Peridynamic Meso-scale Modeling for Degradation in Transverse Mechanical Properties of Composites with Micro-void Defects

The effect of micro-void on transverse stiffness and strength for fiber-reinforced composites subjected to load perpendicular to fiber is studied using the state-based peridynamic(PD)theory.The heterogeneous microstructure with micro-voids are discretized with irregular and non-uniform grids on mesoscale.The PD representative volume element with randomly distributed and sized micro-voids has been established with periodic boundary conditions.A parametric study was performed to evaluate the effects of void fraction and void size on transverse properties.It was found that the transverse modulus decreases with the void fraction in the range of 0.5–2%.A smaller void has a greater impact on the transverse modulus than a larger void under the same void content.The crack growth path for the multi-fiber model subjected to transverse tension was analyzed.The PD predictions show that cracks initiate at the interface between fiber and matrix,then deflect to the micro-voids nearby.The PD predictions match well with the analytical and experimental observations available in the literature.

Interlayer engineering for titanium clad steel by hot roll bonding

Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using pure Nb or Mo interlayer at 950 ~C with a total reduction ratio of 86.7%. Interfacial microstructure and bonding properties of titanium clad steel plates were investigated by electron microscopy and mechanical tests. The results showed β-Ti, TiC and Fe2Ti reaction phases were generated at Ti/steel interface for the clad plates with no interlayer. Inserting Nb or Mo interlayer can effectively suppress the formation of brittle phases, while the weak bonding joint transferred to Nb/steel or Mo/steel interface. And some micro-voids were found at the interface of Nb/steel and Mo/steel. The improved shear strength of clad plates with Nb interlayer might be attributed to the elimination of brittle phases at bonding interface. The small size and little quantities of the micro-voids at Nb/steel interface had a relatively weak effect on shear strength. However, the large number and big size of micro-voids were responsible for the degradation of shear strength for the clad plates with Mo interlayer.

Diffusion behavior at void tip and its contributions to void shrinkage during solid-state bonding

Solid-state diffusion bonding is an advanced joining technique, which has been widely used to join similar or dissimilar materials. Generally, it is easy to observe the diffusion behavior during dissimilar bonding, but for similar bonding the diffusion behavior has yet been observed via experiments. In this study, the diffusion behavior at void tip was firstly observed during similar bonding of stainless steel. Scanning electron microscopy with energy dispersive spectroscopy was used to examine the interface charac- teristic and diffusion behavior. The results showed that a diffusion region was discovered at void tip. Element concentrations of diffusion region were more than those of void region, but less than those of bonded region. This behavior indicated that the diffusion was ongoing at void tip, but the perfect bond has yet formed. The diffusion region was attributed to the interface diffusion from adjacent region to void tip due to the stress gradient along bonding interface. The mass accumulation at void tip transformed the sharp void tip into smooth one at the beginning of void shrinkage, and then resulted in shorter voids.

Improvement of Hole-Expansion Property for Medium Carbon Steels by Ultra Fast Cooling After Hot Strip Rolling

The improvement of hole-expansion properties for medium carbon steels by ultra fast cooling （UFC） after hot strip rolling was investigated.It was found that finely dispersed spherical cementite could be formed after ultra fast cooling , coiling and annealing treatment.Tensile strength of the steel after annealing was measured to be about 440MPa.During hole-expansion test , cracks were observed in the edge region around the punched hole because necking or cracking took place when tangential elongation exceeded the forming limit.Cracks were mainly formed by the coalescence of micro-voids.Fine and homogeneous microstructure comprised of ferrite and spheroidized cementite could increase elongation values of the tested sheets by suppressing the combination of the adjacent micro-voids , resulting in the improved hole-expansion property.

Micromechanical Behavior and Failure Mechanism of F / B Multi-phase High Performance Steel

The deformation and micro-voids formation mechanisms in ferrite / bainite（ F / B） multi-phase steel with the volume fraction of bainite less than 50% were studied by numerical simulation and experimental observation. The results show that the micro-strain concentrates at the soft / hard phase（ F / B） interface in the multi-phase steel,which should be correlated with the mechanism of incoordinate deformation. During the necking of the steel,the micro-voids initially form around the F / B interface,which also form in ferrite and bainite with the severe strain. The micro-voids in bainite are more dense and finer than those in ferrite. The failure mechanism of bainite is the coalescence of micro-voids,and the failure mechanism of ferrite is the growth and tearing of micro-voids. Due to the different failure mechanisms of ferrite and bainite,a suitable part of soft phase would be beneficial to the capability of anti-failure of F / B multi-phase steel during the ductile fracture.