Analysis of Fatigue-Crack Growth in a High-Strength Steel—Part II: Variable Amplitude Block Loading Effects

1976 ◽  
Vol 98 (3) ◽  
pp. 208-212 ◽  
Author(s):  
A. M. Sullivan ◽  
T. W. Crooker
Keyword(s):  
Fatigue Crack ◽  
Cyclic Loading ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Pressure Vessel ◽  
Stress Ratio ◽  
High Strength ◽  
Steel Part ◽  
Variable Amplitude ◽  
Block Loading

To investigate load sequencing effects in fatigue-crack growth, surface flaw or part-through crack (PTC) specimens of a high-strength pressure vessel steel were subjected to variable-amplitude cyclic loading. Blocks of different load sequences and stress-ratio (R) patterns and cyclic lengths were employed. Crack-growth rate (da/dN) data were analyzed within the framework of linear elastic fracture mechanics using the stress-intensity range parameter, ΔK. The da/dN data were found to correlate well with a seauence-independent formulation of ΔK derived using nominal mean stress, σm, for the loading blocks studied. However, it proved necessary to utilize the normalizing relationship discussed in “Analysis of Fatigue-Crack Growth in a High-Strength Steel—Part I: Stress Level and Stress Ratio Effects at Constant Amplitude”, to account for stress-ratio effects observed under the variable-amplitude block loading. Only the sequential block pattern of high-low-intermediate produced a significant crackgrowth retardation. Predictions made using the sequence-independent normalizing procedure compare favorably with those using the Willenborg model. It is concluded that useful predictions for structural life under variable-amplitude cyclic loading sequences can be made for a wide variety of high-strength pressure vessel applications using sequence-independent analysis procedures.

Probabilistic Reliability Evaluation of Space System Considering Physics of Failure: Case Study of Fatigue Analysis

2016 ◽  
Author(s):  
Saeed Kiad ◽  
Mohammad Pourgol-Mohammad ◽  
Hossein Salimi
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Reliability Analysis ◽  
Crack Growth ◽  
Stress Ratio ◽  
Exponential Model ◽  
Space Systems ◽  
Physics Of Failure ◽  
Variable Amplitude ◽  
Uncertainty Range

Mission assurance requires due diligence reliability for space systems taking into account limited accessibility, high uncertainty on the life data and high cost of failure. The methods based on physics of failure are promising approaches for durability evaluation of these systems. In this study, the reliability analysis is aimed for space structures, with the focus on fatigue failure. In this research, the deterministic fatigue simulation is conducted on space systems (satellite in orbit, low-level LEO1, made of aluminum 2024-T3), using models with constant and variable amplitude loading. Walker and Forman models are preliminary utilized in this study for life prediction for benchmarking with the experimental results. In the case of the variable amplitude loading, due to the amount of plastic zone in crack tip, the fatigue crack growth rate will be ceased in case of overload. Deterministic crack growth simulation was numerically simulated by using the MATLAB software and has been compared with commercial AFGROW software for verification and was observed proper match with experimental data. In the analysis of stochastic fatigue crack growth, uncertainty is analyzed by using the Monte Carlo simulation. The universal stochastic crack growth model proposed by Yang and Manning, was used for reliability analysis based on giving probabilistic method for the purpose of power and second polynomial models. In this study, these models are evaluated and three models of I) rational model, II) exponential model and III) global model are proposed. In uncertainty analysis, it is observed that by increasing the crack length, uncertainty range is widening. In case of constant amplitude loading with the same stress intensity factor range but different stress ratio, the uncertainty range was widening with increasing stress ratio. In reliability analysis, the exponential model demands less computational resources however it has a lower accuracy. The fractional model, proposed in this research, is based on the modification to Forman model. However, these models don’t consider geometric factor. The Global model, another model proposed in this research, has the capability of considering this aspect. In multiplicative stochastic factor (Yang and Manning method), accuracy of the approximation is most important. By improving the accuracy of this relation, the result accuracy is enhanced. For this purpose, for increasing efficiency of this method, the accuracy of approximation must be increased by corrections prior models or provide new accurate models.

Analysis of Fatigue-Crack Growth in a High-Strength Steel—Part I: Stress Level and Stress Ratio Effects at Constant Amplitude

1976 ◽  
Vol 98 (2) ◽  
pp. 179-184 ◽  
Author(s):  
A. M. Sullivan ◽  
T. W. Crooker
Keyword(s):  
Fatigue Crack ◽  
Stress Intensity ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Stress Level ◽  
Stress Ratio ◽  
High Strength ◽  
Nominal Stress ◽  
Constant Amplitude ◽  
Stress Intensity Range

High-strength pressure vessel steel surface flaw or part-through crack (PTC) specimens were selected for studies of fatigue crack growth rate (da/dN) under constant amplitude cycling to assess the effects of varied stress ratio R (minimum nominal stress/maximum nominal stress, σmin/σmax) and stress level (maximum nominal stress/yield stress, σmax/σys). Analyzed within the framework of linear elastic fracture mechanics, these studies warrant the following conclusions regarding fatigue-crack growth in this material: • Crack growth does not appear to be influenced by stress level, per se, even for stress levels approaching net section yield. • It is moderately influenced by both positive (tension-tension) and negative (tension-compression) stress ratios. • It is principally related to the tensile range of cyclic stress as expressed by the fracture mechanics stress-intensity range parameter, ΔK. Utilizing the results of this investigation, a normalizing relationship expressing da/dN as a function of both ΔK and R, which is applicable to both positive and negative values, is discussed. It is concluded that the stress-intensity range ΔK provides a viable analytical approach to fatigue crack-growth analyses relevant to high-strength pressure vessels.

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