On Strength at Yield in Condensed Matter
- PDF / 520,816 Bytes
- 7 Pages / 593.972 x 792 pts Page_size
- 73 Downloads / 257 Views
DUCTION
THE challenge of engineering in demanding environments is to design materials and structures capable of surviving high mechanical loads.[1,2] To achieve this requires materials that possess, or are designed to possess a high strength under compression. Theoretical prediction of the strength of matter has been studied for the last century and has considered loading with static and dynamic impulses in order to determine limiting material thresholds.[3–5] Natural environments exist in which structures experience a range of loads from quasistatic to shock such as those subject to volcanic events, tsunami or earthquake but are also found in devices which counter impulsive loads considered across impact engineering.[6–8] Whatever the cause, the strength of solids is controlled by the energy needed to break a bond under mechanical impulse and these values are different in hydrodynamic compression and in static or dynamic shear. It is maximum shear strength that determines the generation, propagation, and interaction of defects under load and the time taken for a material to attain a steady stress state is determined by the deformation mechanisms accessed within the solid as it slips.[9] The strength of metals has been a prime focus for research since the industrial revolution (ca. 1800). Soon simple rate-independent plasticity models were developed to describe behavior and a defined yield strength was a vital component of these. However, a concern was the speed, or the rate, at which a material was loaded NEIL K. BOURNE, Fellow of the American Physical Society, Director of the Centre for Matter under Extreme Conditions, is with the School of Materials, The University of Manchester, Research Complex at Harwell, Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0FA, U.K. Contact e-mail: [email protected] Manuscript submitted May 5, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
and how that affected the resistance to shear. Experiments showed that yield strength was found to vary with the speed of loading. This was represented by the term strain-rate that describes the strain attained over the time taken to do so within a component. The latter is taken to be that required to equilibrate stress over the volume loaded and it represents a process occurring in the head of a loading pulse before the state steadies to a final attained strain (stress-state equilibrium). Key in the use of the term as an experimental descriptor in this manner is that local volumes are equilibrated in measureable times before a strain rate is defined. This contrasts with the mathematical use of the term that describes a state on a continuous history recorded at a particular lagrangian position. Loading pulses span a range of rise times which define length scales from the atomic to that at which structures operate. The observer will almost always be concerned with the response that results in the laboratory and thus this paper will focus on the use of strain-rate as a macroscale descriptor of stress-state equilibration in a range
Data Loading...
