Baoming Gong

A cohesive crack model coupled with damage for interface fatigue problems

An semi-analytical formulation based on the cohesive crack model is proposed to describe the phenomenon of fatigue crack growth along an interface. Since the process of material separation under cyclic loading is physically governed by cumulative damage, the material deterioration due to fatigue is taken into account in terms of interfacial cohesive properties degradation. More specifically, the damage increment is determined by the current separation and a history variable. The damage variable is introduced into the constitutive cohesive crack law in order to capture the history-dependent property of fatigue. Parametric studies are presented to understand the influences of the two parameters entering the damage evolution law. An application to a pre-cracked double-cantilever beam is discussed. The model is validated by experimental data. Finally, the effect of using different shapes of the cohesive crack law is illustrated

A simplified hardening cohesive zone model for bondline thickness dependence on adhesive joints

In this paper, a tri-material adhesive system with nonlinear cohesive springs embedded between two elasto-plastic adhesive layers is proposed to predict the adhesive thickness effects on the fracture energy of bonded joints. The localized plastic and damage behaviours along the interface are described by the hardening cohesive zone models. The thickness dependent interfacial energy release rate is divided into the essential separation energy rate and the energy dissipation rate of the plasticization. The adhesive thickness dependent hardening cohesive zone model is implemented into the proposed numerical method to predict the failure of the adhesive joints. The validation of the model is performed by comparison with the experimental data.

Numerical assessment of fatigue design curve of welded T-joint improved by high-frequency mechanical impact (HFMI) treatment

Abstract In the paper, the fatigue performances of as-welded T-joint and T-joint improved by high frequency mechanical impact (HFMI) were numerically investigated using structural hot spot stress approaches: linear surface extrapolation (LSE) and through thickness at the weld toe (TTWT). The effects of main plate thickness and material strength for HFMI-treated joints were investigated. The results showed that the TTWT method was more effective to study the effect of thickness on T-joints improved by HFMI treatment than LSE method. For as-welded T-joints, the thickness correction exponent n = 0.04 was obtained when the attachment plate thickness was set as constant. For HFMI-treated T-joints, a reverse thickness effect was observed with negative thickness correction exponents, and the thickness correction exponents increased with material strength. In addition, the adoption of S–N slope varying with yield strength was proven to be more proper for HFMI improvement assessment.

Mechanical properties of the hierarchical honeycombs with stochastic Voronoi sub-structures

The introduction of hierarchy into structures has been credited with changing mechanical properties. In this study, periodically hierarchical honeycomb with irregular sub-structure cells has been designed based on the Voronoi tessellation algorithm. Numerical investigation has been performed to determine the influence of structural hierarchy and irregularity on the in-plane elastic properties. Irregular hierarchical honeycombs can be up to 3 times stiffer than regular hexagonal honeycombs on an equal density basis. Both the stiffness and Poissons ratio of the hierarchical honeycomb are insensitive to the degree of regularity, and depend on the cell-wall thickness-to-length ratio of the super-structure. Increasing the relative lengths of the super- and sub-structures results in the increment of Youngs modulus, whereas Poissons ratio almost remains constant varying from 1.0 to 0.7.

Mechanical Metamaterials Foams with Tunable Negative Poisson’s Ratio for Enhanced Energy Absorption and Damage Resistance

Systematic and deep understanding of mechanical properties of the negative Poisson’s ratio convex-concave foams plays a very important role for their practical engineering applications. However, in the open literature, only a negative Poisson’s ratio effect of the metamaterials convex-concave foams is simply mentioned. In this paper, through the experimental and finite element methods, effects of geometrical morphology on elastic moduli, energy absorption, and damage properties of the convex-concave foams are systematically studied. Results show that negative Poisson’s ratio, energy absorption, and damage properties of the convex-concave foams could be tuned simultaneously through adjusting the chord height to span ratio of the sine-shaped cell edges. By the rational design of the negative Poisson’s ratio, when compared to the conventional open-cell foams of equal mass, convex-concave foams could have the combined advantages of relative high stiffness and strength, enhanced energy absorption and damage resistance. The research of this paper provides theoretical foundations for optimization design of the mechanical properties of the convex-concave foams and thus could facilitate their practical applications in the engineering fields.

Experimental evidence and numerical simulation of size effects on the ductile fracture of metallic materials

The results of experimental tests investigating the size effects on single-edge-notched metallic specimens loaded in three-point bending are presented. Five different specimen scales were tested, with dimensions varying within the range 1:16. The samples were subjected to a fatigue pre-cracking to produce a sharp crack stemming from the notch root and, then, a quasi-static loading process was carried out up to the complete failure, in order to capture also the post-peak response. Notable size effects on the overall behaviour were obtained, with a variation of the failure mode from plastic collapse to ductile fracture and brittle failure by increasing the specimen size. An interpretation of the obtained size effects on ductile fracture is proposed based on numerical simulations carried out with a finite-element model that combines the cohesive method and the

Strain-controlled fatigue behaviors of porous PLA-based scaffolds by 3D-printing technology

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Effects of microstructural heterogeneity on very high cycle fatigue properties of 7050-T7451 aluminum alloy friction stir butt welds

J2 plasticity to take into account all the possible mechanisms for energy dissipation. The best-fitting of the experimental results is obtained by scaling the mechanical properties with the specimen size, thus proving the need of considering size-dependent constitutive laws to correctly predict the ductile fracture. Finally, scale-invariant cohesive properties are derived on the base of the fractal approach to the size effect.