膨胀套管的挤毁强度及失效机理分析

Analysis of collapse strength and failure mechanism of expanded casing

为了研究套管膨胀后在均布外挤载荷作用下的挤毁强度,采用弹塑性力学有限元显式动力学分析方法,对常用的L80钢级Φ139.7 mm×9.17 mm套管的膨胀和挤毁过程进行了非线性分析,得到了10%~30%膨胀率下套管材料的包辛格效应、残余应力、套管挤毁强度数值和变化规律,以及套管挤毁失效危险点的应力状态。结果表明,套管膨胀后材料包辛格效应导致套管环向方向的压缩屈服强度降低,且包辛格效应值沿着套管壁厚方向由内至外逐渐减小。膨胀率为30%时,套管内壁和外壁处环向压缩屈服强度由552 MPa分别下降至202和282 MPa;膨胀后,套管内等效残余应力沿着套管壁厚方向由内至外逐渐增加,在环向方向总体上表现为收缩残余应力,在不同膨胀率下,等效残余应力大小不同,其值最大接近材料屈服强度的70%左右。套管膨胀后的挤毁强度下降,且下降幅度随着膨胀率的增加而增加,当膨胀率达到30%时,挤毁强度下降约55.1%,与普通套管发生挤毁时危险点位于内壁不同,膨胀后套管挤毁时的危险点位于套管外壁。分析认为膨胀后套管环向方向材料的包辛格效应及收缩残余应力是导致其挤毁强度下降的主要原因。

To study the collapse strength of casing after expansion under uniform external extrusion load, the nonlinear analysis of the expansion and collapse process of common Φ139.7 mm× 9.17 mm casing of L80 steel grade was carried out by using the elastic-plastic mechanics finite element explicit dynamic analysis method. The Bauschinger effect of casing material, residual stress, collapse strength value and variation rule of casing and the stress state at the dangerous point of casing collapse failure with the expansion rate of 10%~30% were obtained. The results show that the Bauschinger effect of material results in the decrease of the compressive yield strength in circumferential direction of casing after expansion, and the Bauschinger effect value decreases gradually along the direction of casing wall thickness from inside to outside. When the expansion rate is 30%, the compressive yield strength in circumferential direction of the inner wall and outer wall decreases from 552 MPa to 202 and 282 MPa, respectively;after casing expansion, the equivalent residual stresses of casing increases gradually along the direction of casing wall thickness from inside to outside, and the residual stress is shrinkage residual stress overall in circumferential direction. With different expansion rates, the equivalent residual stress is different, and its maximum value is close to about 70% of the yield strength of the material. The collapse strength of casing decreases after expansion, and the decline degree increases with the increase of expansion rate. When the expansion rate reaches 30%, the collapse strength decreases by 55.1%. Different from the ordinary casing with dangerous point at the inner wall when collapsing, the dangerous point of casing after expansion is located at the outer wall of casing. The analysis suggests that the Bauschinger effect of material in circumferential direction and shrinkage residual stress are the main reasons for the collapse strength decrease after expansion.