بررسی تجربی بر روی تاثیر تنش پسماند ناشی از اضافه بار در رشد ترک خستگی در آلیاژ آلومینیوم

The aim of this study is to investigate current methods in linear elastic fracture mechanics (LEFM) and determine a new experimental procedure for their suitability to predict crack growth in aluminium alloy, i.e. Al 7050–T7451, when a compressive residual stress field is introduced by an overload. A basic study has been made on the effect of varying levels of tensile overloads on the crack growth rate in aluminium alloy that are being used extensively in fighter jet aircraft. To the knowledge of the researchers no such results available on Al 7050–T7451 materials. Experiments are performed on the centre-cracked tension specimens at various levels of range of stress intensity factors (ΔK). Typical overload ratios of 1.4, 1.6 and 1.8 that are generally used in service conditions, are investigated for each ΔK level using load shedding to maintain the ΔK value required. Crack growth measurements are performed using both special crack propagation gauges and a newly designed, purpose-built travelling microscope. The average crack growth rate is used to determine an effective ΔK value for each interval using the fatigue crack propagation curve. After the application of overloads, experimental measurements revealed a period of significant retardation before the crack growth rates returned to their original baseline levels. The results from the numerical predictions are compared with the experimental results. The prediction model produces conservative results for both constant amplitude crack-growth and overload induced retarded growth. Wheeler retardation model produces larger improvements and the over predicts the fatigue life in comparison to experimental values.