A B S T R A C TThe stress singularity at the tip of a crack, either lying along or perpendicular to the interface of the two materials, is first investigated by the complex variable method. The order of the singularity is shown to be dependent on both the crack geometry and two parameters ~, fl which are related to the four elastic constants of the two materials. A hybrid crack element is constructed to properly account for the crack tip singularity. The stress intensity factors and energy release rate for cracks in different bi-material continua are then calculated using the finite element method. The results show that the present finite elemenl analysis makes possible a highly accurate and efficient numerical solution of fracture mechanics problems.
The BamA protein of Escherichia coli plays a central role in the assembly of -barrel outer membrane proteins (OMPs). The Cterminal domain of BamA folds into an integral outer membrane -barrel, and the N terminus forms a periplasmic polypeptide transport-associated (POTRA) domain for OMP reception and assembly. We show here that BamA misfolding, caused by the deletion of the R44 residue from the ␣2 helix of the POTRA 1 domain (⌬R44), can be overcome by the insertion of alanine 2 residues upstream or downstream from the ⌬R44 site. This highlights the importance of the side chain orientation of the ␣2 helix residues for normal POTRA 1 activity. The ⌬R44-mediated POTRA folding defect and its correction by the insertion of alanine were further demonstrated by using a construct expressing just the soluble POTRA domain. Besides misfolding, the expression of BamA ⌬R44 from a low-copy-number plasmid confers a severe drug hypersensitivity phenotype. A spontaneous drug-resistant revertant of BamA ⌬R44 was found to carry an A18S substitution in the ␣1 helix of POTRA 1. In the BamA ⌬R44, A18S background, OMP biogenesis improved dramatically, and this correlated with improved BamA folding, BamA-SurA interactions, and LptD (lipopolysaccharide transporter) biogenesis. The presence of the A18S substitution in the wild-type BamA protein did not affect the activity of BamA. The discovery of the A18S substitution in the ␣1 helix of the POTRA 1 domain as a suppressor of the folding defect caused by ⌬R44 underscores the importance of the helix 1 and 2 regions in BamA folding.
Boron/aluminum laminates in symmetric layups of [±45] and [±45/02] containing discontinuities in the form of holes and slits were tested in tension. Both layups exhibited decreases in failure stress with discontinuity size. However the two exhibited quite different failure modes. The [±45 ] failed with interlaminar shear as the dominant mode whereas the [±45/02] specimens reacted to discontinuities catastrophically in a manner suggestive of the effect of cracks on homogeneous metals. A correlation is proposed for the fracture of the [±45/02] laminate.
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