SOME IMPLICATIONS OF THE USE OF THE PD6493 LEVEL 3 FAILURE ASSESSMENT PROCEDURE

Abstract— Under a natural mapping between the standard R-curve analysis diagram and the failure assessment diagrams of R6 and PD6493 Level 3 the R-curve becomes the RCI (R-curve image). It follows that whenever the assessment point moves along the failure assessment line during ductile crack growth, the implication is that the failure assessment line is the RCI. This result is used to test the conservatism of a specific PD6493 Level 3 analysis by two methods. The first calculates the variation during crack growth of the applied elastic-plastic crack tip opening displacement (CTOD) parameter (or‘driving force') which is implied by the PD6493 analysis and then compares this variation with an independent estimate of it. The second uses an assumed driving force to deduce the CTOD resistance curve implied by the failure assessment line. It is shown by both methods and also by a direct R-curve analysis that this particular PD6493 analysis is conservative relative to an R-curve analysis which uses a crack driving force estimated by the EPRI (Electric Power Research Institute) procedures. However there is inconsistency between standard R-curve analysis and PD6493 Level 3 analysis in that the latter implies a material resistance which, for a given material, depends on the geometry of the structure. A similar inconsistency arises in any failure assessment procedures like the Options 1 and 2 with Category 3 of R6, which require the assessment point to move on a geometry independent failure assessment line during crack growth; indeed, even when the failure assessment line is geometry dependent there is full agreement with R-curve analysis only if the correct RCI is used as the failure assessment line. In a brief discussion it is noted that the new failure assessment diagram studies involving multi-parameter fracture mechanics may help to ameliorate these problems.     

Dynamics study on the anti-human immunodeficiency virus chemokine viral macrophage-inflammatory protein-II (VMIP-II) reveals a fully monomeric protein

Encoded by Kaposi's sarcoma-associated herpesvirus, viral macrophage-inflammatory protein-II (VMIP-II) is unique among CC chemokines in that it has been shown to bind to the CXC chemokine receptor CXCR4 as well as to a variety of CC chemokine receptors. This unique binding ability allows vMIP-II to block infection by a wide range of human immunodeficiency virus type I (HIV-1) strains, but the structural and dynamic basis for this broad range of binding is not known. 15N T1, T2 and 15N[-HN] nuclear Overhauser effect (NOE) values of vMIP-II, determined through a series of heteronuclear multidimensional nuclear magnetic resonance (NMR) experiments, were used to obtain information about the backbone dynamics of the protein. Whereas almost all chemokine structures reveal a dimer or multimer, vMIP-II has a rotational correlation time (tauc) of 4.7 +/- 0.3 ns, which is consistent with a monomeric chemokine. The rotational diffusion anisotropy, D parallel/D perpendicular, is approximately 1.5 +/- 0.1. The conformation of vMIP-II is quite similar to other known chemokines, containing an unstructured N-terminus followed by an ordered turn, three beta-strands arranged in an antiparallel fashion, and one C-terminal alpha-helix that lies across the beta-strands. Most of the protein is well-ordered on a picosecond time scale, with an average order parameter S2 (excluding the N-terminal 13 amino acids) of 0.83 +/- 0. 09, and with even greater order in regions of secondary structure. The NMR data reveal that the N-terminus, which in other chemokines has been implicated in receptor binding, extends like a flexible tail in solution and possesses no secondary structure. The region of the ordered turn, including residues 25-28, experiences conformational exchange dynamics. The implications of these NMR data to the broad receptor binding capability of vMIP-II are discussed.