Browsing by Subject "fatigue"

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Comparison of Rotating-Bending and Axial Fatigue Behaviors of LB-PBF 316L Stainless Steel
(University of Texas at Austin, 2019) Shrestha, Rakish; Simsiriwong, Jutima; Shamsaei, Nima
Additive manufactured (AM) materials are prone to internal defects such as entrapped gas pores and lack of fusions along with having a rough surface. There are different types of fatigue tests that are used to characterize the effects of such defects on the structural integrity of AM parts. The present study aims to investigate the effect of stress gradient on the fatigue behavior of 316L stainless steel (SS), fabricated using a laser beam powder bed fusion (LB-PBF) process. Axial fatigue tests are performed on as-built (non-machined) LB-PBF 316LSS round specimens with uniform gage section, while rotating bending fatigue tests are conducted on hourglass specimens (i.e. reduced gage section). Fatigue tests revealed that the specimens subjected to the axial loading exhibited lower fatigue resistance compared to the specimens failed under rotating bending test. Such differences in the fatigue life was attributed to the variation in the stress distribution resulting from different loading types and its effect on the fatigue crack propagation. Fractography analysis conducted to determine the failure mechanism showed that all of the cracks initiated from the surface of the specimen irrespective to the loading conditions. Furthermore, fracture surface observed for LB-PBF 316L SS specimens resembled a typical fracture surface of notched specimens, which supports the fact that for the as-built specimens cracks initiates from the micro-notches as a result of layer wise fabrication in AM process.
Competing Influence of Porosity and Microstructure on the Fatigue Property of Laser Powder Bed Fusion Stainless Steel 316L
(University of Texas at Austin, 2017) Zhang, Meng; Sun, Chen-Nan; Zhang, Xiang; Chin Goh, Phoi; Wei, Jun; Li, Hua; Hardacre, David
Crack initiation constitutes a large portion of the total life for parts under high cycle fatigue loading. Materials made by the laser powder bed fusion (L-PBF) process contain unavoidable process-induced porosity whose effect on the mechanical properties needs to be considered for fatigue applications. Results from this work show that not all pores in L-PBF parts promote fatigue crack initiation. The length scale of local microstructure defects, i.e. grain boundary, could be larger than the pores and in such cases they are the primary cause for crack initiation. Samples were produced in this work to demonstrate the critical defect size responsible for the transition between the porosity-driven and microstructure-driven failure modes.
Effect of Build Orientation on Fatigue Performance of Ti-6Al-4V Parts Fabricated via Laser-Based Powder Bed Fusion
(University of Texas at Austin, 2017) Torries, Brian; Shamsaei, Nima; Thompson, Scott
The effects of build orientation on the fatigue behavior of additively-manufactured Ti-6Al-4V using a Laser-Based Power Bed Fusion (L-PBF) process is investigated. Ti-6Al-4V rods were manufactured in vertical, horizontal, and 45º angle orientations. The specimens were then machined and polished along the gage section in order to reduce the effects of surface roughness on fatigue behavior. Fully-reversed strain-controlled uniaxial fatigue tests were performed at various strain amplitudes with frequencies adjusted to maintain an average constant strain rate throughout testing. Results indicate slight variation in fatigue behavior of specimens fabricated in the different orientations investigated. Fractography was conducted using scanning electron microscopy after mechanical testing in order to investigate the crack initiation sites and determine the defect responsible for the failure. The experimental program utilized and results obtained will be presented and discussed.
Effect of Build Orientation on the Fatigue Behavior of Stainless Steel 316L Manufactured via a Laser-Powder Bed Fusion Process
(University of Texas at Austin, 2016) Shrestha, Rakish; Simsiriwong, Jutima; Shamsaei, Nima; Thompson, Scott M.; Bian, Linkan
n this study, the effects of build orientation on the mechanical properties and fatigue life of stainless steel (SS) 316L, fabricated using the Laser-Powder Bed Fusion (L-PBF) additive manufacturing (AM) process, were investigated under monotonic tensile and uniaxial strain-controlled fully-reversed (R = -1) cyclic loadings. Tensile tests were conducted at a strain rate of 0.001 s-1 , while fatigue tests were performed at strain amplitudes ranging from 0.1% to 0.4% at various frequencies to have a nearly consistent average strain rate in all tests. The comparison between the tensile properties of additively manufactured and wrought SS 316L revealed that L-PBF specimens exhibited higher yield and ultimate tensile stresses as compared to the wrought specimen. In addition, the elongation to failure of the wrought specimen was similar to that of the horizontally oriented specimen, while it was lower relative to specimens built in vertical and diagonal directions. From the strain-life fatigue analysis, the diagonally oriented L-PBF specimens generally exhibited lower fatigue strength as compared to vertical and horizontal specimens. The fractography analysis revealed three major types of defects to be responsible for the crack initiation and failure. These included (1) voids formed due to lack of fusion between the subsequent layers and entrapped gas, (2) inclusions formed due to the partially melted powder particles, and (3) un-melted powder particles clustered near a void.
Effect of Inter-Layer Time Interval on the Mechanical Behavior of Direct Laser Deposited Ti-6Al-4V
(University of Texas at Austin, 2016) Torries, Brian; Shao, Shuai; Shamsaei, Nima; Thompson, Scott M.
Due to its wide applicability in the biomedical and aerospace fields, where unique and/or difficult to machine geometries are required, Ti-6Al-4V continues to be a strong candidate for additive manufacturing. In this study, the effect of inter-layer time interval on the mechanical behavior of Ti-6Al-4V fabricated via Laser Engineered Net Shaping (LENSTM) is investigated. Two sets of specimens were fabricated, each with their own inter-layer time interval, accomplished by depositing either one or two specimens per operation. Tensile tests and fully reversed, strain controlled fatigue tests were conducted on the specimens. Experimental results indicate that specimens fabricated using longer inter-layer time intervals possess a higher ultimate tensile strength, lower ductility, and finer microstructure relative to those fabricated using shorter time intervals. Additionally, specimens fabricated using longer inter-layer time intervals possessed shorter fatigue lives due to presence of more process defects, such as pores and lack of fusion, inherent to additive manufacturing. Such effects are important to consider when producing multi-part assemblies or large parts.
Effect of Specimen Surface Area Size on Fatigue Strength of Additively Manufactured Ti-6Al-4V Parts
(University of Texas at Austin, 2017) Pegues, Jonathan; Roach, Michael; Williamson, R. Scott; Shamsaei, Nima
As additive manufacturing becomes an increasingly popular method for advanced manufacturing of components, there are many questions that need to be answered before these parts can be implemented for structural purposes. One of the most common concerns with additively manufactured parts is the reliability when subjected to cyclic loadings which has been shown to be highly sensitive to defects such as pores and lack of fusion between layers. It stands to reason that larger parts will inherently have more defects than smaller parts which may result in some sensitivity to surface area differences between these parts. In this research, Ti-6Al-4V specimens with various sizes were produced via a laser-based powder bed fusion method. Uniaxial fatigue tests based on ASTM standards were conducted to generate fatigue-life curves for comparison. Fractography on the fractured specimens was performed to distinguish failure mechanisms between specimen sets with different sizes.
Effects of Design Parameters on Thermal History and Mechanical Behavior of Additively Manufactured 17-4 PH Stainless Steel
(University of Texas at Austin, 2018) Shrestha, Rakish; Nezhadfar, P. Dastranjy; Masoomi, Mohammad; Simisiriwong, Jutima; Phan, Nam; Shamsaei, Nima
In this study, the effects of part size on thermal history and mechanical properties of additively manufactured 17-4 PH stainless steel were investigated under monotonic tensile and strain-controlled fatigue loadings. Two sets of specimens were machined from square rods and oversized specimens, which were fabricated using a laser bed powder fusion (L-PBF) process, to introduce variation in specimen geometry and consequently thermal history. Monotonic tensile tests were conducted at a strain rate of 0.001 s-1 . Fully-reversed (Rε = -1), strain-controlled fatigue tests were performed at 0.003 and 0.0035 mm/mm, and varying test frequency to maintain a constant average strain rate in all tests. Experimental results indicated minimal effect of specimen geometry on the tensile properties of L-PBF 17-4 PH SS, which were also found to be comparable to the wrought material. On the other hand, some influence of specimen geometry on fatigue behavior was observed. Specimens machined from square rods exhibited slightly higher fatigue resistance as compared to specimens machined from oversized specimens. Furthermore, thermal simulations demonstrated higher bulk heating in specimens machined from oversized specimens as compared to those from square rods, which indicated the effect of part geometry on thermal history experienced by the fabricated part.
Electropolishing of Ti6Al4V Parts Fabricated by Electron Beam Melting
(University of Texas at Austin, 2016) Yang, L.; Wu, Yan; Lassell, Austin; Zhou, Bin
In this study Ti6Al4V samples fabricated via electron beam melting were subjected to electropolishing under various polishing conditions using the re-designed in-house electropolishing device. The surface finish of the processed samples were characterized, and the tension-tension fatigue properties of the samples were evaluated via experimentation. The results provided preliminary information about the effect of electropolishing on surface crack initiation of the electron beam melted Ti6Al4V parts.
Fatigue Analysis of a Gallop-based Piezoelectric Wind Energy Harvester and Strategies for Long-Term Performance Optimization
(2021-12) Trostle, Andrew; Sirohi, Jayant
With great demand for novel forms of renewable energy, there has been substantial interest in wind energy harvesting based on aeroelastic excitations with the aid of piezoelectric materials. While most research has been dedicated to optimizing power output [17], relatively little research has been done on the device’s behavior in response to fatigue, which may be a key failure mode given its persistent cyclic loading. A Python program was created to simulate a galloping piezoelectric wind energy harvesting (GPEH) under varying wind conditions in real time, monitoring performance metrics until fatigue failure. This can generate Monte Carlo distributions for optimizing GPEH system parameters for durability. This could then determine if such a form of renewable energy is currently feasible and if so, provide a database of effective materials for the device’s implementation. In tandem with this, analytical optimization methods were tested, but remain partially inconclusive, requiring better dimensional analysis to simplify the myriad of system parameters. However, initial runs of the code still provided useful data, identifying an optimal performance state of all those tested, indicating that the tradeoff between performance and durability is not either-or.
Fatigue Behavior and Failure Mechanisms of Direct Laser Deposited Inconel 718
(University of Texas at Austin, 2016) Johnson, Alexander S.; Shuai, Shao; Shamsaei, Nima; Thompson, Scott M.; Bian, Linkan
Fatigue Behavior of Additive Manufactured 304L Stainless Steel Including Surface Roughness Effects
(University of Texas at Austin, 2019) Lee, Seungjong; Pegues, Jonathan; Shamsaei, Nima
The fatigue behavior of additive manufactured parts in the as-built surface condition is typically dominated by the surface roughness. However, the fatigue behavior of 304L stainless steel fabricated by laser beam powder bed fusion shows less sensitivity to surface roughness under strain-controlled loading conditions than other additive manufactured materials. Under force-controlled conditions, however, the high cycle fatigue resistance is much lower for the as-built surface condition than the machined one. This study investigates the underlying mechanisms responsible for fatigue failure for each condition (i.e. strain-controlled or force-controlled). The corresponding cyclic deformation behavior was characterized, and a thorough fractography analysis was performed to identify the features responsible for crack initiation. Results indicate that the crack initiation features in both loading conditions are similar, and that the reduced high cycle fatigue resistance for force-controlled fatigue loading compared to strain-controlled one is related to differences in the cyclic deformation behavior of the material.
Fatigue Behavior of Laser Beam Directed Energy Deposited Inconel 718 at Elevated Temperature
(University of Texas at Austin, 2019) Johnson, Alexander S.; Shrestha, Rakish; Nezhadfar, P.D.; Shamsaei, Nima
Nickle based super alloys such as Inconel 718 are being extensively used to manufacture turbine blades for jet engines due to their superior mechanical properties at higher working temperatures. Furthermore, poor machinability associated with Inconel 718 also makes it an attractive material for additive manufacturing processes, which possess the capability to fabricate near net shaped parts. Hence, in this study, the fatigue behavior of Inconel 718 fabricated using laser beam directed energy deposition (LB-DED) is investigated under strain-controlled, fully-reversed conditions at an elevated temperature of 650° C. Fractography analysis was conducted to determine the failure mechanism for additive manufactured Inconel 718 due to higher working temperatures. The results obtained from the fatigue and fractography analysis were then compared with the results obtained from fatigue tests conducted at room temperature. At elevated test temperature, LB-DED Inconel 718 specimens exhibited lower fatigue resistance compared to the tests conducted at the room temperature, primarily in the low cycle fatigue regime. Whereas, in the high cycle fatigue regime the effect of test temperature was observed to be minimal. Furthermore, secondary cracks resulting from the formation of brittle behaving precipitates on the grain boundaries was also evident from the fractography analysis indicating significant changes in the microstructural features of LB-DED Inconel 718 as a consequence of elevated test temperature.
Fatigue Characterization of 3D Printed Elastomer Material
(University of Texas at Austin, 2012-08-20) Moore, Jacob P.; Williams, Christopher B.
The Objet PolyJet 3D Printing process provides the ability to print graded materials featuring both stiff and elastomeric polymers. This capability allows for a variety of new design possibilities for additive manufacturing such as living hinges, shock absorbing casings, and integrated gaskets. Such design features typically rely upon the ability of traditional elastomers to experience large and repeated strains without permanent deformation or damage. However, voids and other flaws inherent to many Additive Manufacturing (AM) processes can have a significant negative impact on the fatigue life of elastomeric AM materials. In this paper, the authors seek to fill a gap in the literature by characterizing the fatigue life of a direct 3D printed elastomer, and the multi-material interface. Based on the results, the authors offer advice for improving fatigue life of printed elastomeric components.
Locational Dependency of Additively Manufactured Parts: Effects of Surface Roughness on Fatigue Behavior
(University of Texas at Austin, 2021) Lee, Seungjong; Muhammad, Muztahid; Zheng, Jingyi; Shao, Shuai; Shamsaei, Nima
Surface roughness (SR) can vary significantly among parts manufactured at different locations on the same build platform because of the gas flow and powder recoating process in current laser beam powder bed fusion (LB-PBF) technique. In this study, intra-build SR variation across locations was investigated for LB-PBF 316L stainless steel. The build platform was divided into 4 quadrants to assess the location-dependent variation of SR. Uniaxial stress-controlled fatigue tests were conducted using a servo-hydraulic system. The SR from the four quadrants was analyzed using the one-way Analysis of Variance (ANOVA) method and further verified SR's location dependency. Kruskal-Wallis test was also employed since Box-Cox transformed data failed to meet the requirements of the ANOVA test. Kruskal-Wallis test revealed that there are statistically significant differences in SR values across different locations. The results were validated using fatigue test results and gas flow simulation results reported in literature.
Microstructural Sensitive Fatigue Modeling of Additively Manufactured Ti-6Al-4V
(University of Texas at Austin, 2015) Sterling, Amanda J.; Torries, Brian; Shamsaei, Nima; Thompson, Scott M.; Daniewicz, Steven R.
A common issue in powder-based Additive Manufacturing (AM) techniques is porosity. While process parameters can be controlled to limit this occurrence, complete elimination without post-processing is difficult. Because porosity can significantly affect fatigue behavior of AM parts, it is important to understand and model this material trait. In this study, the porosity in various Ti-6Al-4V specimens fabricated via Laser Engineered Net Shaping (LENS) was determined prior to fatigue testing. Void distribution and morphology was quantified. Fractography was performed to determine the specimen’s transition through crack initiation and propagation stages. These results were used to calibrate a microstructure-sensitive fatigue model for predicting the fatigue behavior of as-built and heat treated LENS Ti-6Al-4V.
Microstructure, Mechanical, and Fatigue Properties of a Laser Powder Bed Fused Al-Cu-Mg-Ag-Ti-B (A205) Alloy
(2022) Avateffazeli, Maryam; Khan, Md Faysal; Shamsaei, Nima; Haghshenas, Meysam
This paper aims at assessing the effect of heat treatment on fatigue behavior of a novel laser-powder-bed-fusion (L-PBF) fabricated Al-Cu-Mg-Ag-Ti-B alloy, known as A205. To this end, L-PBF samples were heat-treated including (i) stress-relieving, and (ii) T7 stabilizing over- aging. Upon printing and post-heat treatments, advanced microstructural characterizations, mechanical property measurements and force-controlled fatigue performance studies were conducted on the samples, systematically. The findings in this paper present useful information for the selection of appropriate heat treatment conditions, to facilitate control of the fatigue behavior in the L-PBF A205 material, which is of great significance for their high-demanding applications in aerospace sectors.