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American Society for Testing and Materials

The influence of adhesive constitutive parameters in cohesive zone finite element models of adhesively bonded joints

This paper presents a review of the issues concerning sandwich structures for aeronautical applications. The main questions raised by designers are first recalled and the complexity of sandwich structure design for aeronautics is highlighted. Then a review of applications is presented, starting with early examples from the 1930s and the Second World War. The growth in the use of sandwich materials in civil and military applications is then developed. Recent research and innovations conclude the paper.

https://hal.archives-ouvertes.fr/hal-02952087/document

Review of composite sandwich structure in aeronautic applications

Predictive strength-fracture model for composite bonded joints

A novel continuum-decohesive finite element for modeling in-plane fracture in fiber reinforced composites

A stress-strain delamination constitutive law based on a continuum damage mechanics formulation is postulated for the thin resin layer that exists between adjacent composite laminae to predict initiation and growth of delamination. Delamination is predicted to initiate based on a interlaminar maximum stress obtained from a multiaxial stress criterion and the delamination front is predicted to advance based on a critical fracture energy obtained from a mixed mode fracture criterion. Damage is assumed to be irreversible via the inclusion of a thermodynamically consistent damage parameter. The damage parameter is directly related to the dissipated fracture energy. Although the implementation of the constitutive law in a commercial finite element code is straightforward, the nonlinear solver often fails to converge to a solution using the conventional tangent stiffness matrix. Convergence is achieved by using a stiffness matrix with a modification that improves its condition number. With this novel constitutive law and stiffness modification, the delamination front and the direction of delamination propagation are predicted for various fracture test configurations such as the double cantilever beam, end load split, and end crack torsion.

Interphasic formulation for the prediction of delamination

This report presents the results of a literature review as part of the development of composite hardware fracture control guidelines funded by NASA Engineering and Safety Center (NESC) under contract NNL04AA09B. The objectives of the overall development tasks are to provide a broad information and database to the designers, analysts, and testing personnel who are engaged in space flight hardware production.

Composite Structures Damage Tolerance Analysis Methodologies

An analytical model to predict the bypass and bearing strength of composites is developed. The model includes a contact algorithm, finite deformation theory, nonlinear material behavior, and the progressive damage of composites. The orientation and density of matrix-cracks and fiber damage are represented through a damage tensor, an internal state tensor that is thermodynamically consistent within the framework of continuum damage mechanics. The damage tensor evolves as damage associated with different failure modes accumulates. Matrix-cracks are predicted using a three-dimensional failure criteria, which is based on the stresses acting on a potential fracture plane. For use with the failure criteria, a new anisotropic shear constitutive law is postulated governing the transverse and the in-plane shear nonlinear mechanical behavior. For the prediction of initiation and progression of multiple delaminations through the thickness of the composite, a cohesive-decohesive constitutive law is adopted. The analytical model is implemented in a finite element commercial code via user subroutines. The problem of a notched composite loaded in tension is examined. The general characteristics of the predicted failure modes are in good agreement with the experimental observations obtained from the open literature. Numerical simulations are also conducted for the filled hole tension, double sided bypass and bearing load, and single sided bypass and bearing test configurations. The predicted strength and the strain behavior is in good correlation with in-house experimental data. The predicted failure modes are consistent with observations made in the open literature. A common challenge in the development of aircraft and spacecraft structures is maintaining structural integrity in the presence of mechanically fastened joints. This challenge is amplified when the structures include composite laminates, which have shortcomings from a microscopical and macroscopical standpoint. In the micro-scale, stress concentrations develop between stiff fibers and relatively compliant matrix material, and in the macro-scale stress concentrations develop near discontinuities such as a drilled hole. These stress concentrations lead to failure modes that can initiate at loads below the ultimate strength of the composite material. The focus of this paper is to quantify joint strength via an improved simulation of failure modes in composite structures. The design and analysis of bolted joints in composite structures is complex and uncertain because failure loads depend on a combination of factors such as material selection, stacking sequence, bolt clamping force, loading vector, geometric configuration, and manufacturing defects. The contact between the bolt and the bolt hole may induce large strains and high stress concentrations in the vicinity of the bolt hole boundary. Eventually an accumulation of localized failures ‐ such as fiber failure, delamination, and matrix-cracks ‐ that propagate from the edge of the bolt hole leads to the ultimate failure of the mechanically fastened joint. This complexity requires analytical procedures that can account for these variables in order to reasonably predict the response of new structures. Analytical work needs to be developed based on lessons learned from the extensive experimental work characterizing failure mechanisms that occur in mechanically fastened composite joints. 1‐6 Failure modeling of composite joints is challenging due to the multi-scale damage mechanisms that occur in composite laminates: micro-cracks in the micro-scale, and delaminations and through-the-thickness cracks in the macroscale. Significant work has been conducted in modeling and simulating the failure of composites using progressive

Failure Modeling and Simulation of Composites Subjected to Bypass and Bearing Loads

Burst Pressure Predictions of Composite Cylindrical Vessels with Wrinkled Plies

Composites materials may exhibit a remarkable difference in material properties depending on whether the state of stress is tension or compression. This tension-compression discontinuity in material properties often leads to a nonconvergent finite element solution. Two approaches, an explicit algorithm and an implicit algorithm, are presented in this paper to eliminate this convergence problem. In the explicit algorithm, the material properties are selected based on the sign of the components of the strain tensor from the previous increment while retaining the abrupt change in stiffness from tension to compression. The time step is repeated a few times until the stress tensor and the stiffness matrix are determined to be converged. In the implicit algorithm, a new mechanical constitutive law is developed that replaces the tension-compression discontinuity by a transition region. This produces immediate convergence, the time step does not need to be repeated, and it is time step independent. The efficiency of the algorithms are demonstrated for heat resistant composite materials. Examples are given to demonstrate that the algorithms produce fast convergence, but more importantly to demonstrate how the stress results vary significantly when analyses are conducted with tension material properties only, compression material properties only, and tension-compression material properties.

John C. Klug

Papers

Interphasic formulation for the prediction of delamination

Composite Structures Damage Tolerance Analysis Methodologies

Failure Modeling and Simulation of Composites Subjected to Bypass and Bearing Loads

Burst Pressure Predictions of Composite Cylindrical Vessels with Wrinkled Plies

Efficient Algorithms for Composite Structural Analysis with Different Tension-Compression Material Properties