Failure Stress

Subsection A This subsection contains the general requirements applicable to all materials and methods of construction. Design temperature and pressure are defined here, and the loadings to be considered in design are specified. For stress failure and yielding, this section of the code uses the maximum-stress theory of failure as its criterion. 

Internal-pressure design rules and formulas are given for cylindrical and spherical shells and for ellipsoidal, torispherical (often called ASME heads), hemispherical, and conical heads. The formulas given assume membrane-stress failure, although the rules for heads include consideration for buckling failure in the transition area from cylinder to head (knuckle area). 

Part AD This part contains requirements for the design of vessels. The rules of Division 2 are based on the maximum-shear theoiy of failure for stress failure and yielding. Higher stresses are permitted when wind or earthquake loads are considered. Any rules for determining the need for fatigue analysis are given here. 

In an isotropic material subjected to a uniaxial stress, failure of the latter type is straightforward to predict. The tensile strength of the material will be known from materials data sheets and it is simply a question of ensuring that the applied uniaxial stress does not exceed this. 

In the maximum stress failure criterion, each and every one of the stresses in principal material coordinates must be less than the respective strengths otherwise, fracture is said to have occurred. That is, for tensile stresses,... 

Figure 2-37 Maximum Stress Failure Criterion (After Tsai [2-21])...

The maximum strain failure criterion is quite similar to the maximum stress failure criterion. However, here strains are limited rather than stresses. Specifically, the material is said to have failed if one or more of the following inequalities is not satisfied ... 

The only difference between the maximum strain failure criterion. Equation (2.125), and the maximum stress failure criterion, Equation (2.118), is the inclusion of Poisson s ratio terms in the maximum strain failure criterion. 

As with the maximum stress failure criterion, the maximum strain failure criterion can be plotted against available experimental results for uniaxial loading of an off-axis composite material. The discrepancies between experimental results and the prediction in Figure 2-38 are similar to, but even more pronounced than, those for the maximum stress failure criterion in Figure 2-37. Thus, the appropriate failure criterion for this E-glass-epoxy composite material still has not been found. 

A common use is with a plastic hub or boss accepting either a plastic or metal insert. The press fit operation tends to expand the hub creating a tensile or hoop stress. If the interference is too great, a very high strain and stress will develop. The plastic product will (1) fail immediately by developing a crack parallel to the axis of the hub to relieve the stress, a typical hoop stress failure, (2) survive assembly but fail prematurely when the product is in use for a variety of... 

Moisture can also result in undesirable dimensional changes occurring in AN—K nitrate based expls, caused by changing crystal habit on thermal cycling. The physical stresses produced may cause stress failures of their containers (Refs 26 32)... 

The PF system creates a fracture network by forcing compressed gas into a formation at pressures that cause stress failure. These fractures increase the formation s permeability. Increased permeability can greatly improve contaminant mass removal rates. PF can also increase the effective area that is influenced by each extraction weU and can intersect new pockets of contamination that were previously trapped in the formation. The ARS PF technology is patented and is commercially available. According to the vendor, it has been used at over 135 federal and private sites in the United States, Canada, Japan, and Belgium. 

Contours of maximum principal stress in the first slice (near the gas inlets) and the sixth slice (near the gas outlet) are shown in Figures 5.11 and 5.12 respectively. It can be seen that the stack is partially under compression and partially under tension due to the mismatch in the thermal expansion coefficient of the materials and non-uniform temperature. In each cross-section, the stresses are higher near the top of the stack than near the bottom. Also, the stresses are higher near the gas outlet than near the gas inlets. Maximum tensile and compressive stresses in all the slices are found to be 60 MPa and 57.2 MPa respectively which are in the electrolyte layer of the last slice. The maximum stresses in all the layers are found to be well within the failure limits of their respective materials and hence thermal stress failure is not predicted for this stack. 

The onset of powder motion in a hopper is due to stress failure in powders. Hence, the study of a hopper flow is closely related to the understanding of stress distribution in a hopper. The cross-sectional averaged stress distribution of solids in a cylindrical column was first studied by Janssen (1895). Walker (1966) and Walters (1973) extended Janssen s analysis to conical hoppers. The local distributions of static stresses of powders can only be obtained by solving the equations of equilibrium. From stress analyses and suitable failure criteria, the rupture locations in granular materials can be predicted. As a result, the flowability of granular materials in a hopper depends on the internal stress distributions determined by the geometry of the hopper and the material properties of the solids. 

It is a well-known fact that failure of an aircraft component can have catastrophic consequences such as loss of precious life and aircraft. It is obvious from Table 7.3 that failures due to fatigue are predominant in aircraft components. When the component is no longer able to withstand the imposed stress, failure will occur. Thus failures are associated with stress concentrations which can occur due to ... 

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