Strain-based anisotropic damage modelling and unilateral effects

A new continuum damage mechanics model is developed to describe the behaviour of quasi-brittle materials under general path loading. The induced damage is represented by a second rank symmetric tensor. The constitutive equations are based on irreversible thermodynamics theory. The strain-based model covers in an unified way the unsymmetrical behaviour in tension and in compression and the unilateral response due to crack closure effect. Uniaxial stress tests (in tension as in compression) show realistic non-linear responses in the stress–strain space. The different behaviour in both domains is covered by a single set of equations. A significant volume dilatation is noticed in compression. The model can be generalised to time-dependent phenomena.     

Second-order continuity tensor and stiffness degradation of aluminum alloy 2024T3 under large strain at room temperature

A symmetric second-order continuity tensor is proposed to characterize anisotropic damage of anisotropic materials based on the hypothesis of equivalent elastic strain energy. On the basis of the equivalent elastic strain energy hypothesis, the relation between the effective elastic properties and the continuity tensor has been formulated. The current formulation does not require the assumption that the principal coordinate system of damage must coincide with that of the material. In a two-dimensional damage analysis, the state of damage can be represented by a Mohr’s circle of continuity. The proposed damage characterization technique has been successfully applied to an example case, where aluminum alloy 2024T3 specimens were strain-damaged by uni-axial tension. The experimental results show that the effects of strain damage can cause degradation of the material stiffness. On the other hand, the overall elastic orthotropy of the material does not increase with the degree of damage. The proposed continuity tensor has been found to be capable of describing this phenomenon. The principal values and the principal directions of the continuity tensor have been determined. The mean value of the principal values can represent the magnitude of the damage, while the principal direction of the continuity tensor may provide information about the damage mechanism.     

Shakedown analysis of beams using nonlinear kinematic hardening materials coupled with continuum damage mechanics

The nonlinear kinematic hardening theory of plasticity based on the Armstrong-Fredrick model and isotropic damage was used to evaluate the cyclic loading behavior of a beam under the axial, bending, and thermal loads. Damage and inelastic deformation were incorporated and they were used for the beam shakedown and ratcheting analysis. The beam material was assumed to follow the nonlinear strain hardening property coupled with isotropic damage. The effect of the damage phenomenon coupled with the elastoplastic nonlinear kinematic hardening was studied for deformation and load control loadings. The Bree's diagram was obtained for two different types of loading, and all numerical results confirmed the reduction of the safe loading domain due to material damage.     

Ductile damage micromodeling by particles’ debonding in metal matrix composites

The elastic–plastic behaviour of particle-reinforced metal matrix composites undergoing ductile damage is modelled using a two-level micro-structural approach. The considered heterogeneous material is a polycrystal containing intra-crystalline elastic particles. Ductile damage is initiated by the matrix/particle interface debonding and the subsequent voids growth with plastic straining of the crystalline matrix. Homogenization techniques are used twice: first at mesoscale to derive the equivalent grain behaviour and then to obtain the macroscopic behaviour of the material. Plastic deformation of the crystalline matrix is due to crystallographic gliding on geometrically well-defined slip systems. The associative plastic flow rule and the hardening law are described on the slip system level. The evolution of micro-voids volume fraction is related to the plastic strain. The elastic–plastic stress–strain response of particle composite is investigated. Predictions of the proposed model are compared to experimental data to illustrate the capability of the suggested method to represent material behaviour. Furthermore, specific aspects such as the stress triaxiality and yield surfaces are discussed.     

Modelling of MHS cellular solid in large strains

The stress–strain characteristics of metal hollow sphere (MHS) material are obtained in relatively large strains under uniaxial compression in two characteristic loading directions. Based on the hypothesis of periodic repeatability of a representative block, large deformations of the material are modelled when assuming point connections between the spheres. The elastic deformations are neglected and a rigid perfectly plastic model is assumed for the base material. A structural approach using the limit analysis and the concept of an equivalent structure are then employed to describe the large plastic deformations during post-collapse process of metal hollow spheres, which undergo mainly a snap-through deformation. Stress vs. material density relationships are proposed for different strain levels in each direction of loading. The obtained results can be used to estimate the energy absorbing capacity of MHS materials under quasi-static loading. The theoretical predictions are compared with some test results and reasonable agreement is shown.     

Stress and residual stress distributions in plane strain bending

Stress and residual stress distributions in bending are important in calculating springback and loading capacity of a sheet-metal bending part. Great differences have been found in springback prediction with the same input (benchmark problems) among different researchers. In order to find out the root cause of these differences, stress and residual stress calculation methods in plane strain bending are briefly reviewed or developed. The influence of deformation theory and incremental theory, repeating bending, unbending and re-bending, cyclic material models and springback calculation methods on the stress or residual stress distributions are examined and shown to be large. This emphasizes the importance of careful selection of these variables in a simulation model in addition to other general input variables, such as material properties, tool geometry and friction condition. This fact also helps to explain the great differences among different research results, and presents a challenge to both the programmers and the users of finite element packages.     

Derivation of creep long-term fracture criteria under plane state of stress

A new approach has been suggested for the construction of long-term fracture criteria under creep conditions and multi-axial loading. The method developed overcomes former problems with a view to identifying stresses responsible for failure and material constants, and is based on the equivalent long-term strength diagrams. The criteria are chosen in the form of a mixed invariant relating two stress components, which give rise to brittle and viscous fracture. The values of stress characteristics combined in a mixed invariant take signs of principal stresses into account. The fracture criteria derived have been approved on unified long-term strength diagrams of thin-walled tubular specimens under internal pressure, internal pressure with tension, pure torsion and tension with torsion.     

Analysis of wear-fatigue damage and modification of compressor bearings

Wear-fatigue damage of two-layer (babbitt, bronze) bushings of piston compressor bearings are analyzed. The factors generating fretting wear and fatigue cracking are analyzed. The stress-strain state of a multimodule bushing of the standard and new designs (with auxiliary strengthening by a steel layer) has been calculated with the intent of modifying the friction joints. The proposed modification was found to ensure a twofold reduction of tensile stresses in the babbitt and bronze layers, decreased slippage, and pliability to fretting on the bushing backside. The modified bearings have shown a several-fold increase in their service life in the experimental industrial conditions.     

Fundamental investigation of subsurface damage in single crystalline silicon caused by diamond machining

Single crystalline silicon was plunge-cut using diamond tools at a low speed. Cross-sectional transmission electron microscopy and laser micro-Raman spectroscopy were used to examine the subsurface structure of the machined sample. The results showed that the thickness of the machining-induced amorphous layer strongly depends on the tool rake angle and depth of cut, and fluctuates synchronously with surface waviness. Dislocation activity was observed below the amorphous layers in all instances, where the dislocation density depended on the cutting conditions. The machining pressure was estimated from the micro-cutting forces, and a subsurface damage model was proposed by considering the phase transformation and dislocation behavior of silicon under high-pressure conditions.     

基于小波神经网络的智能复合材料冲击损伤定位的研究

复合材料的力学性能对冲击损伤极为敏感，为此实现复合材料的智能化，从而在线实时监测复合材料的冲击损伤具有重要的意义。应用小波神经网络对智能复合材料冲击损伤进行了定位研究，并与改进的BP网络进行了对比，结果表明小波网络具有非线性建模逼近能力强、识别精度高和推广能力强等优点。小波神经网络为复合材料的进一步智能化提供了更为先进的信号处理方法。     

Minimization of focused ion beam damage in nanostructured polymer thin films

Focused ion beam (FIB) instruments have proven to be an invaluable tool for transmission electron microscopy (TEM) sample preparation. FIBs enable relatively easy and site-specific cross-sectioning of different classes of materials. However, damage mechanisms due to ion bombardment and possible beam heating effects in materials limit the usefulness of FIBs. Materials with adequate heat conductivity do not suffer from beam heating during FIB preparation, and artifacts in materials such as metals and ceramics are primarily limited to defect generation and Ga implantation. However, in materials such as polymers or biological structures, where heat conductivity is low, beam heating can also be a problem. In order to examine FIB damage in polymers we have undertaken a systematic study by exposing sections of a PS-b-PMMA block copolymer to the ion beam at varying beam currents and sample temperatures. The sections were then examined by TEM and scanning electron microscopy (SEM) and analyzed using electron energy loss spectroscopy (EELS). Our empirical results show beam heating in polymers due to FIB preparation can be limited by maintaining a low beam current (≤100 pA) during milling.     

基于分级概率成像的大电机定子绝缘损伤识别

为了正确评估大电机定子绝缘老化程度,提出了一种基于导波复合特征的分级概率成像损伤检测方法。通过提取损伤前后Lamb波信号之间的相关系数,利用全局概率成像初步获取绝缘损伤的分布区域和损伤程度。通过提取Lamb波散射信号波包传播时间和峰值特征,采用局部概率成像方法进一步表征损伤的局部特征。通过对两种损伤概率成像结果进行图像融合获得定子绝缘损伤识别结果。最后,对不同的典型绝缘损伤进行了损伤检测实验。结果表明:利用复合特征和分级概率成像方法可以识别出定子绝缘损伤位置和损伤程度,能够为大电机定子绝缘故障诊断提供更加有效的参考信息。     

Elastic-plastic analysis of the stress and strain distributions in asymmetric rolling

Asymmetric plane strain plate rolling is analysed numerically by using an elastic-viscoplastic finite-element method. The asymmetric conditions are here due to different interface friction conditions of the two rolls. Two sets of different interface friction conditions are used. The influence of the degree of reduction on the curvature of the rolled plate and the corresponding distributions of the stress and strain are examined. For the rolling parameters considered, the curvature of the plate is found to be towards the roll with highest friction. The largest curvature is obtained for the biggest difference in interface friction conditions. The curvature shows a change from increasing to decreasing curvature with increasing reduction followed by a change in the distribution of the strain.