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Michael R Wisnom - One of the best experts on this subject based on the ideXlab platform.
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7.4 Size Effects in Composites
Comprehensive Composite Materials II, 2018Co-Authors: Michael R WisnomAbstract:The effect of test specimen size on the unnotched Strength of continuous fiber reinforced composites is considered. The different fundamental failure mechanisms of fiber direction tension, fiber direction compression, and matrix dominated transverse tension and shear are discussed in turn. The available experimental data is reviewed, concentrating on tests on unidirectional carbon and glass fiber/epoxy materials. The results show a tendency for the Strength to decrease with increasing specimen volume for all failure modes. Size effects in tensile failure have been found in both tensile and flexural tests, and from higher Strengths in bending than in tension. The magnitude of the effect is consistent with Weibull moduli in the range 13–41 for the materials considered. There are some indications that the size effect in tension diminishes with increasing scale. Lower compressive Strengths have been found in large components than in small test coupons, and attributed to defects. Size effects observed in compressive failure in bending are believed to be mainly due to the stress gradient through the thickness. There are large size effects for matrix dominated failures, and Weibull Strength theory predicts the data reasonably well. It is important to consider the effect of specimen size on Strength when carrying out tests on composites, and to take account of it when using Strength data from small coupons in the design of large structures.
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Size effects in the testing of fibre-composite materials
Composites Science and Technology, 1999Co-Authors: Michael R WisnomAbstract:The effect of test specimen size on the unnotched Strength of continuous fibre reinforced composites is considered. The different fundamental failure mechanisms of fibre direction tension, fibre direction compression, and matrix dominated transverse tension and shear are discussed in turn. The available experimental data is reviewed, concentrating on tests on unidirectional carbon and glass fibre/epoxy materials. The results show a tendency for the Strength to decrease with increasing specimen volume for all failure modes. Size effects in tensile failure have been found in both tensile and flexural tests, and from higher Strengths in bending than in tension. The magnitude of the effect is consistent with Weibull moduli in the range 13-29 for the materials considered. There are some indications that the size effect in tension diminishes with increasing scale. Lower compressive Strengths have been found in large components than in small test coupons, and attributed to defects. Size effects observed in compressive failure in bending are believed to be mainly due to the stress gradient through the thickness. There are large size effects for matrix dominated failures, and Weibull Strength theory fits the data reasonably well. It is important to consider the effect of specimen size on Strength when carrying out tests on composites, and to take account of it when using Strength data from small coupons in the design of large structures.
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reduction in tensile and flexural Strength of unidirectional glass fibre epoxy with increasing specimen size
Composite Structures, 1997Co-Authors: Michael R Wisnom, J W AtkinsonAbstract:Abstract Size effects in tensile failure were investigated by means of tensile and four-point bending tests. Tapered tensile specimens with plies dropped off internally showed a reduction in strain at failure with increasing gauge length. Scaled bending tests also showed a reduction in strain with increasing specimen size. These two effects and the relationship between the tensile and flexural results could all be fitted satisfactorily with a Weibull Strength model.
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the effect of specimen size on the bending Strength of unidirectional carbon fibre epoxy
Composite Structures, 1991Co-Authors: Michael R WisnomAbstract:Four-point bending and pinned-end buckling tests were carried out on scaled specimens of 25, 50 and 100 plies. Both types of test gave similar results and showed a significant decrease in Strength with increasing specimen size. The smaller specimens tended to fail progressively in tension. The tensile strain to failure decreased by about 8% for each doubling of specimen size. This corresponds to a Weibull modulus of about 25. However the amount of scatter in the results was much smaller than that expected based on Weibull Strength theory. The larger specimens tended to fail catastrophically in compression. This suggests a reduction in compressive Strength with specimen size which may be even larger than the reduction in tensile Strength.
Marc Anglada - One of the best experts on this subject based on the ideXlab platform.
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Weibull Strength statistics of hydrothermally aged 3 mol% yttria-stabilised tetragonal zirconia
Ceramics International, 2014Co-Authors: Fernando García Marro, A. Mestra, Marc AngladaAbstract:The effect of hydrothermal degradation on the flexural Strength and on the fracture statistics of 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP) has been investigated. To do so, a large number ( > 30) of specimens were tested in four-point bending test after exposure to water vapour at 134 C and 2 bar pressure for different periods of time up to 200 h. The decay in mechanical Strength with hydrothermal degradation material was analysed by Weibull statistics, and a strong increase in the Weibull modulus was observed (from 12 to 20). These results are discussed in terms of the thickness of the surface degraded layer, the size of natural defects in the specimens and the fracture toughness of both the tetragonal and monoclinic phases. The increase in Weibull modulus is associated with a change in the control of fracture from natural defects to the thickness of the surface degraded layer. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Weibull Strength statistics of hydrothermally aged 3 mol% yttria-stabilised tetragonal zirconia
Ceramics International, 2014Co-Authors: Fernando García Marro, A. Mestra, Marc AngladaAbstract:Abstract The effect of hydrothermal degradation on the flexural Strength and on the fracture statistics of 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP) has been investigated. To do so, a large number (>30) of specimens were tested in four-point bending test after exposure to water vapour at 134 °C and 2 bar pressure for different periods of time up to 200 h. The decay in mechanical Strength with hydrothermal degradation material was analysed by Weibull statistics, and a strong increase in the Weibull modulus was observed (from 12 to 20). These results are discussed in terms of the thickness of the surface degraded layer, the size of natural defects in the specimens and the fracture toughness of both the tetragonal and monoclinic phases. The increase in Weibull modulus is associated with a change in the control of fracture from natural defects to the thickness of the surface degraded layer.
Fernando García Marro - One of the best experts on this subject based on the ideXlab platform.
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Weibull Strength statistics of hydrothermally aged 3 mol% yttria-stabilised tetragonal zirconia
Ceramics International, 2014Co-Authors: Fernando García Marro, A. Mestra, Marc AngladaAbstract:The effect of hydrothermal degradation on the flexural Strength and on the fracture statistics of 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP) has been investigated. To do so, a large number ( > 30) of specimens were tested in four-point bending test after exposure to water vapour at 134 C and 2 bar pressure for different periods of time up to 200 h. The decay in mechanical Strength with hydrothermal degradation material was analysed by Weibull statistics, and a strong increase in the Weibull modulus was observed (from 12 to 20). These results are discussed in terms of the thickness of the surface degraded layer, the size of natural defects in the specimens and the fracture toughness of both the tetragonal and monoclinic phases. The increase in Weibull modulus is associated with a change in the control of fracture from natural defects to the thickness of the surface degraded layer. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Weibull Strength statistics of hydrothermally aged 3 mol% yttria-stabilised tetragonal zirconia
Ceramics International, 2014Co-Authors: Fernando García Marro, A. Mestra, Marc AngladaAbstract:Abstract The effect of hydrothermal degradation on the flexural Strength and on the fracture statistics of 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP) has been investigated. To do so, a large number (>30) of specimens were tested in four-point bending test after exposure to water vapour at 134 °C and 2 bar pressure for different periods of time up to 200 h. The decay in mechanical Strength with hydrothermal degradation material was analysed by Weibull statistics, and a strong increase in the Weibull modulus was observed (from 12 to 20). These results are discussed in terms of the thickness of the surface degraded layer, the size of natural defects in the specimens and the fracture toughness of both the tetragonal and monoclinic phases. The increase in Weibull modulus is associated with a change in the control of fracture from natural defects to the thickness of the surface degraded layer.
Henrik Lund Frandsen - One of the best experts on this subject based on the ideXlab platform.
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Influence of porosity on mechanical properties of tetragonal stabilized zirconia
Journal of The European Ceramic Society, 2017Co-Authors: D.n. Boccaccini, Henrik Lund Frandsen, Stefano Soprani, Maria Cannio, Trine Klemensø, Vanesa Gil, Peter Vang HendriksenAbstract:Abstract 3YSZ specimens with variable open porosity (1–57%) were fabricated, and the stiffness, Strength and fracture properties (fracture toughness and R-curve) were measured to investigate their potential use as support structures for solid oxide fuel or electrolysis cells. The ball-on-ring test was used to characterize Young's modulus and Weibull Strength. The variation of fracture toughness with porosity was investigated and modelled using the results from fracture mechanical testing. A distinct R-curve behaviour was observed in dense 3YSZ specimens, in samples with a porosity around 15% and in some of the highly porous samples (porosities ∼45%) reflecting a transformation toughening in the material. For the most porous samples, the “R-curve behaviour” disappeared and subcritical crack growth was observed. The studies indicate that even highly porous 3YSZ structures (porosities exceeding 40%) are feasible supports for SOFC/SOECs from a mechanical point of view.
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Influence of pore former on porosity and mechanical properties of Ce0.9Gd0.1O1.95 electrolytes for flue gas purification
Ceramics International, 2016Co-Authors: Benoit Charlas, Henrik Lund Frandsen, Cristine Grings Schmidt, Kjeld Bøhm Andersen, Dino Norberto Boccaccini, Kent Kammer Hansen, Andreas Roosen, Andreas KaiserAbstract:Abstract Single layered porous Ce 0.9 Gd 0.1 O 1.95 electrolytes were fabricated by tape casting using different types, shapes and sizes of pore formers and their respective Strength and stiffness were compared. The sintered bodies were characterized by scanning electron microscopy, mercury porosimetry, impulse excitation technique (Young modulus) and flexural Strength measurements, to investigate the role of the different pore formers on the properties of the compounds. The compared techniques used to evaluate porosity give consistent results. The ratio between open and total porosities, evaluated from mercury porosimetry, varies depending on the used pore formers. The stiffness and Strength of the compounds show an exponential dependency to the total porosity. By considering the open porosity instead (functional porosity), we observe that samples with platelets shaped pore formers have higher in-plane Strength than spherical pore formers. An optimum can be found in term of Weibull Strength and strain of samples obtained with the various pore formers by considering the dependency on the functional open porosity instead of the total porosity.
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Weibull statistics effective area and volume in the ball-on-ring testing method
Mechanics of Materials, 2014Co-Authors: Henrik Lund FrandsenAbstract:Abstract The ball-on-ring method is together with other biaxial bending methods often used for measuring the Strength of plates of brittle materials, because machining defects are remote from the high stresses causing the failure of the specimens. In order to scale the measured Weibull Strength to geometries relevant for the application of the material, the effective area or volume for the test specimen must be evaluated. In this work analytical expressions for the effective area and volume of the ball-on-ring test specimen is derived. In the derivation the multiaxial stress field has been accounted for by use of the Weibull theory, and the multinomial theorem has been used to handle the integration of multiple terms raised to the power of the Weibull modulus. The analytical solution is verified with a high number of finite element models for various geometric parameters. The finite element model was also used to study the difference from other multiaxial failure theories, i.e. the Batdorf theories.
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Mechanical reliability of geometrically imperfect tubular oxygen transport membranes
Journal of Membrane Science, 2014Co-Authors: Kawai Kwok, Henrik Lund Frandsen, Martin Søgaard, Peter Vang HendriksenAbstract:Abstract Mixed ionic and electronic conductors have potential applications as oxygen transport membranes. Realization of the technology is challenged by mechanical reliability of the components which are subjected to stresses arising from oxygen stoichiometry gradients and external overpressure during operation. This paper investigates numerically the failure risk of tubular oxygen transport membranes under industrial operating conditions using finite element modeling and Weibull Strength analysis. The effects of component manufacturing defects on fracture probability are elucidated by explicit modeling of imperfections in the tubular membrane geometry. A supported membrane made of dense and porous Zr-doped-BSCF is studied as an illustrative example. It is shown that stresses induced by oxygen stoichiometry gradients relax over time due to creep and external pressure is the dominating source of stress in the long term. Therefore, creep has no adverse effect for geometrically perfect membranes. For geometrically imperfect membranes, curl and eccentricity are found to have insignificant influence on fracture risk while ovality is identified as the most critical geometric imperfection. Under the influence of external pressure, ovality may lead to dramatic stress increase and flattening of oval cross sections. Oval membranes can fail in the long term even though the instantaneous fracture risk is tolerable. Based on industrial relevant conditions, the requirements to the material creep rate and component quality (in terms of specification of tolerable deviation from perfect tubular shape) that allows fail-safe operation are deduced.
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Optimization of the Strength of SOFC anode supports
Journal of the European Ceramic Society, 2012Co-Authors: Henrik Lund Frandsen, Tania Ramos, Antonin Faes, Mikko Pihlatie, Karen BrodersenAbstract:Abstract During operation solid oxide fuel cells are stressed by temperature gradients and various internal and external mechanical loads, which must be withstood. This work deals with the optimization of the Strength of as-sintered anode supported half-cells by imposing changes to production parameters, such as powder milling and sintering temperature. The Strength was measured with the ball-on-ring method, and analyzed with a large displacement finite element model. Weibull statistics were used to describe the distribution of Strengths. The influence on the Weibull Strength of the many different processing parameters was found to be quantifiable in terms of cell porosity to a large extent. The results were validated with an independent set of measurements of Strength and stiffness by uniaxial tension and the impulse excitation technique, respectively. For application of the finding in relation to the SOFC technology a mathematical frame to determine the optimal porosity of a SOFC system is presented.
David L Shelleman - One of the best experts on this subject based on the ideXlab platform.
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effect of cleaning and abrasion induced damage on the Weibull Strength distribution of sapphire fiber
Journal of the American Ceramic Society, 1994Co-Authors: Eric R Trumbauer, John R Hellmann, David L Shelleman, D A KossAbstract:Fractographic analysis revealed the presence of concurrent flaw populations in sapphire fibers which were tensile tested in the as-received condition (sized and unsized) and after various cleaning procedures. The following flaw populations were identified: surface flaws attributed to handling and abrasion damage (type A), volume or internal flaws attributed to shrinkage voids which form during the manufacturing process (type B), localized fiber surface reaction flaws introduced during the flame-cleaning procedure (type C), and self-abrasion surface flaws intentionally introduced on unsized fibers (type D). The Strength distribution associated with each flaw type was characterized using a censored data Weibull analysis for both the least-squares and maximum-likelihood estimation methods. The Strength distribution for type C (flame-cleaning) flaws exhibited an approximately 20% degradation in Strength compared to the distribution for type A flaws. The Strength distribution for type D (self-abrasion) flaws exhibited an approximately 35% degradation in Strength compared to the Strength distribution for type A flaws. This result underscores the need for fiber sizings to prevent damage during shipping and handling. However, higher purity sizings and/or improved procedures for sizing removal are required to mitigate cleaning-induced fiber Strength degradation during composite fabrication.
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Effect of Cleaning and Abrasion‐Induced Damage on the Weibull Strength Distribution of Sapphire Fiber
Journal of the American Ceramic Society, 1994Co-Authors: Eric R Trumbauer, John R Hellmann, David L Shelleman, D A KossAbstract:Fractographic analysis revealed the presence of concurrent flaw populations in sapphire fibers which were tensile tested in the as-received condition (sized and unsized) and after various cleaning procedures. The following flaw populations were identified: surface flaws attributed to handling and abrasion damage (type A), volume or internal flaws attributed to shrinkage voids which form during the manufacturing process (type B), localized fiber surface reaction flaws introduced during the flame-cleaning procedure (type C), and self-abrasion surface flaws intentionally introduced on unsized fibers (type D). The Strength distribution associated with each flaw type was characterized using a censored data Weibull analysis for both the least-squares and maximum-likelihood estimation methods. The Strength distribution for type C (flame-cleaning) flaws exhibited an approximately 20% degradation in Strength compared to the distribution for type A flaws. The Strength distribution for type D (self-abrasion) flaws exhibited an approximately 35% degradation in Strength compared to the Strength distribution for type A flaws. This result underscores the need for fiber sizings to prevent damage during shipping and handling. However, higher purity sizings and/or improved procedures for sizing removal are required to mitigate cleaning-induced fiber Strength degradation during composite fabrication.
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Test methodology for tubular components. Volume 2. Fast fracture Strength study. Topical report, June 1, 1986 to August 14, 1990
1991Co-Authors: David L Shelleman, John J. Mecholsky, J.c. ConwayAbstract:Experimental test methods for evaluating the fast fracture Strength of large open-ended tubular components were investigated as a function of temperature. Weibull Strength distributions obtained for simple specimens (c-rings and o-rinds) were successfully scaled to predict the Strength of large tubular components, which were tested in a specially designed high-temperature tube burst test apparatus. Specimen configurations were selected to sample flaw populations on both the inner (i.e., o-ring tested in diametral compression, c-rings loaded in tension, short tubes, and long tubes) and outer surfaces (c-rings loaded in compression) of tubes. The design of the tube burst test apparatus is discussed. The Strengths and fracture toughness behavior of two SiC materials were investigated.