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

疲劳是一种普遍存在的失效模式，影响到许多工业部门，包括高价值的航空航天、核能和汽车部门。一方面，它仍然是现役故障的根源，另一方面，低效的过度设计的保守设计，因此给行业带来了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循环的较低应力状态下，有几个因素使对疲劳失效的基本理解以及如何在工程实践中有效地管理疲劳失效复杂化：(i)测试方法通常只测试到约100万次循环，而安全关键部件的使用周期可能更长，在航空航天和核部门可能要长100到1000倍，迫使外推到未知和未经测试的状态（ii）存在更多分散在疲劳的生活(非常)高循环疲劳相比,低循环疲劳与微观结构的影响更大(3)裂纹萌生过程占用更大比例的总疲劳寿命但没有裂纹很难知道在材料微观结构观察,捕捉局部流程,最终将导致裂纹成核(iv)残余应力处理和加工外载荷越小，对总应力状态的贡献越显著。该研究项目将超声技术与小尺寸微型试件设计相结合，为高周疲劳测试提供一个跨越式的变化。测试将在20千赫的频率下进行，100万次循环只需要不到一分钟，10亿次循环只需要一天。样本维度将有两种形式。首先，聚焦离子束（FIB）的微区切割样品宽度仅为几微米，允许测试微观结构的单个选择特征（晶粒，晶界，夹杂物…）。其次，使用激光微加工技术切割几十到几百微米宽的样品，并允许对微观结构的小块进行测试。细观样品足够小，频繁的间歇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变，最终保证发生裂纹起裂。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所寻求的关键科技成果。这一步骤变化的进展将首先用于表征工艺条件对Ti-6Al-4V中(i)线性摩擦焊缝和（ii）喷喷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.