A Re-examination of the Portevin-Le Chatelier Effect in Alloy 718 in Connection with Oxidation-Assisted Intergranular Cracking

In Alloy 718, a sharp transition exists in the fracture path changing from an intergranular brittle mode to a transgranular ductile mode which is associated with a transition of flow behavior from smooth in the dynamic strain aging regime to a serrated one in the Portevin-Le Chatelier (PLC) regime. In order to better understand both deformation and rupture behavior, PLC phenomenon in a precipitation-hardened nickel-base superalloy was carefully investigated in a wide range of temperatures [573 K to 973 K (300°C to 700°C)] and strain rates (109^-5 to 3.2910^-2 s^-1 ). Distinction was made between two PLC domains characterized by different evolutions of the critical strain to the onset of the first serration namely normal and inverse behavior. The apparent activation energies associated with both domains were determined using different methods. Results showed that normal and inverse behavior domains are related to dynamic interaction of dislocations with, respectively, interstitial and substitutional solutes atoms. This analysis confirms that normal PLC regime may be associated to the diffusion of carbon atoms, whereas the substitutional species involves in the inverse regime is discussed with an emphasis on the role of Nb and Mo

Lichenometric dating (lichenometry) and the biology of the lichen genus rhizocarpon:challenges and future directions

Lichenometric dating (lichenometry) involves the use of lichen measurements to estimate the age of exposure of various substrata. Because of low radial growth rates and considerable longevity, species of the crustose lichen genus Rhizocarpon have been the most useful in lichenometry. The primary assumption of lichenometry is that colonization, growth and mortality of Rhizocarpon are similar on surfaces of known and unknown age so that the largest thalli present on the respective faces are of comparable age. This review describes the current state of knowledge regarding the biology of Rhizocarpon and considers two main questions: (1) to what extent does existing knowledge support this assumption; and (2) what further biological observations would be useful both to test its validity and to improve the accuracy of lichenometric dates? A review of the Rhizocarpon literature identified gaps in knowledge regarding early development, the growth rate/size curve, mortality, regeneration, competitive effects, colonization, and succession on rock surfaces. The data suggest that these processes may not be comparable on different rock surfaces, especially in regions where growth rates and thallus turnover are high. In addition, several variables could differ between rock surfaces and influence maximum thallus size, including rate and timing of colonization, radial growth rates, environmental differences, thallus fusion, allelopathy, thallus mortality, colonization and competition. Comparative measurements of these variables on surfaces of known and unknown age may help to determine whether the basic assumptions of lichenometry are valid. Ultimately, it may be possible to take these differences into account when interpreting estimated dates

The DIRC Particle Identification System for the BABAR Experiment

A new type of ring-imaging Cherenkov detector is being used for hadronic particle identification in the BABAR experiment at the SLAC B Factory (PEP-II). This detector is called DIRC, an acronym for Detection of Internally Reflected Cherenkov (Light). This paper will discuss the construction, operation and performance of the BABAR DIRC in detail

Atomic Species Associated with the Portevin–Le Chatelier Effect in Superalloy 718 Studied by Mechanical Spectroscopy

In many Ni-based superalloys, dynamic strain aging (DSA) generates an inhomogeneous plastic deformation resulting in jerky flow known as the Portevin--Le Chatelier (PLC) effect. This phenomenon has a deleterious effect on the mechanical properties and, at high temperature, is related to the diffusion of substitutional solute atoms toward the core of dislocations. However, the question about the nature of the atomic species responsible for the PLC effect at high temperature still remains open. The goal of the present work is to answer this important question; to this purpose, three different 718-type and a 625 superalloy were studied through a nonconventional approach by mechanical spectroscopy. The internal friction (IF) spectra of all the studied alloys show a relaxation peak P718 (at 885 K for 0.1 Hz) in the same temperature range, 700 K to 950 K, as the observed PLC effect. The activation parameters of this relaxation peak have been measured, Ea(P718){\thinspace}={\thinspace}2.68{\thinspace}{\textpm}{\thinspace}0.05 eV, $\tau$0{\thinspace}={\thinspace}2{\textperiodcentered}10-15 {\textpm} 1 s as well as its broadening factor $\beta${\thinspace}={\thinspace}1.1. Experiments on different alloys and the dependence of the relaxation strength on the amount of Mo attribute this relaxation to the stress-induced reorientation of Mo-Mo dipoles due to the short distance diffusion of one Mo atom by exchange with a vacancy. Then, it is concluded that Mo is the atomic species responsible for the high-temperature PLC effect in 718 superallo

Neutron cross section standards evaluations for ENDF/B-VI

The neutron cross section standards are now being evaluated as the initial phase in the development of the new ENDF/B-VI file. These standards evaluations are following a somewhat different process compared with that used for earlier versions of ENDF. The primary effort is concentrated on a simultaneous evaluation using a generalized least squares program, R-matrix evaluations, and a procedure for combining the results of these evaluations. The ENDF/B-VI standards evaluation procedure is outlined, and preliminary simultaneous evaluation and R-matrix results are presented. 16 refs., 7 figs

Spacing requirements of 0.7 in. (18 mm) diameter prestressing strands

[EN] The use of 0.7 in. (18 mm) diameter strands for pretensioned concrete girders is advantageous when increasing the flexural capacity and extending girder spans. The current codes have no design guidelines for predicting transfer length, development length, and minimum strand spacing for this greater-diameter strand. This study measures transfer and development lengths and evaluates the applicability of using a strand spacing of 2.0 in. (51 mm) for 16 pretensioned concrete beams with 0.7 in. diameter strands. These beams were fabricated with high-strength, conventional concrete or self-consolidating concrete. The Standard Test Method for Evaluating Bond of Seven-Wire Steel Prestressing Strand (ASTM A1081) was used to quantify the strand surface conditions. The experimental results indicate that the current ACI 318-14 and AASHTO LRFD specifications overestimate the measured transfer and development lengths for the beams. Transfer and development lengths of the beams containing two strands placed at 2.0 in. spacing were equal to or slightly greater than those of the beams containing one strand. Finally, the concrete region around the strands showed no sign of cracking.The authors thank Insteel Industries Inc. for providing the strands and RJ Peterman and Associates Inc. for conducting the ASTM A1081 test method for strand bond for this research. The authors thank Don Logan for providing the financial support to conduct the ASTM A1081 test method for strand bond. The authors are also thankful to Richard Deschenes Jr., Cameron Murray, Joseph Daniels III, William Phillips, Doddridge Davis, Alberto Ramirez, and Ryan Hagedorn for helping fabricate the beams at the Engineering Research Center at the University of Arkansas.Dang, C.; Floyd, R.; Hale, W.; Martí Vargas, JR. (2016). Spacing requirements of 0.7 in. (18 mm) diameter prestressing strands. PCI Journal. 61(1):70-87. http://hdl.handle.net/10251/81574S708761

Temperature Gradients in Bridge Concrete I-Girders under Heat Wave

[EN] This paper presents an experimental research work to determine temperature gradients in concrete bridge girders under natural environmental conditions. Three AASHTO Type I-girders having different configurations (with and without wide top flanges) were considered in the experimental program. Temperature was monitored in the bridge girders to determine the vertical and transverse temperature gradients in a predeck placement condition. It was found that uneven heating of optimized bridge girder sections results in large nonlinear temperature gradients. The current AASHTO design standard, which only uses a nonlinear vertical but no transversal temperature gradient, was found inaccurate to predict both shape and magnitude of temperature gradients for the analyzed girders.This research is supported by the University of Arkansas at Fayetteville, Ton Duc Thang University, and Southern Plains Transportation Center. The authors are thankful to a number of graduate students for their help during the experimental program.Hagedorn, R.; Martí Vargas, JR.; Dang, C.; Hale, W.; Floyd, R. (2019). Temperature Gradients in Bridge Concrete I-Girders under Heat Wave. Journal of Bridge Engineering. 24(8):1-14. https://doi.org/10.1061/(ASCE)BE.1943-5592.0001454S11424