Showing papers in "Strain in 1982"

On the correction of residual stress measurements obtained using the centre-hole method

Abstract: Significant errors in interpreting the results of the centre-hole technique may arise where test conditions deviate from a uniform uniaxial low-level stress field. This paper demonstrates that where the stress field varies linearly with depth, the as-measured stress represents the stress at one-quarter hole depth. In cases where surface preparation for strain gauging has induced residual stress or where stress levels are sufficiently high to induce plasticity during hole forming, simple formulae are proposed to correct for these effects.

Effects of self heating when using a continuous bridge voltage for strain gauging epoxy models

Abstract: One source of concern when strain gauging epoxy materials, is that if the gauges are continuously energised at high power densities, then the heat generated by the strain gauges will cause large apparent strains and will reduce the modulus of the underlying material. The magnitude and significance of the heating effect are discussed and methods of reducing this effect are indicated.

A non‐destructive method to determine stresses in a three‐dimensional photoelastic model using a transmission polariscope

Abstract: The determination of stresses within a loaded three dimensional photoelastic model without cutting it into slices has been the ultimate aim of many investigators. The standard methods using a transmission polariscope do not yield much information. Scattered light methods no doubt enable one to completely determine the state of stress at a point within the body. However, the methods proposed up to now necessitate the use of an arrangement where both the body under test and the sensing unit which picks up the scattered light or both will have to be rotated. This leads to a complicated experimental set-up and increases the possibility of making errors, especially while rotating the model. In the proposed method, most of the information needed to determine the state of stress at a point within the body is obtained using the transmission polariscope set-up. The model and the direction of observation of scattered light are fixed. This allows observations to be made in test models which have at least one flat face without using a liquid with matched refractive index and increases the accuracy of measurements. The method is an almost purely transmission polariscope type of method and where space permits tests can be done using ‘live’ loading without having to use the stress freezing technique.

The prediction and measurement of axial forces, bending moments and accelerations in an engine connecting rod

Abstract: An engine connecting rod was fitted with two accelerometers, an axial force bridge and a bending moment bridge. These transducers enabled the acceleration components (along the connecting rod axis and perpendicular to it), the axial force and the bending moment, to be measured at the transducer locations. A theoretical method of analysis was carried out and a computer programme was developed, for use on a PET computer, to execute the repetitive calculations of the above mentioned quantities. The connecting rod was fitted into a Ford two-litre OHC automotive engine. A few components were manufactured to complete the test rig. Three engine tests were conducted using combinations of engine load and cylinder gas pressure. The outputs from the transducers were recorded on photographic film from a galvanometer recorder. The traces were then analysed to reveal the variation of the dynamic properties with crankshaft angle. The experimental and theoretical results of the three tests were compared. The experimental accelerations and axial force were in close agreement with the predicted results. However, the experimental and the predicted bending moment distribution, did not agree at all well.

Measurement of surface strains in GRP products

Abstract: Current work on fibre composite materials required the measurement of surface strains on glass reinforced plastics (GRP) products subjected to static loading. A study undertaken to account for anomalous results obtained with electrical resistance strain gauges showed that strains measured at a given load, increased with decreasing gauge length and foil gauge resistance. Measurement of localised strains using gauges of length up to 25 mm are subject to considerable discrepancy when compared with gauge lengths of less than 13 mm.

Developments in the measurement of reinforcement strain distributions in reinforced concrete members

Abstract: A technique is described for measuring in detail the longitudinal strain distribution along the reinforcement of reinforced concrete members by means of strain gauges installed in a central duct within the reinforcement. Results of a preliminary test are given. (Author/TRRL)

The behaviour of post yield strain gauges

Abstract: The calibration of post yield electrical resistance strain gauges in tension shows that their behaviour is strongly dependent upon gauge construction, bond strength, the presence of instability and the mechanism of plastic deformation. For medium tensile strains in the range 0-5% the calibrations reveal that the gauge sensitivity factor consists of separate elastic and plastic components which are respectively identified with recoverable and irrecoverable strain components.

Experimental comparison of solution methods for the axisymmetric photoelastic problem

Abstract: A method of component separation in axisymmetric photoelastic problems based on the use of the compatibility condition is experimentally tested against the more common methods of oblique incidence and subslicing The results show the method to be comparable in accuracy, but to have the distinct advantage of not requiring additional measurements other than those in the meridional plane

Determination of thermal stress in concrete pavements by means of measurement of in‐situ strains

Abstract: A new method of determining the thermal stress of concrete pavement in-situ has been developed and applied to engineering practice with success. It consists of three parts: (1) a system of theorems was established on the basis of mathematical modelisation that both temperature and thermal strain were sine functions of t, the time of observation, owing to the daily fluctuations of temperature and consequently the corresponding thermal strain. It was found that there exist a time phase and an angle of axes rotation between temperature and thermal strain. (2) A bridge circuit with a strain block and specially designed equipment "free body" was introduced. The strain transducer for every test point consists of one strain gauge in the strain block and one in the "free body", the former is used as the active gauge in the bridge circuit and the latter as the dummy. A temperature probe of resistance type was inserted nearby each of the gauges. Arrangement of observations and procedure of the data treatment were stated. (3) The application was made to concrete slabs. The examples to show the effectiveness of the method. By making use of the newly established theorems calculations were made with the selected data. The effectiveness of the new method was clearly shown by these examples. (Author/TRRL)

Two matrix transformations for the passage of polarised light through a stressed birefringent medium

Abstract: The passage of polarised light through a birefringent slice is dealt with using two simple matrix methods in this paper. These methods enable mechanical transformation procedures when one encounters a series of birefringent slices. The applicability of these methods is shown using classical polariscope set-ups used in photoelastic analysis.

Assessing the deterioration of plain and fibrous concrete subjected to flexural fatigue

Abstract: A flexural cyclic loading machine has been developed for an investigation into the fatigue characteristics of concrete containing small proportios of small steel fibres. Deterioration of large concrete beams caused by cracking was monitored by observation of changes in central deflection, variation of strain across the beam depth, midspan, electrodynamic modulus and ultrasonic pulse velocity. Typical results derived from ultimate load tests whose duration ranged from 0.02 seconds to several minutes and long term fatigue tests have been provided. (Author/TRRL)