Eddy current detection of fatigue cracks in drill pipes

Includes abstract.Includes bibliographical references.The Vertical Marine Mining technique is a lucrative method used by De Beers to recover bottom lying diamonds from the seabed, mainly off the coast of Namibia. The method employs a 200m length pipe-drilling structure rotated from the surface and acting as a conduit through which the disturbed sediment is transported to the surface for processing. However, cyclic operating stresses combined with corrosion have tended to cause circumferential fatigue cracks to develop on the outside surface of the drill pipe and propagate inwards. As no early warning system exists, these cracks propagate undetected until failure occurs. High operational costs and losses associated with down time, provide a strong motivation for a system to detect fatigue cracking prior to failure, especially given that fatigue crack growth rate and fatigue lifetimes can be estimated using Fracture Mechanics principles. Therefore what is required is an early warning of fatigue crack initiation with non-destructive testing (NOT). The eddy current method is an ideal NDT technique as it does not require contact with the test surface and is highly sensitive to fatigue cracks .. However, this method is generally only sensitive to surface and near surface defects. This proves to be a major limitation - the external drill pipe surface is obstructed by flanges and fittings such that continuous inspection of the external surface would be impossible. Therefore two specialised eddy current methods to allow the detection of external fatigue cracks from inside the drill pipe were reviewed, namely: Pulsed Eddy Current (PEC) and Remote Field Eddy Current (RFEC). It was found that unlike PEC, RFEC is highly sensitive to external circumferential planar defects in ferrous pipes. This, above all, highlighted the suitability of RFEC for drill pipe inspection

Measuring fracture properties using digital image and volume correlation: decomposing the J-integral for mixed-mode parameters

Thesis (PhD)--Stellenbosch University, 2017.ENGLISH ABSTRACT: The degradation of material properties over time is one of the core constituents of structural integrity monitoring. For this purpose, the materials’ resistance to brittle fracture is the most widely used property in the determination of safe operation and predicting component life. Measuring such properties by conventional methods presents challenges: large numbers of samples are needed, accounting for mixed-mode loading. Non-contact measurements by digital image correlation shows promise in extracting such parameters from in situ loaded structures, accounting for their complex geometry and loading. This thesis aims to develop a framework that allows for the extraction of mixed-mode brittle fracture properties, using three-dimensional (3D) image correlation techniques: multi-camera Digital Image Correlation (stereo-DIC), and Digital Volume Correlation (DVC). The -integral method was chosen for obtaining the fracture resistance, with the decomposition method used for determining separate − components (opening, sliding and shearing: modes I, II and III), and stress intensity factors (−), as both methods are resilient to crack tip and displacement errors. The first stage of development is a finite element based method for the removal of experimental displacement artifacts from DIC or DVC results, intended as a pre-processor for the -integral. Subsequently, a theoretical derivation is provided to link the two volumetric forms: the volume and the path-area integrals. This results in a proposed ‘hybrid’ integral, which benefits from the resilience to crack-front positional errors of the path-area approach, and resilience to random displacement noise of the volume integral. Initial testing of the -integral based decomposition method was on the surface using stereo-DIC and the Arcan fixture to induce mixed-mode loading. The results are verified with mixed-mode fracture toughness values measured from PMMA and compared to literature and ASTM 1820 tests. The typical image correlation errors close to fracture are quantified by a methodology of applying the -integral to analytical crack tip fields to which displacement errors are added artificially. It was found that the -integral is most prone to DIC errors under anti-symmetric (mode II/III) loading. Testing in the volume used X-ray computed tomography to acquire images and DVC for displacement maps. The measurements were verified on two configurations: a SENT polyurethane composite specimen (mode I), and a shear loaded inclined notch in Magnesium alloy-WE43 (mixed-mode). The -integral was verified against values from finite element fields resulting again in larger errors in mixed-loading. Decomposition of the volume integral requires an approach to separate the anti-symmetric -integral. Two extensions are proposed: the first using a ratio derived from mode II/III Williams series formulas, and the second using the interaction integral. Both approaches are verified on DVC displacements. This thesis finds that based decomposition offers a versatile method to extract − values from non-standard 3D geometries and loadings. However, the pre-processing of fields to minimise errors is essential when mode II-III displacement fields are prominent. Although it is beyond the scope of this thesis, the hope is that this work will assist in the adoption of full-field measurement techniques as a standard testing practice in structural integrity assessments, and lead to better informed maintenance and inspection schedules.AFRIKAANSE OPSOMMING: Die agteruitgaan van material-eienskappe met tyd is een van die kernkomponente van strukturele integriteit monitering. Vir hierdie doel is die materiaal se weerstand teen brosfraktuur die mees algemene gebruikte eienskap vir die bepaling van veilige werking en die voorspel van die komponent se leeftyd. Die meet van sulke eienskappe volgens konvensionele metodes bied uitdagings, bv. groot getalle monsters word benodig en komplekse kragte moet in ag geneem word. Nie-kontakmetings deur digitale-beeldkorrelasie toon belofte om sulke parameters uit in situ gelaaide strukture te onttrek, terwyl komplekse kragte en material vorme in ag geneem word. Hierdie proefskrif beoog om 'n raamwerk te ontwikkel wat die ontginning van gemengde-modus brosfraktuur-eienskappe moontlik maak deur gebruik te maak van driedimensionele (3D) beeldkorrelasietegnieke: Digitale Kamera-Korrelasie (stereo-DKK) en Digitale Volume Korrelasie (DVK). Die -integrale metode is gekies vir die verkryging van die breukweerstand, met behulp van die ontbindingsmetode vir die bepaling van afsonderlike − komponente (opening, gly en skeer: mode I, II en III) en stresintensiteitsfaktore (−), aangesien beide metodes skerm teen kraakpunt en verplasings foute. Die eerste fase van ontwikkeling is 'n eindige element gebaseerde metode vir die verwydering van eksperimentele verplasing artefakte van DKK of DVK resultate, wat bedoel is as 'n voorverwerker vir die -integraal. Vervolgens word 'n teoretiese afleiding verskaf om die twee volumetriese vorms te verbind, naamlik die volume en pad-area integrale. Dit lei tot 'n voorgestelde 'hibriede'-integraal. Hierdie ‘hibriede’-integraal vereis ‘n kompromie tussen die robuustheid van die pad-area-benaderingsfout van die pad-area-benadering en robuustheid van lukrake verplasings geraas van die volume-integraal. Aanvanklike toetsing van die -integraal gebaseerde ontbindings metode is op die oppervlak met behulp van stereo-DKK, en die Arcan greep om gemengde-mode laai te veroorsaak. Die resultate word geverifieer met gemengde-mode-fraktuur taaiheid waardes, gemeet vanaf PMMA en baseer op literatuur sowel as die ASTM 1820 toetse. Die tipiese beeldkorrelasie foute naby aan fraktuur word gekwantifiseer deur die toepassing van die -integraal op analitiese kraakpuntvelde waarby verplasingsfoute kunsmatig bygevoeg word. Daar is bevind dat die -integraal die meeste geneig is tot DKK foute onder asimmetriese (mode II/III) laai. Toets in die volume gebruik X-straal-rekenaartomografie om beelde en DVK vir verplasings kartering te bekom. Die metings is geverifieer op twee konfigurasies: 'n SENT saamgestelde-polyuretaan monster (mode I) en 'n skuinsbelaaide spleet in magnesium-allooi, WE43 (gemengde mode). Die -integraal is geverifieer teen waardes van eindige elementvelde wat weer groter foute in gemengde laai veroorsaak. Ontleding van die volume-integraal vereis 'n benadering om die anti-simmetriese -integraal te skei. Twee uitbreidings word voorgestel: die eerste met behulp van 'n verhouding wat afgelei word vanaf mode II / III Williams reeks formules, en die tweede maak gebruik van die interaksie integraal. Albei benaderings word geverifieer op DVK-verplasings. Hierdie proefskrif bevind dat gebaseerde ontbinding 'n veelsydige metode bied om − waardes uit nie-standaard 3D-geometrieë en ladings te onttrek. Die voorbereiding van velde om foute te beperk, is egter noodsaaklik wanneer die mode II-III verplasingsvelde prominent is. Alhoewel dit buite die omvang van hierdie proefskrif is, is die hoop dat hierdie werk sal help met die aanvaarding van volwaardige metingstegnieke as 'n standaard toetspraktyk in strukturele integriteit assesserings, en lei tot beter ingeligte onderhouds en inspeksieskedules

Interocular transfer of the movement aftereffect in central and peripheral vision of people with strabismus

Purpose. To compare binocularity in central and peripheral vision of people with early-onset strabismus and people with normal binocular vision. Methods. Ten subjects with early-onset strabismus, and nine subjects with normal binocular vision were tested. To assess binocularity, interocular transfer (IOT) of a rotary movement aftereffect (MAE) was measured. The MAE stimuli were either confined to the central 2.8 degrees of the visual field or were presented 10 degrees into peripheral vision. Results. In peripheral vision, there was no significant difference in IOT for the two groups of subjects. In central vision, there was a significant decrease of IOT in subjects with early-onset strabismus. Their IOT was, however, significantly greater than zero. Conclusions. Early-onset strabismus appears to spare binocularity in peripheral vision but reduces it in central vision. It does not abolish binocularity assessed by IOT of MAE, suggesting that some binocular connections survive early-onset strabismus, even in central vision

Aqueous shunts for glaucoma

BackgroundAqueous shunts are employed to control intraocular pressure (IOP) for people with primary or secondary glaucomas who fail or are not candidates for standard surgery.ObjectivesTo assess the effectiveness and safety of aqueous shunts for reducing IOP in glaucoma compared with standard surgery, another type of aqueous shunt, or modification to the aqueous shunt procedure.Search methodsWe searched CENTRAL (which contains the Cochrane Eyes and Vision Trials Register) (2016, Issue 8), MEDLINE Ovid (1946 to August 2016), Embase.com (1947 to August 2016), PubMed (1948 to August 2016), LILACS (Latin American and Caribbean Health Sciences Literature Database) (1982 to August 2016), ClinicalTrials.gov (www.clinicaltrials.gov); searched 15 August 2016, and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) (www.who.int/ictrp/search/en); searched 15 August 2016. We did not use any date or language restrictions in the electronic search for trials. We last searched the electronic databases on 15 August 2016. We also searched the reference lists of identified trial reports and the Science Citation Index to find additional trials.Selection criteriaWe included randomized controlled trials that compared various types of aqueous shunts with standard surgery or to each other in eyes with glaucoma.Data collection and analysisTwo review authors independently screened search results for eligibility, assessed the risk of bias, and extracted data from included trials. We contacted trial investigators when data were unclear or not reported. We graded the certainty of the evidence using the GRADE approach. We followed standard methods as recommended by Cochrane.Main resultsWe included 27 trials with a total of 2099 participants with mixed diagnoses and comparisons of interventions. Seventeen studies reported adequate methods of randomization, and seven reported adequate allocation concealment. Data collection and follow-up times varied.Four trials compared an aqueous shunt (Ahmed or Baerveldt) with trabeculectomy, of which three reported one-year outcomes. At one-year, the difference in IOP between aqueous shunt groups and trabeculectomy groups was uncertain (mean difference (MD) 2.55 mmHg, 95% confidence interval (CI) -0.78 to 5.87; 380 participants; very low-certainty evidence). The difference in logMAR visual acuity was also uncertain (MD 0.12 units, 95% CI -0.07 to 0.31; 380 participants; very low-certainty evidence). In two trials, the difference in visual field score was uncertain (MD -0.25, 95% CI -1.91 to 1.40; 196 participants; very low-certainty evidence). The mean number of antiglaucoma medications was higher in the aqueous shunt group than the trabeculectomy group in one trial (MD 0.80, 95% CI 0.48 to 1.12; 184 participants; low-certainty evidence). The effect on needing additional glaucoma surgery was uncertain between groups in two trials (risk ratio (RR) 0.24, 95% CI 0.04 to 1.36; 329 participants; very low-certainty evidence). In one trial, fewer total adverse events were reported in the aqueous shunt group than the trabeculectomy group (RR 0.59, 95% CI 0.43 to 0.81; 212 participants; very low-certainty evidence). No trial reported quality-of-life outcomes at one-year follow-up.Two trials that compared the Ahmed implant with the Baerveldt implant for glaucoma found higher mean IOP in the Ahmed group at one-year follow-up (MD 2.60 mmHg, 95% CI 1.58 to 3.62; 464 participants; moderate-certainty evidence). The difference in logMAR visual acuity was uncertain between groups (MD -0.07 units, 95% CI -0.27 to 0.13; 501 participants; low-certainty evidence). The MD in number of antiglaucoma medications was within one between groups (MD 0.35, 95% CI 0.11 to 0.59; 464 participants; moderate-certainty evidence). More participants in the Ahmed group required additional glaucoma surgery than the Baerveldt group (RR 2.77, 95% CI 1.02 to 7.54; 514 participants; moderate-certainty evidence). The two trials reported specific adverse events but not overall number of adverse events. Neither trial reported visual field or quality-of-life outcomes at one-year follow-up.One trial compared the Ahmed implant with the Molteno implant for glaucoma over two-year follow-up. Mean IOP was higher in the Ahmed group than the Molteno group (MD 1.64 mmHg, 95% CI 0.85 to 2.43; 57 participants; low-certainty evidence). The differences in logMAR visual acuity (MD 0.08 units, 95% CI -0.24 to 0.40; 57 participants; very low-certainty evidence) and mean deviation in visual field (MD -0.18 dB, 95% CI -3.13 to 2.77; 57 participants; very low-certainty evidence) were uncertain between groups. The mean number of antiglaucoma medications was also uncertain between groups (MD -0.38, 95% CI -1.03 to 0.27; 57 participants; low-certainty evidence). The trial did not report the proportion needing additional glaucoma surgery, total adverse events, or quality-of-life outcomes.Two trials compared the double-plate Molteno implant with the Schocket shunt for glaucoma; one trial reported outcomes only at six-month follow-up, and the other did not specify the follow-up time. At six-months, mean IOP was lower in the Molteno group than the Schocket group (MD -2.50 mmHg, 95% CI -4.60 to -0.40; 115 participants; low-certainty evidence). Neither trial reported the proportion needing additional glaucoma surgery, total adverse events, or visual acuity, visual field, or quality-of-life outcomes.The remaining 18 trials evaluated modifications to aqueous shunts, including 14 trials of Ahmed implants (early aqueous suppression versus standard medication regimen, 2 trials; anti-vascular endothelial growth factor agent versus none, 4 trials; corticosteroids versus none, 2 trials; shunt augmentation versus none, 3 trials; partial tube ligation versus none, 1 trial; pars plana implantation versus conventional implantation, 1 trial; and model M4 versus model S2,1 trial); 1 trial of 500 mm2 Baerveldt versus 350 mm2 Baerveldt; and 3 trials of Molteno implants (single-plate with oral corticosteroids versus single-plate without oral corticosteroids, 1 trial; double-plate versus single-plate, 1 trial; and pressure-ridge versus double-plate with tube ligation, 1 trial).Authors' conclusionsInformation was insufficient to conclude whether there are differences between aqueous shunts and trabeculectomy for glaucoma treatment. While the Baerveldt implant may lower IOP more than the Ahmed implant, the evidence was of moderate-certainty and it is unclear whether the difference in IOP reduction is clinically significant. Overall, methodology and data quality among existing randomized controlled trials of aqueous shunts was heterogeneous across studies, and there are no well-justified or widely accepted generalizations about the superiority of one surgical procedure or device over another