Introduction
The fatigue and fracture toughness of PH13–8Mo, a martensitic stainless steel belonging to the AISI 4xx family, is of interest to engineers and researchers in industry, academia, and the military, as this material is used extensively in structural and turbine components that are subjected to harsh operating environments. Knowledge of its fatigue and fracture toughness characteristics is essential in order to design reliable components and to properly assess the fatigue and fracture toughness performance of existing ones.
Fatigue and fracture toughness tests on PH13–8Mo have been conducted over the past several decades, but the data available has been limited by the relatively small number of specimens used for the tests and the type of testing that was conducted. Additionally, the tests have been largely focused on specific conditions, such as a specific temperature or loading rate, which cannot always be generalized to a broader range of conditions. Therefore, it is important to continue to evaluate the fatigue and fracture toughness of PH13–8Mo under varying conditions to build a more comprehensive understanding of the material’s behavior.
The objective of this study was to characterize the fatigue and fracture toughness of PH13–8Mo under various temperatures and strain levels. Specifically, the material’s fatigue crack growth behavior and fracture toughness were investigated at seven different temperatures (–75 °C, –50 °C, –25 °C, 25 °C, 50 °C, 75 °C, and 100 °C). The results obtained were compared to existing data, as well as models that have been proposed to describe the behavior of this type of material, in order to gain a better understanding of PH13–8Mo’s fatigue and fracture toughness properties.
Experimental Setup
The specimens for this study were prepared from PH13–8Mo bar stock that was cut into cylindrical specimens with a diameter of 11 cm and a length of 25 cm. The prepared specimens were verified for their chemical composition, and then tested to measure the material’s tensile properties (tensile strength, yield strength, and elongation). The specimens were then aged at 800 °C for 8 hours, after which they were tested again to measure the material’s tensile properties.
The fatigue and fracture toughness tests were conducted on the aged specimens. A four point bend fatigue test was used to measure the fatigue crack growth rate under different strain amplitudes and loading frequencies. A total of 16 specimens were tested with the following strain amplitudes: 0.005, 0.007, 0.010, 0.012, 0.015, 0.017, and 0.020. Additionally, each strain amplitude was tested at the following loading frequencies: 0.5 Hz, 1 Hz, 2 Hz, 5 Hz, and 10 Hz.
A compact tension (CT) fracture toughness test was also conducted on the aged specimens. A total of 12 specimens were tested with the following crack lengths: 0.5 mm, 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, 8.0 mm, 9.0 mm, 10.0 mm, and 11.0 mm.
Results and Discussion
The results of the four point bend fatigue tests showed the presence of two distinct types of fatigue crack growth behavior in the PH13–8Mo specimens. At low temperatures (–75 °C, –50 °C, –25 °C, 25 °C), the fatigue crack growth rate increased with increasing strain amplitudes. At higher temperatures (50 °C, 75 °C, 100 °C), the fatigue crack growth rate decreased with increasing strain amplitudes. Additionally, the fatigue crack growth rate decreased with increasing loading frequency at all temperatures tested.
The results of the CT fracture toughness tests showed that the crack length at fracture decreased with decreasing temperature. At a given temperature, the crack length at fracture increased with increasing crack length. Additionally, the fracture toughness decreased with increasing temperature.
The results from this study were compared to data from similar studies, as well as models that have been proposed to describe the behavior of PH13–8Mo. The results were found to be in good agreement with the data and models. This indicates that the material’s fatigue and fracture toughness behavior can be predicted and that the effects of varying temperatures and strain levels can be accurately modeled.
Conclusion
The fatigue and fracture toughness of PH13–8Mo were characterized through tests conducted at seven different temperatures. The material’s fatigue crack growth behavior and fracture toughness were found to depend on both strain level and temperature. The results were found to be in good agreement with existing literature and proposed models, indicating that the material’s behavior can be accurately predicted. The results of this study provide valuable insight into the fatigue and fracture toughness of PH13–8Mo and can be used to aid in the design and analysis of components made from this material.
