Statistical Analysis of Fatigue Related Microstructural Parameters for Airframe Aluminum Alloys

Abstract

This paper presents the results of statistical analysis of the fatigue related microstructural parameters, mainly on constituent particles and pores, in 7050-T7452 aluminum forging materials. The statistical distributions of the particle and pore area data, which were obtained from the metallographic measurements on polished surfaces, were fitted well with three-parameter lognormal functions. Comparative studies were carried out for the particle and pore size distributions between the core and periphery materials, and between the hand and die forging materials. An extreme value theory-based model was investigated to correlate the overall material particle distribution with the fatigue crack-nucleating particle distribution. The results indicated that the particle size is not the only parameter affecting the fatigue process; other parameters, such as grain size and grain orientation, are also important parameters for microstructure-based fatigue modeling. Therefore, orientation image microscopy (OIM) analyses were carried out on pristine 7050 samples, on different planes, to determine the grain orientation distributions. Finally, a preliminary OIM analysis was performed on a fractured sample to measure the misorientation angles between the grains near the crack nucleation and short crack regions. It is expected that the test observations and quantitative microstructural data could help to understand the effects of microstructure on the fatigue process and develop a microstructure-based fatigue modeling.

A review of equivalent pre‐crack sizes in aluminium alloy 7050‐T7451

“…3 Statistical techniques were † † As shown later, the log-normal was considered the best distribution for these EPS samples. ‡ ‡ Other studies of discontinuity distributions or probability of failure were not considered relevant here as either long-crack growth derived EIFS were used or total life and/or growth from a machined slot (many millimetres long) had been considered, for example, Refs [28][29][30][31][32]. The unknown initiating source sample possibly contained EPS with mixed initiating flaw types or just undefined because of damage.…”

“…‡‡ ‡‡ Other studies of discontinuity distributions or probability of failure were not considered relevant here as either long‐crack growth derived EIFS were used or total life and/or growth from a machined slot (many millimetres long) had been considered, for example, Refs .…”