Karl Schulte

Load and failure analyses of CFRP laminates by means of electrical resistivity measurements

Abstract The variation of the mechanical stiffness during fatigue loading has been shown to be an important qualitative and quantitative damage analogue. A correlation between stiffness reduction and the development of the various types of damage, such as transverse and longitudinal intraply cracking or delamination growth, has been made by various authors. However, the in-situ monitoring of fibre fracture occurring during static or fatigue loading in cfrp laminates has not yet been made except by indirect methods. The measurement of the variation of the electrical resistivity during loading promises to be a valuable technique for this purpose. In the case of a conventional metal sample, the conductivity is essentially the same for any direction of current flow through the sample. In cfrp samples, however, the conductivity is not isotropic and depends on the orientation and on the conductivity of the carbon fibres. Changes in the conductivity can therefore be related to fibre fracture. The resistivity also varies with temperature, and it is therefore necessary to correlate resistivity changes with the temperature changes observed during a fatigue test. The resistivity of cfrp laminates is further dependent on the applied load, the fibre volume fraction, the laminate stacking sequence and the fibre type. Knowing all of these factors, it is possible to correlate the change in the electrical resistivity with damage and failure of the load-bearing 0° fibres. However, the technique can also be used to monitor continuously the actual load situation in a composite component.

Agglomeration and electrical percolation behavior of carbon black dispersed in epoxy resin

In conductive polymer compounds, the filler volume fraction at which a network of touching particles is formed is not a constant but depends on the manufacturing process. By applying three main features—particle-particle interaction, dynamics of agglomeration, and structure of agglomerates—which are well known in colloid science to filled polymers, the electrical percolation behavior can be understood. Thus, it is possible to explain the hitherto found low percolation thresholds of less than 0.5 vol% in carbon-black-filled resins and, hence, further reduce the threshold to 0.06 vol%.

Damage detection in CFRP by electrical conductivity mapping

Carbon-fiber-reinforced polymer (CFRP) composites derive their excellent mechanical strength, stiffness and electrical coductivity from carbon fibers. The mechanical deformation and electrical resistance are coupled in these fibers that make them inherently sensors. Thus CFRPs can be considered as a self-monitoring material without any need for additional sensing elements. However, for this to become reality the conductivity map of the entire structure needs to be constructed and the relationships between the conductivity and various use- and damage-related variables need to be established. Experimental results demonstrate that internal damage, such as fiber fracture and delamination, decreases the conductivity of composite laminates. In general, the information about the damage size and position can be obtained by utilizing electrical impedance tomography (EIT), but the traditional EIT is not capable of extracting this information when the medium possesses highly anisotropic electrical conductivity. Above a certain level of anisotropy, it is advantageous to modify the traditional EIT. This paper presents a method of extracting the damage size and position for highly orthotropic (unidirectional) CFRPs. The results are obtained without the need for complex calculations, thus enabling damage detection in real time. Experimental observations indicate that a practical EIT has a potential of being a cost-effective health and usage monitoring technique (HUMT) for CFRPs.

Non-destructive testing of FRP by d.c. and a.c. electrical methods

In situ stress and strain detection, together with health monitoring, would give improved durability and safety of composite structures. Different techniques for in situ observation and non-destructive testing have been used in the past. This paper gives an overview on investigations and possible applications of electrical methods such as d.c. and a.c. measurements on fibre reinforced polymers. In the case of d.c. measurements in CFRP the reinforcing carbon fibres themselves are used as sensors, functioning as electrical resistors. In GRP a conductive matrix filler such as carbon black can take on the role of a resistance sensor. In the case of a.c. measurements, i.e. capacitance and dissipation, the carbon fibres and their connecting points function as resistors, while the spaces between the fibres function as capacitors. It has been shown that it is possible to monitor strain and failure as well as to classify different failure mechanisms in static and dynamic load conditions using electrical methods.

Failure behavior of an epoxy matrix under different kinds of static loading

The yield and fracture behavior of an unreinforced epoxy resin has been investigated. The parabolic Mohr failure criterion was applied to experimental results under different loading conditions. From a plain resin slab, specimens for tensile, torsion and compression tests were manufactured and the failure behavior of the resin was tested and discussed in detail. The results of the mechanical tests and a fractographic study of the fracture surfaces were correlated with the stress-state-dependent strength and fracture stress of the epoxy resin. This plays an important role in fiber-reinforced composites because just after cooling to room temperature the resin matrix is under a tri-axial residual stress state. From the mechanical properties of the plain resin and by using the parabolic failure criterion it is possible to explain the low strain to failure of unidirectional laminates under transverse tensile loading.

Strain concentration factors for fibers and matrix in unidirectional composites

Abstract One of the methods of calculating the stress disturbances due to broken fibers in unidirectional composites is the so-called shear lag analysis. This method has been developed with the approximation that only the fibers carry the applied stress, not the matrix, and that the matrix acts only to transfer stress to the fibers. As a consequences of this approximation, the application of the method has been limited only to composites in which matrix stiffness is low in comparison with that of the fibers, and the volume fraction of fibers is high. In the present work, the ordinary shear lag analysis was modified to introduce the influence of the matrix stiffness. In this modified method, the tensile stress concentration in the fibers and matrix adjacent to cut fibers and matrix and shear stresses at the interface between fibers and matrix were estimated. The influence of interfacial debonding on the strain concentration was also studied.

Alternating electric field induced agglomeration of carbon black filled resins

Abstract This letter reports on our observation that an alternating electric field is able to induce the formation of an electrically conducting network in carbon black (CB) filled resins well below the zero-field percolation threshold. Compared with the recently presented dc method, the ac agglomeration is more efficient in two respects: it proceeds significantly faster under equivalent conditions and is still effective at higher ionic concentration. In contrast to the ramified form of dc-induced CB networks, ac agglomeration favors the formation of parallel CB chains. The experimental results can be explained taking into account ionic conductivities of the matrices as well as charges and field induced dipoles on the CB particles.

In situ observation of electric field induced agglomeration of carbon black in epoxy resin

This letter reports on the influence of a static electric field applied by metal electrodes on the agglomeration process of carbon black CB in epoxy resin. The growth of dendrites from the anode into the material is observed in situ by optical transmission microscopy. A percolating network is seen to form, combined with a drastic reduction in the sample resistivity. This behavior can be explained by taking into account the electrostatic interaction of the charged CB particles. The final resistance for composites with a given CB content can be controlled within a range of several decades by varying the applied voltage and the curing temperature of the mixture.

Finite-element modeling of initial matrix failure in CFRP under static transverse tensile load

The failure of transversely loaded unidirectional CFRP has been investigated by the use of mechanical and thermo-mechanical test methods and finite-element analysis. The case considered here is characterized by a high interfacial strength between fiber and matrix, so that matrix failure governs the fracture process of the composite. On the basis of the experimental results, the parabolic and other failure criteria were applied to the FE calculations. The failure dependence of the resin on the actual stress state could be described. Furthermore, the influence of thermal residual stresses on the initial matrix failure has been investigated, and the actual stiffnesses and thermal expansion changes of the epoxy resins and the composites as a function of temperature have been determined experimentally. The results of the mechanical and thermo-mechanical tests performed on the pure resins and on the composites were incorporated into a finite-element analysis and compared with the transverse tensile properties of the composite laminates. In the FE analysis, the local fiber-volume fraction was varied over a wide range in order to investigate its influence on the thermal residual stresses and transverse composite strength. The results could explain the low strain to failure of transverse laminates under tensile loading.

Morphological investigations of polyethylene fibre reinforced polyethylene

The crystallisation behaviour and morphology of polyethylene-based single polymer composites has been investigated by light microscopy and low voltage scanning electron microscopy techniques. The surface crystals on ultra high molecular weight polyethylene fibres act as nucleation centres for the high density polyethylene matrix, which may result from epitaxial crystallisation. After crystallisation from the melt and independent of air-cooled or isothermal crystallisation conditions, a transcrystalline layer was found having lamellar crystals grown perpendicular to the fibre axis.