Integrating effect of forming in fatigue life prediction: review of present scenario and challenges

Ball-burnishing effect on deep residual stress on AISI 1038 and AA2017-T4

Abstract: Ball-burnishing induces compressive residual stresses on treated materials by the effect of plastic deformation. The result is an increase in the fatigue life of the treated part, retarding the initiation of cracks on the surface. Compressive residual stresses have been previously measured by X-ray diffraction near the surface, revealing considerably high values at the maximum analyzed depth, in relation to other finishing processes such as shot peening. However, the maximum analyzed depth is very limited by using this technique. In this paper, the incremental hole drilling (IHD) technique is tested to measure residual stresses, being able to reach a 2-mm measuring depth. To that objective, a commercial strain gage is used and calibrated using finite element model simulations. A second Finite Element Model based on material removal rate is developed to obtain the equations to calculate the strain release through IHD. Finally, residual stresses are measured experimentally with that technique on two...

Determination of Anisotropic Yield Coefficients by a Data-Driven Multiobjective Evolutionary and Genetic Algorithm

Abstract: The texture induced anisotropy of yield strength in cold rolled sheet metals is modeled using anisotropic yield criteria. The classical and other optimization methods used so far to determine the yield coefficients are limited by fixed set of experimental data, initial guess values, and pre-determined weight factors. A robust multiobjective optimization based on evolutionary algorithm proposed in this paper minimizes the error in yield stress and plastic strain ratio simultaneously and thereby overcomes the limitations in the approaches used before. The new approach is tested using Hill48 and Barlat89 yield criteria for five different materials from literature. The new approach is observed to improve the prediction capability of yield coefficients when compared to earlier approaches. The Pareto frontier obtained in the new approach can serve as a comparative tool to evaluate the accuracy of different yield criteria.

Influence of uniaxial and biaxial pre-straining on the high cycle fatigue performance of DP590 steel

Abstract: Present work aims to assess tensile and high cycle fatigue (HCF) performance of dual-phase (DP590) steel subjected to uniaxial and equi-biaxial pre-strain. Pre-strain increases material’s strength, reduces ductility and simultaneously enhances HCF performance, quantified by increased material’s tensile yield strength 12% & 21%, ultimate tensile strength 6% & 7%, endurance limit 11% & 18%, after 10% uniaxial and equi-biaxial pre-strain. The rotation of one maximum shear stress plane during uniaxial re-loading after equi-biaxial pre-strain results in increased fatigue life. Finite element analysis suggests an alteration in the location of deformation concentration zone during uniaxial fatigue cycling of equi-biaxial pre-strain material.

Post-forming monotonic and cyclic behavior in a HSLA steel sheet after large deformations by in-plane compression

Abstract: The effects of large prestrains (18–40%), produced by in-plane compression, on the asymmetry and the anisotropy of the stress response and on the fatigue life are investigated under fully reversed axial strain for a 345 MPa yield strength V–N high strength low alloy steel sheet. After prestraining, the hysteresis loops are asymmetric and the stress response is anisotropic, i.e., the response differs in directions parallel and perpendicular to that of the compressive prestrain. To understand the cyclic flow stress asymmetry, monotonic tension and compression tests were conducted in these two directions after prestraining. It is shown that the loop asymmetry is related to the Bauschinger effect after prestraining. Two cyclic stress strain curves, one corresponding to the tension side of the hysteresis loops and the other to the compression side, are defined to accurately describe the post-prestraining behavior. The amount of strengthening gained by prestraining is partially retained after cycling. Prestraining increases the fatigue life at low strain amplitudes but decreases it at high strain amplitudes.

A theory of the yielding and plastic flow of anisotropic metals

Abstract: A theory is suggested which describes, on a macroscopic scale, the yielding and plastic flow of an anisotropic metal. The type of anisotropy considered is that resulting from preferred orientation. A yield criterion is postulated on general grounds which is similar in form to the Huber-Mises criterion for isotropic metals, but which contains six parameters specifying the state of anisotropy. By using von Mises' concept (1928) of a plastic potential, associated relations are then found between the stress and strain-increment tensors. The theory is applied to experiments of Korber & Hoff (1928) on the necking under uniaxial tension of thin strips cut from rolled sheet. It is shown, in full agreement with experimental data, that there are generally two, equally possible, necking directions whose orientation depends on the angle between the strip axis and the rolling direction. As a second example, pure torsion of a thin-walled cylinder is analyzed. With increasing twist anisotropy is developed. In accordance with recent observations by Swift (1947), the theory predicts changes in length of the cylinder. The theory is also applied to determine the earing positions in cups deep-drawn from rolled sheet.

Limit strains in the processes of stretch-forming sheet metal

Abstract: The process of the loss of stability is analysed for sheet metal subjected to biaxial tension when the ratio of the principal stresses 0.5 ⩽ σ 2 /σ 1 ⩽ 1 . The loss of stability manifests itself by a groove running in a direction perpendicular to the larger principal stress. In this groove local strains begin to concentrate gradually. In the initial stage of the process the deepening of the groove is associated with a gradually fading strain in the regions adjacent to the groove. This fading strain attains a certain limiting value e∗. This paper contains both experimental results and a theoretical analysis of the process of the generation of the groove based on anisotropic plasticity theory. The system of equations derived was solved numerically with the aid of a computer, which enabled the limiting strain of the sheet metal to be determined as a function of the following properties of the material: (i) Initial inhomogeneity of the sheet metal, (ii) exponent of the strain-hardening function, (iii) coefficient of normal anisotropy, (iv) initial plastic strain, (v) strain at which the fracture occurs. The results are discussed and the properties are described that influence the drawability of sheet metal used in the stretch-forming process.

Fatigue of structures and materials

Abstract: Preface. Frequently used symbols, acronyms and units. 1. Introduction to Fatigue of Structures and Materials. Part 1: Introductory Chapters on Fatigue. 2. Fatigue as a Phenomenon in the Material. 3. Stress Concentrations at Notches. 4. Residual Stresses. 5. Stress Intensity Factors of Cracks. 6. Fatigue Properties of Materials. 7. The Fatigue Strength of Notched Specimens. Analysis and Predictions. 8. Fatigue Crack Growth. Analysis and Predictions. Part 2: Load Spectra and Fatigue Under Variable-Amplitude Loading. 9. Load Spectra. 10. Fatigue under Variable-Amplitude Loading. 11. Fatigue Crack Growth under Variable-Amplitude Loading. Part 3: Fatigue Tests and Scatter. 12. Fatigue and Scatter. 13. Fatigue Tests. Part 4: Special Fatigue Conditions. 14. Surface Treatments. 15. Fretting Corrosion. 16. Corrosion Fatigue. 17. High-Temperature and Low-Temperature Fatigue. Part 5: Fatigue of Joints and Structures. 18. Fatigue of Joints. 19. Fatigue of Structures. Design Procedures. Part 6: Arall and Glare, Fiber-Metal Laminates. 20. The Fatigue Resistance of the Fiber-Metal Laminates Arall and Glare. Subject index.

Metal Forming and the Finite-Element Method

Abstract: Introduction Metal forming process Analysis and technology in metal forming Plasticity and viscoplasticity Methods of analysis The finite element method (1) The finite element method (2) Plane-strain problems Axisymmetric isothermal forging Steady state processes of extrusion and drawing Sheet metal forming Thermo-viscoplastic analysis Compaction and forging of porous metals Three dimensional problems Preform design in metal forming Solid formulation, comparison of two formulations, and concluding remarks Index.

Plane stress yield function for aluminum alloy sheets—part 1: theory

Abstract: A new plane stress yield function that well describes the anisotropic behavior of sheet metals, in particular, aluminum alloy sheets, was proposed. The anisotropy of the function was introduced in the formulation using two linear transformations on the Cauchy stress tensor. It was shown that the accuracy of this new function was similar to that of other recently proposed non-quadratic yield functions. Moreover, it was proved that the function is convex in stress space. A new experiment was proposed to obtain one of the anisotropy coefficients. This new formulation is expected to be particularly suitable for finite element (FE) modeling simulations of sheet forming processes for aluminum alloy sheets.