Fatigue Crack Propagation In Several Titanium And Stainless Steel Alloys And One Superalloy

VARIABLE-AMPLITUDE FATIGUE-CRACK-GROWTH TESTS WERE CONDUCTED ON SIMPLE SHEET SPECIMENS MADE OF 7075-T6 ALUMINUM ALLOY. THE NUMBERS AND THE AMPLITUDES OF THE HIGH-LOAD CYCLES APPLIED IN THESE TESTS WERE SYSTEMATICALLY VARIED TO STUDY THEIR EFFECTS ON SUBSEQUENT LOW-LOAD FATIGUE-CRACK GROWTH. THE HIGH-LOAD CYCLES CONSISTENTLY DELAYED SUBSEQUENT FATIGUE-CRACK GROWTH AT LOWER LOAD LEVELS. FOR A GIVEN LOW-LOAD LEVEL, THE HIGHER THE PRECEDING HIGH-LOAD LEVEL WAS, THE GREATER THE DELAY IN CRACK PROPAGATION. FURTHERMORE, THE DELAY IN CRACK GROWTH INCREASED WITH INCREASING NUMBERS OF HIGH-LOAD CYCLES UP TO A LIMIT. ONE HIGH-LOAD CYCLE CAUSED APPROXIMATELY ONE-FOURTH OF THE MAXIMUM DELAY, AND TEN HIGH-LOAD CYCLES CAUSED APPROXIMATELY ONE-HALF OF THE MAXIMUM DELAY. THESE DELAYS PROBABLY RESULTED FROM RESIDUAL COMPRESSIVE STRESSES GENERATED IN THE MATERIAL IMMEDIATELY AHEAD OF THE CRACK TIP DURING THE APPLICATION OF THE HIGH-LOAD CYCLES. ELECTRON FRACTOGRAPHIC STUDIES SHOWED THAT A GIVEN STRESS LEVEL, FATIGUE CRACKS PROPAGATED MORE SLOWLY IMMEDIATELY AFTER THE APPLICATION OF A HIGH-LOAD CYCLE THAN THEY DID IMMEDIATELY BEFORE ITS APPLICATION. THIS LOWER CRACK-GROWTH RATE IS CONSISTENT WITH THE DELAY IN CRACK GROWTH OBSERVED ON THE MACROSCOPIC LEVEL.

Stress ratio effects on fatigue crack growth in sheet aluminum alloys.

When asked to start teaching a course on engineering fracture mechanics, I realized that a concise textbook, giving a general oversight of the field, did not exist. The explanation is undoubtedly that the subject is still in a stage of early development, and that the methodologies have still a very limited applicability. It is not possible to give rules for general application of fracture mechanics concepts. Yet our comprehension of cracking and fracture beha viour of materials and structures is steadily increasing. Further developments may be expected in the not too distant future, enabling useful prediction of fracture safety and fracture characteristics on the basis of advanced fracture mechanics procedures. The user of such advanced procedures m\lst have a general understanding of the elementary concepts, which are provided by this volume. Emphasis was placed on the practical application of fracture mechanics, but it was aimed to treat the subject in a way that may interest both metallurgists and engineers. For the latter, some general knowledge of fracture mechanisms and fracture criteria is indispensable for an apprecia tion of the limita tions of fracture mechanics. Therefore a general discussion is provided on fracture mechanisms, fracture criteria, and other metal lurgical aspects, without going into much detail. Numerous references are provided to enable a more detailed study of these subjects which are still in a stage of speculative treatment.