冲击试验过程中对试验结果产生影响的因素分析

冲击试验用于考核产品在寿命周期内可能经受的冲击环境下的结构和功能特性。本文作者结合自己在实验室的实际工作经历和经验,利用冲击碰撞试验台对冲击试验过程中影响试验结果的关键因素进行了系统的分析,并提出了保证试验结果准确性的几种方法。     

模糊评价法表征复合材料层合板低能量冲击行为

通过模糊综合评判的方式,建立复合材料低能量冲击表面损伤与其失效的数学判据。复合材料受到冲击后的表面损伤是冲击程度最直观的表现,因此,对复合材料冲击行为的分析,应基于其表面损伤来进行。本研究选择的观测工具为超景深显微镜,观测、探究了复合材料试样受到冲击后的表面损伤情况,建立损伤程度与材料冲击门槛值之间的数值关系。经实验验证,该数值关系可以精确判断材料是否失效。同时也验证了不同厚度的复合材料,其失效时的表面损伤有某些共同特性,本文给出的判据可以在树脂基复合材料中通用。     

高g值微加速度计在冲击环境下的可靠性

详细分析了高g微加速度计在冲击环境下的失效模式和失效机理,对0～150 000g 4端全固支的压阻式梁-岛结构微加速度计进行霍普金森杆(Hopkinson bar)激光干涉冲击试验.由试验结果可知,该结构的微加速度计抗冲击过载能力达100 000 g以上,微加速度计的失效模式表现为梁的裂纹和断裂.微加速度计在冲击环境下的可靠性采用应力-强度干涉模型进行评估.     

基于微观力学失效理论的CFRP多向层合板低速冲击行为预测

基于微观力学失效(MMF)理论对碳纤维增强复合材料(CFRP)多向层合板在低速冲击载荷下失效机制及损伤过程进行分析和预测。建立基于MMF理论的层合板结构冲击损伤行为分析方法。首先, 使用MMF理论对冲击过程中组分的失效类别进行判别; 然后, 根据组分失效的类别制定出相应的材料性能退化方案来实现对复合材料在低速冲击下的逐步失效分析;在ABAQUS平台上开发了基于显示分析的用户材料子程序(VUMAT), 即基于MMF理论的层合板冲击损伤分析程序;最后, 利用MMF理论冲击损伤行为分析方法, 对UTS50/E51碳纤维增强复合材料多向层合板在小能量低速冲击情况下的失效机制和损伤形貌进行预测, 并将预测结果与试验结果进行对比, 分析了利用MMF理论预测冲击损伤这一方法的准确性。结果表明理论预测的凹坑直径与试验测试的凹坑直径误差为4.8%, 预测的失效机制和损伤形貌与实际观察的一致。      

板级封装焊点的振动冲击加速失效方法研究

设计了一种针对板级微电子封装微焊点的振动冲击加速失效试验。对线路板施加定频正弦振动载荷,测量线路板应变值以标定PCB板级载荷水平,采用高速数据采集系统记录了振动载荷作用下的微焊点失效动态过程。结果表明：通过调节振动条件,采用板级振动试验可以获得近似板级跌落冲击试验的峰值形变,其峰值载荷作用频次高于跌落冲击试验;失效数据监测结果显示焊点在振动冲击试验中表现为疲劳失效特征。本加速失效试验在保持焊点失效特征的同时提高了试验效率,可作为跌落冲击条件下微焊点板级可靠性评估的备选试验方案。     

平面编织复合材料层压板低速冲击后的压缩失效

采用超声C扫描检测、断口分析、有限元分析等方法研究低速冲击损伤对G803/5224与G827/5224两种平面编织复合材料层压板失效行为的影响。结果表明,低速冲击后压缩载荷作用下,G803/5224层压板最终为剪切分层失效,G827/5224层压板最终为剪切屈曲失效,两种层压板低速冲击后压缩的失效模式与未受损伤层压板基本相同。建立了平面编织复合材料层压板的损伤扩展与失效模型,该模型的计算结果与试验结果吻合较好,可用于平面编织复合材料层压板的失效仿真。     

Cr12不锈钢零件表面裂纹分析

采用宏观检验、微观检验及化学成分分析等手段对在加工Cr12不锈钢零件时常出现的表面裂纹进行了分析.通过了解这些零件的生产工艺过程、表面裂纹形态及其扩展方式等方面的共性,利用模拟试验,对其裂纹的产生机理进行了剖析.在此基础上建立了Cr12马氏体不锈钢零件热处理后出现的裂纹失效分析的基本流程.     

MEMS微悬臂梁在冲击环境下的可靠性

由于MEMS器件的可靠性成为MEMS产品商业化过程中一个重要问题,而冲击断裂是导致器件失效的一个重要原因.本文主要研究多晶硅微悬臂梁在冲击条件下的可靠性,文中阐述了断裂失效机理,并使用应力一强度模型对可靠度进行建模.通过实验统计在各种加速度冲击下的可靠度,并将实验实测值与理论值进行对比.     

高压水射流参数对材料表面强化性能的影响

高压水冲击强化是一种新的表面强化方法,通过改善零件的表面状态而提高疲劳寿命.本文通过淹没射流的方法,选用铝合金7075T651作为实验材料,研究了高压水射流工艺参数对冲击强化的影响规律,并测试了材料表面强化的效果.结果表明:试件经高压水冲击强化后,其拉伸疲劳极限比未强化和喷丸强化条件下分别提高了22%和6.6%.由于高压水强化比喷丸强化得到了更好的表面状态,强化后的表面轮廓连贯平滑、浅表层压应力高,因而能更大程度提高强化件的疲劳性能.     

冲击荷载下网壳结构的失效模式及其动力响应特性

为系统全面确定网壳结构的冲击失效模式, 通过大量的参数计算研究网壳结构(包括Kiewitt-8型球面网壳、短程线型球面网壳、肋环斜杆型球面网壳、柱壳)的冲击响应特性, 并以Kiewitt-8型单层球面网壳为例, 总结分析网壳结构冲击失效的4个典型算例, 详述各类失效模式下网壳结构冲击失效的全过程及其荷载作用、结构特征响应、能量传递与转化特点;然后依据不同冲击荷载下网壳的冲击失效特点, 定义了网壳结构的3类失效模式(结构局部凹陷、结构整体倒塌、结构冲切破坏)。为揭示网壳结构的冲击失效机理并提出防护建议做了基础性工作。     

Failure Characteristic of Laser Cladding Samples on Repeated Impact

Using self-made impact fatigue test instruments and related analytic devices, the mechanical components with laser cladding layer have been attempted. It is found that, on repeated impact force, several failure modes of the components include the surface cracks, surface plastic deformation, corrosive pitting and coat collapse, etc. The paper reported the test method and initial analysis conclusions about the unique failure characteristics of the mechanical components on repeated impact load.     

Ti6Al4V基体及其改性试样的冲击磨损行为分析

在Ti6Al4V合金基表面制备了N+注入与氮化层、TiN膜及DLC膜,进行小载荷反复冲击,分析不同冲击周次下磨痕形貌及磨坑深度,比较磨损性能。结果表明疲劳磨损是反复冲击条件下实验试样的共同失效机制,主要表现为疲劳剥落。改性后试样的抗冲击磨损能力较基体材料均有提高,其中N+注入与氮化的"长程强化效应"使试样在较高冲击周次保持良好的抗磨性能;晶态结构TiN膜层由于较高的表层硬度,提高了试样的耐磨性,尤其是较低的冲击周次下;DLC膜的特殊非晶态结构,使其在整个冲击过程中具有小的磨损深度,高的失效周次。     

Biaxial impact fatigue of polycarbonate

An impact fatigue test has been designed for studying the repeated biaxial impact load response of polymers. In the test geometry, a thin plate-like sample is rigidly held by an annular clamp and is repeatedly impacted by a hemispherically-tipped plunger. Results have been obtained for polycarbonate. Under impact fatigue, cracks initiate on the bottom surface where high biaxial tensile stresses exist. These cracks propagate along radial lines on the sample surface and also into the sample normal to the surface. The load-bearing capability and the residual impact strength decrease as the cracks propagate. The development of impact fatigue cracks, the lifetime behaviour and the change in mechanical properties after partial impact fatigue are discussed.     

Static and impact fatigue behaviour of borosilicate glass

Failure of a borosilicate glass as a result of repeated impact has been studied. Impact fatigue study was conducted in an improved pendulum type repeated impact apparatus specially designed and fabricated for determining single and repeated impact strength. For elimination of the effect of humidity, repeated impact tests were carried out under liquid nitrogen. Quasi-static measurements were determined under four-point bending. Using a square waveform as applicable to the present impact tests and fracture mechanics interpretation, the number of cycles to failure during impact fatigue tests were predicted from quasi-static fatigue measurements. It has been shown that repeated impact loading has a deleterious effect on the failure cycles compared to slow stressing. The role of an added mechanical effect during repeated impacts has been suggested in controlling the cyclic fatigue behaviour. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Work Required to Cause Failure of Tablets in Diametral Compression

During diametral compression testing load-displacement curves were obtained for tablets made from some direct compression agents, by continually monitoring the force applied to the tablet and the distance moved by the lower compression platten. The area under these curves, termed work of failure, was calculated by numerically integrating the applied force with respect to platten displacement. The resistance to breakage of the tablets was assessed by a semi-empirical multiple diametral impact test in which a tablet was subjected to repeated impact until failure occurred. Work of failure was related to the number of impacts required to cause failure and appears to be a better quantitative assessment of a tablet's mechanical properties than tensile strength.     

Adhesion enhancement of sol-gel coating on polycarbonate by heated impregnation treatment

The main limitation in using coated plastics for optical components, electronic applications and display systems is the softness of the substrate surfaces, which is responsible for the low impact and abrasion resistance and weak adhesion between the coating and the substrate. In this paper, we report a new strategy for surface pre-treatment of plastics using heated vacuum equipment and sol-gel materials to provide both chemical bonds and penetrated hard layer into the plastic surface to increase the overall performance of the coated plastic components. The heated vacuum treatment process involves: (1) surface cleaning and pore opening by heating and vacuum conditions, (2) impregnation of hydrolyzed hybrid precursor into polymer substrate under pressure and elevated temperature, (3) aminolysis of diffused precursor with surface to form chemical bonds and hardened surface layer, (4) formation of chemical bonds at treated surface with sol-gel hard coating. An impregnation depth of 1.5 µm was detected. Water contact angle dropped to below 40° and roughness increased after treatment. These provided better adhesion by increased wettability and contact area. Much increased nanoindentation hardness and Young's modulus after impregnation provided a gradient in mechanical properties between soft substrate and hard sol-gel coating. The hardened substrate delays the plastic deformation in substrate during pencil scratch test, thereby preventing early gouge failure. Both the better adhesion and the delayed gouge failure contributed to the increased scratch resistance from 6B to 8H after sol-gel coating.     

Failure analysis of gears,shafts and keys of centrifugal washers failed during life test

This work presents a failure analysis of a transmission gearbox used in a motor of a food centrifugal dryer tested with a life test procedure developed by Electrolux Professional. The test apparatus consisted in a prototype of the centrifugal dryer where some aluminum cylinders were installed on the drying basket in order to simulate the maximum loads, that in-service are transferred by the food processed in the dryer to the transmission system investigated in this work. This system is employed for the motion of the basket trough an electric motor. The aim of this test apparatus was to simulate the service conditions of the components in order to evaluate their mechanical resistance and to verify that the life of the appliance is consistent with the project specifications. Moreover, the life-test presented in this paper can be used to test new materials for this specific application.The gear and the shafts analyzed in this work are made of low-alloyed steel heat treated by induction quenching followed by tempering.Premature failures occurred during the life test are considered in this work. In particular, the components interested by failure are the gear's teeth or the keys used to transfer force from the shaft to the gear. The failure of these components was observed after about 3000 of ON-OFF cycles.The microstructure of the gears and shafts, that exhibited premature failure together with that of intact components, was investigated by optical microscopy. The chemical composition of these components was characterized by means of Rf-GDOES (Glow Discharge Optical Emission Spectroscopy). The mechanical properties of the components were investigated by Vickers micro-hardness tests.The experimental results obtained in this work indicate that the failure of the components could be related to the quality of the materials of the components. In particular, the life test used in this work proved to be an important tool to evaluate the risk of a possible reduction of the products life due to non-conformity with materials specifications.     

Effects of electropulsing assisted ultrasonic impact treatment on welded components

Abstract

The effect of electropulsing assisted ultrasonic impact treatment (EUIT) on the mechanical properties and microstructure evolution of S50C steel welded components has been investigated. The present paper presents the application of a relatively new post-weld treatment method to eliminate the residual stress and improve the surface mechanical properties. The results show that EUIT exhibits better surface modification capability than does conventional ultrasonic impact treatment. After EUIT, plastic deformation layer with strengthened grains formed on the sample surface, and residual tensile stress was converted into residual compressive stress.     

Experiment-Based Explicit Dynamics Analysis for Bird Strike Damage Prediction in Composite Structures

Bird strike analysis is a common type of analysis performed during the design and analysis of rotorcraft. These simulations are carried out in order to predict whether various designs will pass the necessary certification tests. In the past, the only way to determine whether forward-facing aeronautical composite structures could withstand bird strikes was with time-consuming physical tests. In the research of bird striking, the bird impact test is the most effective method. But the existing data of test results are highly disperse, so that they do less help for the design of aeronautical composite structures and also cost more. Tests usually needed to be repeated several times because components often failed and were required for each new design. There is a large variability in numerical bird models, composite modeling approaches and complexity of simulation processes to design the sandwich structures of an aircraft. This paper investigates the composite structures modeling for bird strike phenomenon by using state-of-the-art modeling tools capable of predicting the experiment-based composite structural damage, damage location, failure size and failure mode due to impact and addresses a critical review on analysis techniques. This paper also demonstrates the state-of-the-art bird strike simulation methodology developed, and the accuracy of modeling approaches available in explicit codes is discussed.     

Modeling and Experimental Testing of Strain Gauges in Operational and Failure Modes

Strain gauges are resistive sensors bonded at critical locations on the surface of structural components to detect surface deformation and, thus, measure mechanical stress. However, strain gauges do not always report expected measurements, even under normal operating conditions. The primary goals of this paper were to develop predictive models for strain-gauge behavior and experimentally test them under controlled laboratory settings. A testing station was developed that generated a mechanical motion on a beam, subjecting strain gauges to a sinusoidally varying strain. Predictive models of the testing station were developed and experimentally analyzed. Models were also developed for two particular failure modes, namely, debonding and wire lead termination. For the cases studied, the models overpredict the output of a strain gauge operating under normal conditions, which is a discrepancy that can be explained by the gauge modifying the surface properties of the test component. Models for debonding and loose lead show agreement in terms of reduction in signal amplitude and histogram modification, respectively. Calculated and experimental data are presented that show characteristic signals in terms of time domain and histogram analysis.