| Effect of high temperature exposure on the mechanical properties of cold expanded open holes in 7050-T7451 aluminium alloy (2008) | |||||||||
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| Cold expansion of fastener holes - also known as cold worked holes - is a mechanical treatment technology widely used in the manufacture of airframe components to provide beneficial compressive residual stresses around the fastener hole that delay or retard initiation and growth of fatigue cracks in that region, leading to extension or enhancement of their fatigue life. Cold expansion is generally used in components, which are exposed only to service conditions at ambient temperature. When cold expanded holes are exposed to elevated temperatures during service, e.g. due to maintenance or accidental factors such as fire, the relaxation of residual stresses could potentially affect their fatigue life. Thermal exposure could also affect the static strength properties of the material. To quantify this effect, this investigation examined the effect of thermal exposure on cold expanded open hole specimens manufactured using aluminium alloy AA7050-T7451 - a high strength aluminium alloy used in airframe components in civil and military aircraft. As a result of exposure to a temperature of 350°F (177°C) for periods of time ranging from 1 to 8 hours: (i) the fatigue life of the specimens decreased; (ii) the yield strength decreased linearly with increased exposure time and fell below the minimum allowable value after 1 hour exposure; (iii) the hardness (Rockwell scale, HRB) decreased as the exposure time increased; and, (iv) the electrical conductivity (%IACS) increased with increasing exposure time, but remained within the specification limits set by AMS 2658B (SAE, 2003b). The decrease in the fatigue life of cold expanded open-hole specimens with increasing exposure time was thought to be primarily due to the relaxation of the beneficial compressive residual stresses around the cold expanded hole. The experimental results indicated that whilst the fatigue life did decrease after exposure to 350°F (177°C), for a period of up to 8 hours, the fatigue life remained at levels of about 3 to 4 times that of non-cold expanded hole specimens. This result suggests that the compressive residual stresses introduced by the cold expansion process are relatively stable at the tested temperature.. Cold expansion of fastener holes has been widely used in the manufacture, maintenance and repair of airframe components to provide beneficial compressive residual stresses around fastener holes and to extend their fatigue life. However, some components may experience exposure to high temperatures due to operational, maintenance or accidental factors. The high temperature exposure could lead to relaxation of compressive residual stresses and potentially affect the component's properties such as fatigue life and static strength properties. This report summaries the investigation on the effect of exposure time at a temperature on the mechanical properties of cold expanded open holes in 7050-T7451 aluminium alloy. The experimental results show that for a given exposure temperature of 350°F (177°C), the fatigue life of the specimens decreased with increasing exposure time. When the exposure time exceeds 3 hours, the fatigue life remained relatively unchanged. The yield strength at the same exposure temperature fell below the minimum allowable value after 1 hour exposure.. DGTA | |||||||||
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