title: Effects of applied stress ratio on the fatigue behavior of additively manufactured porous biomaterials under compressive loading.
abstract: Additively manufactured (AM) porous metallic biomaterials are considered promising candidates for bone substitution. In particular, AM porous titanium can be designed to exhibit mechanical properties similar to bone. There is some experimental data available in the literature regarding the fatigue behavior of AM porous titanium, but the effect of stress ratio on the fatigue behavior of those materials has not been studied before. In this paper, we study the effect of applied stress ratio on the compression-compression fatigue behavior of selective laser melted porous titanium (Ti-6Al-4V) based on the diamond unit cell. The porous titanium biomaterial is treated as a meta-material in the context of this work, meaning that R-ratios are calculated based on the applied stresses acting on a homogenized volume. After morphological characterization using micro computed tomography and quasi-static mechanical testing, the porous structures were tested under cyclic loading using five different stress ratios, i.e. R = 0.1, 0.3, 0.5, 0.7 and 0.8, to determine their S-N curves. Feature tracking algorithms were used for full-field deformation measurements during the fatigue tests. It was observed that the S-N curves of the porous structures shift upwards as the stress ratio increases. The stress amplitude was the most important factor determining the fatigue life. Constant fatigue life diagrams were constructed and compared with similar diagrams for bulk Ti-6Al-4V. Contrary to the bulk material, there was limited dependency of the constant life diagrams to mean stress. The notches present in the AM biomaterials were the sites of crack initiation. This observation and other evidence suggest that the notches created by the AM process cause the insensitivity of the fatigue life diagrams to mean stress. Feature tracking algorithms visualized the deformation during fatigue tests and demonstrated the root cause of inclined (45°) planes of specimen failure. In conclusion, the R-ratio behavior of AM porous biomaterials is both quantitatively and qualitatively different from that of bulk materials.
authors.affiliation: Department of Materials Science and Engineering, Faculty of Mechanical, Maritime, and Materials Engineering, Mekelweg 2, Delft 2628CD, The Netherlands.
authors.forename: Joep
authors.initials: J
authors.lastname: de Krijger
authors.affiliation: Faculty of Aerospace Engineering, Kluyverweg 1, Delft 2629HS, The Netherlands.
authors.forename: Calvin
authors.initials: C
authors.lastname: Rans
authors.affiliation: Department of Mechanical Engineering, KU Leuven, Leuven, Belgium.
authors.forename: Brecht
authors.initials: B
authors.lastname: Van Hooreweder
authors.affiliation: 3D Systems - LayerWise NV, Leuven, Belgium; Department of Materials Engineering, KU Leuven, Leuven, Belgium.
authors.forename: Karel
authors.initials: K
authors.lastname: Lietaert
authors.affiliation: Department of Biomechanical Engineering, Faculty of Mechanical, Maritime, and Materials Engineering, Mekelweg 2, Delft 2628CD, The Netherlands; Department of Orthopedics, UMC Utrecht, Heidelberglaan100, 3584CX Utrecht, The Netherlands.
authors.forename: Behdad
authors.initials: B
authors.lastname: Pouran
authors.affiliation: Department of Biomechanical Engineering, Faculty of Mechanical, Maritime, and Materials Engineering, Mekelweg 2, Delft 2628CD, The Netherlands. Electronic address: a.a.zadpoor@tudelft.nl.
authors.forename: Amir A
authors.initials: AA
authors.lastname: Zadpoor
mesh_terms: D000497:Alloys; D001672:Biocompatible Materials / Q000032:analysis*; D019245:Compressive Strength; D008422:Materials Testing; D016062:Porosity; D011312:Pressure; D013314:Stress, Mechanical; D014025:Titanium / Q000032:analysis*; D055114:X-Ray Microtomography
chemical_list: D000497:Alloys; D001672:Biocompatible Materials; C031462:titanium alloy (TiAl6V4); D014025:Titanium
keywords: Cellular structures; bone grafting; fatigue life; orthopaedic implants; stress ratio