Nominal stresses and Modified Wohler Curve Method to perform the fatigue assessment of uniaxially loaded inclined welds

The present paper summarises an attempt of proposing a simple formula suitable for estimating the fatigue strength of welded connections whose weld beads are inclined with respect to the direction along which the fatigue loading is applied. By explicitly considering the degree of multiaxiality of the nominal stress state damaging the weld toe, such a formula is directly derived from the so-called Modified Wöhler Curve Method (MWCM). The MWCM is a bi-parametrical critical plane approach which postulates that, independently from the complexity of the assessed load history, fatigue strength can accurately be estimated by using the stress components relative to that material plane experiencing the maximum shear stress range. The accuracy and reliability of the proposed design technique was checked against a number of experimental results taken from the literature and generated by testing steel plates with inclined fillet welded attachments. This validation exercise allowed us to prove that the devised formula can successfully be used in situations of practical interest to design against fatigue welded joints whose welds are inclined with respect to the direction along which the cyclic force is applied

Multiaxial Fatigue Assessment of Friction Stir Welded Tubular Joints of Al 6082-T6

The present paper addresses the problem of designing aluminium friction stir (FS) welded joints against multiaxial fatigue. After developing a bespoke FS welding technology suitable for joining aluminium tubes, some one hundred welded tubular specimens of Al 6082-T6 were tested under pure axial, pure torsional and biaxial tension-torsion loading. The influence was explored of two independent variables, namely the proportional or nonproportional nature of the biaxial loading and the effect of axial and torsional non-zero mean stresses. The experimental results were re-analysed using the Modified Wöhler Curve Method (MWCM), with this bi-parametrical critical plane approach being applied in terms of nominal stresses, notch stresses, and also the Point Method. The validation exercise carried out using these experimental data demonstrated that the MWCM is applicable to prediction of the fatigue lives for these FS welded joints, with its use resulting in life estimates that fall within the uniaxial and torsional calibration scatter bands. The approach proposed in the present paper offers, for the first time, a complete solution to the problem of designing tubular FS welded joints against multiaxial fatigue loading

Estimation of fatigue lifetime for selected metallic materials under multiaxial variable amplitude loading

This paper initially investigates the accuracy of two methods, i.e., the Maximum Variance Method (MVM) and the Maximum Damage Method (MDM), in predicting the orientation of the crack initiation plane in three different metallic materials subjected to multiaxial variable amplitude loading. According to the validation exercise being performed, the use of both the MVM and the MDM resulted in a satisfactory level of accuracy for selected three metals. Subsequently, three procedures to estimate the fatigue lifetime of metals undergoing multiaxial variable amplitude loading were assessed quantitatively. Procedure A was based on the MDM applied along with Fatemi-Socie’s (FS) criterion, Bannantine-Socie’s (BS) cycle counting method and Miner’s linear rule. Procedure B was based on the MVM, FS criterion, BS cycle counting method and Miner’s linear rule. Procedure C involved the MVM, the Modified Manson Coffin Curve Method (MMCCM), the classical rainflow cycle counting method and Miner’s linear rule. The results show that the usage of these three design procedures resulted in satisfactory predictions for the materials being considered, with estimates falling within an error band of three

The Modified Manson-Coffin Curve Method to estimate fatigue lifetime under complex constant and variable amplitude multiaxial fatigue loading

This paper investigates the accuracy of the so-called Modified Manson–Coffin Curve Method (MMCCM) in estimating fatigue lifetime of metallic materials subjected to complex constant and variable amplitude multiaxial load histories. The MMCCM postulates that fatigue damage is maximised on that material plane experiencing the maximum shear strain amplitude. In the present investigation, the orientation of the critical plane was determined through that direction along which the variance of the resolved shear strain reaches it maximum value. Under variable amplitude complex load histories, this direction was also used to count the resolved shear strain cycles via the classic Rain-Flow method. Further, the degree of multiaxiality and non-proportionality of the time-variable stress states at the assumed critical locations was directly quantified through a suitable stress ratio which accounts for (i) the mean value and the variance of the stress perpendicular to the critical plane as well as for (ii) the variance of the shear stress resolved along the direction experiencing the maximum variance of the resolved shear strain. The accuracy and reliability of the proposed approach was checked against approximately 650 experimental data taken from the literature and generated by testing un-notched metallic materials under complex constant and variable amplitude multiaxial load histories. The sound agreement between estimates and experimental results which was obtained strongly supports the idea that the proposed design technique is a powerful engineering tool allowing metallic materials to be designed against constant and variable amplitude multiaxial fatigue by always reaching a remarkable level of accuracy. This approach offers a complete solution to the strain based multiaxial fatigue problem

The Theory of Critical Distances to estimate static and dynamic strength of notched plain concrete

The Theory of Critical Distances (TCD) is a well-known design method allowing the strength of notched/cracked components to be estimated accurately by directly post-processing the entire linear-elastic stress fields damaging the material in the vicinity of the stress concentrators being designed. By taking full advantage of the TCD’s unique features, in the present study this powerful theory was reformulated to make it suitable for designing notched plain concrete against static and dynamic loading. The accuracy and reliability of the proposed reformulation of the TCD was checked against a set of experimental results generated by testing, under different displacement rates, square section beams of plain concrete containing notches of different sharpness. This validation exercise has demonstrated that the proposed reformulation of the TCD is capable of accurately assessing the static and dynamic strength of notched unreinforced concrete, with the estimates falling within an error interval of ±20%. The level of accuracy that was obtained is certainly satisfactory, especially in light of the fact that static and dynamic strength was estimated without explicitly modelling the stress vs. strain dynamic behaviour of the concrete being tested

Microstructural length scale parameters to model the high-cycle fatigue behaviour of notched plain concrete

The present paper investigates the importance and relevance of using microstructural length scale parameters in estimating the high-cycle fatigue strength of notched plain concrete. In particular, the accuracy and reliability of the Theory of Critical Distances and Gradient Elasticity are checked against a number of experimental results generated by testing, under cyclic bending, square section beams of plain concrete containing stress concentrators of different sharpness. The common feature of these two modelling approaches is that the required effective stress is calculated by using a length scale which depends on the microstructural material morphology. The performed validation exercise demonstrates that microstructural length scale parameters are successful in modelling the behaviour of notched plain concrete in the high-cycle fatigue regime

On the use of length scale parameters to assess the static strength of notched 3D-printed PLA

This paper aims to investigate the accuracy of the Theory of Critical Distances (TCD) in estimating static strength of notched additively manufactured PLA as both notch sharpness and infill angle vary. The TCD takes as its starting point the assumption that the extent of damage under static loading can be assessed successfully by using two different material parameters, i.e. (i) a critical distance whose length is closely related to the material microstructural features and an inherent (i.e., a defect free) material strength. Plain and notched specimens of 3D-printed PLA were manufactured horizontally by making the deposition angle vary in the range 0°-90°. Using the TCD in the form of the Point Method, failures were predicted by directly post-processing the linear-elastic stress fields estimated through the well-known analytical solutions due to Glinka and Newport. Independently of the notch sharpness, the estimates being obtained were found to be highly accurate, falling within an error interval of about 20%. This result fully supports the idea that the TCD can successfully be used in situations of practical interest to design against static loading notched components of additively manufactured PLA by directly post-processing the results from simple linear-elastic Finite Element (FE) models

A Multiaxial Stress-Based Critical Distance Methodology To Estimate Fretting Fatigue Life

This work presents a methodology for fretting fatigue life estimation based on the evaluation of a multiaxial fatigue parameter at a critical distance below the contact surface. The fatigue parameter is defined using the Modified Wöhler Curve Method together with a measure of shear stress amplitude based on the Maximum Rectangular Hull concept. To apply the approach in the medium-cycle fatigue regime, the critical distance is assumed to depend on the fatigue life. Available fretting fatigue experiments conducted on a cylinder-on-flat contact configuration made of Al-4%Cu alloy were used to evaluate the methodology. Most of the fatigue life estimates were within factor-of-two boundaries