Bend testing
Selecting ductility, weld-bend, and flexural-property tests; controlling specimens, fixtures, measurements, and interpretation.
Tags: inspection, mechanical-testing, bending, welds
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Start with the test objective
Choose the method by the result required. A coupon bent to a prescribed shape can reveal cracking. A flexural-property test uses the force and deflection curve.
ISO 7438:2020, published October 2020, specifies a method for assessing metallic materials' ability to undergo plastic deformation in bending. Its public scope excludes certain products, including full-section tubes and welded joints, for which other standards exist. Do not treat this metallic ductility method as a universal flexural-strength test. ISO: ISO 7438 scope
ISO 5173:2023, published January 2023, covers transverse root, face, and side bends from specified welded and clad specimens. Its purpose includes revealing imperfections on or near the surface placed in tension and/or assessing ductility. It also addresses alternative methods for joints whose constituent regions have substantially different bending-related properties. ISO: ISO 5173 scope
| Objective | Typical evidence | Essential distinction |
|---|---|---|
| Metallic-product bend ductility | Condition after a prescribed bend | Angle, former geometry, specimen, and acceptance criteria belong to the product/method specification. |
| Weld soundness or ductility | Opened imperfections on the stretched surface | Coupon orientation determines which weld region is placed in tension. |
| Flexural properties | Force and specimen-deflection data | Stress, strain, and modulus calculations depend on the prescribed geometry and analysis. |
| Component performance | Load, displacement, damage, or function under a specified setup | A custom component result is not automatically a standardized material property. |
Three-point, four-point, and guided bending
In three-point bending, two supports carry the specimen while a central loading nose applies force. Four-point bending uses two loading points, creating a constant bending-moment region between them under the ideal beam arrangement. Instron's guidance distinguishes the small maximum-stress region in three-point bending from the extended region between four-point loading points. Fixture type therefore changes the material volume being highly stressed and can affect the failure observed. Instron: flexural-test configurations
A guided bend instead constrains deformation around a specified former. TWI describes guided bending for weld qualification and distinguishes it from free bending. The required bend angle and former size must come from the applicable specification, rather than a general instruction to “bend it double.” TWI: bend testing
Record outer support span, inner loading span where relevant, nose and support radii, alignment, loading direction, and any rollers or guides. A fixture's nominal capacity does not establish suitability for a particular specimen, span, or loading nose.
Weld bends: face, root, and side
The named face or root is the region deliberately placed on the outside, stretched part of the bend. A side-bend specimen exposes a different section through the weld. Select specimen position and orientation from the qualification or product requirements; a favorable bend on an unrepresentative coupon does not establish acceptance of the whole joint.
TWI explains that preparation can invalidate a result: cutting-induced heat effects or work hardening, sharp corners, and weld reinforcement can promote unintended failure. It also describes localized deformation or “peaking” when adjacent regions differ in strength. Follow the specified preparation and permitted method for that joint. Do not apply a universal edge radius or rejection crack length taken from an overview article. TWI identifies its article as originating in November 2004, with subsequent figure changes; use current controlled requirements for acceptance. TWI: preparation and interpretation cautions
Flexural-property tests
Instron's ASTM D790 guide, updated 20 March 2026, discusses three-point testing of reinforced and unreinforced plastics and distinguishes flexural strength, stress at break, and different modulus definitions. A reported modulus requires the specified slope interval or construction; the largest load is not itself a modulus. The guide also distinguishes ASTM D790 and ISO 178 requirements, including specimen dimensions and deflection treatment. Do not label results interchangeable merely because both use three-point fixtures. Instron: ASTM D790 guide
Measure actual specimen dimensions at the required locations. Preserve material direction, conditioning, processing history, and which face is loaded. For a laminate or other anisotropic material, orientation and failure mode belong with the reported result. A test dominated by local indentation, shear, support damage, or an unintended defect may not establish the intended flexural property.
Span-to-depth ratio influences shear contribution and the applicability of the selected analysis. Instron's overview identifies this as a method-dependent choice. Use the governing standard's ratio and loading geometry rather than assuming one span works for metals, polymers, composites, and ceramics. Instron: flexural-test analysis
Rate, displacement, and a bounded example
Test rate is part of the method. Instron's D790 guide gives the crosshead-rate relationship R = ZL²/(6d), with support span L and specimen depth d in millimetres, specified outer-fibre strain rate Z in min⁻¹, and crosshead rate R in mm/min.
Illustrative arithmetic only: assume an applicable procedure selects L = 64 mm, d = 4 mm, and Z = 0.01 min⁻¹. The calculated rate is 0.01 × 64² / (6 × 4) = 1.71 mm/min, rounded. These assumed inputs demonstrate the equation; they do not prescribe a valid D790 specimen, procedure, or rate for another material. Instron: rate calculation
Measure the displacement quantity required by the method. Crosshead movement includes deformation elsewhere in the load train; specimen deflection may need a deflectometer or validated compliance correction, particularly for modulus. Record the measurement channel and correction rather than simply labeling both “deflection.”
As an equipment example, Instron identifies 2810-182 as a 100 kN flexure fixture with 30–250 mm adjustable lower span, maximum specimen width 50 mm, and an optional four-point conversion. These are fixture specifications, not a permitted force, span, or specimen prescription for every bend standard. Instron: fixture models
Interpretation and reporting
Establish termination and acceptance criteria before testing: prescribed angle, displacement, strain, fracture, or another defined endpoint. Keep the method standard separate from the product or qualification acceptance requirements. Do not convert “no visible crack under this test” into a quantitative tensile elongation, fatigue-life prediction, or assurance that no internal imperfection exists.
For a failed coupon, document where and how failure began and assess test validity before assigning a material or weld cause. Preserve force-displacement data where acquired and photographs of the relevant surfaces. A broken specimen alone cannot distinguish an incorrect setup from a material limitation.
Report specimen identity, sampling location and orientation, dimensions and preparation, material condition, standard edition, fixture geometry, rate, environment, calibrated measurement channels, endpoint, observations, calculated properties where applicable, and acceptance criterion. This makes the result comparable without obscuring the difference between a qualitative bend check and a quantitative flexural test.