Joint failure

Joint panel shear failure joint shear capacity Vjh (corresponding to the average principle tensile stress pt = 0.42Vfcm (Priestley 1997), with fcm average concrete cylindrical compressive strength) is reached for column shear Vc = 42.8 kN ... 

Abstract To ensure better performance for a range of existing reinforced concrete structures in seismic regions with substandard structural details, seismic retrofit is an economical solution. Hence, this chapter presents some of the available results in which fiber-reinforced polymer (FRP) composites can be used for damage-controllable structures. For example, the performance of existing reinforced concrete structures whose components are vulnerable to shear failure, flexural-compression failure, joint reinforcement bond failure, or longitudinal reinforcement lap splice failure and retrofitted with FRPs is described. Novel concepts of modern constructions with controllability and recoverability using FRP composites are addressed. 

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Heat for soldering is usually obtained from torches. The high conductivity of copper makes it necessary to use large flames for the larger sizes, and for this reason the location in which the joint will be made must be carefully considered. Soldered joints are most widely used in sizes 2 in and smaller for which heat requirements are less burdensome. Soldered joints should not be used in areas where plant fires are hkely because exposure to fires resiilts in rapid and complete failure of the joints. Properly made, the joints are completely impervious. The code permits the use of soldered joints only for Category D fluid service and then only if the system is not subject to severe cychc condions. 

Because of the inherently greater susceptibility of expansion bellows to failure from unexpected corrosion, failure of guides to control joint movements, etc., it is advisable to examine critically their design choice in comparison with a stiff system. 

As discussed in Section 29.2.5, jointing of two different metals (copper being one) causes electrolysis at the joints, leading to corrosion and failure of the joint. To avoid this, it is recommended that the same procedure be adopted as discussed in Section 29.2, and where the electrode and the connecting ground strip are of the same metal, that the joints are riveted or welded with the same metal after making the surface. Soldering is not recommended. 

Aluminium and copper conductors start oxidizing at about 90°C. The oxides of aluminium (AI2O) and copper (CuO) are poor conductors of electricity. They may adversely affect bus conductors, particularly at joints, and reduce their current-carrying capacity over time, and lead to their overheating, even to an eventual failure. Universal practice therefore is to restrict the operating temperature... 

The path of failure of an adhesive joint can give information about the mechanism of failure if analysis of the elemental and chemical composition can be conducted along the path. Several authors have performed such analyses by loading the adhesive joint until it fractures and then using XPS to analyze each side of the fracture. 

Irregular Plant maloperation (e.g. unauthorized venting ) Plant failure - spillages, pipe joint failures Start-up/shutdown Dismantling/demolition Unauthorized waste incineration (rubbish burning) Fires ... 

Another distinction to be made is illustrated with the peel test shown in Fig. 1. Application of stress may cause the joint to fail either adhesively or cohesively . Adhesive failure, shown in Fig. la, is thought ideally to correspond to a perfect... 

Fig. 1, Schematic of commonly u.sed methods for testing the strength of adhesive joints, (a) Peel test. Note that the peel angle can be changed depending on the test requirements, (b) Double overlap shear test. In this test, the failure is predominantly mode II. (c) Single overlap shear test. In this test the failure mode is mixture of mode I and mode II. (d) Blister test.

The interdiffusion of polymer chains occurs by two basic processes. When the joint is first made chain loops between entanglements cross the interface but this motion is restricted by the entanglements and independent of molecular weight. Whole chains also start to cross the interface by reptation, but this is a rather slower process and requires that the diffusion of the chain across the interface is led by a chain end. The initial rate of this process is thus strongly influenced by the distribution of the chain ends close to the interface. Although these diffusion processes are fairly well understood, it is clear from the discussion above on immiscible polymers that the relationships between the failure stress of the interface and the interface structure are less understood. The most common assumptions used have been that the interface can bear a stress that is either proportional to the length of chain that has reptated across the interface or proportional to some measure of the density of cross interface entanglements or loops. Each of these criteria can be used with the micro-mechanical models but it is unclear which, if either, assumption is correct. 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Fig.. 24. Schematic drawing of the visual appearance of the failure surfaces of lap joints prepared from hot-dipped galvanized steel substrates. Reproduced by permission of John Wiley and Sons from Ref. [41].

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