| dc.description.abstract | Advanced composite materials, owing to their high specific strength and modulus, have found their way into several engineering applications, particularly in aircraft structural components. Further, other attractive properties of these materials, such as fatigue and corrosion resistance, formability, and tailorability to suit specific design requirements, have perhaps lured structural engineers into using them for different applications even before fully understanding their mechanics. Consequently, these materials have posed new problems and challenges. As can be observed, most of these problems are due to insufficient understanding of the exact behaviour of these materials under different types of loading conditions, owing to their basic characteristics, namely anisotropy, heterogeneity, and multiphase constitution.
Composite materials are generally considered fatigue resistant. However, this is true only under ideal conditions, such as when the material is free of defects and damage, and the applied load is always aligned with the direction of high strength fibres. The fact that composite materials are susceptible to various types of defects and damage during fabrication as well as during service, some of which may be critical, warrants serious attention. In particular, for aircraft structural applications, where safety and reliability are of paramount importance, reconfirmation and assurance of the integrity and serviceability of components becomes essential.
One type of damage that is very common and of major concern in composite structural components is impact damage. Although the effects of such damage on material performance under static loading conditions have been extensively studied experimentally and analytically by several investigators, consistent and concerted studies addressing these effects under fatigue loading are limited. This may be attributed to the complexity of the problem, which involves examining the influence of a large number of parameters, the significant time and effort required, and the need for sophisticated experimental facilities. Considering that every engineering component is subjected to varying loads throughout its service life, the absolute necessity for such studies cannot be overemphasized. This has therefore become the main theme of the investigations undertaken and presented in this experimental study.
Taking into account the complexity of the problem and its practical relevance, the investigations were formulated and focused through a set of carefully designed and sequential experiments on carbon–epoxy laminates. Initially, low velocity impact damage was induced in these laminates at specific energy levels. The damage and its distribution through the thickness were studied, and the damaged area at different depths was quantified. Subsequently, extension of this damage under constant amplitude cyclic loading was investigated. Ultrasonic imaging was utilized extensively and innovatively to measure and quantify impact induced damage as well as its growth at various stages of fatigue testing. The acoustic emission technique was employed as an online non destructive evaluation (NDE) tool to detect and monitor damage evolution and to study the associated failure mechanisms. Static compression tests were conducted on different sets of laminates, with and without impact induced damage, subjected to various loading conditions.
As stated previously, the investigation is comprehensive and elaborate; however, the outcomes are commensurate in terms of the significant results obtained. Among the key findings, the foremost is the staged measurement of damage extension due to fatigue after impact using ultrasonic imaging. The second significant result is the estimation, comparison, and correlation of strength and stiffness degradation under different loading conditions, namely pure fatigue, pure impact, and fatigue after impact. The third is the successful detection of damage growth during fatigue using acoustic emission monitoring. Additionally, a novel approach has been introduced and validated through quantitative data, involving the measurement of the combined effect of localized damage and distributed material deterioration due to fatigue, along with their individual contributions to loss of strength and stiffness-information that could not otherwise be obtained through conventional strain measurements.
Quantitatively, the results indicate that impact damage induced at energy levels of 5 and 8 joules causes no significant loss in either strength or stiffness of the laminates, whereas an impact energy of 10 joules results in a reduction of approximately 10% to 15% in these properties. One million cycles of fatigue loading applied to impacted laminates lead to a further reduction of about 10% to 15% in residual strength and stiffness. In contrast, fatigue loading of unimpacted laminates does not affect strength, and the reduction in stiffness is less than 5%.
In summary, although fatigue effects alone may often be neglected for composite materials, the results presented in this thesis clearly demonstrate the detrimental influence of fatigue on laminates containing impact damage. The methodology developed and the procedures adopted in this work effectively quantify damage and damage progression, along with their effects on residual strength and stiffness. This establishes a new direction for life assessment of structural components made of composite materials. | |
