Presentation on theme: "ISSUES TO ADDRESS... Stress and strain: What are they and why are they used instead of load and deformation? Elastic behavior: When loads are small, how."— Presentation transcript:
ISSUES TO ADDRESS... Stress and strain: What are they and why are they used instead of load and deformation? Elastic behavior: When loads are small, how much deformation occurs? What materials deform least? Plastic behavior: At what point do dislocations cause permanent deformation? What materials are most resistant to permanent deformation? 1 Toughness and ductility: What are they and how do we measure them? MECHANICAL PROPERTIES Ceramic Materials: What special provisions/tests are made for ceramic materials?
2 1. Initial2. Small load3. Unload Elastic means reversible! ELASTIC DEFORMATION
3 1. Initial2. Small load3. Unload Plastic means permanent! PLASTIC DEFORMATION (METALS)
4 Tensile stress, : Shear stress, : Stress has units: N/m 2 or lb/in 2 ENGINEERING STRESS
15 Simple tension test: (at lower temperatures, T < T melt /3) PLASTIC (PERMANENT) DEFORMATION
16 Stress at which noticeable plastic deformation has occurred. when p = 0.002 YIELD STRENGTH, y
17 Room T values Based on data in Table B4, Callister 6e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered YIELD STRENGTH: COMPARISON
18 Maximum possible engineering stress in tension. Metals: occurs when noticeable necking starts. Ceramics: occurs when crack propagation starts. Polymers: occurs when polymer backbones are aligned and about to break. TENSILE STRENGTH, TS
19 Room T values Based on data in Table B4, Callister 6e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered AFRE, GFRE, & CFRE = aramid, glass, & carbon fiber-reinforced epoxy composites, with 60 vol% fibers. TENSILE STRENGTH: COMPARISON
Plastic tensile strain at failure: 20 Another ductility measure: Note: %AR and %EL are often comparable. --Reason: crystal slip does not change material volume. --%AR > %EL possible if internal voids form in neck. Adapted from Fig. 6.13, Callister 6e. DUCTILITY, %EL
Energy to break a unit volume of material Approximate by the area under the stress-strain curve. 21 TOUGHNESS
An increase in y due to plastic deformation. 22 Curve fit to the stress-strain response: HARDENING
23 Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. Determine elastic modulus according to: MEASURING ELASTIC MODULUS
24 3-point bend test to measure room T strength. Flexural strength: Typ. values: Si nitride Si carbide Al oxide glass (soda) 700-1000 550-860 275-550 69 300 430 390 69 MEASURING STRENGTH
25 Compare to responses of other polymers: --brittle response (aligned, cross linked & networked case) --plastic response (semi-crystalline case) TENSILE RESPONSE: ELASTOMER CASE
26 Decreasing T... --increases E --increases TS --decreases %EL Increasing strain rate... --same effects as decreasing T. T AND STRAIN RATE: THERMOPLASTICS
27 Stress relaxation test: --strain to and hold. --observe decrease in stress with time. Relaxation modulus: Data: Large drop in E r for T > T g. (amorphous polystyrene) Sample T g (C) values: PE (low M w ) PE (high M w ) PVC PS PC -110 - 90 + 87 +100 +150 TIME DEPENDENT DEFORMATION: CREEP
Resistance to permanently indenting the surface. Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties. 28 HARDNESS
Design uncertainties mean we do not push the limit. Factor of safety, N 29 Often N is between 1.2 and 4 Ex: Calculate a diameter, d, to ensure that yield does not occur in the 1045 carbon steel rod below. Use a factor of safety of 5. 5 DESIGN OR SAFETY FACTORS
ISSUES TO ADDRESS... How do flaws in a material initiate failure? How is fracture resistance quantified; how do different material classes compare? How do we estimate the stress to fracture? 1 How do loading rate, loading history, and temperature affect the failure stress? Ship-cyclic loading from waves. Computer chip-cyclic thermal loading. Hip implant-cyclic loading from walking. MECHANICAL FAILURE
4 Evolution to failure: Resulting fracture surfaces (steel) 50 m particles serve as void nucleation sites. 50 m 100 m MODERATELY DUCTILE FAILURE
5 Intergranular (between grains) Intragranular (within grains) Al Oxide (ceramic) 316 S. Steel (metal) 304 S. Steel (metal) Polypropylene (polymer) 3m3m 4 mm 160 m 1 mm BRITTLE FRACTURE SURFACES
6 Stress-strain behavior (Room T): TS << TS engineering materials perfect materials DaVinci (500 yrs ago!) observed... --the longer the wire, the smaller the load to fail it. Reasons: --flaws cause premature failure. --Larger samples are more flawed! IDEAL VS REAL MATERIALS
7 Elliptical hole in a plate: Stress distrib. in front of a hole: Stress conc. factor: Large K t promotes failure: FLAWS ARE STRESS CONCENTRATORS!
t at a crack tip is very small! 9 Result: crack tip stress is very large. Crack propagates when: the tip stress is large enough to make: K ≥ K c WHEN DOES A CRACK PROPAGATE?
10 Condition for crack propagation: Values of K for some standard loads & geometries: K ≥ K c Stress Intensity Factor: --Depends on load & geometry. Fracture Toughness: --Depends on the material, temperature, environment, & rate of loading. GEOMETRY, LOAD, & MATERIAL
12 Crack growth condition: Largest, most stressed cracks grow first! --Result 1: Max flaw size dictates design stress. --Result 2: Design stress dictates max. flaw size. K ≥ K c DESIGN AGAINST CRACK GROWTH
13 Two designs to consider... Design A --largest flaw is 9 mm --failure stress = 112 MPa Design B --use same material --largest flaw is 4 mm --failure stress = ? Use... Key point: Y and K c are the same in both designs. --Result: 9 mm112 MPa 4 mm Answer: Reducing flaw size pays off! Material has K c = 26 MPa-m 0.5 DESIGN EX: AIRCRAFT WING
14 Increased loading rate... --increases y and TS --decreases %EL Why? An increased rate gives less time for disl. to move past obstacles. Impact loading: --severe testing case --more brittle --smaller toughness LOADING RATE
15 Increasing temperature... --increases %EL and K c Ductile-to-brittle transition temperature (DBTT)... TEMPERATURE
16 Pre-WWII: The Titanic WWII: Liberty ships Problem: Used a type of steel with a DBTT ~ Room temp. DESIGN STRATEGY: STAY ABOVE THE DBTT!
17 Fatigue = failure under cyclic stress. Stress varies with time. --key parameters are S and m Key points: Fatigue... --can cause part failure, even though max < c. --causes ~ 90% of mechanical engineering failures. FATIGUE
18 Fatigue limit, S fat : --no fatigue if S < S fat Sometimes, the fatigue limit is zero! FATIGUE DESIGN PARAMETERS
19 Crack grows incrementally typ. 1 to 6 increase in crack length per loading cycle Failed rotating shaft --crack grew even though K max < K c --crack grows faster if increases crack gets longer loading freq. increases. crack origin FATIGUE MECHANISM
1. Impose a compressive surface stress (to suppress surface cracks from growing) 20 --Method 1: shot peening 2. Remove stress concentrators. --Method 2: carburizing IMPROVING FATIGUE LIFE
26 Engineering materials don't reach theoretical strength. Flaws produce stress concentrations that cause premature failure. Sharp corners produce large stress concentrations and premature failure. Failure type depends on T and stress: - for noncyclic and T < 0.4T m, failure stress decreases with: increased maximum flaw size, decreased T, increased rate of loading. -for cyclic : cycles to fail decreases as increases. -for higher T (T > 0.4T m ): time to fail decreases as or T increases. SUMMARY
Joint Replacement: Materials, Properties and Implications This diagrams shows seven locations where total joint arthroplasties (TJAs) are currently used to replace poorly functioning joints.
The history of total hip arthroplasty ins particularly to biomaterials science because it is one of the best illustrations of how an implant first used over a century ago has evolved into the highly successful status it has, primarily because of advances in biomaterials.
Table of most common orthopedic biomaterials Examples of the three types of bearing couples used in modern TJA. From top to bottom: metal-on- polymer, ceramic- 0n-ceramic, and metal-on-metal.
Mechanical properties of dominant orthopedic biomaterials
Approximate weight percent of different metals within popular orthopedic alloys Electrochemical properties of implant metals (corrosion resistance) in 0.1 M NaCl at pH 7. Approximate weight percent of different metals within popular orthopedic alloys Electrochemical properties of implant metals (corrosion resistance) in 0.1 M NaCl at pH 7. Approximate weight percent of different metals within popular orthopedic alloys
Examples of new THA and TKA oxidized zirconium components currently gaining popularity because of enhanced mechanical and biocompatibility properties. Examples of currently used surface coatings on stems of THA to enhance both short- and long- term fixation
Schematic of the interface of a passivating alloy surface in contact with a biological environment Modular junction taper connection of a total hip arthroplasty showing corrosion of the taper connections. Macrograph of deposits of CrPO4 corrosion particle products on the rim of a modular Co-Cr femoral head.
A schematic showing examples of the most common cytokines produced by cells reacting to implant debris acting through a variety of pathways to negatively affect bone turnover. Cytokines are a category of signaling proteins and glycoproteins that, like hormones and neurotransmitters, are used extensively in cellular communication. Cytokines are critical to the development and functioning of both the innate and adaptive immune response. They are often secreted by immune cells that have encountered a pathogen, thereby activating and recruiting further immune cells to increase the system's response to the pathogen.
Photomicrograph (5x) of a section through an acetabular section of a femoral stem retrieved at autopsy, 89 months after implantation. Note that the periprosthetic cavity surrounded development of a granuloma emanating from an unfilled screw hole. TEM images of (a) macrophage containing phagocytized titanium particles and (b) endothelial cell lining with embedded titanium debris. development of a granuloma emanating from an unfilled screw hole.
Approximate average concentrations (ng/ml or ppb) of metal in human body fluids with and without TJA.
Concentrations of metal in body tissue of humans with and without TJA
Polarized light micrograph (190x) of paraaortic lymph node demonstrates the abundance and morphology of birefringent particles within macrophages. The large filamentous particles were identified by IR spectroscopy to be polyethylene. Epithelioid granulomas (A) within the portal tract of the liver (40x) and (B) within the splenic parenchyma (15X) in a patient with a failed Ti-alloy THA and symptomatic hepatitis. (C) Backscattered SEM image of a granuloma in the spleen (3000x) demonstrating Ti-alloy particles.
A compilation of investigations showing the averaged percentages of metal sensitivity among the general population for NI, Co and Cr, among patients after receiving a metal containing implant, and among patient populations with failed implants.
The LINK SB Charite III artificial disk showing the range of standard sizes available. This design consists of an UHMWPE sliding core, which articulates unconstrained between two highly polished metal endplates, simulating the movement of the spine.