Fracture Process in ± θ Laminates Subjected to Mode II Loading

This work studies the behavior of a multidirectional laminate under Mode II loading. We describe the process of delamination in a reinforced composite of glass/epoxy. The stacking sequence (plies orientation [± ] was selected to minimize the coupling effects. The Mode II interlaminar test under three-point bending and cantilever flexure using ENF (End Notch Flexure) and ELS (End Load Split) specimens, respectively, was performed and analyzed. The test procedures and the results of strain-energy-release rate study for crack initiation are presented. The fracture process and the mechanical behavior of the two types of specimens are analyzed. The analysis clearly shows a close link between the angle , the ratio a/L, and the thickness h of the specimen. Fracture by delamination can be obtained only with an optimal choice of these parameters. The analysis of the states of stresses at the tip of crack allows us to explain the phenomenon of bifurcation between plies and is confirmed by the experimental results.     

Mechanical behavior and optimization of multidirectional laminate specimens under delamination by bending

Delamination process in the multidirectional laminates under bending loading conditions is often accompanied by excessive deformations ahead of the crack tip and non-linearities due to large displacements and transverse microcracks, which appear in the most strained plies especially for thin materials. Since linear elastic fracture mechanics cannot be directly used in order to characterize delamination, our objective is to highlight, experimentally and analytically, the delamination peculiarities on DCB specimens, then to optimize specimen thickness to the value, which makes it possible to avoid the appearance of transverse cracking before starting of delamination. The material used is E-glass/epoxy multidirectional laminates. The results of optimization obtained are in good agreement with the experimental data. __________ Translated from Problemy Prochnosti, No. 6, pp. 66–78, November–December, 2006.     

On the mode I delamination test and the importance of laminate lay-ups

The aim of this work has been the study of mode I delamination of multiply double cantilever beam specimens of glass fiber/epoxy. The results show an important influence of laminate lay-ups on delamination resistance. The value of G_Ic at the initiation of delamination varies with laminate curvature coupling K_y/K_x and K_xy/K_x. An empirical model describing this variation has been proposed. In addition, it is seen that the values of G_Ic at the initiation of delamination and at stable crack growth will be very different. The delamination resistance can be characterized by two constants: G_Ic corresponds to the initiation of delamination, and G^S_Ip corresponds to the plateau of stable crack propagation. The correlation between experimental measurement and analysis of compliance and energy release rate results reveals significant three-dimensional effects.     

Characterization by acoustic emission and electrochemical impedance spectroscopy of the cathodic disbonding of Zn coating

Galvanized steel has been tested in a synthetic sea water solution under different cathodic overprotection conditions. The generated hydrogen flux caused the damage of the metal-zinc interface and led to a progressive coating detachment.Scanning electron microscopy, electrochemical impedance spectroscopy and acoustic emission technique were used to characterize the damage chronology under different cathodic potentials.A damage mechanism was proposed and the acoustic signature related to the coating degradation was statistically identified using clustering techniques.     

Monitoring crack growth in pressure vessel steels by the acoustic emission technique and the method of potential difference

The work was performed on pressure vessel steels and has shown that the correlation between acoustic emission (AE) techniques, the method of potential difference and microfractographic observation reveals the different stages of the crack propagation. Based on these correlations, it may be possible to predict the remaining service life of a vessel from the results of short-term acoustic emission monitoring of crack propagation.     

Experimental Analysis of the Influence of Structural Parameters on the Behavior of Glass-Fiber Reinforced Polypropylene Composites

In a composite material reinforced by short random fibers, damage results from different elementary failure mechanisms such as matrix microcracking, fiber pull out, failure of the fiber/matrix interface, failure of fibers, etc. These damages influence greatly the macroscopic behavior of composite materials. To obtain good mechanical performance of a composite material, it is important to optimize the fiber ratio and the quality of the fiber/matrix interface, which have a direct influence on the damage mentioned above. The main objective of this study is to determine the influence of structural parameters on the evolution of damage for two types of polypropylene glass-fiber reinforced composites. In parallel with the classical approach of the mechanical theory of damage, which consists in load–unload tensile tests, the use of acoustic emission allows one to follow in real time the character and the importance of damage mechanisms in the course of loading. In addition, fractographic analysis makes it possible to confirm different assumptions and conclusions from this study.     

Modelling of damage and failure of glass/epoxy composite plates subject to impact fatigue

An investigation has been carried out to study the impact fatigue damage of glass/epoxy laminated composites. Accumulation of damage, such as matrix cracking, delamination and fibre breakage, with repeated impact of the composite material may reduce the overall stiffness. These damage modes have been combined in a very complicated way to describe damage growth and fracture. A model is proposed for characterising the damage as a function of the normalised impact number. The scalar variable D, which characterises the material damage, is written as a function of the life duration β, using a modified form of the Mankowsky empirical law [Int J Solids Struct 32(11) (1995) 1607]. The macroscopic failure mode and the internal damage in laminated specimens of glass/epoxy as a consequence of impact fatigue are analysed at different levels of incident impact energy. The impact fatigue tests have been conducted on an apparatus built in our laboratory.     

Reliability of damage mechanism localisation by acoustic emission on glass/epoxy composite material plate

This investigation focuses on the analysis of the Acoustic Emission (AE) wave propagation and velocity evolution in glass/epoxy composite materials according to the fibre orientation. In this work, several piezoelectric sensors were placed on a large composite laminate in the same direction in order to analyse by one measure, the velocity and wave attenuation over a large distance. Then, the measured velocity was compared with the theoretical values of velocity that have been identified by the theoretical model analysis. This last velocity was used during a three-point bending test to analyse the damage localisation on the plate according to the amplitude distribution model developed in the “Laboratory of Roberval at the University of Technology of Compiègne”. The results of the damage localisation on the specimens were compared to the visually observed damage (delaminating, cracks…) and were corrected on the basis of the amplitude correction. Finally, the error ratio of localisation without correction was quantified. The originality of this study was to use the AE results obtained on a large laboratory specimen in order to improve the AE testing on an industrial structure scale.     

Nickel high temperature oxydation under creep loading using acoustic emission monitoring

We have studied the cracking behavior of oxide scales developed on high purity nickel single crystal during creep deformation under oxygen at 550°C. The oxide microstructure exhibits a network of parallel cracks with a constant spacing between cracks. The deterioration of the oxide scale is monitored by acoustic emission. The creep deformation mechanisms with oxide scale are investigated and compared to vacuum creep experiments. Differences in creep behavior between these two atmospheres are obvious. This study is followed by a stress relaxation analysis. The acoustic emission technique provides a large number of results for the understanding of damage of the oxide scales during creep deformation at 550°C.     

On the Method of Determination of Strain Energy Release Rate During Fatigue Delamination in Composite Materials

In this paper, the problem of calculation of the energy release rate for a fatigue test on composite material has been investigated. The application of the Linear Elastic Failure Mechanics (LEFM) leads to the use of varation of the energy release rate ( G). As the energy release rate is a function of the load squared, the variation of G becomes either a function of variation of the load squared ( G = f((P²))) or a function of the square of the load variation ( G = f(( P)²)).In this paper, we determine, by different fatigue tests, which of the two theoretical results is the best to describe the experiments. These fatigue tests have been made on DCB test-specimen in mode I with different R ratios (R = P_max / P_min) and different maximum loads. The material was a unidirectionnal glass-epoxy.The results show that considering G as a function of ( P)² seems more appropriated to describe a cracking test in fatigue.