Effect of Heat Treatment on the Cyclic Deforming Behavior of As-Extruded ZA81M Magnesium Alloy

In the present work, the effect of heat treatment on the cyclic deformation behavior of as-extruded ZA81M magnesium alloy was investigated. Two heat treatment conditions were applied to the as-extruded ZA81M alloy: a solution treatment (T4, 653 K for 40 h and quenched with 298 K water) and a solution treatment plus artificial aging (T6, 348 K for 32 h (pre-aging at low temperature) and 453 K for 8 h (the second aging) and quenched with 353 K water). The results showed that the fine second phase precipitated after the aging treatment, the tensile yield strength of the T6-treated specimens increased, and the stress amplitude of T6-treated specimens was always higher than that of T4-treated specimens. The T6-treated specimens had a higher total strain energy density and a shorter fatigue life at a strain amplitude of 0.4%, and a lower total strain energy density and a longer fatigue life at a strain amplitude of 0.8%, compared to the T4-treated specimens. All fatigue cracks of the T4 and T6 ZA81M alloy were initiated at the second phase or along the grain boundary and propagated perpendicular to the loading direction.

Mechanical Properties and Energy Dissipation of Sandstone under Cyclic Loading-Unloading

For the bearing rock in geotechnical engineering, it is frequently affected by external loads. This paper adopted different upper limits of stress to carry out cyclic loading-unloading on the sandstone specimens to make them in different damage degrees and analyzed the mechanical mechanism of the damaged sandstone under different stresses. Then, the strength change and energy evolution of sandstone with different damage degrees were analyzed, and the damage of the loaded sandstone was quantitatively characterized. The experimental results showed that the strength and plastic deformation of sandstone after cyclic loading-unloading with different upper-stress limits gradually decreased with the increase of the upper-stress limit. In the loading-unloading stages of cyclic loading-unloading, the elastic modulus increased with the increase of the upper-stress limit. In general, as the number of cycles increased, the total strain energy density and elastic deformation energy density gradually increased, and as the upper-stress limit increased, both of them also increased. The damage factor of sandstone after cyclic loading-unloading, which was characterized by dissipated energy, increased in an S-shape with the increase of the upper-stress limit, and the growth rate first increased and then decreased.

POSSIBILITIES FOR OPTIMAL DESIGN OF A CAR TIRE BASED ON A CRITERION OF SPATIAL STRENGTH BALANCE

A procedure is proposed for calculating the internal profile and optimal distribution of materials for a pneumatic car tire in a mold configuration. To adequately describe the elastic-dissipative properties of tire rubbers and rubber-cord composites, nonlinear elastic deformation Mooney–Rivlin model and viscoelastic Prony model as well as experimental data of static and dynamic tests are considered. An algorithm for finite element analysis of the stress-strain state of a passenger car tire is described in the MSC.Marc software package, and results of numerical solution of applied problems are presented on the tire landing on the rim and its loading with working pressure, as well as on the contact interaction of a passenger car tire with the road surface at maximum operating load at rest and during stationary rolling at a speed of 90 km/h. It was found that the contact loading of the tire when interacting with the road surface does not lead to a significant difference in the deformed state in the zone opposite diametrically to the contact zone from that for a tire mounted on a rim and loaded with excess operating pressure. In this case, the nature of the distribution of strains in the radial section near the contact zone of the tire with the road surface is the same under conditions of compression and stationary rolling. Areas of concentration of equivalent stresses and strains in the bead zone of the tire and in the zone of the edges of the breaker are revealed. For a quick comparison of competing tire designs, it is recommended to calculate the average values of the total strain energy density per wheel revolution. The developed calculation methods make it possible to predict the performance characteristics of automobile tires at the design stage and are tested in the manufacturing of these products.

Low Cycle Fatigue Life Prediction Model of 800H Alloy Based on the Total Strain Energy Density Method

In this paper, a high-temperature low-cycle fatigue life prediction model, based on the total strain energy density method, was established. Considering the influence of the Masing and non-Masing behavior of materials on life prediction, a new life prediction model was obtained by modifying the existing prediction model. With an 800H alloy of the heat transfer tube of a steam generator as the research object, the high-temperature and low-cycle fatigue test was carried out at two temperatures. The results show that the predicted and experimental results are in good agreement, proving the validity of the life prediction model.

Indices to Determine the Reliability of Rocks under Fatigue Load Based on Strain Energy Method

Rock is a complicated material which includes randomly distributed grains and cracks. The reliability of rocks under fatigue load is very important during the construction and operation of rock engineering. In this paper, we studied the deformation and failure process of red sandstone under fatigue load in a laboratory based on a new division method of strain energy types. The traditional elastic strain energy density is divided into two categories: grain strain energy density and crack strain energy density. We find that the proportion of the grain strain energy density to total strain energy density can be used as an indicator of rock yield and the proportion of the crack strain energy density to total strain energy density can be used as an indicator of rock failure. Subsequently, through extensive literature research, we found that such a phenomenon is widespread. We also find the proportion of grain strain energy density to total strain energy density when yielding is affected by rock types and elastic modulus. The proportion of crack strain energy density to total strain energy density in the pre-peak stage is stable and not affected by rock types and elastic modulus, which is about 0.04~0.13. These findings should be very helpful for rock stable state judging in rock engineering.

BIOMECHANICAL STUDY ON PROXIMAL FEMORAL NAIL ANTIROTATION (PFNA) FOR INTERTROCHANTERIC FRACTURE

The proximal femoral nail antirotation device (PFNA) is a typical implant to the treatment of intertrochanteric fractures. However, re-fracture of the femur shaft after nailing are usually been reported. The purpose of this study was to investigate the biomechanical features in the healed proximal femur at different stages in the healing process. Stress and strain distributions, total strain energy density (SED) along the femur and PFNA were analyzed in walking and stair climbing. Results showed remarkable stress concentration occurred near the locking bolt hole with retained PFNA, decreased after PFNA removal. Stair climbing resulted in higher strain at the locking bolt hole than normal walking. The conclusion can be drawn that non-removal of PFNA after healing may result in high fractural risk near locking bolt on femoral shaft. Meanwhile, stair climbing should be avoided during healing.

An Energy-Based Axial Isothermal- Mechanical Fatigue Lifing Procedure

An energy-based fatigue lifing procedure for the determination of full-life and critical-life of in-service structures subjected to axial isothermal-mechanical fatigue (IMF) has been developed. The foundation of this procedure is the energy-based axial room-temperature fatigue model, which states: the total strain energy density accumulated during both a monotonic fracture event and a fatigue process is the same material property. The energy-based axial IMF lifing framework is composed of the following entities: (1) the development of an axial IMF testing capability; (2) the creation of a testing procedure capable of assessing the strain energy accrued during both a monotonic fracture process and a fatigue process at various elevated temperatures; and (3), the incorporation of the effect of temperature into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed in conjunction with the monotonic fracture curve testing procedure to produce fifteen fracture curves at four operating temperatures. The strain energy densities for these fracture curves were compared, leading to the assumption of constant monotonic fracture energy at operating temperatures below the creep activation temperature.

A Model to Predict Fatigue Life of Concrete Based on Total Strain Energy Density

A law on the cumulative damage is presented basing on total strain energy induced as damage parameter to calculate the cumulative damage when the specimens of concrete subjected to fatigue loading.Then the maximum of critical cumulative damage and location of production are determined basing on the equation of cumulative fatigue damage combined with experimental result through using the finite element analysis and the critical plane method in fatigue analysis.The relation equation between the standardized critical total strain energy density and stress level is obtained by considering the impact of loading level.The fatigue life of specimens can be predicted by combining the equation of cumulative fatigue damage with the relation equation of damage and stress level and the prediction results coincide with experimental results very well.

An Energy-Based Axial Isothermal-Mechanical Lifing Procedure

An energy-based fatigue lifing procedure for the determination of fatigue life and critical life of in-service structures subjected to axial isothermal-mechanical fatigue (IMF) has been developed. The foundation of this procedure is the energy-based axial room-temperature fatigue model, which states: the total strain energy density accumulated during both a monotonic fracture event and a fatigue process is the same material property. The energy-based axial IMF lifing framework is composed of the following entities: (1) the development of an axial IMF testing capability; (2) the creation of a testing procedure capable of assessing the strain energy accrued during both a monotonic fracture process and a fatigue process at various elevated temperatures; and (3), the incorporation of the effect of temperature into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed in conjunction with the monotonic fracture curve testing procedure to produce eight fracture curves at three operating temperatures. The strain energy densities for these fracture curves were compared, leading to the assumption of constant monotonic fracture energy at operating temperatures below the creep activation temperature.