Modeling of Early-Age Creep of Shotcrete. I: Model and Model Parameters

In this paper, creep and viscous flow are revisited from the standpoint of constitutive modeling of thermo-chemo-mechanical couplings in early-age concrete. Within the framework of closed reactive porous media, creep is modeled by means of two mechanisms: a stress-induced water movement within the macropores and a relaxation mechanism in the micropores of cement gel, both of which lead to aging effects on creep and viscous flow of concrete. Regarding the first creep mechanism, aging results from chemomechanical couplings. Concerning the second mechanism, long-term aging is attributed to the relaxation of microprestresses in the micropores. Following the formulation of the model, it is shown how the material parameters can be identified from creep tests performed at different ages of loading. Finally, the model is applied to shotcrete, for which proper experimental data are missing.     

A superconducting wavelength shifter as primary radiometric source standard in the X-ray range

For more than 20 years, the Physikalisch-Technische Bundesanstalt (PTB) has been using the calculable radiation of bending magnets from the BESSY I and BESSY II electron storage rings in the visible, UV, vacuum-UV (VUV) and X-ray spectral range for radiometry, especially for the calibration of radiation sources and energy-dispersive detectors. Due to its—compared to bending magnets—higher magnetic field, wavelength shifters (WLS) have the potential to extend the usable spectral range for these applications to higher photon energies. Thus, the characteristic energies of BESSY II bending magnet radiation and a 6 T WLS radiation are 2.5 and 11.5 keV, respectively. Within the scope of this work, the properties of the synchrotron radiation from the 6 T WLS have been investigated and compared to theoretical predictions for photon energies up to 150 keV. Good agreement within the experimental uncertainty of several percent was found. Further improvements for a future radiometric use of WLS radiation with low uncertainties will be discussed.     

Report on ONR Workshop on Fracture Scaling

The paper reports on the discussions at the ONR Workshop on Fracture Scaling, held at University of Maryland in June 1999, under the chairmanship of Z.P. Baant and Y.D.S. Rajapakse. The workshop dealt with size effects in structural failure and scale bridging in mechanics of materials. The lectures at the Workshop were published in Volume 95 of this Journal. The objective of this paper is to present records and interpretations of the extensive discussions prepared by invited specialists. The records show which are the areas of disagreement among leading researchers and which are those where consensus has been reached.     

Ad Hoc File Systems for High-Performance Computing

Storage backends of parallel compute clusters are still based mostly on magnetic disks,while newer and faster storage technologies such as flash-based SSDs or non-volatile random access memory(NVRAM)are deployed within compute nodes.Including these new storage technologies into scientific workflows is unfortunately today a mostly manual task,and most scientists therefore do not take advantage of the faster storage media.One approach to systematically include nodelocal SSDs or NVRAMs into scientific workflows is to deploy ad hoc file systems over a set of compute nodes,which serve as temporary storage systems for single applications or longer-running campaigns.This paper presents results from the Dagstuhl Seminar 17202"Challenges and Opportunities of User-Level File Systems for HPC"and discusses application scenarios as well as design strategies for ad hoc file systems using node-local storage media.The discussion includes open research questions,such as how to couple ad hoc file systems with the batch scheduling environment and how to schedule stage-in and stage-out processes of data between the storage backend and the ad hoc file systems.Also presented are strategies to build ad hoc file systems by using reusable components for networking and how to improve storage device compatibility.Various interfaces and semantics are presented,for example those used by the three ad hoc file systems BeeOND,GekkoFS,and BurstFS.Their presentation covers a range from file systems running in production to cutting-edge research focusing on reaching the performance limits of the underlying devices.     

Biological Structures Mitigate Catastrophic Fracture Through Various Strategies

Gao et al. (PNAS, 100, 5597–5600 (2003)) have argued that load-bearing mineralized hard tissues, including bones, shells, and teeth, are nanocomposites, in which the mineral phase has nanoscale dimensions that ensure optimum strength and flaw tolerance. In particular, it has been claimed that the thickness of these brittle building blocks, being smaller than a critical size, h^*, of the order of tens of nanometers, renders them insensitive to the presence of crack-like flaws and enables them to achieve near-theoretical strength, which is why Nature employs nanoscale features in mineralized biological composites. We find this point of view, which Gao et al. and others have quoted in subsequent publications and presentations, unpersuasive and present several counterexamples which show that biological structures, as a result of being comprised of relatively fragile constituents that fracture at stress levels several orders of magnitude smaller than the theoretical strength, adopt various strategies to develop mechanical responses that enable them to mitigate catastrophic failure. Nanoscale structural features are not a result of an innate resistance to very high stresses.     

Two-Phase Composite Model for High Performance Cementitious Composites

This paper presents a new constitutive model for fiber reinforced cementitious composite materials (FRCC), which is particularly suitable for High Performance Cementitious Composites (HP2C). The model is a two-phase composite model, one phase presenting the matrix, the other the composite fibers. In addition, the matrix–fiber interaction is taken into account as internal cross effects (i.e., thermodynamic couplings) between the irreversible deformations of the composite constituents. From one-dimensional thermodynamics, the partial stresses in the matrix and fibers are derived as thermodynamic forces associated with the irreversible deformations of matrix and fibers, respectively. Next, the identification of the model parameters from tensile data is detailed. In particular, it is shown that the model allows quantification of the ductility enhancement of HP2C in comparison with ordinary FRCC, through two material parameters derived from the constitutive model: a matrix–fiber coupling modulus and a friction-to-fracture strength ratio, to which we refer as ductility ratio. The physical significance of these model parameters is discussed in the context of micromechanical theory. The coupling modulus is found to depend mainly on the fiber volume fraction and the matrix quality. The ductility ratio depends on four material design parameters, which are intrinsic to the matter, and which are not affected by structural size effects. We conclude that any ductility gain, which can be obtained through the use of HP2C, is only related to the mix design, and not to structural dimensions.     

Prenatal diagnosis of a der(X)t(X;15)(p22.2;q11.2) inherited from a maternal translocation X;15

During a 15-y period, 48 children were treated for Wilms' tumour (WT). Seven of them were < 1 y of age at diagnosis. One child presented with non-traumatic haematuria, but in all the other children WT was revealed as a palpable abdominal mass at routine examination or investigation due to another disease. The four children under 6 months of age at diagnosis were primarily operated upon; the others received preoperative chemotherapy. Two children had chromosomal aberrations in the WT tumour specimen. The follow-up revealed that postoperatively six children are healthy with no evidence of relapse from WT, but one child had a contralateral relapse successfully enucleated. The clinical behaviour and management of WT in infants differ compared with that in older children. The diagnosis may be uncertain and it can be difficult to distinguish between malignant and non-malignant lesions. It is essential to realize the possibility of WT, even in children < 1 y of age.