A novel predictive model for mechanical behavior of single-lap GFRP composite bolted joint under static and dynamic loading

Mechanical connection of composite is critical due to its complicated meso-structure and failure mode, which has become a bottleneck on reliability of composite material and structure. Although many researches on composite bolted joints have been carried out, the theory and experiment on mechanical behavior of such a joint structure under dynamic loading were rarely reported. Here, we propose a novel predictive model for quasi-static and dynamic stiffness of composite bolted joint by introducing the strain rate dependent elastic modulus into the mass spring model. Combined with the composite laminate theory and Tsai-Hill theory, the present model was capable of predicting the strain rate dependent stiffness and strength of the composite bolted joint. Quasi-static and impact loading experiments were carried out by Zwick universal hydraulic testing machine and split Hopkinson tension bar, respectively. The stiffness and strength predicted by our model showed good accordance with the experiment data with errors below 12% under quasi-static loading and below 30% under impact loading. The results indicated that under impact loading, stiffness and strength of the composite bolted joint were significantly higher than their quasi-static counterparts, while the failure mode of the joint structure trended towards localization which was mainly bearing failure. Among various lay-up ratios studied, the optimal lay-up ratio for quasi-static and dynamic stiffness was 0:±45:90 = 3:1:1.     

Shear and mode II fracture of PUR foams

Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing and cushioning. They are made of interconnected networks of solid struts and cell walls incorporating voids with entrapped gas. The main characteristics of foams are lightweight, high porosity, high crushability, and good energy absorption capacity. Fracture toughness in mixed mode loading is of particular interest because foam cracking weakens the structure’s capacity for carrying loads.Present paper assesses the shear elastic (shear modulus) and mechanical (shear strength) properties of polyurethane foams. Also, three different types of specimens were used to determine mode I and mode II fracture toughness. The shear modulus, shear strength and fracture toughness increases with increasing foam density. Also the effect of loading direction and loading speed is investigated. The authors propose a micromechanical model to estimate fracture toughness based on the tensile strength of the solid material and the topology of the cellular structure.     

Self-healing property of epoxy/nanoclay nanocomposite using poly(ethylene-co-methacrylic acid) agent

This paper investigates the self-healing repair of cracks in an epoxy/nanoclay nanocomposite using mendable poly[ethylene-co-methacrylic acid] (EMAA) particles. The effects of two different concentrations of EMAA agent on the self-healing efficiency were measured using single edge notch bar (SENB) testing. Inclusion of EMAA particles into the nanocomposite results an increase in the fracture strength and strain of the SENB specimens. Damaged SENBs were healed at 150 °C for 30 min to achieve up to 63% recovery in critical stress intensity and over 85% recovery in sustainable peak load. Also, X-ray diffraction (XRD) analysis and tensile test used in order to examine the nanocomposite structure and investigate the effects of EMAA inclusion on the nanocomposite mechanical properties. The pressure delivery mechanism of the healing agent is shown by scanning electron microscopy (SEM) images. It seems EMAA can be used as an effective self-healing agent for epoxy/nanoclay nanocomposites.     

Numerical investigation of onset and evolution of LVI damages in Carbon–Epoxy plates

In order to study the onset and the evolution of low velocity impact damages in Carbon–Epoxy plates, a numerical investigation has been led. A detailed finite element model has been created by using the finite element code Abaqus® which, thanks to the different implemented algorithms, allowed considering both intra-laminar and inter-laminar failure criteria.In particular, the numerical modelling technique of such failure criteria allowed predicting delamination growth, by using special purpose-elements (cohesive elements) and fiber and matrix failure, by using Hashin criteria.Moreover, with the aim to reduce the required CPU time, a global/local finite element modelling approach has been proposed.For validation purpose, numerical results have been compared with data from two sessions of experimental impact tests. The considered impact energy values are 6 J, 10 J and 13 J respectively.     

Fracture toughness of zirconia from a shallow notch produced by ultra-short pulsed laser ablation

Fracture toughness of submicron grain size tetragonal zirconia polycrystals doped with 3 mol% yttria (3Y-TZP) is measured by the single edge V-notch beam (SEVNB) method from a shallow sharp notch produced by ultra-short pulsed femtolaser ablation (UPLA) on the surface of a bending bar. It is shown that the radius of the notch tip achieved is in the submicron range and the damaged volume in front of the notch tip is characterized by using focus ion beam milling and scanning electron microscopy. It consists of a narrow fully microcracked region less than ∼4 μm wide and ∼15 μm deep in front of the notch. If the extension of this region and the length of the notch are used in the determination of the fracture toughness (K_Ic) in the four bending test, the values obtained for submicron grain size 3Y-TZP are in agreement those obtained by using very sharp cracks. It is concluded that the SEVNB testing method with a sharp notch induced by UPLA may be used for K_Ic testing of submicron grain size ceramics.     

Role of cement content on the properties of self-flowing Al2O3 refractory castables

In this work, Al₂O₃ self-flowing castables (SFCs) were produced based on various cement contents. The SFCs were sintered at 1273 K, 1573 K and 1773 K and the exhibited properties were experimentally determined. Among the properties determined in this work are bulk density (BD), apparent porosity (AP), water absorption (WA), cold crushing strength (CCS), modulus of rupture (MOR) and fracture toughness (K_IC). It is found that additions of 5% cement lead to SFCs with maximum MOR and K_IC values after firing at 1773 K. Firing at 1573 K leads to a reduction in both, MOR and K_IC. In SFC containing 3% cement, maximum K_IC values of 3.53 MPa m^1/2 were achieved after firing at 1573 K. In the low cement SFCs (1 wt%) after firing at 1773 K the exhibited K_IC values were below those obtained in either the SFC-3 or SFC-5, but they were significantly high (3.43 MPa m^1/2).     

In situ fabrication of highly-dense Al2O3/YAG nanoeutectic composite ceramics by a modified laser surface processing

Alumina-matrix eutectic in situ composite ceramics present excellent high-temperature mechanical properties, which have been considered as promising next-generation ultra-high temperature structural materials. A modified laser surface processing is developed to in situ fabricate highly-dense Al₂O₃/YAG bulk nanoeutectic ceramics with large size and homogeneous three-dimensional network of nanoeutectic microstructure by introducing two-side remelting and high-temperature preheating. The crack and porosity are avoided, and the eutectic structure achieves a good continuous growth between two solidified layers. The eutectic phases show sharp interface bonding with a defined orientation relationship. The dislocations and crack deflection at high-density phase interfaces importantly contribute to the enhanced fracture toughness.     

Toughening‐modified epoxy‐amine system: Cure kinetics,mechanical behavior,and shape memory performances

Shape memory epoxy resins are derived on reacting E51 with triethylenetetramine in presence of the toughening agent polypropylene glycol diglycidyl ether (PPGDGE). The curing behaviors are studied with differential scanning calorimetry. The toughening system shows a decrease in activation energy. ?esták–Berggren model is utilized to establish the kinetic equations. The fitting results prove that the equations can well describe the reactions. Tensile tests and dynamic mechanical analysis are used to analyze mechanical performances and thermodynamics. Shape memory properties are characterized by fold‐deploy tests. The elongation at break increases as the concentration of PPGDGE increases. The toughening materials have lower glass transition temperature (T_g). The fixable ratios of all systems are greater than 99.5%. The shape recovery time decreases with increasing the PPGDGE concentration. The optimal system can fully recover its original shape in about 2 min at T_g + 30°C, and exhibit the maximum fold‐deploy cycles as 13 cycles. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40853.     

定向拉伸对有机玻璃性能的影响

介绍了定向拉伸的原理,从工艺角度阐述了定向拉伸对有机玻璃性能的影响。     

正火预处理对高速犁用28MnB5钢组织及力学性能的影响

为解决现有国产高速犁入土部件存在强韧性不足的问题,研究了正火-淬火-回火(N-Q-T)工艺对28MnB5钢微观组织与力学性能的影响机制,并与现有淬火-回火(Q-T)工艺生产的高速犁入土部件进行了对比。结果表明,N-Q-T态和Q-T态28MnB5钢的组织均以板条马氏体为主,N-Q-T态的马氏体板条平均宽度(0.9μm)显著低于Q-T态(1.5μm),且N-Q-T态的强度和韧性均显著高于Q-T态。基于Hall-Petch关系进一步分析可知,板条马氏体细化是导致N-Q-T态28MnB5钢强度提升的主要原因。同时,N-Q-T态28MnB5钢的板条马氏体细化也导致其大角度晶界占比(34.9%)显著高于Q-T态(25.1%),有助于产生更多的残留奥氏体来抑制裂纹的扩展,从而间接地提高韧性。