Stressed skin

A composite concrete slab or a stressed-skin system can also provide a roof diaphragm, but the latter may severely restrict the provision of subsequent roof penetrations. Concrete roof slabs are unusual, due to the greatly increased mass over the more normal metal decking/in-sulation/waterproof membrane or insulated metal decking options. 

SWNTs Pulmonary toxicity HiPCO (30wt% Fe) Mice Topical exposure 5d Oxidative stress Skin thickening Inflammation [46]... 

Sorg, O., Tran, C., and Saurat, J.H., Cutaneous vitamins A and E in the context of ultraviolet- or chemically-induced oxidative stress, Skin Pharmacol. Appl. Skin Physiol., 14, 363, 2001. 

In the future it would seem that greater use will be made of materials combinations and of advanced composite materials -perhaps associated with space exploration. Lighter-weight structures will be increasingly demanded for many applications and bonded stressed-skin concepts are likely to find increasing favour. Adhesives will have an invaluable role to play in all such instances. 

Stressed skin panels of floors, walls, roofs, box beams and trusses are examples of primary structural applications where the connection is expected to carry static and dynamic design loads without failure. 

Structural adhesives are ideal for building up stressed skin or mono-coque assemblies, or just for applying stiffening frames to flat panels. It is a wise precaution to rivet points at which peel separation is a possibility whether under normal stress conditions or overload, as in unforeseen impact situations. 

There are four commonly occurring states of stress, shown in Fig. 3.2. The simplest is that of simple tension or compression (as in a tension member loaded by pin joints at its ends or in a pillar supporting a structure in compression). The stress is, of course, the force divided by the section area of the member or pillar. The second common state of stress is that of biaxial tension. If a spherical shell (like a balloon) contains an internal pressure, then the skin of the shell is loaded in two directions, not one, as shown in Fig. 3.2. This state of stress is called biaxial tension (unequal biaxial tension is obviously the state in which the two tensile stresses are unequal). The third common state of stress is that of hydrostatic pressure. This occurs deep in the earth s crust, or deep in the ocean, when a solid is subjected to equal compression on all sides. There is a convention that stresses are positive when they pull, as we have drawn them in earlier figures. Pressure,... 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

It is becoming common practice to have the cross-section of a plastic moulding made up of several different materials. This may be done to provide a permeation barrier whilst retaining attractive economics by having a less expensive material making up the bulk of the cross-section. To perform stress analysis in such cases, it is often convenient to convert the cross-section into an equivalent section consisting of only one material. This new section will behave in exactly the same way as the multi-layer material when the loads are applied. A very common example of this type of situation is where a solid skin and a foamed core are moulded to provide a very efficient stiffness/weight ratio. This type of situation may be analysed as follows ... 

The surface finish of the specimen may also affect impact behaviour. Machined surfaces usually have tool marks which act as stress concentrations whereas moulded surfaces have a characteristic skin which can offer some protection against crack initiation. If the moulded surface is scratched, then this protection no longer exists. In addition, mouldings occasionally have an embossed surface for decorative effect and tests have shown that this can cause a considerable reduction in impact strength compared to a plain surface. 

Assuming the core does not fail in shear, failure occurs in bending when the stress in the skin reaches its yield strength Oy, ie... 

These parameters are used to evaluate how stressful a given hot working environment is. Depending on the physiological limitations for factors such as sweat rate, total sweat loss, heat storage, and skin wettedness, which are listed in Table 6.8, it is possible to evaluate whether a given environment is acceptable for continuous work. The method also allows calculation of an acceptable working time. Detailed equations for the calculations can be found in the standard (ISO 7933). The relation between the operative temperature and for different... 

Under the steady-state flow conditions, there is an increasing tendency of this fiberlike structure moving toward the capillary wall as shear stress, flow flux, and radial position increase. In fact, we often obtained extru-dates with a very thin TLCP-rich skin layer from the capillary test [8]. 

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