Notched strength of carbon fibre/epoxy composite laminates with a circular hole

This paper improves the two stress fracture criteria proposed by Whitney and Nuismer (known as the point stress criterion and the average stress criterion) to predict the strength of composite laminates with a circular hole. In the point stress criterion, it is assumed that the failure occurs when the stress over some distance (d ₀) away from the notch is equal to or greater than the un-notched laminate strength. In the average stress criterion it is assumed that failure occurs when the average stress over some distance (a ₀) ahead of the notch equals the unnotched laminate strength. Both stress fracture criteria are two parameter models based on the unnotched strength (σ ₀) and a characteristic dimension (d ₀ ora ₀). A simple relation is used for the characteristic length to improve the accuracy while evaluating the notched strength of carbon/epoxy composite laminates. The analytical results are compared well with the existing test results of AS4-carbon/948 Al epoxy [0/90]_4s and [0/ ± 45/90]_2S composite laminates with various hole diameters and specimen widths.     

Notched strength of carbon fibre/epoxy composite laminates with a circular hole

This paper improves the two stress fracture criteria proposed by Whitney and Nuismer (known as the point stress criterion and the average stress criterion) to predict the strength of composite laminates with a circular hole. In the point stress criterion, it is assumed that the failure occurs when the stress over some distance (d ₀) away from the notch is equal to or greater than the unnotched laminate strength. In the average stress criterion it is assumed that failure occurs when the average stress over some distance (a ₀) ahead of the notch equals the unnotched laminate strength. Both stress fracture criteria are two parameter models based on the unnotched strength (σ₀) and a characteristic dimension (d ₀ or a ₀). A simple relation is used for the characteristic length to improve the accuracy while evaluating the notched strength of carbon/epoxy composite laminates. The analytical results are compared well with the existing test results of AS4-carbon/948 A1 epoxy [0/90]_{4 s} and [0/±45/90]_{2 S} composite laminates with various hole diameters and specimen widths.     

Residual strength of aluminum–lithium alloy center surface crack tension specimens at cryogenic temperatures

A three parameter fracture criterion which correlates the stress and the stress intensity factor at failure, is followed for the residual or fracture strength estimations of cracked configurations made of aluminum–lithium (Al–Li) alloys. The three fracture parameters are determined from the fracture data of Al–Li alloy center surface crack tension (SCT) specimens at cryogenic temperatures. It is found that the estimated fracture strength values compare well with the test results.     

On the dominance of the singular dynamic crack tip stress field under high rate loading

The dynamic stress field near a running crack tip is investigated using photoelasticity and caustics. Both quasistatic and dynamic impact loads were considered. Under impact loading, it was found that the range of dominance of the singular term in the asymptotic crack tip stress field expansion was very small. The need for considering higher order transient terms in interpreting the isochromatic fringes is demonstrated. Also, the importance of considering the effect of the higher order transient terms in the characterization of failure criteria is discussed.     

Tensile fracture behaviour of notched boron/aluminum composite laminates

This paper presents modifications in the stress fracture criteria (known as the point stress criterion and the average stress criterion), developed by Whitney and Nuismer to predict the tensile strength of composite laminates containing holes and cracks. A simple relation is used for the characteristic lengths to take care of the notch size dependence, which can improve the accuracy while evaluating the notched strength of composite laminates. The applicability of the simple relation is examined by considering a good amount of fracture data on boron/aluminium laminates. The present procedure correlates well with the test data on circular hole specimens as well as various cracked configurations and confirms that the approach can be applied for tensile strength estimations of notched composite laminates.     

An ultra‐wideband rectenna using optically transparent Vivaldi antenna for radio frequency energy harvesting

This paper presents a novel ultra‐wideband rectenna which consists of a transparent Vivaldi antenna and a wideband rectifying circuit for radio frequency energy harvesting. The antenna is realized on a 2.2‐mm‐thick soda‐lime glass substrate coated with fluorine‐doped tin oxide of thickness 650 nm. It provides an optical transmittance greater than 80% in the visible region. The rectifying circuit with a cascaded matching network and the Greinacher doubler circuit are fabricated on an FR4 substrate with a thickness of 0.8 mm. The antenna provides the best matching characteristics and the realized peak gain is 3.2 dBi. The designed matching network enables maximum power transfer from the antenna to the rectifier. The rectenna provides a peak power conversion efficiency of 69% for ?10 dBm input power. The proposed antenna can be realized on the windscreen of automobiles and glass windows without causing any obstruction to normal view.     

Reconfigurable bow tie‐based filtering antenna for cognitive radio applications

This article presents the design and implementation of an 88 × 85 mm reconfigurable filtering bow‐tie antenna with four operating states. Two PIN diodes are deployed to reconfigure the transmission zeros to realize ultra‐wide band (UWB), 3.5/5.5 GHz with independent switching. This switching prevents the interference of the primary user into the secondary user thereby making the filtenna suitable for cognitive radio applications. The proposed filtenna consists of a UWB bow‐tie antenna integrated with a band‐pass filter with reconfigurable property enabled using two PIN diodes. The prototype‐filtering antenna is fabricated and the simulated results are validated using measurements. The presented results show that the antenna exhibits good impedance and radiation characteristics.