Refined Spherulites of PP Induced by Supercritical N2 and Graphite Nanosheet and Foaming Performance.

The isothermal crystallization properties of polypropylene/graphite nanosheet (PP/GN) nanocomposites under supercritical N2 were systematically studied by a self-made in situ high-pressure microscope system. The results showed that the GN caused irregular lamellar crystals to form within the spherulites due to its effect on heterogeneous nucleation. It was found that the grain growth rate exhibits a decreasing and then increasing trend with the enhancement of N2 pressure. Using the secondary nucleation model, the secondary nucleation rate for spherulites of PP/GN nanocomposites was investigated from an energy perspective. The increase in free energy introduced by the desorbed N2 is the essential reason for the increase in the secondary nucleation rate. The results from the secondary nucleation model were consistent with those acquired through isothermal crystallization experiments, suggesting that the model can accurately predict the grain growth rate of PP/GN nanocomposites under supercritical N2 conditions. Furthermore, these nanocomposites demonstrated good foam behavior under supercritical N2.

Constitutive model of 6063 aluminum alloy under the ultrasonic vibration upsetting based on Johnson-Cook model.

Establishing an accurate constitutive relation in ultrasonic vibration assisted metal forming, can provides a reliable theoretical basis for analyzing the mechanism of the ultrasonic vibration on materials. A constitutive model of 6063 aluminum alloy under the ultrasonic vibration upsetting at room temperature was constructed based on Johnson-Cook Model and experimental results. The influence of amplitude and frequency on the yield strength, hardening coefficient and exponent were analyzed quantitatively. Results showed that the yield strength was reduced due to the softening effect induced by the ultrasonic vibration. The maximum decreasing amount was 68.8% when imposing the maximum ultrasonic energy in this study. The hardening coefficient and exponent increased by 10.9% and 16.6% in maximum, respectively. However, the ultrasonic vibration has little impact on the strain rate hardening. The modified Johnson-Cook constitutive model under the ultrasonic vibration upsetting was established and was in good agreement with the experimental results.

A nanocrystalline-amorphous mixed layer obtained by ultrasonic shot peening on pure titanium at room temperature.

A nanocrystalline-amorphous (NC-A) mixed layer was obtained by ultrasonic shot peening (USP) on pure titanium at room temperature and observed by X-ray diffraction pattern (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). The results showed that the amorphization percentage in the NC-A mixed layer increased continuously with the increase of the peening duration, shot diameter and sonotrode amplitude or the decrease of the peening distance. The maximum amorphization percentage achieved in this study was 44.09%. Moreover, with the amorphization percentage in the NC-A mixed layer increasing, the surface hardness increased constantly. Base on the experimental results, the amorphization mechanism during USP treatment was also analyzed.

Effects of ultrasonic vibration on the compression of pure titanium.

The technology of ultrasonic vibration assisted plastic forming possesses a great many merits, such as reducing the deformation resistance and friction, as well as improving the surface quality of parts. In this study, the ultrasonic vibration assisted compression tests were carried out on pure titanium in order to improve its formability. The results indicating that the ultrasonic vibration had no effort on elastic deformation, and the temperature of material only increased by 6 °C after compression with applying the ultrasonic vibration. Therefore the influence of temperature increase on reduction of flow stress could be ignored. After excluding interface friction and temperature effects, ultrasonic vibration can still decline the flow stress, the mechanism of deformation includes ultrasonic softening, stress superposition and strain hardening. In the intermittent vibration tests, the material shows the residual softening effect after stopping vibration. By observing the microstructure of material with SEM, it shows that the ultrasonic vibration can promote the generation of deformation twins, causing the grain refinement and the reduction of the twins, which is the major factor of affecting the residual softening effect.

The effects of forming parameters on conical ring rolling process.

The plastic penetration condition and biting-in condition of a radial conical ring rolling process with a closed die structure on the top and bottom of driven roll, simplified as RCRRCDS, were established. The reasonable value range of mandrel feed rate in rolling process was deduced. A coupled thermomechanical 3D FE model of RCRRCDS process was established. The changing laws of equivalent plastic strain (PEEQ) and temperature distributions with rolling time were investigated. The effects of ring's outer radius growth rate and rolls sizes on the uniformities of PEEQ and temperature distributions, average rolling force, and average rolling moment were studied. The results indicate that the PEEQ at the inner layer and outer layer of rolled ring are larger than that at the middle layer of ring; the temperatures at the "obtuse angle zone" of ring's cross-section are higher than those at "acute angle zone"; the temperature at the central part of ring is higher than that at the middle part of ring's outer surfaces. As the ring's outer radius growth rate increases at its reasonable value ranges, the uniformities of PEEQ and temperature distributions increase. Finally, the optimal values of the ring's outer radius growth rate and rolls sizes were obtained.

Biomechanical supporting effect of tantalum rods for the femoral head with various sized lesions: a finite-element analysis.

BACKGROUND: Features of necrotic lesions and various interventions could affect the biomechanics of the femoral head. A three-dimensional finite-element analysis was designed to demonstrate necrotic femoral head stress changes with various sizes of necrotic lesions, and evaluate the effect of tantalum rods on preventing femoral head cracking. METHODS: Femoral computed tomography scans were used to build a normal three-dimensional finite-element femoral head model in a computer. Based on the normal model, necrotic models of different lesion diameters (15 mm, 20 mm and 30 mm) were created, as were the repaired models with tantalum rods for each diameter. After a series of meshing and force loading, the von Mises stress distributions, simulating single-legged stance, and stresses on specific points under loaded conditions were determined for each model. RESULTS: Deep exploration into the burdened area of the femoral head indicated that higher stresses to the femoral head were observed with a larger necrotic lesion; the largest stress concentration, 91.3 MPa, was found on the femoral head with a lesion diameter of 30 mm. By contrast, topical stress on the surface of the necrotic regions was lowered following implantation of a tantalum rod, and the changes in stress were significant in models with lesions of 15 mm and 30 mm in diameter, with the best biomechanical benefit from the tantalum rod found with a lesion diameter of 15 mm. CONCLUSIONS: Femoral heads with larger necrotic lesions usually have a higher stress concentration and a higher risk of collapse. Various sized lesions on the femoral head can benefit from the mechanical support offered by the implantation of a tantalum rod; however, femoral heads with smaller sized lesions may benefit more. A thorough evaluation of the lesion size should be conducted prior to the use of tantalum rod implants in the treatment of femoral head necrosis.

[Three-dimension finite element analyses of interior stress of two kinds of Replace implant].

OBJECTIVE: To establish three-dimension finite element model of mandible with two kinds of Replace implant, and to study stress of implant and abutment. METHODS: The data of components of the dental implant was measured, cross section of the mandible was scanned by spiral CT and image reconstruction technique was conducted. Three-dimension finite element analysis software UG and MSC. Marc/Mentat were used to built the conjunction model and bone model of two implant systems. Axial loading (200N) and 30 degrees oblique loading (100N) were applied on the models respectively, the stress distribution patterns of the implant and abutment of two implant systems were analyzed. RESULTS: The peak stress district was concentrated on the structure of the implant cervix, which was more obviously displayed on the Replace Select implant. The peak stress of oblique loading was higher than that of axial loading. The peak stress on the implant cervix of Replace Select implant was higher than that of Replace External Hex implant in all loadings. CONCLUSION: To Replace Select especially, oblique force should be avoided on clinical practice in case of the implant fracture.

[Three-dimensional finite element analyses of bone surface stress of two kinds of conjunction implant].

OBJECTIVE: To establish a three-dimension finite element model of mandible with two kinds of dental implant and to study the stress of implant-bone interface. METHODS: Measuring the data of the components of the dental implant and using spiral CT image reconstruction technique to scan the cross section of the mandible. Three-dimension finite element analysis software Unigraphics and MSC. Marc/Mentat were used to build the conjunction model and bone model of two implant systems. Loading 200 N axially and 100 N 30 degrees obliquely on the models respectively, the stress distribution patterns of the bone interface of two implant systems were analyzed. RESULTS: The stress distribution on the bone interface of two implant systems was similar. The peak stress of oblique loading was higher than that of axial loading. The peak stress district of the bone was concentrated on the stricture of the implant cervix, which was more obviously displayed on the Replace Select implant. The peak stresses on the bone interface of Replace Select implant were higher than that of Replace implant in all loadings. CONCLUSION: To Replace Select especially, oblique force should be avoided in clinical practice in case of the bone absorption.