Showing papers in "Strain in 1987"

Mean stress dependence of the thermoelastic constant

Abstract: The SPATE 8000 thermoelastic stress analyser has been used to demonstrate the mean stress dependence of the thermoelastic constant. This dependence has potential application in the measurement of residual stress provided the material has not yielded.

The measurement of the distribution of residual stresses through the thickness of a welded joint

Abstract: A new method for the measurement of residual stresses in welded joints of complex cross section is described. The method involves the measurement of strain changes in the body as a narrow slot is cut in the plane of interest, in small increments of depth. A finite element model of the cross section is used to relate the strain changes at the measurement points to the residual stresses across the slot plane. The accuracy of the method is demonstrated by using it to measure a known residual stress field in a cold bent bar. The method has been used in combination with a block removal technique to measure the through wall distribution of axial residual stresses at a circumferential butt weld in a cylinder with a protruding root bead. It could also be applied to the measurement of residual stresses in other joint configurations, such as fillet welds or T-butt welds.

Simultaneous combination resonances in a parametrically excited cantilever beam

Abstract: The effects of parametric excitation of a simple structure are such that very large responses may be generated in a plane perpendicular to that of the excitation as a result of relatively small accelerations, provided that the frequency of the excitation is related to that of the excited mode or modes in a certain manner. It is shown that it is possible for two or more resonances to be excited simultaneously, and that an effect generated by a weaker type of coupling can in fact modify that of a stronger coupling to a significant extent. A vertically oriented, thin and flexible cantilever beam of rectangular uniform cross-section with a lumped end inertia, is modelled both to first and second order of approximation, and theoretical and experimental results for the stability zoning of the resonances are presented. An additional model is proposed which examines the combined effect of two resonances for two tuning cases, and this is compared with measurements of vibratory responses and strain on an experimental system. It is thus shown that the theoretical model postulated for the simultaneous excitation of two resonances accurately predicts the observed behaviour of the laboratory system within a specific range of excitation accelerations.

Application guidelines for the parting out step in a through thickness residual stress measurement procedure

Abstract: The first step in an often used destructive procedure for determining through thickness residual stresses in pipes and plates is a parting out step in which a coupon is removed from the parent material while strain gauges monitor the coupon's deformations. The changes in stress that occur at the gauged surfaces when the coupon is parted out, together with information about the stresses in the parted out coupon are used in a back computation method to estimate the stress distributions in the coupon before it was parted out. Either of two back computation methods can be used. One method assumes that the through thickness stress distribution changes that occur when the coupon is parted out lie on a straight line. This method is computationally quick and easy to use, but can be inaccurate if the lateral dimensions of the coupon are too small. The other method uses influence coefficients and greatly extends the range of coupon dimensions for which accurate results can be obtained; but, it is more elaborate than the straight line model and involves two rather time consuming steps. This paper presents guidelines for determining when the more elaborate influence coefficient method must be used in place of the straight line model in terms of the error that the simpler model would produce in the estimated residual stress distribution. This paper also simplifies the application of the influence coefficient method by tabulating the results of the two time consuming steps over the range of coupon dimensions for which this method is typically required. Example problems using the two back computation methods are presented.

A selective diffraction order based lens plane grating shearing interferometer for the study of bent plates

Abstract: A simple, elegant and flexible technique for the determination of partial slope, curvature and twist contours of bent plates is proposed. A single frequency cross-grating (SFCG) placed just ahead of the field lens collects the light reflected by the smooth plate model. From the resultant diffraction pattern formed at the Fourier transform (FT) plane, based on the desired contours, only a few orders are permitted selectively to pass through to the recording plane. This manner of real time in-situ filtering of the diffraction orders is new. While slope contours require double exposures, curvature and twist contours are obtained from single exposures.

Simultaneous X‐ray measurements in‐situ of tri‐axial stresses, Poisson's ratio and the stress free lattice spacing

Abstract: The tri-axial X-ray stress analysis allows, together with boundary conditions and fitting curves, a control of measurements and calculations for the determination in-situ of Poisson's ratio and stress free lattice spacing. A complete internal stress field analysis is discussed in terms of the ‘X-ray integral method’ on rolled steel discs of 100 Cr-6 (ASTM: A 295, BS: 970/2).

Wire tensions and bending moments in axially loaded seven wire strand

Abstract: Carefully instrumented tensile tests were performed on a straight single steel strand of seven-wire construction having core and helical wire diameters of 3–66 mm and 3–33 mm, respectively, and a lay angle of 14-6d. Strain gauges in groups of three with their grid axes parallel and having a common backing were attached to all six helical wires at the mid-strand position. For loading within the linear range of the load-extension response (all tests) and load-rotation response (in tests with free and partial torsional restraint of the ends) the outputs from these strain gauges were used to compute wire tensions and bending moments. Strand extension, rotation and torque generated under axial load (fixed-end and partial torsional restraint) were also recorded. Test results showed that the share of the total strand load taken by the helical wires decreased as the torsional restraint on the strand was reduced. The mean tension in the helical wires was found to be 27-5% smaller in the free-end tests than in fixed-end tests for a given strand load. Theoretical predictions underestimate this reduction in tension by up to 80%. Bending moments in the wires were found to be about 8% less than predicted in the fixed-end condition and over 20% smaller in the free-end condition. As expected, the slope of the linear regions of the load-extension and load-rotation plots decreased as the torsional restraint was reduced. However, the rate of decrease in the case of load-extension plots was found to be less than predicted theoretically and the rotation measured was generally less than predicted theoretically. This is consistent with the helical wire tensions being smaller than the predicted values since the tangential components of these reduced tensions would have less effect in their unwrapping action on the strand under increasing axial load.

Velocity and force measurements in the splash zone

Abstract: A method for measuring fluid velocity in waves in the splash zone is demonstrated. The method uses Laser Anemometry coupled to accurate microcomputer control of the wave generation. Results are presented for the developing flow field in a plunging breaking wave in deep water. The phenomenon of slam when waves hit members of offshore structures causes large forces with small rise times. The design of a rig capable of measuring such forces in laboratory produced breaking waves is presented. Measurements on a small cylinder show that maximum forces are present at the height where the plunging spout forms and that these forces can be up to five times the drag force.

Precision equipment for symmetrical bending— Part 1. The loading system

Abstract: The design and performance of precision bending equipment for testing straight beams with constant rectangular section under a pure bending moment is described. Part 1 deals with the loading arrangements and Part 2 with the rig for measuring beam deflection. The methods for recording deflection of strain gauges applied on the beam have been described elsewhere.1–4 The reactions to the deadweight gravity loads are taken via ball bearings. The total bending moment inaccuracy, i.e. the sum of estimated systematic errors and random errors, the latter expressed as the 2Ga confidence interval about the mean (2Gcim), is about ± 25 x. 10-4%, for 23 tests with a steel beam with 15 mm × 75 mm cross sectional dimensions, loaded to 1-1.5 mm/m strain. The measuring rig stands on the beam via hardened and polished spherical segment feet and uses a capacitive transducer system with less than 5nm resolution. Various application tasks are described. The total inaccuracy of the measured deflection, i.e. apart from bending moment inaccuracy, is about ± 35 times 104% for about 10 reproduced, not repeated, tests with the same steel beam. The deflection for such a beam is of the order of 2 to 4 mm.

A prototype for a resistive extensometer as a strain measuring standard

Abstract: The design, handling and calibration of a prototype to a resistive extensometer with 10 mm measuring length is described. It stands on the test specimen via pins with conical tips. These end in hard metal spherical calottes with 16 micron nose radius. The manufacturing and maintenance of these tips was described earlier. The extensometer is calibrated on a bending beam to 1 mm/m strain, using the loading and deflection equipment developed at the FFA and described earlier. The total extensometer inaccuracy, i.e. the sum of estimated systematic errors and random errors, the latter expressed as the 2σ confidence interval about the mean, is about 400 × 10-4%, for 60 reproduced calibration tests. With the extensometer, it was shown that the beam, which was machined and hardened from an alloy with 1.4% Cr and 1.4% Ni and has a moderate hardness of Hv 430, does not show any kind of inhomogeneity in Young's modulus, within the resolution of the indicator. Therefore, the extensometer is suitable for the determination of bonded resistance gauge factors, even if a total error of only ±0.1% is required. The prototype described can therefore be used as a strain measuring standard.

An instrument for detecting arbitrary peaks and troughs of a fluctuating stress signal

Abstract: An instrument has been developed to detect arbitrary peaks and troughs of a fluctuating stress signal This information is used to perform a fatigue life prediction The instrument uses an analogue method of detecting peaks and troughs This is considered to be cheaper and faster than the digital alternative A microprocessor was employed to read peak/trough values from seven such channels and perform a ‘rainflow’ cycle count of the stress histories The equipment has been tested on the Humber Suspension Bridge and has been shown to work satisfactorily

Self-compensated high plastic resistance strain gauges for use up to 300°C

Abstract: The development of a high plastic strain gauge for use up to 300dC is presented. This kind of gauge can measure strain up to 8% and possesses good self compensation on carbon steel. Basic analysis, gauge wire, adhesive material, gauge structure and examples of application are also described.

Strain distribution along lapped joints in reinforced concrete

Abstract: Strain measurements along lapped joints in reinforced concrete tension members were obtained using a method of internally strain gauging the reinforcing rods. Strain concentration gauges, installed at the lap ends of some specimens, demonstrated the localised nature of force transfer prior to crack formation. Results from ten test specimens of varying lap length and rod diameter have been produced. The changes observed in the longitudinal strain distributions, and the associated bond and concrete stresses as cracks developed are presented.

A technique for dynamic tensile testing of metals

Abstract: A technique is described for obtaining full dynamic stress/strain curves in tension up to strain rates of 100s-1. In conjunction with a high speed servo-hydraulic machine, a low inertia extensometer has been developed to facilitate measurement of specimen tensile strain up to fracture. The benefits of the technique are highlighted in relation to currently available methods for dynamic stress/strain measurement.