Krishnakumar Shankar

Fatigue behaviour of aluminium alloy 7075 bolted joints treated with oily film corrosion compounds

Abstract The effect of oily film corrosion-prevention compounds on the fatigue behaviour of aluminium alloy 7075-T6 mechanically fastened joints was investigated. Unpainted double-lap joints with a single bolt fastener were tested under constant-amplitude fatigue loading with and without treatment with lubricative corrosion-prevention compound (CPC). Fatigue tests were conducted with different levels of fastener clamping force to determine the variation in fatigue life as a function of the clamping force with and without the CPC. Fractography of the failed specimens indicated that there are two distinct modes of fatigue initiation, one dominated by fretting and the other dominated by bearing at the fastener hole. It was found that in joints with high fastener-clamping force, the application of lubricative corrosion-prevention compounds increases the fatigue life, whereas the use of CPC is detrimental to the life of joints with low fastener-clamping force.

Effect of welding and weld repair on crack propagation behaviour in aluminium alloy 5083 plates

Aluminium alloy 5083 is used in the fabrication of lightweight, high-speed marine vessels. The high cyclic service stresses on such structures render welded joints in them fatigue-critical. This paper presents experimental investigations on fatigue crack behaviour of welded 5083-H321 aluminium alloy plates. Crack propagation in the heat-affected zone of welded specimens was determined from tests conducted on single edge-notched tension specimens. Three-dimensional finite-element analysis was employed to determine stress intensity factors for cracks in the weld line. The crack closure effects of weld residual stresses were evaluated. The residual stresses significantly influence the crack growth rates. Fatigue behaviour of weld repairs in cracked plates was also investigated. The weld-repair process significantly increases the grain size and the size of defects in the heat-affected zone. The results indicate that weld repair of cracks in welded joints provides little improvement in residual life.

A study on residual stresses in polymer composites using moiré interferometry

Moiré interferometry is applied along with the hole drilling technique to determine residual cure stresses in symmetric cross ply graphite epoxy laminates. Traditional moiré interferometry set-up using two collimated angle beams was employed to provide the virtual reference grating while a cross grating with a frequency of 1200 lines per mm was replicated on the specimen surface. Holes of different depths, each one penetrating one additional layer of the laminate, were drilled using a high speed air turbine drill to relieve the stresses in each layer sequentially. The strain distribution around each hole was computed from correlation of the undistorted carrier fringe pattern with the distorted fringe patterns around the holes. The measured strain distributions are compared to residual strain distributions by finite element analysis.

Tailoring of Composite Reinforcements for Weight Reduction of Offshore Production Risers

The use of composite materials in offshore engineering for deep sea oil production riser systems would lead to weight savings and improvement in fatigue and corrosion resistance. This paper examines the weight saving potentials offered by varying fibre orientations, stacking sequences, fibre matrix combinations and liner materials for deep sea riser tubulars. Four main load cases, recommended by design standards for the local design of composite offshore risers, are considered in the analysis using three-dimensional finite element modelling: maximum internal pressure, axial tension, top tension with external pressure and maximum external pressure.

Flexure analysis of spoolable reinforced thermoplastic pipes for offshore oil and gas applications

This article is concerned with the numerical modelling and analysis of the mechanical behaviour of composite pipes used for offshore oil and gas applications. Specifically, the bending of the reinforced thermoplastic pipes during the reeling process of reel-lay installation is modelled using non-linear finite-element procedures. In particular, the possible buckling of the reeled composite pipes has been investigated. Composite pipes reinforced with one angle-ply and two angle-ply layers are considered and the effects of different diameter-to-thickness ratios and different angle-ply combinations on the mechanical behaviour of these pipes have been studied.

Global design and analysis of deep sea FRP composite risers under combined environmental loads

The use of composite materials in offshore engineering for deep sea oil production riser systems has drawn considerable interest due to the potential weight savings and improvement in durability that can be achieved. The design of composite risers consists of two stages: (1) local design based on critical local load cases (LCs) to obtain the geometric configuration of the riser which will be analysed in the global design stage, and (2) global analysis of the full length composite riser under global loads including top tension force, platform motion, hydrostatic pressure, gravity, buoyancy, wave and current loads to determine and assess critical locations. This study describes the methodology, LCs, analysis procedure and results of the global design of the composite riser based on the geometries of the tubular optimised in the local design stage. The results show that a careful local design of the tubular using inclined reinforcements in addition to axial and hoop reinforcements can offer substantial weight savings and at the same time ensure that the structure is capable of withstanding the global loads applied on it.

Tailored local design of deep sea FRP composite risers

The use of fibre reinforced polymer (FRP) composite materials in offshore engineering for deep sea riser systems has drawn considerable interest due to the potential weight savings and improvement in durability that can be achieved. The design of FRP composite risers consists of two stages: (1) preliminary local design based on critical local load cases (LCs) and (2) global analysis of the full length composite riser under global loads including platform motion, hydrostatic pressure, gravity, buoyancy, wave and current loads to determine and assess critical locations. The preliminary local design stage is necessary to obtain a first estimate of the laminate configuration – fibre orientations and layer thicknesses of the tube wall, since the deformations and hence the forces and bending moments due to the global loads, depend on the geometric configuration. This paper describes the methodology, LCs, analysis procedure and results of the first stage, the local design of the composite riser. The local design is conducted using five different LCs as prescribed by the standards. In this study, geometric configurations of eight different composite body and liner combinations were optimised to provide minimum structural weight. Previous composite riser designs have mainly focused on axial and hoop reinforcements; in the present study, fibre reinforcements at other orientations are included. The highlight of the current study is that it shows that the use of off angle fibre orientations in addition to axial and hoop reinforcements offer substantial weight savings.

Piezoelectric film sensor for measurement of peel stresses in bonded joints

Adhesively bonded composite patches employed for repairing fatigue cracks in metallic airframe structural components often fail under peel stress generated in the structure. The application of piezoelectric stress sensors embedded within the bonded joint for direct measurement of the peel stresses is reported here. Polyvinylidine fluoride (PVDF) film of about 28 micron thickness coated with nickel copper is employed to construct hin sensors embedded between the composite patch and the metallic surface of the crotch joint specimen. PVDF sensor with varying sizes were constructed and calibrated using polycarbonate test specimens subjected to uniaxial tension and compression. The sensors were then embedded between the composite patch and the metallic surface of the crotch specimen to monitor the peel stresses in the adhesive. The measurements are compare with stresses in the adhesive. The measurements are compared with stresses predicted by finite element modeling.

Frequency measurement based damage detection methods applied to different cracks configurations

With advancements in sensor technology and structural health monitoring systems, the application of modal response measurements for online damage detection and assessment appears more promising than ever. This paper examines the use of natural frequency measurements of the structure for detection of cracks in beams. Two different crack configurations are considered, partial thickness through width edge cracks and partial width through thickness centre cracks. Here, crack is modelled as a torsional spring whose stiffness is proportional to the size of the crack. Changes in modal frequencies depend on the location and the size of crack. With three or more measured natural frequencies, the location as well as the spring stiffness and thus the size of crack is determined. Two different approaches, equilibrium and energy, are used in determination of location of cracks and the results are compared. Previous applications of the energy approach rely on analytical modelling of the structure to determine the mode shapes and modal strain energies. This has the disadvantage of being applicable only to simple structures with simple boundary conditions whose mode shapes can be determined analytically. In the present work it is demonstrated that discrete values of deflection mode shapes, which may be obtained from measurements on the structure, can be employed to determine the location and assess the size of the damage. This makes the proposed technique suitable for possible extension to complex structures which are not amenable to analytical modelling. A method to assess the crack size in elastic beams with through thickness-crack is also formulated. Experimental Modal Analysis (EMA) was performed on beams with simulated edge and centre cracks and the measured frequency changes are used to determine the damage location and size. Good agreement is obtained between predicted and actual locations and sizes of cracks for both crack configurations.

Measurement of residual stresses in polymer composites using moire interferometry

Moire interferometry is employed along with the hole drilling technique to determine residual cure stresses in symmetric cross poly graphite epoxy laminates. Traditional moire interferometry set-up using two collimated angle beams was employed to provide the virtual reference grating while a cross grating with a frequency of 1200 lines per mm was replicated on the specimen surface. Holes of different depths, each one penetrating one additional layer of the laminate, were drilled using a high speed air turbine drill to relieve the stresses in each layer sequentially. The strain distribution around each hole was computed from correlation of the undistorted carrier fringe pattern with the distorted fringe patterns around the holes. The measured strain distributions are compared to residual strain distributions predicted by classical laminate theory.