In the blog article, “Thermal Shock” I used the word ductility. This word is one of a handful of terms commonly used to describe the characteristics of metals and how they react to externally applied forces. Terms such as malleability, hardness, brittleness, toughness, abrasion resistant and impact resistant are commonly used when describing metals but not always completely understood.
THE DIFFERENCE BETWEEN DUCTILITY AND MALLEABILITY
The terms ductility and malleability are often used interchangeably. They are similar in that they both refer to a metal’s ability to withstand stress without rupturing, but opposite in terms of the type of stress being applied. Ductility has to do with tensile stress, whereas malleability deals with compressive stress.
Tensile stress test coupons
Both Terms Applied to Materials
Materials such as lead are very malleable and can be hammered into shape with little chance of fracturing, but they are not ductile and will easily fracture if pulled from two opposite directions.
Compressive test samples from left to right: Untested specimen, gray cast iron, 7075 aluminum and 1020 mild steel.
Gold on the other hand is both ductile and malleable. In fact it is the most ductile and malleable of all known metals. It is said that one ounce of gold can be drawn into a wire so thin it will reach 50 miles before fracturing or be hammered and rolled into a single sheet of gold leaf covering 25 square meters.
In short, metals that are ductile or malleable are both capable of some level of plastic deformation when subjected to stress.
THE MEANING OF BRITTLE AND IMPACT RESISTANCE
Metals that break without significant plastic deformation are said to be brittle. In this sense brittle is the opposite of ductile or malleable. When stress is applied to brittle material and the material fails, there is often a loud snap. Stress may be applied slowly or instantaneously, as in a collision. However, impact resistance is the term more often used when the stress is instantaneous. A so-called Charpy test is used to quantify impact resistance.
|Non-brittle vs. brittle Charpy sample||Overhead view of a typical Charpy test|
HARDNESS AS A GAUGE OF WEAR RESISTANCE
Hardness is somewhat self-explanatory and can be quantified using a number of different scales. The Brinell scales are arguably the most widely used. They characterize the indentation hardness of materials through the scale of penetration of an indenter loaded on a material test piece.
In the pump world, hardness is often used as a guide to wear resistance. Since most metals that are hard also lack ductility, hardness is also often an indicator of brittleness or low impact resistance.
TOUGHNESS: DEFINITION AND APPLICATION
Toughness is a term that describes metals that are strong (can withstand high stress loads) as well as ductile. To be truly tough, a material must be strong and handle stress without permanent deformation. That is to say, it must return to its original dimensions when the stress is removed. In more technical terms, a tough material “has a high modulus of elasticity” as well as high strength.
A stress-strain tensile test focused on the linear part of the stress-strain curve
ABRASION RESISTANCE: DEFINITION AND APPLICATION
The term abrasion resistant, like other terms, is often linked to material hardness. Although tests exist for comparing abrasion resistance (Taber abrasion, rubber wheel abrasion, etc.) these are generally used to compare materials, not specifically quantify this subject. Instead, measured hardness is widely used to predict wear. Easily measured, it has become the “go to” indicator of abrasion resistance and is widely used to predicate wear.
Simplified illustration of Rotary Taber Abrasion Tester
Clearly I cannot even begin to touch upon the science of materials strength in a few short paragraphs. There are volumes of books dedicated to the subject, but I hope this blog will be helpful and guide you in pursuit of answers.