Walter L. Bradley

A Parametric Study of Variables That Affect Fiber Microbuckling Initiation in Composite Laminates: Part 1-Analyses

The objectives of this research focus on the effects of nonlinear matrix constitutive behavior, initial fiber waviness, and fiber/matrix interfacial bond strength on fiber microbuckling initiation in thermoplastic composites. Nonlinear geometric and non linear material two-dimensional finite element analysis is used to model the initiation of fiber microbuckling of an initially wavy fiber. Results show that reductions in the resin shear tangent modulus, larger amplitudes of initial fiber wavinesses, and debonds each cause increases in the localized matrix shear strains; these increases lead to premature fiber microbuckling initiation. These numerical results are compared with experimental data obtained during this investigation. These experimental results and comparisons are presented in a companion paper [1] .

Chapter 5 - Relationship of Matrix Toughness to Interlaminar Fracture Toughness

Abstract The relationship of resin fracture toughness to delamination fracture toughness is considered in this chapter. The resin toughness is seen to play a dominant role in the interlaminar fracture of composite materials. In situ observations of fracture in the SEM indicate that increasing the ductility and decreasing the yield strength of the matrix resin increase the delamination fracture toughness by increasing the plastic (or non-linear viscoelastic) zone size ahead of the crack tip, giving greater load redistribution away from the crack tip and more crack-tip blunting. The low efficiency of translation of resin fracture toughness into delamination fracture toughness for very ductile resins is the result of constraint provided by the fibers in the adjacent plies to the development of a larger plastic zone. The surprisingly high mode-II delamination toughness observed in composites made with brittle resins is due to the nature of the fracture process; namely, the formation of sigmodial shaped microcracks over a considerable distance ahead of the crack tip, giving significant load redistribution.

Determination of the effect of seawater on the interfacial strength of an interlayer E-glass/graphite/epoxy composite by in situ observation of transverse cracking in an environmental SEM

In an effort to understand the failure behaviour and to determine the effects of seawater on composites, a program has been developed to determine the interfacial strength (normal to the fiber) of an interlayer hybrid composite which has been exposed to seawater for two different lengths of time. Specimens were tested in transverse tension in an environmental scanning electron microscope. The specimens were tested in the as-received condition, after they reached saturation, and 8 months after they reached saturation. Observations revealed that damage was initiated at the boundaries of resin-rich regions, regardless of the conditioning process. Analytical results obtained by linear superposition to determine the stress at the fiber/matrix interface revealed several interesting findings. For example, as moisture is added to a composite, the hydrothermal residual stresses in resin-rich inhomogeneities change from tensile to compressive. Assuming a constant interfacial strength, this should make it more difficult to initiate damage in conditioned specimens. As this was not the case, moisture appeared to have a slight degrading effect on the interfacial strength. However, propagation of damage away from the resin-rich regions requires a higher stress than that required to initiate this damage. Moisture actually helped to arrest damage growth causing the ply stress required to cause transverse cracking to increase with increased moisture content and with increased aging time.

In-Situ Observations in SEM of Degradation of Graphite/Epoxy Composite Materials due to Seawater Immersion:

The effect of immersion in seawater on the mechanical properties of three graphite/epoxy composite materials has been studied. The transverse tensile strength was found to be reduced by 17% in one of the systems with essentially no change in the other two systems studied. Direct observation of fracture in the SEM as well as microindentation measurements of the interfacial shear strength have been used to explain these observations. The 17% decrease in transverse tensile strength was associated with degradation of the interfacial strength, resulting in a change in the fracture mechanism from primarily matrix cracking to interfacial failure. Little difference was found in the behavior of composites immersed in distilled water and in seawater at ambient pressure or seawater at 20.7 MPa pressure.

Relating Resin Mechanical Properties to Composite Delamination Fracture Toughness

The relationship between resin mechanical properties and the resultant composite Mode I and Mode II delamination fracture toughnesses in graphite-epoxy systems was examined. The tensile property that best correlated with composite delamina tion toughness was the resin tensile elongation. Resins with higher tensile elongations pro duced composites with a higher delamination toughness, though the relationship between the two may not be a simple one. The method used to toughen the resin affected the resulting composite delamination toughness. A rubber toughened resin would produce a tougher composite than would a resin that had the same tensile elongation (but with a lower crosslink density rather than rubber particle additions).

A detailed investigation of the micromechanisms of compressive failure in open hole composite laminates

Compression failures in composite laminates containing circular holes are often preceded by the development of a damage zone which grows with increasing com pressive load. This damage zone which appears similar to a flat, tension fatigue crack in metals, initiates at the edges of the hole and propagates across the width of the laminates leading to final failure. The damage zone is initiated by local fiber buckling and/or shear crippling into the edges of the hole. The length or size of the damage zone increases with increasing load, propagating stably until it reaches a critical size. Then, unstable growth begins, and the zone completely traverses the specimen. Subsequently, a more cata strophic failure occurs giving brooming and/or delamination. Several tests of 2.54 cm wide AS4/PEEK containing a 0.16 cm diameter hole were inter rupted prior to catastrophic damage growth to allow for a careful examination of the fiber shear crippling and/or microbuckling. These observations were accomplished using scan ning electron microscopy. Direct observation of the specimen revealed the surface size of the shear crippling zone. To determine the extent of this damage through the thickness of the laminate, systematic sectioning techniques were employed. From these sectioning studies, the extent and mode of failure were determined, and a three-dimensional schematic of the damage zone was developed.

Measurements of the stress supported by the crush zone in open hole composite laminates loaded in compression

Compression failures in composite laminates containing circular holes are often preceded by the development of a damage zone which grows with increasing compressive load. This damage zone which appears similar to a flat, tension fatigue crack in metals, initiates at the edges of the hole and propagates across the width of the laminates leading to final failure. The damage zone is initiated by local fiber buckling and/or shear crippling in the edges of the hole. The length or size of the damage zone increases with increasing load, propagating until the zone completely traverses the specimen. Subsequently, a more catastrophic failure occurs giving brooming and/or delamination. Measurements of the load supported by the damage zone were made, allowing a more realistic constitutive rela tionship to be defined for the composite material in the crush zone.

A statistical examination of self-ordering of amino acids in proteins

Steinman and Cole (1967) have claimed that amino acids will naturally assume a nonrandom sequence when polymerized into polypeptides. They based their claim on a comparison of the frequency with which various dipeptide bonds formed in a dilute solution of amino acids to the various depeptide frequencies actually observed in ten proteins. Although the trends in the normalized frequencies go in the same direction, a statistical examination of the two sets of frequencies indicates that there is no correlation at all between experimental dipeptide frequencies and actual depeptide frequencies based on the chi-square tests.

Effect of Moisture on the Interfacial Shear Strength: A Study Using the Single Fiber Fragmentation Test

The effect of absorbed moisture on the interfacial strength of a carbon fiber/ epoxy matrix composite has been studied using the single fiber fragmentation test. The absorption of 1.40% moisture produced a reduction less than 20% in the interfacial shear strength. This reduction in interfacial strength may be associated with a reduction in the residual compressive stresses at the fiber/matrix interface associated with the cooldown from the curing temperature, a weakening of the chemical adhesion, or a softening of the matrix giving failure by interphase yielding. These results suggest that long-term durability of carbon fiber/epoxy matrix composites in seawater may not be much affected by absorbed seawater.

Toughness Properties of Nodular Iron

The German government recently certified ductile iron for construction of nuclear waste transport containers. This approved use of ductile iron for such a critical application represents the culmination of ten years worth of research bringing to light the surprising toughness of ductile iron. This article explains how modern fracture mechanics and microstructure/property relationships have altered the stereotype of ductile iron as a low toughness material.