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Further Fracture Mechanics Applications
Published in Cameron Coates, Valmiki Sooklal, Modern Applied Fracture Mechanics, 2022
Cameron Coates, Valmiki Sooklal
In the safe-life approach, a useful service life is estimated (with a safety factor) based on component testing. The tests use load conditions similar to or which exactly match typical service load spectra. This approach does not consider the damage mechanisms that lead to failure, such as fatigue crack growth; its emphasis is therefore on damage initiation. The safe-life approach is therefore based on the assumption that the structure is initially defect free. The component is replaced at the end of its predicted life, even if failure has not occurred. The standard safe-life approach assumes the availability of a relevant S-N curve and applies a cumulative damage model such as the Palmgren–Miner rule (described in the next paragraph) to calculate the fatigue life. A process schematic of the safe-stress-life approach is shown in Figure 5.1. Note that a safe-strain-life approach can also be adopted.
Effect of Material Properties on Design
Published in Mahmoud M. Farag, Materials and Process Selection for Engineering Design, 2020
Safe-life, or finite-life, design is based on the assumption that the component or structure is free from flaws but the stress level in certain areas is higher than the endurance limit of the material. This means that fatigue-crack initiation is inevitable and the life of the component is estimated on the basis of the number of stress cycles that is necessary to initiate such a crack. Fail-safe design is based on the philosophy that cracks that form in service will be detected and repaired before they can lead to failure. Materials with high fracture toughness, crack-stopping features, and a reliable NDT program should be employed when the fail-safe criterion is adopted. Damage-tolerant design is an extension of the fail-safe criterion; it assumes that flaws exist in engineering components and structures before they are put in service. Fracture mechanics techniques as discussed in Section 2.3 and this section are used to determine whether such cracks will grow large enough to cause failure before they are detected during a periodic inspection.
Cohesive Zone Modelling for Fatigue Applications
Published in Raul D.S.G. Campilho, Strength Prediction of Adhesively-Bonded Joints, 2017
A. Pirondi, G. Giuliese, F. Moroni
The exponent d and the coefficient B depend on the material, temperature, stress ratio R = Pmin/Pmax of the cycle, and frequency (Russel and Street 1987, 1988). An accurate and efficient prediction of fatigue crack growth allows to adopt a “damage-tolerant” design philosophy, i.e. the component or structure may be safely operated even in the presence of some damage up to a limit value before a structure repair or replacement. In other words, a crack may grow in service, but it will not reach critical size before its detection. Essential ingredients of this approach are the knowledge of crack propagation as related with applied loading, and periodical inspections with a frequency ensuring that undetected damage in one inspection will not grow up to critical size before the next inspection. This approach leads to weight savings but also to increased maintenance costs, particularly those related with periodical inspections. Actually, a fail-safe design may not always possible and therefore the “safe-life design” approach must be used. This design philosophy is instead based on the intention of avoiding fatigue crack initiation during the entire lifetime.
Mechanical properties and strength criteria of cement-stabilised recycled concrete aggregate
Published in International Journal of Pavement Engineering, 2019
Yueqin Hou, Xiaoping Ji, Xiuli Su
Theory of fatigue accumulation is the main principle of fatigue analysis and the key theory of estimating safe life under varying stress or strains. Miner’s theory of fatigue accumulation is a typical one. It put forward hypotheses based on fatigue failure of aluminum alloy, thus forming the famous Palingren-Miner linear accumulation damage theory. This theory deems that under a constant stress level (S) and N cycle operations, it will produce complete injury; given n (n < N) cycle operations under S, it will produce partial injury. Meanwhile, it believes that every injury in this process is the same. In other words, the damage rate under S is n/N. If the component is under different stress levels (Si), each stress level will produce a damage rate of D = ni/Ni. When the accumulated fatigue damage reaches 1, the component will be failed. This process was represented in Equations (11). The Miner’s linear accumulation damage theory achieved consistent results with test results in many cases. It also has a simple expression and is widely used: