The Experts below are selected from a list of 249 Experts worldwide ranked by ideXlab platform
Wole Soboyejo - One of the best experts on this subject based on the ideXlab platform.
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Surface topography evolution and Fatigue Fracture of polysilicon
Journal of Materials Science, 2003Co-Authors: Seyed Allameh, Pranav Shrotriya, A. Butterwick, S. Brown, Nan Yao, Wole SoboyejoAbstract:This paper presents the results of an experimental stydy of the micromechanisms of Fatigue crack nucleation and Fatigue Fracture in polysilicon MEMS Structures. The initial stages of Fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO_2 layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are elucidated as functions of Fatigue cycling before discussing the micromechanisms of crack growth and final Fracture.
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Surface topography evolution and Fatigue Fracture in polysilicon MEMS structures
Journal of Microelectromechanical Systems, 2003Co-Authors: Seyed Allameh, Pranav Shrotriya, A. Butterwick, Stuart B. Brown, Wole SoboyejoAbstract:This paper presents the results of an experimental study of the micromechanisms of surface topography evolution and Fatigue Fracture in polysilicon MEMS structures. The initial stages of Fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO/sub 2/ layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are characterized as functions of Fatigue cycling before discussing the micromechanisms of Fatigue Fracture.
Zhigang Suo - One of the best experts on this subject based on the ideXlab platform.
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Fatigue Fracture of nearly elastic hydrogels
Soft Matter, 2018Co-Authors: Enrui Zhang, Xavier Morelle, Ruobing Bai, Zhigang SuoAbstract:Polyacrylamide hydrogels are highly stretchable and nearly elastic. Their stress–stretch curves exhibit small hysteresis, and change negligibly after many loading cycles. Polyacrylamide is used extensively in applications, and is the primary network for many types of tough hydrogels. Recent experiments have shown that polyacrylamide hydrogels are susceptible to Fatigue Fracture, but available data are limited. Here we study Fatigue Fracture of polyacrylamide hydrogels of various water contents. We form polymer networks in all samples under the same conditions, and then obtain hydrogels of 96, 87, 78, and 69 wt% of water by solvent exchange. We measure the crack extension under cyclic loads, and the Fracture energy under monotonic loading. For the hydrogels of the four water contents, the Fatigue thresholds are 4.3, 8.4, 20.5, and 64.5 J m−2, and the Fracture energies are 18.9, 71.2, 289, and 611 J m−2. The measured thresholds agree well with the predictions of the Lake-Thomas model for hydrogels of high water content, but not in the case of low water content. It is hoped that further basic studies will soon follow to aid the development of Fatigue-resistant hydrogels.
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Fatigue Fracture of Self-Recovery Hydrogels
ACS Macro Letters, 2018Co-Authors: Ruobing Bai, Xavier Morelle, Jiawei Yang, Canhui Yang, Zhigang SuoAbstract:Hydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress–stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels or even Fatigue-free hydrogels. Such a hydrogel typically contains a covalent polymer network, together with some noncovalent, reversible interactions. Here we show that self-recovery hydrogels are still susceptible to Fatigue Fracture. We study a hydrogel containing both covalently cross-linked polyacrylamide and un-cross-linked poly(vinyl alcohol). For a sample without precut crack, the stress–stretch curve recovers after thousands of loading cycles. For a sample with a precut crack, however, the crack extends cycle by cycle. The threshold for Fatigue Fracture depends on the covalent network but negligibly on noncovalent interactions. Above the threshold, the noncovalent interactions slow down the extension of the crack under cyclic loads.
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Fatigue Fracture of hydrogels
Extreme Mechanics Letters, 2017Co-Authors: Jingda Tang, Joost J. Vlassak, Zhigang SuoAbstract:Abstract Rapid advances are taking place to develop hydrogels of high stretchability and toughness, but Fatigue Fracture has not been studied for any hydrogels. This negligence hinders the development of hydrogels and their applications. Here we initiate a study of Fatigue Fracture of hydrogels. We choose polyacrylamide hydrogel as a model material. To place Fatigue Fracture in context, we apply monotonic, static, and cyclic load, and observe three types of Fracture behavior: fast Fracture, delayed Fracture, and Fatigue Fracture. Below the critical load for fast Fracture, we find two distinct thresholds, one for delayed Fracture, and the other for Fatigue Fracture. The Fracture behavior of hydrogel is sensitive to both the amplitude of load and the concentration of water. We relate the experimental observations to the molecular picture of swollen polymer networks. Fatigue Fracture of hydrogels is a topic ready for scientific studies and engineering advances.
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Fatigue Fracture of tough hydrogels
Extreme Mechanics Letters, 2017Co-Authors: Ruobing Bai, Jingda Tang, Joost J. Vlassak, Quansan Yang, Xavier Morelle, Zhigang SuoAbstract:Abstract Tough hydrogels of many chemical compositions have been developed in recent years, but their Fatigue Fracture has not been studied. The lack of study hinders further development of hydrogels for applications that require long lifetimes under cyclic loads. Examples include tissue engineering, soft robots, and stretchable electronics. Here we study the Fatigue Fracture of a polyacrylamide–alginate tough hydrogel. We find that the stress–stretch curve changes cycle by cycle, and reaches a steady state after thousands of cycles. The threshold for Fatigue Fracture is about 53 J/m2, much below the Fracture energy ( ∼ 10,000 J/m2) measured under monotonic load. Nonetheless, the extension of crack per cycle in the polyacrylamide–alginate tough hydrogel is much smaller than that in a single-network polyacrylamide hydrogel.
Glen E. Novak - One of the best experts on this subject based on the ideXlab platform.
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Fatigue Fracture of long fiber reinforced nylon 66
Polymer Composites, 1995Co-Authors: M. G. Wyzgoski, Glen E. NovakAbstract:The Fatigue behavior of long fiber reinforced nylon 66 has been investigated by measuring Fatigue crack propagation rates of injection molded samples. Plaques varying in thickness from 3 to 10 mm were employed for nylon 66 containing either glass, carbon or aramid fibers. Both conventional chopped, short fiber reinforcements and pultruded long fiber filled nylon 66 were examined. Long fiber reinforced nylon 66 exhibits improved Fatigue resistance as shown by decreases in Fatigue crack propagation rates compared to short fiber filled composites. Using a Fracture mechanics analysis, it is shown that the improvements are due primarily to the higher moduli of the long fiber reinforced nylon 66, with only a slight increase in the calculated strain energy release rate associated with Fatigue crack growth. For short or long glass fibers, and for short carbon fibers, the effects of fiber orientation on Fatigue crack growth rates can be predicted from the Fracture mechanics model. More significant effects of fiber length on Fatigue Fracture energies are noted for long aramid and long carbon reinforced nylon 66. It is also shown that thicker plaques can exhibit poorer Fatigue Fracture behavior owing to their inferior core sections
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Fatigue Fracture of nylon polymers
Journal of Materials Science, 1991Co-Authors: M. G. Wyzgoski, Glen E. NovakAbstract:The influence of glass fibres on the Fatigue crack propagation rates of injection-moulded nylons has been determined. In contrast to previous results for unreinforced nylons, the cracking kinetics are independent of the oscillating load frequency. The fact that the crack growth rate per cycle is constant, when expressed in terms of the time under load, demonstrates that the contribution of creep crack extension is minimized by the glass fibres. Thus a true Fatigue process is suggested for the Fatigue Fracture of the reinforced system, even when the glass fibres are preferentially aligned parallel to the crack growth direction. A complicating factor in characterizing the Fatigue resistance of the glass-reinforced nylons is the tremendous influence of fibre orientation on crack growth rate. It is shown that the anisotropy problem can be handled by simply expressing the crack growth rate data in terms of the strain energy release rate rather than the usual stress intensity factor representation. Results for four different glass-filled nylons show that the diverse crack growth rates for cracking parallel versus perpendicular to the glass-fibre axes collapse on to individual strain energy release rate curves. Each single relationship therefore characterizes the Fatigue Fracture of the filled material and furthermore permits a prediction of the cracking rates for any glass-fibre orientation based upon the expected change in modulus. Finally it is demonstrated that the increased stress dependence of Fatigue crack propagation (slope of the Paris plot) in filled nylons can be duplicated in unfilled samples under certain conditions. It is concluded that the Fatigue Fracture mechanism is matrix dominated in these chopped glass-fibre reinforced materials.
Seyed Allameh - One of the best experts on this subject based on the ideXlab platform.
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Surface topography evolution and Fatigue Fracture of polysilicon
Journal of Materials Science, 2003Co-Authors: Seyed Allameh, Pranav Shrotriya, A. Butterwick, S. Brown, Nan Yao, Wole SoboyejoAbstract:This paper presents the results of an experimental stydy of the micromechanisms of Fatigue crack nucleation and Fatigue Fracture in polysilicon MEMS Structures. The initial stages of Fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO_2 layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are elucidated as functions of Fatigue cycling before discussing the micromechanisms of crack growth and final Fracture.
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Surface topography evolution and Fatigue Fracture in polysilicon MEMS structures
Journal of Microelectromechanical Systems, 2003Co-Authors: Seyed Allameh, Pranav Shrotriya, A. Butterwick, Stuart B. Brown, Wole SoboyejoAbstract:This paper presents the results of an experimental study of the micromechanisms of surface topography evolution and Fatigue Fracture in polysilicon MEMS structures. The initial stages of Fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO/sub 2/ layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are characterized as functions of Fatigue cycling before discussing the micromechanisms of Fatigue Fracture.
M. G. Wyzgoski - One of the best experts on this subject based on the ideXlab platform.
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Fatigue Fracture of long fiber reinforced nylon 66
Polymer Composites, 1995Co-Authors: M. G. Wyzgoski, Glen E. NovakAbstract:The Fatigue behavior of long fiber reinforced nylon 66 has been investigated by measuring Fatigue crack propagation rates of injection molded samples. Plaques varying in thickness from 3 to 10 mm were employed for nylon 66 containing either glass, carbon or aramid fibers. Both conventional chopped, short fiber reinforcements and pultruded long fiber filled nylon 66 were examined. Long fiber reinforced nylon 66 exhibits improved Fatigue resistance as shown by decreases in Fatigue crack propagation rates compared to short fiber filled composites. Using a Fracture mechanics analysis, it is shown that the improvements are due primarily to the higher moduli of the long fiber reinforced nylon 66, with only a slight increase in the calculated strain energy release rate associated with Fatigue crack growth. For short or long glass fibers, and for short carbon fibers, the effects of fiber orientation on Fatigue crack growth rates can be predicted from the Fracture mechanics model. More significant effects of fiber length on Fatigue Fracture energies are noted for long aramid and long carbon reinforced nylon 66. It is also shown that thicker plaques can exhibit poorer Fatigue Fracture behavior owing to their inferior core sections
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Fatigue Fracture of nylon polymers
Journal of Materials Science, 1991Co-Authors: M. G. Wyzgoski, Glen E. NovakAbstract:The influence of glass fibres on the Fatigue crack propagation rates of injection-moulded nylons has been determined. In contrast to previous results for unreinforced nylons, the cracking kinetics are independent of the oscillating load frequency. The fact that the crack growth rate per cycle is constant, when expressed in terms of the time under load, demonstrates that the contribution of creep crack extension is minimized by the glass fibres. Thus a true Fatigue process is suggested for the Fatigue Fracture of the reinforced system, even when the glass fibres are preferentially aligned parallel to the crack growth direction. A complicating factor in characterizing the Fatigue resistance of the glass-reinforced nylons is the tremendous influence of fibre orientation on crack growth rate. It is shown that the anisotropy problem can be handled by simply expressing the crack growth rate data in terms of the strain energy release rate rather than the usual stress intensity factor representation. Results for four different glass-filled nylons show that the diverse crack growth rates for cracking parallel versus perpendicular to the glass-fibre axes collapse on to individual strain energy release rate curves. Each single relationship therefore characterizes the Fatigue Fracture of the filled material and furthermore permits a prediction of the cracking rates for any glass-fibre orientation based upon the expected change in modulus. Finally it is demonstrated that the increased stress dependence of Fatigue crack propagation (slope of the Paris plot) in filled nylons can be duplicated in unfilled samples under certain conditions. It is concluded that the Fatigue Fracture mechanism is matrix dominated in these chopped glass-fibre reinforced materials.
