自增强处理对厚壁筒裂纹扩展的影响研究

发布时间：2018-06-19 15:48

本文选题：厚壁筒 + 自增强处理　；参考：《长江大学》2016年硕士论文

【摘要】：石油天然气等底层资源日益匮乏,以及对油气资源的开采难度逐渐增大,为稳定现有油气田开采量,提高新开发油气井的采收率,超高压容器设备在化工与石油行业中应用逐渐广泛。为了提高超高压容器设备的承载能力及使用寿命,除了在材料屈服范围内提高容器设备的壁厚,采用自增强处理技术也是在对超高压设备设计加工中不可或缺的一项,通过对内腔加压处理,使内层发生塑性变形,从而在设备内腔处引入残余应力,以降低设备应力水平,提高其疲劳强度。任何构件在加工过程中不可避免出现因表面划痕、金属夹杂、腐蚀坑等加工缺陷而造成裂纹问题,在对超高压设备采用自增强技术时,裂纹尖端处会产生一定程度的应力集中,影响尖端附近位置的残余应力分布,改变自增强处理效果。因此,在以断裂力学为理论基础,假设构件实际存在初始裂纹的条件下,定量研究残余应力下的裂纹尖端断裂参数分布规律,并根据表征参量应力强度因子评估在残余应力作用下裂纹的扩展行为,开拓了自增强技术在对含裂纹超高压设备应用方面的研究领域。实际工程中裂纹的形式主要为三维裂纹问题,对于设备内部型腔结构通常较为复杂,用现有的断裂力学理论无法进行数值计算,且难以通过实验手段完成对理论的验证。根据对复杂内部型腔裂纹产生位置分析,从研究的可行性角度考虑,把存在于复杂型腔内裂纹最核心的位置进行抽象简化成厚壁筒内壁裂纹,为实际工程应用中评判复杂内腔裂纹对自增强处理效果提供理论指导。论文从弹塑性力学角度,基于理想化弹塑性模型以及双线性随动强化模型两种材料类型分别建立了自增强处理压力卸载后残余应力计算公式。通过断裂力学权函数理论,分析并验证了厚壁筒权函数式的准确性,推导出了含裂纹厚壁筒自增强处理后残余应力的应力强度因子计算公式。采用数值模拟方法,引用J积分来表征自增强处理下裂纹尖端断裂特性,对不同裂纹形状下厚壁筒的应力分布以及裂纹尖端处的断裂参数进行数值计算,研究了穿透裂纹及椭圆裂纹的两种含裂纹厚壁筒经自增强处理后残余应力变化规律以及自增强处理引入的残余应力对裂纹尖端J积分值影响规律。对穿透型裂纹分析表明,自增强处理后厚壁筒裂纹的存在主要引起了裂纹尖端沿扩展方向上残余压应力范围内的应力变化,超高该范围应力状态基本与不含裂纹的厚壁筒相近,其中最大残余压应力出现在裂纹尖端沿扩展方向附近,而不是在裂纹应力集中位置处,对降低裂纹尖端的的扩展具有一定促进作用；径比K的变化影响裂纹前缘最大残余切向应力、裂纹尖端残余切向应力以及尖端沿扩展方向的残余压应力区域范围变化,径比K小于1.6时自增强对降低裂纹尖端的起始扩展效果不大,大于1.7时随着K值的增大,自增强处理对降低裂纹尖端的扩展速率逐渐显著,但随着自增强压力的增大,K值的增大对裂纹扩展速率影响较小；径比K取较小值时,选择较小自增强压力对抑制裂纹尖端起裂扩展更明显,而沿厚度方向经自增强处理后,由残余应力场形成的J积分值在厚度小于40mm时为负值并呈现浴盆曲线形状,在该范围内由残余压应力场主导控制裂纹尖端,但靠近约束端位置处主要由残余拉应力场主导控制。对椭圆型裂纹分析表明,当裂纹深度c与裂纹长度的比值小于1时,随着比值的增大,裂纹尖端前缘最大残余压应力逐渐增大,而当比值大于1时反而呈逐渐减少的趋势：椭圆长短轴比例的变化对尖端前缘处的残余切向应力影响较小,变化幅度不大,而沿椭圆曲线方向上,在椭圆两侧端点以及中点位置处附近的残余压应力明显高于其它位置,这是由于在该三点位置裂纹尖端分别处于内壁表面以及椭圆曲率半径最大处；自增强处理对抑制轴向椭圆裂纹远离内层壁面裂纹尖端的起裂扩展相比裂纹其它位置效果更为显著,同时当裂纹深度与裂纹长度比值为1.4时,自增强处理对抑制裂纹尖端扩展最为明显；在相同裂纹形式下过大的增大厚壁筒径比,自增强处理对抑制椭圆裂纹尖端的起裂扩展效果越不明显,而当椭圆裂纹长短轴比例较大时,径比K的增大对抑制表层裂纹尖端起裂扩展影响较明显,所以厚壁筒壁厚尺寸变化对裂纹尖端影响与裂纹结构尺寸有较大关系。
[Abstract]:The underlying resources such as oil and gas are increasingly scarce and the difficulty in exploiting oil and gas resources is gradually increasing. In order to stabilize the production of existing oil and gas fields and improve the recovery rate of newly developed oil and gas wells, ultra high pressure vessel equipment is widely used in chemical and petroleum industries. In order to improve the carrying capacity and service life of UHP vessel equipment, In order to improve the wall thickness of the container equipment in the yield range of the material, the self reinforcing treatment technology is also an indispensable item in the design and processing of the ultra-high pressure equipment. Through the compression treatment of the inner cavity, the inner layer is plastic deformation, and the residual stress is introduced into the inner cavity of the equipment, so as to reduce the stress level of the equipment and improve its fatigue strength. In the process of processing, it is inevitable that cracks are caused by defects such as surface scratches, metal inclusions and corrosion craters. In the use of self reinforcement technology for ultra-high pressure equipment, a certain degree of stress concentration will be produced at the crack tip, which affects the distribution of residual stress in the position near the tip and changes the effect of self reinforcing treatment. On the basis of fracture mechanics, the distribution law of fracture parameters at the crack tip under the residual stress is quantitatively studied under the condition of the actual existence of the initial crack, and the propagation behavior of the crack under the residual stress is evaluated according to the stress intensity factor of the characterization parameter, and the self reinforcement technique should be developed for the ultra high pressure equipment with cracks. In the field of research, the form of cracks in practical engineering is mainly a three-dimensional crack problem. For the internal cavity of the equipment, the cavity structure is usually more complex. It is difficult to use the existing fracture mechanics theory to carry out numerical calculation, and it is difficult to verify the theory by experimental means. Considering the feasibility of the study, the most core position of the complex cavity crack is abstracted into the inner wall crack of the thick wall cylinder. It provides a theoretical guide for the evaluation of the effect of the complex inner cavity crack to the self reinforcing treatment in practical engineering application. The formula for calculating the residual stress after the pressure unloading of the two kinds of material is established respectively. Through the theory of the fracture mechanics weight function, the accuracy of the thick wall cylinder weight function is analyzed and verified, and the calculation formula of the stress intensity factor of the residual stress after the self reinforcing treatment of the thick wall cylinder with a crack is derived. The J integral is used to characterize the crack tip fracture characteristics under the self reinforcement treatment. The stress distribution and the fracture parameters at the crack tip in the thick wall under different crack shapes are numerically calculated. The changes of the residual stress and the self reinforcement of two kinds of cracked thick wall cylinders with crack and Elliptical Cracks are studied. The influence of residual stress introduced into the J integral value at the crack tip. The analysis of the penetrating crack shows that the existence of the crack in the thick wall tube after self reinforcement mainly causes the stress change in the range of the residual compressive stress in the direction of the crack, and the maximum stress state in this range is similar to the thick wall cylinder without crack. The residual compressive stress occurs near the crack tip in the direction of the crack tip, not at the stress concentration in the crack, and has a certain effect on reducing the expansion of the crack tip. The variation of the diameter ratio K affects the maximum residual shear stress in the front of the crack, the residual shear stress at the crack tip and the residual compressive stress zone along the direction of the crack tip. When the diameter ratio is less than 1.6, the effect of self reinforcement on the initiation and expansion of the crack tip is not significant. When the K value increases, the expansion rate of the crack tip decreases with the increase of the K value. But with the increase of the self reinforcing pressure, the increase of the K value has little effect on the crack spreading rate; when the diameter ratio is smaller, the selection of the diameter ratio is smaller. The smaller self reinforcing pressure is more obvious to restrain the crack propagation in the crack tip, and the J integral value formed by the residual stress field is negative and presents the bathtub curve shape when the thickness is less than 40mm, and the residual stress field dominates the crack tip in this range, but the position near the confinement end is mainly controlled by the residual stress field. The residual tensile stress field dominated control. The analysis of elliptical crack shows that when the ratio of crack depth C and crack length is less than 1, the maximum residual compressive stress at the front edge of crack tip increases with the ratio increasing, but when the ratio is greater than 1, it decreases gradually: the change of the ratio of elliptical long and short axis to the remnants at the front edge of the tip The shear stress has little influence on the shear stress, but the residual compressive stress near the elliptical direction is obviously higher than that in other positions near the ellipse and the middle point. This is because the crack tip at the three points is at the surface of the inner wall and the maximum radius of the elliptical curvature, and the autofrettage treatment can restrain the axial ellipsoid. The crack propagation of a circular crack far away from the crack tip of the inner wall is more significant than that of other cracks. At the same time, when the ratio of the crack depth to the crack length is 1.4, the self reinforcement treatment is most obvious for the growth of the crack tip. The increase of the thickness of the thick wall and the diameter ratio of the thick wall under the same crack form, and the self reinforcement treatment for the suppression of the elliptical crack When the ratio of the long and short axis of the elliptical crack is larger, the increase of the diameter ratio K is more obvious to the crack growth of the surface crack tip. Therefore, the influence of the thickness of the wall thickness on the crack tip is closely related to the crack structure size.
【学位授予单位】：长江大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TH49

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