T.P. Philippidis

Damage characterization of carbon/carbon laminates using neural network techniques on AE signals

Abstract Carbon/Carbon (C/C) composites have a large number of microcracks in matrix-matrix or fiber-matrix regions resulting from the thermal processes during manufacturing. Although not harmful to the overall structural integrity, such a network of microcracks creates a noisy background from early load application which ‘covers’ the onset of critical failure mechanisms. Conventional AE analysis based on a sudden activity increase observed in the cumulative events versus load plot, or amplitude distribution histograms, provides limited information on this type of AE activity. Instead, multivariate techniques of unsupervised pattern recognition, taking into account a large number of AE signal descriptors, are proven useful for the clustering of similar AE events. AE results from a systematic fracture mechanics study of 2D woven C/C laminates are analyzed in this paper. Artificial Neural System (ANS) methods are employed for the clustering of similar AE signals, enabling a phenomenological correlation with the actual failure modes. The numerical procedure introduces a modified Learning Vector Quantization (LVQ) technique which was proved fast and suitable for the type of AE data emitted by composites. Fractographic evidence from failed tensile coupons corroborates the predictions of the numerical method in recognizing different failure mechanisms. Cumulative event charts of the various classes versus load demonstrate the criticality of each class on final coupon failure, and may lead to the definition of reliable criteria for the evaluation of remaining strength or life.

Fatigue Strength Prediction under Multiaxial Stress

A multiaxial fatigue failure criterion for composite materials is presented in this paper along with an assessment of the capability it offers for design under multiaxial constant or variable amplitude stresses. The applicability of this criterion, based on the well known quadratic failure tensor polynomial criterion for static loading, is validated through comparisons with uniaxial and multiaxial fatigue experimental data. Static and fatigue tests were carried out during this study on glass/polyester specimens cut off-axis from a multidirectional laminate at different angles. The agreement between experimental values and theoretical predictions is good. The proposed criterion is also compared to existing fatigue criteria and an overall assessment of their performance is given. Some theoretical design considerations for the case of irregular stress spectra, introducing the concept of multiaxial Miner coefficient, are finally presented.

Complex stress state effect on fatigue life of GRP laminates. Part I, experimental

In structural applications of thin-wall, box beam constructions with composites, the effect of complex stress states is not properly taken into account in determining operational life and fatigue response in general, due either to misconceptions or lack of experimental data and theoretical models. Results from a recent research project, presented here in two parts, aim to contribute to the better understanding of fatigue behaviour of GRP laminates under complex in-plane stress states. An initial estimate on the effect of neglecting shear and transverse normal stresses in fatigue life calculations is provided, based on experimental data and theoretical considerations. It is concluded that in structural GRP laminates, shear and transverse normal stresses have an important contribution in reducing operational life, irrespective of their magnitude, usually small compared to axial normal stresses.

Fatigue of composite laminates under off-axis loading

Abstract Results from an experimental program consisting of static and fatigue tests on flat coupons, cut at different off-axis directions from a multidirectional, (MD), Glass/Polyester, (GRP), laminate are presented in this paper. The material is similar to those used by GRP wind turbine rotor blade manufacturers, i.e. hand lay-up and room temperature curing. The stacking sequence of the MD laminate under consideration is [0/(±45)2/0]T. Specimens were cut at five different off-axis directions from that laminate and over one hundred and forty tests were conducted under static and cyclic loading. Based on the test results the effect of off-axis loading on static and fatigue behaviour of the MD laminate is studied. A simple empirical model is used to predict the observed stiffness degradation and to determine stiffness based S–N curves by means of a limited number of test data. For the materials investigated in this program it is shown that E-modulus variation depends on the off-axis loading as much as on the applied cyclic stress level. Stiffness based S-N curves corresponding to 5–20% stiffness reduction are more conservative than standard S-N allowables of 95% reliability.

Acoustic Emission Monitoring of Small Wind Turbine Blades

Wind turbine blade certification tests, comprising a static test, a fatigue test, and finally a residual strength test, often involve sudden audible cracking sounds from somewhere within the blade, without the operators being able to locate the noise source, or to determine whether damage (minor or major) has occurred. A current EC-funded research project is looking at the possibility of using acoustic emission (AE) monitoring during testing of fibre composite blades to detect such events and assess the blade condition. AE can both locate and characterise damage processes in blades, starting with non-audible signals occurring due to damage propagation at relatively low loads. The test methodology is discussed in the context of the blade certification procedure and results are presented from a series of static and fatigue blade tests to failure in the laboratory. Inferences are drawn about small differences in the manufacture of the nominally identical blades and conclusions are presented for the application of the methodology.Copyright

Fatigue design allowables for GRP laminates based on stiffness degradation measurements

Abstract The fatigue behaviour of a multidirectional, [0(±45) 2 /0] T glass/polyester laminate under various loading conditions is studied in this paper. Series of coupons cut on- and off-axis, at seven different orientations, were uniaxially loaded at constant amplitude over a wide range of R ratios. Test data were subjected to statistical analysis and S / N curves at specific reliability levels were calculated. Stiffness changes were continuously monitored during fatigue testing, and design curves corresponding to a certain level of stiffness reduction, and not to failure, were determined. It is shown, for the material studied in this work, that stiffness-based and reliability S ./ N curves are correlated and therefore determination of fatigue design allowables bearing information on both stiffness loss and strength can be formulated.

An acousto-ultrasonic approach for the determination of water-to-cement ratio in concrete

A novel nondestructive procedure for the evaluation of water-to-cement (w/c) ratio in concrete is presented. The experimental setup is based on the method of acousto-ultrasonics; data analysis, however, and recognition of concrete composition from the waveform transmitted through specimen thickness, are achieved by simple time and frequency domain schemes used in this work. Experiments were performed in a number of concrete specimens made at various w/c ratios and at a number of ages starting from 2 up to 90 days. Recognition results were satisfactory and the algorithm introduced was successful in identifying the correct w/c ratio in more than 90% of the test cases. The use of existing spectral analysis techniques such as the coherence function has also proved to be more efficient and fits the purpose. The possibility of water content determination in fresh paste is also discussed along with preliminary evidence from initial tests.

Complex stress state effect on fatigue life of GRP laminates. Part II, Theoretical formulation

The synergistic effect of in-plane stress tensor components on fatigue strength is not traditionally considered in the design of thin-wall box-beam structures, e.g. composite rotor blades in general. Fatigue life calculations account only for the normal stresses due to bending and centrifugal forces, neglecting the contribution of shear and transverse normal stresses. The theoretical formulation of a life prediction methodology accounting for all in-plane stress tensor components, through the use of a multiaxial fatigue strength criterion, is presented here. Comparison of theoretical predictions with experimental results from constant amplitude, uniaxial, off-axis tests demonstrates the drastic effect of shear and transverse normal stresses, besides that of axial normal stress, in reducing operational life of a GRP structural laminate.

Mechanical property distribution of CFRP filament wound composites

Abstract The extending use of composite materials and the simultaneous need for reliability-based design of structural components has introduced the stochastic modelling of material properties, as implied by several design codes and standards. Therefore, for using such materials in advanced applications, the stochastic character of their properties needs to be taken into account. In this work the statistical aspects of strength and elastic properties of carbon/epoxy (C/Ep) filament wound composites are determined through a series of static tensile and compressive tests. Six candidate statistical distributions were used in order to model the strength and elastic properties of the composite, but none of them was proved to be a better fit to the experimental results, as revealed by the Kolmogorov–Smirnov test. Therefore, anyone of them can be used equally well to model mechanical property distributions. Although not expected, the experimental investigation revealed considerable scattering on the elasticities, besides strength, indicating that the variation of the elastic properties should be implemented in an adequate reliability analysis.

Probabilistic failure prediction for FRP composites

Abstract Results are presented from a theoretical study on failure-locus prediction for unidirectional FRP laminae under complex in-plane loading, taking into account statistical aspects of the basic strengths of the material. The purpose of the analysis is to establish simple rules and methodologies for failure prediction under specific reliability requirements. Therefore, the problem one is faced with is the definition of the cumulative distribution function (CDF) of the failure condition if the respective CDFs of the basic material strengths are known by experiment. To this end, two analytical approaches, namely a functional expansion technique and the introduction of Pearson’s semi-empirical distribution function, were developed and implemented in software. Both methods were shown to predict satisfactory results compared with Monte Carlo simulated ones and experimental data, wherever available. Finally, a semi-deterministic approach was examined according to which the failure criterion is used in the usual deterministic way but with basic strength values of a certain reliability level. Results from this simple and fast method were found in good agreement with those derived by expensive pure statistical methods or numerically simulated ones and experimental data.