Creep Failure under Compression

Compressive loading introduces the possibility of another mode of failure— buckling. It is not sufficient to say that an axial member under compression or a shell type configuration under external pressure can merely buckle, but one must say that it will buckle at some critical time, if the material is viscoelastic Uke a polymer. If the instantaneous Euler elastic buckling load for a colunm is 

Let us estimate the creep buckling time for a column whose material is modeled by a Kelvin solid. The differential equation for the buckling deflection of an ideally elastic column (from any Mechanics of Materials book) is 

Using the viscoelastic correspondence principle (Section 3.8), valid for small deflections on which Equation 4.6 is based, the buckling load for a viscoelastic column can be calculated by replacing (l/ ) by 7(t), thus 

The fact that the buckling load decreases with time for viscoelastic materials is a very important design consideration. 

FIGURE 4.5 Buckling loads for Kelvin solid and Maxwell fluid. 

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In a fuel cell, the membrane is sandwiched between two bipolar plates. This structure puts it under compressive stress, which can change the membrane resistance. It has been found that the resistance of Nafion membranes increased when they were compressed, and the increase was consistent with the elastic compression of the membrane (Satterfield et al, 2006). Casciola et al. (2006) also found that membrane conductivity decay occurs only when the membrane is forced to swell anisotropically along the plane parallel to the membrane surface. In addition to the effect of compression on conductivity decay, polymer membranes in fuel cells undergo creep, which can cause membrane thinning, pinhole formation, and other failure. Stuck et al. (1998) proposed that local stress most likely triggered and/or enhanced the nonuniform thinning of the Nafion membranes in a polymer electrolyte membrane electrolyzer. 

Figure 10.7 (a) Failure lines for grouted and ungrouted granular soils, (b) Drained triaxial test results for silicate grouted coarse and medium sands. (From Ref. 11.15.) (c) Typical stress-strain curve from unconfined compression test on chemically grouted sand, (d) Compression versus time data for creep test on chemically grouted sand, under constant load, (e) Failure time versus percent of unconfined compression failure load. (+) indicates unconfined compression tests, and ( ) indicates triaxial tests with S3 = 25% of Si. 

Cyclic tests provide the best representation of the conditions to which sealants are subjected in practice. They are very complex tests, however, and can be designed satisfactorily only if the material properties are well known from the results of tests using simpler loading patterns and if the rates are related to those of actual joints. Tensile extension at constant rate, stress relaxation under constant strain, and creep under constant stress are three of the simpler tests used to obtain the material properties of polymers. Tensile extension is not the simplest of the three tests (of the four basic variables only temperature can be kept constant), but it has been chosen because it is this type of loading that occurs in the sealant in a joint when the chance of failure is most probable. There is less likelihood of failure when the sealant is compressed in summer than when it is extended in winter. In addition, the tensile test is the least time-consuming and most laboratories are equipped for it. 

A failure of the bolted joints will most likely occur because of excessive loads under the bolt and washer. The high loads on the cover will eventually cause the molded Minion to creep under the bolt and make it lose its initial bolt preload. At this point vibration and external loading will cause further loosening of the bolt and a loss of compression between parts. The proper preload on the fastener is required to prevent loosening of the bolt, provide a frictional force between parts to resist bolt shear, and improve the fatigue resistance of the bolt connection. 

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