Deformation and stress evolution during laser powder bed fusion of semi-crystalline polyamide-12

A semi-analytical model for rapid prediction of residual stress and deformation in laser powder bed fusion

Laser powder bed fusion is a promising technique that can produce complex-shaped and integrated part. However, distortion and residual stress are two issues that may decrease the precision and performance of built parts. Classical thermal gradient mechanism offers a basic interpretation toward distortion and residual stress. The evolutions of distortion and residual stress in laser powder bed fusion remain unclear. In this study, we used a physical model with an assumption of constraining force to illustrate the evolutions of distortion and residual stress during the additive process. Based on the model, we are able to understand the phenomenon of X-directional shrinkage, Z-directional distortion, and “tensile-compressive-tensile” distribution of X-directional stresses at the same time. It can be concluded that the shrinkage, distortion, and X-directional residual stress all result from the constraint between previously deposited layer and newly deposited layer, which has a strong shrinkage tendency when cooling. The distortion of part increases with deposition height, especially during first several layers. The “tensile-compressive-tensile” distribution of X-directional stresses can be maintained during the additive process. The magnitude of top tensile stress remains stable, while the tensile stress at bottom increases with the deposition height. This work provides a comprehensive understanding toward the evolutions of distortion and residual stress in laser powder bed fusion.

Powder bed fusion (PBF) is a category of additive manufacturing (AM) that is particularly suitable for the production of 3D metallic components. In PBF, only material in the current build layer is at the required melt temperature, with the previously melted and solidified layers reducing in temperature, thus generating a significant thermal gradient within the metallic component, particularly for laser based PBF components. The internal thermal stresses are subsequently relieved in a post-processing heat-treatment step. Failure to adequately remove these stresses can result in cracking and component failure. A prototype hip stem was manufactured from Ti6Al4V via laser PBF but was found to have fractured during over-seas shipping. This study examines the evolution of thermal stresses during the laser PBF manufacturing and heat treatment processes of the hip stem in a 2D finite element analysis (FEA) and compares it to an electron beam PBF process. A custom written script for the automatic conversion of a gross geometry finite element model into a thin layer- by-layer finite element model was developed. The build process, heat treatment (for laser PBF) and the subsequent cooling were simulated at the component level. The results demonstrate the effectiveness of the heat treatment in reducing PBF induced thermal stresses, and the concentration of stresses in the region that fractured.

Simulation of annealing process on AISI 316 L stainless steel fabricated via laser powder bed fusion using finite element method with creep

Superior Lightness-Strength and biocompatibility of bio-inspired heterogeneous glass sponge Ti6Al4V lattice structure fabricated via laser powder bed fusion

Two case studies were conducted in the Shennan mining area of Shaanxi Province, China to evaluate the surrounding rock deformation and stress evolution in pre-driven longwall recovery rooms. These studies mainly monitored the surrounding rock deformation and coal pillar stress in the recovery rooms of the N1206 panel of 2−2 coal seam at Ningtiaota Coal Mine and the 15205 panel of 5−2 coal seam at Hongliulin Coal Mine. The monitoring results showed that the surrounding rock deformation of the main recovery room and the coal pillar stress in the N1206 and 15205 panels began to increase significantly when the face was 36 m and 42 m away from the terminal line, respectively. After the face entered the main recovery room, the maximum roof-to-floor convergence in the N1206 and 15205 panels was 348.03 mm and 771.24 mm, respectively, and the coal pillar stresses increased more than 5 MPa and 7 MPa, respectively. In addition, analysis of the periodic weighting data showed that the main roof break position of the N1206 and 15205 panels after the longwall face entered the main recovery room was − 3.8 m and − 8.2 m, respectively. This research shows that when the main roof breaks above the coal pillar, the surrounding rock deformation of the main recovery room and the coal pillar stress increase sharply. The last weighting is the key factor affecting the stability of the main recovery room and the coal pillar; main roof breaks at disadvantageous positions are the main cause of the support crushing accidents.

Stress and deformation evolution characteristics of gob-side entry retained by roof cutting and pressure relief

Evolution of Stresses at Cell-Gel Interfaces during Confined Interfacial Migration

Benchmark tests for the numerical simulation of pile installation require clearly defined boundary value problems with corresponding experimental data. These experiments have to provide quantitative information on the soil deformations and stresses. Large scale model tests in dry, granular soil were carried out for this purpose. The interface testing device, that is used for the tests, allows the investigation of selected aspects of pile penetration. The normal and shear forces on the pile structure are measured. The displacements in the surrounding soil zone can be evaluated via Digital Image Correlation (DIC). The test results concentrate on the interface behavior between the soil and the pile and the evolution of stresses and deformations around the pile tip. For rough pile surfaces the occurrence of dilatancy effects in the pile-soil interface is shown. The localization of deformations in the post-peak phase is analyzed for monotonic and cyclic test paths. The influence of the pile driving mode on the evolution of stresses around the pile tip is demonstrated.

This paper discusses computational simulations of the evolution of stresses and deformation in unpassivated and SiO2-passivated Al lines on Si substrates. The finite element model accounts for elastic-plastic deformation in the Al lines during etching, passivation, and subsequent thermal cycling, by recourse to a generalized plane strain formulation within the context of a unit cell with appropriately constrained boundary conditions. The effects of different controlled variations in thermal history, and in the width, height, spacing, and yield behavior of the Al lines are analyzed; all these factors are seen to have potentially strong effects on the evolution of stresses within the lines. The predictions of the computations presented in this work are amenable for direct comparisons with experiments of curvature evolution along and perpendicular to the lines upon patterning, passivation, and thermal loading. The predicted stresses in metal interconnects can be directly used for reliability modeling purposes.

This study aims to experimentally investigate the autogenous deformation and the stress evolution in restrained high-volume ground granulated blast furnace slag (GGBFS) concrete. The Temperature Stress Testing Machine (TSTM) and Autogenous Deformation Testing Machine (ADTM) were used to study the macro-scale autogenous deformation and stress evolution of high-volume GGBFS concrete with w/b ratios of 0.35, 0.42, and 0.50. The early-age cracking (EAC) risk (quantified by stress-strength ratio) and stress relaxation were analyzed extensively based on ADTM and TSTM results. Furthermore, Environmental Scanning Electron Microscopy (ESEM), X-ray Diffraction (XRD), and Mercury Intrusion Porosimetry (MIP) were conducted to explore the micro-scale origin of the autogenous deformation of high-volume GGBFS concrete, which supports the observations on the macroscale measurement of TSTM/ ADTM tests. This study finds that the ettringite formation in the first two days results in autogenous expansion, which can delay the appearance of tensile stress. The magnitude of autogenous expansion depends on the compatibility of ettringite content and pore size. The w/b ratio of 0.42 turns out to be optimal because it produces the highest amount of ettringite and results in the highest autogenous expansion. In comparison, the w/b ratio of 0.35 introduces significant autogenous shrinkage after the expansion peak and therefore corresponds to a high early-age cracking risk.

Stress evolution of restrained concrete is a significant direct index in early-age cracking (EAC) analysis of concrete. This study presents experiments and numerical modelling of the early-age stress evolution of Ground granulated blast furnace slag (GGBFS) concrete, considering the development of autogenous deformation and creep. Temperature Stress Testing Machine (TSTM) tests were conducted to obtain the autogenous deformation and stress evolution of restrained GGBFS concrete. By a self-defined material subroutine based on the Rate-type creep law, the FEM model for simulating the stress evolution in TSTM tests was established. By characterizing the creep compliance function with a 13-units continuous Kelvin chain, forward modelling was firstly conducted to predict the stress development. Then inverse modelling was conducted by Bayesian Optimization to efficiently modify the arbitrary assumption of the codes on the aging creep. The major findings of this study are as follows: 1) the high autogenous expansion of GGBFS induces compressive stress at first hours, but its value is low because of high relaxation and low elastic modulus; 2) The codes highly underestimated the early-age creep of GGBFS concrete. They performed well in prediction of stress after 200 h, but showed significant gaps in predictions of early-age stress evolution; 3) The proposed inverse modelling method with Bayesian Optimization can efficiently adjusted the aging terms which produced best modelling results. The adjusted creep compliance function of GGBFS showed a much faster aging speed at early ages than the one proposed by original codes.

<p>In this study, we conducted injection-driven shear tests on a sawcut fault in granite samples using a triaxial deformation apparatus. The granite samples were drilled from Odenwald basement rocks in Germany. The sawcut fault, inclined 30° to the sample axis, was ground using sandpaper with a particle size of 201 <em>µ</em>m. Two boreholes (nominal diameter 1.8 mm) were drilled near the short edge of each sample half to allow direct fluid access to the fault surface. Eight strain gauges, and eight pairs of acoustic emission (AE) sensors attached on the sample surface were used to monitor the deformation, local strain and AE events. </p><p>During the experiments, we first measured the peak shear strength of the faulted sample by advancing the axial piston at a constant rate of 1 <em>µ</em>m/s under 36 MPa confining pressure and 1 MPa pore pressure. We then adjusted the shear stress to be 90% of the peak shear strength. Subsequently, the piston was fixed, and the first injection-driven shear test was initiated by injecting distilled water from the bottom borehole at a rate of 0.2 mL/min. We observed three full cycles of fast slip events until the injection pressure was increased up to approximately 18 MPa. We then reduced the pore pressure to the initial 1 MPa and the axial force was removed, followed by the second injection-driven shear test conducted at a higher injection rate of 0.8 mL/min using the same procedure as in the first test. We also observed three episodes of fast slip events until the injection pressure was increased to about 20 MPa. Fluid pressures were monitored continuously at the top and bottom boreholes. We employed a COMSOL model to obtain the time-dependent fluid pressure distribution along the sawcut fault during fluid injection.</p><p>For slow fluid injection, we find that the fault surface near the center experiences slight normal dilation and gradual shear stress release prior to the fast slip event. In contrast, for high-rate fluid injection, the same fault patch exhibits normal compaction and shear stress increase preceding fast slip. In both cases, significant normal dilation and abrupt shear stress drops were observed near the fault center during fast slip events. The distinct evolution of local fault deformation and stress are likely attributed to the distribution of slow slipping patches, as signified by the fluid pressure distribution and Mohr-Coulomb failure envelope. At slow injection rate, slow precursory slip may have occurred on the entire fault, initiating a fast slip event. In contrast, at higher rates, slow slip may have been localized around the injection port, resulting in local stress concentration beyond the slow slipping patch. Our results demonstrate that the evolution of local fault deformation and stress can be diverse in different fault patches, depending on the relative location to the fluid pressurized zone and the resulting slow slipping patch. This suggests that the strongly heterogeneous fault deformation should be considered when analyzing the precursors to injection-induced fault reactivation.</p>

<p>Seismic rupture of the lower continental crust requires a high failure stress, given large lithostatic stresses and potentially strong rheologies. Several mechanisms have been proposed to generate high stresses at depth, including local amplification of stress heterogeneities driven by the geometry and rheological contrast within a shear zone network. High dynamic stresses are additionally associated with the subsequent slip event, driven by propagation of the rupture tips. In the brittle upper crust, fracturing of the damage zone is the typical response to high stress, but in the lower crust, the evolution of combined crystal plastic and brittle deformation may be used to constrain in more detail the stress history of rupture, as well as  additonal parameters of the deformation environment. It is crucial to understand these deep crustal seismic deformation mechanisms both along the fault and in the wall rock, as coseismic damage is an important (and sometimes the only) method of significantly weakening anhydrous and metastable lower crust, whether by grain size reduction or by fluid redistribution.</p><p>A detailed study of pyroxene microstructures are used here to characterise the short-term evolution of high stress deformation experienced on the initiation of lower crustal earthquake rupture. These pyroxenes are sampled from the pseudotachylyte-bearing fault planes and damage zones of lower crustal earthquakes linked to local stress amplifications within a viscous shear zone network, recorded in an exhumed granulite-facies section in Lofoten, northern Norway. In orthopyroxene, initial low-temperature plasticity is overtaken by pulverisation-style fragmentation, generating potential pathways for hydration and reaction. In clinopyroxene, low-temperature plasticity remains dominant throughout but the microstructural style changes rapidly through the pre- and co-seismic periods from twinning to undulose extinction and finally the formation of low angle boundaries. We present here an important record of lower crustal short-term stress evolution along seismogenic faults.</p>

Micromechanical modeling approach to single track deformation, phase transformation and residual stress evolution during selective laser melting using crystal plasticity