In mechanics, an impact is when two bodies collide. During this collision, both bodies decelerate. The deceleration causes a high force or shock, applied over a short time period. A high force, over a short duration, usually causes more damage to both bodies than a lower force applied over a proportionally longer duration.
Head impact can cause concussion. Sports helmets help protect against brain injury.[1]
At normal speeds, during a perfectly inelastic collision, an object struck by a projectile will deform, and this deformation will absorb most or all of the force of the collision. Viewed from a conservation of energy perspective, the kinetic energy of the projectile is changed into heat and sound energy, as a result of the deformations and vibrations induced in the struck object. However, these deformations and vibrations cannot occur instantaneously. A high-velocity collision (an impact) does not provide sufficient time for these deformations and vibrations to occur. Thus, the struck material behaves as if it were more brittle than it would otherwise be, and the majority of the applied force goes into fracturing the material. Or, another way to look at it is that materials actually are more brittle on short time scales than on long time scales: this is related to time-temperature superposition. Impact resistance decreases with an increase in the modulus of elasticity, which means that stiffer materials will have less impact resistance. Resilient materials will have better impact resistance.
Different materials can behave in quite different ways in impact when compared with static loading conditions. Ductile materials like steel tend to become more brittle at high loading rates, and spalling may occur on the reverse side to the impact if penetration doesn't occur. The way in which the kinetic energy is distributed through the section is also important in determining its response. Projectiles apply a Hertzian contact stress at the point of impact to a solid body, with compression stresses under the point, but with bending loads a short distance away. Since most materials are weaker in tension than compression, this is the zone where cracks tend to form and grow.
Crane with a pile driver1⁄2 in (12.7 mm) drive pistol-grip air impact wrench
A nail is pounded with a series of impacts, each by a single hammer blow. These high velocity impacts overcome the static friction between the nail and the substrate. A pile driver achieves the same end, although on a much larger scale, the method being commonly used during civil construction projects to make building and bridge foundations. An impact wrench is a device designed to impart torque impacts to bolts to tighten or loosen them. At normal speeds, the forces applied to the bolt would be dispersed, via friction, to the mating threads. However, at impact speeds, the forces act on the bolt to move it before they can be dispersed. In ballistics, bullets utilize impact forces to puncture surfaces that could otherwise resist substantial forces. A rubber sheet, for example, behaves more like glass at typical bullet speeds. That is, it fractures, and does not stretch or vibrate.
The field of applications of impact theory ranges from the optimization of material processing, impact testing, dynamics of granular media to medical applications related to the biomechanics of the human body, especially the hip- and knee-joints.[2] Also, it has vast applications in the automotive and military industries.[3]
Road traffic accidents usually involve impact loading, such as when a car hits a traffic bollard, water hydrant or tree, the damage being localized to the impact zone. When vehicles collide, the damage increases with the relative velocity of the vehicles, the damage increasing as the square of the velocity since it is the impact kinetic energy (1/2 mv2) which is the variable of importance. Much design effort is made to improve the impact resistance of cars so as to minimize user injury. It can be achieved in several ways: by enclosing the driver and passengers in a safety cell for example. The cell is reinforced so it will survive in high speed crashes, and so protect the users. Parts of the body shell outside the cell are designed to crumple progressively, absorbing most of the kinetic energy which must be dissipated by the impact.
Various impact test are used to assess the effects of high loading, both on products and standard slabs of material. The Charpy test and Izod test are two examples of standardized methods which are used widely for testing materials. Ball or projectile drop tests are used for assessing product impacts.
The Columbia disaster was caused by impact damage when a chunk of polyurethanefoam impacted the carbon fibrecomposite wing of the Space Shuttle. Although tests had been conducted before the disaster, the test chunks were much smaller than the chunk that fell away from the booster rocket and hit the exposed wing.
When fragile items are shipped, impacts and drops can cause product damage. Protective packaging and cushioning help reduce the peak acceleration by extending the duration of the shock or impact.[4]
^Consumer Product Safety Commission. (PDF). Final Rule 16 CFR Part 1203. Archived from the original (PDF) on 24 September 2006. Retrieved 3 December 2014.
^Willert, Emanuel (2020). Stoßprobleme in Physik, Technik und Medizin: Grundlagen und Anwendungen (in German). doi 10.1007/978-3-662-60296-6: Springer Vieweg. doi:10.1007/978-3-662-60296-6. ISBN978-3-662-60295-9. S2CID 212954456.{{cite book}}: CS1 maint: date and year (link) CS1 maint: location (link)
^Islam, Muhammed Kamrul; Hazell, Paul J.; Escobedo, Juan P.; Wang, Hongxu (July 2021). "Biomimetic armour design strategies for additive manufacturing: A review". Materials & Design. 205: 109730. doi:10.1016/j.matdes.2021.109730.
^Package Cushioning Design(PDF). Vol. MIL-HDBK 304C. Department of Defense. 1 June 1997.
Sourcesedit
Goldsmith, W. (1960). Impact: The Theory and Physical Behaviour of Colliding Solids Dover Publications, ISBN0-486-42004-3
Poursartip, A. (1993). Instrumented Impact Testing at High Velocities, Journal of Composites Technology and Research, 15(1).
Toropov, AI. (1998). Dynamic Calibration of Impact Test Instruments, Journal of Testing and Evaluation, 24(4).
October 27, 2023
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This article includes a list of references related reading or external links but its sources remain unclear because it lacks inline citations Please help to improve this article by introducing more precise citations June 2013 Learn how and when to remove this template message In mechanics an impact is when two bodies collide During this collision both bodies decelerate The deceleration causes a high force or shock applied over a short time period A high force over a short duration usually causes more damage to both bodies than a lower force applied over a proportionally longer duration Head impact can cause concussion Sports helmets help protect against brain injury 1 At normal speeds during a perfectly inelastic collision an object struck by a projectile will deform and this deformation will absorb most or all of the force of the collision Viewed from a conservation of energy perspective the kinetic energy of the projectile is changed into heat and sound energy as a result of the deformations and vibrations induced in the struck object However these deformations and vibrations cannot occur instantaneously A high velocity collision an impact does not provide sufficient time for these deformations and vibrations to occur Thus the struck material behaves as if it were more brittle than it would otherwise be and the majority of the applied force goes into fracturing the material Or another way to look at it is that materials actually are more brittle on short time scales than on long time scales this is related to time temperature superposition Impact resistance decreases with an increase in the modulus of elasticity which means that stiffer materials will have less impact resistance Resilient materials will have better impact resistance Different materials can behave in quite different ways in impact when compared with static loading conditions Ductile materials like steel tend to become more brittle at high loading rates and spalling may occur on the reverse side to the impact if penetration doesn t occur The way in which the kinetic energy is distributed through the section is also important in determining its response Projectiles apply a Hertzian contact stress at the point of impact to a solid body with compression stresses under the point but with bending loads a short distance away Since most materials are weaker in tension than compression this is the zone where cracks tend to form and grow Contents 1 Applications 2 Impacts causing damage 3 See also 4 References 4 1 SourcesApplications edit nbsp Crane with a pile driver nbsp 1 2 in 12 7 mm drive pistol grip air impact wrenchA nail is pounded with a series of impacts each by a single hammer blow These high velocity impacts overcome the static friction between the nail and the substrate A pile driver achieves the same end although on a much larger scale the method being commonly used during civil construction projects to make building and bridge foundations An impact wrench is a device designed to impart torque impacts to bolts to tighten or loosen them At normal speeds the forces applied to the bolt would be dispersed via friction to the mating threads However at impact speeds the forces act on the bolt to move it before they can be dispersed In ballistics bullets utilize impact forces to puncture surfaces that could otherwise resist substantial forces A rubber sheet for example behaves more like glass at typical bullet speeds That is it fractures and does not stretch or vibrate The field of applications of impact theory ranges from the optimization of material processing impact testing dynamics of granular media to medical applications related to the biomechanics of the human body especially the hip and knee joints 2 Also it has vast applications in the automotive and military industries 3 Impacts causing damage edit nbsp Mock up of a Space Shuttle leading edge made with an RCC panel taken from Atlantis showing impact damage during a test nbsp Chevrolet Malibu involved in a rollover crashRoad traffic accidents usually involve impact loading such as when a car hits a traffic bollard water hydrant or tree the damage being localized to the impact zone When vehicles collide the damage increases with the relative velocity of the vehicles the damage increasing as the square of the velocity since it is the impact kinetic energy 1 2 mv2 which is the variable of importance Much design effort is made to improve the impact resistance of cars so as to minimize user injury It can be achieved in several ways by enclosing the driver and passengers in a safety cell for example The cell is reinforced so it will survive in high speed crashes and so protect the users Parts of the body shell outside the cell are designed to crumple progressively absorbing most of the kinetic energy which must be dissipated by the impact Various impact test are used to assess the effects of high loading both on products and standard slabs of material The Charpy test and Izod test are two examples of standardized methods which are used widely for testing materials Ball or projectile drop tests are used for assessing product impacts The Columbia disaster was caused by impact damage when a chunk of polyurethane foam impacted the carbon fibre composite wing of the Space Shuttle Although tests had been conducted before the disaster the test chunks were much smaller than the chunk that fell away from the booster rocket and hit the exposed wing When fragile items are shipped impacts and drops can cause product damage Protective packaging and cushioning help reduce the peak acceleration by extending the duration of the shock or impact 4 See also editCharpy impact test Coefficient of restitution Compression physical Cushioning Fall factor Impact driver Impact sensor Impact wrench Impulse physics Izod impact strength test Jerk physics Road traffic accident Shock Shock data logger Tension physics Write offReferences edit Consumer Product Safety Commission Safety Standard for Bicycle Helmets PDF Final Rule 16 CFR Part 1203 Archived from the original PDF on 24 September 2006 Retrieved 3 December 2014 Willert Emanuel 2020 Stossprobleme in Physik Technik und Medizin Grundlagen und Anwendungen in German doi 10 1007 978 3 662 60296 6 Springer Vieweg doi 10 1007 978 3 662 60296 6 ISBN 978 3 662 60295 9 S2CID 212954456 a href Template Cite book html title Template Cite book cite book a CS1 maint date and year link CS1 maint location link Islam Muhammed Kamrul Hazell Paul J Escobedo Juan P Wang Hongxu July 2021 Biomimetic armour design strategies for additive manufacturing A review Materials amp Design 205 109730 doi 10 1016 j matdes 2021 109730 Package Cushioning Design PDF Vol MIL HDBK 304C Department of Defense 1 June 1997 Sources edit Goldsmith W 1960 Impact The Theory and Physical Behaviour of Colliding Solids Dover Publications ISBN 0 486 42004 3 Poursartip A 1993 Instrumented Impact Testing at High Velocities Journal of Composites Technology and Research 15 1 Toropov AI 1998 Dynamic Calibration of Impact Test Instruments Journal of Testing and Evaluation 24 4 Retrieved from https en wikipedia org w index php title Impact mechanics amp oldid 1171262875, wikipedia, wiki, book, books, library,
