Jonathan Awerbuch

Notched strength of composite laminates: Predictions and experiments - A review

This study reviews several of the recent and commonly used fracture models for predicting the notched strength of composite laminates. Emphasis has been placed on semi-empirical fracture models which are operationally simple to utilize. The review is sufficiently detailed so that it is self-contained.

Off-axis fatigue of graphite/epoxy composite

Off-axis static and fatigue behavior of AS/3501-5A graphite/epoxy was studied in an effort to characterize the matrix/interface-controlled failure. Seven different off-axis angles were tested: 0, 10, 20, 30, 45, 60, and 90 deg. Initial (static) and post-fatigue residual strength were obtained together with S-N relationships. Fracture surfaces were examined through photomicrographs and stereo (three-dimensional) scanning electron microscope (SEM) photographs, in order to delineate failure modes, and the results of these inspections are discussed. The off-axis static strength, including scatter, was fully characterized by a polynomial and a nondimensional strength parameter. Essentially, no strength or modulus degradation was observed in the specimens surviving fatigue loading of 10 6 cycles regardless of the off-axis angle or fatigue stress level. When fatigue stress level is normalized with respect to static strength, all data seem to fall on the same S-N curve. Fatigue failure occurred without any warning or visible damage. Matrix failure characteristics vary with off-axis angle and appear in the form of serrations and axial and transverse cracks. Large scatter in life was observed at all off-axis angles; however, since the number of specimens employed in the present study is not sufficient to provide meaningful statistical S-N data, a more detailed investigation of the off-axis (and angle ply) behavior of graphite/epoxy composites is warranted.

Crack-Tip Damage and Fracture Toughness of Boron/Aluminum Composites

The fracture behavior of center notched unidirectional boron/aluminum composite under tensile loading has been investigated. Load-crack opening displacement (COD) curves were obtained for crack length-to-width ratios varying from 0.05 to 0.5. The COD was measured across the crack surfaces by means of the laser interferometric technique (gage length of 500 μm). As expected, the interferometric displacement gage (IDG) was found to be very sensitive to the appearance of damage at the crack tip, resulting in highly nonlinear load-COD curves. The damage at the crack-tip has been carefully examined by radiographs, SEM, photomicrographs, interfero metric miscroscope and optical interferometry technique. Fracture strength results and experimental load-COD curves were compared with analytical predictions and an excellent agreement has been established. The resistance curve method has also been tried and found to be non- applicable. An understanding of the correlation between the observed failure modes and deformation characteristics has been achieved.

Acoustic emission during fatigue of Ti-6Al-4V : incipient fatigue crack detection limits and generalized data analysis methodology

The fundamentals associated with acoustic emission monitoring of fatigue crack initiation and propagation of Ti-6Al-4V were studied. Acoustic emission can detect and locate incipient fatigue crack extensions of approximately 10 μm. The technique therefore can serve as a sensitive warning to material failure. There are three distinct stages during which acoustic emission is generated. These stages are: crack initiation, slow crack propagation and rapid crack propagation. The distinction between the stages is based primarily on the rate of acoustic emission event accumulation. These three stages of acoustic emission correspond to the three stages of the failure process that occurs during fatigue loading. That is, changes in acoustic emission event rate correspond to changes in crack extension rate. Acoustic emission event intensities are greater during crack initiation than during slow crack propagation and greatest during rapid crack propagation. In a given fatigue cycle, event intensities increase with increasing stress and most high-intensity events occur near or at the maximum stress. Acoustic emission may therefore be used with confidence to detect, monitor and anticipate failure, in real-time.

Monitoring Progression of Matrix Splitting During Fatigue Loading Through Acoustic Emission in Notched Unidirectional Graphite/Epoxy Composite

In this paper emphasis is placed on the analysis and characterization of the emission associated with matrix splitting, giving special attention to distinguishing the emission generated by actual damage progression from that generated by friction. For this purpose, double-edge notched unidirectional graphite/epoxy AS/3501-6 specimens were subjected to uniaxial tension-tension (R = 0.1) cyclic loadings. Acoustic emission was monitored using the various AE event intensities, i.e., event amplitude, duration, energy, and counts, which were analyzed in detail.

K-Calibration of Unidirectional Metal Matrix Composites

Load-displacement curves were obtained for center notch unidirectional boron/aluminum and borsic/titanium composites. Far field displacements, obtained by means of two standard clip gages, 1 and 0.25 inch long, respectively, were used to establish the variation of global compliance with crack length. The local compliance curve was obtained from the crack opening displacement (COD) measured across the crack surfaces by means of the laser interferometric technique. The Interferometric Displacement Gage (IDG), which is more sensitive to the appearance of damage at the crack tip, yielded nonlinear load displacement curves and seemed to better characterize the fracture behavior of composites. The experimental com pliance curves and the K-calibration results were found to be in good agreement with analytical predictions. The K-calibration curves of the com posites studied were found to be almost equal to that of the isotropic materials; both compliances were predictable by using the isotropic K- calibration.

Monitoring Acoustic Emission in Cross-Ply Graphite/Epoxy Laminates During Fatigue Loading:

Destructive and nondestructive examinations of composite laminates subjected to exter nal loading have indicated that these materials contain a large number of cracks, both on the micro- and macro-scales. Consequently, a significant amount of emission can be generated during fatigue loading not only by damage accumulation (progression) but also by the continuous friction that results from the fracture surfaces grating against each other. Consequently, in order to monitor fatigue damage progression in composite laminates by acoustic emission (AE), the emission caused by the fretting should be distinguished from that generated by actual damage growth.

Monitoring Damage Accumulation in Filament-Wound Graphite/Epoxy Laminate Coupons During Fatigue Loading Through Acoustic Emission

Damage accumulation during low cycle tension-tension fatigue loading has been monitored through acoustic emission for filament-wound graphite/epoxy [±24/90/0/ ±45/0/90/±24] T laminate coupons. Test frequency was 0.1 and 1.0 Hz at different dynamic load levels and specimens were cycled up to 5,000 and 15,000 cycles at a fatigue load ratio of 0.1. Unnotched and double edge notched specimens (300 × 25 mm in dimensions) were tested, with notch length-to-width ratios of 0.15 and 0.25.

Sources of acoustic emission during fatigue of Ti-6Al-4V: effect of microstructure

The fundamentals of acoustic emission (AE) analysis of fatigue cracking were applied to Ti-6Al-4V. The effect of microstructure on the characteristics of the AE events generated and the failure mechanisms which produced AE in Ti-6Al-4V were established. Lamellar microstructures generated one to two orders of magnitude more emission than equiaxed microstructures. The combination of larger grain size, more continuous α/β interfaces, more tortuous crack-front geometry, cleavage and intergranular fracture in lamellar microstructures accounts for the greater amount of emission. For lamellar microstructures, most AE events were generated in the upper 20% of the stress range, whereas in equiaxed microstructures, most events were generated at lower stresses. Most AE events were generated during crack opening and also at low stresses. AE events having high level intensities were also generated at stresses other than the peak stress. This is because in titanium alloys, which have both high strength and toughness, AE events are generated from both plastic zone extension and crack extension.

Fatigue Testing of a Stiffened Lap Joint Curved Fuselage Structure

In April 1988, Aloha Airlines flight 243 experienced an explosive midair decompression that resulted in the separation of an 18-foot section of the fuselage crown of the Boeing 737 airplane. Investigations revealed that the linkup of small cracks emanating from multiple rivet holes in a debonded lap joint contributed to the catastrophic failure. This cracking scenario, known as multiple-site damage, is one of two sources of widespread fatigue damage; a type of structural degradation characterized by the simultaneous presence of fatigue cracks at multiple structural elements that are of sufficient size and density whereby the structure will no longer meet its damage tolerance requirement This study, sponsored by the National Aging Aircraft Research Program initiated by the Federal Aviation Administration in response to the Aloha accident, investigates multiple-site damage initiation and growth behavior in a pristine narrow-body fuselage panel. The test panel, a curved 6 x 10 ft stiffened structure containing six frames, seven stringers, and a longitudinal lap joint, was tested at the Federal Aviation Administration Full-Scale Aircraft Structural Test Evaluation and Research facility. The panel was subjected to a fatigue test with constant-amplitude cyclic loading, simulating the major modes of load associated with fuselage pressurization. Nondestructive inspections were conducted during the fatigue test to detect and monitor crack formation and growth. Multiple-site damage cracks were visually detected after about 80% of the fatigue life. Cracks developed and linked in the upper rivet row of the lap joint in the outer skin layer and formed a long fatigue crack before the termination of the fatigue test A residual strength test was then conducted by subjecting the panel to quasi-static loads until catastrophic failure. Fractographic examinations were conducted to reconstruct crack growth history. Preliminary results show multiple crack origins and significant subsurface crack growth.