Showing papers on "Stress concentration published in 2019"

Fatigue behavior of additive manufactured 316L stainless steel parts: Effects of layer orientation and surface roughness

Abstract: The effects of layer orientation and surface roughness on the mechanical properties and fatigue life of 316L stainless steel (SS) fabricated via a laser beam powder bed fusion (LB-PBF) additive manufacturing process were investigated. Quasi-static tensile and uniaxial fatigue tests were conducted on LB-PBF 316L SS specimens fabricated in vertical and diagonal directions in their as-built surface condition, as well as in horizontal, vertical, and diagonal directions where the surface had been machined to remove any effects of surface roughness. In the machined condition, horizontally built LB-PBF specimens possessed higher fatigue resistance, followed by vertically built specimens, while the lowest fatigue resistance was obtained for diagonal specimens. Similarly, in the as-built condition, vertical specimens demonstrated better fatigue resistance when compared to diagonal specimens. Furthermore, the detrimental effects of surface roughness on fatigue life of LB-PBF 316L SS specimens was not significant, which may be due to the presence of large internal defects in the specimens. Anisotropy of LB-PBF 316L SS specimens was attributed to the variation in layer orientation, affecting defects’ directionality with respect to the loading direction. These defect characteristics can significantly influence the stress concentration and, consequently, fatigue behavior of additive manufactured parts. Therefore, the elastic-plastic energy release rates, a fracture mechanics-based concept that incorporates size, location, and projected area of defects on the loading plane, were determined to correlate the fatigue data and acceptable results were achieved.

Surface Treatment of Powder-Bed Fusion Additive Manufactured Metals for Improved Fatigue Life

Abstract: High-cycle fatigue (HCF) tests were conducted on samples fabricated by two powder-bed additive manufacturing techniques. Samples were tested with as-produced surfaces and after various non-contact surface improvement treatments. Ti-6Al-4V samples were made using both electron beam melting (EBM) and selective laser melting (SLM), while Inconel 625 was fabricated using SLM. Ti-6Al-4V was treated with a commercial chemically accelerated vibratory polishing process, with target material removal of approximately 200 µm from each surface for EBM samples and 100 µm for SLM samples. This technique led to increases in both the number of cycles before failure at a given loading condition and endurance limit (at 107 cycles) compared to samples with as-produced surfaces. The results are interpreted as the reduction in elastic stress concentration factor associated with surface defects where fatigue cracks initiate. SLM 625 was treated with both an abrasive polishing method and laser surface remelting. Both methods led to improvements in surface roughness, but these did not lead to improvements in fatigue properties of SLM 625. For abrasive polished samples, the combination of improved measured surface roughness without fatigue property enhancement suggests that surface material is removed, but the roots of surface defects, where fatigue cracks initiate, were left intact. For laser treatment, the remelted surface layer retained a rapidly solidified microstructure that did not increase the number of cycles before crack initiation even though the surface was smoother compared to the surface prior to polishing.

Effects of heat treatment and build orientations on the fatigue life of selective laser melted 15-5 PH stainless steel

Abstract: Effects of heat treatment (aging and overaging) and build orientations on the fatigue life of Selective Laser Melted (SLM) 15-5 precipitation hardening (PH) stainless steel are reported. Low cycle fatigue (LCF) life of aged specimens is better than specimens in as-built condition for both vertical and horizontal build directions. On the contrary, high cycle fatigue (HCF) life of aged specimens is less than its as-built counterpart. Aging resulted in precipitation strengthening of the matrix through Cu-rich precipitation, but this makes specimen more defect sensitive in HCF regime. Overaging makes specimen ductile through coarsening of Cu-rich precipitates and increases the amount of retained austenite. This results overaged specimens to be less defect sensitive than aged specimens in HCF regime. Presence of irregular shaped defects e.g. voids, un-melted regions etc in SLM specimens reduces its fatigue life than its wrought counterparts. Relatively less number of defects is found for vertically built SLM specimens. However, orientations of defects being perpendicular to the loading axis, creates more stress concentration around a defect leading to lower fatigue life of the vertical SLM specimens. Present study could be used as a guideline to select proper build strategy and heat treatment for SLM 15-5 PH stainless steel for a desired fatigue life.

Modelling of the temperature and residual stress fields during 3D printing of polymer composites

Abstract: Fused deposition modeling (FDM) based 3D printing) technique involves the fabrication of polymer parts using a thermal process which may induce residual stress, stress concentration, distortion, and the delamination between layers. This paper aims to investigate this defect on ASTM D638 polymer composite specimens. For that purpose, a 3D thermo-mechanical model that simulates the process of FDM capable of calculating stresses and temperature gradients during the additive manufacturing of polymer composites was developed. The 3D model considers the temperature-dependent physical properties of composites which consist of density, thermal conductivity, thermal expansion coefficient, yield stress, and Young’s modulus. The simulated process includes the heating, solidification, and cooling phases. Different printed parts were analyzed and compared. The stresses vary continuously because of the temperature gradient occurring through the composite thickness. It appears that the concentration of stresses is higher if the temperatures during printing vary rapidly. Those stresses can favor the delamination between the layers of the printed part and the residual thermal stresses can cause an offset to the failure envelope.

The third Sandia fracture challenge: predictions of ductile fracture in additively manufactured metal

Abstract: The Sandia Fracture Challenges provide a forum for the mechanics community to assess its ability to predict ductile fracture through a blind, round-robin format where mechanicians are challenged to predict the deformation and failure of an arbitrary geometry given experimental calibration data. The Third Challenge (SFC3) required participants to predict fracture in an additively manufactured (AM) 316L stainless steel bar containing through holes and internal cavities that could not have been conventionally machined. The volunteer participants were provided extensive data including tension and notched tensions tests of 316L specimens built on the same build-plate as the Challenge geometry, micro-CT scans of the Challenge specimens and geometric measurements of the feature based on the scans, electron backscatter diffraction (EBSD) information on grain texture, and post-test fractography of the calibration specimens. Surprisingly, the global behavior of the SFC3 geometry specimens had modest variability despite being made of AM metal, with all of the SFC3 geometry specimens failing under the same failure mode. This is attributed to the large stress concentrations from the holes overwhelming the stochastic local influence of the AM voids and surface roughness. The teams were asked to predict a number of quantities of interest in the response based on global and local measures that were compared to experimental data, based partly on Digital Image Correlation (DIC) measurements of surface displacements and strains, including predictions of variability in the resulting fracture response, as the basis for assessment of the predictive capabilities of the modeling and simulation strategies. Twenty-one teams submitted predictions obtained from a variety of methods: the finite element method (FEM) or the mesh-free, peridynamic method; solvers with explicit time integration, implicit time integration, or quasi-statics; fracture methods including element deletion, peridynamics with bond damage, XFEM, damage (stiffness degradation), and adaptive remeshing. These predictions utilized many different material models: plasticity models including J2 plasticity or Hill yield with isotropic hardening, mixed Swift-Voce hardening, kinematic hardening, or custom hardening curves; fracture criteria including GTN model, Hosford-Coulomb, triaxiality-dependent strain, critical fracture energy, damage-based model, critical void volume fraction, and Johnson-Cook model; and damage evolution models including damage accumulation and evolution, crack band model, fracture energy, displacement value threshold, incremental stress triaxiality, Cocks-Ashby void growth, and void nucleation, growth, and coalescence. Teams used various combinations of calibration data from tensile specimens, the notched tensile specimens, and literature data. A detailed comparison of results based of these different methods is presented in this paper to suggest a set of best practices for modeling ductile fracture in situations like the SFC3 AM-material problem. All blind predictions identified the nominal crack path and initiation location correctly. The SFC3 participants generally fared better in their global predictions of deformation and failure than the participants in the previous Challenges, suggesting the relative maturity of the models used and adoption of best practices from previous Challenges. This paper provides detailed analyses of the results, including discussion of the utility of the provided data, challenges of the experimental-numerical comparison, defects in the AM material, and human factors.

Effect of Defects on the Mechanical and Thermal Properties of Graphene

Abstract: In this study, the mechanical and thermal properties of graphene were systematically investigated using molecular dynamic simulations. The effects of temperature, strain rate and defect on the mechanical properties, including Young’s modulus, fracture strength and fracture strain, were studied. The results indicate that the Young’s modulus, fracture strength and fracture strain of graphene decreased with the increase of temperature, while the fracture strength of graphene along the zigzag direction was more sensitive to the strain rate than that along armchair direction by calculating the strain rate sensitive index. The mechanical properties were significantly reduced with the existence of defect, which was due to more cracks and local stress concentration points. Besides, the thermal conductivity of graphene followed a power law of λ~L0.28, and decreased monotonously with the increase of defect concentration. Compared with the pristine graphene, the thermal conductivity of defective graphene showed a low temperature-dependent behavior since the phonon scattering caused by defect dominated the thermal properties. In addition, the corresponding underlying mechanisms were analyzed by the stress distribution, fracture structure during the deformation and phonon vibration power spectrum.

Microstructure-based modelling of fracture of particulate reinforced metal matrix composites

Abstract: This paper aims to develop a microstructure-based model to describe the material deformation and fracture of the particulate reinforced metal matrix composites. The A359/SiC composite was used as a material example. The most important factors, such as particle morphology, distribution and fracture, matrix deformation and failure, and particle-matrix debonding, were comprehensively integrated into the modelling. Relevant experiments were also conducted. It was found that such a microstructure-based model can capture the major material deformation and failure mechanisms. The particle geometry and distribution can be described by normalized shape factor and clustering distance. Stress concentrations occur at the clustered particles, especially at the sharp corners, particle-matrix interfaces and the edges along the loading direction. The stress in the matrix is much lower than that in the particles and concentrates along 45° to the loading direction. Matrix-particle debonding is not noticeable. Particle fracture happens near its sharp corners, while matrix failure initiates around the particle crack tips and propagates to connect the microcracks caused by particle fracture, or through the area with high stress/strain concentration, leading to the fracture of the whole workpiece.

Interrupted fatigue testing with periodic tomography to monitor porosity defects in wire + arc additive manufactured Ti-6Al-4V

Abstract: Porosity defects remain a challenge to the structural integrity of additive manufactured materials, particularly for parts under fatigue loading applications. Although the wire + arc additive manufactured Ti-6Al-4 V builds are typically fully dense, occurrences of isolated pores may not be completely avoided due to feedstock contamination. This study used contaminated wires to build the gauge section of fatigue specimens to purposely introduce spherical gas pores in the size range of 120–250 micrometres. Changes in the defect morphology were monitored via interrupted fatigue testing with periodic X-ray computed tomography (CT) scanning. Prior to specimen failure, the near surface pores grew by approximately a factor of 2 and tortuous fatigue cracks were initiated and propagated towards the nearest free surface. Elastic-plastic finite element analysis showed cyclic plastic deformation at the pore root as a result of stress concentration; consequently for an applied tension-tension cyclic stress (stress ratio 0.1), the local stress at the pore root became a tension-compression nature (local stress ratio −1.0). Fatigue life was predicted using the notch fatigue approach and validated with experimental test results.

Microscale finite element analysis for predicting effects of air voids on mechanical properties of single fiber bundle in composites

Abstract: Air voids produced in the manufacturing process have significant influence on the mechanical performances of single fiber bundle in carbon fiber-reinforced composites. The microscale finite element model of fiber bundle with voids is developed to predict mechanical properties and failure mechanisms. The failure responses of fiber and matrix in bundle are initiated by the maximum stress and Stassi failure criteria. The sudden stiffness degradation law is adopted to capture brittle behaviors. Both available theoretical models for voids and non-voids are used to validate the numerical model without voids and with different percentages of voids including 0.15, 0.5, 1, 2, 3, 4 and 5%. Effects of void contents on stress–strain responses of bundle are studied. The matrix damage development is studied regarding transverse compression and out-of-plane shear loading. The micro-stress analysis of matrix and voids in three-RUC model under different loadings is performed. The simulated elasticity and strength properties correlate well with theoretical results for voids and non-voids. The strengths and moduli gradually decrease with increasing void contents except longitudinal properties. The stress concentrations of bundle are mainly influenced by the loading direction.

Metallic tube-reinforced aluminum honeycombs: Compressive and bending performances

Abstract: Aluminum honeycombs are widely applied in weight sensitive applications, increasing their specific strength and energy absorption capacity are rather important. Thin-walled metallic tube was used to enhance the mechanical properties of aluminum honeycomb and formed a novel tube-reinforced honeycomb structure. Its compressive and three-point bending performances were studied experimentally and numerically. Due to tube filling, the specific compressive strength, elastic modulus and energy absorption have been increased by 16%, 26% and 73%, and the specific bending load and stiffness increased by 42% and 62% respectively. The strengthen mechanism study indicated that the sum of tube and honeycomb caused the increase of compressive properties, and aluminum tube filling changed the stress distribution and expanded the stress concentration region which led to a transformation of bending failure mode. The present reinforcement method will make honeycomb more competitive in light-weight structure applications.

Corrosion pit-induced stress concentration in spherical pressure vessel

Abstract: This paper aims to investigate the effect of pitting corrosion on stress concentration factor (SCF) for a spherical pressure vessel subjected to internal pressure. Stress concentration at semi-elliptical corrosion pit and secondary pit nucleated at bottom of the semi-elliptical base pit are examined systematically by conducting a series of axisymmetric finite element models. SCF occurs at the bottom of the semi-elliptical pits and increases as pit depth enhances but decreases as pit width enlarges. Pit aspect ratio is a principal parameter affecting stress. Presence of a premature pit at the bottom of base pit intensifies the stress distribution and SCF.

Biomechanical analysis of 4 types of short dental implants in a resorbed mandible.

Abstract: Statement of problem Short implants have been increasingly used in the aging society. However, studies which explain the difference of stress distribution according to different connections in short implant treatment are scarce. Purpose The purpose of this finite element (FE) analysis was to evaluate the stress and strain distribution of short implants and surrounding bone under static and cyclic loading conditions with 4 different connections. Material and methods Three-dimensional models of 4 types of implant systems were considered: internal tissue level, internal tissue level wide, internal bone level (IB), and external bone level. Each system had different types of abutment, implant, and screw with the resorbed mandibular segment of the bone block. Static FE analysis was performed under external loads of 200 N (vertical or 30-degree oblique) to each cusp tip. The strain distributions of the peri-implant bone and von Mises stress fields in the abutment, implant, and screw were evaluated. Based on the static FE results, a computational fatigue analysis was performed to predict the risk of fracture caused by fatigue accumulation of repetitive mastication. Results Bone tissues in fatigue failure level (greater than 4000 μe) were observed in the alveolar ridge and the plateaus close to the implant apex in all situations. Under the oblique loading condition, the total volume of the bone tissue in hypertrophy and fatigue failure levels (greater than 2500 μe) was the largest at IB and the smallest at external bone level. Among the 4 situations, the highest stress occurred in the abutment (506.9 MPa) and implant (311 MPa) of IB. In fatigue analysis, fracture was only predicted in the IB abutment model (588 301 cycles), and cracking occurred in the lingual direction, where stress concentration occurred when the oblique load was applied. Conclusions The abutment of IB showed the highest stress of the implant component, and internal tissue level model showed the highest strain of bone. In all groups, the bone strain values mostly appeared within physiologic capacity (under 4000 μe). Various mechanical situations should be considered when using internal bone-level connections in short implants for replacing posterior teeth.