Representative Test Specimens Research Articles

Development of an Open-Source Robotic NDT Solution for Automated Composite Repair Testing

Abstract Non-destructive testing (NDT) plays an important role in aircraft maintenance and repair processes, ensuring the structural integrity necessary for safe operation. The paper presents the design and evaluation of an animated, low-cost robotic NDT system tailored for inspecting composite bonding agents. The system integrates commercially available components, including a three-degree-of-freedom robotic arm and a Raspberry Pi 4B, managed by custom Python software with a user-friendly graphical interface. Mechanical Impedance Analysis (MIA) and Eddy Current Testing (ET) methods were employed to assess the system’s performance on representative test specimens. Results indicate that the system delivers reliable and accurate measurements comparable to commercial tools like the MAUS V, while offering simplicity and modularity. Limitations such as scanning speed and handling of complex geometries are acknowledged, with potential solutions proposed for future enhancement. The system provides an affordable and customizable alternative for NDT automation in the aerospace industry.

Thermal history transfer from complex components to representative test specimens in laser powder bed fusion

AbstractAdditively manufactured components are characterized by heterogeneous mechanical properties due to variations of the microstructure, flaws and residual stresses resulting from the inhomogeneous fabrication process. The large number of influencing factors poses a further challenge in understanding the correlation between material properties, process parameters and component geometry. Therefore, the qualification of components based on witness specimens produced within the same job is questionable. This work aims to present a new strategy for the characterization of PBF-LB/M components based on representative specimens. The key assumption is the feasibility of a transfer of the thermal history from a component to a specimen. It is assumed that similar material properties are determined for components and specimens produced adopting a similar thermal history. After the definition of a region of interest in the component, a combination of thermal analyses by means of finite elements and in-situ experimental determination of the thermal history through infrared thermography is used to produce test coupons with a similar thermal history. The effectiveness of the procedure is demonstrated on a pressure vessel for applications in the chemical industry.

Developing Representative Test Specimen Conditions for Rutting Mechanical Test Methods of Airfield Pavements

Developing Representative Test Specimen Conditions for Rutting Mechanical Test Methods of Airfield Pavements

Predictive maturity of non-linear concrete constitutive models for impact simulation

This paper explores the concept of predictive maturity for non-linear concrete constitutive models employed in the computational prediction of the structural response of reinforced concrete structures to impact from free-flying missiles. Such concrete constitutive models are widely varied in complexity. Three constitutive models were utilized within the same finite element structural model to simulate the response of the IRIS III experiment. Each of the models were individually calibrated with available material testing data and also re-calibrated assuming limited availability of test data. When full calibration is possible, more sophisticated constitutive models appear to provide more predictive maturity; however, when this data is not available (e.g. for an existing structure where representative test specimens may not be available), the expected maturity is reduced. Indeed, this hypothesis is supported by the simulations that indicate good agreement with measured experimental response quantities from the IRIS III tests with complex constitutive models and full calibration, and accordingly poor predictions when less complex models are used or when the more sophisticated models are poorly calibrated. Thus, predictions of structural response where complete material testing data is not obtainable should be understood as less predictive.

Non-invasive internal pressure measurement of 18650 format lithium ion batteries during thermal runaway

The dynamics of 18650 format lithium ion battery pressure build-up during thermal runaway is investigated to inform understanding of the subsequent pressure-driven venting flow. Battery case strain and temperature were measured on cells under thermal abuse which was used to calculate internal pressure via hoop and longitudinal stress relations. Strain measurement is a non-invasive approach with no impact on the chemical decomposition within batteries that leads to thermal runaway. The experimental methodology and test apparatus were developed and validated through predictable, representative test specimens of open cases and pressurized gas cylinders. Live battery experiments reliably captured the final pressure rise before venting onset. The pressure build up was observed to coincide with the beginning of thermal runaway.

Optimizing the acid resistance of concrete with granulated blast-furnace slag

Concrete structures exposed to high levels of chemical attacks are assigned to exposure class XA3, which recommends separate concrete protection or a special expert solution to ensure durability. Due to the partial substitution of Portland cement by blast-furnace slag, an increased resistance to acid attacks could be achieved within the framework of a research project. The technical and ecological advantages of cements containing granulated blast-furnace slag were exploited through chemical, granulometric and concrete technological optimizations. Despite extensive parameters, a statistical test design (DoE) was able to limit the experimental effort, thus defining principles for the conception of binder systems with increased chemical resistance.Mortar prisms indicated that the use of (ultrafine) blast-furnace slags (up to 13,000 cm2/g according to Blaine) with a broad particle size distribution can have a positive effect both on the capillary/gel pore ratio and on the calcium hydroxide content in the cement stone. Furthermore, the chemical composition of the blast-furnace slag as well as the water-binder ratio are decisive influencing factors for the acid-resistance, which was confirmed in accelerated acid resistance tests on concretes (pH-stat method). After 13 weeks of storing concrete specimens in sulfuric acid (H2SO4, pH 3.5), reduced damage depths and lower weight losses were observed compared to conventional binder compositions. The results serve as a basis for the development of highly acid-resistant concretes using blast-furnace slag-containing binder systems. Currently, the acid resistance of those concretes is being investigated in a long-term study by outsourcing representative test specimens into the Emscher sewer.

Towards a Methodology for Component Design of Metallic AM Parts Subjected to Cyclic Loading

The safe fatigue design of metallic components fabricated by additive manufacturing (AM) is still a largely unsolved problem. This is primarily due to (a) a significant inhomogeneity of the material properties across the component; (b) defects such as porosity and lack of fusion as well as pronounced surface roughness of the as-built components; and (c) residual stresses, which are very often present in the as-built parts and need to be removed by post-fabrication treatments. Such morphological and microstructural features are very different than in conventionally manufactured parts and play a much bigger role in determining the fatigue life. The above problems require specific solutions with respect to the identification of the critical (failure) sites in AM fabricated components. Moreover, the generation of representative test specimens characterized by similar temperature cycles needs to be guaranteed if one wants to reproducibly identify the critical sites and establish fatigue assessment methods taking into account the effect of defects on crack initiation and early propagation. The latter requires fracture mechanics-based approaches which, unlike common methodologies, cover the specific characteristics of so-called short fatigue cracks. This paper provides a discussion of all these aspects with special focus on components manufactured by laser powder bed fusion (L-PBF). It shows how to adapt existing solutions, identifies fields where there are still gaps, and discusses proposals for potential improvement of the damage tolerance design of L-PBF components.

Review of the design process of a representative test specimen of a structural joint of a carbon‐epoxy aircraft

The design process of a Representative Test Specimen (RTS) intended to reproduce the same state of stress of a complex‐in‐shape structural joint but with a simpler geometry is here inspected. The RTS corresponds to the joint of the fuselage with the main landing gear and the lower segment of the supporting strut of a strut‐braced, high wing aircraft fabricated with carbon‐epoxy composite. The design process of the RTS is based on the Multi‐Instrumented Technological Evaluators method, developed to reduce testing times and costs with respect to the standard Testing Pyramid approach. Complementary design methods comprise the Quality Functions Deployment articulated with the Functional Analysis. Afterward, once that the parametric design of the RTS is conducted with an iterative numerical analysis based on the Finite Element Method the outcome is a sandwich‐type structure to be tested with a non‐standardized three‐point bending test. In closing, the said design methods plus the numerical analysis foster an integral‐design approach in the outline of composite structures. POLYM. COMPOS., 40:E898–E911, 2019. © 2018 Society of Plastics Engineers

Methodology development to design a representative test specimen for wear damage in spline couplings

Aim of this paper is to develop a new methodology in order to design a representative test specimen for wear damage characterisation in spline couplings. In other words, real component and test specimen (same material) have been considered having a common target that is an isodamage condition reached after an established number of working (testing) cycles.Influence parameters chosen for this aim are, under the hypothesis of equal friction coefficient, hertzian pressure due to the load entity (torque) and the corresponding slidings. Slidings have been determined referring to two different working conditions, traditional fatigue testing with variable torque (aligned conditions) and wear testing in misaligned conditions. Specimen geometry has been firstly stated following DIN 5480 requirements, then profile micro geometries have been varied to tune the established target parameters and the corresponding FEM simulations have been carried on. Hertzian pressure values and corresponding contact areas have been verified by classical formula.A preliminary experimental activity has been done in order to verify the specimen design related to isodamage dimensioning aspects.

Structural Integrity of an Electron Beam Melted Titanium Alloy.

Advanced manufacturing encompasses the wide range of processes that consist of “3D printing” of metallic materials. One such method is Electron Beam Melting (EBM), a modern build technology that offers significant potential for lean manufacture and a capability to produce fully dense near-net shaped components. However, the manufacture of intricate geometries will result in variable thermal cycles and thus a transient microstructure throughout, leading to a highly textured structure. As such, successful implementation of these technologies requires a comprehensive assessment of the relationships of the key process variables, geometries, resultant microstructures and mechanical properties. The nature of this process suggests that it is often difficult to produce representative test specimens necessary to achieve a full mechanical property characterisation. Therefore, the use of small scale test techniques may be exploited, specifically the small punch (SP) test. The SP test offers a capability for sampling miniaturised test specimens from various discrete locations in a thin-walled component, allowing a full characterisation across a complex geometry. This paper provides support in working towards development and validation strategies in order for advanced manufactured components to be safely implemented into future gas turbine applications. This has been achieved by applying the SP test to a series of Ti-6Al-4V variants that have been manufactured through a variety of processing routes including EBM and investigating the structural integrity of each material and how this controls the mechanical response.

A study on the evaluation of performance of the PSC inner containment dome with large openings during prestress transfer

A linear three dimensional finite element (FE) analysis has been carried out for the loading condition at pre-stress transfer of a representative test specimen for a typical pre-stressed concrete (PSC) inner containment (IC) dome with due emphasis on the local behaviour of the steel plate concrete interfaces at the steam generator (SG) openings. The FE model includes the pre-stressing ducts and passive reinforcements. The FE analysis results have been compared with the past experimental results for the same representative test specimen. Due thrust has been put on the objective of predicting the possibility of separation at the steel plate concrete interfaces.An attempt has been made in corroborating the findings of the past two FE analytical studies on the linear behaviour of the PSC IC dome (with large SG openings) of the same actual double containment reactor building of the typical 220MWe capacity Indian nuclear power plant. Finally important conclusions in line with the desired objectives have been made based on the summary observations of all the studies conducted on this typical PSC IC dome.

An experimental investigation on the local behaviour of steel–concrete interfaces at large openings in the PSC inner containment dome

An experimental study has been conducted to know the structural behaviour of a representative test specimen for a typical Prestressed Concrete Inner Containment dome with due regards to the local behaviour at the steel plate–concrete interfaces at large steam generator openings, under initial condition at prestress transfer. The primary thrust of the work has been in the objective of predicting possibilities of separation at the steel–concrete interface zones adjacent to the embedded plates of the steam generator openings of the inner containment dome. Important observations in line with the desired objectives have been made based on the results of the experiment.

Fracture toughness evaluation of precursor-derived Si–C–N ceramics using the crack opening displacement approach

Difficulties in the fabrication of representative test specimens and in the application of suitable test procedures e.g., in the measurement of the test parameters viz., load, crack length etc., have so far limited the intrinsic fracture mechanical characterization of the precursor-derived ceramics (PDC). The present work reports the evaluation of the crack tip toughness ( K I 0) of Si–C–N ceramics, using the novel crack opening displacement (COD) approach. The fully dense PDC test specimens synthesized from a poly(ureamethylvinyl)silazane precursor covered material structures ranging from partly organic amorphous to inorganic nano-crystalline states. Critically loaded cracks were achieved using either a special loading fixture or with Vickers indentation. Crack tip CODs were measured with the atomic force microscopy (AFM). Fractography of the fracture surfaces were performed using topographic, frictional and phase contrast AFM. The measured K I 0 values ranged from 0.6 to 1.2 MPa m 1/2. The net change in crack resistance was affected by the stripping of OFC-hydrogen atoms in the amorphous materials and by the segregated turbostratic graphite phase in the phase-separated materials. Nano-scale crack deflection observed even in the amorphous materials indicated the presence of structural and compositional inhomogeneities within the amorphous network.

Resistance projection welding for TO-Can style package

The effects of the process parameters on the joint quality of TO-Can using resistance projection welding were investigated. A capacitor discharge stored energy welding machine was used for joining caps and headers both coated with nickel SPCC steel. The electrode materials used in this study were brass and W- Cu alloy. Experimental results indicated that the peak current of the W-Cu electrode was higher than that of the brass electrode, irrespective of the operating pressure variation. Increased operating pressure influenced the interfacial heat generation and nugget formation. The nugget area increased rapidly with increasing charging voltage, resulting in expulsions and electrodes getting stuck. An acceptable welding zone was found in the relationship between the charging voltage and the operating pressure. Representative test specimens of the acceptable welding zone satisfactorily passed the helium leak test. The microscopic morphology of the nugget showed a recrystallized fine grain structure resulting in the nugget being harder than with the HAZ.

Design of a Multi-Axial Test Specimen for Fretting Fatigue in Splined Couplings

This paper describes the conceptual, embodiment and detailed design of a representative test specimen to mimic fretting variables in aero-engine splined couplings. A fretting bridge design was chosen for the multi-axial representative specimen from a set of four concepts evaluated. The test specimen was intended to facilitate more rapid testing for design against fretting fatigue and wear in the complex contact geometry of the spline coupling.

Development of a representative test specimen for frictional contact in spline joint couplings

The frictional contact conditions in a helical spline joint coupling under torsional and axial loads are studied using finite element analyses. Comparisons of spline root torsional stresses are made with photoelastic measurements. Surface contact tractions, relative slip distributions and subsurface localized stress component and equivalent stress distributions are presented for both the external and the internal spline contact surfaces. The work is important as a basis for understanding and predicting the fatigue, contact fatigue, fretting fatigue, wear and fretting wear characteristics of the coupling. A representative test specimen concept is presented which seeks to capture the local contact variable distributions in the spline coupling, with a view to predicting the wear and the contact and fretting fatigue performance of the coupling.

Stiffened Steel Plates under Compression and Bending

The buckling capacity of steel plates stiffened with T shape stiffeners was investigated experimentally and analytically. The test specimens, 2000 mm long, were fabricated with a 500×10 mm plate stiffened with a WT 125×12.5 stiffener. The experimental program was designed to investigate the effect of unloaded edges boundary restraint, combination of in-plane and out-of-plane load, and imposed plate damage on the buckling capacity of stiffened steel plates. The residual stresses were measured in a representative test specimen and initial imperfections were measured for each test specimen. The residual stresses and the measured initial imperfections were included in the finite element model. A large deformation and finite strain analysis was performed using the finite element code ABAQUS. The finite element analysis was found to predict very accurately both the behaviour and capacity of stiffened steel plate panels. Stiffener tripping and plate buckling modes were equally well predicted.