Thermo-mechanical Fatigue Crack Growth Research Articles

Effect of Microstructure Size on Deformation Kinetics and Thermo-Mechanical Fatigue of 63Sn37Pb Solder Joints

An increased cooling rate following reflow produced a decrease in the microstructure size including the size of the colonies, the Pb-rich dendrites and the eutectic phases. The decrease in microstructure size gave an increased creep rate at room temperature and a decrease in the creep stress exponent. Also, thermo-mechanical fatigue crack growth rate decreased and fatigue life increased with the decrease in microstructure size. These effects of the increased cooling rate were attributed to an increase in boundary sliding which could occur from the increase in boundary area with decrease in microstructure size.

Crack Growth Rate Behavior of a Titanium-Aluminide Alloy During Isothermal and Nonisothermal Conditions

Observations of isothermal fatigue, isothermal fatigue with superimposed load hold times, and thermomechanical fatigue (TMF) crack growth rate behavior of Ti-24Al-11Nb are presented and compared with results from previous studies on titanium and nickel-base superalloys. Elevated-temperature crack growth mechanisms in this alloy, which involve fatigue, oxidation and creep, and the influence of frequency, temperature, and hold-time are discussed. These mechanisms are used to interpret the observations of TMF crack growth. The limitations of current crack growth rate models based on the linear-elastic fracture mechanics parameter, K, are addressed.

Observations of interfacial damage in the fiber bridged zone of a titanium matrix composite

This paper presents the results of both nondestructive and destructive evaluations performed after a fatigue crack growth test run under out-of-phase thermomechanical fatigue (TMF) conditions. Post test evaluation indicates that the damage along the fiber-matrix interface in the fiber bridged zone in the TMF specimen is of much greater scale than the slip region predicted by the shear lag models in their current form. To evaluate the environmental attack on the fiber-matrix interfacial region, a thermomechanical fatigue crack growth test was performed on a single edge notch specimen with clamped ends (15). The specimen was a 4-ply, unidirectional, fiber reinforced titanium with a fiber volume fraction [approx] 36 %. The matrix material was Ti-15Mo-3Nb-3Al-0.2Si (wt %), and the reinforcing fibers were silicon carbide, SCS-6. During the test, the specimen developed two matrix cracks from the corners of the diamond-saw notch. The cracks then propagated perpendicular to the fiber and load direction. Crack length was monitored using electric potential drop (EDP) over the life of the specimen. The crack length was also verified through periodic optical measurements of the surface cracks. The crack growth rate increased initially as the matrix cracks grew away from the notch. As the matrix cracks continuedmore » across the specimen the crack growth rate began to decrease. This type of behavior in composite crack growth is indicative of a developing fiber bridged zone in the wake of the matrix crack. Since the bridging zone continued to develop across the entire matrix crack region, the crack growth rate further diminished to a point that only small crack extensions (0.1 mm) occurred over long periods (6 days). Upon reaching this slow growth rate, the test was terminated prior to fracture after the specimen was subjected to 14,612 thermomechanical cycles over a period of 36 days.« less

Predicting crack growth under thermo-mechanical cycling

Load-controlled tests were conducted on Inconel 718 under thermo-mechanical fatigue (TMF) to evaluate crack growth rate as a function of phase angle between load and temperature. A model is developed to predict TMF crack growth rates based solely on isothermal data. The model uses a linear summation concept based on contributions to crack growth of the two dominant mechanisms which are active at the minimum and maximum temperature of the cycle: mechanical fatigue and environmentally assisted crack growth. It was found that damage which contributes significantly to crack growth occurs only during the increasing load portion of a TMF cycle and only while the time-dependent contribution is an increasing function. A discussion of the dominant mechanisms and a comprehensive review of the literature in TMF crack growth is presented. Applicability of the modeling concepts to other materials is discussed.