On some basic problems of fatigue research in engineering

Electrical resistivity of an incrementally fatigued lead alloy: Lawson, L. and Meshii, M. Scr. Metall. Mater. July 1991 25, (7), 1713–1717

Fatigue strength of a groove weld on steel backing: Kulak, G. L. and Baker, K. A.J. Constr. Steel Res. 1989 12, (3–4), 261–278

An approach for predicting fatigue crack initiation life of a low alloy steel below room temperature

In view of safety design against fatigue failure in cylindrical storage tank of low temperature use, low cycle and low temperature fatigue bebaviour of shell to annular tee-fillet welded joint in tank was investigated experimentally and analytically, in case of liquefied propane gas tank, using a displacement-controlled bend test of model specimens.The experiments and analyses were done either by strain and displacement measurements around weld toe in annular plate under cyclic deformation or by finite element analysis mainly to clarify quantitatively the effects of type of weld joint as well as dimensions of weld joint, including the effects of annular plate thickness, on the fatigue crack initiation life of tank.Principal results obtained are as follows.(1) In bend test of model specimen, a distribution of strain range in front of weld toe indicates that its maximum value is found at weld toe as far as the deformation range examined, although maximum value of strain distribution is found at 510mm distant from weld toe under large extent of deformation.(2) Experiments and analyses show that there is no difference practically between fillet weld joint and full penetration joint in cyclic deformation as well as in fatigue strength.(3) The expression of evaluating the fatigue crack initiation life in the displacement controlled bend test of model specimen was derived in terms of weld joint parameters, which isNc=2.3×1015·ρ0.35/Δδp3.78t3.86 (SL) 0.19 (unit in mm) where Nc, ρ, Δδp, t, SL denotes fatigue crack initiation life, weld toe radius, displacement range at the loading point, annular plate thickness, and leg length of shell plate respectively.(4) By introducing a parameter θS-A, an angle which defines an extent of deformation around weld toe, the fatigue crack initiation life in tank is predicted from the results on model specimen.(5) The finite element analysis shows that as to the bend test on model specimen, the increase of annular plate thickness decreases the initiation life in displacement controlled test, and increases it in load controlled test. However, the incorporation of the results on model specimen to the behaviour in deformation of the tank indicates that these two expressions are essentially the same as far as the fatigue crack initiation in tank concerned. As of now, it seems that the thicker the annular plate becomes the longer the fatigue crack initiation life in this type of tank.

Fatigue crack initaition and propagation from hole defects

An analysis is described of a welded stainless steel box beam that experienced a structural failure during fatigue testing. Cracks initiated at notches caused by partial penetration welds and grew to a length of several centimetres in about 1000 load cycles. The objective here is to describe the characterization of fracture toughness and fatigue crack initiation for the precipitation-hardening stainless steels used for the welds and parent plate of the beam. Three-point bend specimens were used to measure both fatigue crack initiation life and the J-integral fracture toughness of the parent plate and weld metal in various conditions. The notch fatigue analysis method of Barsom and Rolfe was used to analyze the crack initiation test results. The crack was grown further, side notches were added, and JIc tests were performed using ASTM Test Method for JIc, a Measure of Fracture Toughness (E 813-87) with a modified load-line-displacement unloading-compliance procedure to measure crack growth. Conclusions drawn from the work include the following: (a) JIc can be accurately determined with a three-point bend test using load-line displacement to measure crack growth by unloading compliance (An accurate expression for a/W in terms of load-line displacement was developed.); and (b) fatigue crack initiation life and JIc fracture toughness of stainless steels in various conditions were characterized. The welded and aged condition, with no intermediate solution treatment, showed unstable, cleavage-type fracture and resulted in a low JIc value and a short initiation life.

This study presents the effects of the geometry of the notches on the fatigue life, crack initiation, and growth in mixed mode and first mode of the fracture. For this purpose, some samples were tested with U-shaped notches with different radii and depths and various orientations with respect to the loading directions under fatigue loads. Using the fatigue crack growth rates, the Paris law coefficients were obtained for the used material in different conditions. It was shown that these coefficients are independent of the geometry of the samples. Fatigue crack growth behaviors in the mixed mode tests were also in good agreement with the result of the numerical simulations. Predictions of the maximum tangential stress criterion and numerical simulation for the position of the crack initiation on the notch root were compared with the tested observations, which showed good agreement. The fatigue life of the test samples was compared with the analytical results provided by the Manson-Coffin law. It was shown that a 25% decrease in notch depth increases the fatigue life by 4 to 6 times, also, an increase of 0.5 mm in the notch radius increases the fatigue life by 40%. Finally, the fracture surface of the samples was checked using an optical microscope. This study showed that the fracture surfaces have one or two lateral shear lips and plane stress conditions were established.

Fatigue life estimation accuracy of mechanical parts and assemblies has always been the source of concern in different industries. The main contribution of this article lies in a study on the accuracy of different multiaxial fatigue criteria, proposing and investigating the accuracy of four optimized fatigue crack initiation life estimation methods—volume, weighted volume, surface and point, thereby improving the multiaxial fatigue life estimation accuracy. In order to achieve the goal, the fatigue lives of bolt clamped specimens, previously tested under defined experimental conditions, were estimated during fatigue crack initiation and fatigue crack growth and then summed together. In the fatigue crack initiation part, a code was written and used in the MATLAB software environment based on critical plane approach and the different multiaxial fatigue criteria. Besides the AFGROW software was utilized to estimate the crack growth share of fatigue life. Experimental and numerical results showed to be in agreement. Furthermore, detailed study and comparison of the results with the available experimental data showed that a combination of Smith–Watson–Topper approach and volume method results in lower error values, while a combination of Fatemi–Socie criterion and surface or point method presents estimated lives with lower error values. In addition, the numerical proposed procedure resulted in a good prediction of the location of fatigue crack initiation.

The life of a component subjected to a constant stress can be determined using the S-N diagram. Prediction of cumulative fatigue damage or fatigue life of a component subjected to a given stress sequence of varying amplitude is difficult, if not impossible. Miner's rule and Kramer's equation can predict cumulative fatigue damage with a reasonable degree of accuracy for limited cases. The development of Kramer's equation was studied and a modification suggested. Cumulative fatigue data generated using aluminium alloys 2011-T3 and 2024-T4, were analysed using Miner's, Kramer's and the modified equations. While Miner's rules and Kramer's equation predict cumulative fatigue damage with tolerable error for a few cases, the modified equation has shown close agreement between the experimental and theoretical values of fatigue life or cumulative fatigue damage for the materials studied in this investigation.

Developing a full range S–N curve and estimating cumulative fatigue damage of steel elements

In this paper, fatigue crack initiation and propagation properties of three structural steels with different static strength and cyclic softening behavior were evaluated and compared with the estimation results based on the local strain response. Steel C had the highest cyclic softening rate, which was 10 times that of steel A and twice that of steel B. The relationship between strain range and fatigue life in the fatigue life region shorter than 105 cycles was almost the same regardless of the test steels. The fatigue crack initiation life from the notch bottom of the SENT specimen was almost the same independent of static strength and cyclic softening rate. The crack initiation life estimated from the Strain range versus fatigue life equation using the local strain response measured by DIC was roughly in agreement with the experimental results. Steel C had the highest crack opening load and the slowest fatigue crack propagation rate compared to the other two steels. The local strain range at the fatigue crack tip showed a good correlation with the fatigue crack propagation rate irrespective of the steel grade. In addition, the estimates of fatigue crack propagation rate based on the local strain response were in almost agreement with the experimental results.

The fatigue crack initiation life prediction of butt welded joints with notch under pulsating compression (R=-∞) by bending load has been investigated by paying attention to the local strain at the notch root. The experimental results indicate that the fatigue crack initiation and propagation take place at the notch root in spite of pulsating compression, and there is a tendency that the propagation rate decreases with increasing cycles. These cracks emanating from the notch root under pulsating compression become nonpropagation cracks.From an engineering point of view, the fatigue crack initiation life at the notch root can be estimated by a local strain approach using the fatigue crack initiation data of smooth specimens. It becomes clear that in consideration of the compressive plastic-induced opening the fatigue crack growth behavior at the notch root under pulsating compression can be well explained by the effective stress intensity factor.

Structural failure of a bulk carrier was caused in gale sea under service condition of carrying vehicles enroute from Japan to Canada, where cracked damage was found at upper deck plate and slant plate of topside tank near the after corner of No. 3 cargo hatchway port side. The cracks propagated from hatch side almost up to the stringer angle and total length of the cracks was about 14 m.Investigation on the fractured surface disclosed a fact that fast fracture started near the intersection of the sidecoaming and endcoaming of the hatch. It was observed that low cycle fatigue crack had been initiated there prior to the fast fracture, of which the crack length was estimated about 75 mm as a result of the observation by means of microfractographs.In this paper, the failure analysis is made on the phenomenon such as the fast fracture of upper deck plating and the fatigue crack initiation in upper deck as well as fatigue crack propagation and initiation in hatch endcoaming bracket.Fracture toughness of the failured material as obtained by deep notch test is directly compared with the maximum working stress intensity factor which has been evaluated from hull girder analysis by means of three dimensional FEM analysis on the failured deck structures. Fatigue crack initiation and propagation are analysed by using the cumulative fatigue damage and fatigue crack growth law, respectively. It is assumed that the maximum expectation value of working stress range in her service life corresponds to the value when the fast fracture occurs and that the long term stress range distribution is logarithmic one.From the above estimations, it is considered that the maximum wave height which the ship encountered varies from 10 m to 12 m. The life of fatigue crack propagation on the endcoaming bracket without taking into account of corrosion is long enough as compared with the inspection period after her delivery, but the effect of corrosion on the propagation life becomes of practical significance.It is found from the study that the stress intensity at the end of hatch sidecoaming is increased when the height of hatch coaming is higher than usual and that the fracture mechanics analysis is applicable to predict reliability on the failure life of ships at the stage of design on their new type of structures.

The effect of plastic deformation on crack initiation in fatigue

Prediction of threshold for very high cycle fatigue ([formula omitted] cycles)