Wayne W. Stinchcomb

Elastic moduli of transversely isotropic graphite fibers and their composites

This paper demonstrates that it is possible to calculate the complete set of elastic mechanical properties for graphite-epoxy fiber-reinforced materials at any fiber-volume fraction by modifying equations previously developed to include transversely isotropic graphite-fiber properties. Experimental verification of the modified equations is demonstrated by using these equations to curve fit elastic-property data obtained ultrasonically over a range of fiber-volume fractions. Material systems under consideration are T300/5208, AS-3501 and Modomor II/LY558 graphite epoxy. Using the modified equations it is possible to extrapolate for fiber properties. From Modomor II/LY558 ultrasonic data, it is shown that five out of seven extrapolated graphite-fiber properties are consistent with the assumption that graphite fibers are transversely isotropic. Elastic properties for T300/5208 and AS-3501 are ultrasonically evaluated by propagating stress waves through six individual specimens but at various angles from a block of unidirectional material. Particular attention is devoted to specimen dimensions. To demonstrate the need for accurately calculating or experimentally measuring all lamina elastic properties, a brief discussion is included on the effect that variations in lamina elastic properties have on calculating interlaminar stresses.

A critical-element model of the residual strength and life of fatigue-loaded composite coupons

This paper addresses the basic question of how to develop a mechanistic cumulative damage model that has the capability of describing and predicting the strength and life of high-modulus continuous-fiber composite laminates subjected to general cyclic loading. The paper is a first step in philosophy from phenomenological descriptions of composite laminate fatigue behavior to mechanistic modeling based on the physics and mechanics of the details of the laminate response during cyclic loading. The major point of departure of the present effort from prior modeling activities is the mechanistic approach.

A cumulative damage model to predict the fatigue life of composite laminates including the effect of a fibre-matrix interphase

Abstract Recent experimental efforts have established the significance of the fibre-matrix interface /interphase in the long-term behaviour of polymeric composites. Results indicate that small alterations at the interface level could translate into orders-of-magnitude changes in fatigue life. However, there is no model currently available in the literature to predict these changes. In this paper, a micromechanics model that includes the effects of the fibre-matrix interface is used in a simple cumulative damage scheme to predict the tensile fatigue behaviour of composite laminates. A new parameter called the ‘efficiency of the interface’ is used to model the degradation of the in terface under fatigue loading. A rate equation that describes the changes in interfacial efficiency as a function of cycles is estimated using experimentally determined stiffness reduction data. The influence of this interfacial efficiency parameter on the tensile strength of unidirectional laminates is assessed using a micromechanics model. The effect of damage on the stiffness of the laminate is estimated by solving a boundary value problem associated with the particular damage mode (e.g. transverse matrix cracking). The fatigue life of the laminate is estimated by considering changes in stiffness due to creep and damage in the subcritical elements, and changes in strength associated with the critical element (0° ply). The influence of a fibre-matrix interface is included in the model by considering the degradation in the interface (interfacial efficiency) under fatigue loading. Changes in the interface property are used in the micromechanics model to estimate changes in the in-situ tensile strength of the 0° ply. The stress state and the strength of the 0° ply, calculated including the effects of damage, are then used in a maximum strain failure criterion to determine the fatigue life of the laminate. Predictions from this model are compared with experimental data. The predicted fatigue life and failure modes agree very well with the experimental data.

The Mechanics of Vibrothermography

Vibrothermography is a term coined by the authors to describe a concept and related techniques whereby the internal integrity and uniformity of materials and components is interrogated by observing the heat pattern produced by the energy dissipation which occurs when a specific vibratory excitation is applied to the test piece. In particular, “vibrothermography” consists of the study of thermographic (heat) patterns which are recorded or observed in real time during such an excitation. It has been observed that the details of the mechanisms which produce such dissipative heat are directly related to the mechanisms of material deformation and degradation in several important ways, a fact that provides the basis for the use of this scheme as a nondestructive test and evaluation method and general philosophy. The present paper is an overview of this field which begins with a tutorial explanation of the basic foundations of the method, continues with a development of the manner in which the information provided by the method is related to the mechanics of the response of the test specimens — to the extent that it is understood — and closes with a survey of results and applications of the method.

Thermography — An NDI Method for Damage Detection

Video-thermography, the time-resolved observation of infrared radiation, is a nondestructive inspection technique which offers many potential applications in the study of material behavior. The present work emphasizes detection of damage in both homogeneous and composite materials. This work differs from most others because the materials are subjected to some steady-state mechanical energy, such as fatigue loads or low-amplitude vibrations, that activates heat sources near the damaged regions. Experimental observations are discussed for a variety of materials, including boron/aluminum, boron/epoxy, graphite/epoxy, aluminum, and plastics. Thermography has been used to investigate initiation and progression of subsurface damage caused by fatigue; vibrothermography has been used to locate delaminations and similar damaged regions. Discussed are several analytical studies of the relation between evolution of heat and the stress field in the region of the damaged zone, and the expected surface heat pattern from a subsurface heat source in an anisotropic material.

DAMAGE MECHANICS AND NDE OF COMPOSITE LAMINATES

ABSTRACT The mechanics of the response of composite laminates with damage is discussed in the context of damage initiation, damage growth, stress redistribution, fracture, and nondestructive testing.

Nondestructive evaluation of damage accumulation processes in composite laminates

Abstract This paper presents results from several interdisciplinary studies of the mechanical response of composite materials, in which mechanics and materials science were used in concert with nondestructive evaluation (NDE) to investigate the damage accumulation process. Specific examples are cited for the initiation and growth of damage in notched and unnotched laminates subjected to cyclic loading histories. The process of damage accumulation in composite materials involves a number of damage modes, including matrix cracking, delamination, and fiber fracture, which initiate, grow, and interact to form a complex network of damage details called the damage state. Nondestructive techniques, including replication, light and electron microscopy, X-ray radiography, ultrasonics, stiffness change, and thermography, are used to detect and monitor the progressive development of damage throughout the fatigue life and to evaluate its effect on fatigue response of the laminates.

Response of Notched AS4/PEEK Laminates to Tension/Compression Loading

The response of notched AS4/PEEK specimens to fully reversed, tension/compression loading has been investigated by examining their fatigue lives, damage initiation and propagation, and their residual strength. AS4/PEEK specimens were subjected to R = -1 tension/ compression cyclic loading at 52.4 and 64.3% of their monotonic compression strength. The results indicate a significant difference in the response of the material at the two cyclic stress levels. At the lower fatigue stresses, the predominant damage, as determined by X-ray radiography and by deplying, is characterized by matrix cracking and delamination that initiate at the notch and grow both perpendicular and parallel to the load direction. Stiffness measurements taken during the low-level fatigue history show that compression stiffness and tension stiffness degrade throughout the fatigue lifetime. Further, specimens fatigued at the lower fatigue stresses lost compressive strength as the damage developed while they gained tensile strength. Fatigue life was defined by reduction of compression strength. Damage to specimens fatigued at higher cyclic stresses developed much more predominantly in the direction perpendicular to the loading and much less in the direction parallel to the loading. Stiffness measurements made on these specimens showed a more rapid degradation of tension stiffness than of compression stiffness throughout the fatigue life. As with specimens fatigued at the lower stress levels, residual compressive strength decreased with damage development. However, the residual tensile strength of specimens fatigued at higher stresses decreased with damage development and the fatigue failure modes were tensile. The difference in the response of the graphite-PEEK laminates at the two cyclic stress levels suggests that damage initiation and damage propagation play different roles in defining fatigue response.

Effect of Ply Constraint on Fatigue Damage Development in Composite Material Laminates

It is shown that the effects of constraint on the response of composite materials can be classified as (1) in-plane effects, and (2) through-the-thickness effects; with in-plane constraint being the principal contributor to notched strength and changes in notched strength under quasi-static loading. It is also determined that the constraint situations that produce the greatest static strength do not minimize the extent of damage that develops under either static or cyclic loading, and that through-the-thickness constraint controls the pattern and spacing of transverse cracks in the characteristic damage state that determines those of strength and stress in unnotched laminates. It is concluded that the mode and the extent of damage in notched and unnotched constrained plies is governed by the stress state in those plies, as determined by the constraining ones, and the relationship of stress and strength states.

Response of thick, notched laminates subjected to tension-compression cyclic loads

ASBTRACT: The fatigue response of a [(0/45/90/-45) s ] 4 T300-5208 graphite/epoxy laminate with a drilled center hole subjected to constant-amplitude, fully reversed tension-compression loading was investigated. Damage evaluation techniques such as stiffness monitoring, penetrant-enhanced X-ray radiography, C-scan, laminate deply, and residual strength were used to establish the mechanisms of damage development as well as the relations between this damage and the stiffness, strength, and life of the laminate. Two load levels provided for fatigue lives of 10 5 to 10 6 cycles and significant stiffness reductions. Damage initiated at the hole as matrix cracking parallel to the fibers in all plies. Matrix cracks had a significant effect on delamination initiation and growth. Delaminations initiated near the surface in the densely cracked region at the hole and grew along major matrix cracks. Delaminations of smaller extent developed later throughout the interior of the laminate and followed similar growth patterns as those closer to the surface. Compressive properties degraded more rapidly than tensile properties. At the stress levels used in this investigation. residual tensile strength increased early in the fatigue life and remained approximately constant to near the end of life, when failure was precipitated by excessive laminate instability during the compressive portion of the loading.