Gary R. Halford

Investigation of residual stress relaxation under cyclic load

Abstract Compressive residual stresses induced by mechanical surface treatment such as shot peening, autofretage, hole expansion, laser shock peening, and low-plasticity burnishing can be highly beneficial to fatigue resistance. Cyclic relaxation of compressive residual stress, however, reduces the benefit. An analytical model is proposed for estimation of residual stress relaxation. Parameters considered by the model include the magnitude and distribution of the residual stress, the degree of cold working required, the applied alternating and mean stresses, and the number of applied loading cycles. An elasto–plastic finite element model was used to demonstrate the model.

Cumulative fatigue damage modeling—crack nucleation and early growth

Abstract Prior research into linear and nonlinear fatigue damage accumulation in metals and alloys is briefly reviewed, and general trends are identified. Nonlinear damage rules offer substantially more accurate, and hence less unconservative, fatigue life predictions than the classical linear damage rule (LDR). Increased accuracy does not require excessive cost of analysis or additional databases. The nonlinear damage curve approach and its double linear damage rule (DLDR) equivalent are models that, at most, require only twice the effort of the classic LDR. These models require no increase in complexity of use, nor do they require additional material property or mission loading information to achieve the improved accuracy. Greatest improvements (factors of 5–10 or more in mission lifetime) accrue when missions include considerable high-cycle fatigue with some severe low-cycle fatigue loadings. Criteria are proposed for judging those severe mission loading circumstances when the classical LDR becomes unacceptably nonconservative.

Serrated flow and deformation substructure at room temperature in Inconel 718 superalloy during strain controlled fatigue

Serrated flow was observed in Inconel 718 during strain controlled fatigue at room temperature when tested after a double aging treatment. The material was also tested in the solution annealed condition. In both conditions, unlocking type serrations were seen in the plastic portions of stress-strain hysteresis loops, from the beginning until the end of the fatigue life. The back stress, exerted by accumulated parallel pile-ups of dislocations in planar slip bands and subsequent relaxation of internal stress by initiating new plastic flow in adjacent grains, is proposed to be the cause of serrated flow in this alloy at room temperature.

Structural analyses of Stirling power-convertor heater head for long-term reliability, durability, and performance

Deep space missions require onboard electric power systems with reliable design lifetimes of up to 10-y and beyond. A high-efficiency Stirling radioisotope power system is a prime candidate for future deep space missions and Mars rover applications. To ensure ample durability, the structurally critical Heater Head of the Stirling Power-Convertor has undergone extensive computational analyses of operating temperatures (up to 650 °C), stresses, and creep-resistance of the thin-walled Inconel 718 bill-of-material. Durability predictions are presented in terms of probability of survival. A benchmark structural testing program has commenced to support the analyses. This paper reports the current status of our durability assessments.

Bithermal fatigue - A link between isothermal and thermomechanical fatigue

A technique for bithermal fatigue testing is presented in which the tensile and compressive halves of the cycle are conducted isothermally at two significantly different temperatures. With reference to experimental results obtained for a nickel-base superalloy, B1900 + Hf, it is shown that bithermal fatigue testing is a simple alternative to thermomechanical fatigue and can provide a conservative determination of thermomechanical fatigue life for creep damage dominated failure modes. Bithermal fatigue results can be directly related to thermomechanical fatigue results through the use of an appropriate damage rule.

Structurally compliant rocket engine combustion chamber: Experimental and analytical validation

A new, structurally compliant rocket engine combustion chamber design has been validated through analysis and experiment. Subscale, tubular channel chambers have been cyclically tested and analytically evaluated. Cyclic lives were determined to have a potential for 1000 percent increase over those of rectangular channel designs, the current state of the art. Greater structural compliance in the circumferential direction gave rise to lower thermal strains during hot firing, resulting in lower thermal strain ratcheting and longer predicted fatigue lives. Thermal, structural, and durability analyses of the combustion chamber design, involving cyclic temperatures, strains, and low-cycle fatigue lives, have corroborated the experimental observations.

Prestraining and its influence on subsequent fatigue life

An experimental program was conducted to study the damaging effects of tensile and compressive prestrains on the fatigue life of nickel-base, Inconel 718 superalloy at room temperature. To establish baseline fatigue behavior, virgin specimens with a solid uniform gage section were fatigued to failure under fully-reversed strain-control. Additional specimens were prestrained to 2 percent, 5 percent, and 10 percent (engineering strains) in the tensile direction and to 2 percent (engineering strain) in the compressive direction under stroke-control, and were subsequently fatigued to failure under fully-reversed strain-control. Experimental results are compared with estimates of remaining fatigue lives (after prestraining) using three life prediction approaches: (1) the Linear Damage Rule; (2) the Linear Strain and Life Fraction Rule; and (3) the nonlinear Damage Curve Approach. The Smith-Watson-Topper parameter was used to estimate fatigue lives in the presence of mean stresses. Among the cumulative damage rules investigated, best remaining fatigue life predictions were obtained with the nonlinear Damage Curve Approach.

Long-term durability analysis of a 100000+HR Stirling power convertor heater head

DOE and NASA have identified Stirling radioisotope power systems (SRPS) as a candidate power system for future deep space exploration missions. As a part of this effort, NASA has initiated a long-term durability project for critical hot section components of the Stirling power convertor to qualify flight hardware. This project will develop a life prediction methodology that utilizes short-term (t<20000 hr) test data to verify long-term (t>100000 hr) design life. The project consists of generating a materials database for the specific heat of alloy, evaluation of critical hermetic sealed joints, life model characterization, and model verification. This paper describes the qualification methodology being developed and provide a status for this effort.

Application of a Thermal Fatigue Life Prediction Model to High-Temperature Aerospace Alloys B1900 + Hf and Haynes 188

The results of the application of a newly proposed thermomechanical fatigue (TMF) life prediction method to a series of laboratory TMF results on two high-temperature aerospace engine alloys are presented. The method, referred to as TMF/TS-SRP, is based on three relatively recent developments: the total strain version of the method of Strainrange Partitioning (TS-SRP), the bithermal testing technique for characterizing TMF behavior, and advanced viscoplastic constitutive models. The high-temperature data reported in a companion publication are used to evaluate the constants in the model and to provide the TMF verification data to check its accuracy. Predicted lives are in agreement with the experimental lives to within a factor of approximately 2.

Fatigue Life Estimation Under Cumulative Cyclic Loading Conditions

The cumulative fatigue behavior of a cobalt-base superalloy, Haynes 188, was investigated at 760°C in air. Initially, strain-controlled tests were conducted on solid cylindrical gage section specimens of Haynes 188 under fully reversed, tensile and compressive mean strain-controlled fatigue tests. Fatigue data from these tests were used to establish the baseline fatigue behavior of the alloy with (1) a total strain range type fatigue life relation and (2) the Smith-Watson-Topper (SWT) parameter. Subsequently, two load-level multi-block fatigue tests were conducted on similar specimens of Haynes 188 at the same temperature. Fatigue lives of the multi-block tests were estimated with (1) the linear damage rule (LDR) and (2) the nonlinear damage curve approach (DCA) both with and without the consideration of mean stresses generated during the cumulative fatigue tests. Fatigue life predictions by the nonlinear DCA were much closer to the experimentally observed lives than those obtained by the LDR. In the presence of mean stresses, the SWT parameter estimated the fatigue lives more accurately under tensile conditions than under compressive conditions.