Modeling of residual stresses in structural stainless steel sections

The influence of residual stresses on structural members is to cause premature yielding and loss of stiffness, often leading to deterioration of load carrying capacity. Knowledge of their magnitude and distribution is therefore important for both structural design and finite-element simulations, and hence extensive studies have been performed on structural carbon steel components. With greater emphasis now being placed on durability and reducing consumption of resources, the use of stainless steel in construction is growing, heralding the need for a more precise understanding of its structural response. Stainless steel exhibits differing physical and thermal properties from carbon steel, both of which influence the formation of residual stresses, and it cannot simply be assumed that residual stress models for carbon steel are also appropriate for stainless steel. This paper examines all existing data on residual stresses in stainless steel sections, including data generated from a recent experimental program carried out at Imperial College, London and summarized herein. The collated residual stress data have been used to develop models for predicting the magnitude and distribution of residual stresses in press braked, cold rolled, hot rolled, and fabricated stainless steel structural sections

Life-cycle costing of metallic structures

Structural material selection has traditionally been based on Initial material cost. However, growing pressure on the construction industry to consider the longer-term financial and environmental implications of projects is encouraging a more holistic view. Thus, materials with higher initial costs, but which offer cost savings over the life cycle of a structure, are gaining increasing recognition. The life-cycle costs of structures of two such metallic materials, namely aluminium alloy and stainless steel, are compared with those of ordinary structural carbon steel in the present study. Two structural applications - a typical office building and a bridge - are analysed, while offshore applications are briefly discussed. The ratio of initial material cost per tonne was assumed to be 1.0:2.5:4.0 (carbon steel:aluminium alloy:stainless steel). Following a preliminary structural design to current European design standards taking due account of the material densities and structural properties (principally strength and stiffness), it was found that on an initial cost basis, carbon steel offers the most competitive solution for both the building and the bridge. However, considering the additional life-cycle costs including maintenance costs, end-of-life costs and the residual value of the structure (appropriately discounted to present values), the results indicate that carbon steel offers the most competitive life-cycle solution for the office building but delivers the most expensive life-cycle solution for the bridge. Overall, it is concluded that on a whole-life basis aluminium alloy and stainless steel may offer more competitive solutions than carbon steel for bridges and exposed areas of building structures

Effect of Corner Strength Enhancement on Shear Behaviour of Stainless Steel Lipped Channel Sections

During the cold-forming process of manufacturing, stainless steel sheets undergo plastic deformations, in particularly around corner regions of press braked sections. These plastic deformations lead to significant changes in material properties of stainless steel compared to its flat sheet properties. Consequently, yield strength and ultimate strength increments can be envisaged and this process is termed as cold working. Stainless steel exhibits significant level of strain hardening under plastic deformations. This is the main reason for these strength enhancements. In the structural design process of stainless steel sections, these strength increments are required to be considered to harness the benefits arising from it. Therefore, previous research proposed predictive models for these strength enhancements. In this context, the effect of corner strength enhancement on press-braked stainless steel lipped channel sections under shear was examined in this paper. 120 finite element models were developed. Different corner radii and section thicknesses were taken into account. Results highlighted that the effect of cold working on the shear capacity of stainless steel lipped channel sections is more significant in compact sections compared to slender sections where up to 9% increment was observed. Further analysis was conducted using 40 finite element models to highlight the inelastic reserve capacity available in compact stainless steel lipped channel sections in shear. From the results, it was concluded that when web slenderness is less than 0.25 more than 40% shear capacity increment can be achieved due to strain-hardening of stainless steel

Residual stress analysis of structural stainless steel sections

The magnitude and distribution of residual stresses in structural carbon steel sections have been thoroughly investigated. However, few residual stress measurements have been made on structural stainless steel sections. Stainless steel has differing material stress–strain characteristics and thermal properties to carbon steel, both of which influence the formation of residual stresses. This suggests that established carbon steel residual stress models may not be appropriate for stainless steel. With increased use of stainless steel in load bearing applications, it is important to establish the residual stresses that exist within structural members. An experimental program to quantify the residual stresses in stainless steel sections from three different production routes has therefore been carried out. Comprehensive residual stress distributions have been obtained for three hot rolled angles, eight press braked angles and seven cold rolled box sections, with a total of over 800 readings taken. This paper presents the experimental techniques implemented and the residual stress distributions obtained as well as discussing the assumptions commonly made regarding through thickness residual stress variations. In the hot rolled and press braked sections, residual stresses were typically found to be below 20% of the material 0.2% proof stress, though for the cold rolled box sections, whilst membrane residual stresses were relatively low, bending residual stresses were found to be between 40% and 70% of the material 0.2% proof stress

Early–Middle Holocene vegetation history, climate change and human activities at Lago Riane (Ligurian Apennines, NW Italy)

The radiocarbon-dated palaeoecological study of Lago Riane (Ligurian Apennines, NW Italy) presented here forms part of a wider investigation into the relationships between Holocene vegetation succession, climate change and human activities in the northern Apennines. The record of vegetation history from Lago Riane indicates that, since the end of the last glaciation, climate change and prehistoric human activities, combined with several local factors, have strongly influenced the pattern and timing of natural vegetation succession. The pollen record indicates an important change in vegetation cover at Lago Riane at ~8500–8200 cal. years b.p., coincident with a well-known period of rapid climate change. At ~6100 cal. years b.p., Fagus woodland colonised Lago Riane during a period of climate change and expansion of Late Neolithic human activities in the upland zone of Liguria. A marked decline in Abies woodland, and the expansion of Fagus woodland, at ~4700 cal. years b.p., coincided with further archaeological evidence for pastoralism in the mountains of Liguria during the Copper Age. At ~3900–3600 cal. years b.p. (Early to Middle Bronze Age transition), a temporary expansion of woodland at Lago Riane has been provisionally attributed to a decline in human pressure on the environment during a period of short-term climate chang