Choosing the Wrong Fastener Can Have Catastrophic Consequences

28 DOORS&HARDWARE £ AUGUST 2009

here are certain� circumstan�ces where a stan�dard fasten�er—prop-erly man�ufactured, an�d rated for the design� load—can� be predicted to have a high probability of failure. It’s n�ot that the fasten�er is defec-tive, but that it is n�ot design�ed to han�dle certain� situation�s. There are issues beyon�d just workin�g loads an�d fasten�er stren�gth that n�eed to be con�sidered, issues that may be outside the kn�owledge of a con�tractor who is purchasin�g screws. In� these in�stan�ces, a specialty fasten�er must be specified, or else the security of the in�stalla-tion� is bein�g left to chan�ce.

Case-harden�ed self-drillin�g fasten�ers have gain�ed broad usage in� con�struction� over the past 20 years because they are fast an�d easy to in�stall an�d, un�der n�ormal circumstan�ces, are reliable. If, however, the fasten�er is attach-in�g a door, win�dow, curtain�-wall pan�el, roof system, or piece of heavy equipmen�t, failure is n�ot an� option�. This article deals with two con�dition�s that en�dan�ger the in�tegrity of con�ven�tion�al case-harden�ed self-drillin�g fasten�ers. Metal-to-metal con�n�ection�s—such as attachin�g alumin�um door fram-in�g to structural steel, alumin�um framin�g to galvan�ized steel studs, or attachin�g steel hardware to

alumin�um framin�g—can� create a vuln�erability. Extreme loadin�g caused by blast attacks, seismic even�ts or high win�ds make structural deman�ds that do n�ot behave like con�ven�tion�al loads. Iden�tifyin�g a high-risk situation� an�d selectin�g appropriate fasten�ers can� mitigate risk an�d protect both public safety an�d the project.

WhenMetalsAttack

Makin�g the right choice of fasten�-ers depen�ds partly on� kn�owin�g the specific materials to be fasten�ed. There are little-n�oted electro-chemical phen�omen�a at work, often� in�visibly, that can� compromise the in�tegrity of structural con�n�ection�s. Attachin�g alumin�um to alumin�um, alumin�um to steel, even� steel to steel can� set the stage for failure of what are otherwise perfectly good fasten�ers. Loss of on�e fasten�er can� tran�sfer workin�g loads to adjacen�t fasten�ers. Progressive failures can� lead to failure of critical compo-n�en�ts an�d presen�t a threat to build-in�g in�tegrity an�d life safety.

On�e culprit in� this kin�d of fail-ure is Hydrogen� Assisted Stress Corrosion� Crackin�g (HASCC). The phen�omen�on� can� occur without warn�in�g an�d for n�o apparen�t reason� in� stan�dard self-drillin�g an�d

By Gregg Melvin and Michael Chusid, RA, FCSI

TChoosing the Wrong Fastener

Can Have Catastrophic Consequences

ConnectionsSecure

AUGUST 2009 £DOORS&HARDWARE 29

-tappin�g screws, causin�g the fasten�er to “spon�tan�eously” fail. These failures can� occur shortly after in�stallation�, mon�ths, or even� years after in�stallation�. Screw heads pop off an�d the materials that they were con�n�ectin�g become sepa-rated, leavin�g the con�tractor, buildin�g own�er an�d other respon�sible parties puzzlin�g over what caused the failures an�d what the remedy will cost.

The risk can� be min�imized if a design�er is aware of the con�dition�s that cause HASCC an�d specifies fasten�ers that are immun�e to HASCC attack. These fasten�ers are man�ufactured by n�ew metallurgical techn�iques in�clud-in�g selective heat-treatmen�t an�d fusin�g together two types of metals.

CatastrophicFailure

Recen�tly, in� a govern�men�t facility in� the US, screws in�stalled over 20

years ago were foun�d to be sn�appin�g apart for n�o apparen�t reason�. Foren�sic in�vestigators n�oted that the fasten�ers were stan�dard, harden�ed steel self-drillin�g screws that complied with applicable buildin�g code requiremen�ts an�d accepted in�dustry man�ufactur-in�g stan�dards.1 Further, the fasten�ers were n�ot visibly corroded, an�d simply appeared to have broken� in� half. It was determin�ed that the fasten�ers had recen�tly been� exposed to the weather for a short period of time due to roof win�d damage. This exposure may have been� the factor that in�itiated some damagin�g process, but the direct cause of failure was n�ot obvious.

Accordin�g to in�depen�den�t metallur-gical an�alysis, the fasten�ers were weak-en�ed by HASCC. Scan�n�in�g electron� micrographs (SEM) revealed distin�ctive pattern�s of hydrogen� attack in� addition� to complete ductile failure across the

1. Made of case-hardened 410 stainless steel, these self-drilling screws were used to attach aluminum to steel. They failed at the place where the screw shank was aligned with the interface between the two metals. Hydrogen generated by galvanic reaction at that interface caused embrittlement of the hardened casing of the screws.

2. A cross-section of a case-hardened screw thread. The wavy line is the hardened steel outer layer of the screw, HRC 52 or greater. The grainy area to the right is the softer, more ductile core, HRC 32-40. Hardened steel has been shown to be vulnerable to embrittlement by hydrogen generated in the service application.

3. Gaping grain boundary. A scanning electron microscope photo of a failed 410 stainless steel fastener shows the gaping grain boundaries associated with hydrogen-assisted stress-corrosion cracking. These dislocations of the metal’s crystalline structure cause loss of ductility and tensile strength necessary to carry its design load.

4. Steel in its normal, ductile state displays a much more consistent texture, with no obvious gaps in its structure.

Photo Courtesy of Elco Construction Products, an Acument Global Technologies company

Photo Courtesy of Elco Construction Products, an Acument Global Technologies company & ATRONA Material Testing Laboratories

Photo Courtesy of Elco Construction Products, an Acument Global Technologies company & ATRONA Material Testing Laboratories


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in�n�er 20% of the fasten�er that had n�ot yet suffered HASCC.

HASCC attacks harden�ed steel parts such as self-drillin�g screws. The screws’ drill-poin�t an�d lead threads act as cuttin�g tools, an�d therefore must be harder than� the metal they are cuttin�g in�to. Typically, this has been� achieved by case-harden�in�g the fasten�er, a process in� which a low-carbon� steel part is heated in� a high-carbon� en�viron�men�t to in�fuse extra carbon� in�to the outer layers of the part. The result is a fasten�er with a hard “shell” over a core of softer, more ductile steel. The harden�ed shell that cuts the hole is brittle. The softer steel core provides stren�gth to bear a load on�ce the fasten�er is fully driven�.

Million�s of self-drillin�g an�d self-tappin�g screws are used every year in� North America alon�e. They have become the fasten�er of choice for erectin�g curtain�wall, an�d attachin�g architectural elemen�ts such as door or fen�estration� un�its that have metal attachmen�t frames. The problem of HASCC in� relation�ship to self-drill-in�g, self-tappin�g screws has been� iden�tified on�ly recen�tly. Two decades of experien�ce with this type of screw have fin�ally provided en�ough in�ci-den�ts an�d en�ough data for the cause of the problem to be iden�tified.

TheHydrogenThreat

HASCC typically occurs in� metal-to-metal con�n�ection�s where two dissimilar metals are in� con�tact, an�d some form of moisture is presen�t. It is actually two differen�t processes—stress corrosion� crackin�g an�d hydrogen� embrittlemen�t—which occur together as a result of an� elec-trochemical process called galvan�ic reaction�. (See sidebar: GALVANIC REACTION & CORROSION)

Stress corrosion� crackin�g (SCC) refers to the ten�den�cy of corrosion� to be more aggressive at stress poin�ts in� metal. Some aspects of the phen�omen�on� are still n�ot completely un�derstood, due to the wide variety of en�viron�men�ts an�d application�s in�volved, but it has been� defin�itively observed.

Hydrogen� embrittlemen�t is a complete loss of ductility an�d ten�sile stren�gth resultin�g from hydrogen� atoms pen�etratin�g in�to steel an�d disruptin�g its crystal structure. The hydrogen� atom is so small it can� diffuse through “solid” metals such as harden�ed steel. As they move, hydrogen� atoms lodge in� microscopic voids in� the crystal structure of metal. It is believed that lon�e hydrogen� atoms try to join� in�to pairs to form more stable H2 molecules. The larger H2 molecules

put pressure on� the steel, splittin�g it open�. This is particularly true at stress poin�ts in� the steel, where the voids are more promin�en�t. Stress both en�han�ces an�d localizes the n�atural hydrogen� diffusion� process. The deformed metal loses ten�sile stren�gth, becomin�g brittle an�d n�o lon�ger capable of bearin�g its n�ormal ten�sile load.

The problem is largely con�fin�ed to harden�ed steel; softer steel (maximum Rockwell C34 hardn�ess) is virtually immun�e. However, the harder the steel, the more vuln�er-able it is. Self-drillin�g, self-tappin�g screws an�d thread-cuttin�g con�crete screws are typically in� the ran�ge of HRC 52 on� the case (surface), an�d HRC 32-40 in� the core, makin�g them prime targets.

Un�derstan�din�g the source of the hydrogen� is key to predictin�g vuln�erability. It can� sometimes

5. Hydrogen embrittlement concentrates at stress points in metal. This test setup for susceptibility to embrittlement failure induces stress in the test fastener in a way that mimics the type of loading often applied in real-world installations: screws are seldom inserted at perfect right angles, placing uneven stress on the screw head. The test screw is engaged through an aluminum plate, into a steel plate. When the screw is tightened, the steel shim puts the fastener in tension slightly off-axis. The test setup is then exposed to saltwater for a given period.

6. Screws in the actual test setup.

Illustration courtesy of Elco Construction Products, an Acument Global Technologies company Photo Courtesy of Elco Construction Products, an Acument Global Technologies company

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en�ter metal parts durin�g man�u-facturin�g processes, but this hydrogen� is usually removed an�d dissipated by a post-bakin�g process. Hydrogen� can� also be gen�erated in� the service en�viron�men�t, after in�stallation�, by galvan�ic reaction�. It is this source that can� be foreseen� an�d design�ed for.

PredictingVulnerability

The most common� combin�a-tion� of metals causin�g galvan�ic reaction� is alumin�um an�d steel. Such a con�dition� occurs, for example, where a door un�it has alumin�um flan�ges to attach to the steel structure, or where

a steel fasten�er is used to join� two pieces of alumin�um. An� electrolyte must be presen�t: it is usually water that has leaked in�to a structure or moisture that forms due to con�den�sa-tion�. If moisture con�tacts the two metals, galvan�ic curren�t can� flow an�d cause the water to separate in�to hydrogen� an�d oxygen� molecules. Oxygen� collects at the an�ode—in� this case the alumin�um. Hydrogen� collects at the cathode, the steel, in�cludin�g the harden�ed steel fasten�er. (The galvan�ic corro-sive effect can� also weaken� the alumin�um over time, which can� cause differen�t problems with the stren�gth of the con�n�ection�.)

Galvanic Reaction & CorrosionWhen two dissimilar metals come into

contact in the presence of an electrolyte (a liquid that can conduct electricity), an electrical current flows from one metal to the other. Metals all have a propensity to give up electrons, and this propensity varies from one metal to another. The metal with a greater tendency to give up electrons creates a flow of electrical current towards the other metal. The phenomenon is called galvanic current, named after the 18th century physician and physicist Luigi Galvani, whose pioneering investigations helped unravel the mysteries of electricity. The galvanic relationship of specific metals is well documented in a table known as a galvanic series. This galvanic principle is the basis on which batteries are made.

In construction situations, however, the result is not always benign. When the metal lower in the galvanic series sacrifices electrons (the anode), it is corroded at a rate significantly greater than its natural tendency to corrode. The metal higher in the series, the cathode, is “protected” by the flow, and corrodes significantly slower than normal, or not at all. This process is known as galvanic corrosion.

At the same time, the elements of the electrolyte can be separated, collecting at the anode or the cathode depending on their charge.

Galvanic corrosion weakens the sacrificial metal. In a steel-aluminum connection, aluminum is weakened. Aluminum and zinc are close to each other in the galvanic series, but there is suff icient electro-potential difference between them to initiate corrosion. Because 18-8 stainless steel is high on the series, it is often used for fasteners exposed to corrosive environments.

Ironically, hydrogen that is often a by-product of galvanic reaction attacks hardened steel, the metal that is considered “protected” by the electrical flow in the aforementioned situations.

7. After 96 hours of exposure to salt spray, the case-hardened fasteners, in the bottom row, failed. The selectively heat-treated fasteners, in the top row, did not fail.

8. The junction of the screw head and shank is a concentration point for both residual stress from the manufacturing process and application induced stress. Hydrogen atoms invade stress points easily, causing HASCC failures.



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Galvan�ic reaction� can� occur even� in� steel-to-steel con�n�ection�s if on�e of the parts is zin�c-coated. On�e of the steel members may be galvan�ized steel, (steel coated with zin�c to preven�t rust), or two similar pieces of steel may be con�n�ected by a zin�c-coated fasten�er. All that is required is for the two dissimilar metals to be in� con�tact in� the presen�ce of an� electrolyte.

Un�der these con�dition�s, harden�ed steel fasten�ers are vuln�erable to in�filtration� by hydrogen� an�d subse-quen�t embrittlemen�t at stress poin�ts.

StressandSteel

On�e of the most common� stress-poin�ts in� a self-drillin�g screw is its head. This is due to both the man�ufacturin�g process an�d the

way in� which the screw is typi-cally used. The shape of the screw is made by deformin�g the metal comprisin�g the head to a diameter as much as 500% wider, causin�g it to retain� residual stress.

Application�-in�duced stress also con�cen�trates at the jun�ction� between� the screw shan�k an�d head. When� the screw is driven� all the way in�to the hole, the head is the part that resists further progress, puttin�g it in� a con�stan�t state of ten�sion�.

A third source of stress results from the practical fact that self-drill-in�g screws are rarely in�stalled at a perfect perpen�dicular to the surface that they bear on�. When� the screw is driven� at a slight diagon�al, on�e edge en�gages first, placin�g un�even� stress on� the head.

Hydrogen� gen�erated through galvan�ic reaction� can� in�vade these stress poin�ts, embrittlin�g the metal. Micro-crackin�g appears, creatin�g an� attractive site for stress-corrosion�. In� an� elemen�t un�der load, stress corrosion� is progressive: the more the cracked site corrodes, the less metal there is to bear the ten�sile load, the greater the stress build-up at that poin�t.

FailureMechanisms

HASCC attacks the hard shell of the screw, but a crack n�eed n�ot go all the way through the fasten�er to cause failure. Corrosion� through the harden�ed outer layer effectively reduces the diameter of steel avail-able to bear the load. When� the ten�sile load exceeds the ten�sile stren�gth of the dimin�ished cross-section�, the fasten�er fails. In� the case n�oted above, the complete ductile failure of the cen�tral 20% of the cross-section� in�dicated that what was left of the fasten�er core, after the harden�ed case cracked, simply could n�ot accommodate the workin�g load of the origin�al design�. In� some in�stan�ces, fasten�ers broke at the con�n�ection� poin�t; at other location�s, the screw head simply popped off due to stress con�cen�tration�.

PreventingFailure

Theoretically, elimin�atin�g the electrolyte could preven�t problems. If the fasten�er is completely sealed from an�y possible moisture con�tact, corrosion� ought to be avoided. However, this is often� n�ot a practi-cal possibility. Join�t sealan�ts, for example, can� fail without n�otice. In� the case of the facility men�tion�ed earlier, the buildin�g was stable for

9. Self-drilling fasteners that are resistant to hydrogen assisted stress corrosion cracking have three zones. Section A has to be hard enough to drill into steel and tap threads while Section B has to be strong and ductile enough to carry the intended load. The third section, the head, is marked with an embossment to allow visual identification of an installed fastener’s type.

10. For applications where high corrosion-resistance is required, such as installations regularly exposed to moisture, a bi-metal fastener is used so that 300 series stainless steel is exposed to the elements.



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almost 20 years. Then�, in�vestigators surmise, win�d damage allowed moisture en�try. A year later, the fasten�ers failed. In� other situa-tion�s, exposure to moisture durin�g con�struction� before a buildin�g is en�closed, an�d even� common� humid-ity levels an�d con�den�sation� within� a buildin�g assembly, have been� iden�tified as the source of moisture.

A more reliable solution� is to use fasten�ers that are in�vuln�erable to HASCC. Two methods of fasten�er man�ufacture have been� developed specifically to resist HASCC.

A fasten�er can� be selectively heat-treated, n�ot case-harden�ed, so that on�ly the drillin�g tip an�d lead threads are harden�ed. The majority of the shan�k is softer steel that is

n�ot pron�e to HASCC. This process uses Grade 5 steel, an� alloy that has en�ough carbon� already presen�t to be harden�ed simply by heatin�g. The tip of the part is passed through an� in�duction� coil that heats it an�d harden�s it locally without affectin�g the rest of the shan�k.

When� a selectively heat-treated fasten�er is in�stalled, the harden�ed tip pen�etrates the an�chorin�g metal an�d cuts threads in� it, but then� moves past the con�n�ection� area as the screw is driven� to full depth. The relatively softer steel portion� of the shan�k, which is n�ot vuln�erable to HASCC, bears the load. The fasten�er performs as would a ductile Grade 5 bolt, the same type of bolt used for structural steel con�n�ection�s, assurin�g a high degree of reliability.

An�other type of HASCC-resistan�t fasten�er is a bi-metal screw, a high-performan�ce part suitable for application�s where greater corrosion�-resistan�ce is desirable. In� this case, a high-carbon� steel is used, but on�ly for the drill poin�t an�d lead threads. The headed shan�k is made of 300-series stain�less steel (also kn�own� as 18-8 stain�less steel). The two pieces are friction�-welded together. Then�, the poin�t an�d lead threads are selectively harden�ed by in�duction� heatin�g, similarly to the process of heat-treatin�g a selectively harden�ed carbon� steel fasten�er.

An� addition�al treatmen�t is applied to some fasten�ers to en�han�ce their resistan�ce to galvan�ic reaction�. By coatin�g the fasten�er with a multi-layer, alumin�um-filled, cross-lin�ked baked-on� polymer, the outer surface of the fasten�er acquires high alumi-n�um con�ten�t. This corrosion�-resistan�t fin�ish makes the fasten�er more compatible with alumin�um parts an�d is less pron�e to galvan�ic reaction�.

Photo ©2005 Sherman Sham, used courtesy of Elco Construction Products, an Acument Global Technologies company

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ExpectingtheUnexpectedExtreme loadin�g is a con�dition� where loads far in�

excess of the n�ormal design� load are temporarily applied to a structure, sometimes on�ly for a few milli-secon�ds. The source of the extreme load might be a blast shockwave, an� earthquake, or a hurrican�e-driven� gust of win�d.

In� places where the occurren�ce of an� extreme load can� be predicted, it must be design�ed for. Seismically active region�s an�d locales subject to frequen�t hurrican�es or torn�adoes are well iden�tified. In� recen�t years, build-in�gs that can� be con�sidered likely targets for attack, or are adjacen�t to likely targets, are also receivin�g special design� con�sideration�.

11. Cladding, including glazing units and stone facades, often contain aluminum in the framework that attaches the unit to the building’s steel superstructure. This contact of aluminum and steel sets up the risk of galvanic reactions that generate hydrogen and precipitate hydrogen assisted stress corrosion cracking. To preserve the integrity of the cladding’s attachment to the building, fasteners that resist hydrogen embrittlement are recommended.

12. Screws resistant to HASCC are available in a wide variety of configurations for applications that include self-drilling and self-tapping in structural steel or in concrete.

13. The 88-story Petronas Towers in Kuala Lumpur, Malaysia in 1998. The façade contains literally millions of 300 series self-drilling fasteners with multi-layer zinc rich and aluminum-filled coating, chosen to prevent embrittlement from galvanic reaction between steel and aluminum and to mitigate the potential for sacrificial loss of aluminum in the cladding frame system.

Photo ©1998 Wes Thompson, used courtesy of Elco Construction Products, an Acument Global Technologies company



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Curren�t design� strategy for extreme loadin�g is to make the buildin�g absorb the load by deformin�g but remain�in�g stable lon�g en�ough to allow evacuation� an�d the preservation� of life safety. For threats such as blast shockwave or win�d impact, the design�er typically seeks to tran�sfer load from the vertical elemen�ts of the buildin�g shell to the buildin�g frame-work, often� usin�g the horizon�tal floor diaphragms to spread the load over more of the structure. This mean�s it is vital for the elemen�ts of the buildin�g en�velope to stay in� place an�d the fasten�ers that attach them to tran�sfer load to the frame without failin�g.

Extreme loads are often� impulsive in� n�ature, that is, they hit like a hammer, applyin�g the full force virtually in�stan�tly rather than� gradually. This impulsive load can� make materials perform in� an� atypical fashion�. It can� fracture a brittle material, such as the outer shell of a harden�ed steel fasten�er.

As n�oted above, when� the harden�ed outer case of a fasten�er cracks, the load on� the fasten�er must be born� by the n�arrower ductile in�n�er core. If a sign�ifican�t percen�tage of the fasten�er’s cross-section� is disabled, there is effectively a smaller fasten�er left to bear the load.

A fasten�er that is n�ot case-harden�ed, such as bi-metallic or selectively-harden�ed fasten�ers, has full ductility through the en�tire cross-section� of the shan�k where the fasten�er bears its load. Un�der extreme loadin�g, it is far more likely to deform but remain� in� service, as compared to a more brittle part.

When� design�in�g for extreme loadin�g, it is importan�t to have a realistic design�-threat as the baselin�e for determin�in�g loads. Expert con�sultan�ts may be useful. Con�sideration� of con�n�ection�s that must tran�sfer load in� this situation� should in�clude ductile specialty fasten�ers that can� han�dle impulsive loadin�g more effectively.

FOOTnOTES1. Society of Automotive Engineers SAE J78—Steel

Self-Drilling Tapping Screws.

ABOUT THE AUTHORS: Gregg Melvin is a Senior Applications Engineer with Elco Construction Products, an Acument Global Technologies com-pany. He has been in the fastener industry for over 20 years and works closely with design professionals, system manufacturers and contractors to create safer and more effective fastening systems for construction. He can be reached at [email protected] or www.elcoconstruction.com. Michael Chusid, RA, FCSI, is an architect and a Fellow of the Construction Specifications Institute (CSI). His company, Chusid Associates, provides marketing and technical consulting services for inno-vative building products. He can be reached at www.chusid.com.

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