Lawson criterion for ignition exceeded in an inertial fusion experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion

Improved data reduction for the deep-hole method of residual stress measurement

This paper describes an improved data reduction scheme for the deep-hole method of residual stress measurement. The deep-hole method uses the changes in diameter of a reference hole, drilled through the thickness of a component, to determine residual stress. The diameter changes result from the removal of a cylindrical core from the component, where the core is larger than and concentric with the reference hole. The new data reduction seeks to determine the unknown eigenstrain distribution that gives rise to the residual stress state and to the reference hole deformations; once the eigenstrain distribution is found, it is input to an elastic finite element analysis to provide the residual stress distribution in the original component. The new data reduction relies on expressing the unknown eigenstrain field in a polynomial basis, and finding the unknown basis function amplitudes from the measured reference hole diameter changes. The new data reduction is compared with the current technique, and it is shown that the proposed scheme offers several advantages to the current method of data reduction

Biaxial Residual Stress Mapping for a Dissimilar Metal Welded Nozzle

This paper describes a sequence of residual stress measurements made to determine a two-dimensional map of biaxial residual stress in a nozzle mockup having two welds, one a dissimilar metal (DM) weld and the other a stainless steel (SS) weld. The mockup is cylindrical, designed to represent a pressurizer surge nozzle of a nuclear pressurized water reactor (PWR), and was fabricated as part of a weld residual stress measurement and finite-element (FE) modeling round-robin exercise. The mockup has a nickel alloy DM weld joining an SS safe end to a low-alloy steel cylinder and stiffening ring, as well as an SS weld joining the safe end to a section of SS pipe. The biaxial mapping experiments follow an approach described earlier, in PVP2012-78885 and PVP2013-97246, and comprise a series of experimental steps and a computation to determine a two dimensional map of biaxial (axial and hoop) residual stress near the SS and DM welds. Specifically, the biaxial stresses are a combination of a contour measurement of hoop stress in the cylinder, slitting measurements of axial stress in thin slices removed from the cylinder wall, and a computation that determines the axial stress induced by measured hoop stress. At the DM weld, hoop stress is tensile near the OD (240 MPa) and compressive at the ID (-320 MPa), and axial stress is tensile near the OD (370 MPa) and compressive near the midthickness (-230 MPa) and ID (-250 MPa). At the SS weld, hoop stress is tensile near the OD (330 MPa) and compressive near the ID (-210 MPa), and axial stress is tensile at the OD (220 MPa) and compressive near midthickness (-225 MPa) and ID (-30 MPa). The measured stresses are found to be consistent with earlier work in similar configurations

A Detailed Evaluation of the Effects of Bulk Residual Stress on Fatigue in Aluminum

The fully effective utilization of large aluminum forgings in aerospace structures has been hampered in the past by inadequate understanding of, and sometimes inaccurate representation of, bulk residual stresses and their impact on both design mechanical properties and structural performance. In recent years, significant advances in both computational and experimental methods have led to vastly improved characterization of residual stresses. As a result, new design approaches which require the extraction of residual stress effects from material property data and the formal inclusion of residual stresses in the design analysis, have been enabled. In particular, the impact of residual stresses on durability and damage tolerance can now be assessed, and more importantly, accounted for at the beginning of the design cycle. In an effort to support the development of this next-generation design capability, the AFRL sponsored Metals Affordability Initiative (MAI) consortium has conducted a detailed experimental and analytical study of fatigue crack initiation and fatigue crack growth in aluminum coupons with known, quench induced residual stresses. In this study, coupons were designed and manufactured such that simple 'design features,' such as holes and machined pockets, were installed in locations with varying levels of bulk residual stress. The residual stresses at the critical locations in the coupons were measured using multiple techniques and modeled using detailed finite element analysis. Fatigue crack initiation (FCI) and fatigue crack growth (FCG) tests were performed using both constant amplitude and spectrum loading and the results were compared against computed FCI and FCG lives. © (2014) Trans Tech Publications, Switzerland