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EGA324 – Stress Concentrations Overview ❑ Stress Concentrations & Raisers ❑ Industrial Examples ❑ Infinite Circular Plate ❑ Stress Concentration Factors (SCF) ❑ Neuber’s rule to calculate SCF from stress- strain curves ❑ Tensile Test Experiment ❑ Assignment EGA324 – Stress Concentrations Stress Concentrations & Raisers ❑ What is a Stress Concentration? ▪ A stress concentration (often called a stress raiser) is a location in an object where stress in concentrated ▪ An object is strongest when load is evenly distributed over its areas so any reduction in area results in a localised increase in stress ▪ Stress concentrations can arise from notches, grooves, holes, threads, or any other abrupt changes in geometry or in poor designs of sharp corners and even from inspections/stamping ▪ These discontinuities are known as stress raisers EGA324 – Stress Concentrations Stress Concentrations =
– stress concentration – maximum stress – reference (nominal) stress ? EGA324 – Stress Concentrations Examples of Stress Concentrations ❑ Can anyone name an example? ❑ Pressure Vessels ❑ Poor weld profile ❑Alexander Kielland drilling rig disaster ❑ Bolt/Rivet holes ❑de Havilland DH.106 Comet (Commercial Airliner) EGA324 – Stress Concentrations Alexander Kielland drilling rig disaster EGA324 – Stress Concentrations de Havilland Comet Accident EGA324 – Stress Concentrations Infinite Plate with Small Hole ❑ Kirsch Equations (1898) = ∞ 2 1 − 2 2 + ∞ 2 1 − 2 2 1 − 32 2 cos 2 = ∞ 2 1 + 2 2 − ∞ 2 1 + 34 4 cos 2 = − ∞ 2 1 − 2 2 1 + 32 2 cos 2 r – radial co-ordinate; a – hole radius – loading direction; – stress EGA324 – Stress Concentrations Infinite Plate with Small Hole ❑ at the hole: r = a = 0; = 0 = ∞ 1 − 2 cos 2 ; Maximum stress at = ± 90° ( = 3∞) =
= EGA324 – Stress Concentrations Notch Sensitivity Index ❑ Notch sensitivity (q) of material is a measure of how sensitive a material is to notches or geometric continuities = (ℎ ) (ℎ) = 1 + − 1 1 ≤ ≤ = − 1 − 1 0 ≤ ≤ 1 q is normally between 0-0.1 for ductile materials, 0.5 for cast irons and 0.5-1.0 for brittle materials EGA324 – Stress Concentrations SCF (SCC) Tables W. C. Young and R. G. Budynas, Roark’s Formulas for Stress and Strain, 7th ed. New York: McGraw-Hill, 2002 EGA324 – Stress Concentrations SCF (SCC) look-up graphs for stress- raisers EGA324 – Stress Concentrations Neuber’s Rule ❑ Neuber’s rule states that the product of nominal (elastic) stresses and strains is proportional to the product of the local (elastic plastic) stresses and strains = 2 = & – the nominal stress based on specimen geometry – theoretical stress concentration factor (tables/look-up) & – stress/strain value beyond yield point of the material EGA324 – Stress Concentrations Neuber’s Rule ❑Typically know & ❑Need to determine the effective stress concentration
EGA324 – Stress Concentrations Example 1 – SCE for single hole ❑ or determined by tables/equations: = 3 − 3.140 20 100 + 3.667 20 100 2 − 1.527 20 100 3 = 2.51 ❑ found using net cross sectional area =
− = 64 100 − 20 8 ∙ 103 10−6 = 100 MPa ❑ Based on elastic behaviour peak stress at edge of hole would be = = 2.51 ∙ 100 = 251 MPa EGA324 – Stress Concentrations Example 1 – SCE for single hole ❑ Nominal strain is read from the stress-strain curve at = 100 MPa (A: = 5 ∙ 10 −4) ❑ Applying Neuber’s Rule: = 2 = 2.51 2 100 5 × 10−4 = 0.315 ❑ Intersection of = 0.315 (Point B) yields = 243 MPa & = 13 ∙ 10−4. Effective / experimental stress concentration is: or =
= 243 100 = . ❑ Results show 251 MPa exceeds the yield point of the material, so actual peak stress at hole edge found using Neuber’s Rule EGA324 – Stress Concentrations Example 2 – SCE for two notches ❑ or determined by look up graph ,
= 5 60 = 0.08 ሶ3 → = 2.45 ❑ found using net cross sectional area =
∙ = 110 60∙12.5 ∙ 103 10−6 = 147 MPa ❑ Based on elastic behaviour peak stress at edge of notch would be = = 2.45 ∙ 147 = 360 MPa d=60mm; ρ=5mm; t=12.5mm; P=110kN EGA324 – Stress Concentrations Example 2 – SCE for two notches ❑ Results show 360 MPa exceeds the yield point of the material, so actual peak stress at hole edge found using Neuber’s Rule ❑ A: = 147 MPa; = 7.2 ∙ 10 −4 – can also be done using = when E is known ❑ Applying Neuber’s Rule: = 2 = 2.45 2 147 7.2 × 10−4 = 0.635 EGA324 – Stress Concentrations Example 2 – SCE for two notches ❑ B: Intersection of = 0.635 gives = 275 MPa & = 23 ∙ 10−4. ❑ The effective/experimental stress concentration is: or =
= 275 147 = . = 0.635 EGA324 – Stress Concentrations The Experiment…please watch in your own time https://canvas.swansea.ac.uk/courses/36971/modules/items/1968427 EGA324 – Stress Concentrations Experimental Aim ❑ The aim of this experimental investigation is to assess the effects of introducing a stress concentration feature, such as a hole or notch, to a component or structure ❑ A number of mild steel tensile specimens are provided with and without stress raisers and the load/displacement (stress/strain) characteristics of these specimens under tensile loading need to be compared and contrasted ❑ An extensometer will be demonstrated on the plain samples for more accurate strain measurements that would typically be best practice EGA324 – Stress Concentrations Specimens Plain Sample 2.5 mm single notch 5 mm Hole 2 mm Hole 2x2.5 mm double notch ❑A total of 10 mild steel tensile test specimens will be provided with five different geometries: ❑Key dimensions: thickness – 1mm, length – 180 mm, gauge length – 70mm, width of gauge in plain specimen – 15mm EGA324 – Stress Concentrations Specimens – Manufacture Route 2.5 mm single notch 5 mm Hole 2 mm Hole 2x2.5 mm double notch ❑ Tensile specimens have been manufactured through water jet cutting EGA324 – Stress Concentrations Equipment ❑ The tests are performed using a Hounsfield 25kN electrically operated tensile test machine EGA324 – Stress Concentrations Experimental Procedure ❑ Specimens are located between the jaws (wedge grips on the Hounsfield machine) ❑ Setting the instrumentation to zero, tensile test is started with the cross-head of the machine being driven at a displacement rate of 3 mm/min ❑ Force is measured using a load cell within the Hounsfield. Time, load and displacement are recorded continuously during the test ❑ Once the specimen fractures, yield load and ultimate tensile load should be noted to initially gauge any differences ❑ Start with the plain specimens and repeat for all – additional details given in video & in the lab EGA324 – Stress Concentrations Table of Results Yield Load [N] Ultimate Tensile Load [N] Specimen 1 Specimen 2 Specimen 1 Specimen 2 Plain 2 mm central hole 5 mm central hole Single-sided notch Double-sided notch ❑ Initial table of results gives an insight to the effect of the different stress concentration features ❑ Would expect to see a more complete results table in your final reports, that may not include loads – with stress a better comparator EGA324 – Stress Concentrations Analysing Tensile Data ❑ Data comes from the machine as ▪ time (s), displacement (µm), force (N) ❑ Needs to be converted to engineering stress, and engineering strain, via: =
0 & = ∆ 0 Where is the applied force, 0 is the cross-sectional area of the test piece, 0 is the gauge length and ∆ is the extension EGA324 – Stress Concentrations Analysing Tensile Data ❑ This needs to be converted to true stress, and true strain, which is approximated by: = ( + 1) ∙ & = ln(1 + ) ❑ Use Neuber’s rule as described above and in the examples together with the calculated stress-strain to compare the experimental SCFs with the empirical SCFs and how do they compare? ❑ Nominal stress () values for the Neubers rule analysis for the experimental data are to be taken from the elastic region of the curve – using a consistent approach EGA324 – Stress Concentrations Typical Experimental Results 0 50 100 150 200 250 300 350 0 0.1 0.2 0.3 0.4 0.5 0.6 St re ss ( M P a) Strain Eng Stress/Strain True Stress/Strain 0 50 100 150 200 250 300 350 0 0.1 0.2 0.3 0.4 0.5 0.6 Use Scatter with Smooth Lines, way too many data points to include every point! EGA324 – Stress Concentrations Fracture/Failure Surfaces Available on Canvas: ❑What do they look like? ❑ What do the angles at which the surface orientated tell you? ❑ Ductile or Brittle type fracture? EGA324 – Stress Concentrations Some Open Questions ❑ How does the plain sample properties compare to that of mild steel seen in the literature? ❑How does the size of the hole affect the tensile characteristics of the components and why ? ❑ How does the number of notches affect the tensile characteristics and why ? ❑ How do the tensile characteristics for a 5 mm hole compare with those for two R2.5 mm notches and why ? ❑ Rank the five stress raisers in terms of their impact on the components’ behavior under load. ❑ Compare and contrast the degree and form of the deformation (strain) at failure for the different stress raisers and how do they compare with the plain specimen ? ❑ How do the shapes of the fracture surfaces compare and why might they be different ? ❑ Comment on the repeatability of the tests and how could accuracy be improved ? ❑ In addition – Computational Modelling aspects to be considered… EGA324 – Stress Concentrations Examples Format: ❑ Review experimental protocol & raw data sets ❑ Individual tensile test data analysis ❑ SCFs – empirical & experimental (Neuber’s) ❑ Going further… EGA324 – Stress Concentrations Experimental Procedure ❑ Important points: • Linear measurement – callipers/micrometer • How would this influence your stress/strain results? • Displacement rate – strain rate effects on materials • Data acquisition rate • Consistent jaw separation (returning jaws to same starting position between tests) • Testing standards: • e.g. BS, ISO EGA324 – Stress Concentrations Experimental Practice ❑ Nominal Dimensions – we are using these…