Isothermal Fatigue Experiments Research Articles

Criteria evaluation for the transition of cracking modes in a single-crystal nickel-base superalloy

Single-crystal nickel-base superalloys frequently experience two distinct fatigue crack growth modes. It has been observed that, under certain conditions, cracks transition from a path perpendicular to the loading direction to a crystallographic slip plane. As crystallographic cracking is associated with an increased fatigue crack growth rate, it is important to be able to predict when this transition occurs. In this work three different criteria for crystallographic cracking based on resolved anisotropic stress intensity factors are evaluated in a three-dimensional finite element context. The criteria were calibrated and evaluated using isothermal fatigue experiments on two different specimen geometries. It is suggested by the results, that a threshold value of a resolved shear stress intensity factor can act as a conservative criterion indicating cracking mode transition. Further, a trend hinting towards a loading frequency dependency could be observed.

Constitutive modeling of elastic-visco-plastic behaviors in aluminum alloys subjected to cyclic loadings at various strain rates

In this article, simulations of the stress–strain behavior in aluminum alloys were investigated under cyclic loadings at various strain rates. Four plasticity approaches were applied to simulate cyclic behaviors. To validate obtained results, strain-controlled tensile-compressive fatigue tests were performed in the low cycle fatigue regime. Isothermal fatigue experiments were conducted at various strain rates. Thermo-mechanical fatigue experiments were carried out at different rates of cooling and heating processes. Numerical results demonstrated a good agreement with experimental data at the mid-life cycle of the material. Material constants for different models were also presented for aluminum alloys, at various strain rates.

Numerical simulations of cyclic behaviors in light alloys under isothermal and thermo-mechanical fatigue loadings

In this article, numerical simulations of cyclic behaviors in light alloys are conducted under isothermal and thermo-mechanical fatigue loadings. For this purpose, an aluminum alloy (A356) which is widely used in cylinder heads and a magnesium alloy (AZ91) which can be applicable in cylinder heads are considered to study their stress–strain hysteresis loops. Two plasticity approaches including the Chaboche’s hardening model and the Nagode’s spring-slider model are applied to simulate cyclic behaviors. To validate obtained results, strain-controlled fatigue tests are performed under low cycle and thermo-mechanical fatigue loadings. Numerical results demonstrate a good agreement with experimental data at the mid-life cycle of fatigue tests in light alloys. Calibrated material constants based on low cycle fatigue tests at various temperatures are applied to models to estimate the thermo-mechanical behavior of light alloys. The reason is to reduce costs and the testing time by performing isothermal fatigue experiments at higher strain rates.

Influence of testing and tempering temperatures on fatigue behaviour, life and crack initiation mechanisms in a 5%Cr martensitic steel

The effects of temperature as well as the initial hardness in the range 42 to 50HRC on the fatigue behaviour and life of a 5%Cr tool steel are investigated. Total strain controlled isothermal fatigue experiments between 200 °C and 600 °C are carried out under 1Hz frequency. The significant softening induced by tempering and/or cyclic loading is related to a strong reduction of the dislocation density clearly observed by transmission electron microscopy and x-ray diffraction. The coalescence of secondary carbides is also involved in the yield strength decrease above 550 °C and during cyclic loading. Increasing the initial hardness reduces the cyclic softening rate and increases the fatigue life. Moreover, several isothermal fatigue experiments with regular interruptions at different fractions of life are performed in order to investigate the crack initiation mechanisms. Three main crack initiation sites were observed depending on testing temperatures: non-metallic inclusions, prior austenitic grain boundary and lath boundary.

Characterization of Lead-Free Solders in Flip Chip Joints

The creep and crack propagation behavior of SnAg3.5, SnAg4Cu0.5, and SnPb37 (as reference) was investigated on flip chip solder joints V=1×10−12 m3. The test specimen consisted of two silicon chips (3.3×3.3 mm), bonded to each other by four flip chip joints (one on each corner). The steady-state creep rate was determined by reversible constant load shear tests. The stress exponents were n=11 for Sn96.5Ag3.5, n=18 for Sn95.5Ag4Cu0.5, and n=2 for Sn63Pb37. The apparent activation energies were Q=79.8 kJ/mol for Sn96.5Ag3.5, Q=83.1 kJ/mol for Sn95.5Ag4Cu0.5, and Q=44.9 kJ/mol for Sn63Pb37. Microstructural analyses indicated that small precipitates of Ag3Sn and η-Cu6Sn5 intermetallics are responsible for the high values for n and Q that were found for the Sn96.5Ag3.5 and Sn95.5Ag4Cu0.5. The crack growth rate was determined by isothermal fatigue experiments on Sn63Pb37 and Sn95.5Ag4Cu0.5 flip chip solder joints. The flip chip solder joints were loaded with strain amplitudes ranging from Δε=0.3–4% and test frequencies f=0.0001-100 Hz at a temperature of T=300 K.

Prediction of thermomechanical fatigue lives in metal matrix composites

To identify the role of silicon carbide participate reinforcement on high-temperature thermomechanical fatigue behavior of Al 2xxx-T4, experiments have been conducted under thermomechanical out-of-phase and in-phase loading conditions. A general constitutive representation, based on Eshelby’s inclusion theory, is used for the determination of volumetric average stresses and strains under cyclic loading of the metal matrix composite. This constitutive representation is used with a life prediction model, based on the matrix stress-strain behavior, which predicts contributions of fatigue, creep, and environmental damages to failure under both isothermal and thermomechanical fatigue loading. In isothermal fatigue experiments at 200 °C and 300 °C, pure fatigue damage and creep damage are the dominant damage mechanisms in the short-life regime. In the long-life regime, however, the stress levels are too low to induce considerable creep damage; so, oxidation damage becomes dominant. When fatigue damage is dominant, the model predicts a decrease in life, based on strain range, with increasing volume fraction of reinforcement. Based on stress range, improved fatigue lives are predicted with increasing volume fraction of reinforcement. The reinforced alloy exhibits longer lives when compressive hydrostatic stresses in the matrix at the high-temperature end of the cycle reduce the creep damage.

Thermo-Mechanical Fatigue of Mar-M247: Part 2—Life Prediction

A life prediction model is proposed based on microstructural observations of damage in thermo-mechanical fatigue and isothermal fatigue experiments on Mar-M247 Nickel based Superalloy. The model incorporates damage accumulation due to fatigue, environment (oxidation and γ′ depletion), and creep processes. The model is capable of predicting lives at different temperatures, strain rates and temperature-strain phasing conditions. The model successfully predicted the shorter lives at high strain amplitudes in in-phase thermo-mechanical fatigue cases and the shorter lives at lower strain amplitudes in out-of-phase thermo-mechanical fatigue cases and the associated crossover in life. The prediction of a nonproportional strain-temperature history (diamond shaped) was very satisfactory. A unified constitutive equation was utilized to predict the stresses, which influenced the creep damage term. The oxidation term is a function of mechanical strain range, temperature-strain phasing and incorporated oxidation and γ′ depletion kinetics.