Abstract: By combining modern surface strain measurement techniques with the traditional tensile test mechanical test, a method has been developed whereby the mechanical properties of a test sample with a heterogeneous microstructure such can be characterised in one test. In this paper the applicability to such a method of two such surface strain measurement techniques, image correlation (IC) and electronic speckle pattern interferometry (ESPI), is assessed. Two commercially available systems were used to monitor, simultaneously, the surface strain on tensile
test specimens during testing. Measurements on homogeneous samples were compared with measurements made using strain gauges and excellent agreement was found. Elastic modulus and proof stress values measured in these standard samples showed that the uncertainty in the measurements was below 10%. The method was then applied to an inertia friction weld, whose strength varied linearly as a function of distance from the weld line. The values of proof stress obtained were compared with micro-hardness measurements.
Abstract: A review of the Stress Concentration Factors (SCFs) obtained from normal and oblique holes in thick flat plates loaded in uniaxial tension has been conducted. The review focuses on values from the plate surface and discusses the ramifications of making a plane stress assumption.
Abstract: In this work optical microscopy in situ tensile testing has been carried out to study the localisation of surface strain in fully lamellar titanium alloys. The localised strain was measured by analysing the recorded optical micrographs using image correlation, and the orientation of the a lamellae colony was determined by using Electron Back Scatter Diffraction (EBSD). The results show a localisation of strain, which seems to depend on the crystallographic orientation of the a lamellae colony in the lamellar microstructure. No significant strain was observed in the former b grain boundary region.
Abstract: The advent of new thermoelastic stress analysis (TSA) technology has widened the industrial applications of the technique. Three case studies where data have been taken in previously inaccessible or unfeasible cases are presented. The case studies, taken from both the aerospace and automotive industries, will demonstrate the novelty of these applications and highlight the industrial advantages gained in terms of time saving and quality of results.
Abstract: This paper describes initial investigations into the use of frozen-stress photoelasticity to study the development of slippage within shrink-fitted shaft/hub pairs, with the aim of studying the residual stresses remaining after partial slippage has occurred. Results to date show the feasibility of measuring the shear stresses along the partially-slipping interface between shaft and hub.
Abstract: A joint Brunel-Southampton Universities’ research team has developed digital strain
gauges based on a metallic triple-beam resonator structure with thick-film piezoelectric sensor elements. The resonator, an oscillating structure vibrating at resonance, is designed such that its resonant frequency is a function of the measurand. The resonator substrate was fabricated by a double-sided photochemical etching technique and the thick-film piezoelectric elements were
deposited by a standard screen-printing process. The new metallic digital strain gauges can be used on stiff structures, have high overload capacities, low power consumption, frequency output for digital processing, and offer prospects for wireless-batteryless operation. The device can be easily mass-produced at low cost for use in a wide range of measuring systems, e.g. load cells, weighing
machines, torque transducers and pressure sensors.
Abstract: The impedance-based structural health monitoring method is used to successfully detect different damage mechanisms in composites and to correlate the changes in impedance measurements with the changes in the structure. Specifically, graphite/epoxy composite samples are manufactured and tested. Piezoceramic (PZT) patches are attached to the composite coupons to actuate the structure with high-frequency excitations. Bonding the patches to the sample allows changes in the structural mechanical impedance to be monitored with the electrical impedance of the PZT. Samples are tested using quasi-static tensile loading to introduce damage. To determine the extent of damage incurred, impedance signatures are acquired before and after the tensile load is applied. A change in impedance from the baseline shows the presence of damage. The amount of damage is characterized using a damage metric. Radiography is used to verify the extent of damage.
Abstract: The measurement of changes in the properties of ultrasonic Lamb waves propagating through structural material has frequently been proposed as a method for the detection of damage. In this paper we describe work that uses optical fibre sensors to detect the Lamb waves and show that the directional properties of these sensors allow us to not only detect damage, but also to locate it. We look at two types of optical fibre sensor, a polarimetric sensor and the fibre Bragg grating.
The polarimetric sensor measures the change in birefringence of a fibre caused by the pressure wave of the ultrasound acting upon it. This is an integrated sensor since the fibre length bonded to the sample needs to be greater than the ultrasonic wavelength in order to obtain the required sensitivity. The maximum sensitivity of this sensor is when the fibre is positioned normal to the direction of wave propagation. Fibre Bragg gratings are essentially point sensors since the grating
length needs to be a fraction of the ultrasound wavelength to obtain maximum sensitivity. Ultrasound is detected mainly through the in-plane strain it produces and maximum sensitivity is therefore produced when the grating is aligned parallel to the direction of wave propagation. Holes drilled into sample plates can be detected using both type of sensor by examining the changes in either the transmitted Lamb wave or through detection of the reflections produced by the hole. The sensitivity of the technique is shown to be determined by the relative positions of the acoustic source, the hole and the sensor. If we use fibre Bragg gratings in a rosette configuration (i.e. 3 gratings forming an equilateral triangle) then the direction of the Lamb wave can be determined using the directional sensitivities of the gratings. Using two such rosettes allows us to calculate the source of the wave from the intersection of two of these directions. If the source of the wave is the hole (which acts as a passive source), then the location of that hole can be determined.
Abstract: A means of calibrating the effect of temperature on the thermoelastic signal obtained
from the Deltatherm system is described. A design of a suitable calibration device is covered and sample results presented and discussed.
Abstract: Mechanical test machines play an important role in providing simulation data to underpin careful analysis of high rate processes. In many cases elements of the test frame or testpiece can vibrate when subjected to a high rate force. The transducers attached to the machine frequently register this vibration superimposed on the true signal and thus may mask the signal required for analysis [1-4]. At impact all the parts of the test machine oscillate at their natural frequency
(dependent on stiffness and mass). Ringing in test machines is determined by the rate at which the testpiece is deformed, the elastic characteristics of test material and resonances in components of load train anvils in compression. In the current work high rate hot compression tests are being developed to underpin best
practice measurement guides on test methods for studying the dependence of strength on deformation rate and temperature, [5-7]. At higher rates of deformation ringing is observed in the transducer signals for load and displacement and this measurement note summarises the results of systematic tests to investigate this issue. A purpose built software tool was developed for analysing the time dependence of the transducer signals.