Engineering Material Properties

In this tutorial, you will learn the basics properties used to compare different materials, their definitions, and methods to calculate these properties. Also, you will understand the importance of different material properties and how they affect the behavior of the material.

Contents:

  1. What are Material Properties?
  2. Homogenous and Isotropic Materials
  3. The Tensile Properties of Materials
  4. The Torsional Resistance of a Material
  5. Ductility and Brittleness in Materials
  6. Hardness and Toughness of Materials
  7. The Endurance Limit of a Material
  8. Properties of Materials Related to Time

What are Material Properties?

Whatever you design, you must make it out of some suitable material and be able to manufacture it. A thorough understanding of material properties, treatments, and manufacturing processes is essential to ensure a good machine design.

  • Materials are characterized by their properties. They are unique and are characterized by their properties. Materials may be hard, ductile, or heavy.
  • Some applications may require the material to be elastic while capable of handling load. Some applications may require high resistance to changes in temperature and large amounts of strength.
  • Properties of material are determined experimentally. There exist a plethora of tests that provide different properties of a material as a result. These tests are mostly destructive.
  • It is not feasible to test any material in the middle of the designing phase of any product. Hence, there exists a compendium of properties described for all available material categories which have been acquired via exhaustive testing in research laboratories.
  • The designer of a machine component is not expected to perform each test for his or her chosen material. Theoretical knowledge of the test and the skill to understand the implications of the result for the particular property are sufficient.

Homogenous and Isotropic Materials

Materials are often classified based on their composition and property distribution at a rudimentary level. This helps in segregating standardly available materials into something more easily understood by designers and helps in reducing their search zone for an ideal material.

  • Homogeneous means that the material properties are uniform throughout its continuum, e.g., they are not a function of position. This ideal state is seldom attained in real materials, many of which are subjected to the inclusion of discontinuities, precipitates, voids, or bits of foreign matter from their manufacturing process.
  • One large class of materials that is distinctly nonhomogeneous (i.e., heterogeneous) isotropic is that of composites. Most composites are man-made, but some, such as wood, occur naturally.
  • An isotropic material is one whose mechanical properties are independent of orientation or direction. That is, the strengths across the width and thickness are the same as along the length of the part, for example. Most metals and some nonmetals can be considered to be macroscopically isotropic.
  • Other materials are anisotropic, meaning that there is no plane of material property symmetry. Orthotropic materials have three mutually perpendicular planes of property symmetry and can have different material properties along each axis. Wood, plywood, fiberglass, and some cold-rolled sheet metals are orthotropic.

advertisement
advertisement

The Tensile Properties of Materials

One of the most common tests performed on the material is the Tensile Strength Test. The test involves a bar of material loaded in a Universal Testing Machine and placed under a varying axial tensile load. The relation between the Stress and Strain in the test sample is plotted on the Stress-Strain Graph, which helps show various standard material properties.
A sample Stress-Strain diagram is shown below for reference.

Stress Strain Diagram
  • The slope of the stress-strain curve up to the proportional limit is called Young’s modulus or the modulus of elasticity of the material. It is a measure of the stiffness of the material in its elastic range and has the units of stress.
  • The elastic limit marks the boundary between the elastic behaviour and plastic behaviour regions of the material. Once the limit is crossed, the material cannot regain its original shape on removal of load.
  • At a point y slightly above the elastic limit, the material begins to yield more readily to the applied stress, and its rate of deformation increases. This is called the yield point, and the value of stress at that point defines the yield strength of the material.
  • The stress in the specimen continues to increase non-linearly to a peak or ultimate tensile strength value. This is considered to be the largest tensile stress the material can sustain before breaking.
  • Stiffness is defined as the ability of the material to resist deformation due to the action of an external load. The modulus of elasticity is the measure of stiffness from the material.
  • Resilience is defined as the ability of a material to absorb energy when deformed elastically and release this energy then unloaded. A resilient material absorbs energy within the elastic range without any permanent deformation. It is represented as the area under the stress-strain curve from origin to the elastic limit point

The Torsional Resistance of a Material

The shear properties of a material are more difficult to determine than its tensile properties. A specimen similar to the tensile test specimen is made with noncircular details on its ends so that it can be twisted axially to failure. This helps us measure the Torsional Resistance of the material, and by extension, the shear failure characteristics.

The diagram below shows the setup for the Torsion Test

Torsion Test
  • Poisson’s ratio (ν) is the ratio between lateral and longitudinal strain and for most metals is around 0.3.

    The stress-strain relation for pure torsion is defined by
    τ = \(\frac{G r θ}{l}\)
    Where τ is the shear stress, r is the radius of the specimen, θ is the angle of twist and l is the specimen length. G is defined as the Modulus of Rigidity and can be defined in terms of the Modulus of Elasticity (E) and Poisson’s Ratio (ν) as G = \(\frac{E}{2(1+υ)}\)

  • The breaking strength in torsion is called the Ultimate Shear Strength and is calculated as UTS = \(\frac{T r}{J}\), where T is the Torque applied and J is the Polar Moment of Inertia of the sample.

Ductility and Brittleness in Materials

A material’s failure is typically classified as either brittle failure (fracture) or ductile failure (yield). Most materials can fail in a brittle or ductile manner, or both, depending on the conditions (such as temperature, state of stress, and loading rate). In most practical situations, however, a material can be classified as either brittle or ductile.

  • The ability of a material to sustain a substantially permanent deformation under a tensile load up to the point of fracture, or the relative ability of a material to be stretched plastically at room temperature without breaking, is referred to as ductility.
  • When subjected to stress, a material is said to be brittle if it cracks with little elastic deformation and no considerable plastic deformation. Even high-strength brittle materials absorb relatively little energy before breakage.

Hardness and Toughness of Materials

Material is also characterized by the resistance it offers particular design. Such a design may be exposed to wear and unexpected load, and these factors should not affect the design from fulfilling its function.

advertisement
  • The hardness of a material can be an indicator of its resistance to wear (but is not a guarantee of wear resistance). The strengths of some materials such as steels are also closely correlated to their hardness. Various treatments are applied to steels and other metals to increase hardness and strength. Hardness is most often measured on one of three scales: Brinell, Rockwell, or Vickers.
    • The Brinell test uses a 10 mm hardened steel or tungsten-carbide ball impressed with either a 500 or 3000 kg load depending on the range of the hardness of the material. The diameter of the resulting indent is measured under a microscope and used to calculate the Brinell hardness number.
    • The Vickers test uses a diamond-pyramid indenter and measures the width of the indent under the microscope.
    • The Rockwell test uses a 1/16-inch ball or a 120° cone-shaped diamond indenter and measures the depth of penetration.
  • The ability of a material to absorb energy per unit volume without fracture is called its toughness (also called modulus of toughness) and is equal to the area under the stress-strain curve up to the fracture point.
    • A ductile material of similar ultimate strength to a brittle one will be much tougher. A sheet-metal automobile body will absorb more energy from a collision through plastic deformation than will a brittle, fiberglass body.
    • The Izod and the Charpy tests are two procedures that involve striking a notched specimen with a pendulum and recording the kinetic energy needed to break the specimen at a particular temperature. While these data do not directly correlate with the area under the stress-strain curve, they nevertheless provide a means to compare the energy absorption capacity of various materials under controlled conditions.

The Endurance Limit of a Material

While some machine parts may see only static loads in their lifetime, most will see loads and stresses that vary with time. Materials behave very differently in response to loads that come and go (called fatigue loads) than they do to loads that remain static. Most machine design deals with the design of parts for time-varying loads, so we need to know the fatigue strength of materials under these loading conditions.

  • The Material is tested in a special testing apparatus. The sample is mounted and subjected to varying loads and the number of cycles is counted until material failure. This is plotted on a chart with logarithmic scales, the ordinate representing the Fatigue Strength (S) and the abscissa stating the number of cycles (N). This chart is commonly referred to as the SN curve.
    The diagram below shows an SN Diagram for various materials.

    SN Diagram
  • The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure.

advertisement

Properties of a Material related to Time

Creep (also known as cold flow) is the propensity of a solid material to move slowly or permanently deform under the effect of continuous mechanical forces in materials science. It can happen because of long-term exposure to high levels of stress that are still below the material’s yield strength. Creep is characterized by the following conditions/ features:

  • It is a slow permanent deformation.
  • It occurs below the yield point.
  • It occurs under steady loading conditions as a function of time. Loading condition is one of the salient differences between fatigue and creep.
  • Soft materials like lead, zinc, and tin creep at room temperature, while the tendency to creep in heavy metals like iron and copper increases with increase in temperature.

Key Points to Remember

Here is the list of key points we need to remember about “Engineering Material Properties”.

  • Designing a machine element is not limited to defining the physical dimensions of the element, a proper material also needs to be selected for the element.
  • Homogenous materials have a constant composition throughout their body, and isotropic materials have the same material properties along any direction.
  • A Stress-Strain Curve for a material helps in defining the tensile properties of a material and allows to distinguish between ductile and brittle elements.
  • A Torsional Test helps measure the shear properties of a material and find out the Modulus of Rigidity for the material.
  • Hardness of a material is the ability to withstand frictional wear while Toughness is the amount of energy the material can absorb with failing in case of an impact or shock loading condition.
  • Machine elements are frequently exposed to dynamic loading conditions, and the fatigue strength (or endurance limit) measures the resistance to fatigue failure for a particular fatigue stress range.
  • Materials may fail due to creep when exposed to elevated temperatures under loaded conditions. It is more common in soft materials.

If you find any mistake above, kindly email to [email protected]

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & discussions at Telegram SanfoundryClasses.