Materials Science and Engineering A259 (1999) 209 – 219 Comparison of residual strains measured by X-ray and neutron diffraction in a titanium (Ti – 6Al – 4V) matrix composite Partha Rangaswamy a, *, Michael B. Prime a , Mark Daymond b , M.A.M. Bourke a , Bjørn Clausen a , Hahn Choo a , N. Jayaraman c a Los Alamos National Laboratory, Materials Science and Engineering, Lujan Center, H805, Los Alamos, Mexico b ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK c Department of Materials Science and Engineering, Uniersity of Cincinnati, Cincinnati, Ohio 45221 -0012, USA Abstract This research compares matrix thermal residual strains measured in a continuous fiber reinforced SiC/Ti–6Al–4V titanium matrix composite (TMC) using X-ray and neutron diffraction with finite element predictions. The strain dependence on the strains for several reflections (105, 204, 300, 213 and 312) of the matrix were explored at the surface (X-ray) and in the bulk (neutron). To determine the longitudinal surface strains from the X-ray measurements for comparison with the neutron values, the   versus sin 2  plots were extrapolated to  =90°. Continuum micro-mechanics based multi-ply finite element models (FEM) simulating rectangular and hexagonal fiber distributions were explored for calculating average surface and bulk strains. For different reflections, the experimentally determined surface measured strains ranged from +1904 424 to +2974 321  and the bulk measurements ranged from +2269 421 to +3022 1134 . These values contrast with the single valued FEM prediction of +3200  which was the same for both the surface and the bulk. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Residual stress; Neutron diffraction; Titanium matrix composites; Finite element analysis; X-ray measurement 1. Introduction Residual strains in continuous fiber reinforced tita- nium alloy metal matrix composites (TMCs) are in- duced during the cooling from the consolidation temperature of 900°C and are caused by the 2-fold mismatch in the coefficient of thermal expansion (CTE) between the SiC fiber reinforcement (3.2 ×10 -6 /°C) and the titanium matrix alloy (8.78 ×10 -6 /°C). After cooling, the strain parallel to the fibers (longitudinal) is tensile in the matrix and compressive in the fiber. The magnitude, distribution and stability in service of the resulting residual strains and stresses influence the me- chanical and physical properties of composites [1 – 4]. Despite extensive characterization [1 – 4] on TMC me- chanical behavior, measurement and modeling of resid- ual strains and stresses is a growing challenge. Models that predict residual stresses and their effect on lifetime predictions have to be validated [4,5]. Therefore, it is important for residual strains and stresses to be experi- mentally determined and accounted for to achieve reli- able design parameters. This research explores the range of strains that are measurable depending on the selection of a particular lattice reflection and whether surface or bulk penetrat- ing techniques are used. 1.1. Reiew of diffraction measurements of strain in SiC /Ti TMCs X-ray (XRD) [6–10] and neutron diffraction [10–16] are commonly used to determine residual stresses in continuous fiber reinforced TMCs. XRD is routinely used to monitor residual stresses in the matrix of TMCs, but because of the small penetration depth exhibited by soft X-rays [17], is limited to the near surface region ( 50 m). By contrast, neutron diffrac- tion due to its deep penetration can determine the strains in both the matrix and fiber simultaneously [16]. In this paper we focus only on the matrix strains. * Corresponding author. E-mail: partha@lanl.gov (http:// www.lansce.lanl.gov/mlnsc) 0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00893-4