Thermoelastic investigations for fatigue life assessment

An investigation is presented on the suitability and accuracy of a thermoelastic technique for the analysis of fatigue cracks. The stress intensity factor ranges ΔKI and ΔKII are determined from thermoelastic data recorded from around the tip of a sharp slot in a steel specimen under biaxial load, in order to assess the accuracy of the technique. ΔKI and ΔK II are determined to within 4% and 9% of a theoretical prediction, respectively. The results from a similar test on a fatigue crack under biaxial load are also presented. These show that thermoelastic stress analysis is a rapid and accurate way of analyzing mixed-mode fatigue cracks. A discussion is given on the potential of thermoelastic stress analysis of propagating cracks

Full-field pulsed-magneto-photoelasticity - a description of the instrument

This paper describes a novel instrument used for the analysis of full-field through-thickness stress distributions using the theory of magneto-photoelasticity (MPE) developed by Aben and Clarke et al. [ , , ]. MPE is an experimental stress analysis technique which involves the application of a magnetic field parallel to an electromagnetic wave propagating through a birefringent model within a polariscope. The effect viewed through the polariscope is then a combination of the model’s birefringence and the Faraday rotation created in the model by the magnetic field. Aben developed this technique especially for use in the measurement of stress profiles where the integrated photoelastic pattern alone yields little information. Clarke et al. developed MPE in order to study toughened glass. To date, the technique of MPE has been a single-point measurement and this is of limited utility in the investigation of 3D stress in toughened glasses. The pulsed-magneto-polariscope (PMP), described here, enables the full-field application of MPE. This paper contains a description of the novel apparatus, and demonstrations used to validate the performance of a proof-of-concept PMP instrument. The paper also highlights improvements in the application of MPE which are now possible with this new equipment. These improvements include the extension of MPE to larger areas of analysis, 3D stress analysis and the possibility of analysing a general unknown stress distribution

The impact of loads on standard diameter, small diameter and mini implants: A comparative laboratory study

Objectives: While caution in the use of small-diameter (≤3.5 mm) implants has been advocated in view of an increased risk of fatigue fracture under clinical loading conditions, a variety of implant designs with diameters <3 mm are currently offered in the market for reconstructions including fixed restorations. There is an absence of reported laboratory studies and randomized-controlled clinical trials to demonstrate clinical efficacy for implant designs with small diameters. This laboratory study aimed to provide comparative data on the mechanical performance of a number of narrow commercially marketed implants. Materials and methods: Implants of varying designs were investigated under a standardized test set-up similar to that recommended for standardized ISO laboratory testing. Implant assemblies were mounted in acrylic blocks supporting laboratory cast crowns and subjected to 30° off-axis loading on an LRX Tensometer. Continuous output data were collected using Nexygen software. Results: Load/displacement curves demonstrated good grouping of samples for each design with elastic deformation up to a point of failure approximating the maximum load value for each sample. The maximum loads for Straumann (control) implants were 989 N (±107 N) for the 4.1 mm RN design, and 619 N (±50 N) for the 3.3 mm RN implant (an implant known to have a risk of fracture in clinical use). Values for mini implants were recorded as 261 N (±31 N) for the HiTec 2.4 mm implant, 237 N (±37 N) for the Osteocare 2.8 mm mini and 147 N (±25 N) for the Osteocare mini design. Other implant designs were also tested. Conclusions: The diameters of the commercially available implants tested demonstrated a major impact on their ability to withstand load, with those below 3 mm diameter yielding results significantly below a value representing a risk of fracture in clinical practice. The results therefore advocate caution when considering the applicability of implants ≤3 mm diameter. Standardized fatigue testing is recommended for all commercially available implants

T-stress determination using thermoelastic stress analysis

T-stress and mixed-mode stress intensity factors have been determined experimentally using thermoelastic stress analysis and using a finite element method. Pure mode I, strong mixed-mode I and II, and interacting cracks have been used as the case studies. A new technique has been proposed to identify the crack tip from thermoelastic images. It has also been shown that using three terms of Williams's stress field formulation to determine the T-stress, yields a more accurate solution than using only the first two terms of the expansion

Investigating mixed-mode (I/II) fracture in epoxies using digital image correlation: Composite G(IIc) performance from resin measurements

The digital image correlation technique is applied to investigate mixed-mode (I/II) fracture in five aerospace epoxy formulations, four of which are experimentally toughened. Stress intensity factors are extracted from displacement fields using the Williams method for a range of mode mixities. From these measurements, values of an effective resin KIIc are deduced and these are shown to have a statistically significant relationship with measured composite GIIc mode II toughness values. The differences in constraint between composite and bulk resin specimens are discussed

Repeatable pre-cracking preparation for fracture testing of polymeric materials

Currently, in order to create a sharp pre-crack in a polymeric material for fracture toughness testing, a hand-held razor blade is used. This technique produces cracks of varying angle, length, and crack-front shape. Additionally, there are considerable safety concerns regarding the handling of sharp razor blades. A repeatable, safe method of producing a consistent, sharp pre-crack of a specified length, orientation and crack front is presented here, by use of a simple custom fixture. The specimen preparation procedure has wide applicability for fracture analysis of many brittle materials

A review of using thermoelasticity for structural integrity assessment

The advances in the use of thermoelastic stress analysis (TSA) for fracture mechanics assessment are reviewed. The development of techniques to determine stress intensity factor is presented followed by the application of these techniques to fatigue crack growth, crack closure and the study of mixed mode cracks

A review of using thermoelasticity for structural integrity assessment

The advances in the use of thermoelastic stress analysis (TSA) for fracture mechanics assessmentare reviewed. The development of techniques to determine stress intensity factor is presented followed by theapplication of these techniques to fatigue crack growth, crack closure and the study of mixed mode cracks

In situ observation of NiTi transformation behaviour: A micro-macro approach

A novel experimental investigation is presented of thermally and stress induced transformation behaviour of a Polycrystalline NiTi Shape Memory Alloy (SMA) plate for flexural-type applications: In situ techniques are employed to allow simultaneous macroscopic and microstructural observation of the SMA in a 4-point flexural test. Forming part of a wider research towards realising a NiTi SMA Variable Stator Vane assembly for the gas turbine engine, the study explores variables critical to flexural-type morphing NiTi structures: (1) temperature; (2) strain; and (3) cyclic loading. It builds a relationship between the macro and micro response of the SMA under these key variables and lends critical implications for the future understanding and modelling of shape memory alloy behaviour for all morphing applications. This paper presents the methodological aspects of this study

Developing a soft tissue surrogate for use in photoelastic testing

An improved skin tissue substitute for use in photoelastic testing is required to enable investigation of the mechanics of needle insertion into soft tissue. Current tissue substitutes are mainly used in large scale testing and can neglect the small scale mechanical properties of soft tissue. A series of experiments on konjac glucomannan are performed to characterise its mechanical properties, and the results are compared to published results from similar experiments on skin tissue. The optical properties of the gel, such as its strain optic coefficient, are also assessed using a grey field polariscope (GFP2500). A concentration of 1.5% konjac to water produced a viscoelastic gel whose mechanical response closely matches published data for skin. A strain optic coefficient was recorded and found ideal for the planned testing with a GFP2500. Overall konjac glucomannan was found to be a potential soft tissue surrogate for use in small scale photoelastic testing