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.

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© 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) 979-3249gmelvin@infastech.com

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