Strain rate and temperature dependent strain localization of dynamically stretched lightweight bars: from metal to polymer

This work studies the dynamic strain localization and constitutive relationship of a Ti3Al2.5V alloy in jet engine containment system and a transparent polycarbonate conceived for aircraft canopy application by Digital Image Correlation (DIC) technique from quasi-static condition to high strain rates at different temperatures. The responses of two materials show significant strain rate and temperature sensitivities. Observations of Ti3Al2.5V alloy show that the dynamic local strain rate can reach values up to 1000 % of the nominal strain rate in the necking zone. However, dynamic local strain rate of polycarbonate soars up during strain softening then decreases rapidly with necking propagation, and eventually becomes 20 % of the nominal strain rate until fracture. Appropriate viscoplastic constitutive models are determined for both materials, which are incorporated in finite element simulations to reveal the trend of dynamic local strain rate evolution in dynamic tensile tests. The present work shows two different kinds of strain localization in typical lightweight materials, which should be addressed carefully from Split Hopkinson Tension Bar (SHTB) tests.

Scaling in the Local Strain-Rate Field during Jerky Flow in an Al-3%Mg Alloy

Jerky flow in alloys, or the Portevin-Le Chatelier effect, presents an outstanding example of self-organization phenomena in plasticity. Recent acoustic emission investigations revealed that its microscopic dynamics is governed by scale invariance manifested as power-law statistics of intermittent events. As the macroscopic stress serrations show both scale invariance and characteristic scales, the micro-macro transition is an intricate question requiring an assessment of intermediate behaviors. The first attempt of such an investigation is undertaken in the present paper by virtue of a one-dimensional (1D) local extensometry technique and statistical analysis of time series. The data obtained complete the missing link and bear evidence to a coexistence of characteristic large events and power laws for smaller events. The scale separation is interpreted in terms of the phenomena of self-organized criticality and synchronization in complex systems. Furthermore, it is found that both the stress serrations and local strain-rate bursts agree with the so-called fluctuation scaling related to general mathematical laws and unifying various specific mechanisms proposed to explain scale invariance in diverse systems. Prospects of further investigations including the duality manifested by a wavy spatial organization of the local bursts of plastic deformation are discussed.

Local tectonic deformations measured by extensometer at the eastern foothills of the Alps at the Sopronbánfalva Geodynamic Observatory, Hungary

In Hungary, at the foot of the Eastern Alps, in the Sopronbánfalva Geodynamic Observatory (SGO), a quartz-tube extensometer has been used for recording the Earth’s tides and local tectonic deformations since 1991. The 27-year long strain record (1991–2017) shows a continuous compression of the rock with changing rate. The relations between the measured local deformation and present-day tectonics in the region of the observatory were investigated. The local strain rate variations were also compared with the temporal and spatial distribution as well as with the magnitudes of earthquakes occurred within 200 km from the observatory in two sectors around the azimuth of the extensometer (116°): 116°±15° and 296°±15°. Our investigations show that earthquakes can also influence the strain rate. Earthquakes to the west of SGO generally increase the compressive strain rate, while earthquakes in the Pannonian Basin, with some exceptions, have no significant effect on the local strain rate variations measured in the SGO. It has been found that the recorded compressive strain is in good accordance with the recent tectonic processes in the region of the SGO determined by Global Navigation Satellite System (GNSS) technology and geophysical measurements. From the results it can be concluded that the uplift of the Alps, tectonic processes in the East Alpine region and in the Pannonian Basin play the most important role in the changing local compressive strain rate.

Correlation of Fractographic Examinations With Numerical Calculations Regarding Dynamic Fracture

In practice, cleavage fracture assessment for ferritic reactor pressure vessel steels is standardized by the quasi-static Master Curve concept (ASTM E 1921) on a macroscopic level. This standard is technically valid for dynamic loading conditions, yet recent work has shown that experimentally determined fracture toughness values under rapid loading lead to discrepancies regarding the shape of the Master Curve. It is assumed that one reason for these discrepancies is the profound adiabatic heating. The investigation of this, as well as related phenomena, is subject of this work. Regarding this work, these experimental discrepancies are specified by a wide range of experiments. Moreover, fractographic investigations were performed on this experimental database to determine the exact origin of cleavage fracture, and compared to data from quasi-static experiments. In a second step numerical simulations were conducted for various crack-tip loading rates and testing temperatures, whereupon the temperature development in the cleavage fracture relevant region was quantified. This also allowed a determination of the individual initiation temperatures, which were analyzed and discussed whilst also taking into account the increase in local strain rate at the origin of cleavage fracture. Finally, fractographic examinations also revealed the relevance of local crack arrest (crack stop) which was quantified, and also linked to the mentioned experimental discrepancies regarding the Master Curve concept.