Bonding structures

Vagnhammar B., L.Ericsson, T. Stepinski and B. Grelsson, Improved defect detection in ultrasonic inspection of bonded structures. Report UPTEC 97 105R, Uppsala University, June, 1997... 

A nice example of this teclmique is the detennination of vibrational predissociation lifetimes of (HF)2 [55]. The HF dimer has a nonlinear hydrogen bonded structure, with nonequivalent FIF subunits. There is one free FIF stretch (v ), and one bound FIF stretch (V2), which rapidly interconvert. The vibrational predissociation lifetime was measured to be 24 ns when excitmg the free FIF stretch, but only 1 ns when exciting the bound FIF stretch. This makes sense, as one would expect the bound FIF vibration to be most strongly coupled to the weak intenuolecular bond. 

Porter G 1992 Chemistry in microtime The Chemical Bond Structure and Dynamics ed A Zewail (Boston Academic) pp 113-48... 

It has a covalently bonded structure and is a colourless liquid at room temperature it is hydrolysed reversibly by water, all the germanium being recoverable by distilling the product with concentrated hydrochloric acid GeCl -P 2H2O — Ge02 -P 4HC1... 

The concept of connection tablc.s, a.s shown. so far, cannot represent adequately quite a number of molecular structures. Basically, a connection table represents only a single valence bond structure. Thus, any chemical species that cannot he described adequately by a single valence bond (VB) structure with single or multiple bonds between two atom.s is not handled accurately. 

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

Molecular orbitals (mos) are formed by combining atomic orbitals (aos) of the constituent atoms. This is one of the most important and widely used ideas in quantum chemistry. Much of chemists understanding of chemical bonding, structure, and reactivity is founded on this point of view. 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

The infrared spectra of A-4-thiazoline-2-ones are characterized by a strong absorption around 1650 cm (55, 86, 103, 107. 870). For the N-H derivatives, the whole range 2700 to 3200 cm is covered by a strong absorption related to the dimeric and oligomeric states of the hydrogen-bonded structures (85, 86). 

In most metals the electron behaves as a particle having approximately the same mass as the electron in free space. In the Group IV semiconductors, dris is usually not the case, and the effective mass of electrons can be substantially different from that of the electron in free space. The electronic sUmcture of Si and Ge utilizes hybrid orbitals for all of the valence elecU ons and all electron spins are paired within this structure. Electrons may be drermally separated from the elecU on population in dris bond structure, which is given the name the valence band, and become conduction elecU ons, creating at dre same time... 

In the basic metric matrix implementation of the distance constraint technique [16] one starts by generating a distance bounds matrix. This is an A X y square matrix (N the number of atoms) in which the upper bounds occupy the upper diagonal and the lower bounds are placed in the lower diagonal. The matrix is Ailed by information based on the bond structure, experimental data, or a hypothesis. After smoothing the distance bounds matrix, a new distance matrix is generated by random selection of distances between the bounds. The distance matrix is converted back into a 3D confonnation after the distance matrix has been converted into a metric matrix and diagonalized. A new distance matrix... 

Elastic recoil spectrometry (ERS) is used for the specific detection of hydrogen ( H, H) in surface layers of thickness up to approximately 1 pm, and the determination of the concentration profile for each species as a function of depth below the sample s surfece. When carefully used, the technique is nondestructive, absolute, fast, and independent of the host matrix and its chemical bonding structure. Although it requires an accelerator source of MeV helium ions, the instrumentation is simple and the data interpretation is straightforward. 

In order to make the mathematics tractable, approximations must be made. The choice of approximations has produced a variety of MO methods, the judicious application of which can provide valuable insight into questions of bonding, structure, dynamics, and reactivity. The discussion that follows will not be sufficiently detailed or complete for the reader to understand how the calculations are performed or the details of the approximations. Instead, the nature of the information that is obtained will be described, and the ways in which organic chemists have applied the results of MO theoiy will be illustrated. Several excellent books are available which provide detailed treatment of various aspects of MO methods. 

The primary challenge facing adhesive bonding of metals is to obtain sufficient durability of a bonded structure. Initial bond strength in metal-polymer adhesive joints is almost invariably excellent. Challenging the application of adhesives in polymer-polymer joining, however, is the problem of obtaining a joint that is... 

In silicone adhesives used to bond structural glazing assemblies, the silicone network is made of very long PDMS chains and is filled with silica that improves the elastomeric properties of the adhesive. The strength of such an adhesive is strongly enhanced through various mechanisms of energy absorption. 

The interface/interphase must be stable under the projected use conditions for the lifetime of the bonded structure (Chapter 17 of Volume I). 

Of obvious importance to aircraft is the smoothness of exterior surfaces. Smooth aerodynamic surfaces reduce aerodynamic drag, resulting in higher airspeeds and increased efficiency. Mechanical fasteners, even countersunk flush fasteners, introduce disruptions in the airflow over the exterior surface. Even the slight deformation of thin sheets around fasteners produces drag. Adhesively bonded structure has no fasteners to disrupt airflow and is more capable of producing the smooth continuous contours that are so common on aircraft. 

It is clear that European civil aircraft manufacturers adopted adhesive bonding for major structural elements much more rapidly than their American counterparts, but it is difficult to determine exactly why. Certainly a number of contributing factors are obvious. One was a history of success in incorporating adhesively bonded structure in military aircraft such as the Mosquito. Although the Mosquito was the most unusual and extreme example of adhesively bonded structure, other European wartime aircraft contained bonded structure as well. American military craft of the time were almost exclusively riveted aluminum structure. 

Fig. 3. Adhesively bonded structure (shaded) on the Fokker F27. Reproduced by permission of Stork Fokker.

Current production aircraft include bonded structure designed up to 20 years ago as well as more recent designs on newer aircraft or derivatives. Materials, processes and design philosophy for metallic bonded structure have remained relatively stable over that period, while composite bonded structure has advanced significantly. Both will be reviewed and contrasted in the following sections. 

The aluminum alloys most commonly bonded are 2024 bare, 2024 clad and 7075 bare. Clad 7075 was also used extensively in early bonded structure but was largely abandoned after service performance demonstrated that it was susceptible to rapid dissolution or corrosion of the clad layer. Naturally aged tempers such as T3, in particular 2024-T3 because of its widespread use, are restricted to bonding with adhesives that cure at 250°F or below in order to avoid adversely affecting the temper. Various other alloys and tempers are bonded to a lesser extent, though the dominance of 7075 and 2024 is decreasing as higher-performance alloys and tempers are adopted. 

Design, manufacture and fleet performance of bonded structure... 

Designers must keep in mind that modem aircraft will likely be in service for thirty years and that repair of adhesive bonded structure will be inevitable. They must allow for reasonable disassembly of bonded stmcture for repair access as well as anticipate repair procedures that will not be unduly difficult for airline repair stations. 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

Repair. Repairs for damaged bonded structure can be either mechanical or adhesively bonded. Mechanical repairs are metallic doublers on one or both sides of a damaged component, held on by fasteners. The fasteners transfer the load through the doubler around the damaged site and restore part functionality. Although common for metal-to-metal bonded structure, mechanical repair of sandwich structure is rare because of the risk of further delamination. Unless the doubler and fasteners are perfectly sealed, water can travel into the honeycomb core eventually causing freeze-thaw damage and delamination. 

Bonded repair sizes, shapes and configurations vary greatly depending on the type of structure and size and location of damage. Typically the damaged area is removed and replacement details of like material and size are prepared to fit. Larger doublers and possibly triplers are installed over the replacement details to hold them in place and transfer load to them. Fig. 23 shows a typical bonded repair for a small damaged area on honeycomb bonded structure. 

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