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SUMMARY:Hydrogen induced fast fracture  -   Profs Gábor Csányi  and Vikr
 am Deshpande (University of Cambridge)
DTSTART:20191011T130000Z
DTEND:20191011T140000Z
UID:TALK129352@talks.cam.ac.uk
CONTACT:Hilde Hambro
DESCRIPTION:One of the recurring anomalies in the hydrogen induced fractur
 e of high strength steels is the apparent disconnect between the toughness
  and tensile strength. For example\, the toughness of a high strength stee
 l is typically reduced from approximately 100 MPa√m to about 20 MPa√m 
 in the presence of hydrogen while concurrently the strength reduces from 2
  GPa to about 400 MPa. Traditional fracture mechanics then suggests that q
 uasi-brittle fracture under uniaxial tension occurred by the growth of a p
 re-existing flaw of size ≈1600 μm. There is no evidence of the presence
  of such large pre-existing flaws in high quality steels. This raises the 
 question as to what is the hydrogen-mediated fracture process that reduces
  the strength of such steels? \n\nHere we propose\, supported by detailed 
 atomistic and continuum calculations\, that unlike macroscopic toughness\,
  hydrogen-mediated tensile failure is a result of a fast-fracture mechanis
 m. Specifically\, we show that failure originates from the fast propagatio
 n of cleavage cracks that initiate from cavities that form around inclusio
 ns such as carbide particles. The failure process occurs in two stages. In
  stage-A\, hydrides rapidly form around the roots of stressed notches on t
 he cavity surfaces with hydrogen fed from the hydrogen gas within the cavi
 ty. These hydrides promote cleavage fracture with the cracks propagating a
 t >100 ms^(-1) until the hydrogen gas in the cavity is exhausted. Predicti
 ons of this hydrogen-assisted crack growth mechanism are supported by atom
 istic calculations of binding energies\, mobility barriers and molecular d
 ynamics calculations of the fracture process. Typically\, cracks grow by l
 ess than 1 μm via this hydrogen-assisted mechanism and thus insufficient 
 to cause macroscopic fracture of the specimen. However\, this stage is the
 n followed by a stage-B process where these fast propagating cracks can co
 ntinue to grow\, now in the absence of hydrogen supply\, given an appropri
 ate level of remote tensile stress. This is surprising because the fractur
 e energy is now that of Fe in the absence of H and cleavage fracture requi
 res opening tractions on the order of 15 GPa to be generated. Thus\, fract
 ure is usually precluded due to plasticity around the crack-tip. Here we s
 how via macroscopic continuum crack growth calculations in a rate dependen
 t elastic-plastic solid with fracture modelled using a cohesive zone that 
 cleavage is possible if the crack propagates fast enough. This is because 
 strain-rates at the tips of fast propagating cracks are sufficiently high 
 for the drag on the motion of dislocations resulting from phonon scatterin
 g to limit plasticity. This combined atomistic/continuum model is used to 
 explain a host of well-established experimental observations including (bu
 t not limited to): (i) insensitivity of the strength to the concentration 
 of trapped hydrogen\; (ii) the extensive microcracking in addition to the 
 final cleavage fracture event and (iii) the higher susceptibility of high 
 strength steels to hydrogen embrittlement. \n
LOCATION:Oatley Seminar Room\, Department of Engineering
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