Issue 18

G. Del Piero et alii, Frattura ed Integrità Strutturale, 18 (2011) 5-13; DOI: 10.3221/IGF-ESIS.18.01 8 point x tends to re-enter the elastic range (8). This condition determines the property of elastic unloading . Again, these typical properties of plastic response are not assumed, but come as results of the second-order incremental minimization. Together with the necessary condition   (  ( x ))  0 (14)  imposed by the non-negativeness of the second variation, conditions (11) and (13) determine the solution (   ,   ) of the incremental problem. Here we summarize some properties of the continuations. For a more detailed analysis we refer to the main paper [1].  If at the time t = 0 we start with  (0)=0 from the natural configuration  ( x ) =  ( x ) = 0, from the constitutive assumption   (0) > 0 we have that, initially, inequality (8) is strict. Then   is zero, and a purely elastic evolution occurs. This elastic regime ends when, with growing  , the force  reaches the limit value  =   (0), corresponding to the boundary of the elastic range. This event marks the onset of the inelastic regime .  The response in the inelastic regime strongly depends on the sign of   (0). If   (0) is positive, the deformation is homogeneous, that is, both  and  are constant all over the bar. If 0    , the inelastic deformation is given by the solution of the differential equation )( )) ( )( ('' )) (('' )) ( )( ('' )( t t t w t t t w t               (15) joined with an appropriate initial condition. The slope of the force-elongation response curve is given by )( )) ( )( ('' )) (('' )) ( )( ('' )) (('' )( t t t w t t t w t t                 (16) The slope is positive, that is, the inelastic regime starts with a work-hardening response. If 0    , elastic unloading takes place.   If  (0) is zero, the initial response is perfectly plastic. The force is constant in time, 0 )(  t   , the total inelastic deformation rate is equal to the total elongation rate )( t l   , while the punctual distribution of )( t   remains undetermined.  Finally, if  (0) is negative the necessary condition for equilibrium (14) is violated. The deformation tends to concentrate on a very small portion of the bar, and the slope )( /)( t t     tends to  . This is the model’s representation of the catastrophic failure of the bar. The work-hardening regime ends if, with growing inelastic deformation,   (  ( t )) becomes equal to zero. Of course, this may or may not be possible, according to the shape assumed for the function    Figure 1 : Force-elongation response curves at the onset of the inelastic regime, for different signs of   (0)  The main result of the local model is that the initial part of the inelastic response is determined by the sign of the second derivative   (0). That is, by the initial convexity or concavity of the cohesive energy. The predictions of the local model