1. A method for modeling hydraulic fractures in ductile rocks, comprising: generating, on a computer, a constitutive model for a quasi-brittle material that incorporates unified creep-plasticity (UCP) based on viscoplasticity theory; performing, on a computer, a finite element analysis using the constitutive model, wherein performing the finite element analysis comprises predicting a J-integral value and a stress field that incorporates a plasticity effect from finite element analysis using the constitutive model; determining, on a computer, an effective fracture propagation property for the quasi-brittle material based at least in part, upon a result from the finite element analysis; predicting, on a computer, hydraulic fractures in the quasi-brittle material, wherein predicting hydraulic fractures in the quasi-brittle material comprises predicting a hydraulic fracture of the ductile rocks using a pore pressure cohesive zone model (CZM); and harvesting hydrocarbon from the fractured rock.