Qing Yuan Wang

Effect of pitting corrosion on very high cycle fatigue behavior

Abstract In this study, the effect of pre-existing corrosion pits on the fatigue behavior of 7075/T6 aluminium alloy in very long life range and in the near threshold regime was investigated by using piezoelectric accelerated fatigue test. The results indicate that the presence of pre-existing corrosion pits, produced by 1-day, 4-day, and 7-day immersion in salt water significantly reduces the fatigue life of the aluminum alloy by a factor of 10–100.

Fatigue properties of inconel 718 in long life region at elevated temperature

The fatigue properties of Inconel 718 in long life region up to 10 8 cycles were investigated under rotating bending at room temperature and 500°C. The early propagation of a crack was suppressed by oxide induced crack closure. Consequently, fatigue strength was higher at 500°C than at room temperature. Surface fracture occurred at high st ress levels and internal fracture occurred in long life region beyond 10 7 cycles at 500°C. Correspondingly, S-N curve at 500°C presented in a step-wise shape, compared to the conventional one-step sha pe at room temperature. Intergranular cracking was observed at the origins of internal fracture at 500°C. Introduction Nickel-base superalloy has been widely used in the severe conditions such as components of jet engines, gas turbine engines and so on, because this alloy has excellent proper ties in st ength at high temperature and high resistance to corrosive environment. Therefore, ma ny studies have been carried out on the mechanical properties, especially on the creep and fatigu e properties [1-4]. On the other hand, the fatigue properties of high strength steels and surfa ce modified steels in the long life region beyond 10 7 cycles, are of much interest to machine designers and mater ial engineers. For example, origins of fatigue fracture in high strength steels ch ange from the surface slips or defects in short life region to the interior in long life one, correspondingly S-N curve shows a step-wise shape [5-9]. Moreover, the presence of fatigue limit is not clarifi ed. Therefore, the accumulation of experimental data and the establishment of criteria for design and maintenance of machines and structures are urgent subjects for materials employed under the c ircumstances where high strength steels and surface modified steels find increasing applications. However, the data on the fatigue properties in these materials in long life region, especially t elevated temperature, were very limited [10]. In the present study, the fatigue properties of Inconel 718 in long life region up t 10 cycles were investigated under rotating bending at room temperature and 500°C. Key Engineering Materials Online: 2003-07-15 ISSN: 1662-9795, Vols. 243-244, pp 321-326 doi:10.4028/www.scientific.net/KEM.243-244.321

Super long life fatigue in nitrided high strength steels

In order to investigate the effect of radical nitriding on fatigue behavior of high-strength steels, very long life fatigue tests were carried out for two kinds of steels, 18Ni Maraging steel and SUJ2 bearing steel, by using conventional and ultrasonic machines, respectively. It was shown that radical nitriding played the principal role in the subsurface crack initiation. Fatigue strength of Maraging steel appears to be significantly enhanced by radical nitriding, however, radical nitriding has a destructive effect on the fatigue behavior of SUJ2 steel. Introduction Interest in the nitriding of metal surfaces has been growing rapidly. Several investigations have been carried out to determine the influence of nitriding treatments on fatigue properties [1-5]. In general, improvements of fatigue resistance were found for the stress-controlled high-cycle regime [2-3]. Beneficial effect of ion nitriding on fatigue was attributed to hardness increase of the surface and increase surface residual stress. The residual stress lowers the magnitude of the applied tensile stresses and hence increases the fatigue life of the component. Han [4] studied the effect of ion implantation on the fatigue life of a Ti-24V alloy and observed the improvement in fatigue life in both low cycle and high cycle fatigue. Fatigue crack initiation is usually understood to occur on the specimen surface owing to the irreversible process of extrusions and intrusions through slip deformation. Many tested materials, however, clearly exhibited two kinds of fatigue initiation [6-11]. One was at the specimen surface, and the other was in specimen interior. Subsurface crack initiation was dominant in the long-life range, while the surface fatigue initiation occurred in high-peak stress tests and short-life tests. In case hardened materials the crack initiation usually tends to shift from surface to sub-surface in high cycle fatigue [1-4]. In this study, 18Ni Maraging steel and SUJ2 bearing steel were radical nitrided. Super long life fatigue properties of the steels developed after nitriding was investigated by using a rotating bending fatigue machine, an ultrasonic machine, and scanning electron microscopy (SEM). Key Engineering Materials Online: 2004-10-15 ISSN: 1662-9795, Vols. 274-276, pp 217-222 doi:10.4028/www.scientific.net/KEM.274-276.217

Experimental Study of the High Temperature Mechanical Behaviors of TLP Bonding of Two Ni-Base Single Crystals

In this paper, the mechanical behavior of Ni-base single crystals joint created by TLP bonding is presented. Experimental study has been performed on the TLP single crystal under loading conditions of static uniaxial tension, creep and Three-points-bending (T-P-B) at temperature of 850 °C. Specimens made of the base material, Ni-base single crystal specimens without bonding, were also tested under the same loading conditions. The mechanical behaviors of TLP single crystal is compared to the behaviors of base material. It was found that the static strength of the TLP specimens with the boundary normal to the tensile direction was 63% of the strength of the base material. The creep strength and the fracture ductile strength was more than 57% and 55% of that of the single crystal base material, respectively. The macro and micro graphs of fracture surfaces of specimens indicated that the fracture modes of the samples were brittle fracture.

Influence of loading frequency on fatigue behavior of high strength steel

In order to investigate the influence of loading frequency on the fatigue behaviors of the high strength steel, ultrasonic fatigue tests were carried out for a high-carbon-chromium steel and the results were compared with those of fatigue tests using conventional rotary bending fatigue test machine with a frequency of 52.5Hz. The different of fatigue strength at ultrasonic frequency level and conventional frequency level is very small and the S-N curve obtained from 20 kHz or 52.5 Hz shows the step-wise shape. The fatigue crack occurred from inclusions on the subsurface site in the long life regime and the typical surface fracture occurred in the short life one though the loading frequency level is different. It is indicated that ultrasonic fatigue method is an effective method to investigate the fatigue properties in super-long life region.

Super-Long Life Fatigue Behavior of Structural Aluminum Alloys

The objective of this study is to determine very long life fatigue and near threshold fatigue crack growth behaviors of 7075/T6 and 6061/T6 Al-alloys using piezoelectric accelerated fatigue at 19.5KHz. The experimental results show the fatigue failure can occur beyond 107, even 109 cycles, and endurance limits could not be obtained in the Al-alloys until 109 cycles. Fatigue voids are noticed on fatigue fracture in both alloys. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviors have been examined. Fatigue crack growth rates of small cracks in the Al-alloys are found to be greater than those of large cracks at the same stress intensity factor range.

Very High Cycle Fatigue Crack Initiation Mechanism in Nugget Zone of AA 7075 Friction Stir Welded Joint

Very high cycle fatigue behavior of nugget zone in AA 7075 friction stir welded joint was experimentally investigated using ultrasonic fatigue testing system (20 kHz) to clarify the crack initiation mechanism. It was found that the fatigue strength of nugget zone decreased continuously even beyond 107 cycles with no traditional fatigue limits. Fatigue cracks initiated from the welding defects located at the bottom side of the friction stir weld. Moreover, a special semicircular zone could be characterized around the crack initiation site, of which the stress intensity factor approximately equaled the threshold of fatigue crack propagation rate. Finally, a simplified model was proposed to estimate the fatigue life by correlating the welding defect size and applied stress. The predicted results are in good agreement with the experimental results.

High-Cycle Fatigue and Thermal Dissipation Investigations for Low Carbon Steel Q345

Carbon steel is a kind of metallic material that widely used in construction, machinery, manufacturing and other domains. In the mechanical structure system, long-term cyclic stress may cause the mechanical components failure. In this work, the characteristic of fatigue crack propagate in low carbon steel Q345 and the effect of loading frequency to the fatigue property of Q345 steel were investigated. Meanwhile, the dispersion of high-cycle fatigue of life of the Q345 steel under high fatigue testing frequency was analyzed, and the P-S-N curve with the test data was given out. With the help of infrared camera, temperature rise curve during fatigue test was analyzed to study the thermal dissipation of Q345 steel.

Effect of Temperature and Loading Frequency on the Fatigue Behavior of Ti-17

A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of Ti-17. Ultrasonic (20 kHz) fatigue tests were performed at room temperature, 200°C and 350°C, respectively. The dynamic Young’s modulus and fatigue endurance limit decrease with increasing temperature linearly. Rotating bending (50 Hz) tests were performed to evaluate the influence of loading frequency at room temperature, 200°C and 350°C, respectively. There is an obviously loading frequency effect at elevated temperature, although no loading frequency effect at room temperature.

Effect of Elevated Temperature on the Mechanical Behavior of Natural Aggregate Concrete

Reinforced concrete construction is very common these days and extensively used both in industrial and commercial buildings. With the gradual rise in occurrences of fire accidents in recent years, a more thorough and quantitative understanding of the damage phenomenon in natural aggregate concrete structures is required. However, little research has been done to study natural aggregate concrete behavior under high temperatures. The mechanical behavior of concrete could actually be more complex under high temperature conditions than at room temperature, for instance. Restoration and reinforcement of the structures exposed to fire may have to be based on residual strength analysis and therefore require a correlation between temperature and mechanical properties. Thus, in order to meet the modern challenges of rapid engineering advances and societal development, further research on the concrete material and its structural behavior at high temperatures becomes extremely important. The present paper deals with investigations on the effect of high temperature exposure on the compressive strength of natural aggregate concrete. Experiments were conducted to study the compressive strength variations with increasing temperatures, up to 700 °C, and the subsequent cooling modes such as natural and spray cooling. Results show that the compressive strength gradually decreases with increasing temperatures.