Qiao Shengru

The internal friction of unidirectional CC composites fabricated in a magnetic field

Previous studies have shown that lamellae of mesophase pitch prefer to be oriented parallel to a magnetic field. In the present work the matrix mesophase layers of the C/C composites were oriented perpendicular to the carbon fibers by a 8000 G magnetic field. This study is concerned with the internal friction of unidirectional C/C composites produced by the method of the present research. It is found that the internal friction of unidirectional C/C composites fabricated with mesophase pitch precursor in a 8000 G magnetic field is larger than that of the composites in the absence of a magnetic field; the higher the degree of graphitization of the composites, the larger the internal friction of the composites; and the higher the density of the composites, the larger the internal friction of the composites. The internal friction of the C/C composites decrease with the oscillation frequency from 0.01 to 1 Hz, and then slightly increases up to 5 Hz.

Damage Healing of Aluminum Alloys by D. C. Electropulsing and Evaluation by Resistance

Abstract The damage and cold work hardening of 4043 and 2024 T4 aluminum alloy were introduced by tensile test under the applied stress between the yield strength and ultimate tensile strength. Fatigue damage of 2024 T4 aluminum alloy was introduced by cycling to half of fatigue life (85 000 cycles) at 225 MPa. Then the damaged specimens were healed by D. C. electropulsing. The electric resistances of as-received, damaged and healed specimens were measured by electric bridge, respectively. The results show that the elongation increases, and yield strength and ultimate strength decrease with the increase of healing time for the 4043 aluminum alloy tensile damaged specimen. The mechanical properties of the 4043 aluminum alloy damaged specimens treated by a 0.5 s electropulsing are close to the values of undamaged specimens. Fatigue life is considerably prolonged by a 0.8 s electropulsing treatment for the fatigue damaged specimen of 2024 T4 aluminum alloy. The electric resistance of aluminum alloys increases obviously after fatigue or tension damage, and decreases after electropulsing treatment, but it is still larger than that of the as-received specimen. The treatment time has little influence on the final resistances, but the mechanical properties are quite different for the samples treated by various electropulsing time. Although the resistance can describe the damage of aluminum alloys, it is not very sensitive to the mechanical properties, so it can not be considered as a good criterion for healing evaluation. The observation of microstructure reveals that the damages can be partly healed after D. C. electropulsing, and recrystallizaton phenomenon appears.

Fatigue Properties of Ti17 Alloy Strengthened by Combination of Electric Spark Treatment with Ultrasonic Surface Treatment

Abstract The surfaces of Ti17 alloy were strengthened by an electric spark treatment (EST), then an ultrasonic surface treatment was used in order to improve the fatigue life. The surface morphology was observed by SEM. The surface residual stress was analyzed by a X-ray diffractometer. Tension-tension fatigue tests were carried out in a HT-100 fatigue testing machine at 350 °C. The results show that the surface roughness and the residual stress are reduced for the specimens after the ultrasonic treatment. The fatigue life of the specimens strengthened by ultrasonic treatments is about twice that of the specimens treated by EST only. The fatigue life of the specimen strengthened by YG8 electrode EST combination with ultrasonic treatment is the longest.

Fatigue Life of 6061 Aluminium Alloy Welding Samples both in Atmosphere and Water

Abstract 6061-O and 6061-T4 aluminum alloy sheets with 0.04 inch (1.02 mm) nominal thickness are widely used in the aviation industry. After argon arc welding, the sheets were subjected to a solution heat treatment followed by a quenching and tempering treatment (T62 treatment), afterwards fabricated to specimens. The fatigue tests were carried out at 60 Hz, stress ratio of 0.1 both in atmosphere and water at room temperature, and loading direction was perpendicular to welding bead. The fracture morphologies were examined by scanning electron microscopy (SEM). The results show that the fatigue life in water is considerably decreased, and fatigue striation is not so clear compared with that in atmosphere, which might be attributed to the fact that the oxygen content on the fracture surface tested in water after dry treatment is much higher than that in atmosphere.