Mechanical Properties and Fracture Toughness of Aluminum Vessels by Friction Stir Welding

Mechanical properties and fracture toughness in friction stir welded joint of vessels of structural aluminum alloy type A5083-O are investigated. Welded joint from 25 mm-thick plate is fabricated by one-side one-pass friction stir. Charpy impact energy and critical crack-tip opening displacement (CTOD) in friction stir weld are much higher than those of base metal or heat-affected zone, whereas mechanical properties such as stress-strain curve and Vickers hardness do not have a conspicuous difference. Effects of microstructure on crack initiation and propagation are studied in order to clarify the difference of fracture toughness between stir zone and base metal. Both tensile test and bending test show that the fine-grained microstructure in stir zone induces to increase ductile crack initiation and propagation resistance by analyzing fracture resistance curves and diameter of dimples in fracture surface. It is found that high fracture toughness value in stir zone is affected fine-grained microstructure by friction stirring.

Development of Analytical Evaluation Procedures and Acceptance Criteria for Pipe Wall Thinning in ASME Code Section XI

As the worldwide fleet of nuclear power plants ages, the need to address wall thinning in pressure boundary materials becomes more acute. The 2001 ASME Code Case N-597-1, “Requirements for Analytical Evaluation of Pipe Wall Thinning,” provides procedures and criteria for the evaluation of wall thinning that are based on Construction Code design concepts. These procedures and criteria have proven useful for Code Class 2 and 3 piping; but, they provide relatively little flexibility for Class 1 applications. Recent full-scale experiments conducted in Japan and Korea on thinned piping have supported the development of a more contemporary failure strength evaluation methodology applicable to Class 1 piping. The ASME B&PV Code Section XI Working Group on Pipe Flaw Evaluation has undertaken the codification of new Class 1 evaluation methodology, together with the existing Code Case N-597-1 rules for Class 2 and 3 piping, as a non-mandatory Appendix to Section XI. This paper describes the current status of the development of the proposed new Class 1 piping acceptance criteria, along with a brief review of the current Code Case N-597-1 evaluation procedure in general.

Crack Opening Area for an Isolated Crack Subjected to a Symmetric Mode I Stress Distribution

In developing a leak-before-break case for a component in a pressurized system, a key element is an estimation of the size of through-thickness crack that will give a measurable leakage under normal operating conditions, and this requires a knowledge of the crack opening area. In this context, the paper presents a simple derivation of an expression for the crack opening area associated with an isolated crack that is subjected to a general Mode I symmetric tensile stress distribution which could arise from a combination of applied and residual stresses. The paper also presents a simple derivation of an expression for the crack opening displacement at the crack centre which, coupled with the assumption that the crack opening profile conforms to an elliptical shape, has been used as the basis for a simplified procedure for estimating the crack opening area. The resulting expressions are validated by comparing them with known results for specific stress distributions. They are also used to give new results for a cosine stress distribution.

Delayed Hydride Cracking Initiation at Simulated Secondary Flaws in Zr-2.5 Nb Pressure Tube Material

Flaws in Zr-2.5 Nb alloy pressure tubes of CANDU nuclear reactors are susceptible to a crack initiation and growth mechanism called Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region and fracture of the hydrided region at the flaw-tip. The presence of small surface irregularities, or secondary flaws, at the bottom of service-induced fretting flaws in pressure tubes requires an integrity assessment in terms of DHC initiation. Experimental data and analytical modeling are required to predict whether DHC initiation can occur from the secondary flaws. In the present work, an experimental program was carried out to examine the impact of small secondary flaws with sharp radii on DHC initiation from simulated fretting flaws. Groups of cantilever beam specimens containing blunt notches with and without secondary flaws were prepared from unirradiated pressure tube materials hydrided to a nominal concentration of 50 wt ppm hydrogen. The specimens were subjected to multiple thermal cycles to form hydrides at the flaw-tip at different applied stress levels, which straddled the threshold value for DHC initiation. The threshold conditions for DHC initiation were established for different simulated fretting and secondary flaws. The experimental results are compared with predictions from the engineering process-zone DHC initiation model.

Design Optimization of Magnetic Coupling Using Genetic Algorithm and 2D FEM Model

Magnetic couplings (Figure 1) are widely used to torque transmission between two shafts without any mechanical contact. They are especially well suited for used in hazardous environments, to transmit torque through a separation wall. An additional advantage of a magnetic coupling is that slipping occurs when excessive torque is applied, this can be used to prevent mechanical failure due to torque overloads. This paper deals with influence of temperature on behavior of magnetic coupling and magnetic coupling design optimization. The permanent magnets that are used for torque transmission cannot be used close to Currie point, which is a point of loss of magnetic characteristics. We intend to use the magnetic coupling for pump of radioactive liquid materials for transmutation devices, where the temperature is close to four hundred centigrade. Because of we suggest the design changes for elimination of temperature influence. This paper presents the finite element (FE) parametric model of magnetic coupling, experimental verification of FE model and optimization of the inner part of magnetic coupling in order to increase the maximal torque. The genetic algorithm method in connection with FEM model of magnetic coupling was used for the design optimization procedure.

Influence of the Cutting Variables on the Surface Roughness of Alloy Steels Machined With Cylindrical Grinding

In machining precision mechanical parts, the prediction and the control of the desired surface roughness is very important. The grinding process requires a good control of the operative variables to guarantee best results. This research analyses the influence of the cutting variables such as workpiece speed, wheels grain size, down feed, and workpiece hardness on the surface finish and it is shown how the surface roughness improves when using low values of workpiece speed, transverse feed and down feed, as well as a small wheel grain size. A mathematical expression for roughness, Ra, as a function of the cutting variables and the mechanical properties, is obtained in order to predict the value of surface roughness. AISI 4140, AISI 4340 and AISI O1 alloy steels under different conditions were used for the experiments.

Comparison of Master Curve and Statistical Model Approach for Prediction of Fracture Toughness of ASTM A285 Steel

The master curve approach was utilized to compare fracture toughness of American Society for Testing of Materials (ASTM) A285 as developed from Charpy v-notch (CVN) data and predictive statistical models. The master curves for each of the data sets were developed in accordance with American Society for Testing Materials Specification E 1921 (ASTM E1921, “Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range”), as prescribed by American Petroleum Institute Recommended Practice 579 (API-579, “Fitness for Service”). The results indicate that predictive statistical models developed from compact tension test results express a lower fracture toughness distribution when compared to CVN data.

Fracture Toughness and Crack Path Deviations in Laser Welded Joints

The resistance of laser welded joints to cleavage failure has been examined using fatigue cracked fracture toughness specimens and Charpy tests. The apparent toughness of a range of weld microstructures was determined, the lowest being for a crack located in the weld metal. Sharp cracks deviate into the microstructure with the lowest apparent toughness adjacent to the tip and propagate down the toughness gradient. Charpy tests differ in that the crack consistently propagates into the softer material, and extends down the yield strength gradient adjacent to the tip. Charpy tests can thus give misleading information about the fracture behaviour of mismatched weldments, as crack path deviations may mask a potentially dangerously low toughness weld metal. Recommendations are given on supplementing Charpy weld characterisation with the fracture mechanics tests to ensure conservatism in mismatched weldments.

Hot Isostatic Pressing of Austenitic Stainless Steel Powders for Pressure Retaining Applications

Hot Isostatic Pressing (HIP) has been used for many years to consolidate porosity in cast metal shapes to improve mechanical properties. When the technique is applied to fine metal powders, it becomes possible to produce Near Net Shape (NNS) items and more complex geometry components that are fully dense and offer an attractive set of properties and reduced cost. Manufacture of NNS items from powder delivers cost savings by reducing initial material usage and subsequent machining costs. Powder production and HIP processing are automated methods, which also provide protection against forging route obsolescence. Setup costs are lower and smaller batch sizes possible. HIPped powder microstructures are isotropic and equi-axed, with uniformly fine grain sizes not normally achieved in heavy section components. In austenitic stainless steel materials, this provides significant improvements in ultrasonic NDE (Non-Destructive Examination) in thick sections. Use of the technology has grown, particularly in the off-shore oil industry where it is already established in high integrity applications, but take-up in the more conservative nuclear industry has been slow. In a broad programme of testing, Rolls-Royce has established that HIPped powder 316L components, in items up to several tons in weight, have equivalent or slightly better strength, toughness and corrosion properties across a wide range of test environments. A methodology for developing robust safety justifications for use has been developed. Manufacture of pressure seal components is now in progress and the economics of other applications such as pump bowls are being considered. The quality of HIPped powder items can provide through life cost savings since there is greater assurance of structural integrity compared to welded or wrought components.

Comparison of Fracture Methodologies for Flaw Stability Analysis of Storage Tanks

Fracture mechanics methodologies for flaw stability analysis of a storage tank were compared in terms of the maximum stable through-wall flaw sizes or “instability lengths.” The comparison was made at a full range of stress levels at a specific set of mechanical properties of A285 carbon steel and with a specific tank configuration. The two general methodologies, the J-integral-tearing modulus (J-T) and the failure assessment diagram (FAD), and several specific estimation schemes were evaluated. A finite element analysis of a flawed tank was also performed for validating the J estimation scheme with a curvature correction and for constructing the finite element-based FAD. The calculated instability crack lengths show that the J-T methodology based on a center-cracked panel solution with a curvature correction, and the material-specific FAD, most closely approximate the result calculated with finite element analysis for the stresses at the highest fill levels in the storage tanks (less than 124 MPa or 18 ksi). The results from the other FAD methods show instability lengths less than the J-T results over this range.