Mechanical properties 3 Bend test and hardness test of materials

Transcription

1 MME131: Lecture 15 Mechanical properties 3 Bend test and hardness test of materials A. K. M. B. Rashid Professor, Department of MME BUET, Dhaka Today s Topics Bend test of brittle materials Hardness testing Design/safety factors Reference: 1. WD Callister, Jr. Materials Science and Engineering An Introduction, 5 th Ed., Ch. 6, pp Lec 15, Page 1/9

2 The bend test for brittle materials In a ductile material, the stress-strain curve shows a maximum at the tensile strength, not at the point of failure (or, the breaking strength). In a brittle or moderately material, the maximum load occurs at the point of failure. In extremely brittle materials (e.g., cast irons, ceramics), the yield strength, tensile strength and breaking strength are all have about the same value. stress-strain behavior of brittle materials compared with that of more ductile materials In many brittle materials, the normal tensile test cannot easily be performed due to the presence of flaws difficulties in gripping the test sample high cost in preparing tensile test bar One approach used to minimize these problems is the bend test. Bend test - Application of a force to the center of a bar that is supported on each end to determine the resistance of the material to a static or slowly applied load. Flexural strength or modulus of rupture (MOR) - The stress required to fracture a specimen in a bend test. Flexural modulus - The modulus of elasticity calculated from the results of a bend test, giving the slope of the stress-deflection curve. Lec 15, Page 2/9

3 Since cracks and flaws tend to remain closed in compression, brittle materials are mostly tested in a compression test, not a tensile test. (a) Three point and (b) four-point bend test setup (a) the set-up of 3-point bend test for brittle materials, and (b) the deflection δ obtained by bending Stress-deflection curve for MgO obtained from a bend test Lec 15, Page 3/9

4 rectangular specimen circular specimen The elastic modulus, E E = FL 3 4dwh 3 E = FL 3 12dpr 4 The flexural strength, or modulus of rupture, MOR MOR = 3FL 2wh 2 MOR = FL pr 3 r = radius of sample w = width of sample h = height of sample L = distance between span d = deflection of sample calculations involved in a 3-point bend test Why the flexural strengths of brittle materials are higher than their respective tensile strengths? Lec 15, Page 4/9

5 Hardness test It is the most popular mechanical testing methods 1. simple and inexpensive 2. non-destructive testing 3. possibility of estimating other mechanical properties from hardness data Hardness is a measure of material s resistance to localized plastic deformation by indentation or scratching. The further the indenter sinks into the material, or the more the material is scratched by another material, the softer is the material and lower its yield strength. most plastics brasses, Al alloys easy to machine steels file hard cutting tools nitrided steels diamond increasing hardness The high hardness means : better resistance to plastic deformation or cracking in compression better wear properties Values depends on method of testing; different testing methods different scales and values Macrohardness - Overall bulk hardness of materials measured using loads >2 N. Microhardness - Hardness of materials typically measured using loads less than 2 N using such test as Knoop (HK). Nano-hardness - Hardness of materials measured at 1 10 nm length scale using extremely small (~100 µn) forces. Lec 15, Page 5/9

6 Moh s hardness scale A qualitative and somewhat arbitrary hardness system that measures the ability of one material to scratch another that is softer. 1, Talk softest ; 10, Diamond hardest gradations are uneven (e.g., 10 is not twice as hard as 5) Extensively used to determine the hardness of minerals. The unknown mineral, which hardness is to be determined, is scratched with another mineral of known hardness. If the mineral can be scratched by the known mineral, then the hardness of the mineral will be less than that of the known mineral. Moh s Scale 1. Talc 2. Gypsum 3. Calcite 4. Feldspar 5. Apatite 6. Orthoclase 7. Quartz 8. Topaz 9. Corundum 10. Diamond Two German Captains Fired An Old Queen To Cruel Death Quantitative hardness tests A small indenter is forced into the surface of a material to be tested, under controlled conditions of load and rate of application. The depth or size of resulting indentation is measured and converted into a hardness number. The softer the material, the larger and deeper the indentation and the lower the hardness number Measured hardness value is only relative (not absolute), and care should be taken when comparing values determined by different techniques. Lec 15, Page 6/9

7 Quantitative hardness testing methods Correlation between hardness value and tensile strength Both hardness and tensile strength are indicative of metal s resistance to plastic deformation. Consequently they are proportional to each other. But the proportionality constant is different for different materials. For most steels, TS (MPa) = 3.45 x HB TS (psi) = 500 x HB MME131 / Lec 15, Page 7/9

8 Design stress and safety factor For structural applications, the yield stress is usually a more important property than the tensile strength, since once the yield point is passed, the structure has deformed beyond recovery. Design stress: σ d = N σ c σ c = maximum anticipated stress N is the design factor > 1 (usually 1.2 4) Safe or working stress: σ w = σ y /N where N is factor of safety > 1. want to make sure that, σ d or σ w < σ y Example: Calculate a diameter, d, to ensure that yield does not occur in the 1045 carbon steel rod when a load of 220 kn is applied. Use a factor of safety of 5. Materials data: s y = 310 MPa, s TS = 565 MPa. Lec 15, Page 8/9

9 Next Class MME131: Lecture 16 Dislocation motions and yielding in materials Lec 15, Page 9/9