A technique is described for the measurement of the residual stress tensor averaged over a specified volume within a component. The method involves measurement of small changes in lattice spacing using high resolution neutron diffraction. The stress is inferred from these measurements of the strain, and the theory of the relationship between the two quantities is described, including the effects of crystalline anisotropy. The various types of high resolution neutron diffractometer suitable for the work are described. Experimental results validating the method are given for a simple bent bar of mild steel of known strain, a plastically strained mild steel bar, and a mild steel tube of known torsional strain. Examples of the method in practical use are given by a cracked fatigue test specimen, a double-V test weld and a weld joining a tube to a plate. A more detailed example is the anisotropic response of a polycrystalline sample under elastic and plastic strain; this is illustrated by measurements...

Neutron diffraction methods for the study of residual stress fields

Stresses required to debond the fiber-matrix interface and to pull out a fiber are analyzed as a function of the embedded fiber length for a fiber-reinforced composite. The role of residual clamping stresses at the interface is considered. During fiber pull-out, Poisson contraction of the fiber in the radial direction reduces the resultant compressive stress at the interface and, hence, the interfacial frictional stress. This leads to a non-linear dependence of both the stress required for complete interfacial debonding and the stress for fiber pull-out on the embedded fiber length. Excellent agreement between present theoretical analyses and existing experimental results is obtained. Compared with the stress required for fiber pushout, where the fiber is subjected to compression in its axial direction, the stress required for fiber pullout is lower owing to Poisson's effect.

Interfacial debonding and fiber pull-out stresses of fiber-reinforced composites

http://arc.nucapt.northwestern.edu/refbase/files/Young-2007_9659.pdf

Load partitioning between ferrite and cementite during elasto-plastic deformation of an ultrahigh-carbon steel

This study focuses on interfacial bonding between intergranular silicon carbide particles and an alumina matrix, to determine the creep inhibition mechanism of alumina/ silicon carbide nanocomposites. It is revealed that the silicon carbide/alumina interface possesses much stronger bonding than the alumina/alumina interface through three approaches: investigation of fracture toughness and fracture mode and consideration of internal thermal stresses acting at grain boundaries, estimation of equilibrium thickness of intergranular glassy films by force balance, and direct observation of grain boundaries by TEM. The rigid bonding of alumina/silicon carbide interfaces causes inhibition of vacancy nucleation and annihilation at the interfaces, causing remarkably improved creep resistance of the nanocomposite.

Particle/Matrix Interface and Its Role in Creep Inhibition in Alumina/Silicon Carbide Nanocomposites

The damping response of crystalline metals and alloys is generally associated with the presence of defects in the crystal lattice. The disturbance of these defects, usually in response to an applied cyclic load, dissipates energy, a mechanism known as “internal friction”. The various defects commonly found in crystalline materials include point defects (e.g. vacancies), line defects (e.g. dislocations), surface defects (e.g. grain boundaries) and volume defects (e.g. inclusions). Among these, dislocations are noteworthy because they play a critical role, not only in the damping response of crystalline materials, but also in the overall mechanical behaviour of the materials. Among the various structural materials actively being developed, metal matrix composites (MMCs) have received considerable attention as a result of their potential to combine reinforcement properties of strength and environmental resistance, with matrix properties of ductility and toughness. Of interest is the generally observed phenomenon that MMCs exhibit unusually high concentrations of dislocations, an observation typically attributed to the difference in coefficient of thermal expansion between matrix and reinforcement. The objectives of the present paper are to provide an overview of the sources of dislocation generation in MMCs, and to provide insight into the effects that dislocations have on the damping response of MMCs. The presence of dislocations in MMCs is highlighted on the basis of transmission electron microscopy studies, and the dislocation damping mechanisms are discussed in light of the Granato-Lucke theory.

Dislocation-induced damping in metal matrix composites

Residual thermal stresses and strains, developed during cooling of a SiC whisker-Al2O3 composite, were determined by an experimental neutron diffraction technique and the results were compared with analytically determined values. High compressive residual thermal stresses were generated in the whiskers during cooldown after sintering. Analytical estimates of the residual stresses were obtained by using two self-consistent models: (1) a plane strain composite cylinder model and (2) Eshelby's ellipsoidal inclusion theory. The two models gave almost identical estimates of the stresses and strains in the whiskers. The interpretation of the measured strains in the whiskers by neutron diffraction had some uncertainty because of the lack of a clearly defined crystal structure of SiC.

Determinations of Residual Thermal Stresses in a SiC-Al2O3 Composite Using Neutron Diffraction

An experimental neutron diffraction technique was used to measure residual strains that developed in multiphase composite materials during postfabrication cooling. The reinforcement geometries that were studied include unidirectional fibers, randomly oriented single crystal whiskers, and equiaxed particles. Both metal and ceramic matrices and reinforcements were considered

Application of Neutron Diffraction to Measure Residual Strains in Various Engineering Composite Materials

The fracture toughness (K{sub IC}), strength, and elastic moduli of hot-pressed Si{sub 3}N{sub 4} reinforced with SiC whiskers were measured at room temperature over a range of SiC content from 0 to 20 wt%. The K{sub IC} was determined from cracks produced by Vickers indentation, the elastic moduli were determined by an ultrasonic technique, and the strength was measured by four-point bending. Although K{sub IC} increases from 4 to 7 MN {center dot} m{sup {minus}3/2} with the addition of 20 wt% SiC, the corresponding increase in strength is relatively small, i.e., from 375 to 550 MPa. Scanning electron microscopy reveals that the small increase in strength is partly related to processing-induced critical flaws.

Fracture toughness and strength of SiC-whisker-reinforced Si sub 3 N sub 4 composites

Residual thermal strains in a SiC-whisker-reinforced/Al/sub 2/O/sub 3/-matrix composite were measured as functions of temperature and volume fraction of whiskers by means of a neutron diffraction technique. The residual strains in both phases decrease with increasing temperature. At room temperature, the residual strains in the whiskers decrease with increasing whisker volume fraction, but the tensile residual strains in the matrix increase.

Effects of Temperature and Whisker Volume Fraction on Average Residual Thermal Strains in a SiC/Al2O3 Composite

Fiber/matrix interfacial debonding and frictional sliding stresses were evaluated by single-fiber pushout tests on unidirectional continuous silicon-carbide-fiber-reinforced, reaction-bonded silicon nitride matrix composites. The debonding and maximum pushout loads required to overcome interfacial friction were obtained from load-displacement plots of pushout tests. Interfacial debonding and frictional sliding stresses were evaluated for composites with various fiber contents and fiber surface conditions (coated and uncoated), and after matrix densification by hot isostatic pressing (HIPing). For as-fabricated composites, both debonding and frictional sliding stresses decreased with increasing fiber content. The HIPed composites, however, exhibited higher interfacial debonding and frictional sliding stresses than those of the as-fabricated composites. These results were related to variations in axial and transverse residual stresses on fibers in the composites. A shear-lag model developed for a partially debonded composite, including full residual stress field, was employed to analyze the nonlinear dependence of maximum pushout load on embedded fiber length for as-fabricated and HIPed composites. Interfacial friction coefficients of 0.1--0.16 fitted the experimental data well. The extremely high debonding stress observed in uncoated fibers is believed to be due to strong chemical bonding between fiber and matrix.

D. S. Kupperman

Papers

Determinations of Residual Thermal Stresses in a SiC-Al2O3 Composite Using Neutron Diffraction

Application of Neutron Diffraction to Measure Residual Strains in Various Engineering Composite Materials

Fracture toughness and strength of SiC-whisker-reinforced Si sub 3 N sub 4 composites

Effects of Temperature and Whisker Volume Fraction on Average Residual Thermal Strains in a SiC/Al2O3 Composite

Effect‐of Processing Varcables on Interfacial Properties of an SiG‐Fiber‐Reinforced Reaction‐Bonded Si3N4 Matrix Composite