Basir Shafiq

Corrosion Fatigue of High-Strength Aircraft Structural Alloys

Results of corrosion fatigue characterization of AA7075-T6 and AF1410 steel under different simulated marine environments and loading conditions are presented. In comparison with baseline tests conducted in laboratory air, corrosion fatigue experiments performed at 1-Hz frequency in the presence of 1% NaCl environment indicated a substantial reduction in fatigue lifetime in the case of AA7075-T6, whereas AF1410 corrosion fatigue life was found to be statistically unaffected at 1-Hz frequency in the presence of 1% and 3.5%NaCl. However, a reduction of frequency to 0.5 Hz significantly reduced the lifetime of AF1410 steel. On the other hand, Cd-plated AF1410 tested to study fatigue characteristics in a hydrogen-rich metal surface environment yielded minimal change in the lifetime. Atomic force microscope analysis was performed to discern features in fracture surface morphology leading to changes in lifetime of AF1410 and AA7075-T6 alloys.

Corrosion Fatigue in 7075-T6 Aluminum: Life Prediction Issues for Carrier Based Operations

Fatigue experiments performed on 7075-T6 Al alloy at various frequencies and stress levels and under alternating wet and dry environments indicate that simultaneous action of corrosion and fatigue substantially accelerates crack initiation and growth rates when compared to pure fatigue (dry air) conditions. In experiments performed under alternating wet and dry conditions, fatigue crack growth rate was observed to increase rapidly in the presence of mildly corrosive (salt solution) and decrease sharply when subjected to noncorrosive dry air. Crack arrest of various durations was observed at transition points between dry and wet cycles. Lowering the frequency of fatigue loading significantly reduced crack initiation and overall life time. S-N curves showed a continuous downward trend without reaching a plateau or threshold. These observations led to the conclusion that aircraft structural integrity can be seriously compromised even under mildly aggressive environments and at subcritical stress levels when cracks are present regardless of changing of environments and/or test frequency.

Hydrogen-Assisted Fatigue Lifetime Characteristic of AF1410 Steel

Results of hydrogen-assisted fatigue lifetime testing indicated a substantial but gradual increment in crack growth rate as a function of increasing hydrogen content. Hydrogen was introduced into both sides of the specimen simultaneously via galvanostatic charging. Extensive scanning electron microscope fractographic analyses revealed a clear shift in the modes of failure and changes in fracture surface morphology as a function of increasing hydrogen content. A methodology is outlined that can be used to predict fracture surface features as a function of applied stress intensity factor. A convenient deterministic model is proposed that seems to reasonably accurately capture the crack growth rate behavior under strain controlled testing conditions. In addition, successful application of acoustic emission technique to classify various cracking stages in full spectrum testing was performed on hydrogen charged samples.

Effect of Hydrogen and Hold Time on the Lifetime of AF1410 Steel

Fatigue lifetime of model AF1410 ultrahigh-strength steel as a function of fatigue hold time (time expended at peak loads) and type and severity of environment is presented. Results indicate significant reduction in lifetime as a function of rising hold time for specimens charged with hydrogen, whereas a parabolic trend was observed in the case of specimens tested in air and in the presence of simulated marine environment. Compared with baseline tests in air, a gradual reduction in lifetime was observed as a function of increasing concentration of hydrogen in the specimens. Micrographic, atomic force microscope, and scanning electron microscope analysis is performed to study the various aspects of AF1410 lifetime.

Fatigue and hydrogen embrittlement evaluation of AF1410 steel

AF1410 steel subject to lateral one-sided electrolytic hydrogen-charging and tested under fatigue at a stress ratio of 0.4 and at 1 Hz loading frequency displayed up to four times higher crack growth rate than specimens evaluated without exposure to hydrogen. Additionally, the hydrogen-attacked side underwent a change in the fracture mode from ductile to brittle, which can be either of intergranular, transgranular or quasi-cleavage nature. Scanning electron microscope and atomic force microscope studies as well as fracture surface analysis were conducted to correlate the fracture modes with the hydrogen embrittlement effects on the steel. Since hydrogen did not permeate the entire thickness of the specimens, the experiments provided a more realistic approach to the study of large aircraft components in marine service conditions.

Damage Detection in Sandwich Composites Using Damping Matrix Identification

A semi-empirical iterative computational algorithm along with data obtained from modal analysis is presented to identify the damping matrix and hence a means to quantify FRF changes associated with damage in sandwich composites. Mode shapes, damping ratios and natural frequencies were derived from experimentally obtained FRFs, while mass and stiffness matrices were obtained from FEA in order to formulate the damping matrix. The algorithm was initially tested on a simple two-degree of freedom lumped mass system and later sandwich composite and aluminum beams were tested for undamaged and the damaged cases. Comparison of analytical and empirical FRF plots demonstrated successful correlation. It was concluded that the damping matrix and updated FRF plots obtained through this computational algorithm can be adequately used to represent the structural damage characteristics. K e y W o r d s : Damping Matrix, FRF Modeling, Sandwich Composites

Creep relaxation and fully reversible creep of foam core sandwich composites in seawater

Abstract Foam core sandwich composites were subjected to (i) creep to failure, (ii) cyclic creep-relaxation and (iii) fully reversible cyclic creep loading in seawater in order to mimic an actual ship hull’s service lifetime scenario. The results indicate a strong dependence of lifetime on the mode of loading. A significant reduction in the overall life was observed under cyclic creep as compared with the conventional creep to failure. Creep relaxation (R=1) tests were performed at loading-relaxation periods of 24/24, 24/12, 24/6, 12/12 and 6/6 h, while the fully reversible (R=-1) creep tests were conducted at loading-reversed loading times of 36/36, 24/24, 12/12, 6/6, and 3/3 h. The results suggest that creep-relaxation lifetime characteristics depend predominantly on the relaxation time as opposed to loading times, i.e. longer relaxation periods lead to shorter life. Whereas, fully reversible creep appears to be dependent upon the number of reversals whereby, life is observed to reduce as the number of reversals increase. These significant observations are explained in terms of various possible paths to interface cell wall collapse. Modes of failure were predominantly indentation and core compression in the vicinity of the loading site.

Shift in failure modes in foam core sandwich composites subject to repeated slamming on water

Abstract A test program designed and carried out to mimic the repeated impact of the bow section of fast-moving small boats on the ocean surface provided some unique observations in terms of failure mode transition. Damage progression and modes of failure were evaluated for two types of sandwich composites with comparable global strength and stiffness but different foam density and facesheet strength. Testing was performed on flat rectangular specimens that contained symmetric semi-elliptical edge flaws produced near the end of the specimen held by the rotating cam. Type 1 specimens (softer core/stronger facesheet) consistently failed by interface and through-the-thickness core shear, independent of the flaw size. In contrast, a gradual decrease in flaw size in Type 2 specimens (denser core/weaker facesheet) produced a striking transition in the mode of failure from local buckling in the vicinity of the flaw site along with exponentially increasing lifetime, to interface shear failure at the free end accompanied by a dramatic drop in lifetime. The lifetime of Type 2 specimens was more than two orders of magnitude greater than that of Type 1 specimens.

The Use of Neural Networks to Detect Damage in Sandwich Composites

Composite materials fail in complex failure modes that are difficult to detect. No single NDE technique is capable of detecting all damages. The ability to detect and asses the state of the damage is a key issue in order to improve service life of these materials. A Neural Network (NN) was chosen as a means to interpret and classify the information such that the type of damage, severity and location could be identified. The work describes the implementation of a NN based approach which combines thermal damage detection and vibration signatures in order to detect location and extent of damage in sandwich composites consisting of two carbon fiber/epoxy matrix face sheets laminated onto a urethane foam core. The approach analytically characterized and experimentally validated models for both thermal and vibration response. The numerical models were then used to train the neural networks. This approach is significant as it combines two techniques as opposed to just one as generally performed. Results demonstrated that the multi-component neural network approach successfully detected damage in scenarios in which using just a single method would have failed.

Fatigue Performance and Size Effect in Sandwich Composites

Results of sandwich composite static and fatigue loading are presented. To discern cracking in various constituents of the sandwich composite, AE technique is used. Core damage has been found to be the predominant failure activity. Fiber rupture triggered the onset of catastrophic failure. Mode I cracking was observed in the core while fiber rupture took place in mode I.