Uncovering the creep deformation mechanism of rock-forming minerals using nanoindentation

The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood.Although laboratory creep tests have been carried out to determine the creep deformation of various rocks,these tests are expensive and time-consuming.Nanoindentation creep tests,as an alternative method,can be performed to investigate the mechanical and viscoelastic properties of granite samples.In this study,the reduced Young’s modulus,hardness,fracture toughness,creep strain rate,stress exponent,activation volume and maximum creep displacement of common rock-forming minerals of granite were calculated from nanoindentation results.It was found that the hardness decreases with the increase of holding time and the initial decrease in hardness was swift,and then it decreased slowly.The stress exponent values obtained were in the range from 4.5 to 22.9,which indicates that dislocation climb is the creep deformation mechanism.In addition,fracture toughness of granite’s rock-forming minerals was calculated using energy-based method and homogenization method was adopted to upscale the micro-scale mechanical properties to macro-scale mechanical properties.Last but not least,both three-element Voigt model and Burgers model fit the nanoindentation creep curves well.This study is beneficial to the understanding of the long-term mechanical properties of rock samples from a microscale perspective,which is of great significance to the understanding of localized deformation processes of rocks.

Methodology for credibility assessment of historical global LUCC datasets

Land use-induced land cover change(LUCC)is an important anthropogenic driving force of global change that has influenced,and is still influencing,many aspects of regional and global environments.Accurate historical global land use/cover datasets are essential for a better understanding of the impacts of LUCC on global change.However,there are not only evident inconsistencies in current historical global land use/cover datasets,but inaccuracies in the data in these global dataset revealed by historical record-based reconstructed regional data throughout the world.A focus in historical LUCC and global change research relates to how the accuracy of historical global land cover datasets can be improved.A methodology for assessing the credibility of existing historical global land cover datasets that addresses temporal as well as spatial changes in the amount and distribution of land cover is therefore needed.Theoretically,the credibility of a global land cover dataset could be assessed by comparing similarities or differences in the data according to actual land cover data(the"true value").However,it is extremely difficult to obtain historical evidence for assessing the credibility of historical global land cover datasets,which cannot be verified through field sampling like contemporary global land cover datasets.We proposed a methodological framework for assessing the credibility of global land cover datasets.Considering the types and characteristics of the available evidence used for assessments,we outlined four methodological approaches:(1)accuracy assessment based on regional quantitative reconstructed land cover data,(2)rationality assessment based on regional historical facts,(3)rationality assessment based on expertise,and(4)likelihood assessment based on the consistency of multiple datasets.These methods were illustrated through five case studies of credibility assessments of historical cropland cover data.This framework can also be applied in assessments of other land cover types,such as forest and grassland.