General information

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.

Hopkinson effect: the report

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!

Animation of the image?

Stop of animation?

Does explanations + advance step by step?

Does explanations + move back step by step?

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.

Recalls in connection with the waves

- 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.

Hopkinson effect: explanations

Hopkinson effect: to protect itself

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).

These are the two last solutions which were adopted for the Maginot Line.

Turret of mixed weapons (SF Faulquemont. Bambesch Block 1)

Protection of armourings

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.

Turret of 75/33 of Agaisen (SFAM).

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.

- False note of the GFM 1929, without interior doubling

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.

GFM 1934 (B) type with interior doubling

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.

Standard GFM B. Furnace-with-Lime B6 (SF the Vosges).

Tests of 1937

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!

Case of bells JM 1930

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.

Protection of the concrete

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.

Lesson on the reinforced concrete

Cours Frossard

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).

Disappointing reinforced concrete?

In one of its conferences he speaks thus about the effects produced on the works by the largest German projectiles:

[-] the buildings in masonry of the fortifications of Verdun, built at different times, could be reduced to the three types below:
- type 1 (former to 1885): rubble work a thickness of 1m with 1,5m with the key, covered with a layer of ground of 2 with 5m;
- type 2 (reinforcement after 1885): even masonry that above, reinforced by a special concrete carapace of 1,5m with 2,5m thickness with interposition of a layer of sand of 1m thickness;
- type 3 (after 1885): buildings with special concrete oven walls and cover consisted reinforced concrete flagstones of 1,25m with 1,75m thickness according to the range [-]

The projectiles of 380mm are provided with a rocket of base without delay; they thus burst in contact with a hard body, while hardly penetrating in the obstacle. (-). They destroy naturally masonries of the type 1 [-]. On masonries of the type 2 the projectile of 380mm produces only effects in general surface: hole of 1m of depth.

On masonries of the type 3 they produced major destruction. A shell of 380 made, in the reinforced concrete vault of 1,6m thickness of the corridor of the casemates of the work of Froideterre, a funnel of 0,6m with 0,8m of depth on 4 with 5m of diameter [-] To the work of Thiaumont, a shell of 380, fell on a flagstone from 1,5m from thickness, made there a more considerable funnel involving the disintegration of the reinforced concrete and the rupture of most of reinforcing irons [-]

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:

The projectiles of 420mm are generally provided with rockets with long delay. Masonries of the type 1 are crossed as to the punch and (the braking produced by the obstacle not being enough fast to make function the system of starting) it arrives some time that the projectile does not burst.

Masonries of the type 2 are burst when the special concrete carapace has a thickness lower than 2m. At the height of Douaumont, the part of the barracks which was protected by a carapace from 1,5m thickness was bored in several places; on the contrary, the part protected by a carapace of 2,5m resisted blows isolated from 420. [-]

Masonries of the type 3 are not burst if the thickness of the flagstones is higher than 1,75m. [-] the shell bursts in the flagstones; those indeed present at their upper part a hole of 0,7m approximately of diameter and 0,6 with 0,7m of depth, then a room of bursting in which the concrete is pulverized and the irons destroyed over a length of 1,5m with 1,8m. In the flagstones of 1,5m thickness the last layers of iron were curved before to be broken [-]

[-] In the reinforced concrete which underwent the shock of a very large projectile, the iron bars are generally completely pickled. It remains around them no concrete trace in which they were drowned. It seems that the iron reinforcement facilitated the dislocation of the general mass, probably because the vibrations due to the violent shock and the bursting of the projectile occurred with different intensities and speeds in iron and the concrete, thus bringing the separation of these two materials [-]

In the special concrete, the oven walls, vaults or flagstones are divided most of the time approximately blocks of which some measure sometimes more than one cubic half-meter, often remaining balances some and thus avoiding a total and massive collapse.

If one considers in addition that the thicknesses of special concrete or reinforced concrete not crossed, for example by a projectile of 420, are not very different (more 2m for the special concrete, more 1,75m for the reinforced concrete) one can wonder whether the hopes which one had based from this point of view on the use of the reinforced concrete were not disappointed and if this one is not condemned [-]

Nuances

But at once after Cdt its remarks moderate:

[-] In the forts of Verdun, which, for much, were works reinforced after 1885, the special concrete carapaces were covered a certain thickness of ground and rested on a sand mattress. These two circumstances caused, the first to slow down the speed of the projectile, the second to form an elastic mattress; they had certainly as a result to decrease the effects of the shock and the explosion on the carapace.

The reinforced concrete, on the contrary, was generally reserved for bodies of surface; it was covered with little ground and did not profit, for the damping of vibrations, of the presence of a layer of sand.

It is not less true than the separation of the concrete of the iron bars is inquiêtante. Also, during the war, the Germans and the English have builds slabs concrete, armed only at the upper part and the lower part. The upper part mainstays are intended to decrease the penetration of the projectile in the flagstone before its bursting; the lower part mainstays must be opposed to the detachment meniscuses [-]

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.

Ammunition for Hopkinson purpose

Here we are not any more within the framework of the Maginot Line!

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.

Crushing

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)

Incidence

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).

Teachware. Protection against the Hopkinson effect on the Maginot Line

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