22-Mec-A4 Design and Manufacture of Machine Elements · December 2019
Question 3 of 6: Fracture During Bending of a Blanked Lever
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Paper format. National Examinations, December 2019 — 16-Mec-A4, Design and Manufacture of Machine Elements. Three hours, open book, any non-communicating calculator. Six questions divided into Part A (Q1–Q3, manufacturing processes) and Part B (Q4–Q6, machine-element design); candidates answer two from Part A and two from Part B, and all questions carry equal value (25 % each). All six questions are solved here so the paper can be used as a complete study resource.
Reference texts. R. G. Budynas & J. K. Nisbett, Shigley's Mechanical Engineering Design, 11th ed. (Ch. 3 stress, Ch. 5 static failure, Ch. 6–7 fatigue and shafts, Ch. 9 welded joints); S. Kalpakjian & S. Schmid, Manufacturing Engineering and Technology, 8th ed. (Ch. 15 bulk deformation, Ch. 16 sheet-metal forming); M. P. Groover, Fundamentals of Modern Manufacturing, 7th ed. (Ch. 19–20); R. C. Hibbeler, Mechanics of Materials, 10th ed.
Question 3: Fracture During Bending of a Blanked Lever (25 marks)
Figure 3 — The sheared edge carries the defect; the bend decides whether it is loaded in tension.
(a) The most likely cause
The question deliberately removes the easy answer by stating that all blanks come from the same batch of material, so the change cannot be a change in the incoming steel — not a different heat, not a different temper, not a different rolling direction supplied by the mill. It also happened in the middle of a large production run, which means whatever changed did so progressively while the tools were working. The only thing that changes progressively in a blanking press is the tooling, and therefore the most likely cause is progressive wear (dulling) of the blanking punch and die, which has degraded the quality of the sheared edge on the blank.
The mechanism is worth spelling out, because it is the whole of the answer. A sharp blanking tool with correct clearance produces the four-zone edge of Figure 3 with a modest fracture zone and a small burr. As the punch and die corners wear round, the effective clearance grows and the tool can no longer cut cleanly: it begins to tear the metal instead of shearing it. The consequences are all bad for a subsequent bend. The fracture zone deepens and becomes rougher, so the edge carries a dense population of microcracks and sharp notches. The burr grows tall and ragged. The burnished band and the material immediately behind the whole edge become more heavily cold-worked, so the local ductility — the reduction of area available at that edge — falls, and with it the minimum bend radius that the material can survive, since $R_{min}/T = 50/r - 1$ with $r$ the percentage reduction of area. Finally the increased tool wear generates more heat and can leave the edge locally embrittled.
Then the blank is bent. The outer fibre of a bend is in tension and reaches the largest strain in the part, and the edges of the bend — the corners of the strip — are exactly where the sheared surface is. A rough, work-hardened, micro-cracked, burred edge sitting on the tension side of a bend is a row of ready-made crack starters, and once the strain there exceeds the (now reduced) fracture strain the part splits, partially or completely. That the failures are intermittent rather than universal is consistent with wear: parts blanked early in the tool's life are sound, parts blanked late are not, and the transition is gradual.
Check: This diagnosis assumes the press setup, lubrication and bend geometry were not altered mid-run and that the burr is currently facing outwards on the bend. Both should be confirmed on the shop floor before tooling is condemned — a die that has picked up galled material, or a bend die that has been reset with the burr on the outside, produces the same symptom for a different reason and is cheaper to fix.
(b) Remedies in the blanking operation
The aim in blanking is to restore and then maintain a clean, low-damage sheared edge.
Regrind or replace the punch and die and put them on a scheduled sharpening interval rather than running them to failure. This is the direct fix; the interval should be set from the tool life at which edge quality is measured to have degraded, not from when parts start cracking.
Re-establish the correct punch-to-die clearance, typically 4–8 % of the sheet thickness per side for mild steel and rather less for harder material, and check it around the whole profile. Excessive clearance is what deepens the fracture zone and raises the burr; too little clearance overworks the edge and accelerates wear again.
Reduce the damaged layer directly by shaving or skim-cutting the blank edge as a second operation, or by adopting fine blanking (Question 2) if volume justifies the tooling — a fine-blanked edge is fully burnished and carries essentially no crack starters at all.
Deburr — tumbling, brushing or an edge-rolling operation — so that whichever way the blank is later oriented, the burr is not available as a notch.
Improve the tribology: a better blanking lubricant, harder or surface-coated (TiN, TiCN) tool steel, and control of slug pulling all extend the interval over which the edge stays good.
Orient the blank on the strip so that the bend line runs across (transverse to) the rolling direction of the sheet. A bend whose axis is perpendicular to the rolling direction tolerates a much smaller radius than one parallel to it, because the elongated inclusion stringers are then not aligned with the tensile fibre.
If the material has been driven close to its limit by the blanking work itself, an intermediate stress-relief or full anneal between blanking and bending restores ductility — at a cost, so it is the remedy of last resort.
(c) Remedies in the bending operation
The aim in bending is to reduce the tensile strain seen by the damaged edge, and to keep that edge off the tension side wherever possible.
Increase the bend radius. The outer-fibre strain is $\varepsilon = 1/(2R/T+1)$, so going from a radius of one thickness to two thicknesses cuts the strain from 33 % to 20 %. This is the single most effective change and usually requires only a new punch nose, provided the part function tolerates the larger radius.
Turn the blank over so the burr and the fracture zone face the inside of the bend, i.e. onto the compression side. Compressive stress closes the microcracks instead of opening them. This is free and is often the first thing to try.
Bend across the rolling direction, as above, and where a part needs bends in two directions, lay it out so the more severe bend gets the favourable orientation.
Relieve the ends of the bend — a small notch, scallop or relief hole at each end of the bend line moves the free edge out of the highly strained region, and is a standard cure for edge cracking in bent brackets and levers.
Bend warm. Raising the temperature to a few hundred degrees Celsius increases ductility and lowers the minimum bend radius substantially for steel, at the cost of a heater and a slower cycle.
Apply through-thickness compression during bending — bottoming or coining the bend against a pressure pad, or using a wiping die with a firm pad — so that the outer fibre works under a less tensile mean stress. This is the same hydrostatic-pressure argument as Questions 1 and 2, applied to bending.
Slow the ram down at the bend and check the die-lip radii for wear; a sharp, worn wiping edge scores the outer surface and starts cracks on its own account.
