effect of strain rate on brittle facture.

Plastic Strain is the result of dislocation movement and/or twinning. Brittle fracture occours instead of deformation. Now think: Would you think that the stress necessary for plastic deformation increases or decreases with increasing strain rate? What will happen, when the strain rate increases more and more?

Michael Dahms

thanks for your attaention I know that by increasing strain rate the movement of dislocation should be increased,but they themselves prevent their movement because of their transverse of stress field,.. thus by these difficulties slip and deformation should be decreased,so the yield point of metal should be increaed,but my question is why this cuase brittle fracture and how does it explain by formulas, because I have a ceminerate about it that it should be takes 45 minutes and this is not enough. thanks again.

If the stress for dislocation movement is greater than the cleavage stress, brittle fracture will occur.

Michael Dahms

I saw some women on some of the roller coasters at Busch Gardens (I live about 2 km from the park) that had some serious cleavage stress.

In cases where there is no plastic, viscous, or creep flow it is often the case that increasing strain rate increases fracture stress because of stress corrosion effects.

This is because slow strain rates give lots of time for corrosive chemicals (like water vapor) to attack a stressed flaw tip.

There are also a few counterexamples where chemical corrosion tends to broaden rather than sharpen crack tips in which case slow strain rate gives higher fracture stress.

ok thank u, I find sth in this book" mechanical behavior of metals at extremely high strain rates" by j.buchar,z.bilek,f.dusek. It's about high velocity deformation of metals,but I don't know that in high strain rate what's happen to metal that it causes fracture because fracture need cracks and in this book talking about shock wave in high strain rate,but I still don't know how cracks initiate that high strain rate causes brittle fracture. thank u very much.

Google 'brittle crack initiation' >5000 hits.

HTH

Michael Dahms