Alessandro72 wrote:A difesa degli ingegneri di inizio Novecento c'è da dire che più che di impurità parlerei di fragilità. Aggiungo che le cosiddette sollecitazioni "da fatica" (anche se non è proprio il caso specifico del Titanic) dei materiali erano pressochè sconosciute a quell'epoca. Che io sappia sono state affrontate per la prima volta con il progetto Comet (un aereo pressurizzato che dopo continue pressurizzazioni e depressurizzazioni scoppiava). Probabilmente il materiale con cui ertano costruiti i chiodi ribattuti che tenevano insieme le lamiere erano anche stati resi tanto duri quanto fragili a causa delle temperature prossime allo zero dell'acqua. La forza impressa dall'urto ha causato la rottura degli stessi.
jacopo wrote:Ale però c'è da dire che il cantiere - causa pressioni per consegnare la nave nei tempi stabiliti - ha ordinato pure ferraccio come qui riportato, per di più da "uncertified suppliers":
The source of this poor quality material became clearer when the Harland and Wolff meeting minutes were examined, and it was seen that pressure to finish Titanic caused the company to order wrought iron that was one level below that generally specified for rivets and they had to use suppliers previously uncertified for this application.
Alessandro72 wrote:Sì Jacopo è vero che si dice questo, però sarei curioso di verificare che queste cose siano vere. Innanzi tutto nel 1912 esistevano le certificazioni come le intendiamo oggi? Tra l'altro non è detto che il ferraccio era sbagliato a priori: che quando frequentavo l'ITIS negli anno '80 ci facevano designare i chiodi dei ponti con il "ferro 37" (praticamente ferraccio)... Naturalmente ora non sono più aggiornato rispetto queste cose e non saprei dire di più anche perchè le normative nel frattempo sono cambiate... Ma poi, ripeto, a inizio '900 non c'era la consapevolezza degli sforzi "da fatica" dei materiali quindi la minor qualità del ferro dei chiodi avrà avuto un sapore diverso rispetto quello che possiamo immaginare noi. Non so... cosa ne dite? Magari qualche ingegnere meccanico presente nel forum potrebbe illuminarci...
S-Bahn wrote:I cedimenti a fatica erano sconosciuti ma di sicuro erano anche sconosciuti in marina i limiti stringenti di peso che hanno determianto la riduzione dei coefficienti di sicurezza in aviazione nei decenni successivi.
Poi parlare di rottura per fatica al primo viaggio vero della nave è un po' fuori luogo...
Rottura per fatica intesa come (se vuoi impropriamente!) non sopportare l'impatto ma aprirsi.
In merito alla debolezza del metallo...questo post dovrebbe essere abbastanza esplicativo!
Ricordo un programma trasmesso circa un anno fa da
Nationl Geographic Channel in cui si testavano dei rivetti, costruiti esattamente come venivano costruiti un secolo fa.
Quelli costruiti correttamente e con minori impurità avevano una notevole capacità di sopportare stress, così come era notevole la loro resistenza alla fatica, cosa che non si poteva dire dei rivetti
à la Titanic.
Sicuramente i nostri vecchi non avevano le competenze metallurgiche ed ingegneristiche che si sono sviluppate in questi ultimi decenni, ma sono convinto che il loro lavoro lo sapessero fare!!
A tal proposito, riporto questi due articoli:
The American Society of Mechanical Engineers - Testing the Titanic's Steel http://www.memagazine.org/backissues/me ... sting.htmlIn 1996, several samples of steel from the Titanic—a hull plate from the bow area and a plate from a major transverse bulkhead—were recovered from the wreck site and subjected to metallurgical testing by Prof. H.P. Leighly at the University of Missouri-Rolla, as well as at the laboratories of Bethlehem Steel and the National Institute of Standards and Technology. Chemical testing revealed a low residual nitrogen and manganese content, and higher levels of sulfur, phosphorus, and oxygen than would be permitted today in mild steel plates or stiffeners.
This indicates that the steel was produced by the open-hearth rather than the Bessemer process, most likely in an acid-lined furnace; the steel is of a type known as semi-killed, that is, partially deoxidized before casting into ingots.
(Other fragments of the Titanic's hull have yielded slightly different results, suggesting a degree of variability in the chemical and, hence, the mechanical properties of the steel used in the ship.)
Excess oxygen can form precipitates that can embrittle the steel, and will also raise transition temperatures.
In the absence of sufficient manganese, sulfur reacts with the iron to form iron sulfide at the grain boundaries;
it can also react with manganese, in either case creating paths of weakness for fractures.
Sulfide particles under stress can nucleate microcracks, which further loading will cause to coalesce into larger cracks; in fact, this was found to have been the mode of failure in the shell plating of the Titanic.
Phosphorus, even in small amounts, has been found to foster the initiation of fractures. Of course, much of this metallurgical information has only been learned in the years since the Titanic went down.
To determine the steel's mechanical properties, it was subjected to tensile testing, as well as the Charpy V-Notch Test, used to simulate rapid loading phenomena; the test used samples oriented both parallel and perpendicular to the original direction of the hull plate.
The ductile-brittle transition temperature (using 20 lbs.-ft. for the test) was found to be 20°C in one direction and 30°C in the other, compared with —15°C for a reference sample of modern A 36 steel—and a water temperature of —2°C on the night the ship collided with the iceberg. The Titanic steel was also shown to have approximately one-third the impact strength of modern steel.
When the Titanic samples were also examined with a scanning electron microscope,
the grain structure of the steel was found to be very large; this coarse structure made it easier for cracks to propagate.
Rivet holes were cold-punched, a method no longer allowed (they must now be drilled), nor were they reamed to remove microcracks.
The steel grain size; the oxygen, sulfur, and phosphorus content of the steel; and the cold-punched, unreamed rivet holes were found to have contributed to the breakup of the Titanic, along with the steel's relatively low ductility at the freezing point of water. The shell plates showed signs of brittle fracture, though some plates demonstrated significant plasticity.
Of course, the science of metallurgy has advanced considerably since the Titanic's day, and William Garzke of Gibbs and Cox and his collaborators emphasized in their report that "the steel used in the Titanic was the best available in 1909-1914" when the ship was built.
In fact, they add that when 39,000 tons of water entered the bow, "no modern ship, not even a welded one, could have withstood the forces that the Titanic experienced during her breakup."
New York Times:
In Weak Rivets, a Possible Key to Titanic’s Doom http://www.nytimes.com/2008/04/15/scien ... wanted=allResearchers have discovered that the builder of the Titanic struggled for years to obtain enough good rivets and riveters and ultimately settled on faulty materials that doomed the ship, which sank 96 years ago Tuesday.
The builder’s own archives, two scientists say, harbor evidence of a deadly mix of low quality rivets and lofty ambition as the builder labored to construct the three biggest ships in the world at once — the Titanic and two sisters, the Olympic and the Britannic.
For a decade, the scientists have argued that the storied liner went down fast after hitting an iceberg because the ship’s builder used substandard rivets that popped their heads and let tons of icy seawater rush in. More than 1,500 people died.
When the safety of the rivets was first questioned 10 years ago, the builder ignored the accusation and said it did not have an archivist who could address the issue.
Now, historians say new evidence uncovered in the archive of the builder, Harland and Wolff, in Belfast, Northern Ireland, settles the argument and finally solves the riddle of one of the most famous sinkings of all time. The company says the findings are deeply flawed.
Each of the great ships under construction required three million rivets that acted like glue to hold everything together. In a new book, the scientists say the shortages peaked during the Titanic’s construction.
“The board was in crisis mode,” one of the authors, Jennifer Hooper McCarty, who studied the archives, said in an interview. “It was constant stress. Every meeting it was, ‘There’s problems with the rivets and we need to hire more people.’ ”
Apart from the archives,
the team gleaned clues from 48 rivets recovered from the hulk of the Titanic, modern tests and computer simulations. They also compared metal from the Titanic with other metals from the same era, and looked at documentation about what engineers and shipbuilders of that era considered state of the art.The scientists say
the troubles began when its ambitious building plans forced Harland and Wolff to reach beyond its usual suppliers of rivet iron and include smaller forges,
as disclosed in company and British government papers.
Small forges tended to have less skill and experience.
Adding to the problem,
in buying iron for the Titanic’s rivets, the company ordered No. 3 bar, known as “best” — not No. 4, known as “best-best,” the scientists found. Shipbuilders of the day typically used No. 4 iron for anchors, chains and rivets, they discovered.So the liner, whose name was meant to be synonymous with opulence, in at least one instance relied on cheaper materials.
Many of the rivets studied by the scientists — recovered from the Titanic’s resting place two miles down in the North Atlantic by divers over two decades — were found to be riddled with high concentrations of slag. A glassy residue of smelting, slag can make rivets brittle and prone to fracture.
“Some material the company bought was not rivet quality,” said the other author of the book, Timothy Foecke of the National Institute of Standards and Technology, a federal agency in Gaithersburg, Md.
The company also faced shortages of skilled riveters, the archives showed. Dr. McCarty said that for a half year, from late 1911 to April 1912, when the Titanic set sail, the company’s board discussed the problem at every meeting.
For instance, on Oct. 28, 1911,
Lord William Pirrie, the company’s chairman, expressed concern over the lack of riveters and called for new hiring efforts.In their research, the scientists, who are metallurgists, found that good riveting took great skill. The iron had to be heated to a precise cherry red color and beaten by the right combination of hammer blows. Mediocre work could hide problems.
“Hand riveting was tricky,” said Dr. McCarty, whose doctoral thesis at Johns Hopkins University analyzed the Titanic’s rivets.
Steel beckoned as a solution. Shipbuilders of the day were moving from iron to steel rivets, which were stronger. And machines could install them, improving workmanship.
The rival Cunard line, the scientists found, had switched to steel rivets years before, using them, for instance, throughout the Lusitania.
Harland and Wolff also used steel rivets — but only on the Titanic’s central hull, where stresses were expected to be greatest.
Iron rivets were chosen for the stern and bow.And the bow, as fate would have it, is where the iceberg struck.
Studies of the wreck show that six seams opened up in the ship’s bow plates. And the damage, Dr. Foecke noted, “ends close to where the rivets transition from iron to steel.”
The scientists argue that better rivets would have probably kept the Titanic afloat long enough for rescuers to arrive before the icy plunge, saving hundreds of lives.