Yuh J. Chao

Higher-order asymptotic crack-tip fields in a power-law creeping material

Abstract The higher-order asymptotic crack-tip fields are considered for a mode-I crack in a power-law creeping material under the plane strain conditions. Based on the three-term solution of Yang et al. (1993) and Chao et al. (1994) for hardening materials, this paper develops a three-term solution near a crack tip in creeping materials only with two parameters: C(t)-integral and a constraint parameter A2(t). This solution is then discussed for conditions of small-scale creep, transient creep and extensive creep. In addition, detailed finite element analysis is performed for four specimens, namely, single-edge notched tension, three point bend, center-cracked panel and compact tension. Good agreement, in both angular and radial stresses, with finite element results confirms that the three-term asymptotic solution is universally valid for specimens possessing various crack-tip constraints and from small-scale creep to extensive creep. This statement is especially true for shallow cracked (or low constraint) specimens, where the dominant region for the HHR-type singularity does not practically exit.

Characterization of constraint of fully plastic crack-tip fields in non-hardening materials by the three-term solution

Abstract The J - A 2 three-term asymptotic solution for a crack in a power-law hardening material has been successfully used to interpret the constraint effects due to finite specimen geometry and loading configurations. In the current paper, we study the mechanics behavior of the J - A 2 solution for a plane strain mode-I crack in very low hardening materials. The objective is to investigate the validity of the J - A 2 characterization, in an approximate sense, for the fully plastic crack-tip fields in non-hardening materials. In particular, the constraints at the crack tip for several conventional specimen geometries and loading configurations in non-hardening materials are studied under the framework of the J - A 2 description. The results indicate that within the plastic zone ahead of the crack tip the three-term solution can capture the essential features of fully plastic fields in various finite size specimens in non-hardening materials. Consequently A 2 can be effectively used as a constraint parameter in characterizing the crack-tip field in non-hardening materials.

Fully plastic crack-tip fields for CCP and DECP specimens under tension in non-hardening materials

Abstract Detailed finite element analyses are performed for center cracked plate (CCP) and double edge cracked plate (DECP) in non-hardening materials under plane strain conditions. The objective is to systematically investigate the effects of deformation level, loading type, crack depth and specimen dimension on crack-tip fields and constraints of these two specimens. Special attention is placed on (a) under what conditions the slip-line fields can be present near the crack tip, and (b) determining what deformation mechanism makes the crack-tip fields significantly different in the two specimens at fully plastic state. The results reveal that (a) at load levels much smaller than the limit load (i.e., small-scale yielding) the crack-tip fields are close to the Prandtl field for both specimens, (b) the effects of crack depth a/W on the crack-tip field is not remarkable for CCP, but significant for DECP at the limit load, (c) as L / W ≥2.4 for CCP and L / W ≥2 for DECP, the crack-tip fields are independent of the specimen length L/W , (d) at the limit load, the crack face is under compression for all CCP, and (e) a compression (tensile) zone exists at the crack face of shallow (deep) cracked DECP. Moreover, it is found that there exist tensile and compressive stresses along the vertical centerline of specimen for both CCP and DECP which result in a bending moment M VL . The difference between M VL and the moment generated by the applied far-field loads makes the crack opening stress non-uniform along the remaining ligament. Recall that the slip-line fields for both the CCP and DECP have uniform opening stress along the ligament. At the limit load, therefore, the numerical crack-tip stress fields can only approach to, but cannot attain to, the slip-line fields for both CCP and DECP specimens. In addition, through comparison of the different limit loads given for DECP specimens, the present results indicate that the limit load formula given by Kumar et al. (EPRI, 1981) is valid only for 0.4≤ a / W ≤0.7, whereas the formula of Ewing and Hill (1967) can be used for any crack depth.

Effects of Crack Depth, Specimen Size, and Out-of-Plane Stress on the Fracture Toughness of Reactor Vessel Steels

Cleavage fracture toughness values for A533-B reactor pressure vessel (RPV) steel at -40°C obtained from test programs at Oak Ridge National Laboratory (ORNL) and University of Kansas (KU) are interpreted using the J-A 2 analytical model. The KU test data are from smaller SENB specimens with a/w = 0.1 and 0.5. The ORNL test data are from 1) larger SENB specimens with a/w = 0.1 and 0.5, and 2) a six-point-bend cruciform specimen under either uniaxial or bi-axial loads. The analytical model is based on the critical stress criterion and takes into consideration the constraint effect using the second parameter A 2 in addition to the generally accepted loading parameter J. It is demonstrated that the effects of crack depth (shallow versus deep), specimen size (small versus large), and loading type (uniaxial versus biaxial) on the fracture toughness from the test programs can be interpreted and predicted.

Weld Growth Mechanisms and Failure Behavior of Three-Sheet Resistance Spot Welds Made of 5052 Aluminum Alloy

This paper investigates the weld nugget formation in three-sheet aluminum alloy resistance spot welding. The nugget formation process in three equal thickness sheets and three unequal thickness sheets of 5052 aluminum alloy were studied. The results showed that the nugget was initially formed at the workpiece/workpiece interfaces (i.e., both upper interface and lower interface). The two small nuggets then grew along the radial direction and axial direction (welding direction) as the welding time increased. Eventually, the two nuggets fused into one large nugget. During the welding process, the Peltier effect between the Cu-Al caused the shift of the nugget in the welding direction. In addition, the mechanical strength and fracture mode of the weld nuggets at the upper and lower interfaces were also studied using tensile shear specimen configuration. Three failure modes were identified, namely interfacial, mixed, and pullout. The critical welding time and critical nugget diameter corresponding to the transitions of these modes were investigated. Finally, an empirical failure load formula for three-sheet weld similar to two-sheet spot weld was developed.

Application of Pre-heating to Improve the Consistency and Quality in AA5052 Resistance Spot Welding

Making consistent resistance spot welds of aluminum alloy with good quality and at high volume has several obstacles in automotive industry. One of the difficult issues arises from the presence of a tough non-conducting oxide film on the aluminum sheet surface. The oxide film develops over time and often is non-uniform across the surface of the aluminum alloy sheet, which makes the contact resistance characteristics irregular at the faying interface during welding. The consistency in quality of the final spot welds is therefore problematic to control. To suppress the effect of the irregular oxide film on the spot weld quality, application of a pre-heating treatment in the welding schedule for aluminum alloy 5052 is investigated in this present work. The current level of the pre-heating required to reduce the scatter of the contact resistance at the W/W (workpiece-to-workpiece) faying interface is quantified experimentally. The results indicate that the contact resistance at the W/W faying interface with a pre-heating treatment becomes much consistent and can be reduced by two orders of magnitude. Having the uncertain variation of the contact resistance at the W/W faying surface virtually reduced or removed, the quality of the spot welds in terms of the peak load and nugget diameter is examined and shows a great improvement. The proposed method may provide a robust method for high-volume spot welding of aluminum alloy sheets in auto industry.

Contact Between Vessel Shell and Welded Pad in Nozzle Reinforcement

In the thin shell analysis of welded pad reinforced nozzles in pressure vessels, no contact between pad and vessel is often assumed. The significance of this contact force to the stress distribution in the structure is little known. In this paper, stress results from the finite element analysis, which includes the contact force between the pad and the vessel, are reported. A comparison of the finite element results with those from thin shell analysis and experiments shows that the finite element method with contact assumption yields improved theoretical prediction for the stress distribution. The effect of both the gap and friction between the pad and the vessel are also investigated.

Correlations Between Charpy V-Notch Impact Energy and Fracture Toughness of Nuclear Reactor Pressure Vessel (RPV) Steels

Both Charpy V-notch (CVN) impact energy and fracture toughness are parameters reflecting toughness of the material. Charpy tests are however easy to perform compared to standard fracture toughness tests, especially when the material is irradiated and quantity is limited. Correlations between the two parameters are therefore of great significance, especially for reactor pressure vessel (RPV) structural integrity assessment. In this paper, correlations between CVN impact energy and fracture toughness of three commonly used RPV steels, namely Chinese A508-3 steel, USA A533B steel, Euro 20MnMoNi55 steel, are investigated with two methods. One method applies a direct conversion using empirical formulas and the other adopts the Master Curve method. It is found that when the empirical formula is used, the difference between the predicted fracture toughness (from the CVN impact energy) and actual test data is relatively small in upper shelf, lower shelf and the bottom of transition region, while relatively large in other parts of the transition region. When the Master Curve method is adopted, whether the reference temperature T0 is estimated through temperature at 28J or 41J CVN impact energy, the predicted fracture toughness values of the three steels are consistent with actual test data. The reference temperature T0 is also estimated through the IGC-parameter correlation and through a combination of empirical formula and multi-temperature method. Both procedures show excellent agreement with test results. The mean value of T0 estimated from T28J, T41J, IGC-parameters and the combination method is denoted by TQ-ave and is then used as the final reference temperature T0 for the Master Curve determination. Accuracy of TQ-ave (and therefore the Master Curve method) is demonstrated by comparison with actual test data of the three RPV steels. It is concluded that Master Curve method provides a reliable procedure for predicting fracture toughness in the transition region utilizing limited CVN impact energy data from open literature.Copyright

Effect of specimen width on the failure behavior in resistance spot weld tensile shear testing

Specimen dimensions have significant effect on the tensile shear strength (TSS) of resistance spot welds (RSW) in the commonly employed tensile shear testing. Minimum required dimensions specified in different industrial test standards vary somewhat however. In this paper, the most influential dimension, the tensile shear test specimen width, is investigated for a low-strength steel (galvanized steel Q235), a high-strength steel (TRIP980), and aluminum alloy 5052. The relationships between the specimen width and weld properties, such as TSS, energy absorption, and failure modes, were investigated experimentally. The results show that there is a critical width of the specimen beyond which the TSS of the RSW maintains a constant value and the failure is at the spot weld. This constant TSS is a saturated value. The perceived TSS from the tensile shear testing is lower than the saturated TSS when the width is smaller than the critical value. The displacement at the TSS and the energy up to failure from the tensile shear testing, however, shows opposite trend due to the large plastic deformation from the base sheet metal in narrower specimens. From the results, a minimum width of tensile shear test specimen for determining saturated TSS is recommended.

An Assessment of Mechanical Properties of A508-3 Steel Used in Chinese Nuclear Reactor Vessels

China has very ambitious goals of expanding its commercial nuclear power by 30 Giga-Watts within the decade and wishes to phase out fossil fuels emissions by 40–45% by 2020 (from 2005 levels). With over 50 new nuclear power plants under construction or planned and a design life of 60 years, any discussions on structural integrity become very timely. Although China adopted its nuclear technology from France or US at present time, e.g. AP1000 of Westinghouse, the construction materials are primarily “Made in China”. Among all issues, both the accumulation of the knowledge base of the materials and structures used for the power plant and the technical capability of engineering personnel are imminent. This paper attempts to compile and assess the mechanical properties, Charpy V-notch impact energy, and fracture toughness of A508-3 steel used in Chinese nuclear reactor vessels. All data are collected from open literature and by no means complete. However, it provides a glimpse into how this domestically produced steel compares with western reactor vessel steels such as US A533B and Euro 20MnMoNi55.Copyright