Mechanics of Materials Labs Tensile test Eleni Tsangouri Leonardo Ruggiero

Content Content -Stress/ Strain -Stress- Strain Curves for several materials -Table of Mechanical proprties Strain gauge Wheatstone bridge

Stress force on a member divided by area, which carries the force applied load divided by the material area it is acting on σ= F / A where F is the force [N] acting on an area A [m2 ] unit of measurement: N/mm2 or Mpa Strain deformation per unit length the elongation or contraction of a material per unit length of the material NO unit of measurement

Stress –strain curves During tensile testing of a material , the stress–strain curve is a graphical representation of the relationship between stress and strain. The slope of the elastic (linear) portion of the curve is a property used to characterize materials and is known as the Young's Modulus, E. The nature of the curve varies from material to material. Comparison of the behavior of brittle/ ductile materials

Stress –strain curves Limits The limit beyond which the material will no longer go back to its original shape when the load is removed is called Elastic Limit. The region in stress-strain diagram from O to P is called the Elastic Range. The region from P to R is called the Plastic Range. Yield point is the point at which the material will have an appreciable elongation or yielding without any increase in load. The maximum ordinate in the stress-strain diagram is the ultimate strength or tensile strength.

Stress- strain curves Ductile Materials Ductile: The ability of a material to be subjected to large strains before it ruptures or fails. After the yield point, ductile materials undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin to neck, as the cross-sectional area of the specimen decreases. Steel

Stress- strain curves Ductile Materials Copper Aluminum Engineering alloy

Stress- strain curves Brittle Materials Brittle Materials: The ability of a material to show little or no yielding before failure (do not have a yield point, and do not strain-harden) Failure at small strain !!!! Typical Stress –Strain Curve for Materials like Concrete and Carbon Fibers and Glass

Thermoplastic Materials Stress- strain curves Thermoplastic Materials Nylon –thermoplastic

Universal Tensile Machine Is used to test the tensile stress and compressive strength of materials. It is named after the fact that it can perform many standard tensile and compression tests on materials, components, and structures. Components Load frame: usually consisting of two strong supports for the machine 2. Cross head : a movable cross head (crosshead) is controlled to move up or down (usually at a constant speed) 3. Means of measuring extension : a measure of the response of the test specimen to the movement of the cross head. Extensometers are sometimes used. 4. Output device : a means of providing the test result is needed

Strain Gauge Strain gauge A strain gauge is a device used to measure the strain of an object. Resistence Guge Factor Gf (sensitivity) 12

Detection of ε (strain) Wheatstone Bridge If the voltage difference across or current flow through the meter can be used to calculate the value of Rx, using  Kirchhoff's circuit laws  (also known as Kirchhoff's rules) Vs Small change in Rx Bridge out of balance Detection of ε (strain) Rx: unknown resistance to be measured R1, R2, R3: resistors of known resistance 13

Wheatstone Bridge R1 R3 VG VS R2 R3: a value equal to the strain gauge resistance with no force applied R1 and R2: equal to each other With no force applied to the strain gauge, the bridge will be symmetrically balanced and the voltmeter will indicate zero volts, representing zero force on the strain gauge. As the strain gauge is either compressed or tensed, its resistance will decrease or increase, respectively, thus unbalancing the bridge and producing an indication at the voltmeter.

Wheatstone Bridge Full-bridge strain gauge circuit 15

Types and Configurations 16