Magnetic contactless detection of stress distribution and assembly defects in constructional steel element

The paper describes contactless stress measurement in a fit joint. The stress was investigated on the basis of changes in material's magnetisation. The level of magnetic leakage field changes due to mechanical stress is low. The stress measurements are determined by material's magnetoelastic sensitivity. This sensitivity must be ascertained. The magnetising field intensity is selected with high accuracy in the range where material's magnetoelastic sensitivity is greatest.The fit joint was executed with tolerance kept within planned limits. The elastic deformation of the ring's material at the lateral surface was measured with a strain gauge. At the last stage, the analytic and numerical calculations of the fit joint were compared to the magnetic field measurement results. The experiment has proven that it is possible to carry out contactless magnetic tests in constructional steel characterised by low magnetoelastic sensitivity.     

焊接残余应力对7N01铝合金疲劳裂纹扩展影响

焊接残余应力作为平均应力影响裂纹扩展. 将残余应力与外载平均应力分离,通过构建典型焊接残余应力场,借助扩展有限元计算焊接残余应力场的应力强度因子. 开展了紧凑拉伸（CT）试样的疲劳扩展试验,基于Walker公式将裂纹尖端平均应力强度因子K_m（静态量）和应力强度因子幅值ΔK（动态量）分离,获得疲劳裂纹扩展速率da/dN与K_m及ΔK的非线性关系. 结果表明,不同外载荷下,应力比与裂纹长度为非线性关系;残余应力对裂纹扩展存在尺度效应:CT试样裂纹长度小于2 mm时,残余应力场明显影响疲劳裂纹扩展速率;当裂纹长度大于2 mm,外载荷为主导因素.     

应力和晶粒组织对磁畴动态特性的影响

近年新兴的磁光成像技术,特别是磁畴动态特性可视化技术,为评估铁磁材料的应力状态提供了一种新的磁检测方法。为研究应力和晶粒组织/晶界对磁畴动态特性的影响,通过磁光成像装置,观测取向硅钢片在施加不同的应力和外加磁场时表面磁畴结构的变化。结果表明:应力作用下,附加磁畴数量减少,从点状附加磁畴转变为长条状主磁畴,且应力增加会使主磁畴平均宽度减小,主磁畴完全翻转所需要的磁场减小;外加磁场作用下,可使磁畴向磁场方向转动,磁场强度的增加,使与外加磁场同向的磁畴面积增加。同时,晶粒组织结构和内部的各向异性,以及晶界处的能量和钉扎作用,都会影响磁畴结构和磁畴壁的运动状态。研究结果有助于理清铁磁构件的磁弹耦合效应和磁检测技术的微观物理机制,为提高应力评估的灵敏度和可靠性提供重要的科学依据。     

Mechanism of electric fatigue crack growth in lead zirconate titanate

A series of experiments was performed with through-thickness cracks in ferroelectric double cantilever beam (DCB) specimens. Cyclic electric fields of different amplitudes were applied which resulted in cyclic crack propagation perpendicular to the electric field direction. Crack propagation was observed optically and three regimes were identified: a pop-in from a notch, steady-state crack growth and a decrease of the crack growth rate with increasing cycle number. Crack growth only occurred if the applied field exceeded the coercive field strength of the material. Furthermore, the crack extended during each field reversal and the crack growth rate increased with increasing field. Based on the experimental observations, a mechanistic understanding was developed and contrasted with a nonlinear finite element analysis which quantified the stress intensity in the DCB specimens. The driving forces for crack formation at the notch and subsequent fatigue crack growth were computed based on the distribution of residual stresses due to ferroelectric switching. The finite element results are in good agreement with the experimental observations and support the proposed mechanism.     

Defect assessment of welded specimen considering weld induced residual stresses using SINTAP procedure and FEA

The defect assessment in butt-welded joint of ASTM A36 steel plates and 7075-T7351 aluminum alloy plates containing transverse through thickness crack was analyzed using SINTAP procedure and FEA incorporating weld induced residual stresses. Weld induced longitudinal residual stress profile can be obtained through SINTAP procedure, FEA or experimental analysis. This residual stress profile can be fitted with the trapezoidal residual stress profile available in SINTAP. For three different cases, crack length and residual stress intensity factor (SIF) are calculated and its comparison with the results obtained through FEA is plotted with respect to crack length. The stress intensity factor for mechanical loading is also plotted in the same graph. Using this graphical plot, the total SIF, including residual stress and mechanical loading, can be calculated for any particular crack size. The total SIF can be compared with the fracture toughness of the material for damage tolerance analysis. Also a failure assessment diagram is drawn for welded 7075-T7351 aluminum alloy plates with different crack sizes for as-welded (only residual stress) and mechanical loading along with the existing weld induced residual stresses to show the safety level for a particular crack size and mechanical loading.     

Study on H2S stress corrosion test of welded joint for X65pipeline steel and numerical analysis

The susceptibility of welded joint for the X65 pipeline steel to H2S stress corrosion cracking (SCC) is investigated.SCC tests on the steel are carded out in the environment based on NACE TM-O1-77 solution with saturated gaseous H2S. The threshold stress intensity factor and crack propagation velocity are calculated according to wedge-opening loading (WOL)specimens. The three-dimensional elastic-plastic finite element analysis of WOL specimens is pesformed by using the FEM programming package ANSYS. Stress field and concentration of hydrogen distribution property ahead of the crack tip are obtained. This paper surveyed the microstructure of welded joint and studied on the mechanical properties of X65 pipeline steel.It provides experimental basis for studying stress corrosion. The results of numerical analysis are consistent with conclusions of stress corrosion test.     

新型含钪Al-Mg-Cu合金的抗应力腐蚀开裂特性

对3.5%NaCl溶液中新型含钪Al-Mg-Cu合金的应力腐蚀开裂宏观性能进行测试,并对裂纹尖端的成分与微观形貌进行分析。根据线弹性断裂力学理论,预制疲劳裂纹试样裂纹尖端处于平面应变状态,得到裂纹匀速扩展时的扩展速率、裂纹尖端应力强度因子以及应力腐蚀开裂强度因子的门槛值。扫描电镜及EDS分析表明:应力腐蚀开裂主要是沿晶扩展,预制裂纹与腐蚀介质中的溶解氧生成Al2O3,产生楔入力促使裂纹扩展;裂纹尖端基体主要发生阳极溶解反应,腐蚀产物以氯化铝为主。     

氢扩散与裂纹尖端应力场耦合效应的有限元分析

目的研究氢鼓包形成过程中应力诱导下氢原子的扩散聚集行为,并考虑氢原子扩散聚集后对裂纹尖端区域应力场的影响,探究裂纹尖端区域氢浓度、氢气压强、应力强度因子随时间的演化历程。方法采用有限元软件ABAQUS,通过一个完全耦合分析,探究氢扩散与裂纹尖端区域应力场相互影响的动态过程。借助于断裂力学中的裂纹扩展判据判定氢鼓包是否会继续向前扩展。结果在应力诱导作用下,氢原子源源不断地向裂纹尖端高应力区域聚集,裂纹尖端区域的氢浓度、氢压、应力强度因子随时间呈指数型增长。结论在浓度梯度作用下,氢原子会向材料内部扩散。氢压引起的应力场会促进氢原子的扩散聚集行为,且应力场越大,促进作用越明显,使得缺陷处的氢浓度不断增大,氢压也就越来越大。当应力强度因子达到裂纹起裂的临界值时,就会导致开裂,形成氢鼓包,如此循环往复,直至氢鼓包开裂。     

Effect of shot peening on residual stresses and crack closure in CVD coated hard metal cutting inserts

Chemical vapour deposited TiCN-Al₂O₃ hard coatings applied on hard metals may remain in a residual tensile stress state after production and sometimes they show a surface crack network. Shot peening treatments are used to introduce compressive residual stresses in the coatings and hard metal. The current work investigates dry shot peening treatments of hard metal coated by TiCN as base layer and α-Al₂O₃ as upper layer by means of finite element modelling. The simulations use experimentally parameterized material models for substrate and coatings. The influence of the coating roughness on plastification and stress development after shot peening is regarded and a number of 2D simulations with various combinations of speed, impact angles, and particle sizes are carried out. Additionally, the effect of compressive residual stresses and associated crack closure is investigated. The study reveals a number of recommendations for effective shot peening in terms of introduced compressive stresses and crack closure. Residual stress depth profiles from the calculations and experiments show good agreement for a chosen set of reasonable process parameters.     

A fracture criterion for conducting cracks in homogeneously poled piezoelectric PZT-PIC 151 ceramics

A fracture criterion for conducting cracks in piezoelectric ceramics under combined electrical and mechanical loads is proposed in connection with a measured material specific fracture curve of PZT-PIC 151. The characteristic fracture curve of the investigated material is a function of the stress intensity factor, K_I, which is known from fracture mechanical concepts, and the recently defined electric field intensity factor, K_E, for conducting cracks. A four-point-bending device for fracture tests under combined electrical and mechanical loads serves as the experimental set-up. The measured quantities, force, displacement, applied voltage and crack length, are used to evaluate the experiments. The finite element method (FEM) was used to determine the mechanical stress intensity factor, K_I, and the electric field intensity factor, K_E, taking into account linear piezoelectric coupling in the material. A qualitative process zone model is proposed for a physical interpretation of the fracture curve.     

脆—韧转变的模型化

众多结晶体在低温环境下呈现解理断裂，在高温环境下呈现塑性断裂。在过渡温区，解理断裂的应力随温度增加而增加，反映了屈服应力的下降以及裂纹尖端区塑性的相应增加。裂尖塑性可钝化裂纹并因塑性区中的压应力而屏蔽裂纹。本文简短评述了脆－韧转变模型。在该模型中，计算屏蔽效应的塑性区是由位错的产生、运动和相互作用形成的，这些位错匀在含裂尖的滑移面上运动，并且服从速度／应力／温度定律。对模型在实验结果中的应用也进行     

Two milling time regimes in the evolution of magnetic anisotropy of mechanically alloyed soft magnetic powders

The milling time evolution of magnetic anisotropy of ball milled powders can be described considering two regimes. First, for short milling times, the main factor affecting the magnetic behavior of the alloy is the accumulation of internal stresses. Second, for long milling times, magnetic anisotropy can be explained using three contributions: long-range magnetoelastic, averaged short-range magnetoelastic and averaged magnetocrystalline anisotropies.     

Vibrating ferromagnets in a magnetic field

With the vibrating reed technique we have studied the magnetoelastic behavior and the dynamical response of amorphous and crystalline magnets in the ferromagnetic and re-entrant spin glass states. The measurements have been performed as a function of magnetic field, temperature and angle between the sample’s main area and the applied field. The resonance frequency of the reed is influenced by a static contribution (pole effect), a dynamic contribution with a large effect due to the pinning of magnetic domains and magnetostrictive effects. When the field is applied perpendicular to the main area of the sample, a large decrease of the resonance frequency is observed which is rather independent of the material characteristics and is not related to a change of the Young modulus. A field misalignment of a few degrees strongly affects the field dependence of the resonance frequency. We briefly discuss the influence of the re-entrant spin glass state on the magnetoelastic behavior and the characteristics of the internal friction of vibrating ferromagnets in a magnetic field.     

Crack arrest and velocity oscillations due to local fracture-plasticity effect in polymers

Nizhny Novgorod University, Nizhny Novgorod, 603600 Russia The nonisothermal dynamics of the unstable crack growth in polymers is investigated. The system of general equations (the heat conductivity equation and the crack propagation equation) is formulated taking into account the near-crack-tip plasticity, the specific geometry of the specimen (Chevron) and real temperature dependences of the material parameters (flow stress, critical stress intensity factor, specific heat, etc.). Numerical analysis of general equations reveals the existence of the high-frequency oscillations of the crack velocity and the crack-tip temperature in the course of the crack propagation. The stage of the oscillational crack advance is completed by the arrest. The conditions of crack arrest are discussed.     

航空发动机叶片TC4钛合金振动疲劳裂纹扩展研究及剩余寿命预测

目的研究TC4钛合金的振动疲劳特性及寿命预测。方法通过共振疲劳试验,分析裂纹尖端应力强度因子的变化规律,计算不同应力水平下疲劳裂纹扩展的速率,建立剩余寿命预测计算模型。结果裂纹尖端的应力强度因子是表征裂纹扩展速率快慢的有效参数,与裂纹长度及应力场的大小相关。在裂纹扩展初期应力为274 MPa的条件下,裂纹扩展速率的试验值与计算值吻合较好。通过寿命预测模型计算可知,当初始裂纹为0.5 mm,最终裂纹长度达到5 mm时,在应力为274、366、422 MPa的条件下,振动循环周期分别为36 577、19 090、13 865。结论在应力比为?1的振动条件下,裂纹扩展速率随应力水平的增大而加快,同时初始裂纹长度越长,应力相同时,裂纹扩展速率提高。通过寿命预测模型,可计算出结构件的使用寿命。     

Characterization of drying behavior of granular materials using magnetoelastic sensors

The aim of the present paper is to show that magnetoelastic sensors can furnish important information about the drying behavior of ceramic materials, and are, therefore, possible alternatives to ultrasonic or cantilever techniques. Sensitive measurements of the resonant frequency of coated amorphous ribbons show characteristic structures at different phases of drying. A model relating the resonant frequency of a magnetoelastic sensor to elastic properties of an applied thin film coating is extended to include the magnetic field dependence of the elastic modulus, the ΔE effect. In this manner, the stress developed during the drying process which contributes to the magnetic anisotropy, can be introduced into the problem. The model is applied to hard clay slurries drying on Metglas 2826MB3 ribbons. The long time value of the sensor resonant frequency is found to be consistent with elastic properties of the clay and the sensor appropriately indicates the appearance of stresses when the meniscus of the drying fluid begins to enter the pores of the solid.     

Effect of switching stresses on domain evolution during quasi-static crack growth in a ferroelastic single crystal

The present paper demonstrates the effect of switching stresses on domain evolution and fracture toughening during quasi-static crack growth in elastically isotropic ferroelastic single crystals with transversally isotropic ferroelastic strains. With a simple switching algorithm and crack propagation procedure, domain evolution is simulated in an exemplary material with semi-infinite crack under mode I loading, starting from a mono-domain configuration. Domain reorientation is found to be strongly affected by switching stresses, which therefore have to be considered in the context of domain evolution modelling and fracture toughening. Before the onset of crack growth a needle-like domain is formed at the tip of the stationary crack, but this does not effect the crack tip stress intensity factor. Elongation of this domain during the onset of crack growth causes a large increase of the fracture toughness. Domain separation in a later stage results in toughness reduction. The subsequent domain evolution indicates a periodic formation of needle-like domains as observed in soft ferroelastic materials.     

有裂纹的功能梯度材料板平面问题的随机动态响应(英文)

研究有裂纹的功能梯度材料板平面问题的随机动态断裂,建立平面问题的随机模型。功能梯度材料的物理力学性能沿厚度方向的非均匀随机变化为沿厚度方向的随机场。作用在裂纹面上的动荷载为时间的平稳随机过程,裂纹与功能梯度材料面平行。通过分层法,将随机场的问题转化为多个随机变量的问题进行求解。利用积分变换方法和奇异积分方程技术,得到动态响应的解析形式和裂纹起始扩展的概率计算公式,分析了各参数(裂纹长度、随机场参数和裂纹位置)对动态应力强度因子的影响。结果表明:随着裂纹长度、随机场参数β以及裂纹位置比率 h2/h 的增大,应力强度因子的数学期望和标准方差均相应增大。     

腐蚀疲劳裂尖材料损伤研究

金属环境断裂过程依赖于材料,应力与介质参数间的复杂交互作用,这些交互作用的澄清是实现材料断裂控制的重要基础,而裂纹尖端单元损伤过程的研究则是联接微观机制与宏观断裂规律的桥梁。通过对钝化和活化典型腐蚀体系下恒速长大腐蚀疲劳裂纹受力学与电化学参数扰动后的扩展动力学响应规律的系统研究,并结合多种其它分析手段,考察了不同腐蚀因素对裂尖材料的损伤作用机制。在实验基础上,对腐蚀导致裂纹闭合变化,裂纹扩展过载延     

Random dynamic response of crack in functionally graded materials layer for plane problem

In order to dynamically analyze a crack in a functionally graded materials layer for plane problem under dynamic loadings, a stochastic model is established for plane problem in which the material properties of the functionally graded materials layer vary randomly in the thickness direction, and the crack is parallel to the materials faces. A pair of dynamic loadings applied on the crack faces are treated as stationary stochastic processes of time. By dividing the functionally graded materials layer into several sub-layers, this problem is reduced to the analysis of laminated composites containing a crack, the material properties of each layer being random variables. A fundamental problem is constructed for the solution. Based on the use of Laplace and Fourier transforms, the boundary conditions are reduced to a set of singular integral equations, which can be solved by the Chebyshev polynomial expansions. The stress intensity factor history with its statistics is analytically derived. Numerical calculations are provided to show the effects of the related parameters. The results show that the increase of crack length, random field parameter β and crack location ratio h₂/h leads to the increase of the mathematical expectation and standard deviation of normalized stress intensity factor history.