Test Coupons Research Articles

Tests, simulations and design of aluminium alloy equal-leg angle section stub columns after exposure to fire

This paper reports experimental and numerical studies on the compression resistances of aluminium alloy equal-leg angle section stub columns after exposure to fire. An experimental programme was first conducted, including heating tests, eight post-fire material coupon tests and sixteen post-fire stub column tests. Following the experimental programme, a numerical modelling programme was conducted, where finite-element models were developed and validated. The validated models were then used to perform parametric studies to derive additional numerical data. Based on the obtained test and numerical data, the relevant design rules used in Europe, China, America, Australia and New Zealand as well as the continuous strength method were assessed. The assessment results generally show that the current international standards lead to significant conservative and scattered design resistances when used for the post-fire design of aluminium alloy equal-leg angle sections under compression, particularly for those after exposure to elevated temperatures of 300 °C to 550 °C, due to the neglect of significant post-fire material strain hardening. The continuous strength method is found to provide greatly improved design accuracy and consistency over the European, Chinese, American and Australian/New Zealand standards for the post-fire design of aluminium alloy equal-leg angle sections, since it accounts for the post-fire material hardening.

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.

Assessment of residual resistant capacity of cold-formed Q1100 ultra-high strength steel channel section stub column with corrosion

Experiment and numerical analysis were conducted in this study to reveal the residual resistant capacity of the cold-formed Q1100 ultra-high strength steel (UHSS) channel section stub column with grooving corrosion. Based on the results of tensile coupon test, the strength properties of the Q1100 UHSS were barely changed by the cold-forming process. A clear reduction in the ductile properties was observed, after the cold-forming process. The load-end shortening curve of the cold-formed Q1100 UHSS channel section stub columns with different grooving corrosions were obtained through the axial compressive experiment. The initial resistant stiffness was reduced by the grooving corrosion significantly, because of the reduction in the cross-section area. Decrease in the corrosion angle and increment in the corrosion length enhanced the adverse effects of grooving corrosion on the resistant capacity. Local buckling was observed in the flange and web. The flanges near the corrosion weakening area were more prone to local buckling, because of the stress transfer from web to flange. A numerical modelling method was proposed to simulate the resistant performance of the cold-formed Q1100 UHSS channel section stub columns with different grooving corrosions, which was validated by the comparison in load-end shortening curve, ultimate resistance and failure mode. Effects of corrosion angle, corrosion length, and corrosion location on the resistant capacity and failure mode were investigated through the parametric study. Compared with corrosion location, the corrosion angle and corrosion length exhibited more significant effects on the resistant capacity of the cold-formed Q1100 UHSS channel section stub columns.

Post-fire mechanical properties of titanium-clad bimetallic steel after cold-forming process: Experiment and constitutive model

Titanium-clad bimetallic steel (TCBS), composed of a titanium alloy cladding layer and carbon steel substrate layer, has been proposed to mitigate the effects of corrosion and enhance the durability of steel structures. This study conducted tensile coupon test to explore the coupling effect of the cold-forming process and fire exposure on the monotonic mechanical properties of TCBS. Following the cold-forming process and fire exposure, there was no visible cracking observed in either the cladding or the metallurgical bonding layer. Test results revealed a notable enhancement in the strength properties and a decrease in the ductility of the TCBS specimens after cold forming. Moreover, the study elucidated the effects of the exposure temperature and cooling method on the stress–strain curves and fractures of TCBS corner specimens. Three different fracture sequences of the TCBS corner specimens were observed during monotonic tensile test. The key mechanical parameters were concerned in this study, which included the yield and ultimate strengths, yield and ultimate strains, elastic modulus, elongation after fracture, and section shrinkage. Then, the variation trends of key mechanical parameters were clarified. Predictive equations were developed for these parameters of the TCBS corner specimens after fire exposure. Additionally, two constitutive models were developed for the corner TCBS stress–strain curves with and without the yield plateau, enabling their use in numerical and theoretical investigations of the post-fire performance of cold-formed TCBS components.

A comprehensive investigation of the process and atmospheric coupling corrosion on corroded and mechanical properties of the SPA-H weathering steel

Weathering steel has recently become popular in engineering due to its corrosion resistance, but its mechanical characteristics must alter during double corrosion, process chemical solution corrosion (CSC), and atmospheric corrosion (AC). However, no studies have been published on the coupling effect of process and atmospheric corrosion. The durability and safety of weathering steel structures can be challenging to assess. As a result, we used SPA-H weathering steel as the object, ran process corrosion and double corrosion simulations (process corrosion plus atmospheric corrosion simulation), and created three corrosion conditions: non-corrosion, single-side corrosion, and double-side corrosion. CSC was the corrosion process. Following CSC, the second (120, 360, 720, and 1200 hour) Copper-Accelerated Acetic Acid-Salt Spray Testing (CASS) was performed to simulate atmospheric corrosion. A correlation analysis was conducted using the grey correlation method between indoor and atmospheric corrosion. After corrosion testing, these specimens were subjected to White Light Interferometry (WLI) and tensile coupon tests (TCT) to evaluate the corrosion characteristics and the deterioration of material mechanical properties. Finally, linear fitting was used to produce the calculation formula between the mechanical properties and the mass loss ratio of single-side and double-side corrosion.

Strategy for developing aluminum alloy-based solid structure with homogeneous properties in the overlapping direction using wire arc additive manufacturing

This research investigates the strategy for developing large-scale solid aluminum structures with multi-bead multi-layer (MBML) overlapping using wire-arc additive manufacturing (WAAM). The study addresses challenges, i.e. material accumulation, surface irregularities, and inter-bead and inter-layer gaps, by examining deposits’ geometrical characteristics, process parameters, and deposition methods. Initial test coupon solid blocks are printed and analyzed using optical microscopy, and subsequent statistical optimization leads to the successful printing of a defect-free 140 mm × 150 mm × 28 mm solid block. The flat surface achieved in the MBML structure is attributed to maintaining an optimal overlap coefficient and a parabolic profile with the maximum width at the base. Microstructural analysis reveals the absence of fusion gaps, with an average porosity of 11.7 ± 2.45%, due to hydrogen evolution and reheating effects, with few larger pores near fusion lines. The microstructural and mechanical analyses confirm the homogeneity of properties along the overlapping direction (OD), with microhardness averaging 85.80 ± 6.63 HV0.1 and tensile strength of 161.55 ± 1.19 MPa with 2.18 ± 0.08% elongation. Microhardness fluctuations along the OD are attributed to fusion boundaries, partially melted and heat-affected zones. The research revealed that careful consideration of bead geometry and process parameters is crucial for achieving defect-free aluminum solid structures in WAAM through MBML overlapping.

Application of Mechano-Luminescence-Optoelectronic Composites for Delamination Detection in Fiber Reinforced Polymer Composites

Abstract Fiber reinforced polymer (FRP) composites have attracted significant attention from the aerospace industry in recent years. FRP composites have the advantage of being light weight, high strength and resistant to corrosion making them an attractive option for various applications in the aerospace industry. However, FRP composites are prone to delamination which can eventually compromise structural integrity leading to sudden failure. An effective damage detection methodology is therefore necessary to monitor delamination in structures. In this study, mechano-luminescence-optoelectronic (MLO) composites were integrated with FRP composites to create MLO-FRP test coupons for delamination detection. The MLO composites were prepared by coupling a Poly(3-hexylthiophene) (P3HT)-based sensor with a copper dopped zinc sulfide (ZnS:Cu) composite. MLO composites were attached to the surface of FRP composites to create MLO-FRP test coupons. The MLO-FRP test coupon was mounted on a 3-point bending load frame and subjected to cyclic loading and unloadings at various displacements with the first displacement starting from zero to 5 mm. The second displacement starting from zero to 6 mm and so on. A direct current (DC) voltage was recorded from the MLO composites at a constant loading rate to validate its DC-based strain sensing capability. Other parameters such as applied load, strain energy release rate and strain were also obtained with the aim of tracking delamination propagation in FRP composites. Results showed MLO composites were sensitive to delamination propagation highlighting their potential to be used in structures for health monitoring.

Nondestructive quantification of internal raster path for additively manufactured components via ultrasonic testing.

This work investigates the viability of discerning the raster pattern of additively manufactured components using high frequency ultrasonic nondestructive testing. Test coupons were fabricated from poly cyclohexylenedimethylene terephthalate glycol using the fused filament fabrication process, in which layers were deposited at various predetermined raster angles. Each printed part was scanned using spherically focused, high-resolution, ultrasonic transducers of various peak frequencies between 7.5 and 15MHz. From the captured waveform data, images are extracted to observe the raster pattern in a layer-by-layer manner, with the results from the 10MHz element yielding the best performance. An in-house MATLAB script was developed to analyze the transducer signal to investigate C-scan images at various depths throughout the component. From the resulting C-scan images, one can consistently identify the proper raster orientation within 2°-4° in each of the first 10 deposited layers, with the accuracy decreasing as a function of depth into the component. Due to signal attenuation, there is insufficient data at depths beyond the 11th and 12th layer, to properly analyze the present data sets accurately. Validation was performed using X-ray computed tomography scans to demonstrate the accuracy of the ultrasonic inspection method.

Experimental and numerical investigations of S890 and S960 ultra-high strength steel circular hollow section columns

The experimental and numerical investigations of the buckling behaviour and load-carrying capacities of S890 and S960 ultra-high strength steel (UHSS) circular hollow section (CHS) columns were conducted and reported in this paper. The experimental program encompassed tensile coupon tests, initial global and local geometric imperfection measurements and pin-ended column tests. Parallelly, the numerical modelling program was carried out, where the developed finite element (FE) models were validated with reference to the experimental observations and then employed in parametric studies to result in additional numerical data, considering various cross-sectional dimensions and member lengths. Based on the obtained FE and test data, the applicability of the relevant codified design buckling curves in the current European, Australian and American specifications to S890 and S960 UHSS CHS columns was evaluated, since the aforementioned standards only cover the design rules with steel material yield strength no greater than 700 MPa. The assessment results showed that the European code and Australian standard can generally accurately predict the resistances of S890 and S960 UHSS CHS columns, but result in some unsafe data points, while the American specification yielded accurate and consistent resistance predictions. Finally, a revised Eurocode design buckling curve was proposed, accompanied by the reliability assessment through statistical analyses, with the improved design accuracy and consistency revealed.

Experimental study of in-fire and post-fire material response of high-strength aluminium alloys

This paper presents an experimental study on the residual material properties of high-strength aluminium alloys in and after fire. A testing programme was firstly conducted on grade 7075-T6 high-strength aluminium alloy, consisting of 13 in-fire and 24 post-fire coupon tests, to derive the material stress-strain responses at and after exposure to elevated temperatures ranging from 20 °C to 550 °C. The key temperature-dependent material properties including stiffness and strengths were determined from the measured stress-strain curves and normalised by their room-temperature counterparts, resulting in a series of in-fire and post-fire retention factors. These experimentally obtained retention factors were used to analyse the effects of elevated temperatures on the residual stiffness and strengths of high-strength aluminium alloys. The design in-fire retention factors, as given in the European, American and Chinese standards, and the existing predictive models in previous studies for the post-fire retention factors, were also quantitatively and qualitatively assessed based on the test data. They were found to be inapplicable due to less accuracy; especially when used for predicting the in-fire Young's moduli, a 25 % over-prediction can be yielded, leading to unsafe design. To address the shortcoming, a series of predictive models were developed to offer accurate predictions of the in-fire and post-fire residual stiffness and strengths of high-strength aluminium alloys, with the mean test-to-prediction ratios ranging from 0.995 to 1.074 for different in-fire and post-fire material properties and the design accuracy improved by at least 15 % than existing design models. Then, a two-stage Ramberg-Osgood material model that was proposed for describing the room-temperature stress-strain curves of structural aluminium alloys was considered in this study, with its applicability to high-strength aluminium alloys assessed. The considered Ramberg-Osgood model was found to well predict the in-fire and post-fire stress–strain curves of high-strength aluminium alloys. The proposed models for retention factors and stress–strain curves can be further assessed based on more test data on other new high-strength aluminium alloy grades. Future research can focus on developing new coating technologies and optimising existing treatments to improve the fire performance of high-strength aluminium alloy structures.

Validation of a ray-tracing-based guided Lamb wave propagation methodology in aerostructures

Validation of a ray-tracing-based guided Lamb wave propagation methodology in aerostructures

Elevated-temperature material properties and fracture behavior of fire-resistant steel FR355

Fire-resistant steel (FRS) has been increasingly used in engineering structures due to their advantages of excellent earthquake and fire resistance properties. Accurately characterization of material properties and limit states of steel material is very important for structural-safety design and finite element (FE) simulation. This paper presents the results of experimental and numerical investigations on the elevated-temperature performance of FRS in terms of material properties, full-range true stress-strain curve, ductile fracture behavior, which has not yet been reported. The experimental part of the present paper includes the steady-state tensile testing of conventional structural steel (CSS) and FRS at temperatures up to 800 °C. Hollomon law and weighted average method were adopted to derive full-range true stress-strain behavior of the investigated steels at elevated temperatures, which is required for ductile fracture simulation in ABAQUS. Ductile fracture of tensile coupon specimens was modelled using the stress-modified critical strain model (SMCS). To verify the accuracy and application of the proposed true stress-strain and fracture models, finite element (FE) simulations of tensile coupon tests were performed using ABAQUS/explicit. It shows that the FE simulation incorporating proposed material plastic behavior and ductile fracture model provides accurate predictions for fracture response of the tensile coupons.

Neutralization of ricin toxin on building interior surfaces using liquid decontaminants.

Ricin is a highly toxic protein, capable of inhibiting protein synthesis within cells, and is produced from the beans of the Ricinus communis (castor bean) plant. Numerous recent incidents involving ricin have occurred, many in the form of mailed letters resulting in both building and mail sorting facility contamination. The goal of this study was to assess the decontamination efficacy of several commercial off-the-shelf (COTS) cleaners and decontaminants (solutions of sodium hypochlorite [bleach], quaternary ammonium, sodium percarbonate, peracetic acid, and hydrogen peroxide) against a crude preparation of ricin toxin. The ricin was inoculated onto four common building materials (pine wood, drywall joint tape, countertop laminate, and industrial carpet), and the decontaminants were applied to the test coupons using a handheld sprayer. Decontamination efficacy was quantified using an in-vitro cytotoxicity assay to measure the quantity of bioactive ricin toxin extracted from test coupons as compared to the corresponding positive controls (not sprayed with decontaminant). Results showed that decontamination efficacy varied by decontaminant and substrate material, and that efficacy generally improved as the number of spray applications or contact time increased. The solutions of 0.45% peracetic acid and the 20,000-parts per million (ppm) sodium hypochlorite provided the overall best decontamination efficacy. The 0.45% peracetic acid solution achieved 97.8 to 99.8% reduction with a 30-min contact time.

Cyclic performance study of Q460D steel and ER55-G welds after high temperature

Coupon test specimens with the gauge length part of Q460D high-strength steel or ER55-G weld respectively were heated up to high temperature of 900 °C and then cooled in water. Uniaxial tensile tests were conducted on those cooled specimens. The engineering stress-engineering strain curves were obtained. Meanwhile, twelve smooth round bar specimens made of Q460D high-strength steel and ER55-G welds were heated up to high temperature of 900 °C and then cooled naturally or cooled in water. Cyclic loading tests with two different loading protocols, namely constant amplitude and variable amplitude, were conducted on these specimens at different strain levels. The engineering stress-engineering strain curves of each specimen were obtained, enabling to compare and analyze the differences of the results in cyclic mechanical properties of each specimen in terms of damage morphology, hysteresis performance, and ductility characteristics. The test results show that the different cooling methods after the high temperature of 900 °C have a significant impact on the ultimate strength and elongation of Q460D high-strength steel and ER55-G welds. The ultimate strength of the specimens after water cooling was significantly higher than that of the specimens after natural cooling, while the elongation result was opposite. It was obvious from the test results that water cooling and natural cooling after high temperature led to a decrease in ductility and energy absorption capacity of the base metal and welds to various degrees, while their energy dissipation capacity was significantly improved at low strain level.

Experimental and numerical investigations of high-strength cold-formed steel multi-limb built-up section columns

This paper investigates the buckling behaviour and load-carrying capacity of G550 high-strength cold-formed steel multi-limb built-up section columns under axial compression. Each built-up section consists of multiple lipped and unlipped channel sections connected by self-tapping screws. The C3U1 section is formed by three C-sections (i.e. lipped channel section) and one U-section (i.e. unlipped channel section), while the C2U2 section is formed by two C-sections and two U-sections. The experimental programme involved tensile coupon tests, measurements of initial geometrical imperfection, and column tests on ten C3U1 and ten C2U2 multi-limb built-up section columns. The numerical modelling programme was then performed, with the nonlinear finite element models created and validated against the experimental results. The applicability of the codified Effective Width Method and Direct Strength Method for predicting the strengths of G550 high-strength cold-formed steel multi-limb built-up section columns, as specified in the American Specification and Australian Standard, was then evaluated. The evaluation results showed that the codified Effective Width Method led to rather conservative and less accurate failure load predictions for G550 high-strength cold-formed steel multi-limb built-up section columns, while the codified Direct Strength Method provided accurate and consistent failure load predictions and thus applicable to the design of G550 high-strength cold-formed steel multi-limb built-up section columns. Concerning the lack of design guidelines specifically for multi-limb built-up sections, a modified Direct Strength Method was thus proposed as an alternative design method and shown to provide accurate failure load predictions.

Toward Fatigue-Tolerant Design of Additively Manufactured Strut-Based Lattice Metamaterials

Abstract The advent of additive manufacturing (AM) has enabled the prototyping of periodic and non-periodic metamaterials (a.k.a. lattice or cellular structures) that could be deployed in a variety of engineering applications where certain combinations of performance features are desirable. For example, these structures could be used in a variety of naval engineering applications where lightweight, large surface area, energy absorption, heat dissipation, and acoustic bandgap control are desirable. Furthermore, combining the multifunctional design optimization of these structures with progressive degradation due to cyclic loading could lead to fatigue-activated attritable systems with potentially tailorable performances not yet in reach by current conventional systems. Nevertheless, in order to deploy these complex geometry structures their multiphysics response has to be well understood and characterized. The objective of the current effort is to describe an initial approach for designing a uniaxial metamaterial specimen for fatigue testing as the first step toward the design of multi-axial fatigue test coupons. In order to compare bending- and stretching-dominated structures, two strut-based lattices made of Ti-6Al-4V alloy consisting of the octet and tetrakaidecahedron (or Kelvin) cells are examined. The specimens are designed to fail in the central area of the specimen where edge effects are minimized. Finite element results of the relevant structural mechanics are implemented and exercised to compare the performance of the eight relevant geometries and to evaluate the effect of relative density on fatigue life.

Analysis of Vibratory Stress Relief (VSR) Parameters on Mechanical and Metallurgical Properties of AISI 1008

Analysis of Vibratory Stress Relief (VSR) Parameters on Mechanical and Metallurgical Properties of AISI 1008

Predictive modeling of 3D textile composites using realistic micromechanical representations

An innovative and efficient progressive damage model, informed by coupon tests, and using minimal, readily available material parameters, is proposed in this paper to comprehensively predict the tensile, compressive and shear stress–strain response and strengths of a 3D woven angle interlock composite within 5% of test data. Realistic, high-fidelity microgeometries, with process-induced defects, and devoid of geometric assumptions, are constructed using the digital element approach. Excellent correlations with test micrographs are observed for yarn cross-section areas, aspect ratios and crimp ratios, that are on average within 5.5%, 1.9% and 1% of the micrograph measurements respectively. The mesoscale predictions capture the detailed failure mechanisms under each loading condition. Analytical stiffness expressions are derived in terms of the constituent material elastic properties and architecture geometry, which also predict the stiffnesses within 10% error margin. A robust workflow is established that combines the high-fidelity microgeometry generation, automated conformal finite element model creation and comprehensive property prediction capabilities to provide a holistic solution.

High velocity steel spheres impact and damage analysis of graphene doped T700 composite shielding panels

Composite materials are increasingly being used in aircraft components such as fuselage panels which are subject to collision with debris during take-off/landing or in flight. This article provides an analysis of the impact resistance capability of graphene doped T700 (T700G) multilayer composite, when subjected to steel projectile impacts. Test coupons with 300×300 mm were impacted by steel spherical projectiles, with impact energies ranging from 34 J to 338 J, using a compressed air operated cannon. Test data, such as impact and absorbed energy were determined through impact and residual projectile velocity, acquired using ballistic chronograph and two high speed cameras. Damage assessment was performed using non-destructive techniques such as Ultrasonic testing before and after impact test. Through modelling and simulation, using ABAQUS/Explicit, and recording the change of projectile velocity, the energy loss of projectile is calculated, and the impact resistance is judged together with damage extent. T700G has similar impact resistance to AL2024, underperforming when impacted by larger projectiles and presenting an overall higher damaged area.

Physical simulation of the underclad heat affected zone in a reactor pressure vessel to study intergranular cracking

Underclad cracking in nuclear pressure vessels was of significant concern in the 1970s and 1980s before mitigating adjustments were made both to steel compositions and manufacturing practice. Unfortunately, the cracking mechanisms are still not well understood, and this can undermine confidence when changes to cladding operations are under consideration. In this work, Charpy-sized test coupons of 18MND5 steel were subject to a range of thermal cycles that can be experienced by the substrate immediately adjacent to the interface with the overlay. The Charpy test results are considered in combination with analysis of the samples through field emission gun (FEG) scanning electron microscopy (SEM) and wavelength dispersive X-ray spectroscopy (WDXS). The findings suggest that the heat affected zone subject to the coarse grained plus intercritical thermal cycle, before post weld heat treatment, has a peculiar and potentially brittle microstructure. However, in a steel with no obvious macrosegregation, no clear evidence of intergranular cracking in the early stages of the post weld heat treatment could be found. The second part of the work focuses on the effects of segregation regions in large forgings, more specifically ghost lines, on the tendency for underclad cracking. Ghost line regions that subsequently form a coarse grained heat affected zone (CGHAZ), and are then heated to just under the A1 temperature, seem to be the most prone to intergranular cracking. Cracking susceptibility appears to be associated with elevated concentrations of alloying and impurity elements at grain boundaries.