Piet W.M. Peters

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.

Influence of interfacial stress transfer on fatigue crack growth in SiC-fibre reinforced titanium alloys

Abstract An important damage mechanism during fatigue of unidirectional SiC-fibre reinforced titanium alloys is the formation of matrix cracks transverse to the fibre direction. Due to the relatively low fibre/matrix bond strength these matrix cracks initially do not break the fibres, so that matrix cracks bridged by fibres develop. It is shown experimentally, that the strong drop in fatigue strength is caused by the formation of a bridged crack of a critical size and the crack propagation rate (d a /d N ) for a single load level has been determined. A prediction of d a /d N on the basis of finite element calculation of the stress intensity factor range of the bridged matrix crack Δ K m and the Δ K m –d a /d N relationship of the used titanium alloy (Timetal 834) has been performed. Calculation of Δ K m assuming a negligible fibre/matrix bond strength and considering shear load transfer at the fibre/matrix interface due to Coulomb friction (coefficient of friction μ =0.5 and μ =0.9) led to a large discrepancy between the measured and predicted crack growth rate. It can be concluded, that the assumed conditions of stress transfer at the fibre/matrix interface neglecting bonding is the reason for this discrepancy.

Monte-Carlo simulation of multiple fracture in the transverse ply of cross-ply graphite-epoxy laminates

Graphite-epoxy cross-ply laminates generally show multiple fracture of the transverse ply at higher applied stress. This phenomenon is described by means of a Monte Carlo simulation method based on the assumption that the strength of the transverse ply obeys a two-parameter Weibull distribution function. The main results show that the smaller the scatter of strength of the 90°-ply (i.e. the larger the shape parameter at a constant mean strength of the Weibull distribution), the higher becomes the threshold for the multiple fracture to occur, and the more rapidly the length of 90°-ply segments decreases with increasing applied stress once multiple fracture takes place. The methods to determine the shape and scale parameters of the Weibull distribution for the strength of the 90°-ply proposed by Manderset al. and Peters are proved to be useful even for a small number of test specimens. When the interfacial bond strength between 0°- and 90°-plies is low, saturation of 90°-ply cracking occurs at higher applied stress. The stress-carrying capacity and stiffness of the composites as a whole are reduced by multiple fracture of the 90°-ply. This reduction is more pronounced at increasing applied stress or at a larger number of transverse cracks, especially when the interfacial bond strength is low.

Production of MMCs on the Basis of Fibres Coated with the Metal Alloy by Magnetron Sputtering

Metal matrix composites (MMCs) are attractive materials for specific technical applications. At DLR in recent years MMCs are developed making use of a CVD SiC-fibre (mainly the SCS-6 fibre of Specialty Materials, USA). SiC-fibre reinforced titanium alloys are developed and optimized in the past decade mainly for application in the aircraft gas turbine. As the composite material is light (specific weight 4.0) stiff and strong it is especially suitable for rotating components. A new development for another field of application is the production of SiC-fibre reinforced copper alloys as a heat sink material for application in fusion reactors and rocket propulsion. The technique to produce titanium and copper based MMCs is based on coating the fibre with the respective alloy by magnetron sputtering. In this way the alloy initially produced as a coating is transformed into bulk matrix material, when coated fibres stacked in a preform are hot-isostatically pressed and consolidated at a high temperature and pressure.

Estimation of the 90° ply strength distribution and shear lag parameter from multiple transverse cracking in graphite-epoxy cross-ply laminates

Abstract A method to estimate the Weibull parameters for the strength distribution of a transverse ply and the shear lag parameter (the ratio of the shear modulus G to the thickness b of the shear transfer region) in cross-ply 0 2 /90 4 /0 2 , 0 2 /90 6 /0 2 and 0 2 /90 12 /0 2 graphite-epoxy laminates from multiple transverse cracking was proposed and applied to experimental results. The estimated values of the Weibull parameters were essentially the same as those estimated by the Peters method. The value of the shear lag parameter G / b was found to be 50–100 MPa μm −1 for 0 2 /90 4 /0 2 and 0 2 /90 6 /0 2 laminates. On the contrary, the experimental results on the multiple cracking of the 90° ply in a 0 2 /90 12 /0 2 laminate at high applied stresses could not be described by the value of G / b = 50–100 MPa μ m −1 , possibly owing to the occurrence of plastic deformation and/or shear failure in the 0°–90° interface.

The Effect of Impact on the TiAl Alloy TNBV3B Produced on Three Different Processing Routes

The impact behaviour of the TiAl alloy TNBV3B produced on three different processing routes - cast, forged (with a relatively small degree of deformation) and extruded - has been studied making use of ballistic tests. The damage evolution due to centre and edge impacts on flat and airfoil-like specimens has been investigated with a focus on the influence of the microstructure. Apart from the influence of material properties, the impact location showed a strong effect on the damage evolution. The extension of impact cracks in the interior of the specimens has been studied making use of heat tinting experiments and computer tomography analyses.