Response of Wood Supports in Panel Paintings Subjected to Changing Climate Conditions

Abstract: The finite element method was used to model the moisture movement and strain in the wood supports of panel paintings, in response to changing climate conditions – temperature and relative humidity (RH). The material properties of lime wood (Tilia sp.), determined experimentally, were used in the modelling. Critical amplitudes of cyclic sinusoidal RH fluctuations generating strain of 0.002 in the most responsive tangential direction of the unrestrained, single wood panel, which the pictorial layer was assumed to endure without damage, were derived for the mid‐RH region as a function of cycle duration, panel thickness and diffusion configuration. Panels do not respond significantly to diurnal fluctuations or shorter. The panels respond more and more significantly when the duration of the fluctuations increases until the panel fully responds to each cycle. These fluctuation periods are 14 and 90 days at 20 °C for a panel thickness of 10 and 40 mm, respectively, with two faces of a panel diffusively opened. Sinusoidal RH variations bringing about wood’s full response have the critical amplitude of ±6% RH, that is strain of 0.002 endangering the pictorial layer is produced at such amplitude in the tangential direction of the unrestrained panel.     

Nondestructive Evaluation of Fiberglass Reinforced Plastic Subjected to Combined Localized Heat Damage and Fatigue Damage Using Acoustic Emission

The effect of fatigue damage to unidirectional fiberglass composite specimens with prior contact heat damage was investigated. After damaging the specimens by contacting them to a hot tip at 360°C, the specimens were subjected to fatigue loading at cyclic stress amplitude corresponding to 65% of the specimens' ultimate tensile strength. The fatigue experiments was halted after 3,000 cycles. The specimens were then subjected to tensile tests while monitoring their acoustic emission (AE) activity. In addition, acoustic emission activities of undamaged and contact heat-damaged specimens were monitored during tensile tests for comparison with specimens with combined fatigue and heat damage. AE activities of all specimens can be categorized into three regions: an early rise in activity, a relatively dormant period in activity, and a high exponential activity before failure. The early rise in activity did not appear on the specimens with combined contact heat and fatigue damage. For undamaged and contact heat-damaged specimens, the period of the dormant activity was independent of the contact heat duration of less than 15 minutes. However, the period was a function of the contact heat duration for combined contact heat and fatigue damaged specimens. Analyzing event duration distribution identified micro-mechanisms of the damage growth upon tensile loading. AE-stress delay concept was used to predict the state of the damage in the composite. A correlation between stress delay parameter and damage parameter was obtained for all of the specimens. Fatigue life of contact heat damaged specimen was also studied. It was found that localized heat damage reduced the fatigue life significantly. Loss of matrix to transfer the load to the fibers uniformly was believed to be responsible for the reduction in the fatigue life.     

The Strategy of Widespread Fatigue Damage in FMP System

As is known,widespread fatigue damage ( WFD) is a common phenomenon of aging aircraft being a negative impact on safety and reliability. The current FAR 25. 571( b) requires that"An LOV ( Limit of Validity) must be established,during which it is demonstrated that widespread fatigue damage will not occur in the aircraft structures". The FAA issued AC 91-82A in August 2011 providing guidance on developing and implementing a Fatigue Management Program ( FMP) system to address in-service issues for metallic fatigue critical structures. This action brings new challenges to aircraft manufacturers in China. This paper introduces the problem of widespread fatigue damage including the MSD( multiple site damage) and MED( multiple element damage) and the differences between them and the steps to establish the LOV / ELOV( Extended limit of validity) . Authors in this paper is also proposing some management indicators to control WFD during the design phase and maintenance. Developing and implementing the management indicators to address WFD which are involved in the FMP system may provide guidance on the estimation and management to address WFD for present and future aircraft. Finally,some suggestions are put forward to provide the theoretical basis for large civil aircraft design and airworthiness.     

Acousto-elastic Measurement of the Fatigue Damage in Waspaloy

It has long been speculated that second-order effects should prove an effective measure of a material's fatigue and damage state. One such second-order effect is the acousto-elastic effect. In a series of experiments on the nickel-based superalloy waspaloy, the change in the acousto-elastic coefficient of Rayleigh waves was monitored as the material was fatigued. Our results demonstrate the possibility of using this variation as a mean of measuring fatigue in waspaloy. They also show that waspaloy demonstrates a significantly different acousto-elastic behavior than that expected from a metal. This fact enhances the sensitivity of the technique in this material. It is interpreted as being due to load transfer between the two major phases found in waspaloy, the matrix phase (γ) and the strengthening phase (γ′).     

瓦楞纸板结构疲劳损伤规律的探究

通过对瓦楞纸板结构的疲劳振动试验,对瓦楞纸板结构的疲劳性能进行初步探讨,获得了在一定频率条件下,施加不同载荷时瓦楞纸板结构的疲劳寿命,给出了相应的S-N曲线,提出了瓦楞纸板结构疲劳损伤数学模型。     

Use of Eddy Current Technology to Assist in the Evaluation of the Fatigue Damage of Electrical Conductors

The article deals with the evaluation of fatigue damage on overhead conductors which have been in service for a period exceeding 50 years. Under wind excitation, electrical transmission lines are subjected to aeolian vibrations that may induce wire fatigue of conductor at the exit of supporting equipment. This fatigue phenomenon is a fretting fatigue problem which is located at contact points between wires belonging to adjacent wire layers or between external layer wire and the supporting equipment. This article reports the methodology followed: 1) in selecting wires with the more severe damaged contact point using eddy current technology and 2) in comparing, through a metallographic examination, the contact points of those selected wires coming from in situ worn conductors and from laboratory fatigue tested virgin conductors used as reference.     

疲劳损伤对波纹管耐连多硫酸腐蚀性能的影响

针对催化裂化装置用波纹管使用中所面临的连多硫酸应力腐蚀问题,采用热态连多硫酸腐蚀模拟装置,试验研究了固溶态Incoloy825波纹管的耐连多硫酸应力腐蚀性能,并利用金相显微镜、SEM、EDX等手段分析了腐蚀波纹管的表面形貌和断口特征,探讨了设计疲劳寿命及累积疲劳损伤对波纹管连多硫酸应力腐蚀敏感性的影响规律。结果表明,在60℃的H2SxO6溶液中,设计疲劳寿命及累积疲劳损伤对固溶态Incoloy825波纹管的腐蚀敏感性影响显著。设计疲劳寿命越低、累积疲劳损伤越大,则越易形成应力腐蚀裂纹。     

Analysis of Fatigue Damage under Block Loading in a Low Carbon Steel

Abstract: The fatigue damage accumulation behaviour of the P355NL1 steel is characterised using block loading fatigue tests. First, the constant amplitude low‐cycle fatigue behaviour of the P355NL1 steel is evaluated through strain‐controlled fatigue tests of smooth specimens. Both fatigue and cyclic elastoplastic behaviours are analysed. Then, block loading is applied to identify the key features of the fatigue damage accumulation phenomena for the P355NL1 steel. The block loading is composed of two distinct low‐cycle constant amplitude strain‐controlled blocks. The first block is applied for a predefined number of loading cycles, being followed by a second block which is applied until failure. The block loading illustrates that fatigue damage evolves nonlinearly with the number of load cycles as a function of the strain amplitude. These observations suggest a nonlinear damage accumulation rule with load sequence effects. The linear Palmgren–Miner's rule used extensively in design is not verified for the P355NL1 steel. Finally, using the generated experimental data, the cyclic elastoplastic behaviour of the P355NL1 steel is modelled using a continuum plasticity model with nonlinear kinematic hardening, available in the commercial finite element code ansys ®.     

多孔试件在热循环下的复合疲劳的损伤特性研究

根据航空发动机工作叶片实际工作中的载荷环境,即气冷叶片承受低周的离心力负荷,气动载荷及温度负荷,同时还叠加高弯曲振动载荷,进行了一系列热循环下复合疲劳和常温机械疲劳的组合试验,在试验研究基础上提出复合疲劳中耦合损伤的概念及分析,     

托盘用竹木复合层合板在疲劳/蠕变交互作用下断裂损伤研究

研究了在疲劳/蠕变交互作用下竹木复合层合板的断裂损伤行为.结果表明,在交变载荷最大值为75%的应力水平下,其疲劳/蠕变断裂曲线为3段式曲线;在55%的应力水平下,15小时内其疲劳/蠕变曲线为两段式曲线.随着最大载荷保持时间的增加,层合板的断裂寿命降低.在疲劳/蠕变交互作用下层合板的破坏形式主要为纤维撕裂、界面剪切破坏和层间开裂3种形式的综合表现.影响疲劳/蠕变断裂寿命分散性的因素主要有层合板的铺设结构、木材缺陷和竹材结构等的影响.     

地下工程改扩建中施工爆破对衬砌危害的三维数值模拟研究

应用LS—DYNA3D软件对某地下工程改扩建中施工爆破作业进行了三维数值模拟，分析了围岩与衬砌中爆炸应力波的传播特征，指出了原有洞室混凝土衬砌的破坏主要为施工爆破诱发的反射拉伸波使衬砌拉应力超过混凝土抗拉强度所导致拉裂破坏，且破损现象主要发生在洞室的侧墙部位，为控制衬砌破损的进一步发展，建议采用台阶法开挖，计算表明台阶法开挖能明显改善衬砌拉应力水平和质点振速。     

等离子喷涂Fe基合金涂层组织及接触疲劳损伤性能的研究

使用超音速等离子喷涂设备在45钢基体上制备了铁基合金涂层,采用扫描电镜、透射电镜、X射线衍射仪等设备对涂层的组织结构进行了表征,借助显微硬度仪测定了涂层截面的硬度分布,以球盘式接触疲劳试验机为平台,利用声发射技术实时监测涂层的接触疲劳损伤过程.结果表明,涂层组织主要是由α(Fe,Cr)相和网状的Cr7C3组成,纳米级α(Fe,Cr)相分布于裂纹周围.声发射特征参数RMS可以反映涂层的接触疲劳损伤程度,在转速为2500r/min和应力水平为1.8489GPa实验条件下,分层是涂层失效的最终形式.     

Damage of a screw in the seal coupling

The damage to a screw on the head of a 1.8 liter personal car engine was nucleated as the result of common disadvantageous environmental influences and reversed loads leading to corrosion fatigue.     

10CrNiMo结构钢弹性模量在低周疲劳过程中的变化规律

采用横截面为圆形的光滑试样,在六级轴向系列不同恒应变幅(0.3%~1.0%)控制下,研究10CrNiMo结构钢弹性模量在低周疲劳过程中的变化规律.结果显示:在不同级别应变幅控制下,随着应变幅值的增加,稳定循环下的弹性模量并不是恒量,而是以负幂指数函数规律减小,且最小与最大应变幅下的弹性模量相差超过15%;而在某一恒应变幅控制下的稳定循环区间内,弹性模量随着循环次数的增加而缓慢下降,但在疲劳失稳时,弹性模量迅速大幅降低.分析认为,出现上述现象的原因,与不同应变幅对材料造成不同程度的疲劳损伤及恒定应变幅控制下材料固有的持续疲劳抗力有关.     

低周疲劳过程中弹性模量的损伤特性

加工成圆形横截面光滑试样,采用损伤力学理论,通过轴向不同应变幅控制的低周疲劳试验,研究了10CrNi5MoV高强钢的拉伸卸载模量、压缩卸载模量和循环弹性模量的损伤特性,结果表明:三个弹性模量的损伤特性,依赖于应变幅的大小,应变幅越大,三个模量的损伤均越快;在试样失效之前,三个弹性模量的损伤并无明显变化,只是在试样失效时,各自的损伤变量D才迅速趋向于1。研究结果为10CrNi5MoV高强钢的工程应用提供了参考。     

外爆炸条件下地下目标破坏效应分析

通过比较地下单层C3I设施、地上多层建筑物下面的掩体等目标的防护水平,构造了一个典型的地下目标.固定装药量,在构造的目标上选择典型的爆炸位置,利用经验公式和数值分析相结合的方法,根据不同爆点位置对目标的不同作用模式,分析了爆炸对目标的作用问题,评估了爆炸载荷对目标的破坏程度,可为地下目标毁伤研究和爆炸威力设计提供参考.     

岩石强度对爆炸破坏影响的数值模拟

利用非线性动力有限元分析软件LS-DYNA,模拟了55.9 kg球形TNT药包在花岗岩和白云岩中爆炸时的破坏情况.仿真结果表明:强度对于自由面附近岩石破坏特征有明显影响,强度较大的花岗岩自由面附近反射拉伸波容易引起脆性断裂;强度较小的白云岩对应力波衰减较快,自由面附近不易发生破坏.数值模拟再现了爆炸应力波传播和岩石破坏过程,对爆破工程有一定的借鉴意义.     

Determining the Progression of Damage in Materials Subjected to Sustained Random Loads Using an Adaptive Finite Impulse Response Approach

It is often important to establish the ability of materials to withstand dynamic loads. This ability is best determined by subjecting the elements to sustained random loads under controlled conditions. It is during these fatigue endurance tests that the loss of structural integrity of a material or an element needs to be quantified. The research presented herein uses a recently developed continuous structural integrity assessment technique to evaluate variations in the mechanical properties (namely stiffness) of materials. The effectiveness of the technique was evaluated by undertaking controlled experiments, during which damage was simulated by varying the stiffness (in this case the length of a cantilever beam) of a physical single degree‐of‐freedom vibratory system subjected to random base excitation. Additional experiments where materials were allowed to naturally decay (structurally) under sustained random loads were also performed. Overall, the results presented in this study indicate that the technique can be a practical and effective tool for detecting small variations in the structural integrity of materials subjected to sustained random loads.     

应力比对航空高强LD2CZ铝合金腐蚀疲劳裂纹扩展的影响

利用扫描电镜联合液压伺服试验机,并借助于Walker公式研究了应力比对预腐蚀不同时间航空高强LD2CZ铝合金疲劳裂纹扩展的影响,在应力比分别为0．05,0．5,0．7的条件下对预腐蚀0,15,30d的LD2CZ铝合金单边缺口板状试样进行了疲劳加载试验,得到了其疲劳裂纹扩展速率曲线,并拟合出了Walker公式中的材料常数。结果表明：裂纹扩展速率会随着应力比的增加以及腐蚀损伤的加深而增大,拟舍得到的Walker公式可用来定量化地表征应力比和腐蚀损伤对疲劳裂纹扩展速率的影响。