Experimental investigation of damage progression and strength of countersunk composite joints

An experimental investigation is conducted into the damage progression and strength of bolted joints with fibre-reinforced composite laminates and countersunk fasteners. The main goal of the experimental investigation is to characterise the effect of the countersink geometry on the load-carrying capacity of single lap joints in comparison to the straight-shank case. The effects of bolt torque, clearance and countersink height ratio on the damage progression and joint strength are also studied. Experimental tests and detailed microscopy studies are conducted on a bearing test specimen with a straight-edged hole, and several single-lap joint configurations with countersunk fasteners. It is found that introduction of the countersunk hole roughly halves the bearing stress, and causes delamination for some configurations. This delamination is primarily located at the start of the countersink region, though is found to be triggered by other damage mechanisms and has only minor influence on the results. Bolt torque increases the density of through-thickness damage though limits its extension from the hole edge, whilst bolt clearance causes localisation of the damage region. Increasing the ratio of the countersink depth to the laminate thickness reduces the extent of bearing and promotes bending, with a change to net section failure at large ratios

The effect of clearance on single lap countersunk composite joints

An experimental investigation was conducted into the effects of bolt-hole clearance on the static strength and damage progression behaviour of single lap countersunk composite bolted joints. Joints were manufactured from carbon/epoxy plain weave fabric and tested with three different bolt clearance levels. The experimental results showed that the bolt-hole clearance had a minimal effect on the ultimate failure load of the bolted joint. However, a significant reduction (approx. 22%) was observed in the bearing damage initiation load, consistent with difference in the through-thickness damage profile. Finite element analysis was conducted, and was able to accurately capture the load-displacement behaviour and through-thickness damage profile of the joints

Damage progression in composite bolted joints

Despite the many advantages of adhesive bonding, bolted joints are still used to fasten composite aircraft structures because of the ease of assembly/disassembly, minimal surface preparations, use of common tools between metal and composite structures and airworthiness certification. Joining or repairing external aircraft structures inevitably involves the use of countersunk fasteners, which can induce complex three-dimensional stress fields near the bolt hole. Since bolted joints incur significant penalty of low strength compared to pristine composite laminates, it is important to understand the damage mechanisms and develop design tools to enable better design and optimisation of composite joints so as to take full advantage of composite structures. This investigation focuses on single lap joints using countersunk fasteners, using both experimental testing and computational simulation techniques. Joints were tested in shear to failure at a range of bolt torques, hole clearances and countersunk depths to thickness ratio levels. To assist the development of predictive tools, straight-shank bearing tests were carried out to calibrate model parameters. Detailed microscopy analysis of failed specimens was conducted to characterise the through-thickness failure profile of countersunk bolted joints. Detailed finite element analyses using Abaqus/Explicit were conducted to gain insight into the behaviour of the single lap joints. The models accounted for in-plane and through-thickness composite damage, frictional contact, bolt torque and secondary bending effects in bolted joints under shear. The experimental investigation and finite element analyses showed that the through-thickness damage contained mainly interlaminar and intralaminar shear cracks and delamination. The variations in selected parameters had marginal effects on ultimate failure load of the joints; however the bearing load was significantly affected. The variation in bolt-hole clearance and countersunk depth to thickness ratio can produce significant variation in the through-thickness damage profile. As the countersunk depth to thickness ratio increased, the damage to the bearing plane of the joint increased. Catastrophic bending failure occurred for the highest ratio of countersunk depth to thickness. The finite element investigation showed that stress concentration factor at the hole edge increases with hole clearance. A detailed analysis of initiation and progression of damage, in the plane and through the thickness of the laminate has been performed. A review of the literature indicates that the detailed investigation of damage mechanisms and joint parameters presented in this thesis appears to be the first for joints involving countersunk fasteners. The present research also highlights a new method for determining the fracture energy associated with composite compression failure. The issues associated with the use of literature in determining material properties, friction coefficient and other modelling parameters are identified and discussed. The implications of capturing the overall effect of damage modes without a true mechanistic representation are also discussed. These new findings demonstrate that whilst capturing the overall behaviour and effect of joint parameters is possible, reliable predictive capability such as that required for aerospace design purposes remains a critical aspect for ongoing research

Characterising fibre compression fracture toughness of composites using bearing tests

In this paper we propose the use of a bearing test with a coupled experimental-numerical approach to characterise the critical strain energy release rate, or "fracture toughness", for fibre compression failure in bearing

Numerical analysis of damage progression and strength of countersunk composite joints

A numerical investigation is conducted into the damage progression and strength of bolted joints between fibre-reinforced composite laminates using countersunk fasteners. Experimental tests were previously conducted on a bearing test specimen and countersunk fastener single-lap joints. In this work, computational models are developed for Abaqus/Explicit, with continuum shells employed to model in-plane ply failure. The bolt-nut assembly is modelled with rigid elements, and the models account for bolt torque and frictional contact. The material properties required in the computational model are determined from standard tests, with the compression fracture toughness of composite plies calibrated against experimental data from the bearing test. The analysis approach captures the load-carrying capability of all configurations, and provides reasonable accuracy in predicting damage patterns. The effects of bolt torque, clearance and countersink height ratio are investigated, and the analysis results compare well with experimental findings. Furthermore, the analysis provides rich insight into the damage progression and joint behaviour at the ply level, with the in-plane and through-thickness damage patterns mapped for increasing applied load. Delamination is incorporated using a cohesive element layer at the start of the countersunk region, though has minimal influence on damage progression and load-carrying capability, which agrees with the experimental results

THERAPEUTIC PROPERTIES OF CAPSAICIN: A MEDICINALLY IMPORTANT BIO-ACTIVE CONSTITUENT OF CHILLI PEPPER

Plants are the source of numerous pharmaceutically important compounds that have been employed to cure various human ailments since ancient times. With the assistance of modern chemistry and materials science, such pharmaceutically important compounds have been identified and isolated to produce new drugs. Alkaloids are one of the most significant classes of naturally occurring secondary-metabolites, which are synthesized and widely distributed in various parts of plants. They regulate various metabolic activities and induce physiological responses in the human body. Capsaicin is a naturally occurring alkaloid found in many species of peppers and is attributed to their spicy nature and pungent flavor. This alkaloid is a member of the Capsaicinoids group, which includes capsaicin, homocapsaicin, homodihydrocapsaicin, dihydrocapsaicin, and nordihydrocapsaicin. Capsaicin has a wide range of therapeutic potential against various human ailments. In this article, we provide a comprehensive overview of the capsaicin molecule as well as an examination of its medicinal properties in a variety of human disorders, including pain, various types of cancer, ulcers, diabetes, obesity, inflammation, cardiovascular diseases, and neurodegenerative diseases