DAMAGE EVOLUTION IN COMPOSITE BONDED JOINTS UNDER FATIGUE LOADING
In the present paper the evolution of fatigue damage in composite bonded joints is extensively analysed and discussed, with particular reference to the influence of layer orientation at the adhesive-adherend interface, corner geometry at the end of the overlap area and stacking sequence. Single lap bonded joints manufactured from autoclave-moulded laminates were investigated under tension-tension fatigue loading. Two laminate lay-ups [45/02]s and [452/0]s , two overlap lengths (20 and 40 mm) and two corner geometry (square edge and fillet) were considered. The evolution of fatigue damage, a crack initiation followed by propagation up to a critical length, was investigated at macroscopic and microscopic level, by monitoring stiffness trends and by optical and scanning electron microscopy and it is extensively discussed in the paper. The presence of a 45° oriented layer at the adhesive-interface layer makes damage patterns and crack paths much more complicated with respect to 0° interface joints, previously investigated by the authors. As a result the resistance to crack propagation is increased.
CRACK PROPAGATION ASSESSMENT IN COMPOSITE BONDED JOINTS
In the present paper the problem of crack propagation assessment of single lap bonded joints in composite material has been analysed. Experimental tests on carbon fabric-epoxy 1.6-mm-thick single lap specimens were conduced in tension-to-tension fatigue. Crack length was monitored by means of an optical microscope at scheduled number of cycles during each test. Different over lap lengths (20 mm and 40 mm), configurations of corner geometry (square edge or spew fillet) and stacking sequence ([45/02]s and [452/0]s) were taken into account. Being a scientific definition of the crack initiation and propagation phases beyond our scope, it has been assumed that initiation phase ceases when a technical crack having size equal to 0.3 mm is observed at the adherend-adhesive interface. Since the tested joint geometry induces combined opening and sliding modes at the crack tip, experimental crack growth rates have been summarized in terms of an equivalent value of the Strain Energy Release Rates (SERR). However, crack monitoring by means of manual optical microscope inspection resulted to be a very time –consuming procedure: then it is proposed to adopt the specimen’s stiffness loss, which is easier to be automatically monitored during the fatigue test, as an indicator of the evolution of cracks’ length. Calibration by means of a so-called master curve enabled us to convert the stiffness values into corresponding crack lengths and finally crack growth rates, which resulted consistent with the experimental ones. The existence of a unique Paris-like curve correlating crack growth rates in terms of SERR, to be used for engineering crack propagation assessments, has been finally checked.
Flow in a branching open channel: a review
Branching channel flow refers to any side water withdrawals from rivers or main channels. Branching channels have wide application in many practical projects, such as irrigation and drainage network systems, water and waste water treatment plants, and many water resources projects. In the last decades, extensive theoretical and experimental investigations of the branching open channels have been carried out to understand the characteristics of this branching flow, varying from case studies to theoretical and experimental investigations. The objectives of this paper are to review and summarise the relevant literatures regarding branching channel flow. These literatures were reviewed based on flow characteristics, physical characteristics, and modeling of the branching flow. Investigations of the flow into branching channel show that the branching discharge depends on many interlinked parameters. It increases with the decreasing of the main channel flow velocity and Froude number at the upstream of the branch channel junction. Also it increases with the increasing of the branch channel bed slope. In subcritical flow, water depth in the branch channel is always lower than the main channel water depth. The flow diversion to the branch channel leads to an increase of water depth at the downstream of the main channel. From the review, it is important to highlight that most of the study concentrated on flow characteristics in a right angle branch channel with a rigid boundary. Investigations on different branching angles with movable bed have still to be explored.