The ACPD method is probably the most versatile of all the commercially available NDE techniques. However as applications of such systems increase so does the awareness of serious limitations in present ACPD knowledge. In particular high local crack and notch tip strains can have a marked effect on ACPD response leading to substantial errors in estimates of crackdepth and growth rates. In the present study an investigation has been undertaken into the influence of elastic/plastic notch tip strain on the response of ACPD crack monitoring systems. Experimental work has been undertaken to produce data on the ACPD response observed in two magnetically contrasting materials (EN1A mild steel, NE8 aluminium alloy) using a series of V and U notched bend specimens. An extensive elastic/plastic finite element analysis was conducted to accurately determine the different notch tip strain fields for both materials. A fundamental study was also undertaken into the influence of strain on the electrical resistivity and relative magnetic permeability, the two material parameters governing the ACPD response. The information obtained from the investigations together with results from the FE analysis has made it possible to understand and quantify the influence of elastic/plastic deformation on ACPD response. An electric field model has been successfully developed to explain and predict the effect of increasing strain on the ACPD response in materials where the skin effect is strong. Results have also shown the inapplicability of the compensation method of crack monitoring when levels of plasticity are appreciable and an alternative method has been proposed.

Submitted by Ronildo Prado (ronisp@ufscar.br) on 2017-08-23T12:31:55Z No. of bitstreams: 1 DissRGMS.pdf: 20398919 bytes, checksum: 000d866830c28ab133d11dda08ba6563 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-08-23T12:32:04Z (GMT) No. of bitstreams: 1 DissRGMS.pdf: 20398919 bytes, checksum: 000d866830c28ab133d11dda08ba6563 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-08-23T12:32:10Z (GMT) No. of bitstreams: 1 DissRGMS.pdf: 20398919 bytes, checksum: 000d866830c28ab133d11dda08ba6563 (MD5) / Made available in DSpace on 2017-08-23T12:32:16Z (GMT). No. of bitstreams: 1 DissRGMS.pdf: 20398919 bytes, checksum: 000d866830c28ab133d11dda08ba6563 (MD5) Previous issue date: 2015-12-23 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / The growing demand of the steel industry for cheaper and more efficient production processes creates a necessity for new refractory materials. These requirements are written in terms of more robust concretes, with a longer life cycle, and ease of installation and maintenance. New materials that fulfill these criteria are the Mgo-based refractory castable. However, during curing and drying stages these materials have a critical magnesia hydration reaction that can lead to damage and cracks. The resonance measurement of elastic modulus technique is one of the techniques used to study the effects of magnesia hydration on the castables. As an alternative to this technique is proposed a digital image correlation (DIC) method to monitor the cracks network evolution of refractory castables during the curing stage. Samples (CDPs) of freshly molded castables were subjected to a controlled environment to cure and to dry and images were captured of CDPs with a high resolution digital camera. The DIC is an optical full-field displacement measurement technique. This method is based on the registration of digital images recorded in the undeformed and deformed configurations to measure displacements. A Correli-Q4 based software were developed to detect the cracks and measures their surface crack density and its average crack opening in the sample. The study of some variables such as the paint technique used in the samples (required for application of the CID technique) and the positioning of samples within the thermal chamber were required to evaluate the robustness of the software. As a result of that effort it was possible to compare the CID technique with resonance measurement of elastic modulus. The conclusion is that both detect simultaneously the cracks network beginning. The results claim that the DIC technique is promising because it promotes possibility of measure and compare parameters relative to the crack network (surface crack density and average crack opening) for different compositions of castables, temperatures and relative humidity maintained during curing. The analysis of these cracks network paraviii meters was consistent with the theory in the literature demonstrating the potential of the proposed method. / A crescente busca da indústria siderúrgica por processos de produção mais eficientes cria uma demanda por novos materiais refratários. Isto significa, concretos mais robustos, com ciclo de vida maior e, de maior facilidade de instalação e manutenção. Entre os novos materiais que preenchem estes requisitos estão os concretos aluminosos contendo magnésia. Entretanto, o processamento destes materiais possui algumas etapas críticas devido à reação de hidratação da magnésia que pode causar trincas. Dentre as técnicas já utilizadas para quantificar esse processo, destaca-se o monitoramento do módulo elástico por meio da técnica de ressonância de barra. Este trabalho visa gerar uma nova alternativa de avaliação da fissuração de concretos refratários durante a etapa de cura do processamento desses materiais com o auxílio da técnica de correlação de imagens digitais (CID). Corpos de prova (CDPs) de concreto recém-moldados foram submetidos a um processo controlado de cura e secagem e, durante este, foram capturadas imagens de uma das faces dos CDPs com uma câmera digital de alta resolução. A partir das imagens, o programa Correli-Q4 é utilizado para calcular os campos de deslocamentos de uma região de interesse. Por meio destes campos, um método desenvolvido detecta as trincas formadas e mede o seu comprimento total e a sua abertura média. A partir do método desenvolvido foi possível comparar a técnica de CID com a de ressonância de barra e conclui-se que ambas detectam a formação da rede de fissuras sendo, portanto, complementares. A técnica de CID se mostrou promissora, visto que foi possível medir e comparar diversos parâmetros da rede de fissuras (densidade superficial e abertura média das trincas) para diversas composições de concretos, diferentes temperaturas e umidades relativas mantidas durante a cura. A análise desses parâmetros da rede de fissuras foi coerente com a literatura demonstrando o potencial do método.

The quality of strain measurements using distributed fiber optic sensors (DFOS) depends largely on the bond between the host material and the optical fiber. Experimental investigations were carried out to test the suitability of five different DFOS types for crack monitoring. The DFOS were subsequently bonded to two 4 m long reinforced concrete (RC) beams, so that the influence of the application technique could be evaluated. DFOS measurements were verified by digital image correlation (DIC) and electrical strain gauges (SGs). For the different DFOS types, clear differences in the measured strain curves and determined crack widths were observed. The focus was on two robust DFOS, which can be deployed on construction sites. Compared to the layered sensing cable, a monolithic DFOS showed a clear strain distribution with pronounced strain peaks even for closely spaced cracks. The crack widths obtained by integrating the strain curves showed high agreement with DIC measurements.

The possibility to measure strains continuously using distributed fiber optic sensors (DFOS) offers enormous potential for structural health monitoring. Cracks can be automatically detected, localized and crack widths calculated. To address the relevant questions of choosing the right DFOS and appropriate application technique for monitoring existing structures, two 4 m long reinforced concrete beams were loaded under service loads in a 4-point bending test. The DFOS were either bonded directly to the smooth concrete surface or along a groove. For comparison, another DFOS was integrated into the specimen. It is demonstrated that with the used adhesive, a good strain transfer from the specimen to the DFOS can be ensured even with subsequent installation. In order to detect all cracks with high reliability, robust DFOS with a monolithic cross section should be preferred for the practical use. The accuracy of crack width measurement was verified through Reference measurement via digital image correlation. For the monolithic DFOS, all measurement deviations were within the tolerance range of ±0.05 mm. With layered sensing cables, significant misinterpretations occurred due to strongly damped strain curves.