Cheung Poon

Prediction of Bearing Strength in Fiber Metal Laminates

Finite element (FE) analyses are carried out on bolt bearing testing scenarios based on data found in the literature. Both layer-by-layer and smeared property FE models are created to calculate the compressive characteristic dimension (CCD) for three GLARE variants. A novel re-definition of conventional CCD is proposed which is governed by the yield strength of aluminum. The new definition also incorporates the two-phase nature of GLARE, as well as the delamination/ buckling phenomenon for pin/bolt bearing, in a bearing failure mode. A previously unconsidered, orthotropic plate buckling analysis is also conducted in a conservative, worst case scenario sense on the laterally unsupported prepreg layers. Results of the buckling analysis suggest that the prepreg contribution to bearing strength, in a bearing failure mode, is at best negligible and joint collapse is governed by the yielding and delamination of the aluminum layers. Calculation of a CCD, based on the new yield strength definition, produced consistent values amongst all GLARE variants considered in the layer-by-layer analysis suggesting that the CCD is a property of the material alone.

Experimental Characterization of the Bearing Strength of Fiber Metal Laminates

A novel experimental methodology extended from ASTM D953 was developed and implemented to conduct pin bearing experiments on GLARE3-5/4-0.3 and GLARE3-4/3-0.3 variants with the aim to examine a bearing load configuration from a local perspective as well as a global one. This was accomplished by introducing previously unconsidered testing fixtures and additional instrumentation, including bonded strain gages, as a means of measuring the local strain field generated by a bearing load. Load—displacement curves were produced as per the standard but were subject to numerous plateaus and significant statistical scatter, attributed to pin seating and global displacement measurement. Conventionally defined bearing yield strengths were calculated but lacked physical meaning, prompting the production of novel bearing strength vs. measured strain profiles. These new profiles, derived from the additionally acquired data, were void of the plateau anomalies and depicted a well defined trend, indicative of a more complete characterization of material response. Examination of these profiles resulted in a new, more intuitive definition of bearing yield strength that incorporated influence from the entire curve rather than idealizing it bilinearly. The phenomenological basis of the new definition suggests an analogous extension to additional methodologies such as standard tensile or compressive tests.

Fatigue Damage in On-Axis and Off-Axis Woven-Fiber/Resin Composite

In this study the tensile static and fatigue behaviour of a woven-fabric laminate is investigated in both the on-axis and off-axis material directions. Emphasis is placed on the development of damage and its influence on the stress-strain behaviour of the laminate. The test results illustrate that there is a high degree of anisotropic behaviour due to anisotropic damage development, which is evident by the variation of the material behaviour between the on-axis and off-axis test specimens. The fatigue tests also suggest that the on-axis specimens exhibit noticeable stiffness degradation, while the off-axis specimens do not. The qualitative results provide significant insight into the type of damage mechanism responsible for the observed behaviour.

On the Pin Bearing Behavior of Orthotropic Fiber Metal Laminates

Experiments enforcing a pin bearing loading configuration were performed on a fully orthotropic GLARE 4 variant. The protocol employed in such experiments stemmed from a similar methodology performed on quasi-isotropic GLARE variants though now incorporating a local measurement scheme using biaxial strain gauges rather than uniaxial ones. The aim of this local measurement was both the extraction of novel bearing yield strength values and the detection of buckling within the aluminum layers. The encouragement of delamination and buckling was key since they not only form an integral portion of the proposed yielding through delamination buckling (YDB) mechanism but in addition, the pin bearing configuration — which ipso facto, encourages the former(s) — has been identified in the literature as the most conservative and accurate means for analyzing joint behavior and collapse. Analytical calculations previously performed support the empirical findings and provide direct evidence for the hegemony of aluminum yield strength in joint collapse. The proposed and employed protocol has been shown to be effective across a comprehensive range of GLARE variants and may be extended analogously to other standardized testing methodologies.

Optimization of Nd:YAG-Laser Welding Process for Inconel 718 Alloy

Ni-based superalloys are extensively used in the manufacture of aircraft engine components because of their excellent heat-resistant and corrosion-resistant properties. The principal joining processes for Ni-based superalloys are TIG welding, MIG welding, submerged arc welding, electron beam welding, and CO2 laser welding. In this investigation, a robotic 4-kW continuous-wave Nd:YAG laser system was used to identify the optimal laser welding process for 2.0 mm thick Inconel (IN) 718 sheets. The effect of various processing parameters, which included power input, welding speed, weld geometry and filler wire, was studied using the Taguchi design of experiment (DOE) methodology. The DOE methodology enabled the evaluation of the relationship between the process parameters and the quality of the welded joints, from which the optimal Nd:YAG laser welding process was developed for IN718 alloy. Joint quality was examined by tensile and nondestructive testing methods. Using the optimal process established in this research, mechanically-sound welds with narrow fusion and heat-affected zones were produced. The outcome of this research demonstrates the feasibility of the application of Nd:YAG laser in the joining of IN718 sheets for the manufacture of aircraft engine components.

Application of fiber optic sensors for elevated temperature testing of polymer matrix composite materials

Abstract Advanced polymer matrix composite (PMC) materials have been more frequently employed for aerospace applications due to their light weight and high strength. Fiber-reinforced PMC materials are also being considered as potential candidates for elevated temperature applications such as supersonic vehicle airframes and propulsion system components. A new generation of high glass-transition temperature polymers has enabled this development to materialize. Clearly, there is a requirement to better understand the mechanical behaviour of this class of composite materials. In this study, polyimide-coated fiber optic sensors are employed to continuously monitor strain in a woven carbon fiber bismaleimide (BMI) matrix laminate subjected to tensile static and fatigue loading at elevated temperatures. A unique experimental test protocol is utilized to investigate the capability of the optical sensors to monitor strain and track stiffness degradation of the composite material. An advanced interrogation system and an optical spectrum analyzer are utilized to track the variation in the optical fiber wavelength and the wavelength spectrum for correlation with strain gage measurements. Isothermal tensile static and fatigue tests at room temperature, 105°C, 160°C and 205°C suggest that these optical sensors are capable of continuously monitoring strain and tracking the stiffness loss of a highly compliant PMC specimen during cyclic loading. The results illustrate that employing optical sensors for elevated temperature applications has significant advantages when compared to conventional strain gages.

Considerations for progressive damage in fiber-reinforced composite materials subject to fatigue

Due to the increased use of composite materials in the aerospace industry, numerous attempts have been made to develop fatigue models in order to predict the fatigue behaviour and consequently the fatigue life of these materials. Existing fatigue models have significant deficiencies, thus are not widely acceptable in the industry. A better understanding of the exhibited fatigue behaviour of composite materials is consequently required. The complex nature of fatigue behaviour in fiber-reinforced composite materials is presently investigated. An explicit progressive damage model, that is mechanistic in nature, is currently being developed using the concept of a representative volume element. A micromechanical finite element model that is capable of explicit damage initiation and propagation modeling is utilized for simulation of damage development. The predicted numerical results illustrate the capabilities of the current model. Future work is also outlined in the paper as the development of the fatigue model is continued.