MSE200 Lecture 9 (CH ) Mechanical Properties II Instructor: Yuntian Zhu

Transcription

1 MSE200 Lecture 9 (CH ) Mechanical Properties II Instructor: Yuntian Zhu Objectives/outcomes: You will learn the following: Fracture of metals. Ductile and brittle fracture. Toughness and impact testing. Fracture toughness. Fatigue of metals. The S/N diagram. Mechanisms of fatigue. Stress raisers and stress concentration. Initiation and growth of fatigue cracks. Factors affecting fatigue behavior of metals.

2 Fracture of Metals Ductile Fracture Ductile fracture : High plastic deformation & slow crack propagation. Three steps : Specimen forms neck and cavities within neck. Cavities form crack and crack propagates towards surface, perpendicular to stress. Direction of crack changes to 45 0 resulting in cup-cone fracture. III I II

3 Ductile fracture SEM of ductile fracture

4 Brittle Fracture No significant plastic deformation before fracture. Common at high strain rates and low temperature. Three stages: Plastic deformation concentrates dislocation along slip planes. Microcracks nucleate where dislocations are blocked. Crack propagates to fracture. Crack may propagate through grains (transgranular) or along grain boundaries (inter granular) Examples of brittle fracture

5 Comparison of ductile and brittle fracture (under microscope) SEM of ductile fracture SEM of brittle fracture

6 Ductile and Brittle Fractures/comparison (naked eyes) Ductile fracture Brittle Fracture

7 Toughness and Impact Testing Toughness is a measure of energy absorbed before failure. Impact test measures the ability of metal to absorb impact. Charpy test

8 Impact testing (Cont ) Also used to find the temperature range for ductile to brittle transition. Titanic:

9 Fracture Toughness K 1 = Y a K 1 = Stress intensity factor. = Applied stress. a = edge crack length Y = geometric constant. Fracture toughness K Ic = Y f a c Relationship between fracture stress and crack size

10 Measuring Fracture Toughness A notch is machined in a specimen of sufficient thickness B. B > > a plain strain condition. B = 2.5(K Ic /Yield strength) 2 Sharp crack is introduced Specimen is tensile tested. Used in design to find allowable flaw size.

11 Fatigue of Metals Metals often fail at much lower stress at cyclic loading compared to static loading. Crack nucleates at region of stress concentration and propagates due to cyclic loading. Failure occurs when cross sectional area of the metal too small to withstand applied Fracture started here load. Fatigue fractured surface of keyed shaft Final rupture

12 Fatigues Testing Alternating compression and tension load is applied on metal piece tapered towards center.

13 Fatigue Strength, Stress-Number of circles curve Fatigue strength Endurance limit

14 Cyclic Stresses Different types of stress cycles are possible (axial, torsional and flexural). Mean stress = max + min = 2 a m 2 = max min Stress amplitude = Stress range = r Stress ratio = max min = R = min max

15 Structural Changes in Fatigue Process Crack initiation first occurs. Reversed directions of crack initiation caused surface ridges and groves called slipband extrusion and intrusion. This is stage I and is very slow ( microns/cycle). Crack growth changes direction to be perpendicular to maximum tensile stress (microns/cycle). Sample ruptures by ductile failure when remaining cross-sectional area is small to withstand the stress.

16 Fatigue crack initiation/slip band Formation of slip band extrusion and intrusion

17 Factors Affecting Fatigue Strength Stress concentration: Fatigue strength is reduced by stress concentration. Surface roughness: Smoother surface increases the fatigue strength. Surface condition: Surface treatments like carburizing and nitriding increases fatigue life. Environment: Chemically reactive environment, which might result in corrosion, decreases fatigue life.

18 Homework Example Problems: none Regular Problems, download from the website. Reading assignment: section