Raghuvir Kumar

Influence of stress ratio on fatigue crack growth in mild steel

Abstract Crack propagation experiments were performed on a mild steel side edge notched specimen for various load ranges and stress ratios at constant maximum loads. The life of the specimen increased as the load ratio increased. The crack growth data were analysed in terms of Δ Keff as a function of stress ratio R . Good results were determined for U = 0.7 + 0.15 R (2 + R ) in both cases. Two crack growth rate equations were also developed.

A study of crack closure under constant amplitude loading for 6063-T6 Al-alloy

Abstract Crack closure experiments were performed on 6063-T6 Al-alloy, using a COD gauge for various load ranges ( Δp ) and stress ratios, R . On the basis of the experimental results a model for effective stress intensity range ratio U was developed. This model was found to be a function of the stress ratio, R , and was fitted to existing constant amplitude crack propagation data for 6063-T6 Al-alloy. The crack closure load stabilized after 1 mm initial crack growth.

Effect of single and intermediate tensile overload cycles on effective stress range ratio in 6063-T6 Al-alloy

Abstract In this paper, crack closure experiments were performed on 6063-T6 Al-alloy for various overload ratios at 8 mm crack length. On the basis of these experimental results some power laws are developed to predict minimum effective stress intensity range ratio ( U min ) and number of delay cycles ( N D ). The life of the specimen is also increased by increasing the magnitude of overload cycle. The crack closure load is increased after application of the overload and remains constant for a certain number of cycles. It then decreases to attain the stabilized constant amplitude loading crack closure value.

Effect of stress ratio on effective stress range ratio and crack growth in 6061-T6 Al-alloy

Abstract Crack closure experiments using a COD gauge were performed on 6061-T6 Al-alloy, for various stress ratios. Experimental results show that for a given stress ratio, R , the crack length increases up to 1 mm after which it is stabilized. On the basis of the experimental results a model for effective stress range ratio U , which was found to be a function of stress ratio R , was developed. This model was fitted to existing constant amplitude crack propagation data for the 6061-T6 Al-alloy.

Influence of applied stress ratio on fatigue crack growth in 6063-T6 aluminium alloy

Abstract Crack growth data of 6063-T6 sheet material were analysed with different formulas for ΔKeff as a function of stress ratio R. The data covered R values from 0·1 to 0·5. A good correlation was obtained for d a d N and ΔKeff using the Schijve6 equation for U.

Delay effects in fatigue crack propagation

Abstract The phenomenon of fatigue crack retardation, due to variable single peak overloads in IS-1020 steel, was studied. Selected single edged notched specimens were subjected to single peak overloads for various overload ratios (1·4, 1·6, 1·8 and 2·0). A peak load was found to cause retardation of the crack growth rate, which became more pronounced as the percentage overload was increased. On the basis of these experiments a single power law was developed to predict the delay period for a particular overload ratio. Crack growth also decreased after the application of overload cycles, but after a certain number of cycles it tended to return to the same crack propagation rate for constant amplitude loading.

Investigation of fatigue crack growth under constant amplitude loading

Abstract Crack propagation experiments were performed on 6061-T6 Al alloy for various load ranges and stress ratios. Experimental results show that for a constant load range, the life of the specimen decreased as the stress ratio increased. At constant maximum load, the life of the specimen increased as the load ratio increased. The crack growth data were analysed with a formula for ΔKeff as a function of stress ratio R. The data covered R values from 0 to 0·5. A good correlation was obtained for d a d N and ΔKeff using the Kumar model for U.

Effect of yield stress and stress ratio on fatigue crack closure in 6063-T6 aluminium alloy

Abstract Crack closure experiments were performed on 6063-T6 Al-alloy for different stress ratios, R = 0, 0·1, 0·2 and 0·3 at different prestrains, 0%, 2% and 3%. The growth rate of fatigue crack in the prestrain material was more than that of as-received material. It was shown by crack opening displacement measurements that crack closure occurred to a lesser extent in prestrained material. On the basis of these experimental results a good correlation was obtained for d a d N and ΔKeff for the single edged notched specimen as well as the centrally notched specimen using the developed equation for U. The significance of the increased yield strength for fatigue crack growth is discussed. Fatigue life decreased as the percentage of prestrain increased.

Investigation of yield strength and single cycle overload on crack closure

Abstract Crack closure experiments were performed on 6063-T6 and 6061-T6 aluminium alloys, using a crack opening displacement gauge, for various overload ratios (1·67, 1·88 and 2·06). On the basis of these experiments some relationships are developed. The delay period after application of a single overload increases with increase in the magnitude of overload. The retardation is decreased with increase in prestrain for the same overload ratio. After the overload cycle the increase in U values is less in prestrained material than in the as-received material. Crack growth rate also decreases after application of an overload cycle; it attains a constant amplitude crack growth rate and crack closure value after a certain number of cycles. For the same stress ratio, the delay period in the 6063-T6 alloy was found to be more than in the 6061-T6 alloy. The experimental results when plotted on log-log graph paper, show that N D N CAL vs overload ratio and crack length (when U = Umin) vs overload ratio fit a straight line, from which the power laws are developed.

Effect of variable single cycle peak overload on fatigue life

Abstract Crack propagation experiments were conducted on 6061-T6 Al-alloy, for various overload ratios (1·75, 2·00, 2·15, 2·25 and 2·5). On the basis of these experiments a power law is developed to predict the delay period. The delay period after the application of a single overload increases as the magnitude of overload increases. Crack growth decreases after the application of an overload cycle and after a certain number of cycles it tends to attain its CAL crack propagation rate.