication process. How these atomic defects affect its mechanical behavior, however, remain unknown. Here, we report on a systematic study of the effect of atomic vacancies on the mechanical properties and failure behavior of phosphorene using molecular dynamics simulations. It is found that atomic vacancies induce local stress concentration and cause early bond-breaking, leading to a significant degradation of the mechanical properties of the material. More specifically, a 2% concentration of randomly distributed mono-vacancies is able to reduce the fracture strength by ∼40%. An increase in temperature from 10 to 400 K can further deteriorate the fracture strength by ∼60%. The fracture strength of defective phosphorene is also found to be affected by defect distribution. When the defects are patterned in a line, the reduction in fracture strength greatly depends on the tilt angle and the loading direction. Furthermore, we find that di-vacancies cause an even larger reduction in fracture strength than mono-vacancies when the loading is in an armchair direction. These findings provide important guidelines for the structural design of phosphorene in future applications.

Hybrid specimens eliminating stress concentrations in tensile and compressive testing of unidirectional composites

Abstract: Two novel approaches are proposed for elimination of stress concentrations in tensile and compressive testing of unidirectional carbon/epoxy composites. An interlayer hybrid specimen type is proposed for tensile testing. The presented finite element study indicated that the outer continuous glass/epoxy plies suppress the stress concentrations at the grips and protect the central carbon/epoxy plies from premature failure, eliminating the need for end-tabs. The test results confirmed the benefits of the hybrid specimens by generating consistent gauge-section failures in tension. The developed hybrid four point bending specimen type and strain evaluation method were verified and applied successfully to determine the compressive failure strain of three different grade carbon/epoxy composite prepregs. Stable failure and fragmentation of the high and ultra-high modulus unidirectional carbon/epoxy plies were reported. The high strength carbon/epoxy plies exhibited catastrophic failure at a significantly higher compressive strain than normally observed.

Design of composite structures reinforced curvilinear fibres using FEM

Abstract: This paper describes a method of modelling a curvilinear reinforcement structure, for a composite plate with a hole that allows trajectories of fibres to be adapted to geometric discontinuities (holes, notches, bolts, etc.). For this method, it is assumed that the trajectories of fibres are curvilinear and continuous, as well as located along the trajectories of maximum principal stress. On the basis of these trajectories, the functionally graded material is simulated by means of the finite element method (FEM). Each element of this structure has its own mechanical properties, depending on the fibre direction and a change in the distance between the fibres. It is demonstrated that the maximum value of the stress concentration factor in the fibre direction for the plate with the curvilinear reinforcement structure reduces by 3.2 times in comparison with the same plate with a rectilinear reinforcement structure (orthotropic material).

Brittle film-induced cracking of ductile substrates

Abstract: In the traditional view, hard protective coatings allow improving mechanical performance of ductile materials by increasing surface hardness and wear resistance. However, due to the film extremely low fracture toughness, cracks could easily initiate in the brittle film subjected to tensile stresses, and some of them could propagate towards the substrate. Counter-intuitively, instead of protecting the ductile substrate, a brittle film can cause its premature fracture, as demonstrated here experimentally. If a micro-crack can be initiated in the ductile substrate due to the film cracking, then the whole system would be much easier to fail because of the stress concentration in front of the crack tip under tensile stress. In the present study, cracking of brass ductile substrate induced by brittle TiN film fracture was observed. Analytical calculation based on energy conservation during crack propagation is presented to explain this phenomenon of film-induced cracking. It is shown that crack depth penetrated into the substrate is a function of both crack velocity and the number of dislocations emitted from the crack tip. Relatively thick brittle films and fast propagating cracks favor fracture of the ductile substrates. The critical crack velocity, which can induce brass substrate cracks is 61 m/s. The presence of a film could not only prevent dislocations escaping from the surface of the crystal and inhibit dislocations emitting from surface dislocation sources, but also initiate a channel crack with high velocity due to brittle film fracture. Both contribute to crack propagation in soft substrates. This study provides an alternative view to the notion that a brittle film can protect the ductile substrate from damage. This is an interesting natural phenomenon. Caution must be taken when designing brittle coating protection systems.

Cracks at elliptical holes: stress intensity factor and Finite Fracture Mechanics solution

Abstract: In this work crack initiation at elliptical holes in plates under uniaxial tension is studied by means of a closed form analytical Finite Fracture Mechanics approach. To allow for a detailed study of crack initiation, stress intensity factors available in literature are discussed and compared to extensive numerical results. Based on physical rationale, an improved stress intensity factor solution is proposed that shows a very good agreement with numerical results for a wide range of stress concentration factors of the ellipse and crack lengths. Using the exact solution of the stress field in the notched plate and the proposed stress intensity factor, an efficient Finite Fracture Mechanics solution is obtained. No empiric length parameters are introduced but only the strength and the fracture toughness are required for evaluation. The analysis comprises the case of a circular hole and the limiting cases of a crack tangential and normal to the loading direction. Typical notch size effects are covered by this coupled stress and energy approach. A continuous transition from strength of materials to Linear Elastic Fracture Mechanics can be rendered.