CONSTRUCTION ASSIGNMENT

Material Failure and Embodied Energy



Material Failure

Introduction

When any structural material losses its capability of load carrying, the phenomena refers to the structural failure. This failure occurs when the material stresses to its maximum strength limit, which causes fracture or abnormal deformations. This report discusses three materials and structures, namely welded steel structures, Cast Iron water pipes, and Concrete ties, and the main causes of their structural failure. Also, the report presents the methods used to prevent the failure in these structural materials.

Discussion

Welded Steel Structures

This section of the paper discusses how fatigues impacts the welded steel structures, as this is one of the most degradation in such structures. The main reason behind this degradation of the welded parts is that the welded components are less tolerant for nearing loads as compared to the non welded counter parts. The weld is more prone to the crack as it contains internal flaws. Also, the weld creates external stress which encourages the crack to initiate, as the welding process introduces the residual stresses in the welded region. There are four types of cracks that may occur in the welded steel structures.

The steel structure, which is subject to the fluctuating loads mainly, fails due to fatigue (Shine U.P., 2008, pp.1). After being subjected to the fluctuated loads for a continuous period of time, the welded steel structure often breaks into two pieces. The fatigue occurs when the cyclic stress is less than the material's tensile strength (Craig B.D., 2010, pp.13).

The process of fatigue divides into three stages including, crack initiation, crack propagation and finally the degradation of the material. The cyclic loads introduce the crack in the welding, and if the load condition remains same for significant time, the crack becomes larger and finally the welded structure splits into fragments. The welded steel structures are often repaired using heating methods.

Creep is another mode of failure for welded steel structures. This is a time consuming process that results in the dimensional change of the material. This process is temperature dependent as well, as the rate of dimension change under some applied stress increases as a result of the temperature increases. The relation between increase in temperature and the rate of creep is shown in figure 1. The failure occurs when the welded structure becomes too dimensionless to perform its intended functionality.

Figure 1: Relation Between Temperature and Extension of Material

The creep occurs at the micro scale level. When the microstructure of the material can no longer bear the stress, it begins to move towards the boundaries of the structure, resulting in the change of shape and dimension of the material or the welded structure.

To overcome the creep from the welded structure, the precautions before welding include high pre-heats, and proceeding to stress relief directly after welding.

Grey Cast Iron Pipes

Grey Cast Iron is used in water supply pipes in majority of the countries. This material is most prone to the failures (Makar J.M., et.al, 2001, pp.1). The main cause responsible for the failure of Cast Iron ...