Metallurgical Failure Analysis and Fracture Patterns

Metallic materials such as steel, copper, and aluminum alloy are used in a wide range of products, from home appliances and toys to infrastructure facilities and equipment.
New high-performance materials are being constantly studied, primarily in the automotive and aerospace industries. The studies have become necessary due to requirements to reduce the size and weight of products while also increasing their rigidity and performance. In fields such as automobiles, airplanes, ships, railroad cars, and manned spaceships, breakage of metallic materials may put human lives at risk, so strict material selection and safety design based on strength calculation are required.
Through material tests and fractography, causes of breakage and material properties are analyzed to select or improve materials.

When selecting metallic materials, various material tests regarding the stress of such materials are conducted.
Typical test methods are as follows.

• Mechanical tests:
  Tension test, bending test, compression test, shearing test, creep test, wear test, etc.
• Hardness tests:
  Indentation hardness test, dynamic hardness test
• Chemical tests:
  Corrosion test

A ductile fracture is a fracture pattern observed in many metallic materials and shows large deformation, such as stretching and necking, up to the occurrence of the fracture. Through structure observation, you can see the following features and detailed fracture patterns.

A brittle fracture is a fracture pattern in which cracking has rapidly spread while showing little plastic deformation. As cracking proceeds, no plastic deformation occurs around the fractured surface. In general, brittle fractures occur in many steel materials under normal use. In many cases, the fractured surface consists of quasi-cleavage fracture surfaces, which are observed on large heat-treated steel and general structural steel located in extremely cold environments.

A fatigue fracture is a fracture pattern in which cracking has gradually proceeded under repeated load. It is said that more than 70% of fracture patterns on mechanical structures fall under this pattern.
The appearance of the fractured material does not show stretching or necking, which is similar to brittle fractures, but significant plastic deformation is revealed under microscopic observation.
The fractured surface is generally smooth, compared to the surfaces of other fracture patterns, and beach marks are observed as a macroscopic feature. From the appearance of these beach marks, you can tell where the fracture started and in which direction the cracking proceeded.
As a microscopic feature, a striped pattern called striation is typically observed. This striped pattern is vertical to the direction in which the crack proceeded and is said to occur easily on aluminum alloys and copper alloys while not occurring easily on ferrous alloys.

An environmental fracture is a fracture pattern that occurs with cracking developing under a corrosive environment. Therefore, this fracture may also occur even under an extremely small external stress.
Typical environmental fractures are hydrogen embrittlement and stress corrosion cracking.

• Hydrogen embrittlement
  Hydrogen embrittlement is also called a delayed fracture and is commonly observed on steel materials. This phenomenon is caused by hydrogen entering into materials. This entry is typically observed in material manufacturing processes, such as welding and electroplating, and in corrosion reactions in the usage environment.
• Stress corrosion cracking
  This cracking often occurs in accidents in which austenite stainless steel is the material. In particular, transgranular fractures often occur in usage environments with C1- ions. On the other hand, grain boundary fractures occur in materials other than stainless steel such as pure copper, brass, and aluminum alloys.

Conventional problems with microscopes
Irregular reflection on a metal fracture surface can generate glare, which sometimes makes it difficult to observe cracking. Unclear observation may cause inaccurate analysis due to overlooked cracking.

With the VHX Series 4K Digital Microscope
The glare removal function can suppress unnecessary reflection so that you can clearly capture even minute cracking on a metal fracture surface.

Conventional problems with microscopes
Fractured surfaces on metallic materials are generally three dimensional. To observe each feature of the many irregularities on a fractured surface, you need to adjust the focus repeatedly, resulting in analysis requiring a lot of time. Another problem is that comprehensive observation based on an entire target image is not possible.

With the VHX Series 4K Digital Microscope
The real-time depth composition function makes it possible to bring an entire metal fracture surface into focus. This function not only reduces the time required for adjusting the focus repeatedly, but also enables you to observe and evaluate many composite features that exist on a fractured surface.

Conventional problems with microscopes
Irregularities on a metal fracture surface not only make it difficult to adjust the focus, but also change the shadowing depending on the angle. As a result, it is difficult and takes a lot of time to determine the lighting conditions. In some cases, it is also difficult to explain fracture patterns only with image data captured under a single lighting condition.

Conventional problems with microscopes
Some metal fracture surfaces may have subtle fracture patterns. In such cases, fractography requires a lot of time and the contrast may be too low to carry out observation properly.

With the VHX Series 4K Digital Microscope
The Optical Shadow Effect Mode, a new observation method that combines a specifically designed high-resolution HR lens, a 4K CMOS image sensor, and lighting, analyzes the contrast in an image captured with varied illumination.
This method makes it possible to clearly observe subtle and minute irregularities on a metal fracture surface. Irregularity information can also be displayed in different colors by composing an Optical Shadow Effect Mode image with color information.