中度球化的20g材质疲劳裂纹的扩展速率

Fatigue Crack Propagation Rate for Medially-Spheroidized 20g Steel

对已超期服役且中度球化的焦炭塔 2 0 g材质在常温下的疲劳裂纹扩展速率da/dN进行了试验研究。试样采用CCT试样 ,有效试样数为 3件 ,用七点递增多项式法求da/dN和Δk ,将 3个试样的da/dN和Δk数据点合在一起进行回归分析。回归时对数据点进行了取舍 ,对前几组数据点的异常 (da/dN随Δk的增大而降低 )作了理论上的分析 ,认为是预制裂纹产生过高的残余拉应力影响了裂纹的正常扩展 ,随着裂纹扩展 ,预制裂纹时产生的残留塑性区被消耗掉 ,疲劳裂纹扩展速率才恢复正常。得到了以Paris公式表达的平面应变da/dN与Δk关系式。将此式与另一个不同实验条件下的关系式作了比较 。

Fatigue crack propagation rate on normal temperature for 20 g steel that has served a coke tower over its design life was studied. Center crack tension (CCT) specimens were used in the test. The number of valid specimens was three. In test, regressive analysis of d a /d N and Δ k of each specimen were carried out with seven points incremental polynomial technique. When it was done, some abnormal groups of data points which showed ln(d a /d N ) were not proportional to ln(Δ k ) were rejected. Theoretical analysis was made for it, the result is that the over-high residual stress produced by the process of preparing crack had affected on crack normal growth. And the residual plastic zone produced the same process was used up while the crack grew. Then the crack propagation rate resumed normal. Relation formula of d a /d N and Δ k by means of the Paris crack propagation formula based on plane strain behavior was given. This relation formula, compared with another one, is considered as the one reflecting the fatigue crack propagation law of this material on current condition, and can be accepted.