Effect of hydrogen in aluminium and aluminium alloys: A review

Susceptibility of aluminium and its alloys towards hydrogen embrittlement has been well established. Still a lot of confusion exists on the question of transport of hydrogen and its possible role in stress corrosion cracking. This paper reviews some of the fundamental properties of hydrogen in aluminium and its alloys and its effect on mechanical properties. The importance of hydrogen embrittlement over anodic dissolution to explain the stress corrosion cracking mechanism of these alloys is also examined in considerable detail. The various experimental findings concerning the link between hydrogen embrittlement and stress corrosion cracking are also discussed.     

Improving the properties of aluminium alloys by retrogression and reageing

Retrogression and reageing heat treatments offer the potential of improved tensile properties in combination with greatly increased resistance to stress corrosion cracking. The potential of this technique is reviewed with respect to the current application in the European and North American Aerospace Industry. To illustrate the performance increase associated with RRA treatments, the stress corrosion cracking performance of the established aerospace plate and forging alloy 7010 has been evaluated using an alternate immersion constant load tensile type test (ECSS‐Q‐70‐37A) Specimens were cut from a large aerospace rectilinear forging and tested in three different tempers, T652, T7452 and a retrogressed and reaged condition (RRA). In the T652 condition the material has been shown to be highly susceptible to intergranular corrosion and stress corrosion cracking. In the T7452 and RRA conditions 7010 showed much improved resistance to SCC but pitting corrosion resulted in failure of some specimens within the 30day requirement of the test standard.     

Crack tip mechanics effects on environment-assisted cracking of beta-titanium alloys in aqueous NaCl

The objective of this research is to understand the effects of crack tip mechanics on environment-assisted cracking (EAC) of α-precipitation hardened β-Ti alloys. Precracked Ti–8V–6Cr–4Mo–4Zr–3Al and Ti–15Mo–3Nb–3Al are prone to severe EAC in aqueous NaCl when stressed under fixed or rising displacement. The latter is more damaging and establishes a lower bound threshold of the stress intensity ( K ) well below K _IC . EAC is intergranular and occurs at fast growth rates (d a /d t , up to 150 μm/s) for five orders of magnitude of loading rate, d K /d t . Cracking, due to hydrogen-environment embrittlement, is rate limited by lattice diffusion of hydrogen. EAC at fast d a /d t or high d K /d t requires process zone embrittlement sites very near to the crack tip or enhanced H transport. Subcritical d a /d t versus K depends on loading format through crack tip strain rate (ε˙_CT ) differences governed by d K /d t , d a /d t and creep. A continuum model approximates the contributions of d K /d t and d a /d t to ε˙_CT and EAC, but it is inconsistent with in situ measurements of crack tip strain. Intergranular cracking is exacerbated by high ε˙_CT that destabilizes the crack tip passive film and enables hydrogen uptake from an electrochemical reaction. EAC is mitigated when ε˙_CT is insufficient in these regards.     

Multiaxial fatigue behaviour of AA6068 and AA2017A aluminium alloys under in‐phase bending with torsion loading condition

This paper presents an analysis of the multiaxial fatigue properties of two selected aluminium alloys. Several experimental results were used to perform the analysis e.g. the latest experimental results done in Opole University of Technology on PA6 (2017 A), PA4 (6068) under bending, torsion, and combined bending with torsion. Analyses of the results were done to find similarities of the multiaxial fatigue behaviour of selected aluminium alloys. Based on the (σ_a – τ_a) curves, prepared for a fixed number of cycles, it was possible to show some tendencies of the multiaxial fatigue behaviour of selected material group. This is an important indicator while selecting proper multiaxial fatigue failure criterion suitable to perform fatigue life assessment of aluminium alloys.     

TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

Abstract

Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.     

On the stress corrosion cracking mechanisms of austenitic stainless steels

In this paper, experimental results on stress corrosion cracking in austenitic stainless steels are described. Crack growth data in sodium chloride solution for AISI 304 steel obtained for different metallurgical conditions, acoustic emission data recorded during crack growth and fractographic observations have been discussed with a view to identifying the operating mechanism. Some of the experimental observations such as crack propagation occurring in discontinuous jumps of the order of a few microns, lowering of the threshold stress intensity andJ-integral values on sensitization and cold working, typical transgranular fractographic features, transition in mode of fracture from transgranular to intergranular in sensitized conditions and activation energies of the order of 50 to 65 kJ/mol can all be accounted by hydrogen embrittlement mechanism. Hydrogen generated at the crack tip by corrosion reaction diffuses ahead of the crack tip under hydrostatic stress and influences the deformation process at the crack tip and also leads to the brittle component of the crack advance in jumps.     

碳钢在H2S／CO2体系中的应力腐蚀开裂机理

应力腐蚀开裂是含H2S／CO2天然气输送管线腐蚀破坏的主要形式,探究其开裂机理有利于实施有效的防护,目前相关研究尚不够深入。采用扫描电子显微镜（SEM）和X射线衍射（XRD）等表面分析技术对U形环X52钢试样在H2S／CO2介质中的应力腐蚀开裂（SCC）行为进行了研究,并分析探讨了其发生SCC的机理。结果表明：X52钢...     

Fatigue life estimation of non‐penetrated butt weldments in ligth metals by artificial neural network approach

This study presents a model for estimating the fatigue life of magnesium and aluminium non‐penetrated butt‐welded joints using Artificial Neural Network (ANN). The input parameters for the network are stress concentration factor K_t and nominal stress amplitude s_a,n. The output parameter is the endurable number of load cycles N. Fatigue data were collected from the literature from three different sources. The experimental tests, on which the fatigue data are based, were carried out at the Fraunhofer Institute for Structural Durability and System Reliability (LBF), Darmstadt – Germany. The results determined with use of artificial neural network for welded magnesium and aluminium joints are displayed in the same scatter bands of SN‐lines. It is observed that the trained results are in good agreement with the tested data and artificial neural network is applicable for estimating the SN‐lines for non‐penetrated welded magnesium and aluminium joints under cyclic loading.     

The effect of melt quality and quenching temperature on the Weibull distribution of tensile properties in aluminium alloys

In this study, the effect of melt quality and quenching temperature on the mechanical properties of 2024, 6063 and 7075 was investigated. The alloys were produced by Strain Induced Melt Activated (SIMA) method. T6 heat treatment was carried out by quenching in two different temperatures: 20 °C and 80 °C. Scanning electron microscope analyses were carried out on the fracture surfaces. In addition, reduced pressure tests were carried out to correlate the mechanical test results with metal quality. The results were analysed by Weibull statistics. It was found that samples quenched at 80 °C showed better mechanical properties and there was a good correlation between bifilm index and Weibull distributions.     

Eignungsprüfungen an Bauteilen und Bausystemen

Suitability tests on building components and systems Xella produces building materials according to European product standards and European technical approvals, respectively. Products of a new type or manufactured with a new raw material undergo an internal release process in which the requirements coming from standards, approvals and supplementary national application documents are proved. In addition, the interaction of the Xella building materials with further products applied on construction site are tested. This includes systems like masonry, external thermal insulation composite systems (ETICS), rendering systems, large‐format reinforced components, structures with fixing elements and much more. The scope of the system tests is determined by the intended application and by Xella internal requirements which, in many points, exceed the sets of rules. This article demonstrates the spectrum of these tests and, using the example of the render and mortar tests, the principle of the internal test guidelines.     

Non‐destructive assessment of retaining wall of former coal mine plant

A large retaining wall (appr. 9 m high and 286 m long) of a former coal mine has been investigated using Ground Penetrating Radar (GPR), with two low frequency antennas (200 and 400 MHz), and Large Aperture UltraSound (LAUS) to gather information about the condition and inner structure of the wall. Three vertical lines were selected in representative areas of the wall to evaluate the usefulness of Non‐Destructive Testing (NDT) with these two methods. The LAUS results showed the layer structure at one line where the wall was enforced by a concrete shell and were unspecific regarding the inner structure beyond the first layer. GPR results were able to be collected much faster and showed some internal features. Penetration was limited to 2–3 m due to the high absorption in the material.     

Bond behaviour of a textile‐reinforced mortar for AAC masonry

The bond behaviour of a textile reinforced mortar (TRM) applied to autoclaved aerated concrete (AAC) masonry has been evaluated experimentally. The TRM is composed of a glass‐fibre mesh combined with a cementitious mortar and is intended to strengthen AAC masonry walls subjected to out‐of‐plane bending during an earthquake. The main components have been characterized with preliminary tests. Then, pull‐off and shear bond tests have been performed to determine the bonding properties of the TRM applied to the AAC substrate. Three types of AAC blocks have been used, which differ in the bulk density and compressive strength, to evaluate possible variation in the bond strength. The results of the experimental campaign have shown a good performance of the strengthening system. In most cases, the bonding between TRM and masonry was maintained up to tensile failure of the dry textile. As expected, the masonry samples realized using AAC blocks with a higher bulk density showed better performances. The paper presents and discusses main test results, providing background data for future recommendations for the use of the analysed strengthening system in AAC masonry structures.     

A students' challenge for the estimation of the maximum compressive load of masonry prisms / Studentenwettbewerb zur Ermittlung der maximalen Druckfestigkeit von Mauerwerksprismen

The paper presents the “IMC Students' Challenge” competition held in 2014, during the 9th International Masonry Conference, in Guimarães. The objective of the competition was to predict the maximum compressive load of two masonry prisms built of solid bricks, and of hollow blocks, with mortar joints. To increase the complexity of the problem, all prisms were tested under eccentric load. The students, who enthusiastically participated in the final laboratory tests, presented different approaches to estimate the maximum eccentric compressive force on masonry prisms. The challenge was a great experience, not only for students and conference participants, but also for sponsors and organizers.     

Evaluation of the quality of historic brickwork / Die Ermittlung der Mauerwerksgüte von Bestandsmauerwerk aus kleinformatigen Ziegeln

The quality of historic brickwork is a significant matter for structural engineers for modernisation, also due to its concentration in public areas. One substantial issue for structural inspection is the characteristic masonry strength or allowable masonry stress, which are defined by the compressive strength. There are several ways of assessing the compressive strength of brickwork or its components, but most of these test methods are not described in engineering standards or are only based on standards for new buildings. This article provides information about the various testing methods and gives recommendations for the selection of applicable test procedures and the experimental scope.     

Shear resistance verification of late 19th century masonry according to EC 6 – State of the art

A large part of buildings in Central European cities like Vienna was built in the Gründerzeit period between about 1840 and 1918 [1]. These buildings were constructed according to traditional rules. Current urban development requires historic buildings to be structurally adapted, which requires retroactive analysis of the masonry walls; in Austria according to ÖNORM EN 1998‐3 [2] and ÖNORM EN 1996‐3 (EC 6) [3]. Here, special focus is on the load transfer of horizontal earthquake loads, i. e. the shear strength of masonry walls. This paper describes the verification of historic masonry in detail and discusses individual components. Initial shear strength, load‐influenced friction and the length of the compressed part of the wall are first determined using results from experimental testing and relevant literature and then compared to the approaches in EC 6. Based on this analysis, recommendations are provided to make theoretical approaches more realistic.     

Shear capacity of brick masonry with partially supported slab

In order to investigate the influence of a partially supported slab on the in‐plane shear resistance of masonry walls, six shear tests on full scale walls were performed at the Chair of Structural Concrete of the University of Kassel in cooperation with ”Arbeitsgemeinschaft Mauerziegel“. The walls were made of large chamber clay masonry units and the depths of the partially supported slabs were different. The large chamber units are typically used for thermally insulated exterior walls, with the slabs only being partially supported. The influence of this constellation is not taken into account in shear capacity assessment according to the current version of Eurocode 6 for Germany. This article describes the test results and provides a comparison with Eurocode 6. In addition, the results of older shear tests on masonry walls made of vertically perforated units with eccentric load application were used.