Cracking failure case studies

Equipment Cracking Failure Case Studies Equipment fails either alone or in combination with other factors, including substandard materials, improper material selection, poor design, equipment abuse, unexpected stresses or environmental conditions, and poor maintenance practices and/or neglect. Many failures, in one way or another, involve human error to some extent. 

PHOSPHATE ESTERS AS STRESS CRACK AGENTS -CASE STUDIES IN FAILURE ANALYSIS... 

Case studies in fast fracture and fatigue failure 165 Integration gives the number of cycles to grow the crack from to 02 ... 

This case study involved medical diagnostic parts manufactured from PC resin, which were beginning to break too easily. To determine the cause of the failure, a good sample was submitted for comparison to a cracked part. Two possible causes for the failures were postulated. These include brittleness due to an excess level of filler, or the presence of voids due to insufficient drying of the resin prior to molding. 

P. Jones, T. Hetu. A Case Study of Hydrogen Induced Cracking Failure - Investigation, Remedial Action and Maintenance. NACE International, Calgary Section, One Day Seminar on Failure Mechanisms, Calgary, Alberta, Canada, 1998-04-07. 

Table 1. Probability distributions and parameters (i.e., means and standard deviations) of the uncertain variables xi, X2, x, and X4 of the cracked plate model of Section 4.2 for the four case studies considered (i.e.. Cases 0,1,2 and 3) the last row reports the values of the corresponding exact (i.e., analytically computed) failure probabilities, P(F) (Gille 1999).

A literature search failed to find any other reported examples of Pebax catheters failing by brittle cracking, informal evidence for cracking of other catheters came during the 2001 ANTEC conference in the USA, when a short paper of this case study was read in the Failures Analysis and Prevention... 

Abstract Injection moulding, extrusion and other processes for manufacturing polymer components are discussed. Several case studies are presented where manufacture of polymer components is crucial to product performance, such as injection-moulded polycarbonate coimectors in a catheter, cylinder guard fractures, sight tubes, a crutch failure, and ozone cracking of tubing and a condom. 

Failure cases by carburisation occur mainly at high temperatures (>1000 °C), e.g. in the steam cracking of hydrocarbons for production of olefins (Fig. 1.1). Studies on carburisation have been conducted in CH4/H2 mixtures [2-6], where exact carbon activities can be established below Oc = I, i.e. without carbon deposition. Some researchers have used other hydrocarbons, such as propylene, in their studies on carburisation [7, 8] which are very unstable and decompose on metal surfaces under carbon deposition. In these studies, it is assumed that ac = 1 on the metal surface. 

Because ceramics are brittle, they are susceptible to catastrophic failure under mechanical load. The useful strength of a ceramic is determined by the flaw population stresses are concentrated at flaws, which cause cracks to propagate to failure. The critical property for ceramics in load-bearing uses is not the strength, but the fracture toughness—the resistance of the ceramic to crack propagation. The fracture surface of a ceramic bears the evidence of its failure. One must read the features in a fracture surface to understand the origin and path of the fracture. The case study by E. K. Beauchamp shows how much practical information can be obtained from ceramic fracture analysis. 

Structures constructed from alloys that exhibit this ductile-to-brittle behavior should be used only at temperatures above the transition temperature to avoid brittle and catastrophic failure. Classic examples of this type of failure were discussed in the case study found in Chapter 1. During World War II, a number of welded transport ships away from combat suddenly split in half. The vessels were constructed of a steel alloy that possessed adequate toughness according to room-temperature tensile tests. The brittle fractures occurred at relatively low ambient temperatures, at about 4°C (40°F), in the vicinity of the transition temperature of the alloy. Each fracture crack originated at some point of stress concentration, probably a sharp corner or fabrication defect, and then propagated around the entire girth of the ship. 

With a recent push toward non-brominated flame retardants, phosphorus-based alternatives, such as phosphate esters, are used more frequently for various applications. Their use as plasticizers is also well known. However, their function as environmental stress crack agents of various thermoplastics is less well recognized. Two case studies, one - in which a triaryl phosphate was a component of the formulation, the other - in which it was migrating from an adjacent component illustrate some of the problems with their use. Fractographic analysis and various analytical techniques were used to determine a root cause of each of the two failures. 

We have studied the the fracture properties of such elastic networks, under large stresses, with initial random voids or cracks of different shapes and sizes given by the percolation statistics. In particular, we have studied the cumulative failure distribution F a) of such a solid and found that it is given by the Gumbel or the Weibull form (3.18), similar to the electrical breakdown cases discussed in the previous chapter. Extensive numerical and experimental studies, as discussed in Section 3.4.2, support the theoretical expectations. Again, similar to the case of electrical breakdown, the nature of the competition between the percolation and extreme statistics (competition between the Lifshitz length scale and the percolation correlation length) is not very clear yet near the percolation threshold of disorder. 

In metal alloys the combination of stress and environment can also lead to premature failures, indicated as Stress Corrosion Cracking, SCC [1]. The influence of the environment on SCC is generally of a chemical nature a chemical reaction occurs between the metal and the environment. Most of the research published on the ESC of polymers focuses on ESC in which the environment influences the material only physically [2-8]. In such cases the mechanism of ESC is studied and models are established for ESC prediction [9]. These models for physical ESC are based predominantly on the solubility parameters of the considered polymer/environment combination. In other words, ESC is mainly a consequence of polymer softening, i.e. it is a reduction of the interaction between the polymer chains that lowers the yield stress. 

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