Joint design durability

Morris, C. E. M., Strong, Durable Adhesion Bonding Some Aspects of Surface Preparation, Joint Design, and Adhesive Selection, Materials Forum, vol. 17, 1993, pp. 211-218. 

Procedures for the determination of important properties are essential for the selection of an appropriate product and for joint design and analysis. We have seen that adhesive polymers are sensitive to temperature and moisture, and to the rate at which stress is applied. Their bondline behaviour in joints depends greatly upon the system stiffness, and upon more subtle variations such as bondline thickness. It is therefore important that test methods are relevant to the real application, the fabrication conditions, the actual materials to be joined, and so on. Many standard test methods exist, for both strength and durability assessment, and these were discussed in Chapter 4. 

Aspects of this subject are also dealt with in other articles, notably Acrylic adhesives. Durability - fundamentals. Joint design general. Joint design cylindrical joints. Joint design strength and fracture perspectives. In the article on Toughened acrylic adhesives, some properties are compared with those of Epoxide adhesives and anaerobic adhesives. 

Further consideration relating mechanics to joint design are given in Joint design strength and fracture perspectives, Durabiiity creep rupture. Durability subcritical debonding and Durability fatigue. 

Sealant joint design J C BEECH Design of butt and lap joints Sealants in double glazing G B LOWE Types of materials, durability Silicones properties B PARBHOO Range of properties... 

Strength and durability of the adhesive under various stress modes (tension, shear, creep, impact, cyclic, etc.) and stress levels (low, medium, high, etc.) in the intended joint design and when under the influence of the intended service environment. Although we may speak of the strength and durability of the adhesive, it must be emphasized that it is the serviceability and reliability of the joint or bonded assembly that we are ultimately interested in. It is not at all difflcult to take an adhesive with excellent potential and use it in an improper application or with poor bonding technique and make a very poor joint. 

As already stated, the durability or permanence of a bonded assembly is dependent on the intended use and service conditions to which the bond will be exposed. However, the joint design, choice of substrates, adhesive selection, substrate preparation, and primer selection, where appropriate, plus the method of application and assembly all have significant impact on the service life of adhesively bonded materials. Most or all of these considerations are interdependent, for example, the joint design and substrates chosen will limit the range of suitable adhesives that can be employed. In a similar way, the durability of a sealed joint is only as good as the adhesion of the sealant (and primer) to the surfaces forming the joint. Primers and/or sealants will adhere to surfaces only if those surfaces are properly prepared. A very large proportion of all sealant joint failures result from poor or inadequate surface preparation. 

Joint design is discussed in more detail in Chapter 5 and can have an important influence on the durability of the joint so a few general points relating to durability are discussed here. 

Selection of joint design is influenced by limitations in production facilities, production costs, and the desired final appearance of the part. The strength of an adhesive joint is determined primarily by (1) the mechanical properties of the adherend and the adhesive, (2) the residual internal stresses, (3) the degree of tme interfacial contact, and (4) the joint geometry. Each of these factors has a strong influence on joint performance. Figure 7.1 shows the impact of structural adhesive types and cure temperature on the toughness and durability of the bond. 

The materials used in a total joint replacement ate designed to enable the joint to function normally. The artificial components ate generally composed of a metal piece that fits closely into bone tissue. The metals ate varied and include stainless steel or alloys of cobalt, chrome, and titanium. The plastic material used in implants is a polyethylene that is extremely durable and wear-resistant. Also, a bone cement, a methacrylate, is often used to anchor the artificial joint materials into the bone. Cementiess joint replacements have mote tecentiy been developed. In these replacements, the prosthesis and the bone ate made to fit together without the need for bone cement. The implants ate press-fit into the bone. 

To achieve the goal of required performance, durability, and cost of plate materials, one approach is improvement of the control of the composition and microstructure of materials, particularly the composite, in the material designing and manufacturing process. For example, in the direction of development of thermoplastics-based composite plate, CEA (Le Ripault Center) and Atofina (Total Group) have jointly worked on an irmovative "microcomposite" material [33]. The small powders of the graphite platelet filler and the PVDF matrix were mixed homogeneously by the dispersion method. The filler and matrix had a certain ratio at the microlevel in the powder according to the optimized properties requirements. The microcomposite powders were thermocompressed into the composite plate. 

Let us examine some examples of improved service characteristics of metal-polymer friction joints with the help of electrical fields. A metal-polymer joint (MPJ) is a combination of metal and polymer parts operating in coordination [66]. The durability of an MPJ depends on its design, the properties of constituent materials and the operation conditions, including temperature, pressure, mutual displacement velocity, ambient media, physical fields, radiation etc. During operation, an MPJ undergoes certain changes in its material structure, wearing, etc. that impair its performances and life of the joint as a whole (Fig. 4.18, solid arrows). 

After qualification by acute and chronic biocompatibility and biodurability evaluations, clinical studies were conducted to confirm that the implants were highly durable. Medical grade high performance silicone elastomer has now become used in various biomedical applications including construction of flexible bone and joint implants as designed by Swanson ( 9) (Figures... 

The adherend, adhesive, and interphase between them are major faetors in determining bond durability. For example, the simple disruption of the dispersive forees already described indicates that joints made with eomposite adherends will be inherently more stable than those made with metallie adherends. To inerease durability, most metallie and many polymeric adherends undergo surfaee treatments designed to alter the surfaee ehem-istry or morphology to promote primary eovalent ehemieal bonds and/or physieal bonds (mechanical keying or interlocking) to maximize, supplement, or replaee seeondary dispersive bonds. These treatments are diseussed elsewhere [1,3,12,18 24]. An intent of eaeh treatment is to provide interfacial bonding that is resistant to moisture intrusion. 

Changing the environment to which a bond is exposed is probably the most effective means of ensuring good durability bonded structures are not likely to degrade at moderate temperatures and in low humidity. Unfortunately, this usually is not a viable option. However, it may be possible to protect the bond from its external environment, at least for a period of time. Ways to design a joint to do this are discussed below. 

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