Structures Perspective for Strength and Fatigue Prognosis in Composites with Manufacturing Irregularities

Abstract

Recent advances in understanding deformation and failure mechanisms of polymer-matrix composites used in rotor structures enable accurate and efficient measurement of material stiffness, strength, and fatigue characteristics based on testing small unidirectional laminate specimens. Successful failure predictions provided increased confidence in the development of virtual test methods replacing some of the standard tests of multidirectional laminated composite materials with three-dimensional models accurately predicting deformation, damage topography, strength, and cycles to failure. However, the remaining key questions are related to the ability of transitioning the material-scale virtual test information to larger composite structures. This work presents results of the feasibility assessment targeting the scaling of knowledge and methods acquired at the material scale, to larger structural elements. In particular, manufacturing irregularities such as voids and fiber waviness are typical attributes of composite rotor structures. Therefore, a comprehensive structural analysis should be able to model such irregularities to predict structural strength and fatigue behavior.

Assessing 3D shear stress–strain properties of composites using Digital Image Correlation and finite element analysis based optimization

“…It is worth noting that shear stressestrain relations closely agreeing with the results obtained in this work were used in a recent work by the authors for FEM-based predictions of delamination failure in multidirectional IM7/8552 carbon/epoxy laminate structural elements subject to complex quasi-static and fatigue loading [22]. Accurate non-linear shear stressestrain material response, generated based on unidirectional coupons, was essential for accurately predicting failure initiation in the multidirectional laminates.…”

“…Excellent correlation between the FEM-based structural failure predictions and the experimental data in Ref. [22] increased our confidence in the material properties obtained in this work. 7.…”

Manufacturing-Induced Imperfections in Composite Parts Manufactured via Automated Fiber Placement

“…Fiber angle deviations in laminate thickness direction often emerge as secondary phenomenon (out-of-plane waviness). Typical primary imperfections within a layup causing fiber angle deviations are gaps and overlaps [48][49][50][51], wrinkles and folds [21,30,37,39,52], pores and inclusions [53] as well as ramps (ply-drop off) [30,54]. The tooling [51,[55][56][57] and processing [21,30,[56][57][58]] significantly influence the development of out-of-plane fiber waviness.…”

“…Several studies [52,53,[59][60][61][62][63][64][65] emphasize the need to take fiber waviness into account when evaluating the structural performance of fiber composites. The researchers confirm that out-of-plane fiber waviness reduces the strength and stiffness properties of a material.…”

“…They show both numerically and experimentally that local shear stresses caused by the fiber waviness reduce the tensile and compressive strength of the laminate. Seon et al [53] investigate the interaction of inclusions and fiber waviness in order to determine their effect on the strength and fatigue of a fiber composite structure.…”