The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.     

Natural stress used to obtain linear elastic and optical properties of a rubber under large strains

The mechanical and optical properties of some rubbers, like the polyurethanes, used in experimental stress analysis are usually represented by highly nonlinear relationships. In this paper, uniaxial and biaxial tensile tests on a polyurethane rubber are described and the results are analyzed introducing the new concept of “natural stress.” It has been found that the natural stress-natural strain relationship is linear up to 100 per cent strain and seems to correspond to an intrinsic property of the material. The isochromatic fringes are also shown to be linear with respect to natural stress and natural strain.     

Optimization of hole shapes in circular cylindrical shells under axial tension

Hole shapes are optimized in circular cylindrical shells subjected to axial load considering only the predominantly large membrane stresses present around the holes. Two-dimensional photoelastic isochromatics obtained with a special-purpose polariscope are utilized for the optimization process. The process leads to a significant decrease in the membrane stress-concentration factor and a modest decrease in weight, thus yielding a considerable increase in strength-to-weight ratio. This paper presents results for certain typical ratios of hole diameter to shell diameter. Previous theoretical and experimental studies for the circular hole have also been verified     

A simple holographic interferometer for static and dynamic photomechanics

A particular variation of holographic imaging system is described which, when used as a multiple-exposure holographic interferometer, possesses advantages for applications in static and dynamic photomechanics. Large fields of view can be obtained. Rigid-body motions produced by loading are automatically eliminated. The holograms can be recorded on medium-resolution films which have high sensitivity. Specimens manufactured with readily available materials can be used for the determination of isochromatics and isopachics. An attempt is made to describe this contribution in the background of previous developments in interferometry. Operation of the system is interpreted by showing the equivalence of the holographic interferometer to a combination of two systems presently in use in experimental stress analysis: a Fizeau interferometer and an optical spatial filter. The interpretation of isochromatics and isopachis as moiré phenomena is emphasized. Isochromatics and isopachics are presented as illustrations of the applications of the method to the solution of static-stress problems, and they are used in the solution of some not yet solved dynamic-stress problems. Whole-field static isochromatics obtained as absolute-retardation interference are shown. Also shown are whole-field dynamic isopachics.     

Photography without lenses

Good photographic records fo brittle coating, photoelasticity and moiré patterns can be obtained in certain cases by direct contact of the specimen and the photographic film. No lenses are required. Several examples of application of the technique are given. The method is particularly useful when brittle coating or moiré are applied to transparent curved developable surfaces.     

Extending the stress-analysis range of brittle coatings

In this paper, an attempt is made to extend in both directions the range of the stresses that can be determined using brittle coatings: (1) by sensitizing the coating to determine small stresses, and (2) by the use of a new calibration device that allows the application of very large strains. The increase in sensitivity is obtained by bathing the coating with cold water under load. The coating used in this manner is shown to have a strain sensitivity as low as 100 μin./in., well below the 400 μin./in. limit imposed by crazing of the coating as it is normally used. The fixture to apply large strains is described and the method applied to several brittle coatings to calibrate them up to 10,000 μin./in. (1 percent) strain sensitivity. In conjunction with the above studies, a short test was conducted on the effects of repetitive loading on a brittle coating. Load history was shown to affect strain sensitivity in some cases.     

Log-art as a means of humanization

Log-art is an “artistic” science. By selection and sometimes by deliberate combination of records, methods used to study nature scientifically permit the expression of an artistic feeling. Usually the data are obtained in the form of photographs covering the range of astronomy to electron microscopy. In this paper, particular emphasis will be put on photographic records obtained by experimental stress analysts using photoelasticity, moiré and holography. It is believed that log-art may develop into a powerful means of humanizing scientists by calling their attention to the fact that their scientific records can be looked at other ways than from the scientific point of view.     

Complete determination of the stress tensor in a field using brittle coatings

Brittle coatings are used today, almost exclusively, to determine the directions of the principal stresses at a point located on the surface of a loaded body. This is a strange paradox of history, because no other method can give more complete information about the stress tensor in a whole field. This is so, event if it is also true that the precision of the determinations of some of the components of the tensor may not be as high as that generally obtained using other methods.This paper has the following objectives: (1) to show the application of the brittle-coating method to the determination of very complicated stress distributions like those which develop in thin-walled pressure vessels manufactured using circumferential and longitudinal weldings, (2) to show that the use of brittle coatings is the most practical method to solve some of these problems when boundary conditions are not well known, (3) to show that the method can, in certain cases (when the two principal stresses have the same sign), give the complete determination of the tensor in large parts of the field of complex stress distributions, and that it is the only method that can obtain that amount of information in one experiment, (4) to show how in other cases (when the two principal stresses are of opposite sign) that information can be obtained in two experiments, (5) to show the possibility of using the method to study yielding problems, (6) to review some of the advantages and limiations of the method and (7) to recommend an organized effort to find new materials which may permit the method to become popular again, without exhibiting some of the serious limitations that have significantly restricted its use. Some of the deficiencies in the present state of knowledge are described.     

Linear and non-linear elastic and plastic strains in a plate with a large hole loaded axially in its plane

Strain analysis of the ligament of a plate with a large hole indicates that both geometric and material non-linearity may take place. Geometric non-linearity may occur even when the material is strained elastically. This may explain the disagreement in the results obtained recently, for the stress concentration factors, by several investigators. The experimental analyses were conducted on an aluminum plate, using electrical resistance strain gages, and on a polyurethane rubber sheet using photography. The strain concentration factor was found to vary from 1 to 2 depending on the level of deformation.