Bending strength is defined as the ability of a material to resist deformation under a load. For materials that are significantly deformed but not broken, they are typically reported as load bending strength or bending yield strength in yield measured by 5% deformation / stress of the outer surface. The test beam is under pressure stress on the concave surface and tensile stress on the convex surface.
A similar test to measure flexural strength in the SO system is ISO 178. The values reported in the ASTM D790 and ISO 178 tests rarely differ significantly in the early stages of material selection. These tests also provide the procedure for measuring the flexural modulus of a material (ratio to tensile stress in tensile deformation).
The following table shows the average bending strengths and bending modulus values for some filled and unfilled polymers. These values are a measure of hardness; flexible materials such as elastomers have lower values than fiber-reinforced engineering polymers used as metal substituents such as polyimides or acetals.
Polymer Typical Flexural Strength and Flexural Modulus
|
Polymer Type |
Flexural Strength (MPa) |
Flexural Module (GPa) |
|
ABS |
75 |
2.5 |
|
ABS +% 30 Glass Fiber |
120 |
7 |
|
Acetal Copolymer |
85 |
2.5 |
|
Acetal Copolymer + 30 Glass Fiber |
150 |
7.5 |
|
Acrylic |
100 |
3 |
|
Nylon 6 |
85 |
2.3 |
|
Polyamide-imide |
175 |
5 |
|
polycarbonate |
90 |
2.3 |
|
Polyethylene, MDPE |
40 |
0.7 |
|
Polyethylene Terephthalate (PET) |
80 |
1 |
|
polyimide |
140 |
3 |
|
Polyimide + Glass Fiber |
270 |
12 |
|
Polypropylene |
40 |
1.5 |
|
polystyrene |
70 |
2.5 |
Better quality plastics are generally more flexible.
There is no standard value for all types of plastics.
The four tests listed below are useful in comparing these resins and their flexibility to plastic.
- Impact test: beaten with a plastic device. This test will tell you how hard the plastic is. Higher toughness means zor more difficult to break ”.
Usually from this test, we can also say whether it is broken as a rupture or if there is elongation before rupture. For specific applications, this is an important feature to test.
- Fatigue test: Plastics are flexible back and forth until they break. The higher the number of cycles, the better the property of the plastic.
- Destructive tensile test: This test will tell you that the plastic is broken by longer elongation (better quality plastic) or short elongation (lower quality).
- UV resistance impact test: the plastics are placed in a UV chamber and then an impact test is performed (see No.1 above). What is interesting is the difference between the effect test results with and without UV exposure.