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M107 175mm Self-Propelled Gun

The M107 175mm self-propelled Gun was largely deployed in NATO areas and its ammunition was commonly used throughout NATO forces. Designed to be part of a common family of weapons utilising the same chassis components, the M107 and M110 were essentially the same vehicle mounting different barrels. These guns fired an HE shell weighing 66.6 kilograms at a muzzle velocity of 914 meters per second.

The M107 self-propelled 175-mm. gun and the M110 8-inch howitzer had identical carriages but different tubes. The 175-mm. gun fired a 174-pound projectile almost 33 kilometers. This impressive range made it a valuable weapon for providing an umbrella of protection over large areas. The 8-inch howitzer fired a 200-pound projectile almost 17 kilometers, plus being the most accurate weapon in the field artillery. The 8-inch howitzer was found with most division artilleries, and both the 8-inch howitzer and 175-mm. gun were with field force artillery. At field force the proportion of 8-inch and 175-mm. weapons varied. Since the weapons had identical carriages, the common practice was to install those tubes that best met the current tactical needs. One day a battery might be 175mm.; a few days later it might be half 175-mm. and half 8-inch.

The M107 was used extensively in Vietnam to provide long range firesupport (being able to fire more than 30 km) and took part in numerous cross-DMZ duels with NVA guns. Though withdrawn from American service, vehicles of this type remain in service in the militaries of some other countries. The M107 was exported in Germany, in South Korea, in Spain, in Greece, in Holland, in Iran, in Israel, in Italy, with United Kingdom and in Turkey. In the majority of these countries they are converted besides towards M110A2.

During the 1950s the standard of US Army motorized howitzer of 203mm was the M55, based on the chasis and the turret of the M53 self-propelled howitzer of 155mm gun. The weight of the machines of this family (40t) prohibited, however, their air transport, while their gasonline engines confered a range limit of approximately 260 km.

This brought the Army to build a new series of self-propelled guns, lighter and transportable by air, derive from the same frame and some same mounting. Pacific Car and Foundry company developed a number of prototypes. The self-propelled guns T235 of 175mm and T245 of 155mm, and motorized howitzer T236 of 203mm. The T235 and T236 were driven by a diesel engine and manufactured in series under the respective references M107 and M110.

A certain number of tank recovery vehicles were developed on the frame of M107, but only the light vehicle M578 is used at the present time. The T120E1 was subsequently standardized as the M578. It equiped many countries, in particular the United States. The T119 and T121 unarmored recovery vehicles and the 155mm T245 were not developed further.

The first M107 series were manufactured by Pacific Car and Foundry Company in 1962. The first battalion was formed in 1983 beginning with Fort Still, bases US Field Artillery. Two other firms also produced of M107: FMC, between 1965 and 1980, Bowen-McLaughlin-York. The Army deployed the M107 on level of army corps within battalion equip of twelve guns each, but all these weapon were replaced by the M110A of 203mm which fired a HE projectile with propulsion additional of with a range exceeding 29 KM.

The 175 mm gun points elevation from - 2 to + 65 and in azimuth on 30 on the right and on the left. These operations are assisted, with hand drive operations of help. The only projectile used by the American army is HE M437A1 or M437A2 whose range is approximately 32.7 KM. Israelies employ special ammunition having a range close to 40.0 KM. the projectile normally weighs 66.78 kg, but a hydraulic device assembled to the back of the frame facilitates its loading: it raises it ground, positions it and pushes it in the cylinder head. The powder load is then set up and the shooting can start. M107 transports only two projectiles and their load. It is served by a crew of thirteen people, of which five only (pilot, head of tank, and three gunners) are on board. The others follow normally on board tracked M548 which contains the remainder of the ammunition. Certain country replaces it by trucks 6 X 6, whose possibilities any ground are however quite lower than that of the gun itself. M107 is equipped with infra-red equipment for the night vision.

In general, the higher the strength of the gun steel used in a particular cannon design, the lighter the cannon can be. Also, higher strength material allows the cannon to be fired at greater pressures for improved performance and range. As with most things in life, nothing comes for free. As the strength of gun steel is increased, resistance to fatigue failure and cracking under high pressure decreases. Put another way, as strength, pressure, and performance increase, cannon life decreases, sometimes dramatically.

In order to achieve acceptable fatigue resistance, the majority of artillery cannon designs up to the 1970s used gun steel at strength levels of about 160,000 psi yield strength. Some of the "newer" cannon designs in this era, such as the original 175mm M113 Gun on the M107 Self-Propelled System, experimented with what were considered very high strength steels for the time with gun steel strength levels about 180,000 psi yield strength.

Unfortunately, what was to later become the science of fracture mechanics and related fatigue failure was not well understood or appreciated at this time, and the original 175mm M113 Gun experienced premature catastrophic brittle fracture at a very low round count. Investigation of these in-service failures greatly advanced the understanding of fracture mechanics and the relationship between strength and fatigue resistance in gun steels, essentially making Bent Lab scientists and engineers the world recognized experts in fracture mechanics of high strength pressure vessels.

The firing of the 175mm self-propelled guns on 19 November 1965 in Vietnam was the first time ever fired in combat. Already in April 1965, a 175 mm M113 gun tube failed during firing in Vietnam at a total of 428 Equivalent Full Charge (EFC) rounds and the failure was attributed to fatigue. Almost immediately fatigue limit was reset to 400 EFC (300 maximum Zone 3 rounds) fatigue life on the tube, a major investigation and analysis was launched, and gun tubes with same properties (whole production run) were removed from service.

Cannons that could fire effectively (based on the new wear additive) past 1,000 rounds were now being taken out of service at 400 rounds or less. Given the amount of firing on-going, this was a potential crisis for field units, and the impact was heard at the Secretary of Defense level. Gun tubes with fracture related mechanical properties in the same range as those of the failed tube were removed from service. Immediate additional testing was started From May 1965 - September 1965 - 23 tubes were tested. Additional tubes were added over the next few months.

After some examination it was determined that the lower life tubes exhibited inter granular cracking vs the longer lived tubes that primarily exhibited trans-granular cracking. It was further noted that the lower life tubes were linked to longer timed - lower temperature tempering processes. Temper embrittlement was specifically noted as a concern.

Electron microscopic examination (a very recent tool for the period) revealed that the impurities in the lower life tubes were most prevalent in the grain boundaries, where in the longer life tubes they appeared well distributed longer life tubes, they appeared well distributed. Longer tempering times allowed impurities to migrate to grain boundaries. Weakened grain boundary zones allowed localized (fatigue) crack growth to progress quicker in an intergranular cracking failure mode. Inclusions tended to contribute to the faster crack growth - and brittle failure mode. Temper embrittlement also contributed to brittle failure mode.

The hydraulic autofrettage procedure used in the early 1970s was an expensive, difficult, slow, and sometimes dangerous procedure where pressurized hydraulic fluid, on the order of 100,000 psi or greater, was pumped into the bore of the tube to apply compressive residual stress at the bore. The autofrettage process imparts a deformation to the bore of a cannon tube in order to increase the ballistic pressure sustaining capability of the tube. In other words, the tube that has been autofrettaged will be capable of withstanding a higher internal ballistic pressure than its non-autofrettaged counterpart without going to a thicker tube wall.

The swage autofrettage process developed and implemented in the late 1970s by contrast, is a significantly less expensive, easier, faster, and safer process that produces much more controllable results and is much better suited to high production rates. In the swage autofrettage process, a precision high-strength mandrel is pushed through an undersized bore of the tube to permanently enlarge the bore prior to final machining. By varying the relative size of the cannon bore, the entire length of the tube can be autofrettaged to different amounts in a single pass of the mandrel. Because no special end seals, high pressure pumps and lines, or expensive external restraining fixtures are required, the swage autofrettage process has been greatly simplified, as well as made the process safer for the machinist.

In a very short period of time, the original M113 Gun was significantly redesigned to lower the strength levels of the material, while using the new autofrettage process to make up for the loss of strength. The fatigue life of this replacement design was increased at least ten-fold from the original design, and in service failure was eliminated.

The Army closed out the 1960s with a wide variety of artillery delivery systems such as the 175mm M107 howitzer; the 155mm M126 self-propelled howitzer; the 8-inch M110 howitzer; the 105mm M137 lightweight towed howitzer; and the 105mm M103 self-propelled howitzer. The Watervliet Arsenal, working hand-in-hand with the Army's Bent Laboratories, was instrumental in the research, design, prototype development, and the production of the tubes and various assembly parts for each of these warriors. They were truly state-of-the-art at the time and initially offered a superlative response to any Soviet artillery systems. Although this system is no longer in service with the U.S. military, it continues to provide safe and effective fire power to NATO allies.




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