Measurement and analysis of critical crack tip processes during fatigue crack growth

The mechanics of fatigue crack growth under constant-amplitudes and variable-amplitude loading were examined. Critical loading histories involving relatively simple overload and overload/underload cycles were studied to provide a basic understanding of the underlying physical processes controlling crack growth. The material used for this study was 7091-T7E69, a powder metallurgy aluminum alloy. Local crack-tip parameters were measured at various times before, during, and after the overloads, these include crack-tip opening loads and displacements, and crack-tip strain fields. The latter were useed, in combination with the materials cyclic and monotonic stress-strain properties, to compute crack-tip residual stresses. The experimental results are also compared with analytical predictions obtained using the FAST-2 computer code. The sensitivity of the analytical model to constant-amplitude fatigue crack growth rate properties and to through-thickness constrain are studied

Anti-phosphocholine hybridoma antibodies. I. Direct evidence for three distinct families of antibodies in the murine response

Biochemical and serological studies were performed on more than 400 anti- phosphocholine (PC) hybridoma proteins (HP) derived from six strains of mice; 26 of these HP were examined in detail. All HP possessed specificity for PC, and all those tested contained an H-chain idiotypic determinant, V(H)-PC, which is shared by PC-binding myeloma proteins (BMP) and anti-PC antibodies. Among the HP, three well-defined and distinct families that correlated well with previous studies on serum anti-PC antibodies were identified. The largest group shared idotypic determinants, an L-chain isoelectric focusing (IEF) pattern, and a binding site specificity with the PC-BMP, T15. Using the same criteria, a second group was found to be strikingly similar to another PC-BMP, M603. The third group possessed an idiotypic determinant and an L-chain IEF profile similar to M511, but differences in binding site specificities were observed among the HP. The latter two groups contained members whose L-chain IEF profiles were not identical to other members of that group. Thus, among strains there is a remarkable degree of conservation among responding anti-PC antibodies, in both the kinds of anti-PC families that exist and the immunochemical and structural characteristics of various members within a family. Differences in at least one parameter were observed in each family, demonstrating that even a relatively restricted response is heterogeneous. However, this diversity seems to operate within certain constraints

A comparison of single-cycle versus multiple-cycle proof testing strategies

An evaluation of single-cycle and multiple-cycle proof testing (MCPT) strategies for SSME components is described. Data for initial sizes and shapes of actual SSME hardware defects are analyzed statistically. Closed-form estimates of the J-integral for surface flaws are derived with a modified reference stress method. The results of load- and displacement-controlled stable crack growth tests on thin IN-718 plates with deep surface flaws are summarized. A J-resistance curve for the surface-cracked configuration is developed and compared with data from thick compact tension specimens. The potential for further crack growth during large unload/reload cycles is discussed, highlighting conflicting data in the literature. A simple model for ductile crack growth during MCPT based on the J-resistance curve is used to study the potential effects of key variables. The projected changes in the crack size distribution during MCPT depend on the interactions between several key parameters, including the number of proof cycles, the nature of the resistance curve, the initial crack size distribution, the component boundary conditions (load vs. displacement control), and the magnitude of the applied load or displacement. The relative advantages of single-cycle and multiple-cycle proof testing appear to be specific, therefore, to individual component geometry, material, and loading

Rechargeable aluminum batteries with conducting polymers as positive electrodes

This report is a summary of research results from an Early Career LDRD project con-ducted from January 2012 to December 2013 at Sandia National Laboratories. Demonstrated here is the use of conducting polymers as active materials in the posi-tive electrodes of rechargeable aluminum-based batteries operating at room tempera-ture. The battery chemistry is based on chloroaluminate ionic liquid electrolytes, which allow reversible stripping and plating of aluminum metal at the negative elec-trode. Characterization of electrochemically synthesized polypyrrole films revealed doping of the polymers with chloroaluminate anions, which is a quasi-reversible reac-tion that facilitates battery cycling. Stable galvanostatic cycling of polypyrrole and polythiophene cells was demonstrated, with capacities at near-theoretical levels (30-100 mAh g-1) and coulombic efficiencies approaching 100%. The energy density of a sealed sandwich-type cell with polythiophene at the positive electrode was estimated as 44 Wh kg-1, which is competitive with state-of-the-art battery chemistries for grid-scale energy storage

Most Asked Agronomic Questions

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A SVM and k-NN Restricted Stacking to Improve Land Use and Land Cover Classification

Land use and land cover (LULC) maps are remote sensing products that are used to classify areas into different landscapes. The newest techniques have been applied to improve the final LULC classification and most of them are based on SVM classifiers. In this paper, a new method based on a multiple classifiers ensemble to improve LULC map accuracy is shown. The method builds a statistical raster from LIDAR and image fusion data following a pixel-oriented strategy. Then, the pixels from a training area are used to build a SVM and k-NN restricted stacking taking into account the special characteristics of spatial data. A comparison between a SVM and the restricted stacking is carried out. The results of the tests show that our approach improves the results in the context of the real data from a riparian area of Huelva (Spain)

Measurement and analysis of critical crack tip processes associated with variable amplitude fatigue crack growth

Crack growth retardation following overloads can result in overly conservative life predictions in structures subjected to variable amplitude fatigue loading when linear damage accumulation procedures are employed. Crack closure is believed to control the crack growth retardation, although the specific closure mechanism is debatable. Information on the relative contributions to crack closure from: (1) plasticity left in the wake of the advancing crack and (2) crack tip residual stresses is provided. The delay period and corresponding crack growth rate transients following overloads are systematically measured as a function of load ratio (R) and overload magnitude. These responses are correlated in terms of the local 'driving force' for crack growth as measured by crack tip opening loads and delta K sub eff. The latter measurements are obtained using a scanning electron microscope equipped with a cyclic loading stage; measurements are quantified using a relatively new stereoimaging technique. Combining experimental results with analytical predictions suggests that both plastic wake and residual stress mechanism are operative, the latter becoming predominate as R increases

A Support tool for the reachability and other petri nets- related problems and formal design and analysis of discrete systems

Annotation. In this paper we deal with the PNtool - a tool for a design, analysis and development of concurrent and time-critical systems specified using the Petri nets (PN) formalism. The PNtool supports four Petri nets dialects: Generalized Petri nets, Time-basic nets, Evaluative and Coloured Petri nets. The tool allows to design and simulate a system using any of the supported PN dialects and to perform an invariants-based analysis and reachability analysis for Generalized PN. Each Generalized and Evaluative PN can be also loaded from and saved in a standard interchange format called PNML. The tool is implemented in Java as a part of the mFDT Environment (mFDTE). The mFDTE is a toolset for the formal design and analysis of concurrent discrete and time-critical systems, developed at the home institution of the authors. It integrates three formal methods with complementary features: Petri nets, process algebra and B-Method. This paper also describes interfaces, which connect the PNtool with other parts of mFDTE. The work presented is supported by the grants No. 1/3140/06 and No. 1/4073/07 of the VEGA- The Scientific Grant Agency of Slovakia and NATO CLG 982698 grant.В статье идёт речь о проґраммном средстве – PNtool, предназначенным для синтеза и анализа параллельных и критических во времени систем, которые описываются с помощью формализма сетей Петри (СП). PNtool предназначен для поддержки четырёх диалоґов СП: обобщённые СП (generalized PN), временно- базисные СП (Time-basic PN), эвалюационные СП (evaluative PN) и раскрашенные СП (Coloured PN). PNtool позволяет конструирование и симуляцию системы с применением любоґо из упомянутых диалектов СП и также проделать анализ системы с целью одержания её инвариантов , и решения проблемы достижимости для системы представленной обобщённой СП.Спецификация систем на языке обобщённых, или эвалюационных СП может быть представлена и сохраняться в формате PNML.PNtool реализован на языке JAVA, как одна из частей проґраммной среды mFDTE (multi FDT Environment). Среда mFDTE предназначена для синтеза и анализа параллельных (concurrent) дискретных систем , включая и временно-критические системы. Проґраммная среда mFDTE создана в університете авторов статьи. В ней интеґрированы три формальных метода, которые обладают взаимно-комплементарными свойствами: СП, процессные алґебры и метод B AMN. В статье описываются интерфейсы связывающие PNtool с остальными частями среды mFDTE. Результаты исследований представленные в статье были достиґнуты при поддержке грантов № 1/3140/06 , № 1/4073/07 VEGA – научного грантового агентства Словакии, и гранта NATO CLG 982698

Analysis of small crack behavior for airframe applications

The small fatigue crack problem is critically reviewed from the perspective of airframe applications. Different types of small cracks-microstructural, mechanical, and chemical-are carefully defined and relevant mechanisms identified. Appropriate analysis techniques, including both rigorous scientific and practical engineering treatments, are briefly described. Important materials data issues are addressed, including increased scatter in small crack data and recommended small crack test methods. Key problems requiring further study are highlighted