Optimum joint strength

Optimum Joint strength may not be immediate following assembly. 

The effect of the bondline thickness on single-lap joints is well-documented in the literature. Most of the results are for typical structural adhesives and show that the lap-joint strength decreases as the bondline increases (da Silva et al. 2006 Adams and Peppiatt 1974). Experimental results show that for structural adhesives, the optimum joint strength is obtained with thin bondlines, in the range of 0.1-0.2 mm. However, the classical analytical models such as those of Volkersen (1938) or Goland and Reissner (1944) predict the opposite. There are many theories that attempt to explain this fact and this subject is still controversial. Adams and Peppiatt (1974) explained that an increase in the bondline thickness increases the probability of having internal imperfection in the joint (voids and microcracks), which will lead to premature... 

In a pilot study, it was discovered that an ultraviolet zone (UVO) based method, which has been developed for surface treating wool fibers, could be used to oxidatively modify polymer surfaces. Electron spectroscopy for chemical analysis (ESCA) and contact angle results indicated that the treatment was effective on PE and a polyetheretherketone (PEEK). It produced changes in surface oxygen chemistry and free energy, which increased polarity and improved wettability of the surface. Composite lap shear tests showed that the treatment gave a marked improvement in adhesion and that an optimum joint strength is achieved at low treatment times (<1 min). 

Garnish and Haskins found that exposure of polypropylene to trichloroethylene vapour for 10 s resulted in a sixfold increase in joint strength using an Epoxide adhesives. The authors concluded that the improved adhesion was due to the removal of a weak boundary layer. However, the treatment causes the formation of a very porous surface, and an alternative explanation for the improved adhesion is the mechanical keying of the adhesive into the porous surface (see Mechanical theory of adhesion). Garnish and Haskins found that the optimum treatment time was about 10 s and that after 25 s the adhesion level was similar to that of the untreated polymer. This reduction is probably due to weakening of the surface region of the polypropylene. 

The Martin Company felt that the most critical portion of their design was the welds. Aliiminum alloy 2014 was chosen because it was considered the most weldable of the high strength aluminum alloys. Martin Company engineers had, over a period of time, developed improved welding procedures which, applied to material in the T6 condition of heat treatment, provided joint strengths well above those considered optimum by the industry. These procedures were assessed by means of room temperature testing. This work was to evaluate the welds under the specified conditions of temperature history. 

Irrespective of pretreatment procedure, optimum bond strengths can be attained if the laminates are dried before bonding to remove any moisture absorbed from the atmosphere. This should be carried out, immediately before joint assembly, in an air-circulating oven at a temperature at which no thermal damage will be imparted to the laminate. 

Material thicker than 3 mm will benefit in joint strength if multiple weld runs are made on each side, ie 6 mm—3 runs, on each side 12 mm—6 runs on each side. The same applies to double V butts, although to obtain optimum strength from single V butts three weld runs should be made on 3 mm thick material and five on 10 mm thick. In all cases the prepared welding groove angle should be about 70 with a root gap of about 0.6 mm. Fig. 3 illustrates the various weld types and runs. 

Tapers (internal or external), or more complex adherend shaping, are excellent methods to reduce the peel stresses at the ends of the overlap and, therefore, to increase the joint strength. Internal tapers with a fillet seem to be the more efficient way to have a joint increase, especially with brittle adhesives and when composites are used. The EE method is a convenient technique for the determination of the optimum adherend geometry however, the complexity of the geometry achieved is not always possible to realize in practice. 

Correct surface preparation is critical to achieving optimum bond strength and joint reliability. Cleaners, such as surfactants and solvents are used to ensure that surfaces to be bonded are clean and free from dust and other loose materials, grease, oils, and other impurities that may affect the adhesion and ultimate bond strength. 

The general sequence of surface preparation for ferrous surfaces such as iron, steel, and stainless steel consists of the following methods degreasing, acid etch or alkaline clean, rinse, dry, chemical surface treatment, and priming. The chemical surface treatment step is not considered a standard procedure, but it is sometimes used when optimum quality joints are required. It consists of the formation of a corrosion-preventing film of controlled chemical composition and thickness. These films are a complex mixture of phosphates, fluorides, chromates, sulfates, nitrates, etc. The composition of the film may be the important factor that controls the strength of the bonded joint. 

Figure 5 of the impact test showed that J-9 polyurethane was able to reduce the peak force value down to 36% that of steel balls without sacrificing mechanical strength. The result from the impact test and the measurement damping coefficient indicate that polyurethane J-9 is the optimum sphere material in the new total elbow joint replacement design. 

The same effect as on aluminum adherend was also found with magnesium and PEI adherend. The optimum concentrations of each of the primers studied were applied to PEI (Ultem 1000) and magnesium alloy (AZ-91). Table 15.6 summarizes the single lap shear strengths of the resulting bonded joints. The results show that PAMAMs are effective in improving lap shear strength on... 

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