Acoustic Nonlinearities in Adhesive Joints

Ultrasonic techniques have been used successfully to measure important bond parameters and to detect various defects in adhesive joints for about twenty years. Recent reviews of nondestructive testing of adhesively bonded structures can be found in the literature [1–3]. For direct strength assessment, the reliability of these techniques leaves much to be desired. Linear acoustic parameters are only indirectly correlated to material and bond strength, therefore we must rely on dubious empirical relations between the measured parameter (e.g., velocity or attenuation) and the sought strength parameter on a case-to-case basis. On the other hand, it is well known that failure of most materials and bonds is usually preceded by some kind of nonlinear mechanical behavior, well before appreciable plastic deformation occurs, i.e. within the range of nondestructive testing. This macroscopic nonlinearity is due to a number of different causes such as weakening of covalent bonds with increased atomic spacing, reduction in the number of these bonds, etc. It seems to be reasonable to assume that nonlinear parameters measured at approximately 10–20% of the ultimate stress level are more directly correlated to mechanical strength than linear ones measured at negligibly low ultrasonic amplitudes:</p

Nondestructive Testing for Environmental Degradation of Adhesive Joints

The work described here is an ultrasonics based experimental study which aims to address the lack of a reliable technique for detecting strength loss in adhesive joints after exposure to hot wet environments. This is manifested as a change in the failure mode of an adhesive system from a cohesive failure in the as-made condition, that is failure through the adhesive, to an adhesive failure, failure between the adhesive and adherend, after exposure to a hot, wet environment. This work has been concerned with the bonding of aluminum using two part epoxy adhesive. The reason for the change in failure mode is thought to lie in changes in the oxide layer which is present between the aluminum and the epoxy. The oxide layer generally has a porous structure into which epoxy can penetrate, forming a micro-composite layer, referred to as the interlayer. It is the detection of changes in this interlayer which present the biggest problem to current N.D.T. techniques for adhesive joints [1]. This is largely a problem of size, the interlayer being typically no larger than a few microns thick, sandwiched between several hundred microns of epoxy and several millimetres of aluminum. It is the need to detect changes in such a thin layer through such a thick layer which presents the biggest problem

Who's History: the Auto-Biography and Oral History Accounts Published in Hong Kong in the Last Fifty Years

The use of ultrasonic spectroscopy for the non-destructive determination of the cohesive properties of the adhesive layer in a joint is described. It has been shown that measurements of the through thickness natural frequencies of the joint obtained using ultrasonic spectroscopy can be used to calculate the modulus and thickness of the adhesive layer. The results reported here indicate that the modulus may be determined to an accuracy of ±6%, and the thickness may be found as accurately as it can be measured independently. No other test is available which enables both these parameters to be determined after a joint is made, and it is anticipated that the test will be very valuable in ensuring that the process quality control during the joint manufacture has been satisfactory

Clustering objects on a spatial network

Clustering is one of the most important analysis tasks in spatial databases. We study the problem of clustering objects, which lie on edges of a large weighted spatial network. The distance between two objects is defined by their shortest path distance over the network. Past algorithms are based on the Euclidean distance and cannot be applied for this setting. We propose variants of partitioning, density-based, and hierarchical methods. Their effectiveness and efficiency is evaluated for collections of objects which appear on real road networks. The results show that our methods can correctly identify clusters and they are scalable for large problems.link_to_subscribed_fulltex

International students: A vulnerable student population

This study examines the experiences of international students at The University of Toledo, where international students comprise approximately 10% of the student population. It highlights problems international students experience such as adapting to a new culture, English language problems, financial problems and lack of understanding from the broader University community. Recommendations for improvement include initiatives to raise the profile of international students, improved financial assistance and scholarships, and creating opportunities for international students to improve their spoken English skills. © 2009 Springer Science+Business Media B.V.link_to_subscribed_fulltex

試論城市群區域內的網絡化組織

Under the new circumstances of global economic integration, industrialization and the urbanization promote the rapid development of IT industry, especially ’information expressway and internet’-based on computer and remote-sensing technology among big cities gets faster development. Information network has great influences on the formation and the development of urban agglomeration. The network of urban agglomeration is not only the most superior model in material flow between cities and countries, but also is the optimized urbanization model on the process of formation of urban agglomeration. The network organization of urban agglomeration is being perfected and has all kinds of models in the area of well-equipped infrastructure. The article focally discusses the following issues: ①The conception and the spatial distribution of network in urban agglomeration. ②The four network models of urban agglomeration: core region network, towns of dumbbell network, multiple nuclei network, corridor-type network.③The influences of network on the flows of people, material and information in urban agglomeration. Based on the analysis on the reasonability of network in view of spatial organization, the authors try to adjust the contradiction between the planned economic activities and disorderly management.城市群區域內的網絡化是城鄉之間多種物質的動態流的最高表現形式 ,也是城市群形成發展過程中理想的城市化模式。在基礎設施比較完善的區域內 ,城市群內網絡化組織是比較完善的 ,具有多種網絡化模式。主要研究城市群區域內的網絡化基本概念、空間結構模式、以及網絡化對城市群內人流、物流、信息流的重要作用。通過深層次的網絡化空間組織合理性的研究 ,盡力表現在市場經濟體制下 ,調節有序的經濟活動與無序的指揮之間的矛盾 ,提高城市綜合實力

Towards scheduling virtual machines based on direct user input

We propose a new approach to scheduling virtual machines (VMs) on a provider CPU that is unique in that is based around the use of direct user input. In our system, a user's VM is scheduled as a periodic real-time task. The user can instantaneously manipulate his VM's schedule using a joystick. An on-screen display illustrates the current schedule's cost and indicates when the user's desired schedule is impossible due to the schedules of other VMs or resource constraints. We report on a user study of our prototype system that reveals that even a naive user is capable of using the interface to our system to find a schedule that balances cost and the comfort of his VM. Good schedules are user- and application-dependent to a large extent, illustrating the benefits of user involvement. © 2006 IEEE.link_to_subscribed_fulltex

Role of HIF-1 in physiological adaptation of the carotid body during chronic hypoxia

Ultrasonic spectroscopy has been widely used for characterization of thin layered structures. Most previous studies [1]–[12] have dealt mainly with analysis of the ultrasonic reflection or transmission spectra and the effect of bonding interfaces. More recently the inverse problem for interphase layer property determination from ultrasonic measurements has been addressed. Kinra and co-authors [6],[9] use normally incident ultrasonic waves for determination of the longitudinal properties of the layer. In this case the elastic modulus, thickness and density are coupled and cannot be simultaneously recovered. The method for simultaneous determination of layer thickness, density, longitudinal and shear moduli and wave attenuations has been described in [13]. A thin layer between two thick substrates(Fig. la) was considered. Experimental data at two angles (normal and one oblique) was used for reconstruction

Back Talk - In Your Face

Ultrasonic inspection of adhesive joints is usually done either by normally incident longitudinal or shear waves or by Lamb modes propagating along the joint, i.e. in the adherend-adhesive-adherend sandwich as a whole. This paper discusses the feasibility of using guided interface waves propagating in the adhesive layer itself for nondestructive evaluation of bond quality. This layer is usually less than 5% of the whole joint, but all defects are expected in this region or on its boundary. True guided interface waves are probably the most sensitive to bond imperfections [1,2], but they are inherently very difficult to generate and detect, therefore we should settle for the second best, namely leaky guided interface waves which lend themselves quite easily to practical applications. The main purpose of this paper is to demonstrate the superior sensitivity of the suggested leaky guided interface wave technique over the more conventional Lamb wave inspection via examples of adhesive joints with different cohesive and adhesive type defects

The Entrepreneurial Activities and Business Environment of Late Qing China: The Case of Sheng Xuan-huai

Ultrasonic spectroscopy of a layer between two materials has been developed mostly for nondestructive testing of adhesive joints. For a thin layer separating two substrates signals reflected from front and back surfaces of the layer are overlapped in the time domain and interfere. The velocity of the ultrasonic wave and the attenuation in the layer can be obtained from analysis of the interference signal. Chang [1] and Flynn [2] used ultrasonic velocity and attenuation extracted from the ultrasonic reflected signal for correlation with the joint cohesive strength. The influence of different attenuation functions on amplitude and phase spectra of the signal reflected from the joint bondline was studied in [3]. Through-thickness resonance measurements were used in [4] to calculate both thickness and modulus of adhesive layer. An ultrasonic technique for evaluation of thin layers [5] and adhesive joints [6] was proposed by Kinra et al. A review of the field is given in [7]. In all the studies joints of similar substrates, predominantly Al/ Al or steel/steel, were given the most consideration since these are widely used in industry. Emphasis was given to joint cohesive strength testing by normally incident ultrasonic waves. For adhesive/adherend interface evaluation ultrasonic spectroscopy at oblique incidence was developed [8, 9, 10]. Joints of similar substrates (Al/Al) were used for experiments; the data obtained was correlated with the joint adhesive strength. For general theory on elastic wave propagation in attenuating layered media the reader is referred to [11]