Enhanced assessment of ageing phenomena in steel structures based on materials data and non‐destructive testing

The increasing amount of ageing civil steel infrastructure requests an enhanced assessment of this infrastructure in terms of determining its residual fatigue life in a more realistic way than this has been done in the past. Often the relevant materials data for cyclic loading of such an ageing infrastructure is not available and its retrieval turns out to be relatively cumbersome bearing the urgency in data availability and continuous cost pressure in mind. This article addresses different approaches and techniques on how materials data for cyclic loading can be obtained at a fraction of the effort compared to state‐of‐the‐art techniques, considering load increase tests, non‐destructive testing techniques and finally even a stepped bar specimen allowing a complete set of materials data (stress‐strain behaviour and stress‐ and strain‐life curve) to be obtained with a single specimen in the end only. Options for ’digitizing’ materials data evaluation are discussed and some prospect on application of those novel approaches and techniques in damage accumulation assessments on real steel infrastructure is provided.     

Application of the hybrid process ultrasound enhanced friction stir welding on dissimilar aluminum/dual‐phase steel and aluminum/magnesium joints

Friction stir welding as a solid‐state joining method with its comparatively low process temperatures is suitable for joining dissimilar materials like aluminum/magnesium or aluminum/steel. Such hybrid joints are of great interest regarding lightweight efforts in different industrial fields like the transportation area. The present work investigates the influence of additionally transmitted power ultrasound during the friction stir welding on the joint properties of EN AC‐48000/AZ91 and EN AW‐6061/DP600. Therefore, conventional friction stir welding was continuously compared to ultrasound enhanced friction stir welding. Light microscopic analysis and nondestructive testing of the joints using x‐ray and high frequency ultrasound show different morphologies of the nugget for the aluminum/magnesium joints as well as differences in the amount and size of steel particles in the nugget of aluminum/steel joints. Scanning electron microcopy proves differences in the thickness of continuous intermetallic layers for the aluminum/steel joints realized with and without power ultrasound. Regarding the tensile strength of the joints the power ultrasound leads to increased joint strengths for EN AC‐48000/AZ91 joints compared to a decrease for EN AW‐6061/DP600 joints. Corrosion investigations show an influence of the ultrasound power on the corrosion properties of EN AC‐48000/AZ91 joints which is attributed to a changed aluminum content in the nugget region. Because of the great potential difference between the magnesium and the nugget phase the transitional area exhibits strong galvanic corrosion. For EN AW‐6061/DP600 joints an increased corrosion caused by galvanic effects is not expected as the potentials of the EN AW‐6061 aluminum alloy and DP600 steel are very similar.     

Influence of surface residual stresses on gigacycle fatigue response of high chromium cold work tool steel

The effect of surface compressive residual stresses (RS) induced by surface grinding and polishing on the gigacycle fatigue behavior of medium‐carbon high‐chromium alloy cold work tool steel was evaluated. Two test series were performed: Specimens of series I revealed high compressive RS of about ‐800 MPa at the surface, resulting from grinding with fine emery paper, which treatment had definitely a beneficial influence on the fatigue endurance strength. The existence of surface RS was also revealed to be responsible for the location of the fatigue crack initiation. High compressive RS favored internal crack origins. In this case crack nucleation sites were primary carbide clusters in the interior of the specimen, forming so‐called fish‐eyes at the fracture surface. In contrast, specimens of test series II had only very low RS, which enabled crack initiation near/at the surface at primary carbides/clusters. Furthermore, it has been shown that the high initial RS are prone to partial relaxation through cyclic loading for which the mechanisms are currently unknown. In this case near‐surface induced failure was obtained. It was possible to confirm the experimentally obtained data by the use of the concept of local fatigue strength as function of effective RS. The relaxation of high initial RS was experimentally confirmed through RS measurements at runout specimens (10¹⁰ cycles without failure).     

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.     

Residual stress field determination in welds by means of X‐ray,synchrotron and neutron diffraction

To what extent the welding residual stresses influence fatigue is still unclear and matter of debate. An important reason for this lack of clarity is that the exact determination of residual stress fields in welds is complicated which leads to conservative assumptions about these stresses in the fatigue design codes. The advances in the diffraction analysis of materials offer the opportunity for the full‐field residual strain mapping in welds albeit at the cost of time and technical complexity. In this work residual stress field determination in welded S1100QL specimens by means of the x‐ray, synchrotron and neutron diffraction techniques was undertaken. The results revealed that the maximum values of surface residual stresses are not as frequently assumed, as high as the yield strength in small scale specimens. At the weld toe which could serve as a fatigue crack initiation site, even lower residual stresses than the weld centreline could be present. The in‐depth measurements revealed that the effective part of the residual stress field which could be decisive for the fatigue failure initiation is concentrated at the surface of the weld.     

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.     

Determination of local characteristics for the application of the Weakest‐Link Model

The Weakest‐Link model is based on defects that are statistically distributed within the material with local stress. The failure at least at one defect causes the failure of the total structure. Based on this model, the so‐called statistic size effect can be evaluated in the case of cyclic loading and in the case of static loading the failure behaviour of ferritic steel within the brittle fracture range is highlighted. The application of the Weakest‐Link model requires the allocation of the local characteristics ‐ surface and / or volume ‐ to the discrete points of the stress. By using the method “SPIEL” which is independent from the FE code used, the allocation of couples of values ‐surface and stress and / or volume and stress ‐ by a suitable choice of unit load cases is possible. In consequence of the method “SPIEL” particularities are to be taken into consideration. In the present paper these particularities will be described exemplarily for the FE programs ABAQUS¹ and ANSYS².     

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.     

Size effects in the fatigue behavior of welded steel tubular bridge joints

Tubular space trusses for bridge applications use thick‐walled tubes. The reduction in fatigue resistance due to geometrical size effects is thus an important issue. In order to carry out a thorough study, both fatigue tests on large‐scale specimens and advanced 3D crack propagation modelling were carried out at ICOM/EPFL. The study is limited to circular hollow sections (CHS) K‐joints. An alternate current potential drop (ACPD) system is used to measure crack depth on nodes of the tested truss specimens. The results obtained from the tests are given in the paper in terms of S‐N data, crack depth versus number of cycles and deduced crack propagation rates. The numerical model was developed using the dual boundary elements method (DBEM), software BEASY?, and was validated with fatigue tests data. The stress intensity factors (SIF) along the doubly curved crack front at different crack depths were obtained. With this model, a parametric study investigates the influence of geometry, size and load case on fatigue life. The results of both proportional and non‐proportional sizing effects on fatigue strength are presented. The paper shows that size effects (proportional and non‐proportional) can be expressed as a function of the non‐dimensional parameters and chord thickness.