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.
Failure Analysis of Glass & Other Ceramic Materials - Expert Article
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.
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.
- Coloring – Where glass needs to be colored to achieve a desired aesthetic or to block light, such as amber glass for a beer bottle, iron and sulfur are added to the composition.
- Tempering - Sheets of silica glass can be strengthened by passing them through a tempering furnace to produce a compressive layer on the top and bottom of the glass. Tempered glass contains stresses from the tempering process, and the fracture characteristics of tempered glass are different than for un-tempered glass. When tempered glass breaks, it fractures into many small shards and can be accompanied by a loud noise.
- Laminating – Another common safety treatment for sheet glass is to apply a laminate material on or between layers. When applied correctly, the laminate layer will hold the material together when broken.
- Specialty Glass Products - A low thermal expansion borosilicate glass is created when boron is added to the glass chemistry. The Corning brand marketed this type of glass as “PYREX” as early at 1915. The low thermal expansion properties of this material made it especially useful in culinary and laboratory settings where glass needs to be heated and cooled rapidly without risk of thermal fracture.
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.
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.
Forensic Failure Analysis
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.
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.
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.
Failure Analysis Investigations
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.
For more information, submit an inquiry or contact the author of this article.
Ceramic Scientist & Failure Analysis Expert
James is a ceramic materials scientist with extensive experience in tribology and failure analysis. He has over 25 years of experience analyzing material and equipment failures in the oil, gas, and petrochemical industries, as well as military and aerospace applications. James applies his expertise to forensic investigations involving product performance, personal injury, and intellectual property disputes concerning ceramic and metallic materials.