Larry B. Ilcewicz

Mode I Interlaminar Fracture Toughness of Composites Using Slender Double Cantilevered Beam Specimens

The use of slender double cantilever beam specimens for measuring the mode I in terlaminar fracture toughness was critically evaluated. Experiments were performed with unidirectional composites to judge the validity of using data from multiple crack jumps on a single specimen. Three approaches for calculating the strain energy release rate were compared. These included an energy rate determination of the JI-integral, a compliance calibration procedure and an analytical equation based on linear beam bending. The fracture toughness was overestimated by the latter two approaches. Per manent deflection was seen to accumulate in the arms of the double cantilever beam specimens as the cracked surface area increased. This component of deflection must be accounted for in all forms of analysis with DCB specimens.

Far-field and near-field strain response of Automated Tow-Placed laminates to stress concentrations

In this study, the response of laminates with stress concentrations is explored. Automated Tow Placed (ATP, also known as Fiber Placement) laminates are compared to conventional tape layup manufacturing. Previous tensile fracture tests on fiber placed laminates show an improvement in tensile fracture of large notches over 20 percent compared to tape layup laminates. A hierarchial modeling scheme is presented. In this scheme, a global model is developed for laminates with notches. A local model is developed to study the influence of inhomogeneities at the notch tip, which are a consequence of the fiber placement manufacturing technique. In addition, a stacked membrane model was developed to study delaminations and splitting on a ply-by-ply basis. The results indicate that some benefit with respect to tensile fracture (up to 11 percent) can be gained from inhomogeneity alone, but that the most improvement may be obtained with splitting and delaminations which are more severe in the case of fiber placement compared to tape layup. Improvements up to 36 percent were found from the model for fiber placed laminates with damage at the notch tip compared to conventional tape layup.

Damage tolerance analysis of composite transport fuselage structure

This paper describes the development of a technique for predicting the failure strength of fiber reinforced advanced composite structures which have damage induced cracks and flaws. The technique, described as a Damage Zone Model (DZM) simulates the manner in which the stress concentrations arising from such flaws affect the stable growth of damage and ultimately failure strength of the structure. The concept of a DZM is to describe the multiple damage modes which occur in laminated composite structures via a strain softening material law which permits controlled unloading of material at, for example, a crack tip as applied loads and the size of the crack increase. The technique has been implemented in a commercially available nonlinear finite element code and has been subjected to extensive experimental verification. Experimental fracture strength for a wide range of test specimen sizes and materials have been predicted to a degree of accuracy unobtainable with more conventional fracture mechanics techniques. As an analytical tool it may prove invaluable in designing lighter, safer composite products more quickly and with less testing.

Fracture scaling parameters of inhomogeneous microstructure in composite structures

Abstract The influence of inhomogeneous microstructure on fracture properties is discussed in this study. Previous tests have shown an improvement in the notched tensile strength of laminates manufactured with fiber placement as opposed to hand layup for the same materials and laminate configuration. Fiber placement results in structures with a more inhomogeneous microstructure than in conventional hand layup manufacturing. Equivalent continuum modeling was employed to study this phenomenon. A specialty, couple stress (Cosserat theory) finite element model was developed to explore the influence of inhomogeneities in regions of high stress gradients such as notches. The results indicate that the addition of the couple stresses mitigates singularities at notch tips. In particular, it was shown that geometry alone is not sufficient for proper scaling if inhomogeneities exist. A two-parameter failure criterion, based on critical strain energy release rates and an inhomogeneity parameter, is proposed. Good correlations were found between predictions and experimental results.

Scaling crucial to integrated product development of composite aerospace structrues. Part 1

I dedicate this manuscript to the memory of Ernest F. Dost, who passed away on 18 September 1995. He had numerous achievements prior to his tragic death at the young age of 34. In less than a decade, he helped solve many difficult problems supporting composite structural design, manufacturing and maintenance as an aeronautics engineer at Boeing. Ernie had a critical role on the integrated product development team that performed the work described in both parts of this article. Part 2 will highlight Ernies important contributions to the field of composite impact, as related to structural damage tolerance.

Fracture Scaling Parameters of Inhomogeneous Microstructure in Composite Structures

The influence of inhomogeneous microstructure on fracture properties is presented in this study. Previous tests have shown an improvement in the notched tensile strength of laminates manufactured with fiber placement versus hand layup for the same materials and laminate configuration. Fiber placement results in structures with a more inhomogeneous microstructure than in conventional hand layup manufacturing. Equivalent continuum modeling was employed to study this phenomenon. A specialty, couple stress (Cosserat Theory) finite element was developed to explore the influence of inhomogeneities in regions of high stress gradients such as notches. The results indicate that the addition of the couple stresses mitigates singularities at notch tips. In particular, it was shown that geometry alone is not sufficient for proper scaling if inhomogeneities exist. A two-parameter failure criterion, based on critical strain energy release rates and an inhomogeneity parameter, is proposed. Good correlations were found between predictions and experimental results.