The Experts below are selected from a list of 288 Experts worldwide ranked by ideXlab platform
R. O. Ritchie - One of the best experts on this subject based on the ideXlab platform.
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mechanisms of fatigue Crack propagation in ductile and brittle solids
International Journal of Fracture, 1999Co-Authors: R. O. RitchieAbstract:The mechanisms of fatigue-Crack propagation are examined with particular emphasis on the similarities and differences between cyclic Crack growth in ductile materials, such as metals, and corresponding behavior in brittle materials, such as intermetallics and ceramics. This is achieved by considering the process of fatigue-Crack growth as a mutual competition between intrinsic mechanisms of Crack Advance ahead of the Crack tip (e.g., alternating Crack-tip blunting and resharpening), which promote Crack growth, and extrinsic mechanisms of Crack-tip shielding behind the tip (e.g., Crack closure and bridging), which impede it The widely differing nature of these mechanisms in ductile and brittle materials and their specific dependence upon the alternating and maximum driving forces (e.g., ΔK and K max ) provide a useful distinction of the process of fatigue-Crack propagation in different classes of materials; moreover, it provides a rationalization for the effect of such factors as load ratio and Crack size. Finally, the differing susceptibility of ductile and brittle materials to cyclic degradation has broad implications for their potential structural application; this is briefly discussed with reference to lifetime prediction.
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Cyclic fatigue in monolithic alumina: mechanisms for Crack Advance promoted by frictional wear of grain bridges
Journal of Materials Science, 1995Co-Authors: C. J. Gilbert, R. N. Petrany, R. O. Ritchie, R. H. Dauskardt, R. W. SteinbrechAbstract:The microstructural basis of cyclic fatigue-Crack propagation in monolithic alumina has been investigated experimentally and theoretically. A true cyclic fatigue effect has been verified, distinct from environmentally assisted slow Crack growth (static fatigue). Microstructures with smaller grain sizes were found to promote faster Crack-growth rates; growth rates were also increased at higher load ratios (i.e. ratio of minimum to maximum applied loads). Using in situ Crack-path analysis performed on a tensile loading stage mounted in the scanning electron microscope, grain bridging was observed to be the primary source of toughening by Crack-tip shielding. In fact, Crack Advance under cyclic fatigue appeared to result from a decrease in the shielding capacity of these bridges commensurate with oscillatory loading. It is proposed that the primary source of this degradation is frictional wear at the boundaries of the bridging grains, consistent with recently proposed bridging/degradation models, and as seen via fractographic and in situ analyses; specifically, load versus Crack-openingdisplacement hysteresis loops can be measured and related to the irreversible energy losses corresponding to this phenomenon.
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cyclic fatigue Crack propagation in a silicon carbide whisker reinforced alumina composite role of load ratio
Journal of Materials Science, 1993Co-Authors: Reinhold H Dauskardt, R. O. Ritchie, B J Dalgleish, D Yao, Paul F BecherAbstract:The characteristics of subcritical Crack growth by cyclic fatigue have been examined in a silicon carbide whisker-reinforced alumina composite, with specific reference to the role of load ratio (ratio of minimum to maximum applied stress intensity, R=Kmin/Kmax); results are compared with similar subcritical Crack-growth data obtained under constant load conditions (static fatigue). Using compact-tension samples cycled at ambient temperatures, cyclic fatigue-Crack growth has been measured over six orders of magnitude from ∼10−11–10−5 m cycle−1 at load ratios ranging from 0.05–0.5. Growth rates (da/dN) display an approximate Paris power-law dependence on the applied stress-intensity range (ΔK), with an exponent varying between 33 and 50. Growth-rate behaviour is found to be strongly dependent upon load ratio; the fatigue threshold, ΔKTH, for example, is found to be increased by over 80% at R=0.05 compared to R=0.5. These results are rationalized in terms of a far greater dependency of growth rates on Kmax(da/dN ∞ Kmax30) compared to ΔK(da/dN ∞ ΔK5), in contrast to fatigue behaviour in metallic materials where generally the reverse is true. Micromechanisms of Crack Advance underlying such behaviour are discussed in terms of timedependent Crack bridging involving either matrix grains or unbroken whiskers.
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cyclic fatigue Crack growth in a sic whisker reinforced alumina ceramic composite long and small Crack behavior
Journal of the American Ceramic Society, 1992Co-Authors: Reinhold H Dauskardt, M R James, J R Porter, R. O. RitchieAbstract:The ambient-temperature subcritical growth behavior of both long and microstructurally small Cracks is investigated during cyclic-fatigue loading in a SiC-whisker-reinforced alumina (Al2O3–SiCw) ceramic composite (fracture toughness, Kc∼ 4.5 MPa · m1/2). Based on long-Crack experiments using compact C(T) specimens, cyclic fatigue-Crack growth rates (over the range 10−11 to 10−5 m/cycle) are found to be sensitive to the applied stress-intensity range and load ratio, and to show evidence of fatigue Crack closure. Similar to other ceramic materials, under tension–tension loading the “long”(>3 mm) Crack fatigue threshold, ΔKTH, was found to be on the order of 60% of Kc. Conversely, “small”(1 to 300 μm) Cracks grown from micro-indents on the surface of cantilever-beam specimens were observed to grow at applied ΔK levels some 2 to 3 times smaller than ΔKTH. similar to behavior widely reported for metallic materials. The observed small-Crack behavior is rationalized in terms of the residual stress field associated with the indent, and the restricted role of Crack-tip shielding (from mechanisms such as Crack bridging, closure and deflection) with Cracks of limited wake, analogous to closure effects with small fatigue Cracks in metals. Consistent with the lack of zone-shielding mechanisms in Al2O3–SiCw, under variable-amplitude cyclic loading Crack-growth rates do not exhibit the marked transient response following block overload sequences as do transformation-toughened ceramics or ductile metallic materials. Possible mechanisms for cyclic Crack Advance in reinforced ceramic-matrix composite materials are discussed.
M.m. Hall - One of the best experts on this subject based on the ideXlab platform.
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interacting sensitivities of alloy 600 pwscc to stress intensity factor yield stress temperature carbon concentration and Crack growth orientation alloy 600
Corrosion Science, 2017Co-Authors: M.m. HallAbstract:Abstract Analyses of Alloy 600 primary water stress corrosion Cracking data show there are interacting sensitivities of Crack growth rate to applied stress intensity factor, yield stress, temperature, carbon concentration and Crack growth orientation. The effects of these variables are expressed quantitatively using a Crack Advance model that is rate-limited by Crack-tip-strain rate. Crystallographic texture and dynamic strain aging are considered to account for contrasting behavior observed for ST and LT orientation Crack growth. Crack growth is conjectured to be rate-limited by “junction” pinning of mobile dislocations strengthened by interstitial carbon (ST) and “lineal” pinning by substitutional Cr and Fe (LT).
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critique of the ford andresen film rupture model for aqueous stress corrosion Cracking
Corrosion Science, 2009Co-Authors: M.m. HallAbstract:Abstract The Ford–Andresen film rupture model for aqueous stress corrosion Cracking has obtained a prominent position in the nuclear reactor industry. The model is said to have superior predictive capabilities because it is derived from a fundamental understanding of the film rupture-repassivation mechanism of Crack Advance. However, a critical review shows that there are conceptual and mathematical problems with the Ford–Andresen model development; there are inconsistencies among the stated and implied assumptions, the Crack tip current density expression lacks the necessary dependence on Crack tip strain rate and the fundamental proportionality that exists between Crack tip strain rate and Crack growth rate is overlooked and omitted from the model development. Consequently, the Ford–Andresen model must be considered neither phenomenologically nor fundamentally supported.
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film rupture model for aqueous stress corrosion Cracking under constant and variable stress intensity factor
Corrosion Science, 2009Co-Authors: M.m. HallAbstract:Abstract A film rupture model for aqueous stress corrosion Cracking is developed and used to predict kinetics of Crack growth under constant and variable stress intensity factor. The model predicts that creep is necessary for sustained Crack growth and creep rate limits Crack velocity for constant K and dK / da loading. Contrary to recent thinking, the Crack tip strain due to Crack Advance is viewed as a result, not a cause of Crack growth. The Crack tip strain gradient elevates and maintains Crack tip stress as the Crack propagates, which enables creep and sustained Crack growth. The model provides a basis for understanding effects of positive and negative K – variation on Crack growth.
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Crack tip strain rate equation with applications to Crack tip embrittlement and active path dissolution models of stress corrosion Cracking
Environment-Induced Cracking of Materials, 2008Co-Authors: M.m. HallAbstract:Publisher Summary The mathematical similarity between constitutive equations used to describe high and low temperature creep allows use in stress corrosion Cracking (SCC) modeling of Crack tip strain rate solutions derived for high temperature creep Crack growth. A completely general consideration of Crack growth under constant applied stress intensity factor (K) shows that sustained SCC Crack growth is not possible without a creep contribution to the Crack tip strain rate. The distribution of Crack tip strain rate consists of nested zones with an inner Hui and Riedel (HR) creep zone, Hutchinson and Rice and Rosengren (HRR) elastic-strain hardening plastic zone and an outer K elastic zone. A fracture or damage rate criterion is required to complete development of an SCC Crack growth model. Application of a minimum strain rate criterion and the HR solutions to Crack Advance by a Crack tip embrittlement mechanism such as hydrogen embrittlement is straight forward and results in a power-law Crack growth rate equation similar to the empirical power-law equation often used for SCC data correlations.
Huseyin Sehitoglu - One of the best experts on this subject based on the ideXlab platform.
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deshielding effects on fatigue Crack growth in shape memory alloys a study on cuznal single crystalline materials
Acta Materialia, 2019Co-Authors: J Yaacoub, Florian Brenne, Wael Abuzaid, D Canadinc, Huseyin SehitogluAbstract:Abstract The factors that affect the fatigue performance of shape memory alloys (SMAs), including fatigue Crack growth (FCG) response, is far from being well-understood. In this study, we point to a mechanism that degrades the FCG performance considerably. We introduce the notion of FCG being affected by shielding and deshielding mechanisms, the former enhancing the resistance while the latter reducing the materials’ resistance. We show that the deshielding mechanism creates additional driving forces (positive K contribution) of both Mode II and Mode I types (as much as 5–10 MPa m1/2) which accelerates the Crack Advance. The origin of the positive K component is associated with the localized martensite variant formation that is highly asymmetric with respect to the Crack tip. We derive a resultant ΔK in excellent agreement with that measured based on experimental displacement measurements. Overall, this study represents an Advancement of our understanding in FCG of SMAs by quantifying the deshielding mechanism.
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modeling fatigue Crack growth resistance of nanocrystalline alloys
Acta Materialia, 2013Co-Authors: Piyas Chowdhury, Huseyin Sehitoglu, Richard G Rateick, H J MaierAbstract:The description of fatigue Crack growth in metals has remained an empirical field. To address the physical processes contributing to Crack Advance a model for fatigue Crack growth (FCG) has been developed utilizing a combined atomistic–continuum approach. In particular, the model addresses the important topic of the role of nanoscale coherent twin boundaries (CTB) on FCG. We make the central observation that FCG is governed by the dislocation glide resistance and the irreversibility of Crack tip displacement, both influenced by the presence of CTBs. The energy barriers for dislocation slip under cyclical conditions are calculated as the glide dislocation approaches a twin boundary and reacts with the CTB. The atomistically calculated energy barriers provide input to a mechanics model for dislocations gliding in a forward and reverse manner. This approach allows the irreversibility of displacement at the Crack tip, defined as the difference between forward and reverse flow, to be determined. The simulation results demonstrate that for both refinement of twin thickness and a decrease in Crack tip to twin spacing FCG resistance improves, in agreement with recent experimental findings reported in the literature.
Reinhold H Dauskardt - One of the best experts on this subject based on the ideXlab platform.
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cyclic fatigue Crack propagation in a silicon carbide whisker reinforced alumina composite role of load ratio
Journal of Materials Science, 1993Co-Authors: Reinhold H Dauskardt, R. O. Ritchie, B J Dalgleish, D Yao, Paul F BecherAbstract:The characteristics of subcritical Crack growth by cyclic fatigue have been examined in a silicon carbide whisker-reinforced alumina composite, with specific reference to the role of load ratio (ratio of minimum to maximum applied stress intensity, R=Kmin/Kmax); results are compared with similar subcritical Crack-growth data obtained under constant load conditions (static fatigue). Using compact-tension samples cycled at ambient temperatures, cyclic fatigue-Crack growth has been measured over six orders of magnitude from ∼10−11–10−5 m cycle−1 at load ratios ranging from 0.05–0.5. Growth rates (da/dN) display an approximate Paris power-law dependence on the applied stress-intensity range (ΔK), with an exponent varying between 33 and 50. Growth-rate behaviour is found to be strongly dependent upon load ratio; the fatigue threshold, ΔKTH, for example, is found to be increased by over 80% at R=0.05 compared to R=0.5. These results are rationalized in terms of a far greater dependency of growth rates on Kmax(da/dN ∞ Kmax30) compared to ΔK(da/dN ∞ ΔK5), in contrast to fatigue behaviour in metallic materials where generally the reverse is true. Micromechanisms of Crack Advance underlying such behaviour are discussed in terms of timedependent Crack bridging involving either matrix grains or unbroken whiskers.
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cyclic fatigue Crack growth in a sic whisker reinforced alumina ceramic composite long and small Crack behavior
Journal of the American Ceramic Society, 1992Co-Authors: Reinhold H Dauskardt, M R James, J R Porter, R. O. RitchieAbstract:The ambient-temperature subcritical growth behavior of both long and microstructurally small Cracks is investigated during cyclic-fatigue loading in a SiC-whisker-reinforced alumina (Al2O3–SiCw) ceramic composite (fracture toughness, Kc∼ 4.5 MPa · m1/2). Based on long-Crack experiments using compact C(T) specimens, cyclic fatigue-Crack growth rates (over the range 10−11 to 10−5 m/cycle) are found to be sensitive to the applied stress-intensity range and load ratio, and to show evidence of fatigue Crack closure. Similar to other ceramic materials, under tension–tension loading the “long”(>3 mm) Crack fatigue threshold, ΔKTH, was found to be on the order of 60% of Kc. Conversely, “small”(1 to 300 μm) Cracks grown from micro-indents on the surface of cantilever-beam specimens were observed to grow at applied ΔK levels some 2 to 3 times smaller than ΔKTH. similar to behavior widely reported for metallic materials. The observed small-Crack behavior is rationalized in terms of the residual stress field associated with the indent, and the restricted role of Crack-tip shielding (from mechanisms such as Crack bridging, closure and deflection) with Cracks of limited wake, analogous to closure effects with small fatigue Cracks in metals. Consistent with the lack of zone-shielding mechanisms in Al2O3–SiCw, under variable-amplitude cyclic loading Crack-growth rates do not exhibit the marked transient response following block overload sequences as do transformation-toughened ceramics or ductile metallic materials. Possible mechanisms for cyclic Crack Advance in reinforced ceramic-matrix composite materials are discussed.
Ibrahim Karaman - One of the best experts on this subject based on the ideXlab platform.
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Actuation-Induced stable Crack growth in near-equiatomic nickel-titanium shape memory alloys: Experimental and numerical analysis
International Journal of Solids and Structures, 2020Co-Authors: S. Jape, Theocharis Baxevanis, Dimitris C Lagoudas, B. Young, Behrouz Haghgouyan, C. Hayrettin, Ibrahim KaramanAbstract:Abstract Shape Memory Alloy (SMA) actuation technology requires a thorough understanding of the failure response of these alloys under loading that involves thermal variations, termed “actuation” loading. In this paper, the experimental observation of stable Crack growth in SMA compact tension specimens during temperature changes under constant bias loads is reported for the first time. The intrinsic damage mechanisms that promote Crack Advance are those reported in literature for nominally isothermal overload fracture, i.e., cleavage assisted by ductile void growth. Moreover, a numerical analysis is employed, and the resulting simulations are compared with the experimental data with the purpose of building confidence in the insight provided on the role of extrinsic mechanisms that further promote or impede Crack Advance. It is concluded that phase transformation plays a dual role on the Crack growth kinetics by promoting Crack growth when occurring in a fan in front of the Crack tip and providing the toughness enhancement that results in stable Crack growth when left in the wake of the advancing Crack. While the latter is well known as transformation-induced toughness enhancement, the former has just been recently observed experimentally and is characteristic of SMAs subjected to actuation loading conditions.
