Fastener Testing: Proof Load, Wedge Tensile and Hydrogen Embrittlement

What is proof load testing and how is it performed per ASTM F606?#

Proof load testing is a pass/fail quality control test that verifies a fastener can sustain a specified tensile load without permanent deformation exceeding an allowable limit. Per ASTM F606, a full-size fastener is axially loaded to the proof load, held for at least 10 seconds, and then measured for permanent elongation or assessed with a Go ring gage.

Test method per ASTM F606#

The test is performed on a full-size finished fastener or a machined specimen, depending on the product standard. The fastener is assembled in a tensile testing machine with suitable grips that apply a purely axial load. The load is increased steadily to the specified proof load value and held for a minimum of 10 seconds (some standards require longer holds). After unloading, the permanent elongation or set is measured.

For fasteners with a specified length, the change in overall length is measured with a micrometer or an extensometer. For shorter fasteners, the permanent set is often assessed by checking whether the fastener can be freely threaded into a Go ring gage after testing. ASTM F606 defines the maximum allowable permanent elongation: typically 12.5 µm (0.0005 in.) for fasteners with a nominal length up to 3/4 inch, or a proportionally larger value for longer fasteners, unless the product standard specifies otherwise.

How is the proof load calculated and what are the acceptance criteria?#

The proof load is calculated by multiplying the specified proof load stress (e.g., 85 ksi for SAE J429 Grade 5) by the tensile stress area of the threaded section, which depends on the thread geometry. Acceptance criteria require that the fastener sustain the load without fracture and that permanent elongation not exceed 12.5 µm (0.0005 in.) for short fasteners or a proportionally larger limit for longer ones.

Load calculation#

The proof load is calculated using the tensile stress area of the threaded section. For unified inch threads, the stress area is given by:

A_s = 0.7854 [D - (0.9743/n)]^2

where D is the nominal diameter and n is the number of threads per inch. For metric threads, the stress area is:

A_s = (pi/4) (d_2 + d_3)^2 / 4

using the pitch and minor diameters per ISO 898-1. The product standard specifies the proof load stress (e.g., 85 ksi for SAE J429 Grade 5) which is multiplied by the stress area to obtain the applied load.

Acceptance criteria#

The fastener passes the proof load test if it sustains the load without fracture and the permanent set does not exceed the specified limit. Any fracture or excessive elongation constitutes a failure. Because the test loads the fastener to a high percentage of its expected yield strength, it is sensitive to material defects, improper heat treatment, or thread imperfections that reduce the effective cross-section.

For more on interpreting load and elongation data, see How to Read a Tensile Test Report (Step by Step).

What is wedge tensile testing and how does it assess head-to-shank integrity?#

Wedge tensile testing evaluates the ductility and soundness of the head-to-shank junction in headed fasteners by introducing a bending moment with a wedge placed under the head. This test exposes defects such as forging laps, quench cracks, and hydrogen embrittlement that a straight axial tensile test might miss, forcing failure to initiate at the head fillet.

Test procedure#

Per ASTM F606, the fastener is assembled with a wedge having a specified angle placed under the head. The wedge angle depends on the fastener type, property class, and diameter. The assembly is then loaded in tension until fracture. The location and appearance of the fracture, along with any required reduction-of-area measurements, determine acceptance.

Common wedge angles include:

The wedge must be hardened and have a smooth surface to avoid introducing artificial stress concentrations. Alignment is critical: the load axis must pass through the center of the bolt shank to ensure the bending moment is generated solely by the wedge.

Rationale#

A headed fastener in service may experience some bending due to misalignment, uneven bearing surfaces, or external loads. The wedge test exaggerates this condition to prove that the head-to-shank transition can withstand such stresses without brittle failure. For high-strength fasteners, which are more notch-sensitive, this test is essential to ensure adequate toughness.

How is hydrogen embrittlement detected and prevented in high-strength fasteners?#

Hydrogen embrittlement is detected in production lots primarily through wedge tensile testing, where the multiaxial stress state at the head fillet accelerates hydrogen-induced cracking, often causing delayed brittle fracture. Prevention involves baking plated fasteners at 190-220°C (375-430°F) within hours of plating to drive out diffusible hydrogen, as specified in ASTM B850.

Detection via wedge tensile testing#

The wedge tensile test is a practical screening tool for hydrogen embrittlement in production lots. The multiaxial stress state at the head-to-shank junction created by the wedge accelerates hydrogen-induced cracking. Fasteners that pass a straight tensile test may fail in wedge tension within hours of loading if hydrogen is present. A brittle, flat fracture at the head fillet with little or no necking is a classic indicator. In some cases, a delayed fracture test is used: the fastener is loaded to a percentage of its proof load (often 75-90%) and held for 48 hours to see if it fails. However, wedge tensile provides a faster, standardized method that is integrated into routine quality assurance.

Prevention and mitigation#

Baking plated fasteners at 190-220°C (375-430°F) within a few hours of plating drives out diffusible hydrogen. Standards such as ASTM B850 specify baking times based on fastener size and hardness. Process control during pickling and plating is equally important to minimize hydrogen uptake. Quality engineers should ensure that plated high-strength fasteners are baked and then tested via wedge tensile or a sustained load method to verify embrittlement resistance.

The brittle fracture behavior seen in hydrogen-embrittled fasteners is analogous to low-energy Charpy impact fractures. For more on toughness testing, see Charpy V-Notch Impact Testing: A Practical Explainer.

What acceptance criteria do common fastener standards specify for proof load and wedge tensile tests?#

Standards such as SAE J429, ASTM A193, and ISO 898-1 define proof load stress minima, wedge angles, and failure location requirements. For example, SAE J429 Grade 5 requires a proof load stress of 85 ksi and a 6° or 10° wedge; fracture in the shank or threads is acceptable, while head fracture requires at least 35% reduction of area.

Note: Proof load stresses are nominal; exact values depend on diameter and thread pitch. Always consult the latest revision of the applicable standard.

How do you interpret wedge tensile test results and identify common failure modes?#

Wedge tensile test results are interpreted by examining the fracture location and morphology. A ductile fracture with necking in the shank or threads indicates a sound head-to-shank junction. A brittle, flat fracture at the head fillet with low reduction of area signals hydrogen embrittlement, quench cracks, or forging defects, and is cause for rejection unless the standard’s minimum reduction of area is met.

Ductile fracture (acceptable)#

A normal ductile fracture in wedge tensile testing exhibits necking and a cup-and-cone morphology, typically located in the threaded section or the shank away from the head. This indicates that the head-to-shank junction has sufficient strength and toughness to force the failure into the free length, where the cross-section is slightly smaller due to threads. Such results confirm proper forging, heat treatment, and material quality.

Brittle fracture at head (cause for rejection)#

A flat, shiny fracture at the head fillet or bearing surface with little or no necking signals a brittle failure. Common causes include:

• Hydrogen embrittlement (intergranular facets, often with “fisheye” features)
• Quench cracks from improper heat treatment
• Forging laps or seams that concentrate stress
• Excessive hardness or insufficient tempering

If the fracture occurs at the head, the standard may permit acceptance only if the reduction of area measured on the fracture surface meets a minimum value (e.g., 35% for SAE J429 Grade 5). This measurement must be performed carefully with a point micrometer or optical comparator on the fractured shank.

Thread stripping vs. tensile fracture#

In some assemblies, thread stripping may occur before tensile fracture, particularly if the nut is softer than the bolt. Wedge tensile testing of the bolt alone eliminates this variable, focusing solely on the headed fastener’s integrity.

What practical considerations ensure reliable fastener proof load and wedge tensile testing?#

Reliable testing requires representative sampling per ASTM F1470, calibrated tensile machines with proper wedge fixtures, and precise alignment to avoid invalid bending. Common pitfalls include using the wrong wedge angle, incorrect permanent elongation measurement, and testing plated fasteners without proper baking, which can obscure hydrogen embrittlement.

Sampling#

Sampling plans for fastener testing are typically defined by the product standard or by general standards such as ASTM F1470. A representative sample from each production lot is tested. For critical applications, increased sampling frequency or 100% testing may be specified.

Equipment and setup#

• Use a calibrated tensile testing machine with adequate capacity (typically 60,000 lbf or higher for heavy fasteners).
• Wedge fixtures must match the required angle and be hardened to prevent deformation.
• Alignment is critical: misalignment introduces additional bending that can invalidate the test. Use spherical seating or self-aligning grips where permitted.
• For proof load testing, use a load cell accurate to ±1% of the applied load.

Common pitfalls#

• Using the wrong wedge angle for the fastener size or grade
• Failing to measure permanent elongation correctly after proof load (e.g., measuring before the fastener has cooled if it heated during loading)
• Testing plated fasteners without proper baking, which may mask or reveal hydrogen embrittlement inconsistently
• Overlooking the requirement for reduction-of-area measurement when head fracture occurs

For a broader overview of mechanical testing methods and best practices, see Mechanical Testing of Metals: The Complete Guide.

How do proof load and wedge tensile tests relate to other mechanical tests for fasteners?#

Proof load and wedge tensile tests complement hardness, tensile, and impact testing by evaluating whole-fastener performance under axial and bending loads. Hardness testing alone cannot account for thread geometry or head integrity, while machined-specimen tensile tests do not capture the effects of actual fastener geometry. Charpy impact testing may be required for low-temperature service.

Proof load and wedge tensile tests are part of a larger family of mechanical evaluations for fasteners. Hardness testing (Rockwell, Brinell, Vickers) provides a quick check of strength level but cannot substitute for proof load because it does not account for thread geometry, surface conditions, or head integrity. Tensile testing of machined specimens measures yield and ultimate tensile strength, which correlate with proof load but do not capture the effects of the actual fastener geometry. For fasteners used at low temperatures, Charpy impact testing may be required to ensure adequate toughness. Understanding the complementary roles of these tests helps quality engineers design a complete qualification program.

Key takeaways#

• Proof load testing is a pass/fail evaluation that ensures a fastener can sustain a specified load without unacceptable permanent deformation.
• Wedge tensile testing introduces bending to assess head-to-shank integrity and is the primary production test for detecting hydrogen embrittlement in headed fasteners.
• Hydrogen embrittlement primarily affects high-strength fasteners (above 32 HRC) and can be mitigated by timely baking after plating.
• Acceptance criteria vary by standard: SAE J429, ASTM A193, and ISO 898-1 each define specific proof load stresses, wedge angles, and failure location requirements.
• Proper test setup, calibrated equipment, and adherence to the correct wedge angle are essential for valid results.
• A brittle head fracture in wedge tensile with low reduction of area is a clear signal of potential hydrogen embrittlement or material defects.