Understanding the Fundamentals of Bolt Materials and Design
Stainless steel: For corrosive or wet environments, stainless steel provide superior corrosion resistance over carbon steel. The two main types are austenitic stainless steels like 304 and 316, which are non-magnetic and resist corrosion from most chemicals.
Alloy steel: High-strength alloy steel contain precise amounts of alloying elements like chromium, nickel, and molybdenum for increased hardness, strength, and toughness. Grade 5 and Grade 8 are commonly found alloy steel varieties.
Titanium: Used in marine, chemical, and aerospace applications where light weight and corrosion resistance are important. Titanium can be more expensive than steel but provide excellent strength-to-weight properties.
Design Features
Head styles: Common head styles are hex, square, button/dome, and oval. Hex heads are the most prevalent due to easy wrench access while other low-profile styles fit tight spaces.
Thread type: The most common threading system for Bolts is the Unified Thread Standard (UTS). Finer machine screw threads (UN/UNR) and coarser coupling threads (UNC/UNF) are also used depending on the forces and materials involved.
Finishes: They are often plated, coated, or otherwise treated to prevent corrosion. Zinc and yellow dichromate are popular zinc/zinc-based finishes while hard chromium and titanium nitride provide a harder finish.
Torque requirements: Proper tightening or torquing is critical for maximum clamp load and prevention of loosening over time. Manufacturers specify torque values based on its material, size, thread type, and applied load.
Understanding Its Failure Modes
Unless carefully specified and installed, they can fail due to dynamic loading, vibration, overtightening, or corrosion over time. The primary failure modes include:
Shear failure: Occurs when it is unable to carry the shear forces acting perpendicular to its axis. Symptoms include sheer planes through the shaft or threads.
Tensile failure: Results from stretching forces pulling it apart along its axis. Necking and fractures indicate a tensile overload.
Torsional failure: Twisting or spinning of the bolt causes the threads to strip out or sheer. Improper fitment or overtightening can introduce torsional stresses.
Fatigue failure: Repeated cyclic loading causes microscopic cracks to develop, usually initiating at stress concentrators until complete fracture. Corrosion contributes greatly to fatigue issues.
Proper selection, installation, maintenance, and protection from environmental damage are key to avoiding unexpected failures in critical bolted joints.
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