Low cycle fatigue life improvement of AISI 304 by initial and intermittent wire brush hammering

The effects of hammering by wire brush as a method of improving low cycle fatigue life of highly ductile austenitic stainless steel AISI 304 have been investigated through an experimental study combining imposed strain fatigue tests and assessment of surface characteristic changes under cyclic loading by SEM examinations and XRD analysis. It has been shown that the fatigue life of wire brush hammered surface was increased by 307% at an imposed strain rate of 0.2% and only 17% at an imposed strain rate of 0.5%, comparatively to the turned surface. This increase in fatigue life is explained in terms of fatigue damage that is related to crack networks characteristics and stability which are generated during fatigue on both turned and wire brush hammered surfaces. The improvement of brushed surface is attributed to the role of the surface topography, the near surface stabilized compressive residual stresses and superfi-cial cold work hardening on the fatigue crack network nucleation and growth. It is found that wire brush hammering produces a surface texture that favors, under cyclic loading, nucleation of randomly dispersed short cracks of the order of 40 lm in length stabilized by the compressive residual stress field that reached a value of r0 = 749 MPa. In contrast, turned surface showed much longer unstable cracks of the order of 200 lm in length nucleated in the machining groves with high tendency to propagate under the effect of tensile residual stress field that reached value of r0 = 476 MPa. This improvement is limited to strain rates lower than 0.5%. At higher strain rates, a cyclic plastic deformation induced martensitic phase alters furthermore the fatigue behavior by producing high cyclic strengthening of the bulk mate-rial. This phenomenon lead to a reduction in strain imposed fatigue life. It has also been established that wire brush hammering can be used as an onsite surface treatment to improve the residual fatigue life of components subjected to cyclic loading. The efficiency of this treatment is demonstrated if it is performed at a fraction of service lifetime Ni/Nr lower than 0.5

Effects of temperature and frequency on fatigue crack growth in 18% Cr ferritic stainless steel

The fatigue crack growth behaviour of a ferritic stainless steel has been investigated as a function of test temperature, thermal exposure and frequency at intermediate growth rates. In general, fatigue crack growth rates increased with increasing temperature and in the temperature range 500-700 [deg]C growth rates were described by a kinetic process with an activation energy of 48 kJ/mole. Higher than normal growth rates at 475 [deg]C were observed and attributed to an embrittlement process which is known to occur in this temperature regime in high-chromium ferritic stainless steels. The influence of frequency on fatigue crack growth rates was examined at 500 and 655 [deg]C for a load ratio of 0.1 and over four decades of frequency. A transition from time-independent to time-dependent behaviour was observed at each temperature as frequency was lowered. The frequency at which this transition occurred was dependent on temperature. For all temperatures investigated, near threshold crack propagation occurred by a crystallographic or faceted propagation mechanism. At high crack growth rates, crack-tip plasticity was significant and propagation proceeded by a ductile striation formation process. At intermediate growth rates a mixed-mode fatigue crack growth mechanism was observed where some intergranular fracture occurred.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30806/1/0000464.pd

Near-threshold fatigue crack growth behaviour of a ferritic stainless steel at elevated temperatures

Near-threshold fatigue crack propagation (FCP) behaviour has been studied in an 18%Cr---Nb stabilized ferritic stainless steel at temperatures ranging from room temperature to 700 [deg]C. At a stress ratio of 0.1 increasing the test temperature from room temperature to 500 [deg]C resulted in an increase of the growth rates in the midrange growth regime and a sharply defined threshold at a [delta]K level higher than the room temperature threshold, giving rise to a crossover type of behaviour. At temperatures higher than 500 [deg]C increased crack tip plasticity predominates and the fatigue crack growth rates decrease smoothly with a decreasing value of [Delta]K to thresholds lower than the room temperature value. Crack closure measurements suggest that asperity-induced closure dominates at room temperature but transitions to plasticity-induced closure dominate at 500 [deg]C. A constant-Kmax, increasing R-ratio (CKIR) test procedure was utilized at room temperature and at 500 [deg]C in an attempt to identify near-threshold FCP data in the absence of crack closure. However, the type of crossover behaviour identified with constant R-ratio tests at room temperature and 500 [deg]C was also observed in the CKIR tests. This is attributed to a change in the closure mechanism from a roughness-induced one to one involving crack tip plasticity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30165/1/0000549.pd

Effect of machining processes on the residual stress distribution heterogeneities and their consequences on the stress corrosion cracking resistance of AISI 316L SS in chloride medium

The effects of machining such as grinding and turning on the microstructural and mechanical changes of the machined surfaces of AISI 316L stainless steel (SS) have been studied. Surface aspects and surface defects have been examined by scanning electron microscopy (SEM). Machining-induced nanocrystallization has been investigated by transmission electron microscopy (TEM). Surface and subsurface residual stress distribution and plastic deformation induced by the machining processes have been assessed by X-ray diffraction (XRD) and micro-hardness measurements, respectively. The susceptibility to stress corrosion cracking (SCC) has been assessed by SEM examination of micro-crack networks which are characteristics of a machined surface immersed in boiling (140 ± 2 °C) solution of MgCl2 (40%) during a 48 h-period. The machined surface properties have been correlated to severe plastic deformation (SPD) resulting from specific cutting state of each process. High cutting temperature and plastic rate are considered to be at the origin of near-surface austenitic grain refinement that leads to equiaxed nanograins with a size ranging from 50 to 200 nm. Ground surface residual stress distribution heterogeneities at the micrometric scale are attributed to the random distribution of the density and the geometry of abrasive grains that represent micro-cutting tools in the grinding process. The relationship between residual stress distribution and susceptibility of the AISI 316L SS to SCC has been demonstrated, and an experimental criterion for crack initiation has been established

Near-threshold fatigue crack propagation in ferritic stainless steels at elevated temperatures.

The effects of temperature from 20\sp\circC to 700\sp\circC on the near-threshold fatigue crack growth behavior have been studied in an 18%Cr-Nb stabilized ferritic stainless steel at a stress ratio of 0.1. In general, growth rates at intermediate levels of $\Delta$K increased and $\Delta$K\sb{\rm th} decreased with increasing temperature. An exception occurred at 500\sp\circC where both growth rates at intermediate levels of $\Delta$K and $\Delta$K\sb{\rm th} were 2.5 and 1.4 times higher, respectively, than the room temperature values resulting in a crossover behavior. An extensive evaluation of the crossover using the constant R-ratio, $\Delta$K decreasing tests, and the constant-K\sb{\rm max}, increasing R-ratio, (CKIR), testing procedure and crack closure measurements at 20 and 500\sp\circC indicated that the crossover behavior was caused by plasticity-induced crack closure. The CKIR testing procedure allowed the generation of thresholds at high temperatures at very high R-ratios and without excessive plasticity. The effect of temperature on the fatigue crack growth at intermediate levels of $\Delta$K was investigated at an R-ratio of 0.1 between 20\sp\circC and 800\sp\circC. Crack growth rates were correlated by an Arrhenius relationship involving an activation energy of 47.7 kJ/mole. At 475\sp\circC, growth rates were on the same order as growth rates at 600\sp\circC. This behavior was attributed to 475\sp\circC embrittlement, a well documented phenomenon in ferritic stainless steels with Cr content higher than 15%. This embrittlement was found to affect significantly the room temperature fatigue properties. The effect of frequency from 0.01 Hz to 50 Hz on growth rates was investigated at a constant-R ratio of 0.1 and a constant K\sb{\rm max} of 16 MPa$\surd$m and 24 MPa$\surd$m at 500\sp\circC, and 655\sp\circC. Two crack growth rate regimes were observed: a cycle dependent regime at high frequency where growth rates were independent of frequency, and a mixed regime where creep and environment affected the cyclic damage at the crack tip. In the mixed regime, crack growth rates increased with decreasing frequency from a transition frequency that depended on K\sb{\rm max} and temperature.Ph.D.Materials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105570/1/9135644.pdfDescription of 9135644.pdf : Restricted to UM users only

Deformation behavior of [gamma]-TiAl composites in the solid and mushy states

The deformation behavior of a two phase ([gamma] + [alpha]2) lamellar TiAl alloy reinforced with various amounts of AlTi2 carbide platelets has been examined by determining Vickers hardness at room temperature and in the temperature range 900-1300 [deg]C. The compression flow behavior of these materials has also been examined in the semi-solid state. The hardness values for all materials dropped from about 4 GPa at room temperature to 120 MPa at 1300 [deg]C. The composites exhibited limited increase in hardness with respect to the base material over most of the temperature range investigated. At 900 [deg]C, however, the 13% AlTi2C reinforced composite showed an increase in hardness of about 1.2 GPa over that of the base material. The composites exhibited interfacial as well as transgranular cracks induced by indentations, while the matrix material did not exhibit cracking during the hardness tests. Forgeability of semi-solid Ti-48 Al reinforced with 13% AlTi2C composite was studied by performing compression tests at 1470 [deg]C and strain rates ranging from 0.33 to 2.00 s-1. Stress-strain curves exhibited a peak stress which is associated with breakdown of the carbide platelets. The size of the carbide platelets decreased and their reorientation took place with increasing strain. At strains higher than 0.9, reagglomeration of the carbide platelets was observed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31435/1/0000353.pd

Predicting Students’ Intentions to Create Green Start-ups: A Theory of Planned Behaviour Approach

Start-ups play a fundamental role in the economy and have contributed enormously and effectively to solving economic and social issues. However, despite the significant environmental hazards the world is experiencing, the environmental dimension of its contribution is still significantly low, and the transition to green start-ups is still in its infancy especially in developing countries. The aim of the paper is to identify the factors that influence students’ intentions to create green start-ups. Based on a convenience sample of 248 university students, a multiple regression analysis was performed to test our extended model. The empirical findings show that attitudes, subjective norms, perceived behavioural control (PBC), and environmental awareness positively influence the intention to create green start-ups. In addition, the findings demonstrate that students have a low level of environmental awareness. Based on the results, suggestions are provided on how to motivate university graduates to create green start-ups

Predicting intentions to continue using travel apps in the post COVID-19 crisis

In the era of information technology and the Internet, travel apps have become a significant element influencing modern travel behavior. However, research in this context remains limited in the tourism literature. This paper aimed to investigate the factors that influence intentions to continue using travel apps in the post-COVID-19 crisis in Algeria. A convenience sample of 380 Algerian tourists was surveyed, and a total of 244 questionnaires were collected over three months. The method used to test the proposed hypotheses empirically was the multiple regression analysis using SPSS 26. Results showed that attitude, perceived behavior control (PBC), degree of satisfaction, and trust had significant positive effects on tourists\u27 intentions to continue using travel apps in the Post COVID-19 pandemic. This paper presents a theoretical contribution and affords practical recommendations relevant to academics and hospitality practitioners

Predicting intentions to continue using travel apps in the post COVID-19 crisis

In the era of information technology and the Internet, travel apps have become a significant element influencing modern travel behavior. However, research in this context remains limited in the tourism literature. This paper aimed to investigate the factors that influence intentions to continue using travel apps in the post-COVID-19 crisis in Algeria. A convenience sample of 380 Algerian tourists was surveyed, and a total of 244 questionnaires were collected over three months. The method used to test the proposed hypotheses empirically was the multiple regression analysis using SPSS 26. Results showed that attitude, perceived behavior control (PBC), degree of satisfaction, and trust had significant positive effects on tourists\u27 intentions to continue using travel apps in the Post COVID-19 pandemic. This paper presents a theoretical contribution and affords practical recommendations relevant to academics and hospitality practitioners

Role of sleep endoscopy in obstructive sleep apnea syndrome

Aim: There are limited data about the role of sleep endoscopy in obstructive sleep apnea syndrome (OSAS). The aim of this study was to evaluate the level, degree and shape of obstruction of the upper airway in patients with OSAS by sleep endoscopy and their relation to OSAS severity. Patients and methods: Fifty consecutive patients with OSAS were prospectively enrolled in this cross sectional analytic study. All patients underwent history, a full night-attended polysomnography and sleep endoscopy. The degree of pharyngeal narrowing (grades I–IV) was evaluated at retropalatal, retroglossal and hypopharyngeal levels. Shape of pharyngeal collapse was classified into circular, lateral or antero-posterior at retropalatal and retroglossal levels. Shape of the epiglottis was also observed. Results: All patients showed multisegmental levels of obstruction. Moderate OSAS had a higher percentage of grade II obstruction but a lower percentage of grade I at hypopharyngeal level compared to mild OSAS (P < 0.05). Also, in moderate OSAS, tongue base obstruction was 47.4% which was significantly higher comparing to mild OSAS (16.7%) (P < 0.05). There was no significant difference between different grades of obstruction at all anatomical levels in polysomnographic parameters. Omega shaped epiglottis was associated with the highest apnea hypopnea index, desaturation index, lowest average and minimum O2 level. Conclusion: Sleep endoscopy is a useful tool for the assessment of level, degree and shape of the upper airway obstruction during sleep in OSAS and this could be helpful in preoperative evaluation. Presence of obstruction at hypopharyngeal level or tongue base obstruction is an indicator of OSAS severity