Failure modes compression

The second failure mode to consider is fatigue. The drum will revolve about once every second, and each part of the shaft surface will go alternately into tension and compression. The maximum fatigue stress range (of 2 x 56 = 112 MPa) is, however, only a quarter of the fatigue limit for structural steel (Fig. 28.5) and the shaft should therefore last indefinitely. But what about the welds There are in fact a number of reasons for expecting them to have fatigue properties that are poorer than those of the parent steel (see Table 28.1). 

Different factors contribute to the mechanical properties of plant tissue cell turgor, which is one of the most important ones, cell bonding force through middle lamella, cell wall resistance to compression or tensile forces, density of cell packaging, which defines the free spaces with gas or liquid, and some factors, also common to other products, such as sample size and shape, temperature, and strain rate (Vincent, 1994). Depending on the sample properties (mainly turgor and resistance of middle lamella), two failure modes have been described (Pitt, 1992) cell debonding and cell rupture. 

The failure mechanisms of interest in reinforced masonry wall elements include flexural, transverse shear, in-plane shear and in some cases, combined axial compression and flexure. Buckling failure modes of compression elements and connection failures are to be avoided. 

It is important to operate the fuel cell at different compression pressures in order to determine the correct compression pressure for a DL material. If the applied compression pressures are too high, the DLs may deform, both the porosity and permeability of the DL decrease, and the probability of failure modes increases significantly. On the other hand, if the pressures are too low, then gas leaks and serious contact resistance between the components of the cell may be present. Various studies have been presented in which the compression pressure of the fuel cell is varied in order to observe how the cell s performance is affected [25,183,252]. In general, there is an optimal compression pressure range in which the cell s performance is the highest however, this depends on the DL material and on the MPL thickness (see Figure 4.21). 

Shih, G.C. and Ebert, L.J. (1986). Interface strength effects on the compressive-flexure/shear failure mode transition of composites subjected to four-point bending. J. Mater. Sci. 21, 3957-3%5. 

Sohi. M.M., Hahn, H.T. and Williams, J.G. (1987). The effect of resin toughness and modulus on compressive failure modes of quasi-isotropic graphite/epoxy laminates. In Toughened Composites. ASTM STP 937 (N.J. Johnston cd.). ASTM. Philadelphia, PA. pp 37 60. 

Some of the best uses of Kevlar are in aircraft composites and flak jackets for military and police uses, which require high tensile strength and high modulus per unit of weight. It also has the property of damage tolerance, as it has a ductile compressive failure mode, which is very different from the brittle failure of carbon fibers. 

In all the S4/0 specimens the delamination crack stops after only 5 mm of propagation in the pinned block. The failure mode then changes from delamination crack propagation to a bending failure of the arms. The toughness at the time of failure is 7000 J/m. TTie calculated bending stress in the beam is (Tb=l lOOMPa. The local curvature leads to a compressive failure strain in the outside layers of 1.3%. 

Some agglomerates of different materials have been observed to fail because of internal flaws driven by a number of stresses (e.g., internal tensile stress cracks in the surface plastic flow at the surface between the agglomerate and platen and shear stress within the sphere). For brittle particle agglomerates with significant internal flaws, the tensile strength is small compared to the compressive and shear strength, and failure is likely initiated by the internal tensile stress. In any case, a careful microscopic examination of failed pieces can provide much information on the dominant failure mode (Bika et al., 2001). 

Another feature of AGM separators is their compressibility. With compression of the plate and separator stack, this AGM property guarantees good plate-separator contact, even if the plates are not perfectly smooth. Also, battery assembly is facilitated since the stack can be easily inserted into the cell after compression to a thickness lower than the cell dimension. An undesirable result of the compressibility is that the AGM separator does not exert sufficient resistance against expansion of the positive plate during battery cycle-life. This expansion is particularly prevalent in deep-cycle applications and can cause the battery to suffer premature capacity loss (PCL) via reduced inter-particle conductivity — a phenomenon known as PCL-2 [7]. In the literature, two additional characteristics, which are related to the PCL-2 failure mode, are discussed, namely, AGM separators shrink when first wetted with electrolyte and their fibres can be crushed at high pressure levels [8-10]. These features result in a loss of separator resilience, i.e., a lessening of the ability to display a reversible spring effect. 

FYom the multitude of intricate corrosion processes in the presence of mechanical action (friction, erosion, vibration, cavitation, fretting and so on) it is justified to touch upon corrosion types joined under a single failure mode induced by mechanical stresses. These are the stresses that govern the corrosion wear rate of metals during friction. Such processes are usually called corrosion stress-induced cracking in the case that the mechanical action is effective only in one definite direction, or otherwise termed corrosion fatigue in the case that compressive and tensile stresses alternate within cycles. In spite of the differences between the appearance of these corrosion types, they have much in common, e.g. fundamental mechanisms, the causes, and they overlap to a certain degree [19]. 

The other difficulty concerns the tendency for different failure modes to occur. For example, materials with mat reinforcement will fail in tension, while a fully unidirectional specimen will often fail on the compression face through local microbuckling assisted by the local compression and shear loads at the central loading roller(s). 

In general, the notched Charpy (ISO 179) [26] and Izod (ISO 180) [27] tests are not meaningful for composites, and ISO 179 recommends that only unnotched specimens should be tested. The difficulty with these tests is that in the notched condition the majority of specimens tested perpendicular to the plane of the test panel delaminate at the root of the notch. This reduces the specimen to a thinner version of the unnotched specimen, which as described above for the interlaminar shear test (see Section 5.4) is susceptible to compression-initiated failures under complex local loads. Specimens cut in the plane of the laminate or sheet will be less susceptible to delamination at the notch tip, and crack growth will be possible from the notch tip. However, other compression shear failure modes are still possible in some composites, and they will not be loaded in this direction in most applications. 

The standard mechanical tests, as described in Section 7. can normally be undertaken with care for composites as a function of temperature. The difference between fiber- and matrix-dominated properties can result in different temperature dependencies. Changes in the residual thermal stresses present can occur both between the fibers and the resin, and between layers, in particular between 0" and 90 orientated unidirectional layers. Care needs to be taken in assessing the failure mode, particularly in flexural and compressive tests where there can be changes, particularly at elevated temperatures, due to the matrix providing a lower degree of support to the fibers, thus encouraging compression failure. 

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