Some Exploratory Photoelastic Studies in Stress Wave Propagation

During the last three years the Guggenheim Aeronautical Laboratory of the California Institute of Technology (GALCIT) has been conducting a photoelastic study of stress wave propagation in solids using a high speed framing camera. This paper presents a technical description of the camera, now operating at 100,000 35 mm frames per second at one tenth microsecond exposure time for an elapsed time of approximately two milliseconds. The design capability is expected to approach a half million frames per second. This equipment has been used to record dynamic photoelastic stress fringe patterns in various specimens under impact loadings. Typical experimental records of wave propagation in cracked plates, layered media, compressed bars and beams, and cross sections of rocket heads are included in this report

Additional Exploratory Photoelastic Studies in Stress Wave Propagation

In a previous report to the sponsor, the design and description of a high speed framing camera was presented along with several film strips representing the results of a series of qualitative investigations of dynamic stress wave phenomena. These studies included crack propagation, layered media, compressed bars and beams, and cross sections of rocket heads. As part of a continuing study in these and related fields, a final report is submitted covering (1) exploratory experimental studies of shock wave propagation initiated by explosive caps and by nitrogen shock wave impingement, and (2) theoretical studies of a series of dynamic stress wave problems carried out in conjunction with the overall problem

Long-term characteristics of simulated ice deformation in the Baltic Sea (1962–2007)

[1] The North Atlantic Oscillation (NAO) index is a frequently used measure for the mean winter conditions in Northern Europe. A positive, high index is associated with strong westerlies and anomalous warm temperatures. The effects on sea ice conditions in the Baltic Sea are twofold. Warm temperatures prevent sea ice formation. If ice is present nevertheless, the strong winds can promote the formation of ice ridges which hinders ship traffic. We use an ocean-sea ice model to investigate the NAO impact on the ridged ice area fraction in the Baltic during 1962–2007. Our simulations indicate that in the northern Bothnian Bay, a high NAO index is related to an anomalous accumulation of ridges, while in the rest of the Baltic Sea, the relationship is contrary. The NAO explains locally at most only 20–25% of the ridged ice fraction interannual variability which indicates the systems complexity. However, we find high skill with local correlations around 0.8 for annually averaged ridged ice fraction reconstructed from multilinear regression using winter averaged wind extremes, surface air temperature, and sea surface temperature (SST). This suggests that the amount of ridged ice in late winter can be derived from these routinely measured quantities. In large parts of the basin, it is sufficient to use the atmospheric parameters as a predictor, while in the eastern Bothnian Bay and southern Gulf of Finland, the SST is required to reconstruct the bulk of the ridged ice fraction

A model for the consolidation of rafted sea ice

Rafting is one of the important deformation mechanisms of sea ice. This process is widespread in the north Caspian Sea, where multiple rafting produces thick sea ice features, which are a hazard to offshore operations. Here we present a one-dimensional, thermal consolidation model for rafted sea ice. We consider the consolidation between the layers of both a two-layer and a three-layer section of rafted sea ice. The rafted ice is assumed to be composed of layers of sea ice of equal thickness, separated by thin layers of ocean water. Results show that the thickness of the liquid layer reduced asymptotically with time, such that there always remained a thin saline liquid layer. We propose that when the liquid layer is equal to the surface roughness the adjacent layers can be considered consolidated. Using parameters representative of the north Caspian, the Arctic, and the Antarctic, our results show that for a choice of standard parameters it took under 15 h for two layers of rafted sea ice to consolidate. Sensitivity studies showed that the consolidation model is highly sensitive to the initial thickness of the liquid layer, the fraction of salt release during freezing, and the height of the surface asperities. We believe that further investigation of these parameters is needed before any concrete conclusions can be drawn about rate of consolidation of rafted sea ice features

The buckling of clamped shallow spherical shells under uniform pressure

In view of the wide discrepancy between previous theoretical and experimental results the problem of the buckling of clamped spherical shells under uniform external pressure is reexamined. A theoretical study is carried out to determine if asymmetrical modes participate in the snap-through process. It is shown that asymmetrical buckling does occur in a certain range of a geometric parameter, at loads which are significantly less than those predicted from symmetrical theory. Additional effects can be expected if the shell has symmetrical or asymmetrical imperfections, however, the present study considers only the perfect shell. Experiments were carried out with copper shells fabricated by an electroforming process. The initial imperfections in the test specimens were of the order of 1/10 of the thickness. The buckling loads of these shells exceeded the loads which have previously been reported by as much as a factor of two at higher values of the geometrical parameter [lamda]. Good agreement is found between theory and experiment, and with the recently published asymmetrical theory of Huang