Effect Of Single Overload Research Articles

Effect of a single overload on the cyclic R-curve behaviour of a γ-TiAl TNM alloy

The effect of single overloads on the threshold of stress intensity range of a semi-brittle material, a γ-titanium aluminide, was analysed. This material class exhibits a pronounced R-curve behaviour, which is mainly caused by the formation of shear ligaments, crack deflection and branching. Samples with short cracks on deep sharp notches were subjected to overloads of 80 % of the fracture toughness. Contrary to ductile materials, the experiments revealed no significant increase in the long-crack threshold, however a pronounced increase in the slope of the R-curve for the threshold of stress intensity range.

Fractographic analysis of the overload effect on fatigue crack growth in 2024-T3 and 7075-T6 Al alloys

The effect of single overload on the fatigue crack growth in 2024-T3 and 7075-T6 Al alloys was analyzed. Fatigue tests under constant-amplitude loading with overload peak were carried out on V-notched specimens. Fractographic analysis was used as a principal approach to explain the crack growth retardation due to the overload. Scanning electron microscopy (SEM) analyses were conducted on the fractured surface of failed specimens to study the retardation effect. The obtained results show that the overload application generates a plastic zone in both aluminum alloys. The generated plastic zone is three times larger in the case of 2024-T3 compared to 7075-T6, and thus, a significant crack retardation was induced for 2024-T3. The retardation effect due to the overload for 2024-T3 and 7075-T6 lasted for about 10 mm and 1 mm, respectively, from the point of overload application.

Effect of single overload on fatigue crack growth in QSTE340TM steel and retardation model modification

To analyze the effect of single tensile overload on fatigue crack growth behavior, a series of experiments have been conducted under different load ratios and overload ratios on QSTE340TM automobile steel. The results showed that the application of overload extended the fatigue life of the specimens in all cases. The fatigue crack growth rate curves presented different patterns between the case load ratio R = 0.7 and the cases R = 0.1, 0.3, 0.5. Difference between the trends of the closure load and the fatigue crack growth rate in the cases R = 0.1, 0.3, 0.5 was found and discussed. The images of fracture surface were obtained by Scanning Electron Microscope. A new effective stress intensity factor expression was proposed and discussed. A combined model based on both the residual stress effect and the plasticity-induced crack closure effect was proposed by modifying the Wheeler model and applying the new effective stress intensity factor expression. The proposed model gave reliable estimates of QSTE340TM steel for all overload cases and two other materials from references.

Fatigue crack growth of composite patch repaired Al-alloy plates under variable amplitude loading

In this paper, fatigue crack growth of edge crack in 2024 T351 aluminum alloy plate repaired with bonded composite patch under constant and variable amplitude loading (VAL) was predicted. The Al-alloy plate was repaired with symmetric patch “Boron/Epoxy”. Additionally to constant amplitude loading (CAL), the effects of single overload and band overload were investigated for repaired and unrepaired Al-alloy plates. The obtained results confirm the improvement in repair performances by composite patch on fatigue lives and crack growth rates for all applied cycles (CAL and VAL) compared to unrepaired plates. A retardation effect was observed in application of single overload compared to band overload with the same stress ratio (R=0.2) for unpatched plate and characterized by instantaneous delays. However, this retardation effect is increased by the presence of the patch repair which leads to the higher fatigue life. Retardation effect was neglected for lower overload ratio (ORL>1.8). Comparison in fatigue life and crack growth rates under the same overload ratio (ORL=2.4) between repaired and unrepaired plates show the supplementary beneficial effects in combination of overloading and patch repair

The effect of single overloads in tension and compression on the fatigue crack propagation behaviour of short cracks

The crack propagation behaviour in cast quenched and tempered steel after one overload cycle in tension as well as in compression on short cracks is investigated in deep notched specimens. The overload cycle exhibits a significant influence on the fatigue life endurance, due to the formation of an overload plastic zones in front of the crack tip. The crack propagation after overload cycles is investigated by inspection of the fatigue threshold R-curve and fatigue crack propagation rate. Tension overload increased the long crack threshold and reduced the R-curve effect, whereas overloads in compression reduced the crack growth resistance and shifted the threshold value to larger crack extension. A simple FE simulation was also performed to investigate the variation in the contribution of plasticity induced crack closure during crack propagation after the overload. Macroscopic mechanistic and dislocation models are introduced to explain the results obtained.

Overload-induced retardation of fatigue crack growth in polycarbonate

The effect of single overload within an otherwise constant amplitude loading sequence on the retardation of fatigue crack propagation (FCP) behavior of polycarbonate (PC) was experimentally investigated in this paper. An improved compliance method was employed to measure the length of a fatigue crack, and optical and scanning electron microscopes (SEM) were used to observe the features of crack surface and crack tip deformation. It was shown that there was a small crack increment during the application of overload. After that, the crack growth rate reduced to a low level quickly, and then increased gradually until it reached the normal level of crack growth rate. The waveform and overload ratio of the single overload had different effects on the FCP of PC, with the former having slight effect, and the latter larger effect. The overload-induced retardation of fatigue crack growth was mainly due to the formation of crack tip craze zone. The relationship between overload retardation ratio R r and overload ratio R o had been obtained in the form of R r = 0.95 + 0.056 R o 2.72 .

Overload effect on the fatigue crack propagation of PC/ABS alloy

The effect of single overload within an otherwise constant amplitude loading sequence on the fatigue crack propagation (FCP) behavior of the alloy of polycarbonate and acrylonitrile–butadiene–styrene (PC/ABS) is experimentally investigated in this paper. An improved compliance method is employed to measure the fatigue crack length of the specimen. Optical and scanning electron microscopes are used to observe the features of crack surface and the process of crack tip deformation. The overload waveform has slight effect, while the overload ratio has great effect on the crack growth retardation. A small crack increment is produced during overloading. The crack growth rate reduces quickly, and then increases gradually until it reaches the steady crack growth rate level when the loading recovers to normal constant amplitude fatigue loads. Porous or dimple features govern the fatigue crack surfaces.

Effect of single overloads in ductile metals: A reconsideration

Fatigue crack growth tests were carried out under constant-Δ K loading interrupted by a single overload (OL). The specimen material was a ductile austenitic CrNi alloy. Variables of the experimental program were the severity of the OL and the specimen thickness, the latter one in view of plane strain/plane stress aspects. The results confirm the delayed retardation phenomenon as abundantly observed for aluminium alloys. Fractographic observations were made with special attention to crack extension during the OL and base line cycles following the OL, a topic somewhat neglected in previous studies. Striation observations indicate an increased crack growth rate in a small number of cycles after the application of the OL. This is an unexpected new phenomenon, which could be made with innovative fractographic procedures. It is associated with crack tip blunting and crack tip opening displacements for which a model was proposed. The results should be of interest for fracture mechanics prediction models on fatigue crack growth under variable amplitude loading.

P-11 Effects of single overload on crack propagation of specimens with sharp notch in low carbon steel

The purpose of the present study is to evaluate fatigue crack propagation in tensile overloaded specimen with sharp notch. A 6 mm long u-shaped notch was cut in the center section of the specimen. Notch root radii of 0.1 mm, 1 mm and 2 mm were chosen for testing. These specimens were regarded as center notched specimens. The crack propagation behavior of these specimens was compared with a center cracked specimen. Push-pull fatigue tests were carried out under stress ratios R=0, -1, and -1.5. The stress intensity factors of notched specimen were calculated using the body force method. A special investigation was made into the results for ρ=1 mm. After overload, retardation of crack growth was observed under R=0 and -1, for the notched specimens and center cracked specimens. However, where R=-1.5, crack growth acceleration was observed after overload in both types of specimens. Thus, acceleration and deceleration of crack propagation were observed depending on the applied stress conditions. Even though crack growth was accelerated or decelerated, the rate was able to be evaluated by effective stress intensity factor range after 0.1 mm crack growth from notch root in the present experimental cases. This behavior is related that the stress intensity factors of the present notched specimens are almost same as that of the center cracked specimen after 0.1 mm crack growth.

Effect of single overload on fatigue crack propagation behaviour of aluminium 7039 alloy: Kim, J.-S., Byon, U.S. and Kim, S.H. Korean Inst. Met. Mater. (Apr. 1993) 31 (4), 407–416 (in Korean)

Effect of single overload on fatigue crack propagation behaviour of aluminium 7039 alloy: Kim, J.-S., Byon, U.S. and Kim, S.H. Korean Inst. Met. Mater. (Apr. 1993) 31 (4), 407–416 (in Korean)

Effect of single overload on fatigue behavior in the welded haz of low alloy steel

Room temperature fatigue crack growth behavior was obtained for 4140 parent steel, parent heat treated (same as PWHT), as-welded HAZ and PWHT HAZ material under R≈O constant amplitude loading and single tensile overloads with an over load ratio (OLR: Pover/Pmax) of 2.5. Double pass automatic submerged arc welding with AWS EM2 electrode was used. PWHT was performed at 650°C for one hour. Constant amplitude fatigue crack growth behavior was very similar for all four material conditions in the log-log linear Paris region. All material conditions responded favorably to the single tensile overloads with fatigue crack growth retardation ranging from 2.5×105 to 4.5×105 cycles which corresponded to life increases of 250 to 400 percent. SEM analysis indicated many similarities on the fatigue fracture surfaces with predominant ductile quasi-striation morphology.

Effect of single overload in FCP

Single overload tests are carried out on 7075-T6 Alclad sheets. Overload ratios ranging from 1.3 to 2.2 yield exponentially increasing retardation in fatigue crack propagation. The number of cycles of retarded growth is independent of the geometry as compact specimens and center cracked panels give very similar results. Normalization of the retardation cycles ( N d ∗ ) by constant amplitude growth ( n ca ) leads to better correlation. The proposition N d ∗ N CA = f (overload ratio) enables one to explain some of the observed behavior following single overloads.

The effect of single overloads upon fatigue cracks in 5083-H321 aluminium

The application of a single overload cycle to a growing fatigue crack has long been known to delay future growth of that crack. Although much work has been done in this area, considerably less work has been undertaken with a view to examining the effect of secondary influences such as stress ratio upon such retardation. This paper examines the effect of various load related variables upon both the number of delay cycles and the extent of the delay zone. Results are found which seem to indicate that the basic shape of a delay curve is constant—only the scale being affected by loading variation. The criterion of optimisation is kept in mind throughout the experiments discussed in this paper and several economised testing techniques are discussed. One of the most significant techniques is the use of overload experiments to evaluate the constant amplitude behaviour of the material under consideration. The material chosen for examination was 5083-H321. This material, although not often used in high technology areas, is a common industrial material of particular interest to the transport industry.

The effect of single overloads on fatigue-crack propagation in steels

The effect of single overloads on room temperature fatigue crack growth has been studied in two steels (FV 520B and Ducol W30B) of markedly different yield stresses. For a given overload, the amount of retardation was greater for lower baseline alternating stress intensity. In Ducol, retardation increased with decreasing specimen thickness. The results suggest that overload retardation is primarily due to residual compressive stresses generated in the crack -tip region, and is associated with crack-closure effects. An attempt has been made to rationalize the results in terms of a fatigue crack growing through overload plastic zones of different shapes and sizes associated with plane-stress and plane-strain deformation.