βοΈ Bend Test of Metals
Testing the ability of metallic materials for plastic deformation in bending. Three-point test with mandrel of specified diameter to required angle.
Overview
The bend test is a technological method for testing metals, used to assess the material's ability for plastic deformation in bending. The test involves bending a specimen with rectangular, square or round cross-section on a mandrel (punch) of specified diameter until achieving the required bending angle (usually 180Β°), without cracks appearing on the tension side.
Standard PN-EN ISO 7438 specifies the three-point bending method, in which the specimen rests symmetrically on two supports, and force is applied by a mandrel in the middle of the span. The test is passed (positive) if after bending to the required angle there are no visible cracks on the tension side β no numerical values are determined as in tensile testing.
The bend test is particularly important in testing welded joints (PN-EN ISO 5173) β it allows assessment of plasticity of the weld and heat-affected zone. It is also mandatory when testing concrete reinforcing bars (PN-EN ISO 15630-1) β the bar must bend on a mandrel of diameter 3dβ8d (depending on grade) without cracking.
Testing is also applied to sheets, strips, wires and tubes. Mandrel diameter and bending angle depend on material, specimen thickness and product standard requirements. The smaller the mandrel (sharper bend) and larger the angle β the higher the plasticity requirements.
Method principle
A metal specimen with rectangular cross-section is placed symmetrically on two cylindrical supports in a testing machine. Bending force is applied by a mandrel (punch) of diameter D, perpendicular to specimen axis, in the middle of the span. The specimen is bent at constant crosshead speed until achieving the required bending angle Ξ± (typically 90Β° or 180Β°). After unloading, the tension side of the specimen is visually assessed β absence of visible cracks means positive result.
Applications
- Testing plasticity of reinforcing bars (PN-EN ISO 15630-1)
- Qualification of welding procedures β specimens from welded joints
- Quality control of steel sheets (technological bending test)
- Testing of steel wires and bars (drawn, rolled)
- Assessment of plasticity of non-ferrous metals (aluminum, copper)
- Quality control of tubes β flattening and bending test
Key parameters
| Parameter | Value |
|---|---|
| Bending angle | 90Β° or 180Β° (per product standard) |
| Mandrel diameter D | Dependent on material and thickness: 0.5aβ10a (a = specimen thickness) |
| Support spacing | D + 2.5a Β± 0.5a (a = specimen thickness) |
| Bending rate | Slow, uncontrolled (without impacts) |
| Test temperature | 10β35Β°C (room temperature) |
Standard
- Standard number
- PN-EN ISO 7438:2021
- Title (PL)
- Metale β PrΓ³ba zginania
- Title (EN)
- Metallic materials β Bend test
Step-by-step procedure
1. Specimen preparation
Take specimen from product per product standard. Machine edges (blunt sharp edges with radius r β€ 1/10 thickness). Measure dimensions.
2. Test parameter selection
Based on product standard, select: mandrel diameter D, support spacing, bending angle. Mount appropriate mandrel and set supports.
3. Specimen placement
Place specimen symmetrically on supports. Side with potential defects (weld, rolling side) should be in tension.
4. Specimen bending
Apply force via mandrel at constant, slow crosshead speed. Bend until achieving required angle (90Β° or 180Β°).
5. Target angle achievement
At 180Β° β press specimen arms parallel (with spacer of thickness D). At 90Β° β stop when reaching right angle.
6. Tension side inspection
Remove specimen from machine. Examine tension side under 5Γβ10Γ magnification. Look for cracks, tears and delamination.
7. Result assessment
Positive result: no visible cracks on tension side. Small surface scratches and orange peel acceptable. Cracks at edges β rejection.
8. Documentation
Prepare report: material, specimen dimensions, mandrel D, bending angle, result (positive/negative), photo of bent specimen.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Universal testing machine | ZwickRoell Z100, Instron 5982, MTS Criterion β with bending fixture | 100 000β400 000 PLN |
| Bending test fixture | Set of mandrels Γ6β60 mm + adjustable supports, ZwickRoell, Instron | 10 000β30 000 PLN |
| Protractor / angle template | For measuring bending angle with Β±1Β° accuracy | 100β500 PLN |
| Caliper | Mitutoyo 500-197, accuracy 0.01 mm β for measuring specimen dimensions | 200β600 PLN |
| Magnifying glass 5Γβ10Γ | For visual assessment of tension side after bending | 50β300 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Standardized specimens | β | Flat or round specimens, dimensions per product standard, machined |
| Contrast medium (optional) | β | Color penetrant for detecting microcracks on bent surface |
| Machine oil | β | For lubricating mandrel and supports β reducing friction during bending |
Health and safety (OHS)
- Testing machine β crushing hazard, do not approach mandrel zone
- Bent specimen β sharp edges, cut risk, use gloves
- Specimen springback β impact risk after load release
- Safety glasses mandatory (fragment ejection risk if cracking)