Introduction by V. Honkimäki, ESRF

The conservative field of structural engineering is getting new ideas from new scientific tools such as synchrotron radiation. Gauge volumes down to micrometre dimensions are achievable using microfocussed high-energy X-ray radiation. This even enables the study of individual grains. Fast data acquisition makes in situ investigations possible. Fame38, a joint venture between some British Universities, the ILL and the ESRF, encourages European engineers to take advantage of these new possibilities in their research.

Residual stresses exist in engineering components in the absence of external loads. Generally residual compression near the surface enhances the materials resistance and near surface tension can lead to fracture. Mechanical forming, welding and heat treatment are the major ways to introduce residual stresses in the material. X-ray and neutron diffraction are the only non-destructive methods to determine these residual stresses. The diffraction method of measuring strain relies on the determination of interplanar atomic spacing along different directions and knowing the elastic constants of the material the state of stress in the specimen can be determined. The high penetration power of the high-energy X-ray radiation makes it possible to spatially-resolve stresses deep within material.

Strain measurements are possible at four beamlines at the ESRF: ID11, materials science beamline, ID15A and B, high-energy beamlines and ID31, powder diffraction beamline. These beamlines have several techniques to measure X-ray diffraction patterns: the traditional scanning method with monochromatic radiation, the white beam energy dispersive diffraction method, and monochromatic 2D-diffraction methods with area detectors to collect many reflections simultaneously. The depth resolution is achieved by slits with area detectors. These slits are more sophisticated with a conical or spiral construction. Furthermore, the incident beam can be focussed down to micrometre dimensions using mirrors, multilayers, bent Laue crystals or refractive lenses.

The engineering problems span a wide range of applications and it is impossible to cover all stress and strain studies at the ESRF in this short highlight presentation. One article describes the first direct measurements of the strains at a fibre and metal matrix interface. By combining in situ micro diffraction at ID11 and X-ray tomography at ID19 it has been possible to build up a comprehensive picture of the fibre fracture sequence. Another article describes the novel and efficient method of strain and phase scanning at ID15B and its application to study friction stir-welded joints between dissimilar materials.