Internal friction of foamed aluminium in the range of acoustic frequencies

The internal friction of aluminium foams with various porosities was measured in the range of acoustic frequencies over a wide strain-amplitude range by the bending-vibration method. The measured internal friction shows that aluminium foams have a damping capacity which is enhanced in comparison with bulk aluminium, increases with increasing porosity, decreases with increasing frequency and increases with increasing strain amplitude. In order to explain the behaviour of the internal friction, a mechanism of internal dissipation energy was presented, and an approximate expression for internal friction is derived which is based on the equations of plane waves in elastic material with voids. This expression can account for the dependence of the internal friction on porosity, pore size and frequency. To gain further insight into the dependence of the internal friction on amplitude, the non-linear characteristics of oscillations were observed, and it was found that the resonance curves are asymmetric and the resonant frequencies are proportional to the square of amplitude with a negative slope. On the basis of the equations of the motion and the experimental results, the non-linearity of oscillations was ascribed to a non-linear damping term and an approximate expression for the damping coefficient with respect to amplitude was obtained. © 1998 Chapman & Hall     

Simultaneous monitoring of acoustic emission and ultrasonic attenuation during fatigue of 7075 aluminium

Although ultrasonic attenuation monitoring has been demonstrated to be capable of providing a quantitative assessment of integrity during fatigue, much less success has been achieved in developing the corresponding capability using acoustic emission monitoring. A thorough review of the previous work employing either technique for the examination of aluminium during fatigue is presented. In addition, results from simultaneous monitoring of ultrasonic attenuation and acoustic emission during the fatigue of 7075 aluminium demonstrate the feasibility for utilizing acoustic emission as a means of continually surveying the structural integrity of a piece. It is evident from the results that, due to the passive nature of the acoustic emission monitoring, sensor location relative to failure location is not critical, thus allowing surveillance of structures with irregular geometries. However, the utilization of high frequency sound in ultrasonic attenuation monitoring makes this technique for inspecting known problem areas extremely useful in the presence of intense environmental noise. The overall findings indicate the capability of both techniques for assessing structural integrity during fatigue while providing complementary practical advantages.     

Failure mechanisms and acoustic emission amplitude distributions in automotive finishes

A complete automotive finish consists of a steel substrate with a phosphate coat and three paint coats. The complete finish and several sub-systems of the finish have been tested in tension and the acoustic emission monitored. The results of the acoustic emission, complemented with information from other techniques, such as scanning electron microscopy, have enabled the identification of a number of failure mechanisms and the strain ranges over which they occur. Analysis of the amplitude distributions in terms of overlapping Lorentzian peaks has demonstrated that each failure mechanism is associated with emissions of a characteristic amplitude.     

Analysis of fracture process in single-edge-notched laminated composites based on the high amplitude acoustic emission events

The objective of this study is to analyze the fracture process of single-edge-notched (SEN) laminated composites with different lay-up configurations and different fiber composite systems based on the behavior of high amplitude acoustic emission (AE) signals. The classification of signal type according to the dominant frequency band and its magnitude via FFT, combined with the microscopic observations under reflection and transmission optical microscopy, have been also performed in order to support the effectiveness of the analysis based on the high amplitude AE signals. It has been shown that the behavior of high amplitude event rate describes well the fracture process in the SEN laminated composites with different fiber orientations; the direction of the main crack propagation in SEN laminated composites is decisively affected by the fiber orientation and is irrespective of the initial notch direction. Finally, the AE characteristics for SEN laminated composites have been summarized in association with the individual fracture process.     

Using acoustic emission for the analysis of wear processes during sliding friction

The acoustic response of two friction pairs, in which brittle fracture and intense plastic deformation take place during friction, has been studied. The acoustic signals have been analyzed by calculating median frequencies using the window Fourier transform technique.     

Improvement of very high cycle fatigue properties in an AA7075 friction stir welded joint by ultrasonic peening treatment

The present paper aims to investigate the effect of ultrasonic peening treatment on the very high cycle fatigue resistance of an AA7075 friction stir welded joint. Microscopy observation, microhardness and X‐ray diffraction measurements were carried out to characterize the treated surface of peened specimens. Fatigue crack initiation sites were investigated through scanning electron microscope, and the role of enhanced surface on fatigue resistance was analyzed. The results indicate that a sensible fatigue strength improvement can be obtained through application of ultrasonic peening treatment and that fatigue cracks can initiate from the interior of the specimen. To clarify the fatigue failure mechanism, we analyzed the microstructure characteristics, compressive residual stress profile and intermetallic inclusion distribution in the surface layers, and we discussed the capability of ultrasonic peening treatment to hinder the surface crack initiation.     

SPCC钢滑动摩擦与拉伸声发射信号比较分析

为了将声发射技术应用在金属塑性加工中的摩擦监测中,以SPCC钢在拉伸过程与相对运动速度为100mm/min、正压力为7.5kg的摩擦过程产生的声发射信号特征为研究对象,分别用数据统计、平均值等方法对比分析了两种声发射信号的能量、振铃计数、幅度等参数。实验结果表明:SPCC钢拉伸的声发射能量值要远小于摩擦声发射能量值;拉伸的声发射振铃计数分布范围要略大于摩擦声发射振铃计数分布范围;拉伸的声发射幅度要小于摩擦声发射的幅度。最后对出现该现象的原因进行了探讨。     

Analysis of composite failure mechanisms using acoustic emission and ultrasonic scanning techniques

Acoustic emission data gained during testing of glass fibre-reinforced plastic tubular specimens under internal pressurization loading are presented, along with ultrasonic scanning traces of the test specimens and photomicrographs of samples cut from the test specimens. The data obtained by these tests indicate that fibre-reinforced composites exhibit two levels of failure: the first is that of matrix cracking, the second level is fibre fracture.     

Effectiveness of nonlinear ultrasonic and acoustic emission evaluation of concrete with distributed damages

Nonlinear ultrasonic (NLU) and acoustic emission (AE) techniques are used for nondestructive evaluation of concrete, damaged under compression loading. Experiments were carried out in 18 cubic specimens (150 × 150 × 150 mm) cast with three different w/c (six specimens for each w/c). Three specimens at each w/c were used for AE monitoring and three others for NLU evaluation. The NLU evaluation is based on measuring the change in fundamental amplitude with increasing damage and output power level. In acoustic emission testing technique four sensors were used to listen to the wide range of events under various loading and unloading cycles. An increase in AE hits was observed with increasing damage. Each loading and unloading stage was carefully examined for Kaiser and Felicity effects in order to assess the concrete deterioration. It was proposed to measure Felicity ratio at three different loading levels, corresponding to AE hits at 3%, 5%, and 10% of the AE hits at the previous maximum load, respectively. Normalized values of Felicity ratio were plotted and compared with the NLU test data. Correlation between acoustic emission and nonlinear ultrasonic techniques in assessing damage growth in concrete was investigated.     

Stress corrosion and corrosion fatigue of metals in high temperature aqueous environments

The dissolved oxygen content of water influences the susceptibility of austenitic stainless steels and reactor pressure vessel steels to environment assisted cracking by raising the electrochemical potential. The relevance of this fact to the integrity of welded stainless steel constructions, deaerator vessels and reactor pressure vessels steels is discussed. In particulal, the use of slow strain rate test procedures involving applied electrochemical potential control is discussed as a means of assessing the susceptibility to cracking of materials in a range of aqueous environments.     

用超声导波幅度谱方法评价复合材料板材疲劳损伤的数值研究

基于超声导波的频散方程及位移场表达式, 对单向碳纤维/ 环氧复合板中传播的超声导波的幅度谱进行了数值研究, 得到了能全面、直观反映复合材料板材疲劳损伤程度的超声导波幅度谱的灰度图。数值分析表明,当复合材料板材承受拉伸或弯曲载荷时, 其等效杨氏模量或等效剪切模量的变化率, 与超声导波幅度谱的灰度平均值之间存在单调的对应关系。采用超声导波的幅度谱方法可望能有效定征复合材料板材的初期疲劳损伤。