In this article, career Metallurgist, Dawn DiMarco provides an overview of fracture surface examination by revisiting various analyses involving failed fasteners. As a materials scientist at Robson…
Material science is divided into three categories: metals, ceramics, and polymers. In this article, materials expert James Derby provides an introduction to failure analysis and fractography as they relate to glass and other ceramic materials. Examples of failure analysis investigations from Mr. Derby’s forensic casework are provided to demonstrate the way failure origin and type are identified through failure analysis.
Ceramics are compounds made up of nitrides, carbides, and oxides. Glass is a common ceramic material which contains silicon dioxide.
Ceramics are used in a wide range of applications ranging from consumer goods to specialty technical products. Our most common interactions with ceramics include windows, shower doors, and beverage containers, but specialized ceramics are used in a range of technical applications, such as aerospace coatings, wear parts for rotating machinery, refractories for processing metals, and ballistic armor for military use.
Glass
The most common type of glass is a soda lime silicate glass containing 74% Silicon dioxide, 13% Sodium Oxide, 10.5% Calcium Oxide, 2% Magnesium Oxide, 0.3 % Potassium Oxide, and 0.2 % Magnesium Oxide.
There are specific treatments for glass products that can be applied to make the product safer or more effective in different situations.
In our forensic casework, our experts are commonly tasked with determining how and why glass materials failed within a specific incident, whether or not the materials used were appropriate for the application, and if manufacturing defects or handling contributed to the cause of a loss.
Porcelain
Plumbing fixtures, dishes, and decorative items are commonly made from porcelain, which consists of 70% silicon dioxide, 15% aluminum oxide, 4% calcium oxide, 2% magnesium oxide, and 9% sodium and potassium oxide.
Due to the high amount of silicon dioxide in porcelain, manufacturers must consider phase transformations that cause volumetric changes. When Quartz transforms into Cristobalite in the manufacturing of porcelain, a volumetric expansion takes place at temperatures above 1050 C. If cooling occurs too rapidly, volumetric contraction can cause excessive internal stresses which can result in fracturing.
Quality assurance is essential during the production of porcelain and other glass products. Small cracks and other structural defects can lead to eventual systemic failures that can cause physical harm to the user or substantial monetary loss due to property damage, for example.
Ceramic materials are brittle; they are strong when loaded in compression, but weak when loaded in tension. Failures in ceramic materials often occur when a tensile stress causes a fracture to propagate through the material. Fracture surfaces contain important information about the cause of the fracture, and fractography is defined as the study of fracture surface morphology.
ASTM and the National Institute of Standards and Technology (NIST) have established recommended procedures for conducting a fractographic analysis.
When a fracture propagates through a brittle material, the crack accelerates from the failure origin site, and produces characteristic regions known as mirror, mist, and hackle. The hackle lines point back towards the failure origin site.
Using a stereo microscope to study the fracture surfaces at high magnification, a qualified materials expert can locate a failure origin site and determine if the source of failure was an impact, a material defect, or excessive stress applied to the material.
Testing
ASTM C1161-13 establishes testing procedures for measuring the flexural strength of ceramics. When a force is applied to the top of the specimen, the bottom is put in tension and a crack occurs on the bottom surface of the specimen.
Gathering as much information as available including manufacturing process, circumstances surrounding the failure, pieces of the failed component, and photographs of the failed component will help in determining the cause of the failure.
Visual inspection and examination of the failed component using a Stereo Microscope gives the expert important clues about failure mechanisms. The expert must follow scientific method which often involves taking many photographs during the data gathering phase. The next step is the analysis of the data, which includes review of pertinent technical research. Finally the expert must arrive at a conclusion to form an expert opinion. Conducting testing on an exemplar provides additional insight to the expert and analytical testing including Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and other analytical techniques are often warranted.
Case example
Failure of Porcelain Stool – The plaintiff in this case was fatally injured when the porcelain stool he was leaning on fractured, causing him to fall and sever his femoral artery. In the course of our investigation, our ceramics expert was tasked with analyzing the strength and failure mode of the incident stool.
Our expert found that the diamond shaped holes produced vertical fractures upon failure that expose the user to dangerous shards. The incident stool was unsafe for its intended purpose.
The experts at Robson Forensic follow a rigorous scientific approach to determine how failures occur. The experts can also determine if the failure was due to misuse, a material defect, a manufacturing defect, or a design defect. Investigations are conducted on a broad range of materials and products across a broad range of industries.
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In this article, career Metallurgist, Dawn DiMarco provides an overview of fracture surface examination by revisiting various analyses involving failed fasteners. As a materials scientist at Robson…
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