Explaining Spring-In in Filament Wound Carbon Fiber/Epoxy Composites

Abstract

Filament wound curved composite parts which are autoclave cured tend to exhibit "spring-in," a permanent deformation due to residual stresses (Stover, 1993). Spring-in is the tendency of a circular part to reduce it's radius of curvature upon radial cutting. The causes of spring-in and the associated residual stresses must be understood to ensure dimensional accuracy and safety of the final cured part. The purpose of this paper is to isolate and quantify the principle causes of spring-in in filament wound unidirectional autoclave cured carbon fiber/epoxy composite hoops. This paper first explores the causes of spring-in as given in the literature: through thickness inhomogeneity (Radford, 1993), part compaction (Meink, 1998), and anisotropy with initial curvature (Kollar, 1992). Next, the development of the residual stress profile through the fabrication and cure process is given along with the governing equations for the spring-in phenomenon. Finally, an experimental procedure is developed to obtain the residual stress profile in a thin composite part. This methodology involves progressive cutting (i.e., stress relief) and associated strain gage monitoring. The strain gage data is then combined with a finite element (FEM) analysis to determine the through-thickness residual stress profile. From the residual stress profile experiments, it was found that the principal cause of spring-in is not material anisotropy as was previously thought. Instead, the principle cause of spring-in was found to be the gradual thermal expansion of the mandrel during cure, which causes a "tension-lag" in the curing part. In addition, the compaction of the part during cure was found to have a nominal contribution to total spring-in.

Destructive techniques in the measurement of residual stresses in composite materials: an overview

“…An essential assumption of the incremental slitting method is that stresses are consistent in the direction of the slot width, which allows the material removed by the slot to be treated as a homogeneous continuum. If the heterogeneous structure of a fi brous composite is treated as homogeneous (Ganley et al ., 2000;Kim et al ., © Woodhead Publishing Limited, 2014 2002), the measured residual stresses will be averages of the matrix and reinforcement stresses (Prime and Hill, 2004). This implies that only residual stresses at the meso-scale can be found in the fi ber direction.…”

“…The method employed to create a cut of increasing depth has evolved from sawing (Ersoy and Vardar, 2000;Ganley et al ., 2000;Johnson et al ., 1985;Kang et al ., 1989;Neubrand, 2002) and milling Finnie 1985, 1986) to EDM (Cheng et al ., 1994). Sawing and milling are universally applied but the cutting can cause an undesirable temperature increase and plastic deformation close to the bottom of the cut.…”

“…The radial cutting method is a different version of the sectioning method. It has been used extensively to determine the residual stresses in fi ber reinforced tubes (Cohen, 1997;Ganley et al ., 2000;Short, 2002, Seif et al ., 2006) and rings (Aleong and Munro, 1991;Roy, 1991). Based on the assumption that the residual stresses in such structures are invariant with axial and circumferential position, a single axial cut can result in their release.…”

Impact of nanoparticles on the process‐induced distortions of carbon fiber reinforced plastics: An experimental and simulative approach

“…The Young's modulus and the Poisson's ratio of the matrix are crucial for the transfer of stresses between fibers and resin. In addition, the bending modulus of the CFRP determines to what extent stresses are released by distortions …”