Locating damage in waveguides from the phase of point frequency response measurements

A method for detecting damage in uniform waveguide structures from two or more point frequency response functions (FRFs) is described. Attention is focussed on bending waves in beams although the method can in principle be applied to any waveguide structure. The input FRF is the superposition of directly injected waves and waves reflected from the damage and from other scattering regions in the structure. The phase of this FRF modulates with wavenumber, with the period of modulation in wavenumber space being related to the distances between the excitation point and scattering locations. The phase spectrum of the input FRF is found: the phase is determined, dispersive effects are removed by transforming from the frequency domain to the wavenumber domain, and the inverse Fourier transform from the wavenumber to the space domain found. Peaks in this phase spectrum indicate the distance to the scatterer. Two (or more) input FRFs can be used to determine the location of the scatterer unambiguously. Signal processing issues are discussed. Numerical results for a uniform beam with a breathing crack are given and experimental results for beams with a slot cut into them are presented. The approach lies in the middle ground between low frequency, modal methods and high frequency, ultrasonic methods. It allows one to interrogate a region of a structure rather than the complete structure and there is no requirement for a validated model of the structure in order to locate damage, apart from an estimate of the dispersion relation.     

ATM技术的角色

从前被认为是最完美的高速网络传送方式的ATM(异步传输方式)技术,现在看来还是有其不足。随着帧中继、IPWAN和千兆以太网技术的出现,ATM存在的某些不足就逐渐地显露出来。 ATM的应用范围很广,它可以应用于小到微机桌面系统、服务器网络,大至大型电话公司的交换系统。但是,使用ATM技术的缺陷在于需要巨大的投资,这一点就限制了ATM技术在小型网络中的应用。所幸的是,在这些小型系统中,我们可以使用千兆以太网技术来满足用户对传输带宽越来越高的要求。 ATM除了提供高速的传输以外,还有一个好处,就是它综合了不同的网络业务——话音、图像和数据信息的传输。     

Incidental CT discovery of an isolated thrombus in the aortic arch

The authors report a case of isolated thrombus of the aortic arch discovered incidentally during thoracic CT angiography, complicated by clinical features of embolic renal infarction despite introduction of anticoagulant treatment. This case confirms the value of transoesophageal echocardiography to diagnose and follow these lesions and illustrates the therapeutic approach following discovery of aortic thrombi.     

Identifying subgroups that succeed or fail with three levels of physical activity intervention: The activity counseling trial.

The authors used recursive partitioning methods to identify combinations of baseline characteristics that predict 2-year physical activity success in each of 3 physical activity interventions delivered in the multisite Activity Counseling Trial. The sample consisted of 874 initially sedentary primary care patients, ages 35-75 years, who were at risk for cardiovascular disease. Predictors of 2-year success were specific to each intervention and represented a range of domains, including physiological, demographic, psychosocial, health-related, and environmental variables. The results indicate how specific patient subgroups (e.g., obese, unfit individuals; high-income individuals in stable health) may respond differently to varying levels and amounts of professional assistance and support. The methods used provide a practical first step toward identifying clinically meaningful patient subgroups for further systematic investigation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ratings of psychology journals.

48 psychology department chairpersons rated 64 prominent psychology journals as either adequate, good, or excellent. There was enough variability in ratings to suggest that there is not complete agreement among chairpersons as to the quality of psychology journals. This may be a reflection of departmental orientation or personal differences. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Complex Interactions among Sheep,Insects, Grass,and Fungi in a Simple New Zealand Grazing System

Epichloë fungi (Ascomycota) live within aboveground tissues of grasses and can have important implications for natural and managed ecosystems through production of alkaloids. Nonetheless, vertebrate herbivores may possess traits, like oral secretions, that mitigate effects of alkaloids. We tested if sheep saliva mitigates effects of Epichloë alkaloids on a beetle pest of perennial ryegrass (Lolium perenne L.) in a New Zealand pasture setting. Plants with one of several fungal isolates were clipped with scissors, grazed by sheep, or clipped with sheep saliva applied to cut ends of stems. We then assessed feeding damage by Argentine stem weevils on blade segments collected from experimental plants. We found that clipping plants induced synthesis of an alkaloid that reduces feeding by beetles and that sheep saliva mitigates this effect. Unexpectedly, the alkaloid (perloline) that explains variation in beetle feeding is one produced not by the endophyte, but rather by the plant. Yet, these effects depended upon fungal isolate. Such indirect, complex interactions may be much more common in both managed and natural grassland systems than typically thought and could have implications for managing grazing systems.