Effect of weaving structures on the geometry variations and mechanical properties of 3D LTL woven composites

Abstract

This paper presents a comprehensive experimental study of the effect of weaving structures on the geometry variations and mechanical properties of three dimensional (3D) layer-to-layer interlock (LTL) woven composites. 3D woven composites with four different weaving structures are prepared. Geometry variations of the unit cell structures are investigated based on the micro-computed tomography (micro-CT) images. Tension and compression tests are performed to the four composites. The digital image correlation (DIC) method is used to obtain surface strain fields of the specimens under tension load. Both tension and compression properties of the composites are affected by the waviness of load-bearing yarns. The addition of stuffer yarns significantly improves the warp direction performance. Nonlinear behavior, failure modes and scatter of testing results for the specimens are closely related to the weaving structures.

Introduction

3D woven preforms can be used as reinforcements for complex-shaped composite structures, such as aircraft engine blades, casings and covers, for their high load-bearing capacity and good formability. One of the biggest advantages of these preforms is their near net shape forming ability, which reduces the manufacturing cost and production cycle [1].

Basic 3D woven textiles are composed of two yarn systems: the warp and weft yarns. The warp yarns weave through several layers of weft yarns and bind the weft yarn layers together to form an integrated 3D structure. 3D woven structures can be classified into three main types [2], [3]: (1) Orthogonal (ORT) structures, in which each warp yarn weaves through all the weft yarn layers directly along an orthogonal direction. (2) Through-thickness angle interlock (AI) structures, in which the warp yarns weave through all the weft yarn layers in thickness and cross more than two weft yarn columns in warp direction simultaneously. (3) Layer-to-layer interlock (LTL) structures, in which the warp yarns connect at least two weft layers together without weaving through the whole preform thickness. According to the weave patterns on the surface of the preforms, 3D woven structures can also be classified as plain, twill and satin [4]. A general definition of a 3D interlock structure has been proposed by Bousso et al. in Ref. [5].

Fiber architectures of 3D woven preforms can be adjusted in a wide range by changing the weaving parameters, such as warp/weft density and weaving patterns. The introduction of fiber in thickness direction improves the interlaminar properties. Fiber architectures directly affect the formability of the preforms. Dufour et al. [6] investigated the in-plane and through-thickness behavior of 3D woven preforms through hemispherical forming tests, the results pointed out that the LTL preform has a better stamping behavior. Zhang et al. [7] studied the in-plane shear and interlaminar shear behavior of the LTL preforms with different fabric densities, and found that the lower fabric density, the better deformability. It was concluded by some other studies [2], [8], [9] that the LTL preforms have good deformability properties during forming (or draping) process.

Mechanical properties of 3D woven composites are determined by the weaving structures. Tensile properties of various types of 3D woven composites have been experimentally characterized and compared [10], [11], [12], [13], one of the common conclusions drawn by these researches is that the greater the waviness of the load-bearing fibers, the lower the tensile properties of the composites. The DIC technique was used to measure the full-field strains of the samples during tension [10], [13], [14]. It was found that the strain fields of 3D woven composites were significantly related to the weaving structures of the preforms. The compression properties of 3D woven composites are influenced by the crimp and volume content of the load-bearing fibers [10], [11], [15]. Compression properties of the material would increase with the increase of load-bearing fiber content, but decrease with the increase of the bending degree for load-bearing fibers. The main compressive failure features of 3D woven composites include matrix cracking, delamination and fiber kinking [11], [16], [17]. The kink band caused by the initial misalignment of the yarn is the decisive mechanism leading to compressive failure. Dai et al. [11] investigated the flexural behaviors of 3D woven composites with different weaving structures. The authors indicated that the warp yarns distributed in the thickness can effectively arrest the delamination crack propagation, the weave with angled warp tows shows higher flexural properties. Umer et al. [18] and Zhao et al. [19] analyzed the flexural properties of ORT, AI and LTL composites. They found that the flexural property was closely related to undulation angle. The flexural strength of ORT composite was superior to the AI and LTL composites. Jin et al. [20] presented the comparisons of quasi-static three-point bending and fatigue damage behaviors between the LTL composite and ORT composite. It was found that the ORT composite can carry higher static bending loads but holds shorter period of fatigue life under the specific stress levels than the LTL composite.

Through the observation of high resolution micro-CT, many scholars [21], [22], [23], [24], [25], [26] have found that the internal fiber structures of woven composites are significantly perturbed from the idealistic regular arrangements. These geometry variabilities are caused by yarn tensions in the weaving process and mold pressure during the curing process. Variations in fiber structures are always inevitable and have a certain impact on the mechanical properties of the composites. But so far, the effect of weaving structures on the variations (or stabilities) of 3D woven preforms has not been studied systematically.

The purpose of this paper is to widen knowledge about the effect of weaving structures on structural stability and mechanical properties of 3D LTL woven composites used for aircraft engine blades. Four kinds of LTL structures are designed and prepared, which permit large deformations during the forming process. 3D woven composites are manufactured based on these preforms. Micro-CT measurements are used to analyze the micro-scale variations of the fiber structures. The composites are tested under tension and compression loads. The DIC method is employed to obtain the full-field strains during tension. Mechanical properties and failure mechanisms of the four composites are compared.