X-ray diffraction tests component stress levels

The service life of automotive components varies greatly as a result of “as manufactured”residual stress levels present at failure-critical increased demand by consumers is a need for aitomative manufatures to meet or exceed specified warrranty periods and to reduce the incidence of recalls and warranty repairs.

Thus,the stress state of automotive component must be accurately characterized so that contributing mechanisms and sources of potentially harmful residual stresses can be well understood,fabrication processes can then be modified and optimised so that high quality components can be manufactured at a competitve cost.

When it is suspected that residual stress maybe a contributing factor to a premature failure,such suspicions may be validated by experiment and by measurement. X-ray diffraction provides an accurate, reliable and consistent method to quantify these residual stresses.

Identifying the need for XRD

X-ray diffraction residual stress measurement analysis (XRD) should be used when damaged components:

l         Experience static in service loads that are a significant fraction of the maximum allowable equivalent stress (stress concentrations, overloaded)

l         Distort or form cracks without applied loads

l         Are placed in corrosive environments (stress corrosion)

l         Experience cyclic loading in service (mechanical or thermal fatigue)

l         May have been subject to improper processing upon manufacture (shot peening,grinding, milling, etc.)

l         May have been subject to inappropriate heat treatment (stress relief, induction hardening, service temperature, thermal strains etc.)

XRD can also be used to verify the effectiveness of a heat treatment process, the effects of surface treatments such as nitriding, shot or laser shock peening and surface residual stress measurements can be performed non-destructively for inline quality control and for tracking stresses through processing and in –service cycling.

Robust x-ray technology

Modern, dedicated x-ray diffraction systems are now available to measure residual stress accurately and easily in all types of automotive components and materials. Advances in technology have enabled the creation of instruments which are very robust and capable of measuring on almost any component geometry.

Canadian-based Proto Manufacturing Limited has been manufacturing x-ray diffraction residual stress measurement systems for more than 25 years. Proto’s innovative systems have many features not found on standard XRD systems such as miniaturised goniometers that enable access to cylinder bores; zero maintenance x-ray detectors that do not degrade on exposure to x-ray; residual stress mapping to generate a comprehensive picture of the residual stress state on areas of a component; portable equipment for evaluation on large components and high-speed laboratory systems for high throughput in-line and quality control audit stations.

The gengeration of compressive residual stresses is a function of the processes applied including heat treatment (temperatures, cooling speeds), shot peening (coverage, intensity, shot size and type) or rolling pressure. The examples shown indicate that the applied fabrication processes are not always well controlled. In many cases testing may be required in the final stages of processing using NDT techniques such as x-ray diffraction for residual stress measurements.

The residual stress in automotive components can be a decisive factor in fatigue resistance regardless of the advanced fabrication processes employed. The residual stresses can play an important role in reducing component fabrication cost while enhancing the fatigue life of the mechanical parts.

参考译文：

由于在易发故障点会出现人为地残余应力级，汽车零部件的使用寿命会有很大变化。随着消费者对质量、可靠性和性能方面的要求越来越高，汽车制造商必须保证其服务满足或超过既定的保修期，同时还有减少召回已售出车辆的数量及保修期内的维修次数。

因此，必须精确检测汽车零部件的应力状态，并充分理解其形成机制和潜在危害性残余应力的来源。一旦确定了上述内容，就可以修正并优化制造工艺，从而实现以较低成本来生产高质量的零部件。

当故障尚未发生时，有人怀疑残余应力是故障的成因之一。这种怀疑可以通过实验和测试得以证实。X射线衍射测试提供了一种精确、可靠并且持久的量化残余应力的方法。

明确X射线衍射测试的需求

当零部件发生下列损伤情况时，应该采用X射线衍射(XRD)残余应力测试分析：

l         零部件处于服务加载的停滞期，此时服务加载是所允许的最大当量应力的一个重要部分

l         在没有外加负载的情况下，零部件变形或出现裂痕；

l         零部件放置于腐蚀性环境，而导致零部件破损；

l         零部件在服务过程中处于周期载荷状态；

l         可能由于制造时加工不当而导致零部件破损（如在进行喷丸、磨削、铣削等流程时）；

l         可能由于进行补合适的热处理而导致零部件破损（如在应力释放、感应淬火、服务温度、热应变等过程中）

X射线衍射同时能够用来验证热处理工艺的成效以及表面处理的效果，例如氮化、喷丸或激光喷丸以及表面残余应力测量可以再没有破坏性的前提下，用于内置质量控制，并通过加工处理和循环服务，用于追踪应力。

耐用的X射线技术

先进的专门化X射线衍射系统非常精密，可以用来准确简便地测量各种汽车零部件和材料的残余应力。先进的技术使得本成品健全可靠，几乎能够测量所有零部件的几何数值。

加拿大PROTO制造公司致力于X射线衍射残余应力测试系统的制造已经有25年的历史了，PROTO独创的测试系统拥有许多其他标准X射线衍射系统所没有的特色，如，微型测角仪能够进入气缸腔进行测量；零维修X射线探测器能够在X射线照射下不受腐蚀；残余应力映射可以生成零部件各部位完整的残余应力状况图；以及便携式大型零部件测试仪器和用于高吞吐量的内置高速实验室系统和质量控制审计站。

压缩残余应力的生成是应用工艺，包括热处理（温度，冷却速度），喷丸（覆盖范围、强度、喷丸大小和类型）或者是轧制压力中的一项功能。上述例子表明应用制造工艺通常存在控制不当的状况。在多数情况下，需要在加工的最后阶段，采用无损检测技术，如X射线衍射残余应力测量，来进行测试。

尽管采用了先进的生产工艺，汽车零部件的残余应力仍可能是抗疲劳过程中的一个关键因素。残余应力在降低零部件生产成本，提高机械零件疲劳寿命方面扮演着重要角色。