Papers by Keyword: Cold Working

Abstract: 304 austenitic stainless steel is a common structural material used in the nuclear power plant, and its basic mechanical property would be changed by work hardening, which seriously affects the evaluation of the integrity of the nuclear power structure. The relationships of between the yield strength and the reduced modulus ratio (σ_0.2/E_r) with the residual indentation depth and maximum depth ratio (h_r/h_m) during cold work processing of 304 austenitic stainless steel is analyzed by using theoretical analysis, experiment and finite element simulation, and an approximate analytical model to express σ_0.2/E_r by h_r/h_m is established in this paper. The investigating results indicate that yield strength, Vickers hardness, and σ_0.2/E_r will increase, and h_r/h_m decreases with the material deformation increasing, and the analytical result of the σ_0.2/E_r basically consistent with the experiment result.

Abstract: Co-Ni alloys in which Co, Ni, Cr and Mo are principal elements, exhibit an impressive combination of high strength, high toughness, and excellent corrosion resistance. In this study, the effects of cold-rolled combined with the heat-treatment ranged from 673 to 973 K from 1hour to 10 hours on the mechanical properties and microstructures of Co-Ni alloys were investigated systematically. The relations between the aging temperature and mechanical properties were concluded. The initial ultimate tensile strength of 790 MPa increased to 1808 MPa by cold rolled 80 pct. After aging the cold-rolled alloy (80 pct ) at temperature 773K for 4hours, the ultimate tensile strength and the hardness reached to 2220MPa and 759(HV), respectively. It is found that the material was hardened by the cold working and aging which provided the second hardening. However, TEM observations and X-ray diffractions suggested that no structural change could be found. The cold deformation introduced platelets of a few atomic layers in thickness less than 100 nm, which were identified as stacking faults. A high density of nanoplatelets and dislocations, piled up in the vicinity of twin plate strengthened materials. The aging treatment provided the second major source of strengthening after cold-working (and only after cold-working) by the formation of secondary twins. The ultimate strength resulted from that the intersection of deformation twins and secondary twins blocking the dislocation movement.

Abstract: The main objective of this research was a study the simultaneous influence of cold working and particle reinforcement on mechanical properties of Al. The model composites were fabricated by cold pressing RAl1 aluminium and 5% and 10% SiC powders and hot extrusion with ratio 3.8 in isothermal conditions at 480°C with 90° die angle to 18 mm diameter. Some specimens were then cold drawn with linear velocity 1 m/min to 16% area reduction, and one specimen in 3 passes to 51% reduction in area. Mechanical properties of the near fully dense composites were determined by axial compression, bend tests and hardness measurement and their microstructures examined, showing near homogenous distribution of SiC particles in the aluminium matrix. The increase in Young’s modulus was from 67 to 74 GPa and to 82 GPa for 5% and 10% reinforcement, respectively. Drawing increased average yield strength from 70 to 100 MPa for Al, and from 74 to 110 MPa and from 80 to 115 MPa, for 5 and 10% reinforcement, respectively. The results indicate that there is a significant increase in E, in accord with the law of mixtures, through incorporation of SiC and a synergistic effect of SiC and plastic deformation during drawing on strength. An attempt is made to identify the various contributions to overall strengthening of Al by considering also Al-8.8Cu-6.3%Si-0.7Mg alloy. It appears that alloying and age-hardening have the greatest effect, but that contributions from hot, warm and cold working are not insignificant. As powder metallurgy processing is an important fabrication method, their incorporation into the processing schedule merits consideration.

Abstract: In the present study, the effect of cold-rolling for the amount of reduction in thickness ranging from 25% to 75% on microstructure and mechanical properties of plain low carbon steel processed from dual-phase ferrite-martensite starting microstructure was studied. As the cold-rolling, the microstructure elongated to rolling direction and more compressed with increasing the rolling reduction and strength also increased. After annealing at warm temperature 500°C, the ultrafine grained was obtained in the 75% rolling reduction. Moreover, it was exhibited excellent strength of 82% and hardness of 66.1% higher than as-received condition with adequate uniform elongation 9.6%.

Abstract: Effect of thermomechanical processing on creep resistance at 150°C of an Al-5.6Cu-0.72Mg-0.5Ag-0.32Mn-0.17Sc-0.12Zr (wt. %) alloy was examined. It was shown that increasing strain prior to artificial aging provides achieving high strength. However, a degradation of the creep resistance, i.e., significant decrease in the rupture time and increase in the minimal creep rate, took place, concurrently. The effect of cold rolling on the strength and creep resistance is discussed in relation with the strain effect on the dispersion of secondary phases. The increase in strength and degradation of creep properties of the alloy subjected to cold working before ageing result from superposition of two competitive processes. First, an increase in the lattice dislocation density facilitates the precipitation of Ω-phase plates with high aspect ratio, leading to increase in the static strength at room temperature. Second, acceleration of the diffusion processes results in coarsening of strengthening phase in grain/subgrain interiors and precipitation of Ω-phase on deformation-induced boundaries during creep that deteriorates creep resistance.

Abstract: Effect of cold rolling prior to ageing on a dispersion of secondary phases and mechanical properties at room temperature for an Al-5.6Cu-0.72Mg-0.5Ag-0.32Mn-0.17Sc-0.12Zr (wt. %) alloy, which was solution treated and water quenched initially, was examined. It was shown that cold working leads to significant increase in density of lattice dislocations that induces the formation discrete agglomerates of the θ′-phase on the {100} planes. Strain of 7% provided increased aspect ratio (length to thickness) of plates that leads to moderate increase of strength. Imposing of higher strains leads to increased lattice dislocation density and the formation of deformation-induced boundaries. Precipitation of the coarse particles of secondary phases on these boundaries takes place. The high yield stress (YS) of 535 MPa and ultimate tensile strength (UTS) of 570 MPa, were attained after cold rolling with a reduction of 80% followed by ageing at 190°C for 2 h. The effect of plastic deformation prior to ageing on the precipitation behavior and strengthening of Al-Cu-Mg-Ag alloy is discussed.

Abstract: Over the past 50 years, mitigating cracks from holes has been a major focus of aerospace research, design, and stress engineers. Technology and design philosophies were developed to focus on predicting and increasing fatigue life of metal aircraft structure. One of the innovations to have the biggest impact to slow or arrest crack growth was to induce beneficial residual stresses around a hole. The zone of residual compressive stresses shields the hole from the effects of cyclic loads. The split sleeve cold expansion method was pioneered in the early 1970s by Boeing and Fatigue Technology. The split sleeve cold expansion process has become the bench mark method in mitigating fatigue cracks and enhancing durability and damage tolerance of aerospace metal structures. This paper will review the history of cold expansion methods and the evolution of this technology to the controlled and widely accepted methods used by industry today.

Abstract: The AP-3C Orion aircraft is the oldest aircraft in the Royal Australian Air Force (RAAF) inventory. The planned fleet withdrawal has been extended far beyond the original design service objective. Continued safe and effective operation has required the development of a robust ageing aircraft management approach. A fundamental aspect was supplementing the structural certification basis with appropriate standards in the form of fatigue management requirements from Federal Aviation Regulations (FAR) 25.571 and Federal Aviation Administration Advisory Circular (FAA AC) 120-93. To develop and underpin the ageing aircraft management plan and transition to the supplementary fatigue management standards, the RAAF collaborated with the Original Equipment Manufacturer, Lockheed Martin Aeronautics Company, the United States Navy (USN) and other operators to form the P-3C Service Life Assessment Program (SLAP). This program provided Full Scale Fatigue Test (FSFT) data, associated analyses and analysis tools to support management in accordance with FAR 25.571. An important element of the ageing aircraft management plan included the introduction of a rigorous Safety By Inspection (SBI) maintenance regime to assure structural airworthiness. FAA AC 120-93 requires assessment of structural repairs to determine revised fatigue management and inspection requirements. Often, this information is derived using tailored analysis tools and detailed models on a case-by-case basis. This approach is specialized, expensive and usually occurs after the repair has been designed and installed. To avoid these limitations, the AP-3C Repair Assessment Manual (RAM) was developed to provide the repair designer with a design handbook approach to fatigue analysis. In conjunction with some simple Finite Element (FE) models, the RAM supports complete repair analysis prior to an aircraft leaving the maintenance venue. This paper will present the history of the SBI program, the genesis of the RAM and actual examples of assessing structural repairs on the P-3 platform using the RAM.

Abstract: Rolling bearings in wind turbine gearboxes occasionally fail prematurely by so-called white etching cracks. The appearance of the damage indicates brittle spontaneous tensile stress induced surface cracking followed by corrosion fatigue driven crack growth. An X-ray diffraction based residual stress analysis reveals vibrations in service as the root cause. The occurrence of high local friction coefficients in the rolling contact is described by a tribological model. Depth profiles of the equivalent shear and normal stresses are compared with residual stress patterns and a relevant fracture strength, respectively. White etching crack failures are reproduced on a rolling contact fatigue test rig under increased mixed friction. Causative vibration loading is evident from residual stress measurements. Cold working compressive residual stresses are an effective countermeasure.

Abstract: The cold-working expansion methods are extensively used in the aerospace and mechanical industry to obtain forced couplings of steel bushings into holes made on mechanical components, by means of the passage of an oversized mandrel. This work describes a calculation algorithm, able to correlate the selected interference level, as the difference between the maximum radius of the mandrel and the inner radius of the bushing, to the residual stresses expected on the hole surface. The designer, taking into account the different design choices already made, can choose the optimal interference, estimating the uncoupling resistance and the fatigue strength of forced components. Two original and different systems bushing-mandrel, in low and high interference, have been tested for determining the characteristics of fatigue resistance in the finite life part of the Wohlers diagram.The results have been compared with previously estimated data, getting a good agreement of the series. The increase of the adopted interference value determine a corresponding increase of residual stresses on the hole, both in the radial than in the circumferential direction, and an appreciable and predictable improvement of the fatigue strength of components.