From time immemorial the problem of the fortifications is closely related to that of the weapon (arms against armour). One creates a first armour which resists a first weapon. Blow the armour, fortification, enters the strategy of defense and becomes flourishing. Then one creates a second weapon which bores the armour. The latter becomes obsolete and the fortification is temporarily abandoned. Then, technology helping, appears a new armour which resists the second weapon and the fortification reappears. etc
The shortly after the war of 14-18, the fortifications given up at the beginning of war have the wheel in motion, inter alia thanks to their victorious resistance to Verdun. Also all the countries will want to build some, more especially as the preferences of the public opinion lean often more towards defense that towards aggressiveness.
But before building of the nine one analyzes the effects of the weapons on what exists. And these effects are of two types:
- penetration of the projectiles in masonry,
- Hopkinson effect.
2 men believe themselves well protected behind a thick steel or concrete bed. But suddenly arrives a shell and here are killed by a hail of glares without their protection not being bored!
The report is clear. One can believe that materials and men are with the shelter, behind a thickness of concrete or steel which no projectile can bore, and it of it is nothing! Because of the waves produced at the time of the shock, even without penetrating, the projectile can make major damage on other side of the screen - guard. It is what is called the Hopkinson effect.
- A wave which arrives at the interface between 2 mediums divides into 2 waves: one, reflected, turns back in the medium of origin and the other, refracted, is propagated in the new medium.
- Two waves which meet interfere and the forces which they convey add vectoriellement.
To protect itself from the Hopkinson effect one can:
- to minimize the shock waves by absorbing them as with the composite shieldings of the current tanks (and thus to minimize the formation of the meniscus)
- to avoid the departure of the meniscus by retaining it either with a metal trellis-work or by doubling the inner face to be protected (in red below).
To avoid the departure of the meniscus one can retain it by doubling the inner face to be protected or by replacing steel by composite materials which absorb the shock waves. As these composite materials do not exist at the time, the only possible solution is the doubling of the inner face of armourings.
The diagram of vertical cut of this turret highlights the 30cm thickness of its wall and its roof (out of chestnut) and the 4,5cm thickness of its interior doubling (in yellow).
Block 5 of Galgenberg (SF Thionville). Bell JM (1930) in the foreground and GFM (1929) on the right. One realizes very well that the GFM is not likely to pass unperceived.
Bells GFM (Guet Rifle Machine gunner), are the most visible parts of the works of the Maginot Line (and one can hide them with difficulty since they are used as observation posts on 360°). Also, at the meetings of the CORF, since 1929, the Belhague general worried it in connection with their effectiveness. In fact, it then seems more worried by the multiple missions than one assigned with the GFM (overall observations and shootings, the whole with only one man, for lack of place) that by their need for reinforced protection against the Hopkinson effect.
And thus 1000 of these bells were set up, vis-a-vis the enemy, without interior doubling, whereas, obviously, they were going to be the first targets of the next conflict; what did not fail besides to occur in 1940.
With discharge for the CORF, one can suppose that these bells, of a diameter external of 1,80m at most, were considered to be enough small not to be seriously reached by the tended shootings of a field artillery considered as not very precise. But we did not find any documentary evidence, on this subject.
In 1934 the artillery shootings largely gained in precision and the protection of the bells became a crucial problem that the engineers of the Genious solved with the development of a bell of the type B (model 1934). The latter is internally a little vaster than its elder (10cm moreover) what makes it possible to counter the Hopkinson effect by doubling his armouring by a sheet of 20mm thickness. At the same time one modifies the shape of his crenels to make them more resistant to the tended shootings.
But the bells of the type has, they, are already installed and equipped. Their low internal diameter (1,20m), combined with the constraints imposed by the presence of the crenels of observation and shooting, then prevent from conceiving an effective and practical doubling.
In 1937, at the time of the installation of the J2 periscopes in the auxiliary bells of observation armoured of type has, a project of interior doubling is studied in order to be used at the same time of support for J2 and protection against the Hopkinson effect.
The built prototype consists of a sheet segment of a sphere of 20mm thickness which takes seat with the top of the crenels. The cap rests on spacers connected to a ring fixed at the bell, under the crenels.
Installed with difficulty in August 1937, this prototype does not give satisfaction and is abandoned. The standard GFM have will remain without protection against the Hopkinson effect!
Just like the GFM, bells JM are not doubled. But contrary to the GFM they are enchased in the concrete and offer only little catch to the shootings. Always it is that, when they are transformed into mixed bells weapons, as from 1936, they receive a doubling then.
Benefitting from the English and German experiments, the reinforced concretes of the Maginot Line are reinforced by two series of reinforcements, one near to outside and the other near to the interior, this last series having for role to retain the meniscuses.
But for more safety, the inner faces of the flagstones and walls most exposed to the blows are doubled by armour-plates.
AC47 Simserhof at the entry of men (SF Rohrbach). With make on the right, side which can receive blows, the wall is papered armour-plates; on the left, side which considering its situation cannot receive blows, it is not it.
Staff waiting room of Cape-Martin (SFAM). The wall where this grenade chute is installed is directed vis-a-vis in Italy. It is doubled by a whole of armour-plates of 4,5mm thickness which, on the left, some anchor bolts are seen. With the site of the chute the plates were flame-cut.
The vaults, under the flagstones, are also protected by armour-plates. Room of troop to the Shelter of Bichel-South (SF Thionville).
B1 Bambesch (SF Faulquemont) after the German attack of June 20, 1940. With the top of the crenel one notes that the flagstone, whose face was destroyed, consists of reinforced concrete at its upper part and her lower part. There is no reinforcement in the center of the concrete.
The analysis of the effects produced by German artillery on the fortifications of 14-18 is source of lesson for the originators of the Maginot Line.
The shortly after the Great War certain interrogations relating to the concrete of the strengthened works appear clearly expressed in the courses of the Commander Frossard. (In the 1920 this last is professor at the Military academy of the Genius where it exempts a teaching on the fortification. Named thereafter Brigadier general, it will occupy the important post of technical inspector of work of fortification).
In one of its conferences he speaks thus about the effects produced on the works by the largest German projectiles:
At first sight thus the reinforced concrete of before 1914 does not seem to have been as powerful as one could have believed it. And Cdt Frossard continues are speech in this direction:
But at once after Cdt its remarks moderate:
It is interesting to note that as of the end of the Great War, before even as the observations relating to the destroying effects of the German projectiles all were not analyzed, before even as from the experiments did not come to supplement these observations, the remarks - with heat on the behavior of the concrete already precedes what will be the future casemates of the Maginot Line: thick concrete blocks to two layers of reinforcements (inner face and external face). The internal layer protects from the Hopkinson effect.
It is as interesting to note as, it seems, the English and the Germans became aware of the phenomenon before French.
After 1945 the British develop projectiles for purpose of crushing HESH (High Explosive Squash Head) intended to produce a Hopkinson effect reinforced on the shieldings.
As the dimension of the meniscus depends on the surface of attack of the projectile, all that increases this surface is likely to accentuate the Hopkinson effect.
More the head of the projectile is crushed and spread out over its target, more the detached meniscus is important. Also ammunition HESH acts it in two times:
the 1st time: the head of the shell is crushed and spread out
the 2nd time: the load explodes (on the well spread out head)
A shooting under incidence increasing the surface of contact between the explosive and the shielding. It thus improves the performance of the projectile!
According to Colonel Roland Gras, who documented us, the studies in situation show that the optimal angle of incidence is of 40° (starting from 60° the risks of rebound strongly reduce the effectiveness of the HESH).
In yellow: ground, rubble. In red: metal reinforcements.
What one observes
Theory on the Hopkinson effect
How to preserve Hopkinson effect
Errors of 1929 and 1930
Lesson drawn from the war of 1914-1918
To test its knowledge on the Maginot Line and the Hopkinson effect.
Click: to take/release the object to move it
Click: return to the banner page
Click: post/erases the reference marks of the legends
Click: post the whole of the files of the site
Click: increase the size of the image
Click: decrease the size of the image
Click: firm the window
Click: move and the zoom centre of rotation of the model
Click: coward the object
Click: fact of turning the model
Click: fix the model
Click: post the lexicon of the site, in another window
Click: modify the dimension of the window
Click: animate the image
Click: stop animation
2 men believe themselves well protected behind a thick steel or concrete bed. Suddenly a shell arrives.
The shell explodes without penetrating in the obstacle. But it creates a wave train of shock.
The train of compression waves (and associated elastic strain) are propagated, the such waves P of the earthquakes, until the interior of the casemate.
Arrived in the casemate, to the change of medium between the concrete (or steel) and the air, each wave gives rise to two new waves; one, refracted, carries on its way in the interior air of the room (and noise generates), the other is considered in material.
The reflected waves, by turning back, interfere with the other waves, which continue to arrive, and create constraints which, in certain points, exceed the limit of mechanical resistance of the concrete (or of steel).
The concrete (or steel) breaks (HOPKINSON EFFECT). A meniscus is detached from the wall.
The meniscus is projected on the men of crew. Sometimes the meniscus splits up and gives a hail even more dangerous, because more largely spread out.
It is noted that the concrete (or steel) is not bored.
It is noted that the shell did not penetrate in the casemate.
In spite of the resistance of the wall the men are killed and the damaged material!
Neither the thickness of the wall, nor its resistance, can counter the Hopkinson effect.
The ammunition does not need to be special, it is enough for him to strike the wall.
The effect is, of course, the same one when it is about a flagstone on which a bomb explodes.
All the walls which can be reached by a projectile are thus likely to be dangerous.
There exist however parades for this purpose, the whole is to use them!
Maginot line - Hopkinson Effect; Document carried out thanks to the technical assistance of Colonel d' Artillerie Roland Gras which we thank warmly. B-E-R Cima ©2000-2008
0_*; Local files; 1_*; General information; 2_*; The report; 4_*; To protect itself; 5_*; Protection of armourings; 6_*; False note of the GFM; 7_*; Protection of the concrete; 8_*; Concrete with 2 reinforcements; 9_*; Ammunition for Hopkinson purpose; 10_*; Teachware