【作者】 贾义庚；

【导师】 王东坡；

【作者基本信息】 天津大学 ， 材料加工工程， 2009， 硕士

【摘要】 在船舶及海洋工程结构等许多工业领域其零部件受交变载荷的作用,在服役期内构件常常发生疲劳破坏。而结构发生疲劳破坏的地方往往是比较薄弱的焊接接头。本文介绍了改善焊接接头疲劳强度的超声冲击方法,并比较了Q235和Q345焊态和超声冲击态下的疲劳强度。对于Q235,在1×107循环周次下由焊态的211MPa提高到超声冲击态的330MPa,提高了56%,在1×109循环周次下由焊态的82MPa提高到了224MPa,提高了173%;对于Q345,在1×107循环周次下由焊态的225MPa提高到超声冲击态的383MPa,提高了70%,在1×109循环周次下由焊态的89MPa提高到了226MPa,提高了154%。根据试验结果,建议对于普通焊态接头,以1×107作为设计曲线转折点,循环周次低于1×107时S-N曲线斜率采用m=3进行设计,循环周次高于1×107时S-N曲线斜率采用m=5进行设计;对于超声冲击强化接头,在整个疲劳寿命区间都采用m=10的斜率进行寿命设计。分析了超声冲击改善焊接接头疲劳强度的机理,发现超声冲击主要通过在焊趾区形成的表层压应力、表层形变硬化和塑性变形、增加了接头处过渡半径等几个方面影响焊接接头疲劳行为的。对Q235和Q345钢焊态和超声冲击态下的疲劳试样断口进行了观察分析发现:表面型疲劳裂纹源是主要的疲劳裂纹萌生方式;超声冲击处理态的一些试样中的裂纹起源于超声冲击的振动挤压作用造成的表层或次表层微裂纹,有着与焊态不同的断裂模式。最后初步探讨了Q345在焊态和超声冲击态下的双周疲劳强度,无论双周载荷按外包络线表征还是只按低周载荷部分计算而忽略高周成分,只要最大应力值小于材料的屈服强度,冲击处理试件的疲劳强度总是高于焊态试件的疲劳强度,疲劳强度高出的程度随着载荷应力水平的增加而减小。更多还原

【Abstract】 In the industrial field of ship and marine engineering structures, many of its parts and components are affected by alternating load. The fatigue failure often occurs in service enlistment. However, fatigue failure often occurs in unsubstantial welded joints. This paper introduces an ultrasonic peening method to improve the fatigue strength of welded joints, and compare the fatigue strength of Q235 and Q345 in treated and as-welded.For Q235, the fatigue strength of which has been enhanced from 211Mpa to 330Mpa in 1×107 cycle times, it increases 56%. The fatigue strength of which has been enhanced from 82Mpa to 224Mpa in 1×109 cycle times, it increases 173%. For Q345, the fatigue strength of which has been enhanced from 225Mpa to 383Mpa in 1×107 cycle times, it increases 70%. The fatigue strength of which has been enhanced from 89Mpa to 226Mpa in 1×109 cycle times, it increases 154%. Based on the test results, the paper suggests using 1×107 cycle times as a turning point in the design for as-welded joints. When the cycle times is less than 1×107, you’d better use m=3 to design S-N slope of curve. When the cycle times is more than 1×107, you’d better use m=5 to design S-N slope of curve. For the ultrasonic peening strength welded joints, you’d better use m=10 to design S-N slope of curve in the whole range of fatigue life. This paper analysis the mechanism of improving the fatigue strength of welded joints in an ultrasonic peening method. There were several aspects which had an important impact on fatigue behavior, such as surface compressive stress which is formed in toe zone, surface deformation hardening and plastic deformation, increasing transition radius of joint.For Q235 and Q345 steel in as-welded and ultrasonic peening state, the observation of fatigue fracture tells that surface fatigue crack source is the main mode of fatigue crack initiation. Some specimens who are in ultrasonic peening state, crack originate from surface or sub-surface micro-cracks caused by compression of the ultrasonic vibration. The fracture mode is different from which of as-welded state.Finally, the paper preliminarily studies the combined fatigue strength of Q345 in as-welded and ultrasonic peening state. Whether combined load is characterized by outsourcing curves or only by low-cycle load when ignoring the high-cycle components, as long as the maximum stress value is less than the material yield strength, the fatigue strength of treated specimen is always higher than that of as-welded specimen, the degree may increase with the decrease of loading stress level.更多还原