Charpy vs Izod, tensile vs flexural modulus, dry vs conditioned, XY vs Z: those details are not footnotes. They change what a number means.
WORKPIECE ENGINEERING TERMS
See what the numbers actually mean.
Engineering properties describe different failure modes. The important question is not only “what is the value?” but also “what test produced it, in what direction, over what time, and what would the part actually do?”
A tensile test may take minutes. Creep, stress relaxation, fatigue and environmental aging can change a part over months or years.
Geometry, notches, thickness, print path and load direction determine how material properties become real deflection, damage or failure.
VISUAL ENGINEERING GLOSSARY
Learn the property through the test and the part.
Key terms use synchronized test-rig animations. The graph and the specimen share one state, so what you see on the curve corresponds to what the part is doing.
INTERACTIVE TEST RIG
Follow a ductile material through a tensile test
This is a conceptual engineering stress–strain curve. The marker is mathematically constrained to the plotted SVG path. The dogbone specimen and the functional cantilever below it are driven from the same test progress.
CURRENT REGIONElasticDeformation is recoverable.
Gauge length1.00×Sectionuniform
Conceptual ductile response; exact curve shape and yield definition depend on material and test standard.
SAME MATERIAL STATE · DIFFERENT GEOMETRYWhat would a loaded bracket do?
The cantilever is not the tensile specimen. It is a conceptual translation from material state to part behavior: elastic bending is recoverable; once yield is crossed, an unloaded part can retain permanent set.
MASS / VOLUME
Density
Density is mass divided by volume. For equal part volume, a higher-density material produces a heavier part.
What it does not tell you
Density alone says nothing about strength, stiffness or impact resistance. Those are separate properties.
ρ = m / VLOAD / AREA
Stress
Stress is internal force divided by the loaded area. It converts an external force into the intensity of loading carried by the material.
Why geometry matters
The same force produces more stress in a thinner section. Holes, notches and sharp corners can create local stresses much higher than the nominal average.
σ = F / ADEFORMATION / ORIGINAL LENGTH
Strain
Strain is deformation relative to the original dimension. It is dimensionless and is often reported as a percentage.
Stress vs strain
Stress describes how intensely the material is loaded. Strain describes how much it deforms as a result.
ε = ΔL / L₀RECOVERABLE
Elastic deformation
Elastic deformation disappears when the load is removed. A part can deflect substantially and still be elastic if it returns to its original geometry.
Connection to the test lab
In the initial stress–strain region, unloading approximately retraces the elastic slope back toward zero strain.
PERMANENT
Plastic deformation
Plastic deformation remains after the load is removed. A part may still be intact but can already have failed functionally because alignment, fit or preload changed permanently.
Yield is the boundary
Plastic strain begins once the material passes its yield criterion. Different standards may define yield in different ways, including offset methods.
START OF PERMANENT DEFORMATION
Yield strength
Yield strength marks the onset of significant permanent deformation under the chosen test definition. For many ductile components, service stress is kept below yield so dimensions and alignment remain recoverable.
Not fracture
Open the synchronized tensile test ↑Yield does not mean the specimen breaks. A ductile material may carry considerably more load after yielding before it reaches UTS and then fractures.
Before yield
Elastic strain dominates. Unload → shape recovers.
At / beyond yield
Plastic strain accumulates. Unload → permanent set remains.
MAXIMUM ENGINEERING TENSILE STRESS
Ultimate tensile strength (UTS)
UTS is the maximum engineering stress reached in a tensile test. In a typical ductile material it occurs after yield and strain hardening, around the onset of localized necking.
Why the curve may fall after UTS
Watch necking in the tensile test ↑Engineering stress divides force by the original area. Once necking localizes deformation and load capacity falls, the engineering curve drops even though the local true stress can behave differently.
Yield
Permanent deformation begins.
UTS
Peak engineering stress.
Fracture
Final separation; not necessarily at UTS.
DUCTILITY
Elongation at break
The percentage increase in a specified gauge length when the specimen fractures. High elongation generally indicates the ability to undergo substantial tensile deformation before separation.
Strength and ductility are different
A material may have high strength and low elongation, or modest strength and very high elongation. Both matter to toughness and failure style.
ELASTIC STIFFNESS
Young’s modulus / tensile modulus / elastic modulus
For uniaxial tension, Young’s modulus is the slope of the initial elastic stress–strain response. A steeper slope means more stress is required to create the same strain: the material is stiffer.
Use the test-specific modulus
“Elastic modulus” is a broader family. Flexural, shear and compressive moduli come from different loading modes and are not automatically interchangeable.
E = Δσ / ΔεENERGY TO FRACTURE
Toughness
In a tensile context, toughness is the energy per unit volume absorbed before fracture—the area under the complete stress–strain curve.
Not the same as impact strength
Tensile toughness and pendulum impact tests both involve energy absorption, but they use very different loading rates, geometries and test definitions.
CONSTANT LOAD · DEFORMATION GROWS WITH TIME
Creep
Creep is time-dependent deformation under sustained stress. The load in this rig stays constant while both the curve marker and the cantilever deflection advance together.
CURRENT REGIONInstantaneous + primaryInitial elastic deformation followed by a decreasing creep rate.
*A full creep-to-rupture sequence can develop under sufficiently severe conditions; not every service creep test reaches tertiary creep.
Why short-term strength is not enough
A polymer bracket can survive a quick tensile or bending load yet continue to sag under a much smaller constant load. Long-term design needs the actual time, stress, temperature and conditioning context.
CONSTANT DEFORMATION · FORCE FALLS WITH TIME
Stress relaxation
Stress relaxation is the counterpart to creep. Hold a viscoelastic material at a fixed deformation and the stress required to maintain that deformation can decrease over time.
Common consequence
Plastic clips, seals and elastomer preloads can lose clamping force even when their installed displacement does not change.
REPEATED LOAD CYCLES
Fatigue
Fatigue is progressive damage under repeated or fluctuating loading. Failure can occur after many cycles at stress levels well below one-time tensile strength.
Look for local stress raisers
Holes, sharp corners, layer interfaces, scratches and notches are common places for fatigue cracks to start.
SUSTAINED LOAD TO FAILURE
Creep rupture
Under sufficiently severe sustained stress, time-dependent deformation can progress to damage and eventual fracture. Time-to-rupture depends strongly on stress, temperature, environment and material state.
Design implication
The long-life allowable stress can be far below short-duration tensile strength.
Short duration
High apparent strength may be available.
Long duration
Lower stress can still produce unacceptable deformation or rupture.
FAST ENERGY ABSORPTION
Impact strength — simplified Charpy rig
The specimen rests on two anvils. The pendulum striker hits the midpoint on the face opposite the notch. The difference between entry and exit height represents energy absorbed by the specimen/test system.
TEST STATERaised pendulumKnown starting potential energy.
Illustrative geometry; exact Charpy dimensions and method are standardized.
Comparison rule
Do not compare Charpy directly with Izod, or notched with unnotched values, as if they were one scale. Fixture, notch, specimen geometry, temperature and orientation all matter.
STRESS CONCENTRATION
Notched vs unnotched impact
A notch deliberately creates a stress concentration and makes crack initiation easier. The same polymer can therefore show very different notched and unnotched impact results.
Part-design translation
Sharp internal corners, scratches, holes and machining damage can make a real part behave more like a notched specimen.
LOCAL INDENTATION / SCRATCH RESISTANCE
Hardness
Hardness describes resistance to localized indentation or scratching under a specified test. Shore A/D, Rockwell and ball-indentation methods use different indenters and scales.
Not stiffness
A hard surface can still belong to a flexible material, and a stiff material can still fracture in a brittle way.
POLYMER MOBILITY
Glass transition temperature (Tg)
Tg is a transition range where the amorphous portion of a polymer gains much greater molecular mobility. Stiffness can drop and creep can accelerate as temperature approaches and exceeds this region.
Not a service-temperature rating
Tg alone does not tell you whether a loaded part is safe at a given temperature. Geometry, load, time and properties such as HDT or creep modulus may be more directly useful.
glassy / stiffTg rangemore mobile / compliant
HEAT + SPECIFIED BENDING STRESS
Heat deflection temperature (HDT)
HDT is the temperature where a standardized specimen reaches a specified deflection under a specified flexural stress. The applied stress and method belong with the reported temperature.
Why “HDT 100°C” is incomplete
Different load levels can produce different HDT values for the same material.
INDENTATION WHILE HEATING
Vicat softening temperature
Vicat records the temperature at which a loaded needle penetrates a thermoplastic specimen to a specified depth while temperature rises.
Different from HDT
Vicat is an indentation-based softening test; HDT is a flexural deflection-under-load test. They answer different questions.
DIMENSION VS TEMPERATURE
Coefficient of thermal expansion (CTE)
CTE describes how much a material changes dimension per degree of temperature change. Plastics often expand much more than metals.
Mixed-material assemblies
Different expansion rates can create binding, looseness, warping or thermal stress.
ΔL ≈ α L₀ ΔTMOISTURE ABSORPTION
Hygroscopicity / moisture conditioning
Hygroscopic polymers absorb water from the environment. Moisture can change dimensions and mechanical response, and in FDM it can also degrade print quality during extrusion.
Dry and conditioned data can differ
Nylons are a key example: datasheets may report meaningfully different mechanical values in dry-as-molded and moisture-conditioned states.
LOAD PATH THROUGH DEPOSITED ROADS
Layer adhesion and Z-strength retention
FFF/FDM parts are direction-dependent structures. This rig compares a load carried largely along continuous roads with a Z-direction load that must cross many deposited interfaces.
KEY IDEASame material, different load pathZ/XY tensile retention is useful only when both directions were tested comparably.
The animation illustrates load paths, not a universal numerical adhesion coefficient.
How Workpiece reports it
Where comparable supplier tensile results exist, the Materials Guide reports Z tensile ÷ XY tensile as Z-strength retention. It remains an orientation-strength indicator, not a transferable material constant.
Z-strength retention = σZ / σXY
DIRECTION-DEPENDENT PROPERTIES
Anisotropy
An anisotropic material behaves differently depending on direction. In FDM, bead orientation, interfaces, void geometry and part build orientation create different mechanical responses along different axes.
Orientation is a design decision
Changing build orientation can change support needs, surface finish, dimensional accuracy and mechanical load path at the same time.
BEAD / TOOLPATH DIRECTION
Raster or bead orientation
Raster orientation is the direction of deposited roads within a layer. Continuous roads aligned with the main tensile load can carry load differently from roads oriented across it.
Not only infill angle
Perimeters, seams, holes, local toolpaths and solid regions determine the actual path through which force is transmitted.
INTERNAL PRINTED STRUCTURE
Infill
Infill is the internal structure between outer walls. Density and pattern affect mass, local stiffness, buckling and load transfer, but more infill does not automatically solve a weak Z-direction interface.
Perimeters often dominate
For many loaded parts, wall count, local reinforcement and continuous load paths matter at least as much as the headline infill percentage.
BUILD DIRECTION
Print orientation
Print orientation sets how a part’s geometry sits relative to XY deposition and Z stacking. It changes both process behavior and the directions in which critical service loads cross deposited interfaces.
For loaded parts
Avoid placing a critical tensile load straight across weak interfaces unless the actual process, material and geometry have been validated for it.
READING MATERIAL DATA RESPONSIBLY
Keep the test context attached to the property.
Modulus, yield, UTS and elongation come from a tensile response but describe different parts of that response. A single strength number cannot replace the curve.
Time, stress, temperature and conditioning are part of the result. Short-duration tensile strength does not predict long-term deformation by itself.
Charpy/Izod, notch geometry, specimen geometry, temperature and orientation must remain attached to the number.
XY/Z comparisons describe printed structure and process conditions as much as polymer chemistry. They are not universal bulk-material constants.
Animations are conceptual teaching diagrams, not test simulators and not design allowables. Exact specimen geometry, strain measurement, yield definition and test conditions are governed by the applicable standard and material datasheet.