Alpay Ankara

Effects of short cracks on fatigue life calculations

In this study, the potential impact of the short crack problem in En7A steel with a high content of elongated MnS inclusions has been observed. Experimental short crack data were compared first with the predicted S-N curves determined using long crack growth parameters and second with the calculated lives using only long crack data and using both short crack and long crack data. All these life predictions revealed that long crack data alone cannot be used in fatigue lifetime analyses that treat short crack growth, because they can all lead to dangerous over-predicted lives.

Initiation and early growth of short fatigue cracks at inclusions

In this study, the initiation and early growth behaviour of short fatigue cracks in En 7A steel with a high content of elongated MnS inclusions was investigated, by generating and evaluating data on the growth of short fatigue cracks under various stress levels and stress ratios for the six principal specimen orientations. Short cracks usually initiated at the debonded interfaces between the matrix and the inclusions. If there was no debonding, cracking sometimes occurred in the inclusions. In the early stages, short cracks propagated by a mechanism of inclusion influenced growth. Under low stress levels, usually one short crack was initiated which dominated most of the fatigue life, while under high stress levels there was multicrack interaction.MST/3249A

Short fatigue crack growth in AL 2024-T3 and AL 7075-T6

Abstract Short fatigue crack growth behavior was investigated. The crack growth was monitored using a replica technique for cracks propagating on the specimen notch from an initial length of 40–50 μm to the specimen thickness of 2.3 mm. As part of the AGARD round robin test program Al 2024-T3 and Al 7075-T6 specimens with semi-circular notches on one edge were tested under R-ratios of 0.5, 0, −1 and −2. For each R-ratio several stress levels were used. In order to compare the behavior of short and long cracks, their crack growth rate data were drawn on the same graph. The general conclusion for both aluminum alloys is that the growth rates of short cracks were faster than the rates of long cracks for R-ratios of −1 and −2, no significant differences were observed for R = 0, and the short cracks actually grew more slowly at R = 0.5.

A comparison of short and long fatigue crack growth in steel

In this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions has been investigated by generating and evaluating short crack growth data for the six principal specimen orientations. The anomalous initiation and growth behaviour of short cracks was also compared with that of long crack data for the same material and loading conditions determined before. This comparison revealed the sub-threshold initiation and faster growth rate of short cracks compared with long cracks under the same stress intensity factor range. In order to simulate this discrepancy between short and long cracks, a crack closure model for long crack data was used.

Determination of hot crack susceptibility in various kinds of steels

Abstract The aim of this investigation was to study the hot crack susceptibility of some Turkish and German structural steel plates with a modified varestraint-transvarestraint test method. Testing was accomplished using a Tungsten Inert Gas (TIG) welding process. The results of this study revealed that total crack length increases as welding current increases or punch radius or welding speed decreases. An attempt is made to provide explanations for these phenomena.

Analysis of structural lap joints in metals

Abstract The behaviour of single-lap mechanical joints in mechanical joints in metallic structures was investigated under static tensile loading. To estimate the margins of safety for a given joint an analytical approach was used. In this approach experimental data were also utilized for the analysis of laps. Thus the established method is semi-empirical and the most important characteristics of the approach is that it includes the fastener tilting in single-lap joints as well as the fastener shank deformation that contributes to the total deflection of the joint under an applied load. The experimental data required for the development of the method were obtained from aerospace design sources. The method is compact and easy to use, making it valuable for preliminary design work by facilitating many design iterations to be made for single-lap metallic joints in structures for aerospace and other applications.

The Thermal Loading Response of CFRP Structures

IN AEROSPACE APPLICATIONS widely used fiber reinforced polymer matrix composites may face thermal loading or heat damage due to accidents, engine exhaust impingements and during improper damage repair cycles. Composites subjected to thermal loading undergo changes in the form of chemical reactions (oxidation, degradation, pyrolysis, out-gassing) and physical effects (charring, cracking). These changes in turn cause strength degradation, physical appearance change and structural integrity loss. Heat damage problem in composites is further complicated by the difficulty of detecting the extent of damage. Usual NDT methods such as ultrasonic inspection can only reveal cracks formed within the laminate. The onset of the damage cannot be detected by usual NDT methods thus the understanding of mechanisms of chemical reactions and physical changes is of prime importance for confident application of composites in aerospace applications. The experimental results of a study aimed at providing an insight to the effect of thermal loading of CFRP structures is summarized here. For this purpose CFRP laminates and adhesively bonded structures were exposed to temperatures well above the glass transition point of the resin matrix and then mechanically tested.