Fu-Kuo Chang

A Progressive Damage Model for Laminated Composites Containing Stress Concentrations

A progressive damage model is presented for notched laminated composites subjected to tensile loading. The model is capable of assessing damage in laminates with arbitrary ply-orientations and of predicting the ultimate tensile strength of the notched laminates. The model consists of two parts, namely, the stress analysis and the failure analysis. Stresses and strains in laminates were analyzed on the basis of classical lamination theory with the consideration of material nonlinearity. Damage accumulation in laminates was evaluated by proposed failure criteria combined with a proposed property degradation model. A nonlinear finite element program, based on the model, was developed for lami nates containing a circular hole. Numerical results were compared with the experimental data on laminates containing an open circular hole. An excellent agreement was found be tween the analytical prediction and the experimental data.

Finite element analysis of composite structures containing distributed piezoceramic sensors and actuators

A finite element formulation is presented for modeling the dynamic as well as static response of laminated composites containing distributed piezoelectric ceramics subjected to both mechanical and electrical loadings. The formulation was derived from the variational principle with consideration for both the total potential energy of the structures and the electrical potential energy of the piezoceramics. An eight-node three-dimensional composite brick element was implemented for the analysis, and three-dimensional incompatible modes were introduced to take into account the global bending behavior resulting from the local deformations of the piezoceramics. Experiments were also conducted to verify the analysis and the computer simulations. Overall, the comparisons between the predictions and the data agreed fairly well. Numerical examples were also generated by coupling the analysis with simple control algorithms to control actively the response of the integrated structures in a closed loop.

Pitch-catch Active Sensing Methods in Structural Health Monitoring for Aircraft Structures

This study presents active sensing methods in structural health monitoring, for detecting cracks and debonds in metallic and composite structures, which can be potentially implemented into airframe structures. First, a pitch-catch method using a pair of piezoelectric actuator and sensor is introduced to generate a damage indeX which can be used to characterize damage at a known location. Tests on airbus fuselage panels are conducted to verify the method and damage indeX. The damage indeX relates changes in the energy content of a specific Lamb wave mode selected by group velocity analysis to the eXtent of damage. Second, an imaging method based on multiple pitch-catch information, a network of piezoelectric actuator/sensors, is presented for characterizing damage (location and size) without need for a structural or damage model. The imaging method with an autofocusing feature is applied to aluminum plates and a stiffened composite panel for method verification.

A Model for Predicting Damage in Graphite/Epoxy Laminated Composites Resulting from Low-Velocity Point Impact:

An investigation was performed to study the impact damage of graphite/epoxy laminated composites caused by a low-velocity foreign object. The impact damage in terms of matrix cracking and delaminations resulting from a point-nose impac tor was the primary concern. A model was developed for predicting the initiation of the damage and the extent of the final damage as a function of material properties, laminate configuration and the impactors mass. The model consists of a stress analysis and a failure analysis. A transient dynamic finite element analysis was adopted for calculating the stresses and strains inside the composites during impact resulting from a point-nose im pactor. Failure criteria were proposed for predicting the initial intraply matrix cracking and the size of the interface delaminations in the composites. Experiments were also per formed to verify the model and the computer simulations. The predictions agreed fairly well with the test data.

Damage Tolerance of Laminated Composites Containing an Open Hole and Subjected to Compressive Loadings: Part II—Experiment

An analytical investigation was performed to study the damage in laminated composites containing an open hole and subjected to compressive loading. A progressive damage model was developed during the investigation to predict the extent and the failure modes of the internal damage in the laminates as a function of the applied load and to simulate the in-plane response of the laminates from initial loading to final collapse. The model consists of a stress analysis and a failure analysis. Stresses and strains inside the laminates were calculated by a nonlinear finite element analysis which is based on finite deformation theory with consideration of material and geometric nonlinearities. The types and extent of damage in the material were predicted by a failure analysis which includes a set of proposed failure criteria and material degradation models. Numerical results from the model were compared with the data which were obtained during the investigation and are presented in a companion paper [1]. Good agreements were found between the predictions and the test results. A computer code was developed based on the model which can be used as a tool for sizing and designing composite plates containing holes and subjected to compression.

Detection and monitoring of hidden fatigue crack growth using a built-in piezoelectric sensor/actuator network: I. Diagnostics

A piezoelectric based built-in diagnostic technique has been developed for monitoring fatigue crack growth in metallic structures. The technique uses diagnostic signals, generated from nearby piezoelectric actuators built into the structures, to detect crack growth. It consists of three major components: diagnostic signal generation, signal processing and damage interpretation. In diagnostic signal generation, appropriate ultrasonic guided Lamb waves were selected for actuators to maximize receiving sensor measurements. In signal processing, methods were developed to select an individual mode for damage detection and maximize signal to noise ratio in recorded sensor signals. Finally, in damage interpretation, a physics based damage index was developed relating sensor measurements to crack growth size. Fatigue tests were performed on laboratory coupons with a notch to verify the proposed technique. The damage index measured from built-in piezoceramics on the coupons showed a good correlation with the actual fatigue crack growth obtained from visual inspection. Furthermore, parametric studies were also performed to characterize the sensitivity of sensor/actuator location for the proposed technique.

A New Approach toward Understanding Damage Mechanisms and Mechanics of Laminated Composites Due to Low-Velocity Impact: Part II—Analysis

An investigation consisting of both analysis and experiments was performed to study impact damage mechanisms and mechanics of laminated composites subjected to low-velocity impact. In order to fundamentally understand the impact damage of laminated composites, a unique test program was developed and performed by using a specially designed line-loading impactor. Experimental findings from the tests were obtained and presented in the first part of this series on the impact study. In this paper, an analysis will be presented for predicting the impact damage resulting from the line-loading impact and for determining essential parameters governing the impact damage. The model consists of a transient dynamic finite element analysis for calculating stresses, strains and deformations of composite plates during impact, and a failure analysis for predicting the threshold of impact damage and initiation of delaminations and micro-cracks. The predictions based on the analysis agreed with the test results reasonably well. Based on the unique approach, the proposed analysis, coupled with the experiment, was able to provide comprehensive information for fundamentally understanding the impact damage mechanisms and mechanics of laminated composites.

Post-failure analysis of bolted composite joints in tension or shear-out mode failure

A progressive damage model was developed for bolted joints in laminated composites which may fail in either tension mode or shear-out mode. The model is capable of assess ing damage accumulated in laminates with arbitrary ply orientations during mechanical loading and of predicting the ultimate strength of the joints which failed in tension or shear-out mode. The model consists of two parts, namely, the stress analysis and the failure analysis. Stresses and strains in laminates were analyzed on the basis of the theory of finite elasticity with the consideration of material and geometric nonlinearities. Damage accumulation in laminates was evaluated by the proposed failure criteria combined with a proposed property degradation model. Based on the model, a nonlinear finite element code was developed. Numerical results were compared with available experimental data. An excellent agreement was found between the analytical predictions and the experimental data.

Detection and monitoring of hidden fatigue crack growth using a built-in piezoelectric sensor/actuator network: II. Validation using riveted joints and repair patches

A built-in diagnostic technique for monitoring hidden fatigue crack growth in aircraft structures has been developed in part I of the study. The technique uses diagnostics signals, generated from nearby piezoelectric actuators built into the structures, to detect crack growth. In this second part of the study, the proposed diagnostic technique was applied to monitor fatigue crack growth in riveted fuselage joints and a cracked metallic plate repaired with a bonded composite patch. A complete built-in diagnostic system for the tests was developed, including a sensor network, hardware and the diagnostic software. Predictions were correlated quite well with measurements from the eddy current test and the ultrasonic scan methods as well as visual inspection. The damage index successfully detected both crack growth and debond damage for the structures considered.

Bearing Failure of Bolted Composite Joints. Part I: Experimental Characterization

An investigation was performed to study the bearing failure of mechanically fastened fiber-reinforced laminated composite joints. Only double-lap metal/composite/metal bolted joints were investigated. The results of this study will be presented in two parts: experimental characterization and analytical prediction. This paper summarizes the experimental work. The major focus of the experiments was to characterize the bearing failure mechanism and mechanics, and to evaluate the effect of clamping pressure on the bearing response and bearing strength of bolted joints. The bearing damage was characterized either as pure bearing, which had no lateral supports, or as bolt bearing, which contained lateral supports with various degrees of clamping pressure. A specially designed semi-circular notched specimen was proposed to characterize pure bearing damage. For bolt bearing, a load cell was designed and manufactured which was mounted on the fastener to monitor the bolt clamping pressure as a function of the applied load. T800/3900-2 graphite/epoxy prepregs were selected to fabricate the specimens. All the specimens were x-rayed and sliced at different load levels to examine internal damage. Based on the experiments, it could be concluded that lateral support is crucial for bolted laminated composite joints. Bearing damage can be catastrophic if there is no lateral support. Shear cracks induced by accumulated compression failure appeared to be the primary failure mode of the bearing damage. Lateral supports could suppress the shear crack propagation and change failure from a catastrophic to a progressive failure mode. Clamping pressure could increase bearing strength.