Key Engineering Materials Vols. 340-341

Abstract: This paper describes a model of large-strain cyclic plasticity and its verification by some experiments of cyclic plasticity and biaxial stretching. The performance of this model in springback simulation is discussed by comparing the calculated results for S-rail forming with the experimental data on high strength steel sheet (HSS) of 980MPa-TS. The results of numerical simulations of the springback agree well with the corresponding experimental data, including the torsion-type springback appearing in S-rail forming.

Abstract: Based on the experimental results of uniaxial time-dependent ratcheting behavior of SS304 stainless steel at room temperature and 973K, three kinds of time-dependent constitutive models were employed to describe such time-dependent ratcheting by using the Ohno-Abdel-Karim kinematic hardening rule, i.e., a unified viscoplastic model, a creep-plasticity superposition model and a creep-viscoplasticity superposition model. The capabilities of such models to describe the time-dependent ratcheting were discussed by comparing with the corresponding experimental results. It is shown that the unified viscoplastic model cannot provide reasonable simulation to the time-dependent ratcheting, especially to those with certain peak/valley stress hold and at 973K; the proposed creep-plasticity superposition model is reasonable when the creep is a dominant factor of the deformation, however, it cannot provide a reasonable description when the creep is weak; the creep-viscoplastic superposition model is reasonable not only at room temperature but also at high temperature.

Abstract: In the present paper, in order to better understand the third type “dynamic strain aging” occurring during the plastic flow of metals, the uniaxial compressive experimental data ever obtained in University of California, San Diego using an Instron servo-hydraulic testing machine and the Hopkinson technique are systematically analysed. These experimental data cover the plastic flow stress of several fcc, hcp, bcc polycrystalline materials and several alloys at a broad range of temperatures (77K – 1,100K) and strain rates (0.001/s – 10,000/s). In analysis, the appearing region of the “dynamic strain aging ” under different temperatures and strain rates are respectively plotted by the curves of stress vs temperature, and stress vs strain for fcc, hcp and bcc metals. The results show that: (1) this third type “dynamic strain aging ” occurs in all hcp, bcc and fcc polycrystalline or alloy materials, and there are different profiles of stress-strain curve; (2) the “dynamic strain aging ”occurs in a matching coincidence of the temperature and strain rate, its temperature region will shift to higher region with increasing strain rates; (3) bcc materials do not have an initial pre-straining strain as the onset of work-hardness rate change for the “dynamic strain aging ”; and (4) based on the explanations of dynamic strain aging with serration curves (Portevin-Lechatelier effect) and other explaining mechanisms of references, The mechanism of third DSA is thought as the rapid/continuous formation of the solute atmospheres at the mobile dislocation core by the pipe diffusion along vast collective forest dislocations to result in a continuous rise curve of flow stress. Finally, several conclusions are also presented.

Abstract: An accurate prediction of plastic anisotropy induced by initial texture in sheet metal forming operations depends on the constitutive models adopted. Models of engineering interest include both phenomenological formulations and crystal plasticity based on dislocation slip. In addition to the above two approaches that are commonly adopted in FE analysis, now an alternative is available which describes anisotropic behavior of polycrystalline sheet metals still by an analytic yield function to keep the computing time as low as possible but at the same time which also takes explicitly into account the crystallographic texture of the material to give a more precise description of plasticity anisotropy. However, the locus of such a yielding potential determined by constitutive coefficients upon invoking the rate-independent crystal plasticity may exhibit an unrealistic concave shape, which will make it impossible to obtain a convergent solution. To circumvent the difficulty, a detailed computation procedure is presented to calculate the constitutive coefficients based on rate-dependent crystal plasticity. The combination of the coefficients obtained with experimentally measured texture coefficients of an annealed FCC polycrystalline sheet metal will provide a complete constitutive characterization of the material. As an application of the calibrated model, the process of deep drawing by hemispherical punch is simulated, in which plastic anisotropy (earring) corresponding to typical texture type is observed, thus demonstrating the applicability of the coefficients found.

Abstract: Textures developed during hot rolling process may affect the high temperature deformation behaviors of Ti alloys, but their relation has not been well understood or quantitatively analyzed yet. A series of load relaxation and creep tests for hot rolled Ti-6Al-4V alloy has been conducted in this work to clarify the effect of textures on the deformation behaviors of the alloy under 700 °C and the result was analyzed by using an internal variables approach. The internal strength σ* was found to vary significantly by the textures, but not by the temperature change, while the texture effect was found to decrease at higher temperatures.

Abstract: A common treatment to restore normal blood flow in an obstructed artery is the deployment of a stent (i.e. small tube-like structure). The vast majority of stents are crimped on a folded balloon and laser cut from 316L stainless steel tubes. Although, several numerical studies (exploiting the Finite Element Method) are dedicated to the mechanical behaviour of balloon expandable stents, there seems to be no consensus regarding the mechanical properties to describe the inelastic material behaviour of SS316L. Moreover, as the typical dimensions of stent struts (e.g. 100 μm for coronary stents) are of a similar order of magnitude as the average grain size in stainless steel (i.e. 25 μm), continuum approaches relying on macroscopic material properties may be questionable. In addition, an experimental study on stainless steel stent strut specimens showed a size-dependency of the failure strain. In this study the impact of the magnitude of the yield stress on the stent expansion behavior is examined. An increase in the yield stress (from 205 N/mm² to 375 N/mm²) results in an increase of the pressure (from about 0.3 N/mm² to approximately 0.4 N/mm²) which the clinician needs to exert for the balloon to unfold and to reach its cylindrical expanded shape. Furthermore, the effect of the size dependency behavior of the material is studied by monitoring the nominal strain during stent expansion. The maximum value of the nominal strain in the expanded stent (e.g. εn = 23 %) does not exceed the critical value of the failure strain, (i.e. εn = 33 %), moreover the critical values are nowhere exceeded in the whole stent during the expansion. Our numerical results - accounting for the presence of the balloon in its actual folded shape - correspond very well with pressure/diameter data supplied by the manufacturer. Consequently, this study shows that the free expansion of new generation balloon-expandable stents can be studied accurately with computational analysis based on the Finite Element Method (FEM) and relying on macroscopic material properties. In this context, there is no need to implement a size-based constitutive material model, but before accepting the results of the study, one should check in any case the maximum strain against the limit as shown above.

Abstract: In Western countries, cardiovascular disease is the most common cause of death, often related to atherosclerosis which can lead to a narrowing of the arteries. To restore perfusion of downstream tissues, an intravascular stent (i.e. a small tube-like structure) can be deployed in the obstructed vessel. The vast majority of stents are balloon expandable and crimped on a folded balloon to obtain a low profile for deliverability and lesion access. Several studies have exploited the finite element method to gain insight in their mechanical behaviour or to study the vascular reaction to stent deployment. However, to date – to the best of our knowledge – none of them include the balloon itself in its actual folded shape. Furthermore, literature on the effect of the crimping process on the expansion behaviour of the stent is even scarcer. Our numerical results - accounting for the presence of the balloon in its actual folded shape - correspond very well with data provided by the manufacturer and consequently our approach could be the basis for new realistic computational models of angioplasty procedures. The plastic deformation, prior to the stent expansion and induced by the crimping procedure, has a minor influence on the overall expansion behaviour of the stent but nevertheless influences the maximum von Mises stress and nominal strain. The maximum von Mises stress drops from 440 N/mm² to 426 N/mm² and the maximum nominal strain value lowers from 0.23 to 0.22 at the end of the expansion phase when neglecting the presence of the residual stresses. Depending on the context in which to use the developed mathematical models, the crimping phase can be discarded from the simulations in order to speed up the analyses.

Abstract: Double curvature nacelle skin components are often produced in 2024-T3 Alclad aluminium alloy (clad both sides), using simple stretch forming. For this process uniaxial-tension is assumed to be the dominant mode of deformation. The observed springback in these components is generally minimized by inducing high levels of strain within the forming limits of the material. Experimental investigations to quantify the level of the springback have revealed an underlying aspect of material behaviour, which to the authors’ knowledge, has not been reported elsewhere. The results of this investigation have shown a remarkable dependence on specific material parameters unique to the Alclad material. Whilst previous investigations [1, 2, 3] have shown the influence of cladding, it has been assumed that cladding layers on both sides produce effects that are symmetrical about the mid-plane of the sheet. However, the current investigation has revealed a distinct asymmetry in springback behaviour. A detailed study of cladding thickness and strength, residual stresses and through-thickness material property changes has revealed that this asymmetry results from a complex combination of these parameters.

Abstract: In this study, plastic flow joining using a shot peening process was investigated. Surface treatment is necessary to improve the surface properties. Shot peening is one of the surface treatments. Since the surface of substrate is hit repeatedly with a large number of shots, the substrate undergoes a large plastic deformation near its surface. Therefore, plastic flow characterized by a shear droop occurs at the edge of the substrate due to shot peening. If an implant made of a dissimilar material is set in a hollow space on the surface of the substrate and then shot-peened, it can be joined to the substrate due to the peening droop generated by the large plastic deformation during shot peening. In this method, the availability of the plastic flow, i.e., the peening droop makes the joining of the implant possible. In the experiment, a compressed-air-type shot peening machine was employed. To examine experimentally the influence of working temperature on bondability, equipment with a heating furnace was produced. The influence of processing conditions on the joining of the implant and the substrate was examined. The joint strength increased with the kinetic energy of shots and processing temperature. The improved implant with a step was effective in improving in bondability. The dissimilar material was also successfully joined to a thin sheet by using of the interaction of peening droops. It was found that the present method using the peening droop was effective for joining the dissimilar materials.