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In this paper, we give examples of innovating forging sequences developed by numerical simulations, including the investigation of damage in tools and forged parts. In case of specific processes with very determined geometry — such as wire drawing — we show how systematic numerical studies may lead to predictive models of force, local strains and residual stress…
However, reliable predictions from numerical simulations require reliable input data, including constitutive laws, friction conditions and propensity to ductile damage. These data must be characterized under realistic sollicitations. Typical cold forging loadings are indeed very severe: local strains up to 600%, strain rates locally greater than 1000 s−1, and subsequently, plastic heating over 500°C.
To characterize the constitutive behaviour, the standard upset test between grooved dies is used along with an original methodology to derive the strain hardening curve from the experimental force-displacement recording. Tool elastic deformations, specimen strain heterogeneity… are taken into account. This enables a precise determination of the strain hardening curve up to about 100% of strain. The extrapolation of the flow stress to greater deformations is then very easy and reliable. Such a test can be performed under quasi-static and isothermal conditions (0.1 s−1 but also adiabatic and rapid conditions (up to 10 s−1). This procedure was adapted to a Pellini hammer, which enables very simple characterization at 800 s−1. The comparison of all these flow curves lead to the formulation of an original constitutive model, which accounts for the effects of plastic heating, strain rate, dynamic aging…
In order to predict ductile fracture during the forging process, the most classical criteria were tested over a wide range of experimental loading conditions. None of them were general enough to solve all the cold forming problems. On the other hand, mesoscopic models describing the deformation of the metal matrix around inclusions or second phases have proved to be in good agreement with the various experimental observations. An original plasticity criterium, based on the recent works in porous plasticity theory, has been developed and already displays promising capabilities. Simple experimental procedures enable a reliable classification of steel qualities, heat treatments… in term of forgeability.
Finally, the friction problem is treated using different methodologies based on the forming process considered. For forging operations, a fine analysis of the force-displacement curves in direct extrusion stages may lead to a precise measurement of the friction coefficient under pressures from about 200 MPa up to 1000 MPa and for sliding rates between 1 and 100 mm/s. For wire drawing process, a model relying on an analytical approach using a significantly improved slice method has been developed: the comparison of the experimental drawing force and the predicted one gives the friction coefficient in industrial processing conditions (speed up to 5 m/s). 相似文献
喷丸的主要贡献是提高疲劳强度,公认残余压应力起主导作用。表面纳米化认为结构细化,屈服强度提高,相应延缓疲劳裂纹萌生,提高了疲劳强度。却没有试验证实此机理的正确性。实际上,获得纳米化的强喷丸表面伴随有裂纹,不存在裂纹萌生阶段。文中展示了喷丸强度和出现微裂纹几率的关系。明确强变形表面属裂纹体,应该用断裂力学而非经典力学的连续体处理。介绍表面形变各影响因素对疲劳裂纹扩展作用的试验,结果表明残余压应力仍是提高疲劳性能的主导因素。追求纳米化的加工硬化,牺牲塑性换取强度,导致裂纹扩展加速,对疲劳有害无益。 相似文献