Papers by Keyword: Microvoids

Abstract: Two fractured pins of impact wrench were evaluated by fractography using scanning electron microscopy. This pneumatic wrench is used to the fabrication of automotive components being characterized by its power and tightening capacity. The important part of the wrench is the pin clutch impact mechanism used to generate high torque. An original pin manufactured with AISI E52100 steel adjusted 2,580 nuts before fracture and a substitute pin of E52100 modified steel with S adjusted 7,095 nuts before failure. Fractographic analysis in both fracture surfaces indicated a ductile-brittle mixed fracture mode. Original pin surface exhibited a granular appearance while substitute pin surface showed fibrous and rough morphology. Precipitation of nearly rounded, elliptical and elongate second – phase particles containing Cr, C and Mn measured as intergranular and transgranular particle density was observed, promoting some brittle failure zones and ductile fracture measured as the volume fraction of grain boundaries and microvoids, respectively. A larger amount of intergranular medium precipitates was found on the original pin which favored the brittle failure among the grain boundaries compared to that of the substitute pin. The ductile fracture by larger transgranular fine particle density which acted as nucleation sites of higher volume fraction of microvoids was found in the substitute pin.

Abstract: An experimental method was developed in this work to study the PET fibers by synchrotron small-angle X-ray scattering (SAXS). Compared with the traditional measurement in air, the new method measured in the glycerin is helpful to eliminate edge scattering arising from the fiber edge. The results showed that intensity from edge scattering is so high as to cover the microstructure information. PET fiber with a diameter of 25 μm was measured by the new experimental method, and a lobed shape can be seen clearly on the meridian which didn't emerge in traditional measurement. The SAXS data were evaluated to extract parameters of lamellar structure and microvoids. In addition, the effect of fiber diameter on edge scattering was also investigated.

Abstract: The introduction of advanced high strength steels, e.g., into the automotive industry initiated a huge interest in analyzing and understanding ductile fracture of sheet metals to greater details. This demands for the development of experimental methodologies that provide microvoid evolution parameters, which also serve as crucial input parameters for advanced forming simulation that can predict damage evolution. Therefore, this work scrutinizes the reliability and applicability of an increasingly popular damage characterization methodology, in which microindentation tests are carried out to measure hardness and elastic modulus degradation as a function of accumulated strain, relating this degradation to damage evolution. To accomplish this goal, this methodology is applied to several different sheet metals of different formability (an interstitial-free steel, a dual phase steel, an aluminum-magnesium-silicon alloy and a ferritic stainless steel). To analyze and verify the results of indentation based methodology, damage evolution in these metals is monitored also via different experimental techniques, i.e. scanning electron microscopy, micro-ct tomography and sensitive density measurement. Moreover, finite element simulations are carried out to understand the effect of void accumulation in the degradation of hardness and elastic modulus. In the case of using the hardness as a damage probe, the degradation due to damage is always coupled to other effects (strain hardening, grain shape change, texture development) causing an increase in the obtained hardness value for all of the sheet metals tested, thereby complete obscuring any degradation of the hardness due to damage. In the case of elastic modulus, all the sheet metals tend to pile-up upon indentation when they are severely deformed, leading to large systematic errors in the Oliver-Pharr methodology based modulus determination, whereas the elastic modulus is also intrinsically altered by the grain shape change and texture development seen for increasing deformation. Therefore, it can only be concluded that, contrary to the published results in the literature, neither the hardness degradation nor the elastic modulus degradation can be used as a precise probe for damage accumulation, at least when the indentation based methodology is carried out in the originally-proposed manner that is commonly used in the literature.

Abstract: Various hold periods in a cyclic wave of fatigue load were introduced to investigate loading frequency effects on crack growth behavior and microstructural damage. The crack growth path and microstructural damage characteristics at 600°C in tempered martensitic 9Cr-2W (P92) HAZ of welded steel were studied. Generally, low frequency effect with increasing hold periods affects microstructural damage with microvoids/cavities nucleation due to the effect of creep. Results showed that the fatigue crack growth behavior was sensitive to the loading frequency. As frequency decreased, the fatigue crack growth rate increased and the crack path mode changed from transgranular to intergranular in terms of microstructural damage. As the loading frequency decreased, it was found that the microvoids /cavities and microcracks that formed along the prior austenite grain boundaries ahead of the main crack contributed to the intergranular crack growth.

Abstract: This work investigates the relationship between the creep-fatigue life and ultrasonic test of creep-fatigue damage. Under the creep-fatigue interaction, the main cause of life reduction is the initiation and growth of microvoid with increasing hold time. The number/size of microvoid/cavity, the fraction of cavity area varied with the hold time. Therefore, the life evaluation using the microvoid with the variation of hold time is very informative for safety of components in power plants. In this study, using the heat resisting alloy, P122 steel for USC (ultra super critical) power plant, the creep-fatigue tests with various hold times and their ultrasonic inspection were carried out for the purpose of evaluation for creep-fatigue life. The results obtained by Rayleigh surface wave of backscattered ultrasound were compared and analyzed with the experimental parameters. The good agreement between the experimental life and the predicted life was obtained.

Abstract: The application of nondestructive evaluation to creep-fatigue damage was examined in this paper. Generally, as the hold time of static load increases, the degradation of material becomes more rapid and the creep-fatigue life decreases. Therefore, in the evaluation of creep-fatigue strength and life of high-pressure vessel such as main steam pipe at high temperature is very important in power plants. In this study, the creep-fatigue behavior of P92 steel was evaluated nondestructively by the backscattered ultrasound using the creep-fatigue specimens. The results obtained by Rayleigh surface wave of backscattered ultrasound were compared and analyzed with the experimental parameters. Also, the relation between the SDA (slope of degraded area) and creep-fatigue life was examined. From the result of nondestructive test, we suggest that SDA would be used as the new parameter for the evaluation of creep-fatigue damage. As the degradation increased, the SDA decreased and also the creep-fatigue life decreased.

Abstract: In this study, the creep-fatigue crack growth behavior was investigated at 600°C under the maximum load with various dwell times. Test material was P92 steel (9%Cr-2%W) weldment. The creep-fatigue crack growth behavior of the HAZ(heat affected zone) and base metal was compared. The relationship between the crack growth behavior and dwell time was studied and it was characterized using ΔK and (Ct)avg parameters. The area fraction of micro-voids/cavities at the crack tip and fracture modes were examined and the relationship between crack growth rate and dwell time was investigated. The cycle dependant crack growth rate increased with the increase in dwell time. From the results of creep-fatigue test, the crack growth rate of the HAZ was found to be faster than that of base metal. As the dwell period increased, the intergranular fracture was observed along the crack growth path with the effect of creep during the dwell period.

Abstract: In the present work, positron annihilation spectroscopy (PAS) is employed for microstructure investigations of various ultra fine grained (UFG) metals (Cu, Ni, Fe) prepared by severe plastic deformation (SPD), namely high-pressure torsion (HPT) and equal channel angular pressing (ECAP). Generally, UFG metals prepared using both the techniques exhibit two kinds of defects introduced by SPD: dislocations and small microvoids. The size of the microvoids is determined from the PAS data. Significantly larger microvoids are found in HPT deformed Fe and Ni compared to HPT deformed Cu. The microstructure of UFG Cu prepared by HPT and ECAP is compared and the spatial distribution of defects in UFG Cu samples is characterized. In addition, the microstructure of a pure UFG Cu prepared by HPT and HPT deformed Cu+Al2O3 nanocomposite (GlidCop) is compared.

Abstract: In this study, the crack growth behavior in P92 steel (9%Cr-2%W) weldment was investigated at 600ı under the load of trapezoidal wave shape with various holding times. The relationship between the crack growth behavior and holding time was studied and it was characterized using the ΔK and (Ct)avg parameters. The number of micro-voids/cavities at the crack tip and fracture modes were examined and the relationship between crack growth rate and holding time was investigated.

Abstract: Tensile properties of SA508 Cl.3 reactor pressure vessel (RPV) steel were investigated at room temperature and at 288 °C before and after hydrogen charging by electrolysis. At room temperature, the charged hydrogen induced distinct hardening and ductility loss, where quasi-cleavage features were observed around inclusions. These results may be due to interactions between the dissolved hydrogen and dislocations and an increase of hydrogen concentration near the inclusions. On the other hand, at 288 °C, the charged hydrogen induced some softening, which was explained in terms of the hydrogen shielding effect, and of strain localization by dynamic strain aging (DSA). Further, at 288 °C, the fracture surfaces of the hydrogen-charged specimens showed brittle regions, where the hydrogen might have been trapped in microvoids, leading to internal pressurization.