Materials & Fracture

The selection of materials is a critical step in the design process, and it directly affects the performance, cost, and reliability of the final product. The choice of material should be based on several factors, including the functional requirements, operating conditions, manufacturing process, and environmental impact. The mechanical, thermal, electrical, and chemical properties of materials, as well as their availability, cost, and sustainability, should be carefully evaluated to ensure the right material is chosen for the application.

For example, consider the chassis of a car. It is designed to support the engine, passengers and other components. It is also designed to absorb impact in the event of an accident to protect the passengers inside. Given these requirements, metal is a good candidate as the manufacturing material - it has a higher ultimate strength and can stretch to carry more load. For this particular application, ceramics would not work well due to their brittlness, higher density and lower ultimate strength. The figure below shows how the chassis absorbs the shock and load during an accident. Old and new versions of the car are compared to better understand how much the development has improved.

The ability of ductile materials to elongate is compared with that of brittle materials. This ability to stretch and absorb impact is used in cars. The cabin in the old car is heavily deformed compared to the new version - keeping the passengers safe (from

Some of the key requirements for material selection in design are listed below.

  1. Material properties: The material should be able to meet the functional requirements of the product, such as mechanical strength, stiffness, toughness, and wear resistance, as well as thermal, electrical, and chemical properties.
  2. Availability and cost: The material should be readily available and cost-effective for the application, and should not be subject to supply chain disruptions or price fluctuations.
  3. Operating conditions: The material should be able to withstand the operating conditions, such as temperature, pressure, humidity, and exposure to chemicals or radiation.
  4. Regulatory compliance: The material should meet regulatory requirements, such as safety, health, and environmental standards, and should not pose any risks to human health or the environment.
  5. Sustainability & Environmental impact: The material should be sustainable, such as using recycled materials or renewable resources, to reduce waste and conserve natural resources. It should also have minimal environmental impact, such as low carbon footprint, low toxicity, and high recyclability.

Key properties

Material selection is based on the design requirements and the mechanical properties of the material. Some of the most relevant properties of a material are listed below.

  • Strength: The material should be strong enough to withstand the loads and stresses that it will be subjected to during use.
  • Stiffness: The material should have sufficient stiffness to maintain its shape and resist deformation under load.
  • Toughness: The material should be able to absorb energy without fracturing or breaking under impact.
  • Hardness: The material should be resistant to scratching, abrasion, and wear.
  • Ductility: The material should be able to deform under stress without fracturing.
  • Thermal properties: The material should have appropriate thermal conductivity, expansion coefficient, and melting point for the intended application.
  • Electro-mechanical properties: The material should have appropriate electrical conductivity, insulation and magnetic permeability and so on, for the intended application.
  • Corossive resistance: The material should be able to resist the effects of chemical reactions and environmental exposure.


Through various projects in structural mechanics, I have come across a variety of materials for different applications. Some of the work and topics are outlined below.

  1. Brittle materials
  2. Brittle materials undergo little or no plastic deformation before breaking under stress. They are characterised by high stiffness, high strength and low ductility. Fracture in brittle materials occurs along the planes of maximum shear stress, resulting in sudden and catastrophic failure. Examples of brittle materials include ceramics, glass, concrete and some types of metals such as cast iron.

    During my PhD I worked with brittle materials and how they fail under different types of loading. I carried out experiments and compared the data with several damage models using FEM. The study was useful in understanding how the crack propagates during fracture and how it can be controlled or guided to an extent.

  3. Ductile materials
  4. Ductile materials are materials that are capable of undergoing significant plastic deformation under stress without breaking. They have the ability to deform plastically without fracturing and can sustain large strains before failure. Ductility is a desirable property in many engineering applications as it allows for greater flexibility and design options. Examples of ductile materials include metals such as copper, aluminum, and steel.


    Metal forming is a common method used in industry to develop most of the systems around us. Common items that we see around us in everyday life are mostly made from sheet metal, from a candy box to parts of an aeroplane. The sheet metal is shaped into various required shapes by forming, which is the process of stretching the metal plastically into the desired shape. I have studied and developed some material models to simulate the forming of sheet metal. The model takes into account the role of environmental parameters such as temperature and strain rate. The ductile material model is based on the GTN model where the evolution of the material during plastic deformation is taken into account to be more accurate compared to other existing models. The model is particularly useful for achieving higher accuracy in sheet metal forming under extreme environments.

    Simulation of a sheetmetal forming using a spherical tool. GTN damage model is used to define the material behaviour and implemented using FEM. The fracture in the sheet due to overstretching is visible.

  5. Composite materials
  6. Composite materials are materials composed of two or more constituent materials with different physical and chemical properties. They are designed to combine the unique properties of each material to create a material with enhanced performance characteristics. Composite materials are widely used in various industries, including aerospace, automotive, and construction, due to their high strength-to-weight ratio, corrosion resistance, and durability. Examples of composite materials include fiberglass, carbon fiber reinforced polymer, and sandwich panels. Applications of composite materials range from aircraft and spacecraft components, such as wings and fuselages, to sports equipment, such as golf clubs and tennis rackets, and even in the construction of buildings and bridges.

    Unidirectional composites

    During my Masters I worked on modelling uniaxial composites, where the fibres are aligned in one direction only. The study focused on how the composite fails when it has different fibre types and strength properties. The numerical study was done with a large number of simulations (Monte Carlo simulations) to obtain accurate results.


    Unidirectional composites contains fibers alighned in single direction and they are represented by the cells as in the picture. The nature of the load distribution and the load carrying capacity of fibers are studied using Voronoi cells.

  7. Viscoelastic & viscoplastic materials
  8. Viscoelastic and viscoplastic materials are two types of materials that exhibit time-dependent deformation behaviors under stress. Viscoelastic materials deform elastically under short-term loading, but exhibit significant time-dependent deformation, or creep, under long-term loading. They are used in a wide range of applications, including adhesives, sealants, and damping materials in structures and vehicles. Viscoplastic materials, on the other hand, exhibit both elastic and plastic deformation under stress and can undergo significant plastic deformation without breaking. They are used in a variety of industrial applications, including the shaping and forming of metals and polymers, and in the design of impact-resistant materials for helmets and body armor. Both types of materials are important in many engineering applications, where their unique properties can be utilized to enhance performance and durability.

  9. 3D printed materials
  10. 3D printing, also known as additive manufacturing, allows for the creation of complex structures and geometries by layering materials in a precise manner. A wide range of materials can be used for 3D printing, including plastics, metals, ceramics, and composites, which enables a variety of applications. 3D printed materials can be used in aerospace and automotive industries to create lightweight, high-performance components. Medical and dental industries use 3D printed materials to create customized prosthetics, implants, and surgical models. Additionally, 3D printed materials are used in the creation of consumer products, such as jewelry and home decor. Many of the 3D printed polymers exhibit visco elastic or plastic behavior

    3D printed material used in the experiments. This specimen contains two types of material - a soft interface (black) and a hard shell (white). These samples will be tested under different loading conditions. 3D printing enabled the creation of these specimens in different configurations.

  11. Metamaterials
  12. Metamaterials are artificial materials engineered to have properties that do not exist in natural materials, such as negative refractive indices or cloaking devices. They are created by manipulating the geometry, composition, and arrangement of materials at a sub-wavelength scale. Metamaterials have a wide range of applications, including in telecommunications, where they can be used to enhance antenna performance and in data storage, where they can be used to increase storage density. They can also be used to create ultrathin lenses and super-resolution microscopes in the field of optics, and in the development of invisibility cloaks and soundproofing materials. Additionally, metamaterials have potential applications in energy harvesting and sensing technologies.

    Gyroid metamaterials are engineered structures that can be fabricated using techniques such as 3D printing, and they exhibit unique properties that are not found in natural materials. These properties include high stiffness, low weight, and the ability to manipulate electromagnetic waves and acoustic waves.

    Gyroid has better isotropic behaviour compared to other similar structures such as honeycomb. Gyroid and similar structures are currently used in various systems to create lattice structures due to their better strength to weight ratio.