198 research outputs found
Directions of high cycle fatigue cracks emanating from circular notches studied by optical profilometry
Current models for predicting the fatigue endurance of notched solids use the stresses along a straight line, beginning at the notch root, as a simplification of the real crack propagation path. In this work, the experimental crack paths for hollow notched samples were analysed through different microscopy techniques, with the objective of establishing high cycle fatigue crack growth directions in a mild steel. Fully reversed tension–compression fatigue tests
of thin-walled tube specimens with a passing-through hole were carried out. The crack paths observed in the outer cylindrical surface were studied in each case, with special attention to the crack initiation point and the crack direction along the first grains. Moreover, the analysis of the fracture surfaces allowed the same analysis to be performed to determine the internal crack paths. It was observed that the crack initiation point was close to the maximum principal stress point at the hole contour as obtained from linear elastic finite element analysis, and the crack direction in its initiation was generally close to Mode I direction, contrary to the conventionally accepted 45
crack growth direction.The authors would like to thank the European Union, the Spanish
Government and the Junta de Andalucía for its financial support through
grantsDPI2017-84788-PandPID2020-117407GB-I00(FEDER/Ministerio
de Ciencia e Innovación - Agencia Estatal de Investigación) and P18-FR-
4306 (‘‘Fondo Europeo de Desarrollo Regional (FEDER) y Consejería de
Economía, Conocimiento, Empresas y Universidad de la Junta de Andalucía,
dentro del Programa Operativo FEDER 2014-2020’’). N.O. Larrosa
would also like to thank the UK Engineering and Physical Sciences
Research Council (EPSRC) for its financial support through grant number
EP/S012362/1
3D characterisation of hydrogen environmentally assisted cracking during static loading of AA7449-T7651
In this investigation, synchrotron X-ray microtomography was used to perform 3D in situ observations of crack initiation and growth during hydrogen environmentally assisted cracking (HEAC) in tensile samples of AA7449-T7651. Two smooth tensile samples with a 1 mm diameter gauge section were held at a fixed displacement (≈30% of yield stress) in warm, moist air (≈76∘C, 73% relative humidity). The samples were then imaged repeatedly using X-ray tomography until they fractured completely. The tomograms showing the nucleation and evolution of intergranular cracks were correlated with electron microscopy fractographs. This enabled the identification of crack initiation sites and the characterisation of the crack growth behaviour relative to the microstructure. The samples were found to fracture within an environmental exposure time of 240 min. Some cracks in both samples nucleated within an exposure time of 80 min (33–40% of the total lifetime). Many cracks were found to nucleate both internally and at the sample surface. However, only superficial cracks contributed to the final fracture surface as they grew faster owing to the direct environmental exposure and the larger crack opening. HEAC occurred prominently via brittle intergranular cracking, and cracks were found to slow down when approaching grain boundary triple junctions. Additionally, crack shielding from nearby cracks and the presence of coarse Al–Cu–Fe particles at the grain boundaries were also found to temporarily reduce the crack growth rates. After prolonged crack growth, the HEAC cracks displayed ductile striations and transgranular fracture, revealing a change in the crack growth mechanism at higher stress intensity factors
Reducing chaos in SAT-like search: finding solutions close to a given one
Motivated by our own industrial users, we attack the following
challenge that is crucial in many practical planning, scheduling
or timetabling applications. Assume that a solver has found a solution
for a given hard problem and, due to unforeseen circumstances (e.g., rescheduling),
or after an analysis by a committee, a few more constraints
have to be added and the solver has to be re-run. Then it is almost always
important that the new solution is “close” to the original one.
The activity-based variable selection heuristics used by SAT solvers
make search chaotic, i.e., extremely sensitive to the initial conditions.
Therefore, re-running with just one additional clause added at the end
of the input usually gives a completely different solution. We show that
naive approaches for finding close solutions do not work at all, and that
solving the Boolean optimization problem is far too inefficient: to find
a reasonably close solution, state-of-the-art tools typically require much
more time than was needed to solve the original problem.
Here we propose the first (to our knowledge) approach that obtains
close solutions quickly. In fact, it typically finds the optimal (i.e., closest)
solution in only 25% of the time the solver took in solving the original
problem. Our approach requires no deep theoretical or conceptual innovations.
Still, it is non-trivial to come up with and will certainly be
valuable for researchers and practitioners facing the same problem.Postprint (published version
An efficient procedure for reducing in-line-inspection datasets for structural integrity assessments
Apoptosis resistance in HIV-1 persistently-infected cells is independent of active viral replication and involves modulation of the apoptotic mitochondrial pathway
<p>Abstract</p> <p>Background</p> <p>HIV triggers the decline of CD4<sup>+ </sup>T cells and leads to progressive dysfunction of cell-mediated immunity. Although an increased susceptibility to cell death occurs during the acute phase of HIV infection, persistently-infected macrophages and quiescent T-cells seem to be resistant to cell death, representing a potential reservoir for virus production.</p> <p>Results</p> <p>Lymphoid (H9/HTLVIII<sub>B </sub>and J1.1) and pro-monocytic (U1) HIV-1 persistently-infected cell lines were treated with hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) and staurosporine (STS) for 24 h, and susceptibility to apoptosis was evaluated and compared with uninfected counterparts (H9, Jurkat and U937 respectively). When exposed to different pro-apoptotic stimuli, all persistently-infected cell lines showed a dramatic reduction in the frequency of apoptotic cells in comparison with uninfected cells. This effect was independent of the magnitude of viral replication, since the induction of viral production in lymphoid or pro-monocytic cells by exposure to TNF-α or PMA did not significantly change their susceptibility to H<sub>2</sub>O<sub>2</sub>- or STS-induced cell death. A mechanistic analysis revealed significant diferences in mitochondrial membrane potential (MMP) and caspase-3 activation between uninfected and persistently-infected cells. In addition, Western blot assays showed a dramatic reduction of the levels of pro-apototic Bax in mitochondria of persistently-infected cells treated with H<sub>2</sub>O<sub>2 </sub>or STS, but not in uninfected cells.</p> <p>Conclusion</p> <p>This study represents the first evidence showing that resistance to apoptosis in persistently-infected lymphoid and monocytic cells is independent of active viral production and involves modulation of the mitochondrial pathway. Understanding this effect is critical to specifically target the persistence of viral reservoirs, and provide insights for future therapeutic strategies in order to promote complete viral eradication.</p
On Tackling the Limits of Resolution in SAT Solving
The practical success of Boolean Satisfiability (SAT) solvers stems from the
CDCL (Conflict-Driven Clause Learning) approach to SAT solving. However, from a
propositional proof complexity perspective, CDCL is no more powerful than the
resolution proof system, for which many hard examples exist. This paper
proposes a new problem transformation, which enables reducing the decision
problem for formulas in conjunctive normal form (CNF) to the problem of solving
maximum satisfiability over Horn formulas. Given the new transformation, the
paper proves a polynomial bound on the number of MaxSAT resolution steps for
pigeonhole formulas. This result is in clear contrast with earlier results on
the length of proofs of MaxSAT resolution for pigeonhole formulas. The paper
also establishes the same polynomial bound in the case of modern core-guided
MaxSAT solvers. Experimental results, obtained on CNF formulas known to be hard
for CDCL SAT solvers, show that these can be efficiently solved with modern
MaxSAT solvers
- …