component failure analysis

8/11/2019 Component Failure Analysis

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2/422 OCT-DEC 2010engine professional

COMPONENT FAILURE ANALYSISBY DUANE BOES

a minimal amount of bending inducedfatigue a crack that initiates in this sharptransition will quickly propagate alongthis stress riser. When viewing the broken follows the sharp radius blend preciselyaround the journal. The break is sosmooth and true that it almost appears to

have been cut with a saw. The true andprecise nature of this break is the firstindicator that a stress riser mechanicallyinduced this failure. Your best protectionagainst this situation is a careful andinexpensive visual inspection of all filletsafter grinding. As is always the case alittle prevention is worth a lot of cure.

You can avoid this outcome bytelling the machinist who is regrindingand polishing your crank exactly whatyou expect and dont want to see. Look improperly dressed grinding wheels. Keep

in mind the goal in dressing a wheel is

to create a tangent blend point between example of an over blended radius on conditions will result in an unacceptableradius contour that will leave a stress

riser at the blend point. After beingground the journals will be polished,make it perfectly clear that you expectthe fillets to be polished to a high lusterfinish that is every bit as fine as the fine finish you will greatly improve theendurance of any crankshaft. The type of eliminated.

Our second problem involves ajournal that had been repaired by done properly this common practice is

quite capable of yielding a durable repair.

When done incorrectly, weld repairs canbe short lived. Every weld produces aHeat Affected Zone (HAZ); within thiszone the grain structure of the parentmaterial is significantly altered. Thisalteration of grain creates an internalstress riser, for this reason the HAZneeds to be moved away from areas of

can see a visible lap created by a weldthat did not penetrate seamlessly intothe parent material. This lap serves asa witness mark which verifies this asthe last pass made during the weldingprocess. Unfortunately for this crank theHAZ of this weld is in a heavily stressedfillet. With any crank journal repair, theweld must extend up the journal sidewallto insure the HAZ is beyond the fillet the weld did not extend beyond the fillet,

and Second the weld was penetrating

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Figure 3: Arrow indicates the areaof fracture origin, notice the trueand crisp edge that preciselyfollows the sharp OD tangencypoint of an under blended radius.

Figure 4: Left illustration is of anOver Blended radius, Illustrationon right depicts an Under Blended(sharp)radius.

Figure 5: Open weld flawindicated by arrow.



the parent material inconsistently. Tohave an exposed weld lap is absolutelyunacceptable, regardless of location.Think of these type flaws as openinvitations to disaster. When sending acrankshaft out for journal welding yourvendor must clearly understand that youexpect the HAZ to be out of the fillet andthat any type of open weld flaw near afillet will not be tolerated.

The last failure we will review can beextremely destructive. We will be lookingat crankshaft post separations. The engine. The operating circumstances ofthis engine were fairly mild, the tune-up was not radical and a very gooddamper had been installed. Under these

circumstances there should have been no

way for this type of failure to occur, but we are looking at the fracture face, in thisview a brownish area can be seen just tothe right of the keyway. This area wasthe initiation site for this failure. Thisbrown coloration is a result of heat andoil staining, generated as the two faces area we can see beach marks (fracturestriations) emanating outward. The speedat which the fracture began to progresscan be witnessed by the increasingdistance between each striation. Thispost stayed together until all that wasleft holding it was the dark gray circle atroughly 7:00 Oclock in relation to thekeyway. This is impressive when you stopto think that this small section was able

to briefly hold that much energy. Another

7), an arrow is pointing to the section ofthe circumference that coincides exactlywith the brownish area on the fractureface. Notice the true and precise edgeof the broken post, it almost appears asthough Superman scored this post with aglass cutter, then simply snapped it off. Ina sense that is exactly what happened. Inthis situation and many others like it thechamfer of the timing gear had cut intothe fillet radius at the back of the postwhen the gear was installed.

with the timing gear still attached can Initially this fracture progressed slowly,but once the striations reached the

multiple keyways the pace of this failure


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Figure 6: Fracture facefrom broken post ofnaturally aspiratedengine.

Figure 7: Crisp edge atfailure initiation site.

Figure 8: Red Arrow =Primary initiation site,Yellow arrows areSecondary initiationsites.




increased significantly. The reason forthis increase in speed is that Secondaryfracture points had already been initiatedat each of the nine keyway locations.Arrows have been placed at a few ofthese secondary points that can bewitnessed by the striations that emanate

outward from each. Once again withthis post the last area to let go is darkgray and rough in texture as though itwas simply torn apart. In this examplethe timing gear was left on to show howperfectly the keyways and secondarystriations are aligned. Both initiation result of a stress riser that was scoredinto the post radius when the timing gear point became such by the fact that itcovered a greater distance along thecircumference of post.

Many timing gears are made ofcarburized and hardened steel whichleaves them very hard for wearresistance. At the same time these gearsare often considered to be a commodityitem, produced with minimal steps inmanufacture by the most efficient means

possible. You will always find the insidediameter edge of the chamfer on thesegears to be very crisp. When you placea nice, well defined edge on a hardenedpiece of steel youve just created theperfect instrument to score a radius. Twothings could have been done to prevent

make sure the gear chamfer is largeenough to accommodate the post radius.Second, take a minute to break bothedges of the gear chamfer. By eliminatingthose sharp edges and verifying yourclearance you will have eliminated thepossibility of your gear plowing anengine destroying stress riser into yourcrankshaft.

These examples did not fail on theirown, but rather as the result of someoutside influence being brought tobear upon them. As mentioned in the

first paragraph, how an item is usedor maintained will often determine be careful not to prematurely reacha conclusion by simply saying thecomponent failed. By taking timeto understand the applied loads and

carefully examining the fracture site wemay discover the actual failure beganwith a human.

Duane Boes is from Callies PerformanceProducts in Fostoria, Ohio. For moreinformation, please call 419-436-8402

or e-mail [email protected].

component failure analysis

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