Quantitative evaluation of augmented strain at the weld metal during the Trans-Varestraint test

The hot cracking susceptibility in the Trans-Varestraint test was evaluated using the nominal strain calculated using the curvature radius of a bending block and the thickness of a specimen based on the theory of material mechanics. The nominal strain was calculated using the material properties at room temperature. Thus, in the Trans-Varestraint test, the non-uniformity of the strain around the weld part due to the temperature distribution is not considered. Therefore, the strain in the Trans-Varestraint test cannot be evaluated correctly. The aim of this study is to reveal the loaded strain at the weld metal to understand the evaluation of hot cracking susceptibility in the Trans-Varestraint test. The loaded strain around the trailing edge of the weld pool of pure iron was measured in-situ using a high-speed camera and high-resolution optical lens. The results of strains measured using image analysis and the finite-element method at the center of the weld bead were compared. Accordingly, it was clarified that the strain was concentrated on the weld part owing to the bending occurring along the weld line, and the strain exceeding the nominal strain was loaded to the trailing edge of the weld pool.

Stretchable Carbon and Silver Inks for Wearable Applications

For wearable electronic devices to be fully integrated into garments, without restricting or impeding movement, requires flexible and stretchable inks and coatings, which must have consistent performance and recover from mechanical strain. Combining Carbon Black (CB) and ammonia plasma functionalized Graphite Nanoplatelets (GNPs) in a Thermoplastic Polyurethane (TPU) resin created a conductive ink that could stretch to substrate failure (>300% nominal strain) and cyclic strains of up to 100% while maintaining an electrical network. This highly stretchable, conductive screen-printable ink was developed using relatively low-cost carbon materials and scalable processes making it a candidate for future wearable developments. The electromechanical performance of the carbon ink for wearable technology is compared to a screen-printable silver as a control. After initial plastic deformation and the alignment of the nano carbons in the matrix, the electrical performance was consistent under cycling to 100% nominal strain. Although the GNP flakes are pulled further apart a consistent, but less conductive path remains through the CB/TPU matrix. In contrast to the nano carbon ink, a more conductive ink made using silver flakes lost conductivity at 166% nominal strain falling short of the substrate failure strain. This was attributed to the failure of direct contact between the silver flakes.

Control of corrosion features by forming parameters

During forming operations, the microstructure of metal parts is usually changed. Effects of cold hardening result in different mechanical properties, whereas the deformed microstructure also changes the electromechanical properties. The latter is responsible inter alia for the chemical corrosion behavior in terms of breakdown potential. In this study, the principle of corrosion resistance of steel E355 (EN 10305-1) was analyzed after rotary swaging with the same nominal strain but different process settings. Especially higher feed rates (forming increments per stroke) and the additional application of shear strain by eccentric rotary swaging increased the pitting potential significantly and thus the corrosion resistance. The introduced methods are assumed as prospective candidates for industrial production of parts that provide higher durability without further anti-corrosion treatment.

Strain rate and temperature dependent strain localization of dynamically stretched lightweight bars: from metal to polymer

This work studies the dynamic strain localization and constitutive relationship of a Ti3Al2.5V alloy in jet engine containment system and a transparent polycarbonate conceived for aircraft canopy application by Digital Image Correlation (DIC) technique from quasi-static condition to high strain rates at different temperatures. The responses of two materials show significant strain rate and temperature sensitivities. Observations of Ti3Al2.5V alloy show that the dynamic local strain rate can reach values up to 1000 % of the nominal strain rate in the necking zone. However, dynamic local strain rate of polycarbonate soars up during strain softening then decreases rapidly with necking propagation, and eventually becomes 20 % of the nominal strain rate until fracture. Appropriate viscoplastic constitutive models are determined for both materials, which are incorporated in finite element simulations to reveal the trend of dynamic local strain rate evolution in dynamic tensile tests. The present work shows two different kinds of strain localization in typical lightweight materials, which should be addressed carefully from Split Hopkinson Tension Bar (SHTB) tests.

Heat Transfer Modeling and Optimization of a Carbonized Microvascular Solar Receiver

Concentrating solar power is an emerging renewable energy source. The technology can collect and store thermal energy from the sun over long durations, generating electricity as needed at a later time. Current CSP systems are limited to a maximum operational temperature due to constraints of the working fluid, which limits the maximum possible efficiency of the system. One proposed pathway forward is to utilize a gas phase for the working fluid in the system such as supercritical carbon dioxide. A composite gas phase modular receiver is being developed by researchers at Boise State University and the University of Tulsa. The receiver uses supercritical carbon dioxide as the working fluid, which can operate at temperatures greater than 1000 ˚C. The unique carbon-carbon composite material has high thermal conductivity and is structurally durable at extreme temperatures. A model has been developed in this work to simulate the thermal and hydraulic performance of a composite receiver unit cell. The model is built as a thermal resistance network that solves more quickly than traditional computational fluid dynamics simulations. The thermal and hydraulic models are compared with CFD simulations and show close agreement over a wide range of inlet velocities and path architectures. A genetic algorithm has been developed to optimize the design of the receiver. The algorithm optimizes the fluid channel diameter, inlet velocity, and the path architecture design of a unit cell. The optimization scheme weighs the thermal performance of the receiver with the hydraulic performance, maximizing the thermal efficiency and minimizing the pressure drop. The nominal strain is also calculated and constrained. The algorithm produces an optimal design from a constrained set of architectures. The optimal design is a simple three-channel parallel path with an acceptable pressure drop, less than 17 kPa. The thermal efficiency of the design is 75.6% with a 1,000,000 W/m2 solar flux and the nominal strain is an allowable 0.03%. Future work will be done to expand the path design space and remove arbitrary constraints from the optimization process.

Influence of the loading direction on the uniaxial compressive strength of sea ice based on field measurements

Sea ice is composed of columnar-shaped grains. To investigate the influence of the loading direction on the uniaxial compressive strength and failure processes of sea ice, field experiments were performed with first-year level ice. Loads were applied both horizontally (parallel to the grain columns) and vertically (across the grain columns) with various nominal strain rates. Two failure modes have been observed: a ductile failure mode at low nominal strain rates, and a brittle failure mode at high nominal strain rates. However, the failure pattern of sea ice was clearly dependent on the loading direction. At low nominal strain rates (ductile failure mode), the sea-ice samples yielded due to the development of wing cracks under horizontal loading and due to splaying out at one end under vertical loading. When sea ice fails in the ductile mode, the deformation is driven by grain boundary sliding under horizontal loading and by grain decohesion and crystal deflection under vertical loading. At high nominal strain rates (brittle failure mode), the sea-ice samples failed in shear faulting under horizontal loading and in cross-column buckling under vertical loading. The nominal strain rate at the brittle–ductile transition zone is about ten times higher under vertical loading.

Integrity Assessment of Post-Peak-Moment Wrinkles

For a pipeline experiencing a ground movement event, high longitudinal strain can be developed in the pipe longitudinal direction. When prerequisite requirements are met, ASME B31.4 allows up to 2% (nominal) longitudinal strain in a pipe. However, such high strain may be beyond the compressive strain capacity (CSC) of the pipe which is defined as the compressive strain corresponding to the maximum bending moment. Furthermore, wrinkles are usually formed at such a high strain level. Excessive local strain can accumulate around the wrinkles when the nominal strain goes beyond the CSC which can lead to significant wrinkle growth or even tearing of the pipe wall. Therefore, integrity of the pipes containing post-peak-moment wrinkles need to be assessed in order to confirm that the 2% nominal strain permitted in the ASME codes can be safely tolerated. A number of failure modes are possible. Firstly, a pipe must be capable of tolerating the nominal strain up to 2% under static loading without leak or rupture. Secondly, if a buckle or wrinkle is formed in the initial event of ground movement and no leak or rupture occurs, the buckle or wrinkle can be subjected to fatigue loading during the continued operation of the pipeline. The pipe should have sufficient remaining life till the anomalies are discovered (through inline inspection, for example) and mitigated. The fatigue loading can come from fluctuations in operation pressure, temperature, and/or other sources. In this paper, the immediate and long-term integrity of selected pipelines were assessed. The work has demonstrated that for the selected pipelines: (1) all lines meet the prerequisite conditions outlined in ASME B31.4 for the nominal strain limit up to 2%; (2) all lines are capable of tolerating nominal longitudinal strain up to 2% without immediate negative consequences; (3) for the wrinkles corresponding to nominal strain up to 2%, the wrinkles are expected to have finite fatigue lives and intervention within 5 to 7 years should be sufficient to prevent fatigue failures; and (4) locating and mitigating wrinkles corresponding to nominal longitudinal strain greater than 2% after a ground movement event may be necessary to ensure the safety of the pipelines.

High Peak Strain on Pipeline Material During Reel-Lay: Acceptable or Not?

The nominal strain occurring during installation of a pipeline by the reeling method is expected to be limited to 2–3% strain. This is only true if the pipeline has a perfect geometry (diameter, thickness) and homogeneous material properties along its length, resulting in a uniform bending stiffness. There will however always be a stiffness mismatch at the joints between pipes. Different scenarios can be considered as the cause of this stiffness mismatch: differences in average wall thickness and average yield stress of two pipe joints welded together, counterbored/machined pipe ends or field joint coating for pipes with thick coating. To some extent these scenarios can initiate high peak strains in the pipeline material far above the level of the expected nominal strain, exceeding in some cases 5% strain. Questions which might arise are: Could this high peak strain occurrence be ignored?, or: What is the impact of the high peak strain on the performance of the material after reeling? This paper presents FEA results illustrating the concerns of the occurrence of large peak strains which can still be significant even after averaging these strains over the thickness as well as over a certain length of the pipe. The methodology of averaging strains, as proposed together with DNV GL, correlates the length of the averaging pipe section with the maximum length of the test specimen geometry as allowed when performing strain aging tests of specimens with high tension and compression strains. A series of cyclic plastic deformation tests with pre-strained specimens from 4% to 7% strain was performed with seamless pipe material, followed by the tensile, Charpy and hardness tests of strain aged samples. One of the challenges is the setup of the test machine to avoid buckling of specimens during high compression pre-straining. The results from material tests (tensile, hardness and Charpy) have been evaluated against the DNV-OS-F101 Supplementary requirement for plastic deformation (P). The consequences of material modification due to plastic strain is further discussed and evaluated referring to the DNV GL limit state design and criteria for pipeline installation after reeling as well as during the lifetime of the pipeline.

True and Nominal Stress during Low Cycle Fatigue Tests of Metals

In this paper results of P91 cast steel after static and fatigue tests were presented. During the tests longitudinal and transverse strains of the specimen were measured. Basing on the results a comparative analysis of nominal stresses σn,true stresses σrz, nominal strain energy ΔWpl (n) and true strain energy ΔWpl (n) was carried out. It was stated that differences between determined paramters rise with increasing strain.

Influence of Recycled Irradiated HDPE on Mechanical Behavior of LDPE/Hdpex Blends

This research paper deals with utilization of recycled irradiated high-density polyethylene (HDPEx). Grit prepared of irradiated HDPEx was used as a filler into virgin low-density polyethylene (LDPE). Concentrations from 10 to 60 % were made and their influence on mechanical properties was investigated. Tensile test at ambient and elevated temperature was used to describe mechanical properties of resulting blends. Results show that there is an upward trend of elastic modulus and ultimate tensile strength and downward trend of nominal strain at ambient temperature. Similar findings were observed at elevated temperature, which might suggest possible utilization of such modified thermoplastic materials. However other material properties have to be tested to make final conclusion.