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Brecht Van Hooreweder - One of the best experts on this subject based on the ideXlab platform.

  • Fatigue life of additively manufactured Ti6Al4V scaffolds under tension-tension, tension-compression and Compression-Compression Fatigue load
    Scientific Reports, 2018
    Co-Authors: Karel Lietaert, Antonio Cutolo, Dries Boey, Brecht Van Hooreweder
    Abstract:

    Mechanical performance of additively manufactured (AM) Ti6Al4V scaffolds has mostly been studied in uniaxial compression. However, in real-life applications, more complex load conditions occur. To address this, a novel sample geometry was designed, tested and analyzed in this work. The new scaffold geometry, with porosity gradient between the solid ends and scaffold middle, was successfully used for quasi-static tension, tension-tension (R = 0.1), tension-compression (R = −1) and Compression-Compression (R = 10) Fatigue tests. Results show that global loading in tension-tension leads to a decreased Fatigue performance compared to global loading in Compression-Compression. This difference in Fatigue life can be understood fairly well by approximating the local tensile stress amplitudes in the struts near the nodes. Local stress based Haigh diagrams were constructed to provide more insight in the Fatigue behavior. When Fatigue life is interpreted in terms of local stresses, the behavior of single struts is shown to be qualitatively the same as bulk Ti6Al4V. Compression-Compression and tension-tension Fatigue regimes lead to a shorter Fatigue life than fully reversed loading due to the presence of a mean local tensile stress. Fractographic analysis showed that most fracture sites were located close to the nodes, where the highest tensile stresses are located.

  • Effects of applied stress ratio on the Fatigue behavior of additively manufactured porous biomaterials under compressive loading
    Journal of The Mechanical Behavior of Biomedical Materials, 2016
    Co-Authors: Joep De Krijger, Brecht Van Hooreweder, Karel Lietaert, Calvin Rans, Behdad Pouran, Amir A. Zadpoor
    Abstract:

    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.

Karel Lietaert - One of the best experts on this subject based on the ideXlab platform.

  • Corrosion Fatigue behavior of additively manufactured biodegradable porous zinc.
    Acta Biomaterialia, 2020
    Co-Authors: Y. Li, Karel Lietaert, W. Li, F. S. L. Bobbert, J-h. Dong, M.a. Leeflang, Jie Zhou, Amir A. Zadpoor
    Abstract:

    Abstract Additively manufactured (AM) biodegradable porous zinc exhibits great potential as a promising bone-substituting biomaterial. However, there is no information whatsoever available regarding its corrosion Fatigue behavior. In this study, we used direct metal printing to fabricate topologically ordered biodegradable porous zinc based on a diamond unit cell. We compared the Compression-Compression Fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The Fatigue strength of AM porous zinc was high in air (i.e., 70% of its yield strength) and even higher in r-SBF (i.e., 80% of its yield strength). The high value of the relative Fatigue strength in air could be attributed to the good ductility of pure zinc itself. The formation of corrosion products around the strut junctions might explain the higher Fatigue strength of AM zinc in r-SBF. Furthermore, we compared the Fatigue behavior of a uniform design of the AM porous zinc with a functionally graded design. The functionally graded structure exhibited higher relative Fatigue strengths than the uniform structure. The inspection of the Fatigue crack distribution revealed that the functionally graded design controlled the sequence of crack initiation, which occurred early in the thicker struts and moved towards the thinner struts over time. The theoretical Fatigue life models suggest that optimizing the functionally graded structure could be used as an effective means to improve the Fatigue life of AM porous zinc. In conclusion, the favorable Fatigue behavior of AM porous zinc further highlights its potential as a promising bone-substituting biomaterial. Statement of Significance Additively manufactured (AM) biodegradable porous zinc exhibits great potential for the treatment of large bony defects. However, there is no information available regarding its corrosion Fatigue behavior. Here, we compared the Fatigue behavior of AM porous zinc in air and in revised simulated body fluid (r-SBF). The Fatigue strength of AM porous Zn was even higher in r-SBF than in air, which were attributed to the formation of corrosion products. Furthermore, we found that the functionally graded structure controlled the sequence of crack initiation in differently sized struts and exhibited higher relative Fatigue strengths than the uniform structure, suggesting that optimizing the functionally graded structure could be an effective means to improve the Fatigue life of AM porous Zn.

  • Fatigue life of additively manufactured Ti6Al4V scaffolds under tension-tension, tension-compression and Compression-Compression Fatigue load
    Scientific Reports, 2018
    Co-Authors: Karel Lietaert, Antonio Cutolo, Dries Boey, Brecht Van Hooreweder
    Abstract:

    Mechanical performance of additively manufactured (AM) Ti6Al4V scaffolds has mostly been studied in uniaxial compression. However, in real-life applications, more complex load conditions occur. To address this, a novel sample geometry was designed, tested and analyzed in this work. The new scaffold geometry, with porosity gradient between the solid ends and scaffold middle, was successfully used for quasi-static tension, tension-tension (R = 0.1), tension-compression (R = −1) and Compression-Compression (R = 10) Fatigue tests. Results show that global loading in tension-tension leads to a decreased Fatigue performance compared to global loading in Compression-Compression. This difference in Fatigue life can be understood fairly well by approximating the local tensile stress amplitudes in the struts near the nodes. Local stress based Haigh diagrams were constructed to provide more insight in the Fatigue behavior. When Fatigue life is interpreted in terms of local stresses, the behavior of single struts is shown to be qualitatively the same as bulk Ti6Al4V. Compression-Compression and tension-tension Fatigue regimes lead to a shorter Fatigue life than fully reversed loading due to the presence of a mean local tensile stress. Fractographic analysis showed that most fracture sites were located close to the nodes, where the highest tensile stresses are located.

  • Improving the Fatigue performance of porous metallic biomaterials produced by Selective Laser Melting
    Acta Biomaterialia, 2017
    Co-Authors: Brecht Van Hooreweder, Yanni Apers, Karel Lietaert
    Abstract:

    This paper provides new insights into the Fatigue properties of porous metallic biomaterials produced by additive manufacturing. Cylindrical porous samples with diamond unit cells were produced from Ti6Al4V powder using Selective Laser Melting (SLM). After measuring all morphological and quasi-static properties, Compression-Compression Fatigue tests were performed to determine Fatigue strength and to identify important Fatigue influencing factors. In a next step, post-SLM treatments were used to improve the Fatigue life of these biomaterials by changing the microstructure and by reducing stress concentrators and surface roughness. In particular, the influence of stress relieving, hot isostatic pressing and chemical etching was studied. Analytical and numerical techniques were developed to calculate the maximum local tensile stress in the struts as function of the strut diameter and load. With this method, the variability in the relative density between all samples was taken into account. The local stress in the struts was then used to quantify the exact influence of the applied post-SLM treatments on the Fatigue life. A significant improvement of the Fatigue life was achieved. Also, the post-SLM treatments, procedures and calculation methods can be applied to different types of porous metallic structures and hence this paper provides useful tools for improving Fatigue performance of metallic biomaterials. Statement of Significance Additive Manufacturing (AM) techniques such as Selective Laser Melting (SLM) are increasingly being used for producing customized porous metallic biomaterials. These biomaterials are regularly used for biomedical implants and hence a long lifetime is required. In this paper, a set of post-built surface and heat treatments is presented that can be used to significantly improve the Fatigue life of porous SLM-Ti6Al4V samples. In addition, a novel and efficient analytical local stress method was developed to accurately quantify the influence of the post-built treatments on the Fatigue life. Also numerical simulation techniques were used for validation. The developed methods and techniques can be applied to other types of porous biomaterials and hence provide new and useful tools for improving and predicting the Fatigue life of porous biomaterials.

  • Effects of applied stress ratio on the Fatigue behavior of additively manufactured porous biomaterials under compressive loading
    Journal of The Mechanical Behavior of Biomedical Materials, 2016
    Co-Authors: Joep De Krijger, Brecht Van Hooreweder, Karel Lietaert, Calvin Rans, Behdad Pouran, Amir A. Zadpoor
    Abstract:

    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.

Rajendra K. Bordia - One of the best experts on this subject based on the ideXlab platform.

  • Compression-Compression Fatigue of selective electron beam melted cellular titanium (Ti-6Al-4V).
    Journal of Biomedical Materials Research Part B, 2011
    Co-Authors: Nikolas W. Hrabe, Peter Heinl, Brian Flinn, Carolin Korner, Rajendra K. Bordia
    Abstract:

    Regular 3D periodic porous Ti-6Al-4V structures intended to reduce the effects of stress shielding in load-bearing bone replacement implants (e.g., hip stems) were fabricated over a range of relative densities (0.17–0.40) and pore sizes (∼500–1500 μm) using selective electron beam melting (EBM). Compression-Compression Fatigue testing (15 Hz, R = 0.1) resulted in normalized Fatigue strengths at 106 cycles ranging from 0.15 to 0.25, which is lower than the expected value of 0.4 for solid material of the same acicular α microstructure. The three possible reasons for this reduced Fatigue lifetime are stress concentrations from closed porosity observed within struts, stress concentrations from observed strut surface features (sintered particles and texture lines), and microstructure (either acicular α or martensite) with less than optimal high-cycle Fatigue resistance. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2011.

  • Compression-Compression Fatigue of selective electron beam melted cellular titanium (Ti-6Al-4V)
    Journal of Biomedical Materials Research - Part B Applied Biomaterials, 2011
    Co-Authors: Nikolas W. Hrabe, Peter Heinl, Brian Flinn, Carolin Korner, Rajendra K. Bordia
    Abstract:

    Regular 3D periodic porous Ti-6Al-4V structures intended to reduce the effects of stress shielding in load-bearing bone replacement implants (e.g., hip stems) were fabricated over a range of relative densities (0.17-0.40) and pore sizes (approximately 500-1500 μm) using selective electron beam melting (EBM). Compression-Compression Fatigue testing (15 Hz, R = 0.1) resulted in normalized Fatigue strengths at 10(6) cycles ranging from 0.15 to 0.25, which is lower than the expected value of 0.4 for solid material of the same acicular α microstructure. The three possible reasons for this reduced Fatigue lifetime are stress concentrations from closed porosity observed within struts, stress concentrations from observed strut surface features (sintered particles and texture lines), and microstructure (either acicular α or martensite) with less than optimal high-cycle Fatigue resistance.

Afsaneh Rabiei - One of the best experts on this subject based on the ideXlab platform.

  • Infrared evaluation of heat generation during the cyclic deformation of a cellular Al alloy
    MRS Proceedings, 2011
    Co-Authors: Afsaneh Rabiei, J. W. Hutchinson, A. G. Evans
    Abstract:

    Heat generation from a notch during the Compression-Compression Fatigue of a cellular Al alloy has been measured and compared with a model. The measurements indicate that heat is generated because of hysteresis occurring in narrow cyclic plastic zones outside the notch. This process continues until the notch closes. At closure, a brief period of heat generation arises because of friction along the notch faces. A plasticity model based on the Dugdale zone is shown to provide a reasonably accurate characterization of the heat generated, with the proviso that an “ineffective” zone be transposed onto the notch tip. It is found that the temperatures generated are too small to cause Fatigue by thermal softening. A Fatigue mechanism based on either geometric softening of the cells or crack growth in the cell walls is implied.

  • Fatigue in aluminum–steel and steel–steel composite foams
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2009
    Co-Authors: Lakshmi Vendra, Brian P. Neville, Afsaneh Rabiei
    Abstract:

    Abstract The compression–compression Fatigue behavior of two classes of composite metal foams (CMF) manufactured using different processing techniques, was investigated experimentally. Aluminum–steel composite foam processed using gravity casting technique comprises of steel hollow spheres and a solid aluminum alloy matrix. Steel–steel composite foam, processed using powder metallurgy (PM) technique consists of steel hollow spheres packed in a steel matrix. Under compression Fatigue loading, the composite foam samples showed a high cyclic stability at maximum stress levels as high as 90 MPa. The deformation of the composite foam samples was divided into three stages – linear increase in strain with Fatigue cycles (stage I), minimal strain accumulation in large number of cycles (stage II) and rapid strain accumulation within few cycles culminating in complete failure (stage III). Composite foams under cyclic loading undergo a uniform distribution of deformation, unlike the regular metal foams, which deform by forming collapse bands at weaker sections. As a result, the features controlling the Fatigue life of the composite metal foams have been considered as sphere wall thickness and diameter, sphere and matrix materials, processing techniques and the bonding strength between the spheres and matrix.

  • A comparison of composite metal foam's properties and other comparable metal foams
    Materials Letters, 2009
    Co-Authors: Afsaneh Rabiei, Lakshmi Vendra
    Abstract:

    Abstract New closed cell composite metal foams are processed using casting and powder metallurgy (PM) techniques. The foam is comprised of steel hollow spheres packed into a random loose arrangement, with the interstitial spaces between spheres occupied with a solid metallic matrix. The characterization of composite metal foams was carried out using monotonic compression, compression–compression Fatigue, loading–unloading compression, micro-hardness and nano-hardness testing. The microstructure of the composite metal foams was studied using optical, scanning electron microscopy imaging and electron dispersive spectroscopy. The composite metal foams displayed superior (5–20 times higher) compressive strengths, reported as 105 MPa for cast foams and 127 MPa for PM foams, and much higher energy absorbing capability as compared to other metal foams being produced with similar materials through other technologies.

  • Fatigue in aluminum-steel and steel-steel composite foams
    Materials Science and Engineering A, 2009
    Co-Authors: Lakshmi Vendra, Brian Neville, Afsaneh Rabiei
    Abstract:

    The Compression-Compression Fatigue behavior of two classes of composite metal foams (CMF) manufactured using different processing techniques, was investigated experimentally. Aluminum-steel composite foam processed using gravity casting technique comprises of steel hollow spheres and a solid aluminum alloy matrix. Steel-steel composite foam, processed using powder metallurgy (PM) technique consists of steel hollow spheres packed in a steel matrix. Under compression Fatigue loading, the composite foam samples showed a high cyclic stability at maximum stress levels as high as 90 MPa. The deformation of the composite foam samples was divided into three stages - linear increase in strain with Fatigue cycles (stage I), minimal strain accumulation in large number of cycles (stage II) and rapid strain accumulation within few cycles culminating in complete failure (stage III). Composite foams under cyclic loading undergo a uniform distribution of deformation, unlike the regular metal foams, which deform by forming collapse bands at weaker sections. As a result, the features controlling the Fatigue life of the composite metal foams have been considered as sphere wall thickness and diameter, sphere and matrix materials, processing techniques and the bonding strength between the spheres and matrix. © 2009 Elsevier B.V. All rights reserved.

  • Heat generation during the Fatigue of a cellular Al alloy
    Metallurgical and Materials Transactions A, 2000
    Co-Authors: Afsaneh Rabiei, J. W. Hutchinson, A. G. Evans
    Abstract:

    The heat generation from a notch during the Compression-Compression Fatigue of a cellular Al alloy has been measured and compared with a model. The measurements indicate that heat is generated because of hysteresis occurring in narrow cyclic plastic zones outside the notch. This process continues until the notch closes. At closure, a brief period of heat generation arises because of friction along the notch faces. A plasticity model based on the Dugdale zone is shown to provide a reasonably accurate characterization of the heat generated, with the proviso that an “ineffective” zone be transposed onto the notch tip. It is found that the temperatures generated are too small to cause Fatigue by thermal softening. A Fatigue mechanism based on either geometric softening of the cells or crack growth in the cell walls is implied.

Alkiviadis Paipetis - One of the best experts on this subject based on the ideXlab platform.

  • Impact and after-impact properties of carbon fibre reinforced composites enhanced with multi-wall carbon nanotubes
    Composites Science and Technology, 2010
    Co-Authors: Vassilis Kostopoulos, Antonios Vavouliotis, Petros Karapappas, Athanasios Baltopoulos, Alkiviadis Paipetis
    Abstract:

    Abstract The goal of the present study was to investigate the influence of multi-wall carbon nanotubes (MWCNTs) on the impact and after impact behaviour of carbon fiber reinforced polymer (CFRP) laminates. About 0.5% per weight MWCNTs were dispersed via a high shear device in the epoxy matrix (Bisphenol A) of carbon reinforced quasi-isotropic laminates. Subsequently, the modified CFRPs were subjected to low-energy impact and directly compared with unmodified laminates. In previous studies, the beneficial effect of the MWCNT inclusion to the fracture properties of CFRPs has been demonstrated. In terms of the CFRP impact performance, enhanced performance for the CNT doped specimens was observed for higher energy levels. However, the after-impact properties and more specifically compression after impact were improved for both the effective compression modulus and the compression strength. In addition, compression–compression Fatigue after impact performance of the CNT modified laminates was also improved, by extending the Fatigue life.

  • Impact and after-impact properties of carbon fibre reinforced composites enhanced with multi-wall carbon nanotubes
    Composites Science and Technology, 2010
    Co-Authors: Vassilis Kostopoulos, Antonios Vavouliotis, Petros Karapappas, Athanasios Baltopoulos, Alkiviadis Paipetis
    Abstract:

    The goal of the present study was to investigate the influence of multi-wall carbon nanotubes (MWCNTs) on the impact and after impact behaviour of carbon fiber reinforced polymer (CFRP) laminates. About 0.5% per weight MWCNTs were dispersed via a high shear device in the epoxy matrix (Bisphenol A) of carbon reinforced quasi-isotropic laminates. Subsequently, the modified CFRPs were subjected to low-energy impact and directly compared with unmodified laminates. In previous studies, the beneficial effect of the MWCNT inclusion to the fracture properties of CFRPs has been demonstrated. In terms of the CFRP impact performance, enhanced performance for the CNT doped specimens was observed for higher energy levels. However, the after-impact properties and more specifically compression after impact were improved for both the effective compression modulus and the compression strength. In addition, Compression-Compression Fatigue after impact performance of the CNT modified laminates was also improved, by extending the Fatigue life. © 2009 Elsevier Ltd. All rights reserved.

  • A simple model for the prediction of the Fatigue delamination growth of impacted composite panels
    Fatigue & Fracture of Engineering Materials & Structures, 2004
    Co-Authors: D. G. Katerelos, Alkiviadis Paipetis, Vassilis Kostopoulos
    Abstract:

    The Fatigue behaviour of composite panels that have been subjected to low-velocity impact was studied. Impacted specimens were tested under Compression-Compression Fatigue. A delamination propagation model based on the derivation of the strain energy release rate was used. The stress distribution around the initially induced delamination was derived analytically. The shape of the delamination was experimentally monitored by c-scan imaging and is assumed to be an ellipse. The orientation and aspect ratio of the ellipse were used to calculate the corresponding strain energy-release rates, which were subsequently used to predict the direction of delamination propagation.