Three “directions” of stress Compression Tension Shear Materials may react differently to different stresses
Two Deformation Regimes BRITTLE Cold hard crust Fracture Permanently deformed (once broken) DUCTILE Hot soft crust Flow Permanent (plastic) or recoverable (elastic) These are End Members! Look for simultaneous or overlapping effects of these regimes.
What determines whether brittle or ductile mechanisms will prevail? Temperature Rate Material properties Stress heterogeneity Material heterogeneity Confining pressure Other factors…..
Brittle Deformation Shape change by breaking rather than flowing Manifests as fractures - faults and joints Occurs when rate of deformation exceeds rate at which material can deform plastically Occurs when deformation exceeds elastic threshold of material
Ductile Deformation Shape change through flowing (on km down to atomic scales) rather than breaking Manifests as flow patterns, coherent shape/size change Occurs when material can flow fast enough to keep up with deformation applied to it.
Features of Brittle Structures Hard, broken edges Cracks, grain breakage (=cataclasis) NON recoverable Sometimes Systematic, ie. regular spacing of joints May begin with small cracks which line up to form one major crack Dilation or compaction common in granular or broken materials Volume/ layer thickness conserved
Features of Ductile Structures May be fluidy-looking, marbled Usually asystematic Volume change or re-distribution (ie. thickening in the hinge of a fold) May be layered or banded Usually smooth countours Ductile material may flow around brittle grains (= augen) Ancient structures - non recoverable
Slide show… As each image appears, please yell out “BRITTLE” or “DUCTILE” or “BOTH”