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Automobile manufacturers have experienced increasing consumer and regulatory pressure to improve fuel efficiency and crashworthiness while simultaneously decreasing overall vehicle body weight. As such, the use of advanced high strength steels (AHSS) in body panels and other structural elements is becoming more and more prevalent because these advanced materials present an economical and elegant solution to the problem. To ensure the quality and safety of AHSS components, residual stress (RS) specifications (among others) have been introduced with the intent to minimize failures experienced both in the field and during production. Moreover, when welding processes are applied to AHSS components, the localized loss of ductility in combination with tensile RS can result in localized cracking, distortion, and/or failures.

It is well known that machining and cold working operations produce surface conditions that can either enhance or debit the fatigue life of production components. When surfaces are abusively machined, the resultant tensile residual stresses (RS) and microstructural damage often cannot be detected reliably by conventional nondestructive testing (NDT). However, nondestructive surface RS measurements via x-ray diffraction (XRD) can detect process induced surface damage or abusive machining in very thin layers before cracking occurs. Heretofore, XRD techniques have been limited in their ability to characterize stresses in small diameter holes and other limiting geometries. Recent advancements in XRD technology and instrumentation have allowed nondestructive RS measurements to be performed on the inner diameter (I.D.) surface of small diameter through holes. The same advanced XRD instrumentation can also be used to perform RS measurements at relatively low diffraction angles.

Residual stress plays an important role in fatigue life resistance of automotive components. Due to the complex nature and combination of multiple processes used in the manufacturing of these components the residual stresses present may not always be uniform. Typically most components contain both surface and sub-surface stress gradients. This leads us to the conclusion that characterizing the residual stress at one location will not generate a sufficient understanding of the stresses present. Characterizing and understanding the stress gradients will help to manage the fatigue life of various components as well as help establish proper quality control practices to ensure the presence of beneficial residual stresses at critical locations.

The processing of certain features in automotive components such as crankshafts, gears, shafts, springs, rotors, cylinder heads, engine blocks etc. pose several difficulties for manufacturers and it is often a challenge to produce a finished product with the superior material characteristics that may be required for a given application. Among material characteristics of interest, the residual stress can have a significant impact on the effective service life of a component. Since residual stresses are introduced in nearly every step in manufacturing, it follows that the effect of processing applied to failure-critical locations must be well understood, controlled and optimized. This paper discusses the key aspects of applying XRD to the measurement of residual stress and will cite examples where XRD has been applied to the characterization of some typical automotive components.