Sheareography - a Practical Optical Testing and Measuring Method

Shearography is a laser optical method which is suited for either nondestructive testing or for strain analysis. Contrary to holography which measures surfach displacements, shearography measures derivatives of surface displacements. Since strains are functions of displacement derivatives, shearography allows strains to be determined without numerical differentiating displacement data. Defects in object normally create strain concentrations; it is easier using shearography to correlate defects with strain anomalies rather than displacement anomalies applying holography. Furthermore, rigid body motions do not produce strain, thus shearography is insensitive against such motions and does not need adopting any particular device for vibration isolation. It is an industrial tool suited very well for the following ares: 1. strain measurement and strain analysis 2. nondestructive testing and quality assurance system 3. optimization ofmachine parts and structures of any materialMechanical Engineerin

Photoelastic Investigation Using New STL-Resins

Stereolithography is not only ideal to study the function and the design of a simple or complex component, but also for stress and strain analysis by means of photoelasticity. The basis for using Stereolithograpy components is the biremngent property of the photoPOIYffiers, which has been discovered in 1991 in the Lab ofPhotoelasticity and Holography [5]. Therefore, a few acrylate and epoxy resins develoPed by Ciba-Geigy were calibrated and compared with the most commonly used resin, Araldite B (manufactured by Ciba-Geigy, too). The experience shows that static and dynamic photoelastic investigations by using the new STL-resins are possible. The time saving for photoelastic investigations amountsto values about 10 months and the cost saving is equivalent to 90010.Mechanical Engineerin

Two P or Not Two P: Understanding Regulation by the Bacterial Second Messengers (p)ppGpp

Under stressful growth conditions and nutrient starvation, bacteria adapt by synthesizing signaling molecules that profoundly reprogram cellular physiology. At the onset of this process, called the stringent response, members of the RelA/SpoT homolog (RSH) protein superfamily are activated by specific stress stimuli to produce several hyperphosphorylated forms of guanine nucleotides, commonly referred to as (p)ppGpp. Some bifunctional RSH enzymes also harbor domains that allow for degradation of (p)ppGpp by hydrolysis. (p)ppGpp synthesis or hydrolysis may further be executed by single-domain alarmone synthetases or hydrolases, respectively. The downstream effects of (p)ppGpp rely mainly on direct interaction with specific intracellular effectors, which are widely used throughout most cellular processes. The growing number of identified (p)ppGpp targets allows us to deduce both common features of and differences between gram-negative and gram-positive bacteria. In this review, we give an overview of (p)ppGpp metabolism with a focus on the functional and structural aspects of the enzymes involved and discuss recent findings on alarmone-regulated cellular effectors