Fracture Mechanics Analysis of Reinforced DCB Sandwich Debond Specimen Loaded by Moments

Saseendran, Vishnu; Berggreen, Christian; Carlsson, Leif A.

doi:10.2514/1.j056039

Cited by 18 publications

(14 citation statements)

References 23 publications

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“…108,137,138 This difference is due to a dependency on the thickness of the doubler in estimating the fracture energy. 139 Therefore, it is not recommended to use doublers for bi-materials to evaluate the effectiveness of stitching without further investigation.…”

Section: Test Methods and Analysis To Characterize Stitched Compositesmentioning

confidence: 99%

Influence of stitching on the out-of-plane behavior of composite materials – A mechanistic review

Drake

Sullivan

Lovejoy

et al. 2021

Journal of Composite Materials

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Fiber-reinforced polymer composites are widely used in the aerospace industry due to their high stiffness and strength-to-weight ratios. However, their applicability can be limited by their relatively low interlaminar properties when compared to metallic alternatives. Through-thickness reinforcement approaches, such as stitching, z-pinning, needling, tufting, and three-dimensional weaving, have been developed in recent decades to enhance the interlaminar properties of composites. Stitching is considered to be an efficient and cost-effective method to reinforce composites in the through-thickness direction. Additionally, stitch parameters (stitch density, linear thread density, thread material, pretension, etc.) highly influence the in-plane and out-of-plane properties. This paper summarizes results from over one hundred papers on the influence of stitch parameters on fracture energy, interlaminar strength, and impact characteristics of stitched composite laminates, sandwich composites, and high-temperature composites. Much of the research on the influence of stitch parameters has focused on thermoset polymer matrix composites (PMCs), while fewer studies have investigated the impact of stitch parameters on high temperature or sandwich composites. Modification of existing and new test methods have been developed to adequately measure the effectiveness of stitching on the out-of-plane behavior of PMC panels. Results demonstrate that out-of-plane properties of PMCs are highly dependent on stitch parameters and can be enhanced by through-thickness stitching.

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Section: Test Methods and Analysis To Characterize Stitched Compositesmentioning

confidence: 99%

Influence of stitching on the out-of-plane behavior of composite materials – A mechanistic review

Drake

Sullivan

Lovejoy

et al. 2021

Journal of Composite Materials

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“…Therefore, by holding the MR constant throughout the crack propagation during the fracture testing, toughness characterization can be performed at a fixed phase angle ( ψ ). Closed form expressions for both energy release rate and mode mixity phase angle for an un-reinforced (see Figure 1(a)) [20] and reinforced (Figure 1(b)) [21] sandwich DCB-UBM specimens exist in the literature. Attachment of reinforcement layers, referred to as “doublers,” on both sides of the specimen reduces excessive rotations and displacements, especially for specimens with thin face sheets [22].…”

Section: Introductionmentioning

confidence: 99%

“…A large range of MR values was explored to vary the mode mixity. It should be noted that in addition to the numerical method, the phase angle, ψ , can also be obtained using closed form expressions for a moment loaded DCB sandwich specimen [20, 21]. The mechanical properties of face sheet and core materials are provided in Tables 1 and 2, respectively.…”

mentioning

confidence: 99%

Mixed-mode fracture evaluation of aerospace grade honeycomb core sandwich specimens using the Double Cantilever Beam–Uneven Bending Moment test method

Saseendran

Berggreen

2018

Jnl of Sandwich Structures & Materials

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Fracture testing of aerospace grade honeycomb core sandwich composites is carried out using the Double Cantilever Beam specimen loaded with Uneven Bending Moments, and a Double Cantilever Beam–Uneven Bending Moment test rig capable of applying pure moments is utilized. Specimens with carbon fiber-reinforced plastic face sheets are employed with a range of honeycomb core grades comprising of Nomex® and Kevlar paper. The sandwich specimens are reinforced with steel doublers to reduce excessive rotation of the face sheets. The mode mixity phase angle pertaining to a particular ratio of moments between the two arms of the Double Cantilever Beam specimen is determined using the numerical mode mixity method—Crack Surface Displacement Extrapolation method. For Nomex® honeycomb core sandwich specimens, it is observed that the mode I interface fracture toughness increases with increase in core density. The interface fracture toughnesses for Nomex®-based honeycomb cores are also compared against specimens with Kevlar paper-based honeycomb cores. Crack propagation is observed at the interface just beneath the meniscus layer for the majority of the tested specimen configurations. The Double Cantilever Beam–Uneven Bending Moment test methodology with the concept of direct application of moments on both crack flanks has proven to have a significant potential for mixed mode face/core fracture characterization of aerospace grade sandwich composites.

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“…In all other cases, the presence of the oscillatory terms in Eqs (4). implies that the ratio of normal and shear stress components depend on the distance r from the crack tip, namely define the ratio of shear and normal stresses at a fixed distance r ahead of the crack tip.…”

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confidence: 99%

The effects of shear and near tip deformations on interface fracture of symmetric sandwich beams

Barbieri

Massabò

Berggreen

2018

Engineering Fracture Mechanics

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The effects of shear on energy release rate and mode mixity in a symmetric sandwich beam with isotropic layers and a debond crack at the face sheet/core interface are investigated through a semianalytic approach based on two-dimensional elasticity and linear elastic fracture mechanics. Semianalytic expressions are derived for the shear components of energy release rate and mode mixity phase angle which depend on four numerical coefficients derived through accurate finite element analyses. The expressions are combined with earlier results for three-layer configurations subjected to bending-moments and axial forces to obtain solutions for sandwich beams under general loading conditions and for an extensive range of geometrical and material properties. The results are applicable to laboratory specimens used for the characterization of the fracture properties of sandwich composites for civil, marine, energy and aeronautical applications, provided the lengths of the crack and the ligament ahead of the crack tip are above minimum lengths. The physical and mechanical significance of the terms of the energy release rate which depend on the shear forces are explained using structural mechanics concepts and introducing crack tip root rotations to account for the main effects of the near tip deformations.Composite sandwich structures are widely used in marine, energy, aeronautical and civil engineering applications. Their main advantages over traditional metallic materials or monolithic composites are the high stiffness to weight and strength to weight ratios, which make them key enablers for present and future lightweight structures.Sandwich structures, however, are highly susceptible to manufacturing flaws as well as inservice damages. One of such flaws and damages is debonds, which may arise at the face/core interfaces and will degrade the load carrying capacity and integrity of the sandwich structure, and may even grow catastrophically during both quasi-static and fatigue loading depending on their debond location and loading scenario in the structure.In order to assess the criticality of debond flaws and damages, the fracture properties of the face/core interface have to be measured and used as input properties in fracture mechanical analysis models. Several mixed mode fracture mechanical characterization tests have been proposed in the literature and are currently used for fracture characterization of sandwich face/core interfaces (see review in [1]). The analysis of most of the proposed test specimens relies on approximate structural theories and/or numerical finite element analyses to define energy release rate and mode mixity phase angle. Knowing energy release rate and mode mixity angle allows the reduction of the fracture toughness and crack propagation rate vs. energy release rate from measured data for different crack tip mixed mode loading conditions. Semi-analytic solutions based on 2D elasticity for sandwich beams subjected to generalized end forces are already available in the literature [2][3]. These solut...

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Fracture Mechanics Analysis of Reinforced DCB Sandwich Debond Specimen Loaded by Moments

Cited by 18 publications

References 23 publications

Influence of stitching on the out-of-plane behavior of composite materials – A mechanistic review

Influence of stitching on the out-of-plane behavior of composite materials – A mechanistic review

Mixed-mode fracture evaluation of aerospace grade honeycomb core sandwich specimens using the Double Cantilever Beam–Uneven Bending Moment test method

The effects of shear and near tip deformations on interface fracture of symmetric sandwich beams

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