David W. Hoeppner

The effect of environment on the mechanism of fretting fatigue

Abstract A randomized complete block fretting fatigue test program was designed to determine statistically the relative roles of the mechanical factor and the chemical factor in the mechanism of fretting fatigue. Fretting fatigue tests were performed in both laboratory air and vacuum environments with the normal pressure and horizontal maximum fatigue stress kept constant at 6000 lbf in−2 and 50 klbf in−2 (within experimental error), respectively. It was found that the number of cycles to failure of the specimens tested in vacuum is between 10 and 20 times longer than that tested in laboratory air. It was determined that the three parameter Weibull distribution best fits the fretting fatigue data for both test environments. The difference in the mean lives of the specimens tested in vacuum and in laboratory air was determined to be significant with 95% confidence. Thus, the alternative hypothesis which states that the chemical factor plays the dominant role in reducing specimen life when fretting occurs simultaneously with fatigue is accepted. Fractographic analysis on the wear surface of the specimen tested in air revealed that pitting was very common while that performed on the wear surface of the specimen tested in vacuum indicated that rewelding of metal was very common. The overall fretting fatigue damage in vacuum was found to be less than that in laboratory air. The results of the fractographic analysis support the alternative hypothesis.

Microstructural and environmental effects on fretting fatigue

Abstract Fretting fatigue characteristics of two microstructures of a .40 .50 carbon steel are discussed in terms of the relative effects of mechanical and corrosive damage. Comparison of vacuum and laboratory air fretting fatigue data of each microstructure verifies the hypothesis that the mechanical damage incurred during the fretting process is of more consequence than the chemical damage due to oxide formation. A microstructural effect is evidenced in that the region of data over which decreases in life become significant is different for each microstructure. The microstructures investigated were a ferrite-pearlite structure in the as-received sheet form and a tempered martensite structure of higher strength.

The effect of crystallographic orientation on fatigue and fretting-initiated fatigue of copper single crystals

Abstract The effect of varying the crystallographic orientation of copper single crystals on their fatigue and fretting-initiated fatigue behavior was studied. Single crystal fatigue specimens were tested under both uniaxial fatigue loading and uniaxial fatigue loading with superimposed fretting. The results show the extreme degree of anisotropy of the individual grains and the strong dependence of both fatigue life and fretting-fatigue life on crystallographic orientation and they help to provide a base on which further models can be built, in order to understand the complexities involved in polycrystalline materials. It is suggested that surface deformation characteristics, slip mode and slip character are extremely important variables in the frettingfatigue process.

Sustained load crack growth in A533B-1 steel under neutron irradiation in a water environment

A study of the combined effects of radiation, water and temperature on sustained load crack growth behavior of reactor pressure vessel steel A533B-1 is reported. To complete this study wedge opening loading (WOL) T-type fracture toughness specimens were prepared from a sample of A533B-1 steel which had a copper content of 0.13%. The crack length change was measured after 939 hr of irradiation in a water environment. An electrical potential method was successfully used to measure the crack length of rusted radioactive specimens. Sustained load crack growth occurred at initial stress intensity factor KIi as low as 50 MN/m32. The value of stress corrosion cracking threshold factor KIscc after neutron irradiation in a water environment appears to be in the range of 47MN/m32. The results of neutron irradiation in a water environment are to apparently increase the susceptibility of A533B-1 steel to stress corrosion cracking and hydrogen embrittlement.

Contact stress solutions using computer techniques

Abstract The use of a computer program to solve example problems involving contact stresses is presented as an initial step in the development of computerized stress analyses involving fretting fatigue loadings. The utilization of the computer greatly simplifies the calculation of contact stresses.

An apparatus for investigating fretting fatigue in vacuum

Abstract An apparatus needed for investigating the effect of environment on fretting fatigue is described.