Causes and Control of Fastener Failures
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Transcript of Causes and Control of Fastener Failures
Causes and Control of FastenerFailures
Gregg Melvin – Senior Applications Engineer
Elco Construction Products
© 2012 Elco Construction Products
Better Buildings from Better Fasteners
AIA Credits: Elco Construction Products an Infastech company is Registered Provider with American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members available on request.
This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.
Copyright Materials
This presentation is protected by US and International copyright laws. Reproduction, distribution, display, and use of the presentation without written permission of the speaker is prohibited.
© Elco® Construction Products 2012
Learning Objectives1. Identify where A/E must specify fasteners
instead of leaving product selection to contractor or supplier.
2.Learn techniques for reducing fastener rust and galvanic corrosion damage.
3.Be able to explain causes of hydrogen embrittlement and HASCC.
4.Be able to specify high-performance, HASCC-resistant fasteners.
5.Explain role of fasteners in building sustainability.
PART ONE
SPECIFYING FASTENERSRisks and Responsibilities
For want of a fastener…
“For want of a nail, the shoe was lost.
For want of a shoe, the horse was lost.
For want of a horse, the rider was lost.”
“God is in the details.”Mies van der Rohe - 20th Century Architect
Your reputation hangs onspecifying the right fasteners.
Standard types and grades of fasteners are sufficient in many construction applications. In these instances, fastener selection can usually be based upon industry standards, building code requirements, and design loads.
It may even be acceptable in some cases to leave fastener selection to the discretion of contractors or building product fabricators, allowing them to select fasteners based on cost, convenience, and trade practices.
The Petronas Towers in Kuala Lumpur, the world’s tallest building when constructed, required careful engineering to assure that the millions of fasteners used to install cladding would not be the installation’s weakest link.
Fastener specification are critical to project success.
Your reputation hangs onspecifying the right fasteners.
At other times, however, fastener selection can be one of the most crucial aspects of building design.
Professional judgment is required to specify fasteners that:
Perform as intended
Are dependable and economical
Last the life of the buildingThe Petronas Towers in Kuala Lumpur, the world’s tallest building when constructed, required careful engineering to assure that the millions of fasteners used to install cladding would not be the installation’s weakest link.
Fastener specification are critical to project success.
Structural capacity: Static and dynamic load.Pull-out and pull-over.Shear on fastener and attached materials.
Corrosion resistance.
Seal against liquid or gas.
Constructability, practicality, and inspectability.
Design intent and aesthetics.
Removability and other special requirements.
Service life exceeds that of attached materials.
Specifying fasteners is part of designing safe, durable, and practical connections between components.
A big job for such a small part
HASCC Another Threat to Fasteners
HASCC = Hydrogen-Assisted Stress Corrosion Cracking.
HASCC has only been identified as a fastener failure mechanism within the past few decades.
“Best Industry Practices” now require consideration of HASCC when specifying fasteners.
Before considering HASCC, let’s review some basics of ordinary fastener corrosion:
Oxidation (Rusting)
Galvanic Corrosion
PART TWO
VISIBLE CORROSIONRust and Galvanic Action
Must it Rust?Most architectural fasteners are made from grades or alloys of steel that will react with oxygen to create ferrous oxide, commonly know as rust. Rust is a type of corrosion that weakens and deteriorates steel. Rusting is accelerated when steel is also exposed to moisture, especially if the moisture contains chlorides (salts), a condition that is common in marine, industrial, and urban atmospheres.
When designing steel-to-steel connections, both the fasteners and the items being joined must be protected against rusting. Steel building panels, for example, can be protected by using zinc galvanizing and a high-performance coating.
As a general guideline, fasteners should have greater corrosion resistance than items being joined so that fasteners do not become weakest link in connection.
Once rust starts, it can spread rapidly to adjacent exposed steel surfaces.
Corrosion-Resistant PlatingHow they work:
1. Barrier against air and moisture.
2. Sacrificial, galvanic protection.
Available Types:
Hot-dipped galvanizing: Thick zinc clogs threads.
Electroplated Zinc: Insufficient protection for exterior or high performance fasteners.
Cadmium and Hexavalent Chromates: Greater protection, but environmental concerns.
Plating is contraindicated for case-hardened fasteners.
Plating is not acceptable protection for high performance
fasteners.
Corrosion-Resistant CoatingsBetter protection for high-performance fasteners.
Base Coat: Baked-on, zinc-rich, cross-linked polymer.
Top-Coat: Baked-on, aluminum-rich, cross-linked polymer. Pigments optional.
Eco-friendly: Minimize or eliminate heavy metals.
Lubricate threads: Eases installations
Colors provide product identification and
aesthetics.
Corrosion-Resistant Stainless Steel300-series stainless steel provides greater corrosion resistance.
Use for critical structural connections, exposed exteriors, and in locations with corrosive environments.
Can require coating for protection against galvanic corrosion of materials they are attaching.
Occurs when dissimilar metals are in presence of an electrolyte (an electrically conductive medium) and form a galvanic cell.
Like an electric battery, except two pieces of metal form a circuit.
Galvanic Corrosion
H+ H+
H+
(electrolyte)
Metals conduct electricity because they have a tendency to give up electrons easily. Some metals give up electrons more easily than others. They can be listed in order of their potential to yield electrons, in a table known as a Galvanic Series.
Aluminum is more active (anodic) than steel or stainless steel. Where aluminum and steel form a galvanic cell, aluminum will corrode, sacrificing electrons that will deposit onto and protect the steel.
Galvanic Series
More “noble”(protected and less corroded)
Less “noble”(sacrificial and more corroded)
Gold
Silver
Stainless Steel
Bronze
Copper
Brass
Nickel
Lead
Steel & Iron
Aluminum
Zinc
Magnesium
Anode (Most Active)
Cathode (Least Active)
This aluminum plate received two screws made from 300-series stainless steel, a grade that is highly resistant to oxidation and rusting. It was then exposed to 1000 hours of salt-water spray that acted as an electrolyte to form a galvanic cell between the aluminum plate and stainless steel fasteners. The screws
were them removed to allow examination of the plate.
Stainless Steel Screw: The
aluminum sacrificed electrons to the
steel. The surface of the aluminum is
visibly deteriorated.
Stainless Steel Screw With Anti-Corrosion Coating: The coating prevent formation of a galvanic cell and prevented corrosion of the aluminum plate.
Using ordinary steel fasteners in architectural aluminum elements (such as windows, curtain walls, or wall cladding) can also create galvanic corrosion of the aluminum, weakening the connection until it fails at well below design loads.
Avoid this problem by specifying fasteners with high-quality anti-corrosive coatings.
Galvanic Corrosion
Dissimilar metal combinations are common in construction. For example:
Aluminum framed glazing units and cladding get attached to structural steel framing using steel, self-drilling, self-tapping screws.
Aluminum parts are often attached to concrete or masonry with self-tapping screws made of steel.
Even steel-to-steel connections can involve dissimilar metals, if either of the steel parts, or the fasteners joining them, has been coated with zinc.
Thin veneers can be attached with self-drilling fasteners in the support
structure.
Galvanic Corrosion
It is difficult to prevent electrolytes from contact with fasteners.
Moisture can enter construction due to:Rain or dew during ConstructionCondensation inside a wall or roof Leaks in building envelopeWater from building maintenance or operationsPlumbing failuresFlooding and spillsPerspiration from workers who installed the part Air pollutants can make atmospheric moisture more conductive
If dissimilar metals are in contact, assume that an electrolyte will also be in contact with the fasteners.
Specifying fasteners with adequate corrosion resistance protects against catastrophic failure, loss of use, injury, death, and liability.
Galvanic Corrosion
PART THREE
HASCC“The Invisible Corrosion”A secondary effect of the galvanic reaction can also lead to fastener failures. Hydrogen, a by-product of galvanic corrosion, can weaken standard, hardened fasteners and cause them to fail. It produces a type of corrosion that is not readily apparent…until it is too late.
Construction Scene Investigation20 years after installation, self-drilling screws began snapping for no apparent reason.
Screws complied with code requirements and industry standards.
They had been exposed to weather due to roof damage, but were not visibly corroded.
Scanning electron micrographs revealed fractured fastener surfaces and separated grain boundaries in steel.
Screws had also lost ductility.
The Verdict: HASCC
Hydrogen & EmbrittlementHASCC starts with hydrogen.
Hydrogen embrittlement is associated with galvanic action. However, steel fasteners are not necessarily weakened by galvanic corrosion itself. Rather, hydrogen generated by galvanic action attacks the steel.
Even if steel is protected from galvanic corrosion, hydrogen can attack it rapidly.
Specialized fasteners have been developed to avoid this risk.
Screws attacked by hydrogen lose ductility and become embrittled They typically show no visible corrosion.
1. Galvanic action creates electrical current.
2. Water in electrolyte separates into oxygen and hydrogen.
3. Oxygen bonds with anode and oxidizes metal. Since oxides have little structural strength, anode weakens and corrodes.
4. Hydrogen is attracted to cathode and penetrates into metal.
Source of Hydrogen
Process is similar to laboratory procedure for separating water into hydrogen and oxygen by passing electrical current through it, a process called electrolysis.
Hydrogen Diffuses into SteelHydrogen, the smallest atom, penetrates voids in steel’s crystal structure.
Hydrogen atoms are unstable and bond with other hydrogen atoms.
The larger H2 molecules put pressure on the surrounding steel structure.
Internal tension in steel enlarges grain boundaries and reduces ductility.
Steel can no longer bear design load, and the fastener can fail.
Scanning electron microscope image of steel
shows enlarged grain boundaries indicative of hydrogen embrittlement.
The Role of Case HardeningHydrogen has little effect on “mild” (soft) steel.
Many building fasteners are case hardened,
In case hardening, low-carbon steel is heated in a high-carbon environment to infuse extra carbon into metal’s surface.
This hardens “case,” but makes it brittle.
Case hardened self-drilling and self-tapping screws are hard enough to cut into structural steel or concrete.
CoreRockwell Hardness HRC 32-40
Case Rockwell Hardness HRC 52 min.
HASCC affects steel of Rockwell hardness HRC 35. The harder the steel, the more susceptible is it to HASCC.
In case-hardened fasteners, the hard outer layer is most vulnerable to HASCC.
Cross-section of failed case-hardened fastener:
A. HASCC Failure Mode
B. Strength loss due to Ductile and Embrittlement Mode
C.Ductile failure due to reduced cross-section of fastener.
Hydrogen Embrittlement & Stress
Stress Concentration
Stress increases steel’s susceptibility to embrittlement.
Stress concentration occurs at screw heads for 3 reasons:
1. MANUFACTURE
Deformations required to form steel rod into screw heads induces stress into metal.
Stress Concentration
2. CLAMPING FORCES
When screws are tightened, their heads bear on the surface of the object being attached.
This places the area of the shank immediately under the head into tension, inducing stress.
Stress Concentration
3. OUT OF ALIGNMENT
Screws are rarely perfectly perpendicular to surfaces they are attaching.
When tightened, uneven pressure is put on head by oblique angle at which it is installed.
This puts additional stress on one side of fastener at juncture of head and shank.
A test for resistance to hydrogen embrittlement closely mimics this real-world situation. The fastener is screwed through two plates of dissimilar metals. A shim is placed under one edge of the top plate,
creating an angle between the two plates and placing the screw at an oblique angle with
respect to one of them.
Galvanically generated hydrogen at stress points leads to microscopic cracking.
Micro-cracks further concentrates stress points.
Once propagated, cracks can spread quickly through hardened steel.
Metallurgists call this cracking “Hydrogen Assisted Stress Corrosion Cracking” (HASCC).
Stress Cracking
Plating and Hydrogen EmbrittlementCase hardened fasteners must still be protected against rust and galvanic corrosion.
Standard plating of case hardened fasteners generates hydrogen that could contribute to “Hydrogen Embrittlement”.
This is unlikely, because platers bake hydrogen out of fasteners @ 400+ for several hours after treatment.
But you cannot bake-out Hydrogen from continuous Galvanic reactions on the job.
In low stress, completely dry areas, zinc or other plated, case hardened fasteners, may work well, or, if exposed to moisture it can pose a risk for delayed failures.
Case hardened fasteners can be protected with high-performance anti-corrosion coating. But it does not stop hydrogen diffusion into hardened steel.
However, new innovations – described in next section – will.
Galvanic action between dissimilar metals generates hydrogen.
Hydrogen penetrates hardened steel and creates internal stresses that weaken fasteners.
This process occurs primarily in high hardness/case hardened parts.
Stress concentrations initiate micro-cracking that can propagate across fastener.
HASCC can occur days, months or years after fastener installation if connection is exposed to moisture.
Failure is often sudden and without warning. No rust is seen.
HASCC Recap
PART FOUR
HASCC SOLUTIONSFor Self-Drilling Screws
Benefits of Self Drilling ScrewsSelf-drilling screws require just seconds of installation time and labor VS that required for nuts-and-bolts, rivets, and other fasteners requiring pre-drilled holes.
Fasteners install in single, fast operation using a power driver.
They are much faster than two-handed bolt-and-nut installations, and more practical where the back side is not accessible for installing a nut.
With hundreds of thousands of fasteners used in large buildings, this represents an enormous savings in labor.
Use to install windows, cladding, curtain wall, framing, anchors, equipment, fixtures, and other building components.
Dril-Flex.exe
Design of Self Drilling Screws
Tip drills pilot hole.
Lead threads tap threads into substrate, so substrate acts as nut.
Available in configurations to suit a wide range of construction applications.
Tip drills hole and removes shavings
Lead threads cut threads into (tap) substrate for pull-out resistance
Select head style for easy installation,
acceptable profile, and pull-over
resistance
Select thread style
to hold in substrate
The Remedy for HASCCSelf-drilling fasteners have to be case hardened in order to drill and tap substrates – but case hardening makes them vulnerable to failure.
Fortunately, there is a solution to fastener HASCC hazards.
HASCC-Resistant Fasteners
Virtually Immune
to HASCC
A Metallurgical Marvel
Hardened for drilling and
tapping
Ductile for structural performance and
HASCC-resistance.
There are two ways to achieve this performance:
1. Selectively Hardened Fasteners
2. Bi-Metal Fasteners
1. Selectively Hardened Fasteners
Made from special, high-carbon steel, they do not have to be placed in a high-carbon environment during heat treatment.
Tip of screw is passed through electrical induction coil that heats and hardens drill-point and lead-threads without affecting rest of shank.
HARDENED WHERE NEEDEDSelectively hardened point and tapping threads HRC 52
DUCTILE WHERE NEEDEDGrade 5 heat treat in load-bearing area of fastener
2. Bi-Metal FastenersUse where stainless steel is required for increased resistance to corrosive environments.
A high-carbon steel tip is fused onto stainless shank.
High carbon tip is then selectively hardened using induction-coil heating.
300 Series StainlessSteel Head andShank
High-CarbonSteel Tip,Selectively Hardened
Recommended for exposed fasteners.
Use corrosion-resistant coatings on both selectively hardened and bi-metal fasteners.
Coating provides galvanic protection and lubricates threads.
Coating can be color matched as required.
Bi-Metal Fastener: Before coating, above. After coating, below.
Corrosion-Resistant Coatings
PROOFSelf-drilling fasteners joining steel and aluminum plates, exposed to salt-spray test.
Conventional fasteners fail due to stress concentration at screw heads that accelerates embrittlement and HASCC.
Neither selectively-hardened nor bi-metal fasteners fail.
Standard case-hardened fasteners failed
HASCC-resistant fasteners are not affected
PROOF
PROOFHASCC-resistant screws were used to secure crown at top of building and building envelope.
Exposed to marine and urban atmosphere, high winds, and earthquakes.
Over two decades of service.
Zero failures.
US Bank Building/ Los Angeles, CA
Pei Cobb Freed Partners
Best Industry Practices
Where dissimilar metals will be in contact, specify: selectively-hardened self-drilling fasteners to resist HASCC.
Where stainless steel is required for protection against visible corrosion or extreme environments, specify: bi-metal self-drilling fasteners with a selectively-hardened tip to resist HASCC.
• Galvanic action in dissimilar metals generates hydrogen.
• Hydrogen penetrates case hardened steel and creates internal stresses that weaken fasteners.
• Stress concentrations initiate micro-cracking that propagate across fastener and lead to failure.
• HASCC can occur days, months or years after fastener installation if connection is exposed to moisture.
• Failure is often sudden and without warning.
• Failures can be avoided by using selectively-hardened or bi-metal self-drilling screws with corrosion-resistant coating.
HASCC Recap
PART FIVE
SPECIAL FASTENERS
Extreme LoadsStructure is subjected to brief loads far higher than normal operating loads.
Risks include:• Hurricane, tornado, and windblown missile impact• Earthquake, tsunami, landslide, avalanche• Industrial accidents and vehicular collisions• Load redistribution due to failure of building elements• Explosion, including boiler, natural gas leaks, attack• Located near possible targets
If there is reason to suspect a risk, design for it.
Determinationof Risk
Estimate of Risk Loads
Risk-ResistantDesign
Extreme Loading of FastenersIf a structure is designed to withstand extreme load, load is transferred to fasteners that connect it to other building elements.
Ductile fasteners can deform to absorb some load while maintaining connection.
If fasteners are brittle, extreme loading may cause them to fail.
Using ductile, HASCC-resistant fasteners helps assure full design strength of fasteners can be used to resist extreme loads.
QuickTime™ and a decompressor
are needed to see this picture.
Extreme loads can affect all building components, not just structural members.
Where to Specify
Blast-resistant windows
Curtain wall framing
Building equipment
Critical life safety and communications equipment
To assure continuous service of essential facilities
Concrete and Masonry Screws
Concrete Masonry
Choice of head
styles
Hi-lo thread
taps into masonry
Specifying concrete and masonry anchors also
requires consideration of dissimilar metals
and corrosion resistance.
A variety of special designs are
available to satisfy most construction
requirements.
Choice of head styles
Grade 5 hardness for strength and
ductility
Higher hardness for
tapping threads
Pressure-Relief FastenersIn buildings with potential for interior explosions, special panels can be designed to blow off and relieve pressure, a built in safety valve.
Examples include places where highly volatile liquids are in use, or storage for extremely fine dry, materials subject to dust-explosion.
Pressure-Relief FastenersPanels have an holes larger than fastener heads.
A special aluminum washer retains the panel in position.
Under explosive pressure, washers fail and allow panels to blow off, venting the pressure.
Panel is tethered to keep it from becoming a dangerous missile.
panel
panel separates
from framing member
Normal Explosion
washer collapses
framing
aluminum pressure-relieving washer
centering device
Threaded rods are used to support fire-sprinklers and other essential building services. These fasteners simplify installation while protecting against potential
failures of other fastening methods that are not positively engaged in the structural materials.
Threaded Rod Anchors
1/4-20 & 3/8-16
Sealing Washers
Special head design plus high performance washer material help prevent leaks through building envelope.
SUMMARY
Fasteners are critical components of buildings.
A/Es must be aware of their professional responsibility to
specify fasteners that fulfill design intent and provide safe and
durable connections.
Manage Your Risk
Economy
The lowest cost fastener may not be the most economical when labor and service life are considered.
Specialty fasteners can often save money by providing lower installed cost.
Fasteners are typically less than .0025% per dollar of project cost, but specifying inadequate fasteners can cause up to 100% of construction defect costs.
Sustainability
Most metal fasteners have recycled-material content and are recyclable.
New corrosion-resistant finishes eliminate toxic heavy metals plating.
Durability over life of structure is the most important measure of sustainability.
Fasteners are critical building components
A/Es have professional responsibility to specify fasteners that fulfill design intent and provide safe and durable connections.
It is not safe to assume that contractors will be aware of these issues and will select anything other than standard fasteners.
Specify accordingly.
For More Information
Elco Construction ProductsDril-Flex® and Bi-Flex™
HASCC-Resistant Fasteners
www.elcoconstruction.comGregg Melvin
(815) [email protected]
Ask for reprint of article from Construction Specifier.