Don H. Morris

Time-temperature behavior of a unidirectional graphite/epoxy composite

A testing program to determine the time-temperature response of unidirectional T300/934 graphite/epoxy materials is presented. The short-term creep test results of tension specimens with the load at various angles to the fiber direction and at various temperatures are reported, showing that the material is elastic at all temperatures when the fiber is in the load direction. However, when the load is transverse to the fibers, the viscoelastic response varies from small amounts at room temperature to large amounts at temperatures above the 180 C transition temperature. The time-temperature superposition principle or the method of reduced variables were used to determine compliance master curves for each fiber angle, and a viscoelastic analog to the elastic orthotropic transformation equation was used incrementally to predict the master curves for the tensile compliance of the off-axis specimen.

Predicting Viscoelastic Response and Delayed Failures in General Laminated Composites

Although graphite fibers behave in an essentially elastic manner, the polymeric matrix of graphite/epoxy composites is a viscoelastic material which exhibits creep and delayed failures. The creep process is quite slow at room temperature, but may be accelerated by higher temperatures, moisture absorption, and other factors. Techniques are being studied to predict long-term behavior of general laminates based on short-term observations of the unidirectional material at elevated temperatures. A preliminary numerical procedure based on lamination theory is developed for predicting creep and delayed failures in laminated composites. A modification of the Findley nonlinear power law is used to model the constitutive behavior of a lamina. An adaptation of the Tsai-Hill failure criterion is used to predict the time-dependent strength of a lamina. Predicted creep and delayed failure results are compared with typical experimental data.

The fracture behavior of graphite/epoxy laminates

The results of uniaxial tensile tests conducted on a variety of graphite/epoxy laminates containing narrow rectangular slits and square or circular holes with various aspect ratios are discussed. The techniques used to study stable-crack or damage-zone growth—namely, birefringence coatings, COD gages, and microscopic observations are discussed. Initial and final-fracture modes are discussed as well as the effect of notch size and shape and laminate type on the fracture process. Characteristic lengths are calculated using the point, average and inherent flaw theories and comparisons with observations are discussed. Further, the effect of flaw geometry on stress states and deformations is assessed.

Creep-rupture of polymer-matrix composites

An accelerated characterization method for resin-matrix composites is reviewed. Methods for determining modulus and strength master curves are given. Creep-rupture analytical models are discussed as applied to polymers and polymer-matrix composites. Comparisons between creep-rupture experiments and analytical models are presented.

A comparison of the fracture behavior of thick laminated composites utilizing compact tension, three-point bend, and center-cracked tension specimens

An experimental study of the effects of specimen configuration and laminate thickness on the fracture behavior of notched laminated composites is presented. The behavior of (0/+, 45/90)ns, (0/90)ns and (0/+, - 45)ns graphite/epoxy T300/5208 laminates was studied. When fracture toughness was computed at the failing load of the center-cracked tension specimen, laminate thickness significantly influenced fracture toughness. If fracture toughness was computed at the interception of the 5 percent secant line with the load-COD record, fracture toughness was found to be independent of both laminate thickness and specimen configuration. Defined in this manner, fracture toughness can be physically interpreted as an indicator of the onset of significant notch-tip damage.

On the use of crack-tip-opening displacement to predict the fracture strength of notched graphite/epoxy laminates

The fracture strength and crack-opening displacement of notched graphite/epoxy laminates were measured experimentally using the center-cracked tension-specimen geometry. Four replicate tests were conducted for a variety of laminate stacking sequences, thicknesses, and notch lengths. Most laminates exhibited extensive notch-tip damage prior to fracture. Values of crack-tip-opening displacement (CTOD) at fracture were estimated from values of crack-opening displacement measured at the crack center line. CTOD was independent of specimen crack length for the [0/±45/90]s, [0/±45/90]15s, [0/±45]s, [0/±45/]15s, and [0/90]24s laminates. In addition, notched laminate strength was accurately predicted using a Dugdale-type model along with the estimated CTOD.

A Fractographic Investigation of the Influence of Stacking Sequence on the Strength of Notched Laminated Composites

The fracture behavior of T300/5208 CFRP laminate panels with 12 different combinations of ply orientation and stacking sequence is investigated experimentally, using optical microscopy, SEM, and X-ray radiography to characterize the notch-tip damage zones and fracture surfaces of center-cracked tension specimens subjected to tensile loading at constant crosshead displacement rate 20 micron/s. The results are presented graphically and analyzed in detail. Significant differences in notched strength are found for different ply fiber orientations and stacking sequences; the laminates with few major delaminations had a greater percentage of fracture due to broken fibers and also higher notched strength.

Effect of laminate thickness and specimen configuration on the fracture of laminated composites

The effect of laminate thickness on the fracture behavior of laminated graphite/epoxy (T300/5208) composites has been studied. The predominantly experimental research program included the study of the [0/′45/90] n s and [0/90] n s laminates with thicknesses of 8, 32, 64, 96, and 120 plies and the [0/′45] n s laminate with thickness of 6, 30, 60, 90, and 120 plies. The research concentrated on the measurement of fracture toughness utilizing the center-cracked tension, compact tension, and three-point bend specimens. The development of crack-tip damage prior to fracture was also studied. Test results showed fracture toughness to be a function of laminate thickness. The fracture toughness of the [0/′45/90] n s and [0/90] n s centercracked laminates decreased with increasing thickness and asymptotically approached lower bound values of 1043 MPa √mm (30ksi√in.) and 869 MPa√mm (25 ksi√in.), respectively. The fracture toughness of the [0/ ′45] n s center-cracked laminate increased with increasing thickness but reached an upper plateau value of 1390 MPa√mm (40 ksi√in.). The fracture toughness of all laminates was independent of crack size except the [0/90] 2 s laminate that split extensively. The fracture surface of all thick laminates was uniform in the interior and self-similar with the starter notch. With the exception of the [0/′45] n s laminate, the fracture toughness of the thicker laminates was relatively independent of specimen configuration.

The Viscoelastic Response of a Graphite/Epoxy Laminate

An accelerated characterization method for resin matrix composites is reviewed. Methods for determining compliance master curves are given. Creep rupture analytical models are discussed as applied to polymer-matrix composites. Comparisons between creep and rupture experiments and phenomenological models are presented.

Surface crack analysis applied to impact damage in a thick graphite/epoxy composite

The residual tensile strength of a thick graphite/epoxy composite with impact damage was predicted using surface crack analysis. The damage was localized to a region directly beneath the impact site and extended only part way through the laminate. The damaged region contained broken fibers, and the locus of breaks in each layer resembled a crack perpendicular to the direction of the fibers. In some cases, the impacts broke fibers without making a visible crater. The impact damage was represented as a semi-elliptical surface crack with length and depth equal to that of the impact damage. The maximum length and depth of the damage were predicted with a stress analysis and a maximum shear stress criterion. The predictions and measurements of strength were in good agreement.