Md. Shafiul Ferdous

Improvement of Fatigue Crack Growth Behavior in the Case of the Cracked Specimen with Relatively Narrow Width

The improvement of acceleration behavior of crack growth was investigated with constant stress amplitude under negative stress ratio R=-1. Then a technical method to detect the fatigue crack growth was discussed. For example, when the stress amplitude exceeds a critical value, crack growth rate of overloaded specimen became higher than that of baseline which was obtained by crack growth test without applying overload. In some experimental cases, the acceleration of crack growth was observed and that could be happened on practical cases. Stop-holes were drilled at crack tips or in the vicinity of crack tips to remove the plastic zone and the effect of that on crack growth behavior were investigated. Also, steel pins were inserted into the stop-holes and its effect was discussed. Finite element method (FEM) was used to analyze the stress concentration at the edge of stop-holes. Positions of center of the stop-holes were varied at different distance from the crack tips to investigate the effect of stop-holes on fatigue crack growth. Also, stress intensity of base and stop-holed specimen was calculated. Then, the effect of stop-hole was discussed by both the experimental and analytical results. Specially, it was discussed whether the crack growth behavior was improved or not in the case of relatively smaller width specimen.

IMPROVEMENT OF FATIGUE LIFE OF A HOLED SPECIMEN OF ALUMINUM-ALLOY 2024-T3 BY INDENTATION AND HOLE EXPANSION

A method of improving the fatigue life and crack growth behavior of a center holed specimen was investigated. Local plastic deformation was applied around the hole by indentation to achieve the purpose. A series of fatigue tests was conducted on aluminum-alloy 2024-T3. Push-pull tests were performed under a stress ratio of R= -1 and a frequency of 10Hz. The observations of the crack initiation and growth were performed with a microscope, and hardness around the hole was measured by Vickers hardness testing machine. In the present study, the longest fatigue life was observed in the case of an indentation specimen with the highest load. The indentation was performed on both sides of the hole edges. The crack growth rate was decreased by indentation or expansion of the material around the hole. From the experimental results, it is found that the fatigue life and crack growth behavior of a holed or notched specimen can be improved by a simple technical method that is related to the local plastic working.

Method of Detecting Unexpected Load Leading to Crack Growth Acceleration

The crack growth can accelerate after applying unexpected overload or underload during usual operation of machine structure. An acceleration or deceleration of the crack growth was observed depending on the experimental conditions. In the present study, a method of detecting the application of an unexpected hazardous load, that is high levels of overload or underload, was investigated. It is expected that the acceleration of crack growth brings catastrophic failure in machine structure. Therefore, it is useful for machine maintenance to know whether an unexpected load was applied or not by doing daily inspection. In the present study by using center-crack specimens, a simple method of detecting application of overload or underload was investigated by using strain information.

EFFECT OF NaCl SOLUTION SPRAYING ON FATIGUE LIVES OF SMOOTH AND SLIT SPECIMENS OF 0.37% CARBON STEEL

The fatigue crack initiation life and growth rate are affected by experimental conditions. A corrosive environment can be created in a laboratory by means of dropping salt water onto the specimen surface, spraying chloride mist into the experimental chamber, etc. In the case of smooth specimens of some metals, fatigue life is shortened and the fatigue limit disappears under such corrosive experimental conditions. In this study, the effects of intermittent spraying of 3% NaCl solution-mist on corrosion fatigue behavior were investigated. The material used was 0.37% carbon steel. This is called JIS S35C in Japan. Spraying of 3% NaCl solution-mist attacked the surface layer of the specimen. It is well known that the pitting, oxidation–reduction reaction, etc. affect the fatigue strength of metals in a corrosive environment. We carried out corrosion fatigue tests with smooth specimens, holed specimens and slit specimens. Then the effects of such specimen geometry on the fatigue strength were investigated when the NaCl solution-mist was sprayed onto the specimen surface. In the case of lower stress amplitude application in slit specimens, the fatigue life in a corrosive atmosphere was longer than that in the open air. It is discussed that the behavior is related to the crack closure which happens when the oxide builds up and clogs the crack or slit.

FATIGUE BEHAVIOR AND THE RELATIONSHIP BETWEEN CRACK PROPAGATION AND THE SLIT CONFIGURATION OF C/C COMPOSITES

The fatigue and fracture behavior of C/C composites fabricated using fine-woven carbon fiber laminates with α = 0/90° direction were investigated. Also, the phenomenon of crack growth behavior and the shear damage in the fiber bundle was discussed. Slits of several sizes were cut on both sides of a test section and different sizes of slit length were chosen. The effect of the slit configuration on crack initiation and growth behavior was observed. Specimens with blunt-notches and center-holes were also used to compare the fatigue strength and crack growth behavior. Non-propagating cracks were observed and fatigue limit was defined as the maximum stress at which specimen did not break for N = 107 cycles stress application. The longest fatigue life was obtained in the case of specimens with shorter slits. The relationships between fatigue strengths and specimen shapes were analyzed by stress concentration, Kt, and stress intensity factor, KI. The effect of slit configuration on fatigue strength was then discussed regarding both the experimental and calculated consequences.

Hole Expansion by Using Al-Alloy Specimen and 0.45% Carbon Steel Specimen

The enhancement method of fatigue life and the crack initiate and growth behavior of a holed specimen was investigated by using the 2024 Aluminum alloy and 0.45% Carbon steel. The purpose of present study is to propose a simple technical method for enhancement of fatigue life in a notched specimen. Also, the effect of local plastic deformation by cold work on fatigue crack initiation behavior was examined. This paper presents a basic experimental kinematic cold expansion method by inserting and removing a pin through the specimen hole. The shape of cross-section of pin was a circle or an ellipse. It was shown that the fatigue life of the specimen with the cold-worked hole was longer than that of the specimen with non-cold-worked hole for the case of same stress level in aluminum alloy and carbon steel. Also, the fatigue strength was higher in the case of the cold expanded hole. In this study, a methodology of lengthening of fatigue life of holed specimen is shown. Also, the improvement conditions of fatigue life were significantly affected by shape of pin, local hardening and residual stress conditions. The fatigue life improvement of the damaged component of structures was studied.