Revealing the Pressure-Induced Softening/Weakening Mechanism in Representative Covalent Materials

Diamond, cubic boron nitride(c-BN), silicon(Si), and germanium(Ge), as examples of typical strong covalent materials, have been extensively investigated in recent decades, owing to their fundamental importance in material science and industry. However, an in-depth analysis of the character of these materials' mechanical behaviors under harsh service environments, such as high pressure, has yet to be conducted. Based on several mechanical criteria, the effect of pressure on the mechanical properties of these materials is comprehensively investigated.It is demonstrated that, with respect to their intrinsic brittleness/ductile nature, all these materials exhibit ubiquitous pressure-enhanced ductility. By analyzing the strength variation under uniform deformation, together with the corresponding electronic structures, we reveal for the first time that the pressure-induced mechanical softening/weakening exhibits distinct characteristics between diamond and c-BN, owing to the differences in their abnormal charge-depletion evolution under applied strain, whereas a monotonous weakening phenomenon is observed in Si and Ge. Further investigation into dislocation-mediated plastic resistance indicates that the pressure-induced shuffle-set plane softening in diamond(c-BN), and weakening in Si(Ge), can be attributed to the reduction of antibonding states below the Fermi level, and an enhanced metallization, corresponding to the weakening of the bonds around the slipped plane with increasing pressure, respectively. These findings not only reveal the physical mechanism of pressure-induced softening/weakening in covalent materials, but also highlights the necessity of exploring strain-tunable electronic structures to emphasize the mechanical response in such covalent materials.

Finite deformation analysis of crack tip fields in plastically compressible hardening–softening–hardening solids

Crack tip fields are calculated under plane strain small scale yielding conditions. The material is characterized by a finite strain elastic-viscoplastic constitutive relation with various hardening-softening-hardening hardness functions. Both plastically compressible and plastically incompressible solids are considered. Displacements corresponding to the isotropic linear elastic mode I crack field are prescribed on a remote boundary. The initial crack is taken to be a semi-circular notch and symmetry about the crack plane is imposed. Plastic compressibility is found to give an increased crack opening displacement for a given value of the applied loading. The plastic zone size and shape are found to depend on the plastic compressibility, but not much on whether material softening occurs near the crack tip. On the other hand, the near crack tip stress and deformation fields depend sensitively on whether or not material softening occurs. The combination of plastic compressibility and softening (or softening-hardening) has a particularly strong effect on the near crack tip stress and deformation fields.

Damage-induced material softening and its effect on seismic performance of steel structures

A numerical approach for simulating the seismic performance of steel truss structures, considering damage-induced material softening, is developed based on a ductile damage constitutive model by applying the backward Euler explicit algorithm. It is implemented in ABAQUS through a user-defined material subroutine, by which damage evolution could be incorporated into the analysis of seismic performance of steel structures. The case study taken up here is the investigation of a steel connection with a reduced beam section(RBS) and a steel frame with such connections. The material softening effect during the failure process is particularly investigated. The results show that material softening in the vulnerable zone has a significant effect on the distribution of stress and strain fields, as well as on the carrying capacity of the steel connection with RBS. Further, material softening is found to have almost negligible effect on the seismic performance of the steel frame in the early stages of the loading process, but has a large effect when the steel frame is about to fail. These findings offer a practical reference for the assessment of seismic structural failure, and help in understanding the damage mechanism of steel structures under seismic loading.