Technical English Week 1 Unit 1 Shahrood University

Technical English Week 1: Unit 1 Shahrood University of Technology 12/1/2020 English for Civil Engineering Students 1

Modes of Failure • When a structural member is subjected to loads, its response depends not only on the type of material from which it is m~de, but also on the environmental conditions and the manner of loading. 12/1/2020 English for Civil Engineering Students 2

• Depending on how the member is loaded, it may fail by excessive displacement, which results in the member being unable to perform its design function; it may fail by plastic deformation (yielding), which may cause a permanent undesirable change in shape; it may fail due to a fracture (break), which depending on the material and the nature of loading, may be of a ductile type preceded by appreciable plastic deformation or of a brittle type with little or no prior plastic deformation. 12/1/2020 English for Civil Engineering Students 3

• Materials such as glasses, ceramics, rocks, plain concrete, and cast iron are examples of materials (brittle materials) that fracture in a brittle manner under normal environmental conditions and the slow application of tension load. In uniaxial compression, they also fracture in a brittle manner, but the nature of the fracture is quite different from that in tension. Depending on a number of conditions such as environment, rate of load, nature of loading, and presence of cracks or flaws, structural metals may exhibit ductile or brittle fracture. 12/1/2020 English for Civil Engineering Students 4

• One type of loading that may result in brittle fracture of ductile metals is that of repeated loads. For example, if a uniaxially loaded bar with smooth surface is subjected to repeated cycles of alternately applied tensile and compressive loads of equal magnitude, it may fail by fracture (usually in a brittle manner for high cycle fatigue) at a stress level considerably below the magnitude of stress that causes failure by fracture under a noncyclic static load. 12/1/2020 English for Civil Engineering Students 5

• Fracture of a structural member under repeated loads is commonly called fatigue fracture or failure. Fracture by fatigue may start by the initiation of one or more small cracks, usually in the neighborhood of the maximum critical stress in the member. Repeated cycling of the load causes the crack or cracks to propagate until the structural member is no longer able to carry the load across the cracked region, and the member ruptures. 12/1/2020 English for Civil Engineering Students 6

• Another manner in which a structural member may fail is that of elastic or plastic instability. In this failure mode the structural member may undergo large displacement from its design configuration when the applied load reaches a critical value, the so called buckling load (or instability load). 12/1/2020 English for Civil Engineering Students 7

• This type of failure may result in excessive displacement or loss of ability (because of yielding or fracture) to carry the design load. In addition to the above failure modes, a structural member may fail because of environmental corrosion (chemical action). 12/1/2020 English for Civil Engineering Students 8

• To elaborate upon the modes of failure of structural members we discuss more fully the following categories of failure modes: – 1. Failure by excessive deflection – 2. Failure by yielding • (a) Ordinary (room) temperatures • (b) Elevated temperatures (creep) – 3. Failure by fracture • • 12/1/2020 (a) Sudden fracture of brittle materials (b) Fracture of cracked or flawed members (c) Progressive fra~ture (fatigue) (d) Fracture with time at elevated temperatures English for Civil Engineering Students 9

• These failure modes and their associated failure criteria are most meaningful for simple structural members (for example, tension members, columns, beams, circular cross section torsion members). For more complicated two and three dimensional problems, the significance of such simple failure modes is open to question. 12/1/2020 English for Civil Engineering Students 10

• Many of these modes of failure for simple structural members are well known to engineers. However, under unusual conditions of load or environment, new types of failure modes may occur. For example in nuclear reactor systems, cracks in pipe loops have been attributed to stress assisted corrosion cracking, with possible side effects attributable to residual welding stresses. 12/1/2020 English for Civil Engineering Students 11