Problems for Chapter 3

 

Constitutive Models: Relations between Stress and Strain

 

 

 

3.8.  Viscoplasticity

 

 

3.8.1.      Suppose that a uniaxial tensile specimen with length made from Aluminum can be characterized by a viscoplastic constitutive law with properties listed in Section 3.8.4.  Plot a graph showing the strain rate of the specimen as a function of stress.  Use log scales for both axes, with a stress range between 5 and 60 MPa, and show data for room temperature; 1000C, 2000C, 3000C,  4000C and 5000C.   Would you trust the predictions of the constitutive equation outside this range of temperature and stress?  Give reasons for your answer.

 

 

3.8.2.      A uniaxial tensile specimen can be idealized as an elastic-viscoplastic solid, with Young’s modulus E, and a flow potential given by

g( σ e )= ε ˙ 0 ( σ e Y ) m MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rkY=xi pgYlH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8ku c9pgc9q8qqaq=dir=f0=yqaiVgFr0xfr=xfr=xb9adbaqaaeGaciGa biaabeqaaiqabaWaaaGcbaGaam4zaiaacIcacqaHdpWCdaWgaaWcba GaamyzaaqabaGccaGGPaGaeyypa0JafqyTduMbaiaadaqhaaWcbaGa aGimaaqaaaaakmaabmaabaWaaSaaaeaacqaHdpWCdaWgaaWcbaGaam yzaaqabaaakeaacaWGzbaaaaGaayjkaiaawMcaamaaCaaaleqabaGa amyBaaaaaaa@4037@

where Y, ε ˙ 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaH1oqzgaGaamaaBaaaleaacaaIWa aabeaaaaa@355C@  and m are material properties.  The specimen is stress free at time t=0¸ and is then stretched at a constant (total) strain rate η ˙ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaH3oaAgaGaaaaa@347B@ .  

3.8.2.1.            Show that the equation governing the axial stress in the specimen can be expressed in dimensionless form as d σ ˜ d t ˜ + σ ˜ m =1 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaadaWcaaqaaiaadsgacuaHdpWCgaacaa qaaiaadsgaceWG0bGbaGaaaaGaey4kaSIafq4WdmNbaGaadaahaaWc beqaaiaad2gaaaGccqGH9aqpcaaIXaaaaa@3D20@ , where σ ˜ =(σ/Y) ( ε ˙ 0 / η ˙ ) 1/m MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaHdpWCgaacaiabg2da9iaacIcacq aHdpWCcaGGVaGaamywaiaacMcacaGGOaGafqyTduMbaiaadaWgaaWc baGaaGimaaqabaGccaGGVaGafq4TdGMbaiaacaGGPaWaaWbaaSqabe aacaaIXaGaai4laiaad2gaaaaaaa@4339@  and t ˜ =( E η ˙ t/Y ) ( ε ˙ 0 / η ˙ ) 1/m MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaaceWG0bGbaGaacqGH9aqpdaqadaqaai aadweacuaH3oaAgaGaaiaadshacaGGVaGaamywaaGaayjkaiaawMca amaabmaabaGafqyTduMbaiaadaWgaaWcbaGaaGimaaqabaGccaGGVa Gafq4TdGMbaiaaaiaawIcacaGLPaaadaahaaWcbeqaaiaaigdacaGG VaGaamyBaaaaaaa@4484@  are dimensionless measures of stress and time.

3.8.2.2.            Hence, deduce that the normalized stress σ ˜ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaHdpWCgaacaaaa@3498@  is a function only of the material parameter m and the normalized strain ε ˜ =ε(E/Y) ( ε ˙ 0 / η ˙ ) 1/m MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaH1oqzgaacaiabg2da9iabew7aLj aacIcacaWGfbGaai4laiaadMfacaGGPaGaaiikaiqbew7aLzaacaWa aSbaaSqaaiaaicdaaeqaaOGaai4laiqbeE7aOzaacaGaaiykamaaCa aaleqabaGaaGymaiaac+cacaWGTbaaaaaa@43CB@ .

3.8.2.3.            Show that during steady state creep σ ˜ =1 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaHdpWCgaacaiabg2da9iaaigdaaa a@3659@ .

3.8.2.4.            Obtain an analytical solution relating σ ˜ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaHdpWCgaacaaaa@3498@  to ε ˜ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaH1oqzgaacaaaa@347C@  for m=1.

3.8.2.5.            Obtain an analytical solution relating σ ˜ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaHdpWCgaacaaaa@3498@  to ε ˜ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaH1oqzgaacaaaa@347C@  for very large m (note that, in this limit the material behaves like an elastic-perfectly plastic, rate independent solid, with yield stress Y).

3.8.2.6.            By integrating the governing equation for σ ˜ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaHdpWCgaacaaaa@3498@  numerically, plot graphs relating σ ˜ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaHdpWCgaacaaaa@3498@  to ε ˜ MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaH1oqzgaacaaaa@347C@  for a few values of m between m=1 and m=100.

3.8.2.7.            Estimate the time, and strain, required for a tensile specimen of Aluminum to reach steady state creep at a temperature of 4000C, when deformed at a strain rate of 10-3s-1

 

 

3.8.3.      The figure shows a thin polycrystalline Al film on a substrate.  The film can be idealized as an elastic-viscoplastic solid in which the steady-state uniaxial strain rate, stress temperature relation given by ε ˙ = ε ˙ 0 exp(Q/kT) ( σ/Y ) m MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaH1oqzgaGaaiabg2da9iqbew7aLz aacaWaaSbaaSqaaiaaicdaaeqaaOGaciyzaiaacIhacaGGWbGaaiik aiabgkHiTiaadgfacaGGVaGaam4AaiaadsfacaGGPaWaaeWaaeaacq aHdpWCcaGGVaGaamywaaGaayjkaiaawMcaamaaCaaaleqabaGaamyB aaaaaaa@468B@ , where ε ˙ 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacuaH1oqzgaGaamaaBaaaleaacaaIWa aabeaaaaa@355C@ , Q and Y are material constants, and k is the Boltzmann constant.  Suppose that the film is stress free at some temperature T 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFKI8=feu0dXdh9vqqj=hEeeu0xXdbba9frFj0=OqFf ea0dXdd9vqaq=JfrVkFHe9pgea0dXdar=Jb9hs0dXdbPYxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaiaadsfadaWgaa WcbaGaaGimaaqabaaaaa@37F9@  time t=0.  Its temperature is then raised at steady rate T= T 0 (1+βt) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFKI8=feu0dXdh9vqqj=hEeeu0xXdbba9frFj0=OqFf ea0dXdd9vqaq=JfrVkFHe9pgea0dXdar=Jb9hs0dXdbPYxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaiaadsfacqGH9a qpcaWGubWaaSbaaSqaaiaaicdaaeqaaOGaaiikaiaaigdacqGHRaWk cqaHYoGycaWG0bGaaiykaaaa@3F72@  .   Using the material properties listed in Section 3.8.4, calculate and plot the variation of stress in the film with time, for various values of T 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFKI8=feu0dXdh9vqqj=hEeeu0xXdbba9frFj0=OqFf ea0dXdd9vqaq=JfrVkFHe9pgea0dXdar=Jb9hs0dXdbPYxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaiaadsfadaWgaa WcbaGaaGimaaqabaaaaa@37F9@   and β MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFKI8=feu0dXdh9vqqj=hEeeu0xXdbba9frFj0=OqFf ea0dXdd9vqaq=JfrVkFHe9pgea0dXdar=Jb9hs0dXdbPYxe9vr0=vr 0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaiabek7aIbaa@37DB@  .   You will need to integrate the first-order differential equation for the stress numerically, e.g. using the ODE solvers in MATLAB.

 

 

 

 

 

 

3.8.4.      A cylindrical, thin-walled pressure vessel with initial radius R, length L  and wall thickness t<<R is subjected to internal pressure p.  The vessel is made from an elastic-power-law viscoplastic solid with Young’s modulus E, Poisson’s ratio ν MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8srps0l bbf9q8WrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacqaH9oGBaaa@3377@ , and a flow potential given by

g( σ e )= ε ˙ 0 ( σ e Y ) m MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rkY=xi pgYlH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8ku c9pgc9q8qqaq=dir=f0=yqaiVgFr0xfr=xfr=xb9adbaqaaeGaciGa biaabeqaaiqabaWaaaGcbaGaam4zaiaacIcacqaHdpWCdaWgaaWcba GaamyzaaqabaGccaGGPaGaeyypa0JafqyTduMbaiaadaWgaaWcbaGa aGimaaqabaGcdaqadaqaamaalaaabaGaeq4Wdm3aaSbaaSqaaiaadw gaaeqaaaGcbaGaamywaaaaaiaawIcacaGLPaaadaahaaWcbeqaaiaa d2gaaaaaaa@4036@

where σ e MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rk0le9 v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0=yr0R Yxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGa caGaaeqabaGadeaadaaakeaacqaHdpWCdaWgaaWcbaGaamyzaaqaba aaaa@359F@  is the Von-Mises eequivalent stress. Recall that the stresses in a thin-walled pressurized tube are related to the internal pressure by σ zz =pR/(2t) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rkY=vi pgYlH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqaq=JfrVkFH e9pgea0dXdar=Jb9hs0dXdbPYxe9vr0=vr0=vqpWqaaeaabiGaciaa caqabeaacmqaamaaaOqaaiabeo8aZnaaBaaaleaacaWG6bGaamOEaa qabaGccqGH9aqpcaWGWbGaamOuaiaac+cacaGGOaGaaGOmaiaadsha caGGPaaaaa@3AF2@ , σ θθ =pR/t MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaebbnrfifHhDYfgasaacH8rkY=vi pgYlH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqaq=JfrVkFH e9pgea0dXdar=Jb9hs0dXdbPYxe9vr0=vr0=vqpWqaaeaabiGaciaa caqabeaacmqaamaaaOqaaiabeo8aZnaaBaaaleaacqaH4oqCcqaH4o qCaeqaaOGaeyypa0JaamiCaiaadkfacaGGVaGaamiDaaaa@3A4B@ . Calculate the steady-state strain rate in the vessel, as a function of pressure and relevant geometric and material properties.  Hence, calculate an expression for the rate of change of the vessel’s length, radius and wall thickness as a function of time.