Papers by Keyword: Cyclic Deformation

Abstract: A finite element model is developed in this paper for simulating cyclic deformation in ultra-fined polycrystalline metals. The grain material is characterized by conventional theory of mechanism-based strain gradient plasticity (CMSG). A cohesive interface model was used to simulate the initiation and propagation of intergranular cracks. The simulation results show that inhomogeneous plastic deformation induces high strain gradient effects and severely plastic hardening in the grain interior, and the intergranular crack has a significant influence on the overall mechanical properties of ultra-fined polycrystalline metals subjected to cyclic loading.

Abstract: Cyclic deformation for two magnesium alloys AZ91 and AM50 with different processing status has been performed under total strain amplitude control mode and at room temperature. A serrated flow can be observed in both tensile and compressive directions of the stress-strain hysteresis loop for as-extruded AZ91 and AM50 magnesium alloys. It means that the so-called dynamic strain aging occurs during cyclic deformation. In addition, the dynamic strain aging phenomenon can also be observed in two extruded magnesium alloys subjected to aging treatment as well as the AZ91 alloy subjected to solution treatment. However, the dynamic strain aging seems not to take place in the extruded AM50 alloy subjected to solution treatment because there exists no significant serrated flow behavior in either compressive or tensile direction of the stress-strain hysteresis loop. It is suggested that the occurrence of the dynamic strain aging is associated with collective behavior of many mobile dislocations as well as interactions between solute atoms and dislocations.

Abstract: Isothermal low cycle fatigue and thermomechanical fatigue tests are performed on Alloy617B in the solution-annealed and stabilized condition at temperatures between room temperature and 900 °C. In addition, the replica technique is applied to study the growth of microcracks. The Chaboche model is found to describe the cyclic viscoplastic behavior of both heats, except the pronounced cyclic hardening in the low-temperature branches of the TMF tests. A lifetime model based on the cyclic crack-tip opening displacement and the cyclic J integral is used to describe the measured lifetimes and crack growth rates. However, the description is not fully consistent, since the data for room temperature and for temperatures above 400 °C fall into two separate scatter bands.

Abstract: Cyclic deformation behaviour is experimentally investigated using an incremental step method under a spectrum straining block loading mode for China grade B cast steel wheel. Results reveal that the material acts cyclically as non-Masing behaviour with a Bausinger effect. Different from monotonic tensile situation, material under fatigue loads was subjected to a plastic dissipation process without a yielding sign. Fatigued areas of specimen fracture surfaces are around 35%. Cyclic stress-strain (CSS) curve exhibits constant lower than monotonic tensile curve. Difference is enlarged as strain level increasing in elastic regime of the monotonic deformation. And this difference sustains a level of around 100 MPa as over yielding point. In addition, randomness is also observed for the present cyclic deformations. It indicates that random CSS relations should be comprehensively applied even if under stress based fatigue analysis.

Abstract: The moderate corrosion rate exhibits by Mg-RE alloys make it candidate material for biodegradable implant. Owing to the operating environment, implants are subjected to stochastic cyclic load and chemical condition. The alloys response to the subjected condition determines its degree of applicability. This work studies the cyclic deformation of newly developed Mg10GdxNd alloys both in air and under corrosive environment. The corrosion fatigue test was carried out in Ringer-Acetate solution and was evaluated using mechanical hysteresis and pH-value measurements. The microstructural changes in correlation to the deformation parameters and fracture surfaces were characterized using SEM. Results show that alloying Mg10Gd with Neodymium improved its fatigue live both in air and corrosive medium. Chilled casted Mg10Gd and Mg10GdxNd were found to undergo brittle fracture in both media. Loading in Ringer-Acetate was found to reduce the fatigue life of the investigated alloys due to the interaction of corrosion and fatigue processes on the microstructure of the alloys. EDX analysis suggests that the improved fatigue life observed on the Mg10GdxNd is connected to the new ternary Mg-Gd-Nd phase observed in the microstructure.

Abstract: With the increase of interest in using ultra-fine and nano-grained metals for structural purposes, the need to build on the knowledge pool regarding the response and behaviour of those metals under a mechanical load becomes more vital. However, it is well known that, especially for this type of materials such as the ECAPed and ARBed materials, the thermo-mechanical history affects the mechanical behaviour of the product strongly. Although ECAP and ARB are different techniques under the category of severe plastic deformation, similarities in their cyclic deformation response is observed from time to time. Specifically, the microstructural mechanisms involved in accommodating cyclic plastic strain in these two types of materials is seemingly comparable. The similarities arise from the similar microstructures in the majority of the volume of the bulk. In this report, the cyclic deformation response, and the related microstructural mechanisms of ECAPed copper will be discussed first and those of ARBed second. A comparison between ECAPed copper and ARBed copper will then be performed. Furthermore, the differences due to the unique features of ARBed material will be discussed. Lastly, the reasons behind the observed similarities in cyclic deformation behaviour and the related micro-mechanisms for metals process with the two different techniques will also be explored.

Abstract: Cyclic slip irreversibility is one of the most important features of fatigue processes in ductile metals because it induces surface relief evolutions during cycling which are mainly responsible for crack initiation. The reversible and irreversible parts of the slip within persistent slip bands (PSBs) in polycrystalline nickel are measured directly after half-cycle deformation and one full cycle on specimen surfaces once more well-polished after 60% of fatigue life using atomic force microscopy (AFM) and different techniques of scanning electron microscopy as electron channelling contrast imaging and electron backscattered diffraction. Using AFM measures on the same slip steps after half-cycle and full cycle, the cyclic slip irreversibility factor is directly evaluated and discussed with respect to the literature.

Abstract: Cu-2.2wt%Ni-0.5wt%Si alloy single crystals were grown by the Bridgman method and aged at 723 K for 10 h to form Ni2Si precipitates. Fully reversed tension-compression fatigue tests were conducted on the aged single crystals with a single slip orientation under constant plastic-strain amplitudes at room temperature. Cyclic softening occurred at plastic-strain amplitudes between 2.5x10-4 and 2.5x10-2. Using the maximum stress amplitude in each cyclic hardening/softening curve, a pseudo cyclic stress-strain curve (CSSC) was obtained. The CSSC was found to exhibit a plateau region with a stress level of about 167 MPa. Transmission electron microscopic observation revealed the formation of persistent slip bands (PSBs) in the plateau regime. It was found that the Ni2Si precipitate particles were intensively sheared by glide dislocations within the PSBs and were eventually re-dissolved into the Cu matrix. The macroscopic cyclic softening can be attributed to the local softening induced by the re-dissolution of the Ni2Si particles in the PSBs.

Abstract: Cyclic deformation characters and description method are investigated to the grade B cast steel for Chinese railway rolling wagon bogie frames. Incremental step test method with 8 strain amplitude loading mode were employed. Results reveal that the material acts as non-Masing behaviour with a significant Bausinger’s effect and appears a slight cyclic softening. Great of dimples in the transient fracture district indicate that present material is ductile. Significant discrepancy between the monotonic and cyclic beaviour was revealed to indicate that there is different monotonic and cyclic deformation behaviour. The scattered cyclic stress-strain relations indicate that appropriate description for the cyclic deformation of present material should be a probabilistic modeling. And then, a probabilistic modeling with measurements of survival probability and confidence is developed to give a good description to the cyclic deformation behaviour.

Abstract: For investigating the twinning and detwinning behaviour under cyclic loading conditions in-situ stress measurements with different applied strain amplitudes were performed at the beamline 7T-MPW at BESSY II. Intensity measurements of the (10-10), (11-20) and (0002) reflections served for the determination of the twinned volume fraction. The measurements of the (hkil) dependent strains and stresses gave information on the load partitioning between different grain orientations. The intensity measurements performed in the compression regime showed that the evolution of the (0002) reflection intensity depends on the strain amplitude. For strain amplitudes ≥ 0.75 % an increase in intensity is observed whereas for smaller strain amplitudes the (0002) intensity decreases with increasing cycle numbers. In the tensile regime an increase in the (0002) reflection intensity is found for the whole range of applied strain amplitudes.