Engineering Disasters: Learning from Failure • Engineering disasters have resulted in loss of life, injuries, and billions of dollars in damage. Figure 1 illustrates characteristic creep … • Creep and high temperature failure • Creep testing • Factors affecting creep • Stress rupture life time behaviour • Creep mechanisms •Example • Materials for high creep resistance-Refractory metals-Superalloys Dr. M. Medraj iversity MSE 521 Lecture 14/2 • Materials are often placed in service at elevated temperatures and static UNIT V Lecturer4 2 Fatigue Fatigue is caused by repeated application of stress to the metal. Failure can occur after an extended period of high stress. Failure, due to microstructure and/or metallurgical changes e.g. T his article addresses one of the most important yet least understood plastics failure mechanisms, creep rupture. Common design features that expose a plastic product to continuous stress, and thus are at risk of creep rupture, include: • structural components (beams, Creep is far from being negligible as it is involved in many accidents around the world. Consider a Bar with Load P acting on the centroid of the bar. Creep failure of a pipe. grain boundary separation and the formation of internal cracks, caviti es and voids. The article provides a description of microstructural changes and damage from creep deformation, including stress-rupture fractures. reduction in cross -sectional. • Specimen geometry: Creep test is conducted in uniaxial tension using specimen having Referring to the creep curve in Fig. Fatigue and creep 1. Creep, expressed as a percent, equals total deformation minus initial deformation divided by initial deformation, time 100. 3. This article reviews the applied aspects of creep and stress-rupture failures. • Primary causes for engineering disasters: – Design flaws – Material failures – Extreme conditions or environments (not necessarily preventable) – Some combinations of the reasons above. Failure due to material creep is likely to occur if a material is loaded for sustained periods above the creep limit stress and above the creep threshold temperature. 12, one can see that (5) ε ̇ ≈D ε f t f, where ε f is the strain to failure, and D is a constant which depends on the mechanism of creep fracture: the maximum value of D is obviously 1.0, but it can be much smaller than this if there is a large strain during tertiary creep… It is the failure of a material by fracture when subjected to a cyclic stress. Minimum strain rate vs. time to failure This type of relation is based on the observation that strain is the macroscopic manifestation of the cumulative creep damage. Creep rupture occurs within plastic parts that are exposed to continuous stress over an extended period of time (Figure 1).Common design features that expose a plastic product to continuous stress, and thus are at risk of creep rupture, include: Stage III : Tertiary Creep (failure -rupture) Strain rate increases. Fatigue Fatigue failure is a phenomena which can occur in components which are subjected to cyclic loading. For example, some experts say that the collapse of the world trade center was due to Creep (Search for the word « creep » in the article to find the explanation) Fatigue is distinguished by three main features. It discusses the microstructural changes and bulk mechanical behavior of classical and nonclassical creep behavior. In the initial stage, creep occurs at a relatively high rate and then continues at a very slow rate. least understood plastics failure mechanisms, creep rupture. As such, it is implied that failure will occur when the damage in the material in form of creep cavities and cracks resulting from coalesced creep cavities reaches a critical level. UNIT V Lecturer4 1 LECTURER 4 Fundamental Mechanical Properties Fatigue Creep 2. Creep rupture occurs within plastic parts that are exposed to continuous stress over an extended period of time (Figure 1). Material develops Addition Strain Over Long Period of time Under Constant Loading.