Damage Tolerance A design measure of crack growth rate. Cracks in damage tolerant designed structures are not permitted to grow to critical size during expected service life. Growth of a crack to critical size would be expected to cause failure of a structure under continued or subsequent loading.
Deflect Bend or twist under load.
Deformation A change in the shape of a structure or structural element caused by a load or force acting on the structure.
Deformation Energy Energy required to deform a material a specified amount. It is the area under the stress-strain diagram up to a specified strain, as shown in the following diagram.
Deformation under load Measure of the ability of rigid plastics to withstand permanent deformation and the ability of nonrigid plastics to return to original shape after deformation. Standard test methods for determining both types of deformation under load are given in ASTM D-621. For rigid plastics, deformation (which can be flow or flow and shrinkage) is re-ported as % change in height of specimen after 24 hours under a specified load. For nonrigid plastics, results are reported as % change in height after 3 hours under load and recovery in the 1-1/2 hour period following removal of the load. Recovery is % increase in height calculated on basis of original height. Crushing Strength.
Degassing (breathing) The opening and closing of a mould to allow gases to escape early in the molding cycle. Also called Breathing. When referring to plastic sheeting, "breathing" indicates permeability to air.
Denier The unit of linear density equal to the mass in grams per 9000m of fiber, yarn, or other textile strand.
Density Compactness.
Density and Specific Volume Density is the measure of the weight per unit volume of material at 23ºC (73ºF) usually expressed as grams per cubic centimeter (gm/cm3) or as pounds per cubic inch (lb/in3). Density and specific gravity are used interchangeably, which is formally incorrect.
Specific Gravity
Specific gravity is the ratio of the weight of a given volume of material compared to an equal volume of water, both measured at 23ºC (73ºF).
Specific gravity is useful to the designer in calculating cost-weight and strength-weight ratios. The often-used English term “relative density” has the same meaning as “specific gravity.” The difference is the following: density is mass per unit volume of a material at 73°F (23°C); specific gravity is the mass of a given volume of material at 73°F (23°C) divided by an equal volume of water at the same temperature.
Density and Specific Volume of Various Thermoplastics1
| . |
Density
|
Specific Volume
|
|
Resin
|
S.I. g/cm
|
English lb/in3
|
Metric g/cm
|
S.I. cm/g
|
English in/lb
|
Metric cm/g
|
|
ABS
|
1.05
|
0.038
|
1.05
|
0.95
|
26.3
|
0.95
|
|
Acetal
|
1.40
|
0.051
|
1.40
|
0.71
|
19.7
|
0.71
|
|
Acrylic
|
1.16
|
0.042
|
1.16
|
0.87
|
23.8
|
0.86
|
| HDPE |
0.96
|
0.035
|
0.96
|
1.04
|
28.8
|
1.04
|
| LDPE |
0.92
|
0.033
|
0.92
|
1.09
|
20.1
|
1.09
|
| Nylon |
1.15
|
0.042
|
1.15
|
0.87
|
24.0
|
0.87
|
| PBT |
1.30
|
0.047
|
1.30
|
0.77
|
21.3
|
0.77
|
| PC |
1.20
|
0.043
|
1.20
|
0.83
|
23.0
|
0.83
|
| PP |
0.90
|
0.033
|
0.90
|
1.10
|
30.7
|
1.10
|
| PS |
1.05
|
0.038
|
1.05
|
0.95
|
26.3
|
0.95
|
| SAN |
1.08
|
0.039
|
1.08
|
0.93
|
25.6
|
0.93
|
1 Typical average values are shown. Consult literature for exact values of specific resin grades.
Density, bulk The weight per unit volume of a material, including voids inherent in material as tested.
Deployment The act of arranging, placing, or moving strategically, as in the optimal deployment of mass to absorb stress.
Designer Material Engineered material (see Materials Science).
Die Any of various devices for forming materials in a press or a stamping or forging machine.
Dihedral Angle The dihedral angle of a polyhedron is the angle between two faces.
Distributed Mass The precision distribution of mass in FCC alignment in an RMT architecture.
Distributed Porosity The precision distribution of voids in a mass in FCC alignment in an RMT architecture.
Dome An arch rotated about its vertical axis to produce a structure shaped like an inverted bowl.
Dual Two polyhedra are duals if the vertices of one can be put into a one-to-one correspondence with the centers of the faces of the other.
Ductility Ability of metals and alloys to retain strength and freedom from cracks when shape is altered.
Durability Able to exist for a long time without significant deterioration.
Dynamic Creep Creep that occurs under fluctuating load or temperature.