Risk Management of Composite Structures

Abstract

The two aspects of risk assessment for an event are likelihood of occurrence of the event and the consequence of the event given its occurrence. A typical risk matrix where several events would usually be plotted is shown in Figure 45.1.Composite structures are often used in situations where they are subjected to multiple environments and combined loads. They can fail in a variety of modes, some of which have dire consequences. Knowledge of the risk-likelihood/consequence space is required for proper risk management. For example, space vehicles face combined environment loading that includes dynamic/acoustic/static thermalmechanical loads applied in a wide range of random atmospheric (e.g., hygral) conditions that gradate to the vacuum of space during a mission. These complex conditions and the consequences of possible system failure make risk management to achieve mission-readiness quality both costly and time consuming. This complexity is present to varying degrees in most military and commercial products employing advanced composite designs. Much of the cost and time of assessing/managing risk is due to physical testing. Physical testing is usually done in the design stage to obtain an adequate design and again during production to ensure that the design criteria have been met. Tests that give useful failure information (destructive tests) are usually limited to small portions of a system and a few failure modes. Acceptancetype tests that do not involve failure give minimal information about either the likelihood or consequence of failure for multiple complex loadings and failure mechanisms. The penalty is paid in cost, weight, and quality trade-offs -limited risk management. ProblemThe present method of structural design is based on tailoring structural components and joints to provide the required strength and stiffness response based on finite element stress analyses. Safety factors are -2Engineering Design Reliability Handbook used locally to provide sufficient strength for each component to remain safe under occasional overloading. The factors of safety for each failure mode are usually rules of thumb based on limited experience and test data. The focus is often on local rather than on system considerations. For example, during the initial design phase, the effects of possible local failures due to overloads and discrete source damage are not considered with regard to global structural response and damage tolerance.Addressing combined environments, such as those described for spacecraft, usually involves modifying (increasing) the factors of safety. There is generally little data to define how much to increase the factor of safety. The risk strategy/culture of the enterprise or regulatory agency may drive the decision, or "similar" experience with metals may be used. This drives design risks such as cost and weight that can result in a product that cannot satisfy the customer or market requirements (market failure). Another aspect of the risk is designing products that perform satisfactorily for a require...