The T-failure criterion is a set of material failure criteria that can be used to predict both brittle and ductile failure.

These criteria were designed as a replacement for the von Mises yield criterion which predicts the unphysical result that pure hydrostatic tensile loading of metals never leads to failure. The T-criteria use the volumetric stress in addition to the deviatoric stress used by the von Mises criterion and are similar to the Drucker Prager yield criterion. T-criteria have been designed on the basis of energy considerations and the observation that the reversible elastic energy density storage process has a limit which can be used to determine when a material has failed.

Description

curve, represent the total amount of energy stored. In all other cases, there is a divergence between the actual and calculated stored energy in the material, which is maximum in the case of pure hydrostatic loading, where, according to the von Mises criterion, no energy is stored. This paradox is resolved if a second constitutive equation) are essential for a complete description of material behaviour up to failure. Thus, two criteria must be accounted for when considering failure and two constitutive equations that describe material response. According to this criterion, an upper limit to allowable strains is set either by a critical value ΤV,0 of the elastic energy density due to volume change (dilatational energy) or by a critical value ΤD,0 of the elastic energy density due to change in shape (distortional energy). The volume of material is considered to have failed by extensive plastic flow when the distortional energy Τd reaches the critical value ΤD,0 or by extensive dilatation when the dilatational energy Τv reaches a critical value ΤV,0. The two critical values ΤD,0 and ΤV,0 are considered material constants independent of the shape of the volume of material considered and the induced loading, but dependent on the strain rate and temperature.

Deployment for Isotropic Metals

For the development of the criterion, a continuum mechanics approach is adopted. The material volume is considered to be a continuous medium with no particular form or manufacturing defect. It is also considered to behave as a linear elastic isotropically hardening material, where stresses and strains are related by the generalized Hooke’s law and by the incremental theory of plasticity with the von Mises flow rule. For such materials, the following assumptions are considered to hold: respectively: (1) is given by Hooke’s law:(2) the shear modulus the Poisson’s ratio the Krönecker delta. is proportional to the respective deviatoric stress:(3) an infinitesimal scalar. (3) implies that the plastic strain increment:

• depends on the value of stresses, not on their variation

• is independent of the hydrostatic component of the Cauchy stress tensor

• is collinear with the deviatoric stresses (isotropic material)

(d) The increment in plastic work per unit volume using (4.16) is: (4):(5) and for metals following the von Mises yield law, by definition(6)(7) curve in a pure shear experiment. The hardening function after point “y” in Figure 1 is then:(8) (9) is the infinitesimal increase in plastic work (see Figure 1). The elastic part of the total distortional strain energy density is: (10) is the elastic part of the equivalent strain. When there is no nonlinear elastic behaviour, by integrating (4.22) the elastic distortional strain energy density is: (11), the dilatational strain energy density is: (12) is the elastic straining, proportional to the hydrostatic pressure, p (Figure 2): (13) the bulk modulus of the material. In summary, in order to use (12) and (13) to determine the failure of a material volume, the following assumptions hold:

• The material is isotropic and follows the von Mises yield condition

• The elastic part of the stress-strain curve is linear

• The relationship between hydrostatic pressure and unit volume change is linear

• The derivative (hardening slope) must be positive or zero

Limitations

are derived from experimental data. A drawback of the criterion is that elastic strain energy densities are small and comparatively hard to derive. Nevertheless, example values are presented in the literature as well as applications where the T-criterion appears to perform quite well.

References