Mechanical Properties of Materials PDF

Mechanical Properties of Materials It is very important to know the mechanical behaviour of materials during design and manufacturing. Main mechanical properties: – Tensile / compression / bending – Hardness – Impact – Fatigue – Creep Tensile test and S...

Mechanical Properties of Materials It is very important to know the mechanical behaviour of materials during design and manufacturing. Main mechanical properties: – Tensile / compression / bending – Hardness – Impact – Fatigue – Creep Tensile test and Stress - Elongation concepts Stress - Elongation curve  Tensile test It is used to test the strength of the material under static forces. Tensile test  Tensile test  : Stress  : Strain F  Ao  l l  lo   lo lo F - L (force-elongation) curve  -  (stress-strain) curve obtained obtained from tensile test from the data in F-L  Tensile test The stress value at which the material begins to undergo plastic deformation is called "yield strength". 1. Materials without significant yielding 2. Materials with significant yielding 0.2 1 2 a Significant yield point In case it is not evident, the yield strength is equal to the stress value generating 0.2% permanent pshd.. p =0.002 = 0.2% e  Elastic Giving Zone Plastic Zone neck 𝜎𝜎 (necking)  Homogeneous plastic deformation Heterogeneous plastic T deformation x Yield point (yield strength) Tensile strength a = 0.2 x (beginning of x neck giving) Fracture- Elastic rupture limit applied stress < yield strength ⇒ elastic applied stress > yield strength ⇒ plastic + elastic  5 0.002   Elastic Shape Change Ç K 0.2 Plastic Zone  0.002  Elastic Shape Change  a E, Modulus of elasticity Elongations in atomic bonds results in elastic strain.  Elastic strain Hooke's law is valid in the elastic region. The strain varies linearly with stress. When the force is removed, elastic elongation disappears. E, Modulus of Elasticity, equal to the slope of the linear part – Characteristic of the material (varies from material to material) – The larger the E, the more rigid the material becomes, i.e. it shows less shape change with stress. Normal stress Shear stress   E    G   = Normal stress  = Shear stress  = Unit shape change  = Shear unit strain E = Modulus of elasticity G = shear modulus  Parameters affecting E: Different elements (e.g. different E for Al and steel) It is affected by ambient temperature It is not affected by heat treatment (the same steel has the same E in the mild and hardened state) E is a material property. For example, steel is more rigid than aluminum.  Poisson Ratio 𝜀 𝜀 𝜈=− = − (in isotropic materials) 𝜀 𝜀 It is another parameter that determines the elastic properties of materials. During elastic deformation the volume of the material changes (in plastic deformation the volume remains constant). While the material elongates in the tensile direction, shortening occurs in the direction perpendicular to it. The ratio between them is determined by the Poisson's ratio.  For metals it varies between around 0.25 and 0.35. It is usually close to 0.3.  Plastic Shape Change Plastic deformations, PD (permanent-irreversible) begin when stresses are applied above the yield strength, which refers to the maximum stress of materials that is used for engineering components. At this point, PD occurs when the dislocations start to slip.  a There are different deformation mechanisms in PD depending on the temperature levels. These are; 1. Cold plastic deformation 2. Warm plastic deformation 3. Hot plastic deformation These temperature levels are determined by the equivalent temperature.  Equivalent temperature 𝑇 𝐾 TM = Melting temperature of the material 𝑇 = TW = Operating temperature 𝑇 𝐾 0 < TA < 0.25 Cold Shape Change 0.25 < TA < 0.5 Warm Shape change 0.5 < TA < 1 Hot Shape change Room temperature; While for many metals such as Fe, Cu, Al, etc. it is the cold deformation zone For materials with low melting temperatures such as Pb, Sn, etc., it becomes a hot deformation zone.  Cold Deformation Two types of deformation mechanisms can be active in cold deformation. 1. Slip 2. Twinning PD is caused by slip, i.e. sliding movement of the dislocations. In cases where sliding is difficult, plastic deformation takes place by twinning.  Slip: The Concept Of Work Hardening During plastic deformation, the dislocations move by sliding in shear planes. But in the meantime, new dislocations are created and their density increases. As their number increases, they start to block each other's movement or get stuck in other obstacles (vacancies, substitutional sites, interstitial sites, grain boundaries, precipitates, etc.). Thus, higher stresses are required for their movement. This is called deformation hardening or work hardening.  Homogeneous PD Zone  -  is the part of the curve between the yield point and the peak (necking). Description: In PD, the part length increases continuously. The volume remains constant and the increase in length is balanced by a reduction in cross-sectional area. After the yield point, the material work hardened until the peak (Tensile strength) and more stress is required, but as the PD increases, the cross- section reduces. Heterogeneous PD Zone  -  is the part of the curve between the peak (necking) and the break point. Description: Plastic instability starts after the peak point (tensile strength). The cross- section starts to shrink rapidly in the region and the material gives a neck. The force required for the shape change decreases. Therefore the curve turns downwards. At a certain point, rupture occurs.  Heterogeneous PD after max. point Homogeneous PD after the yield point Beginning of necking Ç 0.2  a Fracture (rupture) 18 0.002 Fracture Surface Tensile Strength Values  Data obtained from the tensile curve E, Modulus of elasticity Y , Yield strength T , Tensile strength Also, at any point F , Tensile stress at break Amount of elastic strain  , Elongation at break Amount of plastic  , Section contraction deformation, etc. can be  , Uniform elongation found Static toughness Resilience  Plastic deformation Tensile strength Elastic Limit Point A Fracture Elastic deformation stress Yield strength Modulus of elasticity Elongation at break  T (= E +  )P  Ductility / Brittleness / Toughness / Resilience Ductility: refers to the ability to undergo plastic deformation. The larger the ductility value, the greater the plastic deformation until the material failure. It can be expressed in terms of the elongation at break and the area contraction parameters. Brittleness: Refers to the absence of plastic deformation capability. The curve can end at the elastic limit or sometimes at a point very close to the elastic limit for the brittle materials Toughness: It refers to the total energy absorbed by the material until failure. It is equal to the area under the  -  curve. Generally speaking, ductile materials have higher toughness than brittle materials. Resilience: It is the energy stored by the material during elastic deformation. It is equal to the area under the elastic region in the  -  curve.  Ductility Elongation at break; lk , can also be found from the curve. lk  lo   lk = Gauge length at break lo lo = first gauge length Cross-section contraction: AC cannot be found from the curve. A A Ao = Initial cross-sectional area  o k Ak = Cross-sectional area A o measured after rupture  Static Toughness Toughness refers to the energy the material expends until it breaks the area under the curve  - .  Tokluk Toughness     d   Static Toughness It is an indication of how much energy the Tokluk Toughness     d material will absorb until it breaks.  Resilience Resilience is the area under the elastic region in the  -  curve. It refers to the energy stored during elastic behavior. Resilience 𝜎.𝜀 𝑈 = 𝜎. 𝑑𝜀 = Spring steel 2 Simple carbon steel