Mechanical Properties: Hardness and Fatigue PDF

Mechanical Properties Hardness and Fatigue Objective  Discuss mechanical properties  Hardness  Fatigue  Understand the criticality of such properties  Tabulate the values of such properties Hardness What is it?  Hardness is a resistance to deformation  Hardness is mor...

Mechanical Properties Hardness and Fatigue Objective  Discuss mechanical properties  Hardness  Fatigue  Understand the criticality of such properties  Tabulate the values of such properties Hardness What is it?  Hardness is a resistance to deformation  Hardness is more likely to mean the resistance to indention  Deeper or larger impression indicates softer material Hardness Industrial applications  Abrasion  Wear and high friction  Subjected to high pressures and sharp impacts  Theft protection to be resistant to cutting Hardness Industrial applications Hardness How it was measured before?  Qualitative vs Quantitative analysis  Mohs scale  1-10 as materials get harder (Diamond is the hardest) Hardness Measuring devices  Indenter  Comparing the depth/shape/area of the indentation to the load will tabulate the Hardness number Hardness Measuring devices Vickers Brinell Rockwell Hardness Measuring devices  Depth/shape/area of the indentation will be compared to the load  The calculations will always be the applied load divided by the area  The area could be surface area or cross-sectional area  The applied load is usually a weight  The load is left for a certain dwell time to get the needed value in terms of N/mm2  Since each measuring device will have a different area, the unit is not MPa and is named after the device itself  For example Al measured by Vickers will have 80 Vickers Hardness (HV)  Brinell HB, Rockwell HR Fatigue Strength and Failure Fatigue What is it?  Fatigue (as defined medically) is a term used to describe an overall feeling of tiredness or lack of energy.  Fatigue (as defined in mechanics) is based on the concept that a material becomes “tired” and fails at a stress level below the nominal strength of the material. Fatigue What is it?  Fatigue occurs when a material experiences:  Lengthy periods of cyclic or repeated stresses  Failure occurs rapidly and without warning.  Fatigue is estimated to be responsible for approximately 90% of all metallic failures  The stresses acting repeatedly upon the material may be due to:  Tension-compression type stresses  Bending or twisting type stresses Fatigue In real life and industry  Any application with dynamic loading is subjected to Fatigue failure  Airplanes  Bridges  Cars and trains  Bearings and gears Fatigue Stress that induces failure  Sinusoidal varying Fatigue Stress variations  Only the Stress peaks above the threshold will cause fatigue cracking Fatigue Failure vs Tensile failure S-N Curves  Stress which can lead to failure at stress levels much lower than the tensile strength of the material. S-N curves for different materials Fatigue Calculations  The mean stresses given by:  Stress amplitude is given by:  σr being the range of stress.  The stress ratio of the maximum and minimum stress amplitudes is given by:  Note: that tensile stresses are positive while compressive stresses are always negative Fatigue Problem.1  A fatigue test was conducted in which the mean stress was 70 MPa (10,000 psi), and the stress amplitude was 210 MPa (30,000 psi).  a) Compute the maximum and minimum Stress levels  b) Compute the stress ratio  c) Compute the magnitude of the stress range Fatigue Problem.1 solution Fatigue Problem.2  A cylindrical 70Cu-30Zn brass bar is subjected to axial tension– compression stress testing with reversed-cycling. If the load amplitude is 10,000 N, compute the minimum allowable bar diameter to ensure that fatigue failure will not occur at 107 cycles. Assume a factor of safety (N) of 2.5, data in the Figure were taken for reversed axial tension– compression tests, and that S is stress amplitude.  Stress amplitude= Stress*Factor of Safety Minimum allowable bar diameter?? Givens:  70Cu-30Zn  Cylinder  Axial tension–compression stress testing with reversed-cycling (Fatigue)  F=10000N  N=107 cycles Fatigue Problem.2 solution Recap  Hardness in the resistance to scratches or indentation  Measured using multiple devices like Rockwell  Hardened materials in use as gears and locks  Fatigue as an unpredictable failure mechanism of parts  The fatigue limit vs the ultimate strength