定向有机玻璃疲劳断裂性能的频闪法研究

疲劳裂纹扩展前应力集中区域变形场的测试对材料疲劳性能的研究起到至关重要的作用。但由于疲劳试验为动态的过程,在动态下测量比在静态下要复杂得多,这样就给变形场的测量带来了困难。通过调节频闪光源的频率使其与疲劳试验机的频率相同,可把此动态过程转换为准静态过程,采集不同循环周期内的相同时刻的灰度图像,并运用数字图像频域相关分析法进行测试。本文运用此方法对定向有机玻璃缺口应力集中区域的变形场进行了测试,并分析得出其层间疲劳断裂力学性能薄弱的结论。     

An indentation technique for estimating the energy density as fracture toughness with Berkovich indenter for ductile bulk materials

A technique is proposed to estimate the energy density as fracture toughness for ductile bulk materials with an indentation system equipped with a Berkovich indenter based on the theory of plastic deformation energy transforming into the indentation energy of fracture. With progressive increase of penetration loads, the material damage is exhibited on the effective elastic modulus. A quadratic polynomial relationship between the plastic penetration depth and penetration load, and an approximate linear relationship between logarithmic plastic penetration depth and logarithmic effective elastic modulus are exhibited by indentation investigation with Berkovich indenter. The parameter of damage variable is proposed to determine the critical effective elastic modulus at the fracture point. And the strain energy density factor is calculated according to the equations of penetration load, plastic penetration depth and effective elastic modulus. The fracture toughness of aluminum alloy and stainless steel are evaluated by both indentation tests and K_IC fracture toughness tests. The predicted S_cr values of indentation tests are in good agreement with experimental results of CT tests.     

Toughness amplification in natural composites

Natural structural materials such as bone and seashells are made of relatively weak building blocks, yet they exhibit remarkable combinations of stiffness, strength and toughness. This performance can be largely explained by their “staggered microstructure”: stiff inclusions of high aspect ratio are laid parallel to each other with some overlap, and bonded by a softer matrix. While stiffness and strength are now well understood for staggered composites, the mechanisms involved in fracture are still largely unknown. This is a significant lack since the amplification of toughness with respect to their components is by far the most impressive feature in natural staggered composites such as nacre or bone. Here a model capturing the salient mechanisms involved in the cracking of a staggered structure is presented. We show that the pullout of inclusions and large process zones lead to tremendous toughness by far exceeding that of individual components. The model also suggests that a material like nacre cannot reach steady state cracking, with the implication that the toughness increases indefinitely with crack advance. These findings agree well with existing fracture data, and for the first time relate microstructural parameters with overall toughness. These insights will prove useful in the design of biomimetic materials, and provide clues on how bone fractures at the nano and microscales.     

Some sufficient conditions for lower semicontinuity in SBD and applications to minimum problems

We provide some lower semicontinuity results in the space of special functions of bounded deformation for energies of the type and give some examples and applications to minimum problems, arising in Fracture Mechanics for linearly elastic materials. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation of mechanical parameters of an elastic thin film system by modeling and numerical simulation of telephone cord buckles

The morphology of telephone cord buckles of elastic thin films can be used to evaluate the initial residual stress and interface toughness of the film-substrate system. The maximum out-of-plane displacement δ, the wavelength λ and amplitude A of the wave buckles can be measured in physical experiments. Through δ, λ, and A, the buckle morphology is obtained by an annular sector model established using the von Karman plate equations in polar coordinates for the elastic thin film. The mode-mix fracture criterion is applied to determine the shape and scale parameters. A numerical algorithm combining the Newmark-β scheme and the Chebyshev collocation method is adopted to numerically solve the problem in a quasi-dynamic process. Numerical experiments show that the numerical results agree well with physical experiments.     

富钾卤水成因探讨及绿豆岩中钾元素浸出特性

为探讨四川盆地卤水中钾元素的来源,提出了卤水中钾元素来源之一是原始沉积水溶滤绿豆岩中的钾元素的可能性.并将峨眉山龙门洞绿豆岩作为岩石样品,通过模拟实验,将相同的岩石样品装配在不同盐度的浸出液中,通过盐度、温度、时间的不同,得出峨眉山绿豆岩中钾的浸出率并将浸出率进行比对.结果表明,绿豆岩中的钾可以被浸出,进而证明了四川盆...     

Finite element simulation for yield stress of hard poly(vinyl chloride)/acrylonitrile-butadiene-styrene blends at different crosshead speeds

Hard poly(vinyl chloride)(PVC)/acrylonitrile-butadiene-sryrene(ABS) blends were prepared using injection-molding and influence of crosshead speed on mechanical properties was examined.Based on morphology parameters obtained from transmission electron microscopy photography and the material parameters from true stress-strain curves of neat PVC and ABS,yield stresses of the blends at different crosshead speeds were simulated employing a two-dimensional nine-particle model based on the finite element analysis(FEA).The FEA results were compared with the experimental yielding stress and the good agreement validated the simulation approach.The FEA approach allowed establishing a yielding criterion related to local yielding of the interstitial matrix between ABS particles.     

Accelerated ageing due to moisture absorption of thermally cured epoxy resin/polyethersulphone blends. Thermal, mechanical and morphological behaviour

A model epoxy resin/anhydride system, modified with a polyethersulfone (PES) engineering thermoplastic toughening agent, has been studied under hydrothermal ageing in order to investigate the modification of the thermal, morphological and mechanical behaviour through dynamical mechanical thermal analysis, SEM microscopy and fracture toughness test respectively. Two different concentrations of the toughening agent were used in the blends and two ageing conditions have been considered, consisting of the immersion of the samples in distilled water at constant temperature of 70 °C for 1 week and for 1 month. Dynamical mechanical thermal analysis results on hydrothermally aged materials indicated the occurrence of progressive segregation effects with the formation of regions with different cross-linking degrees.Fracture toughness tests showed an increase of the K_IC value with the increase of the toughening agent concentration, revealing both a dramatic decrease of the same parameter after 1 week ageing for all the materials and the tendency to reach an almost constant value after 1 month ageing for all the formulations, with a slight increase with respect to 1 week ageing. These results have been interpreted on the basis of the SEM analysis, showing the presence of a well defined micrometric PES particles distribution in the epoxy/anhydride matrix, and discussed in the light of different water absorption mechanisms at short and long ageing times.     

Extraordinary mechanical performance in charged carbyne

Carbyne, the linear chain of carbon, promises the strongest and toughest material but possesses a Peierls instability (alternating single-bonds and triple-bonds) that reduces its strength and toughness. Herein, we computationally found that the gravimetric strength, strain-to-failure, and gravimetric toughness can be improved from 74 GPa·g^-1·cm³, 18%, and 9.4 kJ·g^-1 for pristine carbyne to the highest values of 106 GPa·g^-1·cm³, 26%, and 19.0 kJ·g^-1 for carbyne upon hole injection of +0.07 e/atom, indicating the charged carbyne with record-breaking mechanical performance. Based on the analyses of the atomic and electronic structures, the underlying mechanism behind the record-breaking mechanical performance was revealed as the suppressed and even eliminated bond alternation of carbyne upon charge injection.