The nonlinear behaviour of epoxy resins is studied on standard tensile tests. A strain field measurement system is applied (Aramis) in order to monitor local strains. The residual strain is measured ...by recovering the specimens for up to 68 hours after unloading. The time span the specimen is exposed to load has a large influence on the creeping process and the residual strain after recovering. This is studied by comparison of instantaneous unloading with keeping the specimen under permanent load for thirty minutes. It is shown that moderate differences in the initial strain can lead to large differences in the creep behaviour as well as in the residual strain.
As a result of the high stress concentrations at the fiber entry point, a failure-stress analysis of pull-out test data is problematic. As an alternative, the energy release rate is determined by a ...fracture-mechanics-based analysis in which the interfacial debonding process is studied with the aid of finite elements. The energy release rate is calculated for glass and carbon fibers embedded in a thermoplastic matrix. The influences of thermally induced stresses and of interfacial friction are analysed. It turns out that the thermal stresses result in a large amount of stored strain energy, leading to a remarkable increase of the energy release rate. The dependence of the crack stability on the compliance of the specimen and the testing device, and on the level of interfacial friction has been studied. Furthermore, the experimental findings concerning the change of the force/displacement trace indicating the onset of crack propagation are confirmed.
Widely-used methods for characterising the fibre/matrix interface in polymeric composites are the fragmentation test and the droplet test as a special kind of the single-fibre pull-out test. A severe ...disadvantage of these tests is that non-realistic model samples are investigated which contain only one fibre in the matrix. In order to obtain data about the effect of the different residual stress situations for fibres in such samples and in composites, pull-out tests of E-glass fibres in polystyrene and polycarbonate are performed using samples, where the investigated fibre is surrounded by 0 to 3 other near fibres. Neighbouring fibres can increase the pull-out forces by a factor of three and the interfacial toughness by a factor of four. This has to be taken into account, if the tests are performed not only for comparison reasons but for measuring interface properties.
As a result of the high stress concentrations at the fiber entry point, a failure-stress analysis of pull-out test data is problematic. As an alternative, the enregy release rate is determined by a ...fracture-mechanics-based analysis in which the interfacial debonding process is studied with the aid of finite elements. The energy release rate is calculated for glass and carbon fibers embedded in a thermoplastic matrix. The influences of thermally induced stresses and of interfacial friction are analyzed. It turns out that the thermal stresses result in a large amount of sorted strain energy, leading to a remarkable increase of the energy release rate. The dependence of the crack stability on the compliance of the specimen and the testing device, and on the level of interfacial friction has been studied. Furthermore, the experimental findings concerning the change of the force /displacement trace indicating the onset of crack propagation are confirmed. Materials discussed include glass fiber and carbon fiber reinforced polycarbonate.
Partitioning uncertainty in projections of future climate change
into contributions from internal variability, model response uncertainty
and emissions scenarios has historically relied on making ...assumptions about
forced changes in the mean and variability. With the advent of multiple
single-model initial-condition large ensembles (SMILEs), these assumptions
can be scrutinized, as they allow a more robust separation between sources
of uncertainty. Here, the framework from Hawkins and Sutton (2009) for
uncertainty partitioning is revisited for temperature and precipitation
projections using seven SMILEs and the Coupled Model Intercomparison Project CMIP5 and CMIP6 archives. The original approach is shown to work
well at global scales (potential method bias < 20 %), while at
local to regional scales such as British Isles temperature or Sahel
precipitation, there is a notable potential method bias (up to 50 %), and
more accurate partitioning of uncertainty is achieved through the use of
SMILEs. Whenever internal variability and forced changes therein are
important, the need to evaluate and improve the representation of
variability in models is evident. The available SMILEs are shown to be a
good representation of the CMIP5 model diversity in many situations, making
them a useful tool for interpreting CMIP5. CMIP6 often shows larger absolute
and relative model uncertainty than CMIP5, although part of this difference
can be reconciled with the higher average transient climate response in
CMIP6. This study demonstrates the added value of a collection of SMILEs for
quantifying and diagnosing uncertainty in climate projections.
Continuous estimates of the oceanic meridional heat transport in the Atlantic are derived from the Rapid Climate Change–Meridional Overturning Circulation (MOC) and Heatflux Array ...(RAPID–MOCHA)observing system deployed along 26.5°N, for the period from April 2004 to October 2007. The basinwide meridional heat transport (MHT) is derived by combining temperature transports (relative to a common reference) from 1) the Gulf Stream in the Straits of Florida; 2) the western boundary region offshore of Abaco, Bahamas; 3) the Ekman layer derived from Quick Scatterometer (QuikSCAT) wind stresses; and 4) the interior ocean monitored by “endpoint” dynamic height moorings. The interior eddy heat transport arising from spatial covariance of the velocity and temperature fields is estimated independently from repeat hydrographic and expendable bathythermograph (XBT) sections and can also be approximated by the array.
The results for the 3.5 yr of data thus far available show a mean MHT of 1.33 ± 0.40 PW for 10-day-averaged estimates, on which time scale a basinwide mass balance can be reasonably assumed. The associated MOC strength and variability is 18.5 ± 4.9 Sv (1 Sv ≡ 10⁶ m³ s−1). The continuous heat transport estimates range from a minimum of 0.2 to a maximum of 2.5 PW, with approximately half of the variance caused by Ekman transport changes and half caused by changes in the geostrophic circulation. The data suggest a seasonal cycle of the MHT with a maximum in summer (July–September) and minimum in late winter (March–April), with an annual range of 0.6 PW. A breakdown of the MHT into “overturning” and “gyre” components shows that the overturning component carries 88% of the total heat transport. The overall uncertainty of the annual mean MHT for the 3.5-yr record is 0.14 PW or about 10% of the mean value.
The Atlantic meridional overturning circulation (AMOC) makes the strongest oceanic contribution to the meridional redistribution of heat. Here, an observation-based, 48-month-long time series of the ...vertical structure and strength of the AMOC at 26.5°N is presented. From April 2004 to April 2008, the AMOC had a mean strength of 18.7 ± 2.1 Sv (1 Sv ≡ 10⁶ m³ s−1)with fluctuations of 4.8 Sv rms. The best guess of the peak-to-peak amplitude of the AMOC seasonal cycle is 6.7 Sv, with a maximum strength in autumn and a minimum in spring. While seasonality in the AMOC was commonly thought to be dominated by the northward Ekman transport, this study reveals that fluctuations of the geostrophic midocean and Gulf Stream transports of 2.2 and 1.7 Sv rms, respectively, are substantially larger than those of the Ekman component (1.2 Sv rms). A simple model based on linear dynamics suggests that the seasonal cycle is dominated by wind stress curl forcing at the eastern boundary of the Atlantic. Seasonal geostrophic AMOC anomalies might represent an important and previously underestimated component of meridional transport and storage of heat in the subtropical North Atlantic. There is evidence that the seasonal cycle observed here is representative of much longer intervals. Previously, hydrographic snapshot estimates between 1957 and 2004 had suggested a long-term decline of the AMOC by 8 Sv. This study suggests that aliasing of seasonal AMOC anomalies might have accounted for a large part of the inferred slowdown.
The rate of global surface warming is crucial for tracking progress towards global climate targets, but is strongly influenced by interannual-to-decadal variability, which precludes rapid detection ...of the temperature response to emission mitigation. Here we use a physics based Green's function approach to filter out modulations to global mean surface temperature from sea-surface temperature (SST) patterns, and show that it results in an earlier emergence of a response to strong emissions mitigation. For observed temperatures, we find a filtered 2011-2020 surface warming rate of 0.24 °C per decade, consistent with long-term trends. Unfiltered observations show 0.35 °C per decade, partly due to the El Nino of 2015-2016. Pattern filtered warming rates can become a strong tool for the climate community to inform policy makers and stakeholder communities about the ongoing and expected climate responses to emission reductions, provided an effort is made to improve and validate standardized Green's functions.
We introduce an improved initialization to the decadal predictions performed for the Mittelfristige Klimaprognosen (MiKlip) project based on the Max‐Planck‐Institute Earth System Model and ...furthermore test the effect of increased ocean and atmosphere model resolutions. The new initialization includes both a more sophisticated oceanic initialization and additionally an atmospheric initialization. We compare the performance of retrospective decadal forecasts over the past 50 years with that of the previous system. The new oceanic initialization considerably improves the performance in terms of surface air temperature over the tropical oceans on the 2–5 years time scale, which also helps to improve the predictive skill of global mean surface air temperature on this time scale. The higher model resolution improves the predictive skill of surface air temperature over the tropical Pacific even further. Through the newly introduced atmospheric initialization, the quasi‐biennial oscillation exhibits predictive skill of up to 4 years when a sufficiently high vertical atmospheric resolution is used.
Key Points
The new initialization improves temperature predictions in the tropics
A higher model resolution leads to further improvements in the tropical Pacific
The quasi‐biennial oscillation exhibits prediction skill of up to 4 years
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