The bellows flattening test is a fundamental mechanical evaluation used to verify the ductility, weld integrity, and structural soundness of corrugated tubes and bellows by compressing a specimen between rigid parallel plates to a specified deflection. Conducted in accredited third-party laboratories in accordance with standards such as ISO 8492, GB/T 246, GB/T 19472.2, and EN 13476, this test confirms that bellows products can tolerate flattening deformation during installation and service without cracking, splitting, or leaking. This article explains the test principle, applicable sample types, testing items, procedures, acceptance criteria, and typical industrial applications.
Technical Overview
The bellows flattening test, also referred to as a flatten test or crush test, is a mechanical examination in which a short section of corrugated tube or bellows is placed between two rigid, parallel plates and compressed until a specified plate separation or percentage deflection is reached. The specimen is then inspected, either while still loaded or after the load is released depending on the governing standard, for cracks, fissures, weld failure, delamination, and other structural defects. Because bellows convolutions are produced by hydroforming, rolling, or specialized extrusion processes, the flattening test is a sensitive indicator of material ductility, forming quality, and longitudinal weld soundness.
Purpose of the Test
The test provides a clear pass or fail verdict on several quality attributes: whether the convolution material retains sufficient ductility after forming and heat treatment, whether the longitudinal weld seam can withstand severe plastic deformation without opening, and whether the corrugated structure will survive crushing loads encountered during burial, clamping, bending, or installation. For bellows used in fuel gas service, the flattening test is typically followed by a tightness verification to prove that leak-tightness is preserved even after the tube has been severely deformed.
Governing Standards
- ISO 8492 and GB/T 246, which define the flattening test method for metallic tubes
- ISO 10380, covering corrugated metal hoses and hose assemblies
- GB/T 14525, the general specification for bellows metal hoses
- CJ/T 197 and GB/T 26002, covering stainless steel bellows and corrugated tubes for fuel gas applications
- GB/T 19472.2 and EN 13476, which include flattening requirements for structured-wall corrugated plastic pipes used in buried drainage systems
- ASTM D2412, the parallel-plate loading method for plastic pipe, frequently used alongside flattening evaluations to characterize load versus deflection behavior
Testing Samples
Metallic Bellows and Corrugated Metal Hoses
Typical specimens are annular or helical corrugated tubes manufactured from austenitic stainless steels such as 304 and 316L, as well as carbon steel and copper alloys. These samples are cut from flexible metal hoses and expansion joint bellows, and particular attention is paid to the position of the longitudinal weld, which is oriented at the angle required by the applicable standard before compression begins.
Stainless Steel Bellows for Fuel Gas
Thin-wall stainless steel corrugated tubes used for residential and commercial gas connections, often supplied with a polymer oversheath, are subjected to flattening followed by a leak test. This combination demonstrates that the tube will not crack or lose gas tightness if it is accidentally crushed, kinked, or pinched during handling and installation.
Structured-Wall Plastic Corrugated Pipes
High-density polyethylene, polypropylene, and PVC double-wall or structured-wall corrugated pipes for non-pressure buried drainage and sewer applications are tested to verify that the inner wall does not crack and that the corrugated outer wall does not separate from the liner when the pipe is compressed. Specimens are cut to a specified length, commonly several hundred millimeters or a defined multiple of the nominal diameter.
Flexible Corrugated Conduits for Cable Protection
Corrugated polyamide, polyethylene, and polypropylene conduits used to protect wiring harnesses in vehicles, machinery, rail vehicles, and electrical installations are evaluated for flattening resistance and shape recovery in accordance with manufacturer and industry conduit specifications.
Testing Items
Depending on the product standard, a bellows flattening test program may include the following examination items:
- Resistance to flattening-induced cracking, which reflects the ductility of the convolution material
- Integrity of the longitudinal weld seam after severe plastic deformation
- Leak tightness or gas tightness verified by a subsequent pressure or tightness test
- Delamination or separation between the walls of double-wall and structured-wall pipes
- Elastic recovery of the cross section after the load is released
- Visual condition of the inner wall and corrugation structure, including splits at corrugation roots and crests
- Load versus deflection behavior when the test is instrumented, providing data complementary to ring stiffness measurements
Testing Methods
Specimen Preparation
Representative tube sections are cut with ends perpendicular to the tube axis, and all burrs are removed to avoid stress concentrations that could produce false failures. For plastic corrugated pipes, specimens are conditioned at standard laboratory temperature, commonly 23 degrees Celsius, before testing. The outside diameter, wall thickness, and corrugation geometry are recorded prior to loading.
Metallic Tube Flattening Procedure
The specimen is placed between two flat, hardened, parallel plates that are wider than the specimen, with the weld seam oriented as specified, commonly at 90 degrees or 45 degrees to the loading direction. A steady compressive force is applied using a universal testing machine until the distance between the plates reaches the value H specified by the governing product standard, which is typically calculated from a formula based on the deformation factor and the diameter-to-thickness ratio, or expressed as a percentage of the original outside diameter. After unloading, the external and internal surfaces are examined without magnification, and for gas bellows a tightness test is then repeated to confirm that no leakage path has been created.
Plastic Corrugated Pipe Flattening Procedure
The specimen is positioned under the parallel plates and compressed at a constant rate, commonly in the range of 10 to 20 millimeters per minute as defined by the standard, until the vertical dimension is reduced to a specified fraction of the original outside diameter. The load is held briefly at the target deflection and then released. Inspectors examine the inner wall for cracking, splitting, and exposure of reinforcement, and check whether the corrugated outer wall has separated from the smooth liner. Where required, dimensional recovery is measured after a defined resting period.
Acceptance Criteria
- No cracks or fissures visible to the unaided eye on the outer or inner surfaces
- No opening, rupture, or indication of incomplete fusion along the longitudinal weld
- No delamination between the structural walls of structured-wall pipes
- For fuel gas bellows, no leakage detected in the subsequent tightness verification
- Shape recovery values, where specified, within the limits defined by the product standard
Equipment
Testing is performed with a universal testing machine or a dedicated compression fixture fitted with flat, parallel, hardened plates of sufficient size to cover the full specimen width. Instrumented setups record force and displacement throughout the test, and an auxiliary leak test rig using pressure decay or water bath immersion is employed when tightness after flattening must be demonstrated.
Applications
Flattening test results directly support qualification and quality control in a wide range of industries that rely on corrugated tubes and bellows:
- Fuel gas distribution, including flexible stainless steel connectors for residential and commercial installations
- Building plumbing and flexible water connection hoses
- Buried drainage and sewer systems using HDPE, PVC, and polypropylene corrugated pipes, including culverts and cable ducting trenches
- Cable protection conduits in automotive wiring harnesses, rail vehicles, industrial machinery, and electrical installations
- Industrial piping systems employing flexible metal hoses and expansion joint bellows
- HVAC, vacuum, and aerospace ducting where corrugated conduits must survive handling and clamping loads
FAQ
What is the bellows flattening test?
It is a mechanical test in which a bellows or corrugated tube specimen is compressed between rigid parallel plates to a specified deflection, after which it is inspected for cracking, weld failure, delamination, and, where relevant, loss of leak tightness. It is one of the most direct ways to confirm the deformation tolerance of a corrugated product.
Which standards specify the bellows flattening test?
Common references include ISO 8492 and GB/T 246 for metallic tubes, ISO 10380 and GB/T 14525 for corrugated metal hoses, CJ/T 197 and GB/T 26002 for fuel gas bellows, and GB/T 19472.2 and EN 13476 for structured-wall corrugated plastic pipes, with ASTM D2412 frequently used for parallel-plate loading characterization of plastic pipe.
How is the target plate separation determined?
For metallic tubes, the required distance between the plates is usually calculated from a formula that accounts for the deformation factor and the diameter-to-wall-thickness ratio, or is stated as a percentage of the nominal diameter by the product standard. For plastic corrugated pipes, the standard specifies a target deflection expressed as a defined fraction of the original outside diameter.
Is the flattening test destructive?
Yes. The specimen undergoes permanent plastic deformation and is sacrificed during the test, which is why flattening is performed as a type test or routine sampling test on representative sections rather than on every delivered product.
How does flattening differ from ring stiffness testing?
Flattening is primarily a qualitative integrity test with a pass or fail outcome focused on visible structural damage, whereas ring stiffness tests described in ISO 9969 and ASTM D2412 quantify the load required to achieve a given deflection. The two methods are complementary and are often performed together on corrugated pipe products.
What common quality problems does the test reveal?
Typical findings include brittleness caused by improper annealing or overworked material, incomplete weld penetration or weld inclusions, cracking at corrugation roots due to excessive thinning during forming, delamination between extruded layers in structured-wall pipes, and poor fusion at the interface between the corrugated outer wall and the smooth liner.