Impact Properties:
1. Tensile and flexural testing, the material absorbs energy slowly. materials often absorb applied forces very quickly: falling objects, blows, collisions, drops etc. The purpose of impact testing is to simulate these conditions.
2. The impact properties of the polymeric materials are directly related to the Overall toughness of the material.
3. Toughness is defined as the ability of the polymer to absorb applied energy.
4. The area under the stress-strain curve is directly proportional to the toughness of a material.
5. The impact resistance is the ability of a material to resist breaking under a shock loading.
There are basicallyy four types of failures encountered due to impact load.
Brittle Fracture -The product fractures extensively without yielding
Slight Cracking -The product shows evidence of slight cracking and yielding without losing its shape.
Yielding -The product yields showing formation and stress whitening.
Ductile Failure -A definite yielding of material along with cracking.
TEST METHOD
Test Method for Impact Resistance of Plastics & Electrical Insulating Material
(ASTM D 256 A & B), ASTMD1822, JISK-7111 &7112
The impact test methods are as following:
(11) Pendulum impact tests
(i) Izod impact test
(ii) Charpy impact test
(iii) Chip impact test
(iv) Tensile impact test
(2) High-rate tension test
(3) Falling weight impact test
(a) (a) Drop weight (top) impact test
(4) Instrumented impact tests
(5) High- rate impact testers.
(a) High speed ball impact tester
(b) (b) High speed pllunger impact tester
(6) Miscellaneous impect test.
ASTM D 256, ISO‐R‐180, BS‐2782
Method 306 A, DIN 53453, JIS‐K 7110
IMPACT TEST: Impact test is a method of determining the behavior of material subjected to shock loadingg in bending or tension. The quantity usuallyy measured is the energy absorbed in fracturing in a single blow.
IMPACT STRENGTH: Energy required fracturing a specimen subjected to shock loading.
Unit : J/m
SIGNIFICANCE
(11) The excess energy pendulum impact test indicates the energy to break std. Test specimen of specified size under stipulated conditions of specimen mounting, notching and pendulum velocity at impact.
(22) The energy lost by the pendulum during the breakage of the specimen is the sum of
energy required,
(i) To initiate fracture of the specimen.
(ii) To propagate the fracture across the specimen.
(iii) To through the free end of the broken specimen.
(iv) To bend the specimen.
(v) To produced vibration in the pendulum arm
(vi) To produced vibration ‘or’ horizontal movement of the machine frame ‘or’ base.
(vii) To overcome friction in the pendulum bearing and in the excess energy indicating mechanism and to overcome pendulum air drag (wind age).
(viii) To indent ‘or’ deformed plastically the specimen at the line.
1. Impact properties can be very sensitive to test specimen thickness and molecular orientation. The differences in specimen thickness as used in ASTM and ISO methods methods may may affect affect impact impact values values strongly strongly.
2. A change from 3 to 4 mm thickness can even provide a transition in the failure mode from ductile to brittle behaviour ‐ through the influence of molecular weight and specimen thickness on Izod notched impact.
3. Materials already showinga brittle fracture mode in 3 mm thickness – such as mineral and glass filled grades ‐ will not be affected. Neither will impact modified materials.
CHARPY IMPACT STRENGTH
Test Test Method Method:: ASTM ASTM DD 6110 6110, ISO ISO-RR-179 179, BSBS-2782 2782
Method 306 B & 307
Charpy impact is less common in US but is widely used in Europe. The test is identical to Izod test except that the specimen is a simply supported beam that is impacted, midlong between the supports.
Specimen Size: 12.7 x 6.4 x 127.0 mm
The main difference between Charpy and Izod tests is the way the test bar is held. In Charpy testing the specimen is not clamped, but lies freely on the support in a horizontal position.
FORMULA AND CALCULATIONS
Energy required breaking the sample (J)
Impact strength (J/m) (Izod / Charpy)= ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
Thickness (m)
Dart Impact Test=
Calculate Wf = WL ‐ [ ΔW (S/100 – ½)]
Where,
Wf = impact failure weight, gms,
ΔW = uniform weight increment used, gms,
ΔW used L = lowest missile weight, gms, according to the particular ΔW used, at which 100% failure occurred and
S = sum of the percentages of breaks at each missile weight (from a weight corresponding to no failures upto and including WL )
Factors Affecting Impact Results:
Rate of loading: At low rates high impact strength vice versa.
Notch Sensitivity: A notch in a test specimen drastically lowers the impact strength. Radius of notch and angle of notch.
Temperature: Higher the temperature more impact strength.
Orientation: The impact strength is highe
r in the direction of flow.
Temperature and Humidity: Recommended Temperature and Humidity is 23℃ and 55 –65%. Flexural Strength decreases as Temperature increases. Moisture workks as plasticizer, so it causes then increase in impact.
Method of specimen Preparation: Injection moulded specimens will have higher value than the specimen compression specimen. Molecular Orientation has a significant effect on impact strength values. A load-applied parallel to the direction of molecular orientation may yield higher value than the load applied perpendicular to the orientation.
Impact Properties
Falling-Weight Impact Test
1. The fallingg impact test, also known as the drop impact test or the variable-height impact test, employs a falling weight.
2. This falling weight is a tip with a conical nose, a ball, or a ball-end dart.
3. The energy required to fail the specimen is measured by dropping a known weight from a
known height onto a test specimen.
4. This test is also very suitable for determining the impact resistance of plastic films, sheets and laminated materials.
Three basic ASTM tests are commonly used depending upon the application:
ASTMD 3029 - Impact resistance of rigid plastics sheeting
ASTMD 1709 - Impact resistance of poly ethylene film by the free falling dart method
ASTMD 244 - Test for impact resistance of thermoplastics pipe and fittings by means of a tip.
Impact Properties
DROP IMPACT TEST
1. The test is carried out by raising the weight to a desired height manually or automatically with the use of motor-driven mechanism & allowing it to fall freely on to the others side of the round- nosed punch.
2. The punch transfers the impact energy to the flat test specimen, which is positioned, on a cylindrical die or a part lying on the base of the machine.
3. The kinetic energy is possessed by the falling weight at the instant of impact is equal to the energy used tto raise to the height of drop and is the potential energy possessed by the weight as it is released.
4. Since the potential energy is expressed as the product of weight and height, the guide tube is marked with a linear scale representing the impact range of the instrument in in-lb.
5. Thus, the toughness or the impact strength of a specimen or a part is read directly off the calibrated scale in in-lb.
6. The energy loss due to the friction in the tube or due to the momentary acceleration of the punch is negligible.
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