How To Define Fastener Head Strength?

High-strength threaded fasteners are critical components in almost all forms of transportation and industrial machinery assembly. While industry-recognized standards for measuring and evaluating these components have been established for decades, recent failures of threaded fasteners in usage and qualification testing have renewed focus on the critical area between the fastener head and the shank.

Large-diameter (0.5-inch or greater) high-strength externally tightened bolts (e.g., hex head, 12-point) used in automotive and aerospace applications have been extensively analyzed. However, bolts and screws below this size threshold often receive less scrutiny.

The recent failure of M5-diameter fasteners manufactured according to European standards has highlighted this issue. Aerospace applications, for instance, often emphasize lightweight structures, requiring thin materials and specialized fasteners like 100-degree countersunk head screws. Such thin-head designs pose unique challenges in designing effective torque transfer mechanisms while maintaining the structural integrity of the head-shank interface.

Fastener standards organizations and engineers focus on developing part standards with measurable attributes (e.g., length, diameter, head dimensions) to confirm compliance with design requirements. However, accurately measuring the critical stress regions at the head-shank connection or weight-reduction features without destroying the bolt remains challenging.

Therefore, there is a need for consensus on how to calculate the Head Strength Ratio (HSR) to ensure the minimum acceptable tensile strength at the head-shank interface, along with the methods and measurable data to be used in these calculations.

1. The Problem

Recent qualification testing of aerospace screws (100-degree countersunk head, six-lobe recess, threaded head, titanium alloy TI-P64001, anodized, molybdenum disulfide-coated, rated at 900 MPa from ambient to 350°C) revealed that a batch of M5-diameter screws failed to meet the tensile test limits, with head-shank interface failures occurring below the required threshold.

An investigation found that similar parts, rated at 1,100 MPa, had previously passed tests based on analogous standards for different aerospace screws with dissimilar head and shank geometries. These included pan head screws with coarser tolerances and different thread-to-head transitions. The significant differences in design and material raise the question:

What methods or data should be used to evaluate the potential tensile strength of a given head-shank geometry or assess part similarity for qualification by analogy?

Poorly designed head or recess geometries may lead to failures at the head-shank interface, with tensile strength levels lower than expected.

2. Defining Variables

Key variables influencing tensile strength at the head-shank interface are the effective geometry of the interface and the tensile strength of the threaded section, which can be calculated based on the thread’s effective tensile stress area.

For fasteners transitioning from threaded to unthreaded shank: The effective tensile stress area is determined by the diameter at the transition.

For fully threaded fasteners: The weakest tensile stress area is typically located one or two threads below the transition.

For fasteners where the threads extend to the head, the minimum tensile stress area often occurs in the transition between the threads and the underside of the head. The effective tensile stress diameter at this transition may equal the thread pitch diameter or be slightly smaller if a fillet radius is added to reduce stress concentration.

This distinction is critical for calculating the HSR, comparing the fastener's effective tensile area at the head-shank transition to the full shank or thread pitch area. Ideally, the HSR should be 1 or greater, ensuring that tensile failure occurs at or above the minimum strength of the thread pitch diameter.

3. Calculating HSR

To calculate the HSR, it’s important to use worst-case values for dimensional variables, such as the smallest head diameter and the largest groove depth, to identify the lowest possible tensile strength point.

For fully threaded fasteners, the cross-sectional area below the head is determined by the blank diameter. For fasteners with a smooth shank, the larger body diameter is used. The HSR is then calculated as the ratio of the effective tensile area to the thread pitch area.

Key Consideration: Fasteners with an HSR less than 1 are generally unsuitable for critical applications unless application-specific analysis proves otherwise.

4. Impact of Recess Geometry

The geometry of the recess drive system significantly impacts head strength. Cross-drive systems like Phillips and Pozidriv have a defined cruciform shape, with the stress area at the closest intersection between the head-shank transition radius and the recess. For calculation simplicity, the cross-sectional area is approximated as circular based on the wing outer diameter.

Straight-wall systems like Torx maintain a consistent profile from the head’s top to the recess’s bottom, with the effective area determined similarly. Fasteners with smooth shanks are often used in high-stress applications, where an HSR of 1 or greater is recommended.

Designs with shallow recesses or head configurations like button or pan heads generally achieve higher HSR values due to increased head mass. However, designs emphasizing reduced head height for weight savings or tighter clearances may compromise HSR if recess depth is not adequately minimized.

Conclusion

Although the example uses a 100-degree countersunk aerospace fastener, the same principles should apply during initial evaluations of other head configurations (e.g., pan head, button head). Protruding heads generally offer higher HSR due to increased material volume at the head-shank interface. Recess systems with shallow designs, such as AS6305 MORTORQ or NAS33750 dovetail drives, can further enhance strength in thin-head applications.

For high-torque applications, it’s essential to design recess systems with substantial contact area between the drive tool and recess to achieve the necessary clamping load.