Application of high-frequency mechanical impact treatment for fatigue strength improvement of new and existing bridges

This thesis investigates the application of High-Frequency Mechanical Impact (HFMI) treatment for fatigue strength improvement of weldments in existing and new bridges. In the former case, the welds have already been subjected to fatigue loading and accumulated damage before treatment. A fatigue testing program is set up, comprising welded specimens subjected to fatigue loading before HFMI treatment to investigate the efficiency of HFMI treatment on existing structures. Moreover, additional fatigue test results are collected from the literature and analyzed. HFMI treatment is found to be very efficient in extending the fatigue lives of existing structures regardless of the accumulated fatigue damage prior to treatment, given that any surface cracks, if exist, have not grown more than 2.25 mm in depth. For practical applications, HFMI treatment is only recommended if the critical details are verified to be free of any surface cracks. Remelting the surface with a tungsten electrode before HFMI treatment is another solution which has rarely been studied on existing structures. Therefore, several experimental investigations are conducted including fatigue testing, measurement of residual stress, hardness testing and scanning the welds topography to study the effect of combining these two post-weld treatment techniques. The combined treatment is found to be efficient as it induces higher and deeper compressive residual stress and local hardening. These aspects are all considered in numerical simulations conducted to investigate the fatigue behaviour of new and existing weldments treated using this combination. The results verify the superiority of the combined treatment to both individual treatments (TIG & HFMI). Nonetheless, because of the complexity associated with TIG remelting, the combination is only suggested for existing structures containing shallow fatigue cracks which can be fused by a tungsten electrode. One major hindrance to applying HFMI treatment on weldments in steel bridges is the lack of design rules and recommendations such as consideration of stress ratio (mean stress) and overloads.\ua0 Therefore, a correction factor (λHFMI) to account for the mean stress effect is derived. This factor is used to magnify the design stress range for fatigue verification of HFMI-treated welded details existing in road and railway bridges. λHFMI is calibrated using measured traffic data that includes millions of vehicles and hundreds of trains. In addition, the characteristic load combination associated with the serviceability limit state is found to be the most appropriate for verifying the maximum stresses in road bridges. Based on the work conducted in this thesis, a complete methodology is proposed for the design and assessment of HFMI-treated welded details in new and existing steel bridges. Finally, the effect of corrosion on the performance of HFMI-treated weldments is studied by analyzing collected test results. Despite the observed reduction in fatigue endurance of HFMI-treated details due to the removal of top layers improved by residual stresses, the obtained fatigue lives are still longer than the design lives assigned for new welded details even in extreme corrosion conditions. However, corrosion protection and removal of sharp HFMI groove edges via light grinding are still necessary to reduce the susceptibility of weldments to corrosion

Literature review on crack retrofitting in steel by Tungsten Inert Gas remelting

Welded metallic structures are usually prone to different forms of degradation in the mechanical properties during their life span such as fatigue. Tungsten Inert Gas remelting is a post-weld treatment method that gained increasing interest in the last decades. However, more light should be thrown on the behaviour of the existing cracked structures retrofitted by TIG-remelting. Therefore, fatigue test results of several welded details improved TIG-remelting have been extracted. Around 130 test results with different existing crack sizes are presented and analyzed. The gain factor in fatigue is defined and calculated for the collected data. TIG-remelting shows high potential in life extension. In fact, TIG-remelting is found to give a fair life extension greater than the design life of the new details even when 2 crack mm remains. Besides, toe radius and residual stresses are found to vary significantly after TIG-remelting depending on the remelting parameters and the steel type

Corrosion Effect on the Efficiency of High-Frequency Mechanical Impact Treatment in Enhancing Fatigue Strength of Welded Steel Structures

Fatigue and corrosion are two material degradation phenomena that occur in welded steel structures. High-frequency mechanical impact (HFMI) treatment is a post-weld treatment method that aims to increase the fatigue strength of welded details. This paper investigates the effect of steel corrosion on the efficiency of this method in enhancing fatigue resistance. More than 150 fatigue test results on corroded and HFMI-treated welded details are collected from several research articles and analyzed for both transverse welded attachment and butt-welded details. The efficiency of HFMI treatment decreases in corroded details as the corrosion level increases. However, HFMI treatment is found to have a high potential in prolonging the fatigue life, even in circumstances of an extremely corrosive environment

Fatigue life extension in existing steel bridges. High-Frequency Mechanical Impact treatment and Tungsten Inert Gas remelting in life extension and fatigue crack repair of welded steel structures

This thesis investigates the performance of improved welds with two post-weld treatment methods for application on existing structures. High-Frequency Mechanical Impact (HFMI) treatment and Tungsten Inert Gas (TIG) remelting were used for fatigue life extension of welded structures. Axial fatigue testing was conducted on transversal non-load-carrying attachment treated via the investigated methods. Furthermore, more than 250 test results on different treated welded details were collected, sorted and analysed. HFMI-treatment was found to give a significant fatigue life extension even with the presence of cracks up to 2.25 mm. On the other hand, the efficiency of TIG-remelting was also proven when the crack was completely eliminated after remelting. Even if a small part of the crack remains after remelting, fair fatigue life could be expected. However, it is recommended to use HFMI-treatment or TIG-remelting only when the crack inspection is negative before and after treatment respectively.Complimentary studies showed that the investigated methods induced compressive residual stress, increased the smoothness of the weld toe, increased the local hardness and changed the angular distortion status locally. Moreover, TIG-remelting changed the microstructure in both the fusion zone and the heat-affected zone. HFMI-treatment changed the crack orientation, induced compressive plasticity at the crack tip and caused crack narrowing or even closure. However, these effects were less significant for deeper cracks. Moreover, some practical aspects of the treatment application were investigated. Unlike treating new structures, TIG-electrode should be placed at the weld toe to secure that the maximum fusion depth corresponds to the crack plane. On the other hand, HFMI-indentor should be slanted more toward the base metal than the weld to avoid unintentional crack opening. Moreover, the IIW recommendations for both HFMI-treatment inclination and indentation depth could be extended to cracked structures. The aforementioned investigated parameters (i.e. residual stress, distortions, local hardness and toe\u27s smoothness) were incorporated in fatigue life predictions for both treatment methods. The base metal S-N curve was used to predict the life of specimens treated via TIG-remelting, while Paris law was used to track the crack propagation of HFMI-treated details. The results corresponded well with fatigue test results. Combining TIG-remelting with HFMI-treatment resulted in welds with higher fatigue strength because of the combined effects of crack closure via TIG-remelting and compressive plasticity via HFMI-treatment

Fatigue design and assessment guidelines for high-frequency mechanical impact treatment applied on steel bridges

Structural design of bridges in Europe should be carried out in accordance with Eurocode regulations. However, there is no guideline demonstrating how the fatigue design is to be conducted when high-frequency mechanical impact (HFMI) is used to enhance welded joints in steel bridges. The aim of this paper is to present the different design rules and equations and apply them to some example bridges enhanced by HFMI treatment. Fatigue verification of some welded details in these bridges is carried out via either “damage accumulation” or “λ-coefficients” methods in the Eurocode. Four fatigue load models are used in the fatigue verification (FLM3 and FLM4 for road bridge assessment, and LM71 and traffic mix, for railway bridge assessment). The effect of steel grade, mean stresses, self-weight, variation in stress ratios, and maximum stress on the treatment efficiency is considered in both examples. It is found that HFMI treatment causes a significant increase in fatigue lives in all studied cases

Fatigue life estimation of welded structures enhanced by combined thermo-mechanical treatment methods

Different post-weld treatment methods are used to strengthen welded joints that are subjected to cyclic loading. Combining High-Frequency Mechanical Impact (HFMI) treatment with Tungsten Inert Gas (TIG) remelting is rather a new concept. In this paper, the fatigue lives of welded transverse attachments treated by HFMI-treatment, TIG-remelting, or the combination of both are estimated using fatigue damage modelling and finite element deletion. The change in local topography and residual stresses due to treatment are evaluated numerically and incorporated in the analysis. The local hardness is measured by a Vickers tester and incorporated by increasing the elemental ultimate strength. The analysis demonstrates the superiority of the combined treatment because of the introduced compressive residual stress and the improvement in topography. The analysis also shows that the damage is less distributed after the combined treatment than both individual treatments. Besides, the capability of the TIG-HFMI combination in treating existing welded structures with remaining embedded fatigue crack is proven. Besides, available fatigue test results on combined TIG-HFMI treatment shows that this combination gives always longer fatigue life than the characteristic fatigue lives of the treated details by any of the treatment methods. However, many aspects such as TIG arc and HFMI indenter positioning, and indentation and fusion depth should be taken into consideration when the combined treatment is to be applied to existing structures

Material and residual stress improvement in S355 welded structural steel using mechanical and thermal post‐weld treatment methods

Welding is by far the most widely used metal joining method. High-Frequency Mechanical Impact (HFMI) treatment and Tungsten Inert Gas (TIG) remelting are two post-weld treatment methods that aim to enhance the strength of the steel. In this paper, the improvement in residual stress and material characteristic obtained with these methods are studied by conducting several experimental investigations such as hole drilling, hardness testing and microscopy. Hole drilling shows that HFMI treatment improves the status of residual stress at the weld toe in the first 1 mm from the surface. Furthermore, Vickers testing shows a remarkable improvement in the hardness values at the weld toe in the first 2 mm. This can be attributed to the reduction in grain size after treatment. Moreover, acicular ferrite and tempered bainite are found to be the main constituents in the fusion and heat-affected zones after TIG-remelting

Comparison of costs of personal protective equipment for all workers to avoid costs of fall accidents

The construction industry is one of the most important industries in the United States. It is the biggest contributor in the growth of the U.S. economy. Despite the great achievements and prominent role for this sector in the U.S. economy, the construction industry is suffering from neglect and dereliction by related government institutions and private companies. This neglect led to formation of a negative image about the construction industry that has caused a deep impact in productivity and economic growth. One of the reasons is a high percentage of accidents that occur within the construction industry which leads to injuries and fatalities. The construction industry consists of about 5% of the U.S.A work force, and accounts for some 20% of the work fatalities and 12% of disabling injuries. The largest percentages of fatalities or injuries in the construction industry were the result of a fall. The falling fatality rate was about 33.3% of the total attributed most common accidents in this sector at 2010 (CPWR, 2013), and this percentage increased in 2012 as the number of fatalities due to falls was to 280 out of 775 people killed in industrial constructions (United States Department of Labor, 2012). This thesis is a comparison between the total cost of fall accidents and costs of protective programs to reduce a fall accidents. The lack of accurate information about costs is one of the main reasons that employers show little interest in supporting the protective systems to avoid a fall hazards. The mechanism that was used in this thesis is a realistic comparison between costs that may occur as a result of fall accident in the construction industry, and costs of use of Personal Protective Equipment (PPE). --Abstract, page iii

Stacked Cross Validation with Deep Features: A Hybrid Method for Skin Cancer Detection

Detection of malignant skin lesions is important for early and accurate diagnosis of skin cancer. In this work, a hybrid method for malignant lesion detection from dermoscopy images is proposed. The method combines the feature extraction process of convolutional neural networks (CNN) with an ensemble learner called stacked cross-validation (CV). The features extracted by three different CNN architectures, namely, ResNet50, Xception, and VGG16 are used for training of four different baseline classifiers, which are support vector machines, k-nearest neighbors, artificial neural networks, and random forests. The stacked outputs of these classifiers are used to train a logistic regression model as a meta-classifier. The performance of the proposed method is compared with the baseline classifiers trained individually as well as AdaBoost classifier, another ensemble learner. Feature extraction with Xception architecture, outperforms all other benchmark models by achieving scores of 0.909, 0.896, 0.886, and 0.917 for accuracy, F1-score, sensitivity, and AUC, respectively