Low and high neutron energy for damage detection by chemical composition analysis and residual stress measurements

The use of neutrons as an investigative tool has received considerable attention over the last few years. Neutrons, due to their capability to penetrate thick layers of materials, are particularly suited to investigate inside or beneath the surface of an object without damaging it, determining structure at the microscopic scale, thus providing fundamental information. In particular, neutron measurements such as Neutron Diffraction (ND) or Neutron Tomography (NT), can study structural characteristics like composition, presence of alteration, inclusions, structure of the bulk, manufacturing techniques and presence of those elements which give us an overall fingerprint of the object’s characteristics. A neutron beam impinging onto any heterogeneous object is differently transmitted depending on neutron energy and on thickness, density, chemical composition and total cross section of the material along the line of sight. Recording the transmitted beam is possible to reconstruct the internal feature of the objects. Contrary to the photon case, a neutron beam can transmit through centimetres of metal but it is easily attenuated by small amounts of light elements like hydrogen, boron and lithium. The investigation of moisture and corrosion, the detection of explosives and adhesive connections and the inspection of defects in objects or in thick metallic samples are examples where neutron can be utilized favourably. For this reason neutron analysis is an unique tool for non-destructive testing with multidisciplinary applications. Furthermore, they can be used for physical problems such as residual stress measurements, study of mechanical behaviour in materials, archaeometry and cultural heritage. Introduction The use of neutrons to investigate the fundamental properties of materials began in the 1940s. The pioneering applications [1,2] were limited to studies of the physical properties of matter, and in particular to phase transitions, magnetic structures and especially the hydrogen bond. In the last three decades the use of neutrons has vastly expanded following the development of new technologies for the production of thermal and epithermal neutrons. Neutrons with wavelengths of the order of angstroms are able of probing molecular structures and find applications in a wide array of scientific fields, including biology, cultural heritage materials, environmental sciences, engineering, material sciences, mineralogy and solid state and soft matter physics (figure 1). Figure 1: neutrons for pure and applied science. The special nature of neutron interaction with matter provides important complementary and supplementary data to other techniques. The large penetration depth and selective absorption of neutrons make them a powerful tool in NDT (Non Destructive Testing) of materials. For example, the residual stress formed in a material during manufacturing, welding, utilization or repairs can be measured by means of neutron diffraction. In fact neutron diffraction is the only NDT method, which make possible a 3D mapping of residual stress in a bulk component. The experimental techniques are described in this paper. Neutron diffraction Neutron scattering is the most suitable method for resolving 3D samples and it mainly consists in Neutron Diffraction (ND). ND [3] is based on Bragg law 1 and allows to resolve matter crystallographic structures, determining the atomic and/or magnetic structure of a material. Moreover ND can be applied to study crystalline solids, gasses, liquids or amorphous materials. The method requires irradiating the analyzed sample with a collimated beam of low energy (cold or thermal) neutrons. The revealed intensity pattern gives information about the material structure. Neutron diffraction uses neutrons generated by fission or spallation. The first is mostly employed in steady-state nuclear reactors while the second usually in pulsed sources. In both cases the neutrons produced are moderated until to the thermal energy range, i.e. λ ≥ 0.05 nm. When a neutron beam of wavelength λ, comparable with the inter-planer spacing dhkl, impinges a crystalline material, a diffraction pattern is observed and the position of each plane (hkl) is obtained by the Bragg law:    hkl hkl d sin 2 Figure 2 shows 2θhkl angle related to Bragg peak and is linked to the direction of the incident 1 Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. neutron beam. Thus, all dhkl are established from the angle θhkl at which the reflection is detected. Figure 2: Schematic illustration of Bragg scattering. Neutron diffractometers A polycrystalline sample consists of small (few μm) crystallites randomly oriented with respect to each other. When a monochromatic radiations strike a sample, the diffraction from a Bragg plane results in a cone shape, the Debye Scherrer cone, with semi-vortex angle 2θ. The intensity profile is recorded as a circle on a two dimensional detector (Figure 3). Figure 3: Diffraction from polycrystalline sample in a Debye Scherrer cone The polychromatic neutron beam is first monochromated to a chosen wavelength by diffraction from a suitable monochromator. The divergence and size of the monochromatic beam is suitably adjusted using appropriate neutron optical devices and is then diffracted from the specimen. In a similar way, the diffracted beam is shaped using suitable optical devices, before being captured by the neutron detector. The gauge volume over which the strain measurement is made is given by the intersection of the incident and diffracted beams (Figure 4). Strain measurement and determination The strain is measured towards the scattering vector, Q = kf ki, which splits the angle between incident and diffracted beams and is perpendicular to the diffracting planes, as shown in Figure 2. Lattice spacing is determined from the measured angular position of the diffraction peak (Bragg reflection) by irradiating the specimen with a monochromatic collimated neutron beam. If the specimen contains no strain, the lattice spacing is the strain free (stress free) values for the material and are denoted by d0,hkl. In a stressed specimen, lattice spacing is altered and a shift in each Bragg peak position occurs and the elastic strains then are given by:

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