Joint stresses

Over time a large variety of materials have been used, including ivory, stainless steel, chromium—cobalt, and ceramics for the acetabular component. None proved sufficient. The implant material composition must provide a smooth surface for joint articulation, withstand hip joint stresses from normal loads, and the substance must disperse stress evenly to the cement and surrounding bone. 

For purposes of this specification, stresses in the individual members of a latticed or trussed structure resulting from elastic deformation and rigidity of joints are defined as secondary stresses. These secondary stresses may be taken to be the difference between stresses from an analysis assuming fully rigid joints, with loads applied only at the joints, and stresses from a similar analysis with pinned joints. Stresses arising from eccentric joint connections, or from transverse loading of members between joints, or from applied moments, must be considered primary stresses. 

Viscosupplementation is a clinical procedure that is being used increasingly in the treatment of osteoarthritis. This technique uses a substance known as hyaluronan to restore the lubricating properties of synovial fluid in osteoarthritic joints.6,41 Hyaluronan is a polysaccharide that can be injected into an arthritic joint to help restore the normal viscosity of the synovial fluid.6 This treatment helps reduce joint stresses, thus limiting the progression of articular destruction seen in osteoarthritis.106 Viscosupplementation has therefore been shown to reduce pain and improve function in osteoarthritis.1,95... 

D 2918 Practice for Determining Durability of Adhesive Joints Stressed in Peel... 

Only very rarely are bonded joints stressed exclusively under normal conditions. For this reasons, testing under the environmental effects of temperature, natural and artificial climates are required. Ageing tests are usually carried out on standardized test pieces, mainly on single-lap bonded joints exposed to respective environmental conditions and then tested according to the provisions of the respective standards. 

Powers, C.M. et al, The effect of bracing on patellofemoral joint stress during free and fast walking. Am. J. Sports Med., 32,224,2004. 

E is Young s modulus. An incorrect interference can result on the one hand in cracking of the part, and on the other in loosening of the joint. Stress relaxation in the polymer and differences in thermal expansion coefficient must both be taken into account. Knurled metal surfaces enable the polymer to creep into depressions and increase joint strength. 

An adhesively bonded joint is typically more structurally efficient than a mechanically fastened joint. Fasteners introduce discrete, or at least concentrated, points at which load is transferred from one component to another. Therefore the peak stress at each fastener is much higher than overall joint stress and the joint must be suitably designed to withstand these peak stresses. Compounding this is the fact that aerospace components are typically quite thin with proportionately low fastener bearing strength. The most efficient way to mechanically fasten thin members is with numerous small fasteners which add weight and cost. 

Because it is more corrosive than fresh water, salt water not only attacks metals but also degrades even the so-called inert polymers polyethylene, polypropylene, polystyrene, polymethyl methacrylate, etc. (26). Indeed, salt water is very corrosive to adhesive joints. For example, it was reported (27) that exposure to a 5% salt spray for three months had a more severe effect on aluminum joints (stressed DCB) than exposure to a semi-tropical environment for three years. In Table 1, McMillan (27) compares the severity of salt spray with a natural, semi-tropical environment and various in-service conditions. 

Most theoretical analyses have been carried out on single or double lap joints, which are the primary types of joints used for determining the strength of adhesives. Properly designed joints stress the adhesive in shear. Adhesives are especially weak in peel, and are also weak under tensile loads applied normal to the plane of the joint. The earliest theoretical lap-joint work involved simplifying assumptions that (1) the joint was a... 

Figure 7.17 Distribution of tensile and compressive stress along the joint. Stress is distributed evenly along the joint for both tensile and compressive stress, as shown by the straight line. ...

ASTM D2918-99 Standard practice for determining durability of adhesive joints stressed in peel. 

ASTM D 1828-70—Atmospheric exposure of adhesive-bonded joints and structures Determining durability of adhesive joints stressed in peel -Determining durability of adhesive joints stressed in shear by... 

The investment needed in terms of cost and time means that few such comprehensive programmes are undertaken in the short-term culture of contemporary industry. Still important are classic reports of such weathering trials undertaken by UK government laboratories. These involved bonded aluminium double overlap joints, stressed and unstressed, peel and honeycomb specimens (see Peel tests). These were exposed at the Royal Aircraft Establishment (RAE), Famborongh (temperate), and at the Joint Tropical Research Unit (JTRU) sites at hmisfail (hot-wet) and Cloncurry (hot-dry), Aushalia. Periods of up to 6 years were employed, exposing a variety of adhesive systems to the different climates. 

For a given adhesive and adherend, the strength of a joint stressed in shear depends primarily on the width and depth of the overlap and... 

Atmospheric Exposure of Adhesive-Bonded Joints and Structures. Practice for (D1828) Determining Durahility of Adhesive Joints Stressed in Peel, Practice for (D 2918) Determining Durability of Adhesive Joints Stressed in Shear by Ihnsion Loading, Practice for (D 2919)... 

Determining DurabiUty of Adhesive Joints Stressed in Shear by Tension Loading, Practice for (D 2919)... 

Strength retention (%) after 6 years for aluminum double-overlap joints. Stress level 20% (vinyl phenolic 15%). 

This theory is more complicated than it may seem for three reasons. In order to predict failure of the joint it becomes necessary to know the fundamental properties of the adhesives in bulk form. Measuring properties such as tensile strength and modulus, yield strength, and shear strength is difficult due to specimen fabrication limitations. As mentioned, typical test methods measure properties of the bonded joint rather than just the adhesive s properties. Another complexity arises in the determination of local stresses in the adhesive joint. Stresses typically occur from the application of loads on a system however, deformation of the adherends with respect to the adhesive and stress concentrations in the joint can also produce large local stresses. The last reason for complications is that each joint geometry or design can produce different types of stresses and in different locations. 

ACA flip chip bonding exhibits better reliability on flexible chip carriers because the ability of flex provides compliance to relieve stresses. For example, the internal stress generated during resin curing can be absorbed by the deformation of the chip carrier. ACA joint stress analysis conducted by Wu and co-workers indicated that the residual stress is larger on rigid substrates than on flexible substrates after bonding (14). 

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