Applications of dynamic fracture mechanics for the prediction of crack arrest in engineering structures

The compatibility of the static and dynamic approaches to crack propagation and arrest is investigated. For clarity, the term kinetic is introduced for analyses that focus on the entire crack initiation/propagation/arrest process — as opposed to static analyses which consider only the end points — as dynamic effects are not always significant in a crack run/arrest event. The importance of integrating experimental and computational work in this field is also discussed and a differentiation is given between the ways in which this can be done. An example computation on a thermal shock problem where dynamic effects are minimal, i.e., where the static and kinetic approaches are nearly the same, and a computation for a DCB specimen where significant differences occur between the two approaches are described. It is concluded that the static and kinetic approaches are entirely compatible as long as reflected stress waves do not reach the crack tip prior to crack arrest. But, when this is not the case, it is the kinetic approach that must be used. Similarly, when inelastic effects are important, only the kinetic approach can properly admit them.

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Résumé On étudie la compatibilité des approches statiques et dynamiques pour la propagation et l'arrêt d'une fissure. Pour des raisons de clarification, le terme cinétique est introduit dans une analyse qui couvre l'entièreté du processus d'amorçage, propagation et arrêt d'une fissure, par opposition à l'analyse statique qui ne considère que les points terminals et dans la mesure où les effets dynamiques ne sont pas toujours appréciables dans un processus de course et d'arrêt d'une fissure. On discute également l'importance d'intéger le travail expérimental et le travail par calculs dans ce domaine et l'on établit une différence entre les différentes voies qui peuvent être suivies. A titre d'exemple, on établit le calcul pour un problème de choc termique où les effets dynamiques sont minimes, c'est-à-dire où les approches statiques et cinétiques sont à peu près les mêmes et un calcul pour une éprouvette Cantilever où des différences significatives surviennent entre les deux approches. On conclut que les approches statiques et cinétiques sont entièrement compatibles pour autant que les ondes de contrainte réfléchies n'atteignent pas l'extrémité de la fissure avant que ne se produisent son arrêt. Cependant, comme lorsque cela n'est pas le cas, c'est l'approche cinétique qui doit être utilisée. De même, lorsque les effets inélastiques sont importants, seule l'approche cinétique peut les prendre en compte.

     

Stress and strain state of concrete during freezing and thawing cycles

The objective of this work is to calculate the pressures, stresses, and strains induced into moist concrete during freezing and thawing. The applied theory is based on thermodynamics and the linear theory of elasticity. If no additional salts are dissolved in the pore water the inputs needed in the theory are relative humidity and temperature measured in the sample chamber and inside concrete and evaporable water amount in the pore structure. Theoretical results were compared with the test results made with two concretes cured under water or at 96% relative humidity. One of the concretes was air entrained and in the comparison concrete no air-entraining agents were used. In the test cylinders cured under water the largest tensional stresses in freezing occurred on the surface of the test cylinders both in the axial and tangential direction. The largest tensional stress was 2.2 MPa, both in air-entrained and in non air-entrained concretes. The largest tensional stresses in the warming phase took place at the end of the thawing period when the chamber temperature was around +5 °C. Then the maximum tension occurred in the middle of the concrete cylinder in the axial direction of the cylinder. This maximum tensional stress was over 2.5 MPa in the air-entrained concrete cured in the relative humidity of 96%. The thermodynamic pumping effect at the end of the thawing phase in every cycle can increase the pore water amount remarkably if free water or moisture is available on the surface of the structure or in the environment vapor. The thermodynamic pumping effect seems to be remarkably greater and more dangerous in air-entrained concretes.     

Reaction engineering studies in a polytropic fixed-bed reactor over a highly active and selective catalyst for oxidative methane coupling

The oxidative coupling of methane (OCM) was carried out in a polytropic fixed-bed reactor applying a Zr/La/Sr catalyst developed by the Neste company. Over this catalyst the OCM reaction follows a complex reaction scheme which includes primary parallel reaction steps to CO, CO₂ and C₂H₆ and consecutive reactions of ethane to ethylene or CO_x. Yield of higher hydrocarbons C₂₊ obtained with this catalyst strongly depended on reaction conditions, i.e. low partial pressures of methane and oxygen obtained by diluting the feed gas with nitrogen and high reaction temperatures promoted C₂₊ selectivity and yield. The maximum yield amounted to 21.4% (20 Vol.-% CH₄, 9 Vol.-% O₂, 71 Vol.-% N₂, T = 860°C; X_CH4 = 41.8%, S = 52.5%). This result belongs to the highest yields reported in the open literature.     

Validation of the Climate-SAF surface broadband albedo product: Comparisons with in situ observations over Greenland and the ice-covered Arctic Ocean

This paper describes a validation study performed by comparing the Climate-SAF Surface Albedo Product (SAL) to ground truth observations over Greenland and the ice-covered Arctic Ocean. We compare Advanced Very High Resolution Radiometer (AVHRR)-based albedo retrievals to data from the Greenland Climate Network (GCN) weather stations and the floating ice station Tara for polar summer 2007. The AVHRR dataset consists of 2755 overpasses. The overpasses are matched to in situ observations spatially and temporally. The SAL algorithm presented here derives the surface broadband albedo from AVHRR channels 1 and 2 using an atmospheric correction, temporal sampling of an empirical Bidirectional Reflectance Distribution Function (BRDF), and a narrow-to-broadband conversion algorithm. The satellite product contains algorithms for snow, sea ice, vegetation, bare soil, and water albedo. At the Summit and DYE-2 stations on the Greenland ice sheet, instantaneous SAL RMSE is 0.073. The heterogeneous surface conditions at satellite pixel scale over the stations near the Greenland west coast increase RMSE to > 0.12. Over Tara, the instantaneous SAL RMSE is 0.069. The BRDF sampling approach reduces RMSE over the ice sheet to 0.053, and to 0.045 over Tara. Taking into account various sources of uncertainty for both satellite retrievals and in situ observations, we conclude that SAL agrees with in situ observations within their limits of accuracy and spatial representativeness.     

Overlap-freeness in infinite partial words

We prove that there exist infinitely many infinite overlap-free binary partial words containing at least one hole. Moreover, we show that these words cannot contain more than one hole and the only hole must occur either in the first or in the second position. We define that a partial word is $k$-overlap-free if it does not contain a factor of the form $xyxyx$ where the length of $x$ is at least $k$. We prove that there exist infinitely many 2-overlap-free binary partial words containing an infinite number of holes.     

Extension of the decidability of the marked PCP to instances with unique blocks

In the Post Correspondence Problem (PCP) an instance (h,g)$(h,g)$ consists of two morphisms h$h$ and g$g$, and the problem is to determine whether or not there exists a nonempty word w$w$ such that h(w)=g(w)$h\left(w\right)=g\left(w\right)$. Here we prove that the PCP is decidable for instances with unique blocks using the decidability of the marked PCP. Also, we show that it is decidable whether an instance satisfying the uniqueness condition for continuations has an infinite solution. These results establish a new and larger class of decidable instances of the PCP, including the class of marked instances.     

BOAR: an advanced HW/SW coemulation environment for DSP system development

The BOAR emulation system is targeted to hardware/software (HW/SW) codevelopment of advanced embedded DSP and telecom systems. The challenge of the BOAR system is efficient customization of programmable hardware, and dedicated partitioning routine to target applications and structures, which allows quite high overall system performance. The system allows multiple configurations for communication between processors and field programmable gate arrays (FPGAs) making the BOAR system an efficient tool for real-time HW/SW coverification. The reprogrammable hardware of the emulation tool is based on four Xilinx 4000-series devices, two Texas TMS320C50 signal processors and one Motorola MC68302 microcontroller. With current devices the BOAR hardware provides approximately 40–70 kgates of logic capacity in DSP applications. The emulation capacity can be expanded by connecting several similar boards in chain. The system has also a versatile internal reprogrammable test environment for test bench development, performance evaluations and design debugging. The logic development environment is based on the Synopsys synthesis tools and an automatic design management software, which performs resource mapping and performance-driven design partitioning between FPGAs. The emulation hardware is currently connected to logic and software development environments via an RS-232C bus. The BOAR emulation system has been found a very efficient platform for real-life prototyping of different types of DSP algorithms and systems, and validating correct functionality of a VHDL macro library.     

The Strategic Incrementalism of Lahti Master Planning: Three Lessons

ABSTRACT

The city of Lahti, Finland, has developed a unique policy of combining city strategy work with strategic master planning in an iterative process. It thereby offers insights to research on strategic spatial planning, exemplifying how institutional frameworks of statutory planning can be utilized as resources in strategic planning. Three lessons from the Lahti case are drawn: (1) utilize the moments of opportunity in the institutional environment of statutory planning, (2) shift the focus from the level of ‘strategic plans’ to the policy level of strategy work, (3) develop strategic planning as a platform for diverse ‘languages’.