With respect to engineering, manufacturing, and product design, the properties of various materials greatly affect their usefulness which defines how their value is appropriated. Consequentially, hardness and strength are two of the most fundamental features of a material that are very often confused. Even though both features constitute vital metrics to evaluate material performance, there are very distinct differences in the definition, method of measurement, practical application, and implications of each attribute. This analysis seeks to explore the differences between strength and hardness and provide an appreciation for the two terms with respect to material science.
What is Hardness in Material Science?
In the field of material science, hardness is defined as the resistance to deformation of a material, specifically permanent indentation, scratching or wear under an applied force .hardness might be determined by using the Mohs, Vickers, Brinell or Rockwell methods depending on the application at hand. It describes a material’s resistance to permanent deformation and wear. Stepwise increases in applied force till the deformation becomes permanent is conducted to measure hardness. This helps to understand a material’s longevity, and whether it can withstand mechanical force or abrasion.
How is hardness measured
Different standardized procedures exist for measuring hardness but each set of criteria is meant for a particular material or usage. Here are a few approaches that are widely used:
The Mohs scale ranks materials in terms of their hardness, specifically, the ability to be scratched by another material. It employs descriptive scale from 1(talc) to 10(diamond the hardest naturally existing material). This method is used mainly in the case of minerals as it offers speed when estimating degree of hardness.
Every material rated higher than Mohs 1 Talc can scratch it.
Quartz Mohs 7 can scratch orthoclase Mohs 6 but can’t scratch topaz Mohs 8.
The Vickers test assesses hardness by pressing a diamond-shaped indenter into a material’s surface. The indentation is evaluated with respect to the diamond’s edges, and the material’s hardness is expressed in HV units, which is highly accurate for thin materials or coatings.
Indentation dimensions (diagonal): 0.5 mm
Test force applied (F): 30 kgf
Resulting Vickers Hardness (HV): 300 HV ± 5%
This test utilizes a steel or tungsten carbide ball, which is pressed into the material under a fixed load. The indentation’s diameter is calculated to determine the Brinell Hardness Number (BHN). It is frequently used for metals with coarse structures, like castings.
Material A (pure aluminum): HB ~ 15
Material B (mild steel): HB ~ 130
This test measures the depth of indentation made by an indenter under load. Different types of indenters and loads give rise to different Rockwell scales such as softer metals being HRB and harder materials HRC.
Copper (HRB): 50
Hardened steel (HRC): 60
All these methods are based on specific principles, each having clear pros and cons. The method selected depends on the material’s properties, geometry, and intended requirements.
Various Hardness Tests Applied in Industries
Hardness testing is crucial across several industries. It determines the adequacy of a material for a specific application. For instance in manufacturing, tests are carried out to determine the wear resistance, strength of the material, and the quality of heat-treated parts. In aerospace and automotive industries, the Vickers or Rockwell tests are frequently used to determine the strength of components subjected to high stress to evaluate durability and safety. In construction materials like steel and iron, Brinell testing is used due to the ability to measure wider areas. Each test type optimized material selection, which improves product reliability and performance due to precision and relevance of each test type.
Importance of Hardness in Material Selection
Materials selection is influenced greatly by hardness value as it determines a material’s resistance towards deformation, wear and surface damage. This would assure that components maintain their structural integrity and functionality while enduring mechanical forces. Products selected would undergo a reduction in maintenance, enhanced durability, and prolonged lifespan if materials with suitable hardness values are chosen. Material selection would require taking into account the right values of hardness to provide ensure performance and safety.
How are Strength and Hardness Related?
Understanding tensile strength and hardness
While tensile strength and hardness are interconnected, they pertain to measuring different material properties. The capacity of a solid to resist breaking when stretched is called tensile strength while the ability of a solid to withstand localized deformation or indent is called hardness. In many cases, the relationship is present because tougher metals more often have greater tensile strength and hardness. This relationship exists because of the atomic bonding and microstructure of the material affecting both properties. This relationship is, however, not absolute and differs based on the material’s type as well as the heat treatment methods applied to the material. It is very important to understand this relationship for proper selection of materials for engineering and industrial applications that have specific performance requirements.
Effects of yield strength on hardness
Yield Strength (YS):
The stress at which a solid begins to deform plastically.
Unit of measure in megapascals (MPa) or pounds per square inch (psi).
Depends on the type of material such as steel, aluminum, or titanium and their processing.
Hardness (H):
Inability of a solid to be deformed, scratched, or indented.
It is measured using scales such as Rockwell (HR), Vickers (HV), or Brinell (HB).
Toughness of a material correlates with the amount of abrasion and wear resistance it has.
Empirical Correlation Between YS and H:
For steel, there is an approximate relationship:
“Hardness (HV)” is equal to “3 times Yield Strength (MPa)”, or
“HV” = 3“σy”.
For Aluminum: There appears a weaker correlation which is more processing dependent.
For Alloys: Microstructure and alloying elements have considerable impact upon this relationship.
Low Carbon Steel:
“Yield Strength” is equal to 250MPa, “Hardness” is approximately 75HRB.
Tempered Stainless Steel:
“Yield Strength” is equal to 1,500MPa, “Hardness” is approximately equal to 45HRC.
Annealed Aluminum 6061:
“Yield Strength” is equal to 55MPa, “Hardness” is approximately 25HRB.
Factors Affecting Correlation:
heat treatments (e.g. quenching, or tempering, and annealing),
Grain size and internal crystalline architecture,
Impurities or alloys.
Grain size and internal crystalline architecture help rationalize the softening impact of these materials on mechanical performance.
These alloys together reveal enhanced understanding of material selection and design optimization against defined conditions of mechanical performance.
Correlation between Hardness and Tensile Strength
The correlation between hardness and the tensile strength can be examined from empirical relations based on converting different hardness values, such as Brinell or Rockwell, to related tensile strength. In case of Steel Alloys, one can define:
Tensile strength in Megapascal can be approximated with “3.45 times HB”; where HB is Brinell hardness.
Considerable interplay exists among the parameters and characteristics of hardness, which measures a material’s resistance to deforming externally, and tensile strength—the material’s resistance to stress until failure—while interdependently being a factor of the relationships. However, these relationships are dependent on materials and wrought microstructural features like composition of alloys, degree of grain size reduction, and heat treatment. For precise estimates, accurate material-specific conversion tables or empirical data must be relied upon.
Why is Toughness Different from Hardness?
Defining toughness in materials
Toughness describes the maximum energy that can be absorbed by a material before fracturing, physically deforming, or yielding, unlike hardness, which only measures the deflection of deformation its resistance to. Toughness incorporates strength and ductility, thus becomes crucial while considering impact or any dynamic load on the material.
Comparison of hardness vs toughness
These two terms describe interrelated but completely different mechanical properties of a material. Hardness describes the tendency of a material to resist surface deformation (scratch or indentation) and is defined with the help of Mohs or Vickers scales. Toughness is the capacity to withstand energy or deformation without fracturing. A hard material may not easily wear off, and even withstand indentation, but undergo severe fracture when hit; thus, hardness without toughness renders a material brittle. For example, tough ceramics are very hard but not tough, while steel, a metal is said to possess added superiority due to its balance of moderate hardness and massive toughness. This immense toughness is due to being alloyed and purposely designed to high-metric conditions.
The significance of toughness concerning material properties
When assessing toughness, it is vital to consider particular data points and associated properties relevant to the conditions of interest in order to achieve optimal performance. A description of the most important parameters and measurements is provided below:
- Determined with the use of Charpy and Izod impact tests.
- Bottom measurement units are generally in joules (J) or foot-pounds (ft-lbf).
- Higher numbers in reported results correspond to better material capability for energy absorption during impact.
- It indicates a material’s ability to resist crack propagation from the tam/damage force under some stress.
- It is measured in MPa√m – Megapascals times the square root of meters.
- Usually used with structural works to assess failure risk.
- The possibility of plastic deformation prior to rupture and of externally applied tensile stresses, usually considered as elongation (%) or reduction in area (%).
- Greater ductility translates to higher toughness values.
- Evaluates the extent to which flaws or notches weaken a material’s toughness.
- Low notch sensitive alloys are more appropriate for quasi-static or stress-concentrated environments.
- The toughness properties are very sensitive to temperature.
- The brittle-to-ductile-transition temperature (BDTT) is particularly important for materials that operate at lower temperatures.
Through this analysis, in combination with the environment in which the material will be used and the loads it may encounter, engineers are able to select materials optimally balancing endurance and safety for performance requirements.
How Does Stiffness Compare to Hardness?
Understanding stiffness in materials
The resistance that a material offers to deformation under applied load is defined as stiffness, which is quantitatively defined by the modulus of elasticity or more commonly referred to as Young’s modulus \(E\) which is measured in units of pressure such as MPa or GPa. FOr example, the stiffer the material, the higher the modulus it will have:
- Aluminum – Young’s Modulus \(\approx 69 GPa\)
- Steel – Young’s Modulus \(\approx 200 GPa\)
- Titanium – Young’s Modulus \(\approx 110 GPa\)
Stiffness is different than hardness since it deals with elastic deformation and not surface penetration, cutting and scratching. Furthermore, stiffness can change the way a material behaves under certain loads and affect deflection, vibration, structural stability, and overall stiffness of a material. Stiffness is one of the parameters to be calculated by an engineer in a structure where rigidity matters most like bridges, parts of an aircraft, and precision instruments.
Differences between stiffness and hardness
Here is a list detailing the regions of some materials that are mostly used for engineering purpose:
– Young’s Modulus or Elastic Modulus:
Young’s Modulus (\approx 69 GPa\)
Density (\approx 2.7 g/cm³\)
Common Applications: Aerospace Structures, Automotive Parts, Packaging.
Young’s Modulus \(\approx 200 GPa\)
Density (\approx 7.8 g/cm³\)
Common Applications: Construction Beams, Machinery, Pipelines.
Young’s Modulus:
Uses: Medical implants, aircraft components, marine equipment.
Value: 110 GPa
Density: 4.5 g/cm³
Brinell Hardness: 160 MPa
Brinell Hardness:
- In comparison to steel, it is weaker; however, it is considered sufficient for lightweight applications.
- Ranges approximately from 400 MPa to 900 MPa with mild and hardened steel.
- Due to its high hardness, steel is considered durable for applications that bear heavy loads and warrant resistance to wear.
- Brinell Hardness: 270 MPa
Titanium:
- Offers balance between hardness, excellent resistance to corrosion and demanding environments.
- Ultimate Tensile Strength: 310 MPa
- Works best for designs that bear weight due to low tensile strength.
- A wide range spanning 400 MPa to over 1,200 MPa, depends on graded structural and industrial use.
- Ultimately, 900 MPa provides high strength to weight ratio, ideal for aerospace and medical applications.
It is important to note that properties of each material need to be taken into consideration during the selection process to meet the required design and performance standards, as well as environmental criteria, the thorough comparison presented gives aid to engineers evaluating trade-offs optimizing materials for a particular purpose.
Effect of stiffness on mechanical properties
Resistant materials undergo minimal deformation as they resist external forces, and stiffness plays a key role. For instance, metals such as steel or titanium have high stiffness and deform negligibly when stressed, making them ideal for load-bearing scenarios. On the other side of the spectrum, polymers and aluminum have low stiffness which allows greater movement. This flexibility is beneficial for energetic absorbing applications, allowing the material to adapt to changing conditions. This balance is critical to fulfill structural performance regarding stability and functionality per the requirement.
What Are the Types of Strength in Materials?
Explanation of compressive strength and its relevance
Compressive strength is the ability of a material to withstand axial compressive forces without failure. This property is important for construction materials like concrete, stone, and metals, as compressive strength indicates their ability to withstand considerable loads. It is measured in units such as Pascals (Pa) or pounds per square inch (psi) and is determined using accepted methods such as the crushing of cylindrical or cubical stone specimens under specific conditions.
Selection of materials with appropriate compressive strength for construction ensures structural application safety and reliability. For example, concrete has a compressive strength value of 3,000 to 6,000 psi while specialized high strength concrete has compressive strength exceeding 10,000 psi which makes it useful in bridges or skyscrapers. Engineers need to understand and specify values of compressive strength in order to design structures that optimize force application without resulting in mechanical failure.
Examining the shear strength of different materials
Shear strength are the basic elemental phenomena that restrain a material from being sheared and sliced through sliding or other deformation forcing one portion of the material over the other portion. Along with compression and tension, shear strength constitutes significant features when classifying parts like beams, bolts, and adhesives which are subjected to tangential forces acting perpendicular to the axis of the part. Properties such as shear strength are measured in units of Pascal, Pa, and psi. Shear strength can measure using a standard procedure also known as shear testing.
As an example, alloys of aluminum offer a choice when weight saving is required with yield strength in psi from 40,000 to 50,000 offering alloys with strong structural properties were expected. On the other hand, controlled grade epoxies usually offer more than 5,000 psi of shear bond strength and other adhesives show variable shear bond strength depend upon the make-up.
The shear strength of concrete usually falls in the range of 500 to 1,000 psi, but can change due to the mix design or even the reinforcements. For example, the addition of steel reinforcements into concrete makes it so much stronger that shear resistance becomes its forte. Knowing the shear strength of materials helps engineers prepare designs that provide stability and operational safety in different conditions.
The importance of ultimate tensile strength
To better quantify and compare the strength properties among different materials, the table, and list below provide the relevant data for both shear strength and ultimate tensile strength, where applicable:
Aluminum Alloys:
Shear Strength: 15,000 to 30,000 psi
Offers lightweight properties ideal for aerospace and automotive industries.
Adhesive Materials (Industrial-grade Epoxies):
Shear Strength: 5,000+ psi
Dependent on specific formulation and curing conditions.
Concrete (Non-reinforced):
Shear Strength: 500 to 1,000 psi
This ratio changes with the mix ratio and curing process.
Reinforced Concrete:
Enhanced shear strength significantly above 1,000 psi
Steel Alloys:
Ultimate Tensile Strength: 60,000 to 100,000 psi
Basedon grade and treatment.
Aluminum Alloys:
Ultimate Tensile Strength: From 30,000 up to 70,000 psi
Moderate tensile properties coupled with a light weight.
Industrial Adhesives/Epoxies:
Ultimate Tensile Strength (for select formulas): Approximate 4,000 to 8,000 psi.
Best designed for joining high-stress surfaces.
Concrete (Reinforced and Non-reinforced):
Ultimate capacity of tensile strength of reinforced concrete is about 300-600 psi (effective tensile strength due to steel reinforcement).
These datasets provide clear guidance to engineers regarding the selection of materials with ample regard to the most optimal balance between their strength, weight, and specific intended use. Understanding shear and tensile strength is important for many engineering designs and structures.
Frequently Asked Questions (FAQs)
Q: How would you define the difference between strength and hardness in engineering?
A: Both strength and hardness are related to the properties of materials, but they differ in scope. Strength is defined as the ability of a material to sustain an applied load without failing or undergoing permanent deformation. It is usually expressed as the maximum stress a material can sustain before yielding. On the other hand, hardness signifies the ability of a material to resist surface indentation or deformation. In engineering, hardness is tested using the Vickers hardness test or the Rockwell hardness test.
Q: Discuss how hardness and strength are correlated in materials?
A: Properties such as hardness and strength are related to each other, but that does not mean they will always correlate directly. To some degree, it is observed that the harder a material is, the stronger it is, but not always. Some materials are harder and have lower tensile strength while others are more tendinous but softer. The correlation of these properties depends on the material and its microstructure.
Q: Can a material have high strength but low hardness?
A: A material can have both high strength and low hardness. For instance, certain metals and some polymers exhibit high tensile strength, meaning they are capable of enduring substantial stress before failing, yet they may possess lower hardness levels, which makes them more prone to surface deformation. Such materials might be useful in industries that benefit from greater flexibility and resistance to impact.
Q: What does hardness refer to in material science?
A: Hardness is the ability of a material to withstand an external compressive force that aims to indent or scratch the surface. It assesses the material’s capability to endure wear and abrasion at its surface. In engineering, hardness of a material is usually calculated through standardized tests like the Vickers or Rockwell hardness tests that assess the material’s resistance to penetration by a certain indenter at controlled conditions.
Q: How is the strength of materials measured?
A: Strength is assessed using one or many mechanical tests. The most popular is tensile strength, defined as the maximum stress a material can endure due to stretching without failing. Other measurements of strength include compressive, yield, and flexural strength. Each of these tests entails the application of a controlled amount of force to a material specimen and recording the response, frequently to the point of failure or irreversible change in shape or position.
Q: Is it possible to increase the strength of a material by simply increasing its hardness?
A: In many situations, greater hardness translates to greater strength; however, this is not universally so. In some cases, increasing the hardness of a material too much beyond a threshold makes it brittle, which is a reduction in strength. The interplay of hardness and strength varies with the type of material and the hardening processes employed. Each situation requires its own specific equilibrium between hardness and strength.
Q: How is the yield point important in understanding material strength?
A: The plastic deformation occurs at a certain stress level and The yield point is that value which reflects the stress value at which a material starts to deform plastic. Yield strength, or yield point stress, is stress measured in the yield point and plastic deformation occurs. In plastic deformation condition material fail permanently. On the other hand yield point is taken as standard strength of structure or material. In mechanical engineering design yield point is termed as engineering strength.
Q: What is the difference of application of strong versus hard material?
A: Strong materials are meant for withstanding high load without failure, hence used in structural components, high pressure vessels and rigid frame structures. Weak materials, also termed E-cast metals are used where surface durability and resistance to wear surface is critical such as bearing, cutting blades. Certain steel alloys are best example which are both strong and hard which makes them applicable to more demanding situations.
Reference Sources
1. Hardness as an Indicator of Material Strength: A Critical Review
- Authors: Giuseppe Pintaude
- Journal: Critical Reviews in Solid State and Materials Sciences
- Publication Date: June 7, 2022
- Citation: (Pintaude, 2022, pp. 623–641)
- Summary:
- This review critically examines the relationship between hardness and strength in various materials. It discusses the empirical and theoretical correlations between hardness and uniaxial strength, emphasizing the constraint factor commonly used in metals. The paper highlights the challenges in applying these correlations to non-metallic materials and suggests future research directions to better understand the relationship between hardness and strength.
2. A General Method for Determining Probabilistic S-N Curve Based on Static Strength Data of Material
- Authors: Jianxiong Gao et al.
- Journal: Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science
- Publication Date: February 28, 2022
- Citation: (Gao et al., 2022, pp. 4862–4875)
- Summary:
- This study presents a method for determining the probabilistic S-N curve of materials based on their static strength data. The authors explore the relationship between static strength and fatigue life, providing insights into how these properties can be correlated. The findings suggest that understanding the static strength can significantly reduce testing time and costs associated with fatigue testing.
3. Effect of Material and Process Specific Factors on the Strength of Printed Parts in Fused Filament Fabrication: A Review of Recent Developments
- Authors: Muhammad Harris et al.
- Journal: Materials
- Publication Date: May 1, 2019
- Citation: (Harris et al., 2019)
- Summary:
- This review analyzes the factors affecting the strength and ductility of parts produced through Fused Filament Fabrication (FFF). It discusses how material properties, including hardness, influence the mechanical performance of printed parts. The paper identifies gaps in current research and suggests future directions for improving the strength of 3D-printed materials.
