22-Mec-A4 Design and Manufacture of Machine Elements · May 2013

Question 3 of 8: 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 Examination, 07-Mec-A4 Design and Manufacture of Machine Elements, May 2013 — 3 hours, open book, any non-communicating calculator permitted. Eight questions on six pages, divided into Part A (manufacturing processes, Q1–Q4) and Part B (machine-element design, Q5–Q8). The rubric asks for three from Part A and two from Part B, five questions constituting a complete paper, all of equal value (20 % each). All eight questions are solved here, because this document is a study resource rather than an examination script.

Reference texts.

Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed. — grinding (Ch. 26), sheet-metal forming and the forming-limit diagram (Ch. 16), sand casting and casting defects (Ch. 11–12).
Budynas & Nisbett, Shigley's Mechanical Engineering Design, 10th/11th ed. — combined stresses and static failure theories (Ch. 3, 5), fatigue (Ch. 6), shaft design (Ch. 7), bolted joints in shear (Ch. 8), welded joints (Ch. 9).
Hibbeler, Mechanics of Materials, 10th ed. — stress transformation and combined loading.
ASM Handbook Vol. 15, Casting — gas, penetration and mould-wall-movement defects in no-bake sand systems.

Check: Part B is figure-driven. Every dimension used below was read from the printed figures (Fig. S4–S7). Where the drawing dimensions a distance from a face rather than from a bolt centre (Q7), the reading is stated explicitly in Given so a grader can substitute a different interpretation without redoing the method.

Question 3: Fracture during bending of a blanked lever (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Most likely cause

The decisive clue is embedded in the question: the material is the same batch throughout, yet the fractures begin in the middle of the run. Whatever changed is therefore not the sheet — it is the tooling. The most likely cause is progressive wear of the blanking punch and die, which has degraded the quality of the sheared edge that subsequently ends up on the tension (outer) side of the bend.

The mechanism is worth setting out carefully because it explains both the timing and the appearance of the failures. A sheared edge is not a clean cut. It consists of four zones through the thickness: a rollover, a smooth burnished band where the punch rubbed, a rough fracture band where the crack from punch and die met, and a burr. The burnished band is severely cold-worked; the fracture band carries a population of fine microcracks left by the shearing fracture itself. Together they form a damaged, low-ductility layer whose depth is a strong function of the punch-to-die clearance.

As the tooling wears, the cutting edges round over and the effective clearance grows. Large clearance produces excessive rollover, a taller and sharper burr, and — critically — a deeper zone of cold work and a rougher fracture band with more and larger microcracks. When that edge is then placed on the outside of a bend, it experiences the maximum tensile strain in the part, $\varepsilon = 1/(2R/T + 1)$. A material that comfortably survived that strain with a sharp-tool edge will crack once the damaged layer deepens. Because tool wear is gradual, the defect appears part-way through the run and then worsens — exactly the reported symptom.

A secondary contributor, which should be checked at the same time, is bend orientation relative to the rolling direction. Rolled sheet contains elongated inclusion stringers and a preferred grain texture; bending with the bend axis parallel to the rolling direction stresses those stringers transversely and can halve the achievable minimum bend radius. If the blank layout was nested differently to improve material utilisation part-way through the run, this becomes the primary cause rather than a secondary one.

(b) Remedies in the blanking operation

Regrind or replace the punch and die and restore the correct clearance. For low-carbon steel this is roughly 5–8 % of the sheet thickness per side; too tight causes secondary shear and rapid tool wear, too loose causes the deep damaged layer described above. Introduce a scheduled sharpening interval based on the observed part count, rather than sharpening on complaint.
Control burr direction at the layout stage. The burr always forms on the die side. Lay the strip out so that the burr ends up on the inside (compression side) of the bend, where a crack cannot propagate. This is a free fix and is often sufficient on its own.
Remove the damaged layer. Add a shaving station (a second, light cut of about 10 % of thickness) or specify fine blanking with a V-ring blank holder and counterpunch, which produces a fully burnished edge with essentially no fracture band. Tumbling or vibratory deburring is a cheaper partial measure.
Restore ductility. A stress-relief or process anneal between blanking and bending removes the cold work in the sheared edge. This is the standard remedy when the geometry forces a tight bend radius.
Re-nest the blank so that the bend axis is perpendicular to (or at least at 45° to) the rolling direction.

(c) Remedies in the bending operation

Increase the bend radius. The tensile strain on the outer fibre is $\varepsilon = 1/(2R/T+1)$, so raising $R/T$ from, say, 1 to 2 cuts the outer-fibre strain from 33 % to 20 %. Confirm the new radius against the material's published minimum bend radius, expressed as a multiple of $T$.
Bend with the burr and the rougher sheared band on the inside — the die-side face of the blank goes against the punch. This complements the blanking-side layout fix and costs nothing.
Reduce the severity of the punch nose and improve support. Replace a sharp air-bend punch with a larger-radius punch, or move from air bending to bottoming with matched radii so that the deformation is spread rather than concentrated at a line.
Apply superposed compression. Bending under a small superposed compressive or tensile-restraint condition — for example wiping dies or a urethane-pad die that presses the sheet against a form block — raises the hydrostatic pressure at the outer fibre and suppresses crack opening.
Warm the blank. Bending at a few hundred degrees Celsius restores ductility for grades that are marginal at room temperature and is a practical last resort when radius and orientation are both fixed by the design.
Slow the press. Lower strain rate raises the fracture strain of most sheet steels and reduces the chance of a partial crack running.

Topic: Sheared-edge quality, its interaction with bendability, and the role of tool wear in progressive defects.

Key relations: Outer-fibre engineering strain in bending $\varepsilon = 1/\left(2R/T + 1\right)$; minimum bend radius expressed as $R_{\min} = c\,T$ with $c$ tabulated by grade and temper; blanking clearance per side $\approx 0.05\!-\!0.08\,T$ for mild steel; the depth of the cold-worked edge layer scales with clearance.

Why this works: Bendability is limited by the ability of the outer fibre to sustain tensile strain without cracking, and a sheared edge presents that fibre with a pre-cracked, work-hardened notch. Everything in the answer either reduces the demand (larger radius, burr inside, superposed compression) or restores the capacity (sharper tooling, shaving, annealing, warming). Recognising that a mid-run onset with a single material batch must be a tooling change is the piece of reasoning the examiner is testing.

Common pitfall: Blaming the material when the question has already excluded it. The second pitfall is treating clearance as "smaller is always better"; excessively tight clearance causes secondary shear, a double burnish band and very rapid punch wear, which brings the same problem back within a shorter interval.

Source: Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed., §16.2 (Shearing: clearance, burr and edge quality) and §16.5 (Bending: minimum bend radius, anisotropy, springback).

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