Miniature Ultrasonic Fatigue Analysis of Local Modified Regions near Welds and Surfaces in Ti alloys

疲劳是一种普遍的失效模式，影响许多工业部门，包括高价值航空航天、核能和汽车行业。一方面，它仍然是使用中故障的来源，另一方面，效率低下的过度设计的保守设计，因此给工业带来相当大的风险和成本（资本和运营）。在发生高或非常高的失效周期的较低应力状态下，有几个因素使疲劳失效的基本理解以及如何在工程实践中有效管理疲劳失效变得复杂：（i）测试方法通常只测试到约100万次循环，而在航空航天和核领域，安全关键部件的使用期可能要长得多，为100至1000倍，这迫使外推到未知和未经测试的状态;（ii）（非常）与低周疲劳相比，高周疲劳与微观结构的更大影响有关（iii）裂纹萌生过程占总疲劳寿命的很大一部分，但是由于没有裂纹存在，因此很难知道在材料微观结构中的何处进行观察，（iv）当外部载荷较小时，加工和机械加工产生的残余应力对总应力状态的贡献更大。这项研究计划将把超声波技术与小尺寸微型试件设计结合起来，为高周疲劳测试带来一个飞跃。测试将在20 kHz下进行，一百万次循环只需不到一分钟，十亿次循环只需一天。样品尺寸将在两个制度。首先，采用聚焦离子束（FIB）的微区切割样品宽度仅为几分之一至几微米，允许测试微结构的各个选定特征（晶粒、晶界、夹杂物.）。其次，使用激光微加工切割具有几十到几百微米宽的样品的介观状态，并允许测试小块的微观结构。介观样品是足够小的，频繁的间歇性微观表征方法，可以用于局部变形，应力和位错含量的局部演化的区域中，裂纹萌生是保证最终发生。更好地理解导致裂纹萌生的过程以及微观结构的局部变化如何控制疲劳裂纹萌生寿命是寻求的关键科学和技术成果。首先利用这种阶跃变化来表征工艺条件对Ti-6Al-4V中（i）线性摩擦焊缝和（ii）喷丸表面的疲劳裂纹萌生响应的影响。

Fatigue is a pervasive failure mode that affects many industrial sectors including the high value aerospace, nuclear and automotive sectors. It remains a source of in-service failure on the one hand and inefficient over-engineered conservative design on the other and so generates considerable risks and cost (capital and operating) to industry. In the lower stress regimes where high or very high cycles to failure occur there are several factors that complicate fundamental understanding of fatigue failure and how to manage it effectively in engineering practice:(i) testing methodologies generally only test to ~1 million cycles while safety critical components may see much longer service periods of a hundred to a thousand times longer in the aerospace and nuclear sector forcing extrapolation into unknown and untested regimes(ii) there is considerably more scatter in fatigue lives in (very) high cycle fatigue compared to low cycle fatigue which is linked to a greater influence of microstructure(iii) the crack initiation process takes up a much larger fraction of the total fatigue life but as no crack is present it is difficult to know where in the material microstructure to make observations that will capture local processes that will eventually lead to crack nucleation(iv) residual stresses from processing and machining make a more significant contribution to the total stress state when the external loading is smaller.This research programme will deliver a step change in high cycle fatigue testing by combining ultrasonic technology with small scale miniature test-piece designs. The tests will be conducted at 20 kHz at which a million cycles takes just less than a minute and a billion cycles takes only 1 day. The sample dimension will be in two regimes. Firstly, a micro-regime with Focused Ion Beam (FIB) cut sample widths only a fraction to a few micrometres across allowing testing of individual selected features of a microstructure (grain, grain boundary, inclusion...). Secondly, a meso-regime with samples a few tens to a few hundreds of micrometres wide cut using laser micro-machining and allowing small patches of microstructure to be tested. The meso-samples are sufficiently small that frequent intermittent microscopic characterisation methods can be used to the local evolution of local deformation, stress, and dislocation content in regions where crack initiation is guaranteed to occur eventually. Greater understanding of processes leading to crack initiation and how local variation in microstructure control fatigue crack initiation lifetimes are the key scientific and technological outcomes sought. This step change advance will be exploited in the first instance to characterize effects of process conditions on the fatigue crack initiation response of (i) linear friction welds & (ii) peened surfaces in Ti-6Al-4V.