Ductility (Earth science)
In Earth science, ductility refers to the capacity of a rock to deform to large strains without macroscopic fracturing.
[1] Such behavior may occur in unlithified or poorly lithified sediments, in weak materials such as halite or at greater depths in all rock types where higher temperatures promote crystal plasticity and higher confining pressures suppress brittle fracture.
In addition, when a material is behaving ductilely, it exhibits a linear stress vs strain relationship past the elastic limit.
[1] Ductile deformation is typically characterized by diffuse deformation (i.e. lacking a discrete fault plane) and on a stress-strain plot is accompanied by steady state sliding at failure, compared to the sharp stress drop observed in experiments during brittle failure.
[1] The brittle–ductile transition zone is characterized by a change in rock failure mode, at an approximate average depth of 10–15 km (~ 6.2–9.3 miles) in continental crust, below which rock becomes less likely to fracture and more likely to deform ductilely.
The depth of the material does exert an influence on the mode of deformation, but other substances, such as loose soils in the upper crust, malleable rocks, biological debris, and more are just a few examples of that which does not deform in accordance to the transition zone.
1.1, different geological formations and rocks are found in accordance to the dominant deformation process.
Mathematically, it is commonly expressed as a total quantity of elongation or a total quantity of the change in cross sectional area of a specific rock until macroscopic brittle behavior, such as fracturing, is observed.
It is important to understand that even the same type of rock or mineral may exhibit different behavior and degrees of ductility due to internal heterogeneities small scale differences between each individual sample.
External conditions include temperature, confining pressure, presence of fluids, etc.
while internal conditions include the arrangement of the crystal lattice, the chemical composition of the rock sample, the grain size of the material, etc.
In viscous deformation, stress is proportional to the strain rate, and each rock sample has its own material property called its Viscosity.
[1] In addition to rocks, biological materials such as wood, lumber, bone, etc.
can be assessed for their ductility as well, for many behave in the same manner and possess the same characteristics as abiotic Earth materials.
"[2] The study aimed to analyze the behavioral rheology of 2 wood specimens, the Sitka Spruce and Japanese Birch.
In the past, it was shown that solid wood, when subjected to compressional stresses, initially has a linear stress-strain diagram (indicative of elastic deformation) and later, under greater load, demonstrates a non-linear diagram indicative of ductile objects.
[2] Controls included moisture content in the lumber, lack of defects such as knots or grain distortions, temperature at 20 C, relative humidity at 65%, and size of the cut shapes of the wood samples.
[2] Results obtained from the experiment exhibited a linear stress-strain relationship during elastic deformation but also an unexpected non-linear relationship between stress and strain for the lumber after the elastic limit was reached, deviating from the model of plasticity theory.
First, since wood is a biological material, it was suggested that under great stress in the experiment, the crushing of cells within the sample could have been a cause for deviation from perfectly plastic behavior.
This may have also been induced by other factors like irregularities in the cellular density profile and distorted sample cutting.
[2] The conclusions of the research accurately showed that although biological materials can behave like rocks undergoing deformation, there are many other factors and variables that must be considered, making it difficult to standardize the ductility and material properties of a biological substance.
It is defined as the amount of ductile deformation a material must be able to withstand (when exposed to a stress) without brittle fracture or failure.
[4] This quantity is particularly useful in the analysis of failure of structures in response to earthquakes and seismic waves.
[4] It has been shown that earthquake aftershocks can increase the peak ductility demand with respect to the mainshocks by up to 10%.
