Kenneth L. Reifsnider

Stiffness-Reduction Mechanisms in Composite Laminates

It now is recognized widely that stiffness changes during the service loading of composite laminates can be significantly large, especially as those changes affect deflections, dimensional changes, vibration characteristics, and load or stress distributions. Several generic sources of stiffness change can be identified, in various degrees, in fibrous composite materials. The source which occurs quite early in the life of a specimen or component is matrix cracking, the subject of this paper. While most laboratories now report stiffness changes, very little systematic philosophy has been developed to account for and explain such stiffness changes. The complexity of this situation requires systematic study, and motivates the search for a model, or models, which can describe the behavior and predict unfamiliar response. The present paper reports the results of an experimental program and an analytical modelling exercise which indicate that much of the observed matrix cracking can be predicted and the effects on stiffness calculated with various degrees of accuracy depending upon the sophistication of the model used.

Thermographic investigation of high-power ultrasonic heating in materials

Abstract Results are reported on the use of an infrared thermovision imaging system to observe the surface temperature distribution caused by the application of high-power 20 kHz ultrasound to a variety of metal specimens and one fluoroplastic. Temperature increases of the order of 200°C were found to occur 20 to 30 s after initiation of insonation in resonant specimens of fine-grained polycrystalline brass, copper, and steel. Observation of extremely rapid temperature increases localized to the points of attachment of thermocouples to the test specimens casts strong doubt on all previous thermocouple temperature measurements made during similar experiments by earlier investigators. Fatigue cracks, artificially induced defects, and grain boundaries were also found to be sites of rapid localized heating during insonation.

An Investigation of Cumulative Damage Development in Quasi-Isotropic Graphite/Epoxy Laminates

Results of an experimental investigation of cumulative damage development in two unnotched quasi-isotropic graphite/epoxy laminates subjected to quasi-static tension and tension-tension fatigue are presented. Damage development in the form of transverse cracking in all off-axis laminae, longitudinal cracking, and delamination was monitored via the surface replication technique. Results of the study include a detailed description of the chronology of damage development prior to failure. Evidence also is presented in support of a damage model based on the concept of a characteristic damage state.

Fracture of fatigue-loaded composite laminates

Abstract While the quasi-static fracture load of many composite laminates can be estimated with engineering accuracy, the fracture event itself has not been clearly characterized and is incompletely understood. When cyclic loading is present, the pre-fracture damage state is altered significantly, so that estimating strength (or residual strength) is greatly complicated. The present paper examines this complexity and attempts to assess the manner in which pre-fracture fatigue damage affects residual strength and the fracture event. It is found that the large strength reductions observed prior to failure at low load levels can be accounted for by internal stress redistribution and material degradation events. A careful chain of physical evidence in support of this approach is presented.

Analysis of fatigue damage in composite laminates

Abstract The degradation of laminated composite materials under service loading occurs by means of a complex combination of damage modes and damage mechanisms. These damage events combine to produce a damage state which controls the state of stress and the state of strength of the degraded laminate. Although data have been generated dealing with various details of this process, an understanding or philosophy of the precise nature of damage development has not, heretofore, been developed. This paper makes such an attempt for the general case of damage development in laminated plates under cyclic (fatigue) loading. The philosophy is based on the discovery of a ‘characteristic damage state’ in such plates which forms independently of load history, and is determined only by the properties of the laminae, their orientations, and their stacking sequence. The detailed nature of this characteristic damage state, the nature of its formation, and its influence on strength, life and stiffness is discussed. The general need for further work in this, and related fields is also assessed.

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.

Fatigue Damage Evaluation through Stiffness Measurements in Boron-Epoxy Laminates

Stiffness reductions, resulting from fatigue damage, were measured for unnotched [±45]s, [0/90] s, and [0/90/ ±45 ] s boron/epoxy laminates. Deg radation in the various in-plane stiffnesses (Exx, Byy, Gxy) were measured using a combination of uniaxial tension, rail shear, and flexure tests. An attempt was made to predict stiffness loss at failure from a secant modulus criterion. Damage growth and stiffness loss were load-history dependent, hence, the secant modulus criterion is not a valid criterion for general application.

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.

Damage and Damage Mechanics

Abstract This chapter attempts to establish the fundamental nature of fatigue as a physical process in composite materials, and to identify some of the fundamental aspects of mechanics representations of that process. The chapter begins with a discussion of the physical details of “damage”, and uses those details to define “the fatigue effect” for composite materials, especially composite laminates. Requirements for mechanics representations of the fatigue process are discussed, and limitations of classical representations are indicated. Strength reduction is introduced as a method of estimating remaining life, and the computational aspects of that approach are explored. A specific mechanistic example of such an approach, the “critical element method”, is described, and some results of that modeling scheme are examined. Horizons for continuing work are identified and discussed.

The critical element model: A modeling philosophy

Abstract The advent of composite primary structures has presented the technical community with several challenges. Not the least of these is the need to answer the question, “How long will this composite component last?”. One of the most basic issues associated with this question is the matter of how to describe (and predict) behavior that is influenced and controlled by complex combinations and interactions of the multiple damage modes know to occur in composite materials. The present paper suggests a mechanistic approach to this problem which is generally known as the “critical element model” (CEM). The CEM is not so much a single model as a modeling philosophy which proposes that the damage process that ultimately leads to initiation of the fracture event can be modeled by using a micro-mechanical analysis to determine local stress redistribution caused by the failure of “subcritical elements”, and phenomenological (constitutive) information to characterize the condition of the “critical elements” which control fracture. This approach has a variety of advantages, and provides a framework for the incorporation of improved descriptions and analysis of micro-events as they become available. At the same time, it is presently operable at the elementary engineering level in convenient coded form and has been used to predict the behavior of composite coupons subjected to a variety of tensile and compressive load cycles including block loading. Some 270 tests have been compared with the predictions with encouraging results. A summary of these activities and the general philosophy will be presented in the paper.