Investigations On Damage And Failure Mechanisms In Self-reinforced Polyethylene Thermoplastic Composites

Ultra-high Molecular Weight Polyethylene (UHMWPE) has an excellent integrated property. Self-reinforced polyethylene (PE/PE) thermoplastic composites with UHMWPE as the reinforcing fiber can transfer the excellent properties of UHMWPE fiber to the composites commendably because of the chemical compatibility of fiber and matrix. Therefore, PE/PE composites have a promising future in application. Investigations on the damage and failure mechanisms of PE/PE composites are important to the assurance of mechanical safety of the composite structures during serviced, and the obtainment of optimal design of structures. Researches hitherto mostly focus on carbon fiber and glass fiber reinforced thermoset composites. However, because of the factors such as the plasticity of matrix and the thermoplastic of the composites, there exists a big difference between thermoplastic and thermoset composites in mechanical behaviors. On the other hand, as the faster development of thermoplastic composites in the recent years, thermoplastic composites are more and more used in the engineering field. So investigations on thermoplastic composites are a need of time. There are realistic meanings in the study on the PE/PE thermoplastic composites.The damage and failure process of fiber-reinforced composites is a complicate and progressive one. It may include many damage modes such as matrix cracking, fiber breakage, fiber pullout, fiber-matrix debonding and delamination. In order to reveal the progressive damage process of PE/PE composites nicely, this investigating study carried out two parts of work: the first part was numerical modeling study based on the Progressive Failure Analysis Methodology using Finite Element Method (FEM); the second part was Acoustic Emission (AE) monitoring and signal analysis based on Pattern Recognition (PR) technique. Efforts were also made to settle many existing problems in the two researching fields. The studies are expected to provide an effective and convenient way for the explaining of damage mechanisms and the analyzing of AE signals for thermoplastic composites. In the modeling study, several types of unidirectional UHMWPE/LDPE composites (0°, 90°and 45°) were firstly tested for the normal mechanical properties. Results showed that the composites have an obvious nonlinearity in longitudinal, transverse and shear stress-strain behavior. Therefore, a FEM based progressive damage model was developed for the PE/PE laminates subjected to tensile loading, by considering the nonlinear properties of unidirectional composites in the three tensile modulus(E₁, E₂ and G₁₂) as piecewise linear-elastic properties.Then, ANSYS software was used for simulating the progressive failure of UHMWPE/LDPE laminates to study the damage mechanisms, according to the established progressive failure model. The FEM analysis program was developed by using the APDL program language. Comparison was made to the data obtained from the tensile tests. Results showed as follows: the nonlinear properties of the compositesin the three tensile modulus (E₁,E₂ and G₁₂) have an important effect on the numericalmodeling of mechanical behaviors of UHMWPE/LDPE laminates; the nonlinear behavior of the laminates were remarkably affected by the longitudinal nonlinearity of the composites; the nonlinearity of composites and structures was conveniently and effectively included by the piecewise linear elasticity treatment; the progressive failure model can predict damage modes such as matrix cracking, fiber-matrix shearing to debonding and fiber fracture, and reveal the damage propagation; the analytical prediction showed an excellent agreement with the experimental data in the tensile stress, rupture strain and the developing trend of the stress-strain curve.In the AE monitoring and study, several simple lay-up of UHMWPE/LDPE composites (0°, 90°and [±45°]) were firstly investigated for the AE features and damage mechanisms of the composites. A common analytical procedure of PR technique was used for the preprocessing and clustering analysis of the AE data. In the clustering analysis, scanning electron microscope (SEM) technique was utilized for the assuring of the classification. Results showed as follows: the PR technique is objective and suitable for the analysis of AE data from UHMWPE/LDPE thermoplastic composites; it also can perform noise reduction effectively; the PR technique is able to identify damage modes such as matrix cracking, fiber-matrix debonding, fiber pullout, fiber breakage, etc. in the specimens, and the identification results are the same with the observation results by SEM; by the cumulative AE hits of each damage mode vs. strain curves, the damage process of the specimens can be reviewed clearly; in addition to the features of the specimen, a correlation between the clustered AE signal classes and their original damage modes could be established reasonably, and a clear understanding of damage mechanisms in the composites could be finally reached.Therefore, investigations were further carried out for the simple lay-up laminate of 0°, 90°and [±45°] UHMWPE/LDPE composites. Fracture waveforms of pure PE resin and UHMWPE fiber bundle were collected analyzed, in order to establish, by using Unsupervised Pattern Recognition (UPR) technique, a unified analytical procedure which can recognize the mechanical souses of the clustered AE signal classes objectively. Typical waveforms and AE signal samples of different damage modes in UHMWPE/LDPE composites were obtained from the analysis. All the information and the unified analytical procedure using UPR technique were proposed to be assistant in the analysis of the cases of complicated lay-up laminates. Then, AE data from UHMWPE/LDPE quasi-isotropic laminates ([0/90/±45]_s) was analyzed to study the damage mechanisms, by compared analysis using the established UPR procedure and Supervised Pattern Recognition (SPR) technique respectively. Results showed that the established UPR procedure can recognize the mechanical sources in the complicated lay-up laminates effectively; that on the other hand, by selecting a suitable sample data, the SPR technique can reach a fast separation of AE data from complicated lay-up laminates. In this study, typical waveforms and AE signal samples of different damage modes in UHMWPE/ LDPE composites were set to be obtained from the analyses of matrix, fiber and simple lay-up laminates. By this way the critical problems in the use of the UPR and SPR technique for the AE classification were solved and reasonable classifying results were obtained. Classifying results by the two techniques reached a good coherence. It showed that the PR technique is objective to the AE data analysis of UHMWPE/ LDPE composites, and that AE signals of the composite laminates are of separableness.The analytical result showed that, as for the [0/90/±45]_s laminates, the main damage modes existing were matrix cracking, fiber breakage and fiber-matrix debonding, with a little fiber pullout. All these damage modes presented a progressive failure process. The damage and failure of fibers was the dominant failure mode in the laminates and it account for the final fracture of the laminates. These results showed by the AE technique were used for the validation of the established progressive failure model. The investigating studies finally showed that, the established progressive failure model, considering the material nonlineanty of the UHMWPE/LDPE composites, can correctly predict the failure process and reveal the damage mechanisms. The numerical modeling results were supported by the AE monitoring and analyzing (including the SEM analyzing) results. At the same time, the model can predict the tensile stress, rupture strain and the developing trend of the stress-strain curve for the laminates perfectly, proving that the numerical modeling is valid.