Showing papers in "Strain in 1980"

The application of optical fibres as witness devices for the detection of plastic strain and cracking

Abstract: Low cost optical fibres have recently become readily available for telecommunications purposes. Silica fibres are characterised by high elastic strains to failure. The feasibility of using these fibres for structural integrity monitoring particularly for off-shore structures is investigated. The basis of the technique is that a fibre may be bonded to a critical part of a structure and provides an optical path which will be broken if the fibre fails due to plastic strain or crack opening in the critical area. Groups of fibres which have been given predetermined fracture strains by surface etching were encapsulated in special packs. These packs were bonded to steel and concrete tensile specimens. Strain transfer occurred successfully between the specimens and individual fibres. The distribution of strain to fibre fracture appeared to be uniform along the fibre. The use of several fibres with a range of fracture strains caused fibres to break progressively with increasing strain. For applications to offshore structures it has been found possible to use water-repellent adhesives which can be applied and cured in sea water and suffer no deterioration. The advantages of this system include versatility, relatively low cost, adaptability to continuous monitoring and the possibility of being fitted retrospectively and refitted after repair operations.(a)

Behaviour of underwater enclosures of optimum design

Abstract: Experiments are described which support the application of the membrane theory of shell analysis in determining optimum shapes for underwater structures of constant strength. A relationship between marine life and such structures is considered and design procedures for shape selection are given. Proposals for the investigation of the behaviour of such structures under loadings other than the initial design forces are outlined both from an analytical! numerical and an experimental approach.

Polycarbonate as a photoelastic material

Abstract: The use of commercial grade polycarbonate for photoelastic models is evaluated and hints given on methods of annealing, machining, stress freezing and preparation of slices for viewing.

A note on curvature and twist measurements using the Ligtenberg‐Moiré method

Abstract: In this note an alternative technique for measuring curvature and twist is discussed. The method relies on measuring the line spacing in a single exposure reflected pattern. Use of this technique appears to be preferable in many applications.

DIY strain gauge transducers (Part 1)

Abstract: Although ‘commercial’ transducers are readily available it is sometimes necessary to build a transducer by bonding gauges to part of an existing structure, eg. a tension link or a propeller shaft.

A semidestructive method for three dimensional photoelasticity

Abstract: Using the principle of optical equivalence and the experimentally determined characteristic parameters, it is possible to determine the variations of the secondary principal stresses and their directions across the thickness of a slice taken from a three dimensional photoelastic model. With these data, the shear difference method can be applied without cutting the slice into a subslice. This technique is applied to determine the stresses along a general line in a cone subjected to a compressive load at the apex. In practice, a fairly thick slice can be taken for analysis.

Measuring and calculating GTOD and the J-integral with a double clip gauge

Abstract: This paper describes a technique for measuring and calculating CTOD (crack tip opening displacement) arid the J-integral with double clip gauge. CTOD (or δ) and the rotation factor (r), the relation between δ and J-integral, as well as the R-curves δ - Δa and J. Δa are discussed. From the measurements with double clip gauge, one can calculate CTOD and J througout the entire loading process. The present technique seems to be a better approach than DD19, in which r is taken as a constant (r = 1/3).

Effect of temperature on vibrating wire strain gauges

Abstract: Vibrating wire strain gauges attached to buried pipe can exhibit changes in strain readings when subjected to changes in ground temperature. It is essential that the correct interpretation is placed upon the readings. An analysis is given which, it is suggested, will give true pipe strain values from the indicated strain readings. Two solutions are outlined, a graphical analysis which tends to be intuitive and a theoretical analysis which derives a solution from first principles. True strain in the pipe is influenced by thermal effects due to changes in ground temperature, and mechanical effects due to the constraint placed upon the pipe by the surrounding soil. Both analyses consider the thermal and mechanical components of true strain and the theoretical analysis enables these individual components to be determined from the indicated strain readings. (a) (TRRL)

An electrical demountable strain transducer

Abstract: This paper describes the design and application of a simple transducer developed by the Cement and Concrete Association to study the properties of reinforced and prestressed concrete structures. Minimal surface preparation and a simple method of fixing was required. The transducer was to be able to span any cracks that may develop in the specimen, have an output comparable with the Demec gauge, be capable of easy construction and repair. The transducer evolved was basically a clip gauge, and was made in three lengths: 50 mm, 100 mm and 200 mm. A flexible strip is used, mounted on two blocks with conical pins driven into them to provide a means of location to the Demec stand. The assembly is attached to the specimen by means of tension springs hooked into short lengths of aluminium angle bonded onto the specimen. A bridge circuit, energised by 5 V DC, is used to give full scale output of approximately plus or minus 10 millivolts corresponding to plus or minus 12700 microstrain. The author describes a calibration rig and shows that the transducers can be made to meet the specification for a grade C extensometer. (TRRL)

A study of the saddle stresses arising from the support of a cylindrical vessel

Abstract: The design method widely used for the support of cylindrical vessels is that contained in BS 55001 which is based on the work of Zick2 published in 1951. In recent years, however, the theoretical analysis of this problem has received some attention,3–7 thus enabling the vessel stresses and displacements to be determined for supporting saddles of various degrees of flexibility. In this paper the saddle itself is examined, 1) experimentally, using photoelasticity, and 2) analytically by the shell analysis of refs5–7, together with a Finite Element approach. It is found that the theoretical approach used to determine saddle stresses is very sensitive to— (a) the type of finite element used for the saddle, and (b) the boundary assumption at the saddle base. From a general stress analysis point of view, the paper should provide a warning to designers that the results obtained from finite element analyses always contain the above inherent problems. A comparison with the photoelastic results enables this to be studied. In particular the agreement between the theoretical and experimental approaches lends credence to the analytical approach used in refs5–7 and to the value of an optimum design approach for the vessel and its support.

Steady state strains in power transmitting flat belts made of composite material

Abstract: Strain data for a simple two pulley belt drive was obtained using a photoelastic coating technique. The operating strains present were not constant along the driving and load sides but varied in an irregular manner. The average strain gradually increased on the driving side and decreased on the load side as the power transmitted was increased.

The torsional stiffness of prismatic beams of isosceles triangular cross section: a survey of methods for small included angles

Abstract: Six approximate methods of calculating the torsional stiffness of prismatic beams of isosceles triangular cross section are compared with the exact solution for the cross section in the form of a sector of a circle.

Stresses in a 60° perforated wedge

Abstract: This paper deals with the determination of stresses around a circular hole in a cantilevered wedge of uniform thickness subjected to end loading, using electrical resistance strain gauges. The authors have previously reported a similar investigation on a 45 wedge. The results obtained are compared with the theoretical analysis for an infinite perforated wedge given by Ling and Hsu. The stress concentration factors are calculated on the basis of nominal stress in the wedge at the hole boundary (using the theory for calculating stresses in a wedge without a hole). A maximum stress concentration factor of 3–2 was indicated for an end vertical load; and a maximum factor of 2 6 was indicated for an end horizontal load.