Stephen B. Clay

Cohesive modeling of damage growth in z-pinned laminates under mode-I loading

In this paper, a traction-separation-based cohesive modeling approach is proposed to predict the effect of z-pinning on laminated composites. A detailed experimental characterization of the z-pin pullout process using the flatwise tension test is presented. Utilizing these flatwise tension results, numerical simulation of the progressive damage due to delaminations in a double cantilever beam with z-pinning has been performed. Experimental details of the z-pinned double cantilever beams are presented for IM7/977-3 graphite/epoxy. The approach taken in this study utilizing the cohesive elements within the Abaqus® finite element software has proven that the models can predict the behavior of z-pinned composites close to experimental observations. It was found that the discretization of the fracture resistance curve along the z-pin field is essential to capture the dynamics of the delamination accurately.

Comparison of composite damage growth tools for static behavior of notched composite laminates

This paper provides overall comparisons of the static results of an Air Force Research Laboratory exploration into the state of the art of existing technology in composite progressive damage analysis. In this study, blind and re-calibration bench-marking exercises were performed using nine different composite progressive damage analysis codes on unnotched and notched (open-hole) composite coupons under both static and fatigue loading. This paper summarizes the results of the static portion of this program. Comparisons are made herein of specimen stiffness and strength predictions against each other and the test data. Overall percent error data is presented, as well as a list of observations and lessons learned during this year-long effort.

Experimental results of quasi-static testing for calibration and validation of composite progressive damage analysis methods:

The Air Force Research Laboratory directed a research program to evaluate nine different composite progressive damage analysis methods under both quasi-static and fatigue loading. This paper describes the coupon tests that were performed at the Air Force Research Laboratory for calibration and validation of the methods under quasi-static conditions. The basic elastic and failure properties of unidirectional IM7/977-3 graphite/epoxy were first determined in order to properly calibrate the models. Validation tests were then performed on unnotched and open-hole coupons with three different laminate stacking sequences under both tension and compression loading conditions. This paper summarizes these experimental results and provides X-ray computed tomography images at subcritical load levels.

High Fidelity Composite Bonded Joint Analysis Validation Study Part II: Analysis Correlation

The analysis/prediction phase of a high fidelity analysis validation activity conducted by the Composites Affordability Initiative (CAI) Program on composite bonded structure was presented in a prior paper and the correlation to test results are presented here. The CAI program has developed two improved approaches for the analysis of bonded joints, (1) high fidelity stress analysis combined with the incorporation of the new composite strain invariant failure criteria for the prediction of failure initiation, and (2) an interface fracture finite element (IFE) for predicting the delamination propagation and residual structural strength. The test articles are that of an all composite skin and stiffener run-out geometry with both bonded and co-cured configurations. The focus of this paper is on analysis/test correlation results and discussion of the findings. Extraordinary means were employed to detect the early failure events to be consistent with the fidelity of the analysis methods. The interpretation of sensors readings was not straight forward and the led to a question of how one defines “first failure” in a bonded composite joint where the scale of failure occurs on a level below the size of features that are being controlled during the manufacturing process. The IFE analysis displayed good correlation with test results in regards to the extent of delamination growth and prediction of the major load drop associated unstable delamination growth. Analysis of z-pin reinforced stiffener terminations using the CAI developed Interface Traction Elements (ITEs), a type of cohesive element, required modification of the analysis to get reasonable agreement. Both types of analyses, strength based and fracture based approaches, provided exceptional insight into the performance of the structure.

Interlaminar Fracture Toughness Characterization of Z-Pinned and Flocked Composite Laminates

The paper presents the improvements in the fracture toughness under Mode-I and Mode-II loading conditions due to flocking and z-pinning. Fracture toughness values for the z-pinned and flocked composite coupons have been assessed using the Double-Cantilever Beam (DCB) test and the End-Notched Flexure (ENF) test. Degradation of in-plane tensile property due to translaminar reinforcement with flocking and z-pinning has been assessed using the standard tension test. Details of these experiments, estimated fracture toughness values, and the comparison of each through-thickness reinforcement technique to improve the delamination resistance are presented in this paper.

Modeling Rate Dependent Damage Evolution in Composite Structures

In this paper we propose a reduced order multiscale model for prediction of damage evolution in composite structures. Eigendeformation-based reduced order homogenization is employed in conjunction with rate-dependent constituent damage evolution to model the accumulation of diffuse damage in the microstructure. The homogenized microstructural constitutive response is exercised in the macroscopic finite element simulation modeling the composite behavior. We utilize Markov chain Monte Carlo simulation in the calibration of the material parameters for the proposed computational model.

Benchmarking of composite progressive damage analysis methods: The background

This article introduces an Air Force Research Laboratory study, which performed static and fatigue benchmark exercises for nine composite progressive damage analysis methods. Air Force Research Laboratory is interested in exploring the feasibility of these progressive damage analysis methods to predict composite damage growth for the purposes of improved durability and damage tolerance analysis of composite aircraft structure. This article gives the background, goals, motivation, and guiding principles of the study and provides brief descriptions of the teams that participated and the tools that were utilized.

Low-Velocity Impact Damage and Delamination Crack Arrestment with Translaminar Reinforcements

The process of z-pinning has been used to improve the delamination strength of laminated composites. Improvements in Mode-I and Mode-II fracture toughness and the degradation of in-plane properties have been studied in the past and documented in the literature. This paper presents a preliminary study on the use of z-pinning to improve the delamination resistance against barely visible impact damage. Experimental results indicated that the use of pinning can considerably improve the compression after impact strength. Non-destructive observations revealed a lower delamination area after impact when the samples were fully covered with z-pins. Additionally, samples with a surrounding z-pin boundary were able to provide a containment mechanism for an area with sub-surface delamination.

Fast Life Prediction Model for Composites Based on Multiple Temporal Scale Homogenization

This manuscript presents a fast temporal homogenization methodology for failure analysis of materials and material systems subjected to fatigue loadings. Computational homogenization theory with multiple temporal scales is employed to devise the proposed methodology. Multiple temporal scales address the disparity between the characteristic loading period and overall life under fatigue loading. The fast temporal homogenization method is presented in a general format for a large class of constitutive relationships. The proposed method is verified in a series of numerical tests. The method is shown to gain orders of magnitude in computational efficiency while numerical error remains small.

Probabilistic Cohesive Zone Model to Capture Steady State Energy Release Rate Variations of DCB Specimens

Delamination between plies is the most common failure mode in composite laminates that can cause ber breakage and reduction in life of the composite. Currently Mode-I interlaminar fracture toughness or critical energy release rate of a composite is measured using double cantilever beam (DCB) with unidirectional composites (ASTM D5528). Unlike metals, the energy plot of a DCB specimen shows increase in energy release rate with increase in crack length. Also during testing the steady state energy release rate from each of the samples of the same batch shows a lot of variation due to ber cross-over bridging that occurs only in unidirectional composites. In this study, a probabilistic Cohesive Zone Model (CZM) is developed to capture steady state energy release rate variations of 51 mm crack size DCB specimens based on unidirectional composite (IM7/977-3) test data. Then using the probabilistic CZM, the energy release rate variations of 76.2 mm crack size DCB model are predicted and compared to the test results. The predictions showed good agreement with the experiments suggesting a probabilistic CZM is capable of simulating the strength scatter during the delamination process in unidirectional composites that were of interest in this research.