The high-speed deformation behavior of a commercially available dual phase steel composition was studied by means of split Hopkinson bar testing in shear punch mode. The as-received cold-rolled material was subjected to a variety of heat treatment conditions to produce several different microstructures, namely ferrite plus pearlite, ferrite plus bainite and/or acicular ferrite, ferrite plus bainite and martensite, and ferrite plus different fractions of martensite. A strong effect of microstructure on both static and dynamic properties and on the dynamic factor (ratio of dynamic to static strength) was observed. According to the variation of dynamic factor with static strength, three groups of microstructures with three distinct behaviors were identified. Under the same static strength level, the classic dual phase microstructure was found to have the highest value of dynamic factor and therefore can absorb more dynamic energy than other microstructures.

Effect of microstructure on the dynamic deformation behavior of dual phase steel

The high strain rate properties of high strength steels with various microstructures and static strength levels were studied by means of split Hopkinson bar apparatus in shear punch mode. The as received cold rolled sheet steels were subjected to a variety of heat treatment conditions to produce several different microstructures, namely ferrite plus pearlite (F+P), ferrite plus bainite (F+B), ferrite plus martensite (F+M) and ferrite plus bainite and retained austenite (F+B+RA). According to the variation of dynamic factor (ratio of dynamic to static strength) with static strength, two groups of microstructures with two distinct behaviours were identified, i.e. classic dual phase (ferrite plus martensite) and multiphase (including ferrite–pearlite, ferrite–bainite, etc.). It was also observed that the general dependence of microstructure on the dynamic factor was strongly influenced by chemical composition in the case of ferrite plus martensite microstructures.

Effect of microstructure and chemical composition on dynamic factor of high strength steels

Computational modeling of the drop-weight tear test: A comparison of two failure modeling approaches

xxxvii I Literature Review 1 1 Fracture Control 3 1.1 Fracture Initiation Control 6 1.2 Fracture Propagation Control 14 1.3 Material Testing 42 1.4 Summary 51 2 Separations 53 2.1 Origins and Consequences of Separations 59 2.2 Effect on Stress State 74 2.3 Modified Charpy Specimens Evaluating the Effect of Separations 79 2.4 Rising Upper-Shelf Phenomenon 86 2.5 Characterizing Separation Severity 88 2.6 Summary 97 3 Numerical Modeling 101 3.1 Cohesive Zone Model 101

The effect of separations on the assessment of Charpy impact tests

This paper reports results of tests on flow and ductile fracture of a very high toughness steel with Charpy V-notch absorbed energy (CVN energy) at room temperature of 471 J. The microstructure of the steel is bainite/ferrite and its strength is equivalent to X80 grade. The flow stress was determined using tensile tests at temperatures between 150°C and −147°C and strain rates of 0.00075, 0.02 and 1 s−1 , and was fitted to a proposed constitutive equation. Charpy tests were carried out at an initial impact velocity of 5.1 ms−1 using drop-weight machines (maximum capacity of 842 J and 4029 J). The samples were not broken during the test, i.e. they passed through the anvils after significant bending deformation with only limited crack growth. Most of the absorbed energy was due to deformation. There was little effect of excess energy on absorbed energy up to 80% of machine capacity (i.e. the validity limit of ASTM E 23). As an alternative to the CVN energy, the crack tip opening angle (CTOA) measured using the drop-weight tear test (DWTT) has been proposed as a material parameter to characterize crack propagation resistance. Preliminary work on evaluating CTOA using the two-specimen CTOA test method is presented. The initiation energy is eliminated by using statically precracked test specimens. Account is taken of the geometry change of the specimens (e.g. thickening under the hammer) on the rotation factor and of the effect of strain rate on flow stress.Copyright © 2004 by ASME

W. R. Tyson

Papers

Constitutive equation for structural steels