Design failure criteria

Finally, failure analysis is the process of comparing actual performance with the desired performance. Thus, failure analysis is a nontrivial part of the structural design process. Facets of failure analysis including what failure means for a structure are addressed in Section 7.6 on Design Requirements and Design Failure Criteria. 

DESIGN REQUIREMENTS AND DESIGN FAILURE CRITERIA 7.6.1 Introduction... 

The area of design failure criteria impacts, and is a quantitative measure of, the success of a design. Fundamentally, design failure criteria are the statement of the design requirements. The manner in which individual laminae as well as laminates fail is but a part of design failure criteria. Failure of laminae and laminates, as in Chapters 2 and 4, is a fundamental portion of all strength-related failure criteria, but those failures are also determining factors in stiffness-related failure criteria. 

Rules for the design of shells of revolution under internal pressure differ from the Division 1 rules, particularly the rules for formed heads when plastic deformation in the knuckle area is the failure criterion. Shells of revolution for external pressure are determined on the same criterion, including safety factors, as in Division 1. Reinforcement for openings uses the same area-replacement method as Division 1 however, in many cases the reinforcement metal must be closer to the opening centerline. 

A possible adjunct to the laminate design procedure is a specific laminate failure criterion that is based on the maximum strain criterion. In such a criterion, all lamina failure modes are ignored except for fiber failure. That is, matrix cracking is regarded as unimportant. The criterion is exercised by finding the strains in the fiber directions of each layer. When these strains exceed the fiber failure strain in a particular type of layer, then that layer is deemed to have failed. Obviously, more laminae of that fiber orientation are needed to successfully resist the applied load. That is, this criterion allows us to preserve the identity of the failing lamina or laminae so that more laminae of that type (fiber orientation) can be added to the laminate to achieve a positive margin of safety. 

The FPL vertical wall furnace used in our study was described in some detail by Brenden and Chamberlain (6). This furnace is normally used to evaluate the fire endurance of wall assemblies. The basic guidelines for the furnace test method are given in the ASTM E-119 standard (5). The method was designed to evaluate the ability of a structure to withstand a standard fire exposure that simulates a fully developed fire. The furnace is gas fired, and its temperature is controlled to follow a standard time-temperature curve. A load may be applied to the assembly. The failure criterion can be taken as time at burnthrough, structural failure, or a specified temperature rise on the unexposed side of the wall—whichever comes first. The construction of the furnace is not specified in the ASTM E-119 standard. 

In general, a failure criterion (or limit state) defines the conditions under which a structural component can no longer fulfill its design function. Let... 

Design parameters of structures containing defects can be established through the stress intensity factor, K. The failure criterion for structures that contain defects and are subjected to static or monotonically increasing loads is as follows ... 

Failure Criterion. In the design of a propellant structure, the output of the calculation is a tensor quantity in terms of either stress or strain, which must be compared with some experimentally available failure properties. These properties are generally obtained from uniaxial and multiaxial extension... 

The main aspect of design and dimensioning of CMC combustion liners is the integration of CMC components into metallic structures. Therefore the different thermal expansion of the CMC components and the metallic support structure has to be taken into account carefully Based on an anisotropic material model and a failure criterion suitable for the CMC material, Finite Element Analysis supports the design of the liner. 

P(l) The Hart-Smith failure criterion should be used for design. Design strengths shall be determined by experimental testing or taken from manufacturers data, and divided by the appropriate partial safety factor. 

The choice of failure criterion for design will be governed by the amount of measured data available to the designer. If measured data are available, the designer should use the Hart Smith failure criterion. 

Note that these calculations are conservative and are included to allow the designer to use the Tsai-Wu failure criterion in the absence of measured data. However, for an economic design it is recommended that the lamina strengths be determined experimentally and that the HartSmith failure criterion be employed. 

The failure criterion parameter unnotched strength in tension, compression, or shear at any location and direction around the hole circumference is predicted on the basis of the laminate (extensional) stiffness equation in conjunction with a critical strain value (design value) in tension, compression or shear, respectively. Failure of the unnotched laminate is assumed to have occurred when the strain exceeds the critical value (design value). 

The material will have a given strength expressed as stress or strain, beyond which it fails. In order to postulate the failure, it is necessary to have a failure criterion with an associate theory to be able to effect a satisfactory design. Such theories include maximum stress, maximum strain, Tsai-Hill (based on deviatoric strain energy theory) and Tsai-Wu (based on interactive polynomial theory). The Tsai-Wu theory is the most commonly used. 

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