Defects, Metrology and Material Characterization

Introduction and Context

Additive manufacturing (AM) produces components layer by layer from digital geometry. This enables complex shapes, internal channels, lattice structures, and customized products, but it also introduces defect mechanisms that differ from conventional machining, casting, and forming. A part may conform closely to its nominal computer-aided design (CAD) model and still fail because of lack-of-fusion pores, residual stress, unmelted particles, weak interlayer bonding, surface-connected defects, or anisotropic microstructure.

Quality assurance in AM therefore requires more than visual inspection. It combines dimensional metrology, surface measurement, microscopy, density determination, computed tomography (CT), and mechanical testing. These techniques answer different questions:

• Dimensional inspection: Was the intended geometry produced?
• Surface metrology: Is the texture suitable for function, sealing, fatigue life, or subsequent processing?
• Microscopy: What phases, grains, pores, and bonding features exist?
• Density measurement and CT: How much internal porosity exists, and where is it located?
• Mechanical testing: Does the material possess the required strength, ductility, toughness, and fatigue resistance?

A reliable assessment links the observed result to process history, material behavior, and intended service conditions. No single measurement is sufficient for every AM part.

Learning Objectives

By the end of this chapter, the learner should be able to:

1. Classify common AM defects and explain their formation mechanisms.
2. Distinguish geometric error, surface error, material defect, and process-induced damage.
3. Select suitable dimensional, surface, microscopic, density, CT, and mechanical tests.
4. Calculate basic density, dimensional error, roughness, and mechanical properties.
5. Explain anisotropy, residual stress, microstructure, and fatigue behavior in AM components.
6. Interpret inspection results while recognizing measurement uncertainty and method limitations.
7. Develop a practical inspection plan for an AM component.

1. Defect Formation in Additive Manufacturing

1.1 What is a defect?

A defect is a condition that causes a part to depart from a specified requirement or reduces its expected performance. A discontinuity is not automatically a defect: a small pore may be acceptable in a noncritical polymer housing but unacceptable in a pressure vessel or fatigue-loaded aerospace bracket. Acceptance must therefore be related to function, material, process capability, and applicable standards.

AM defects can be grouped into four broad classes:

1.2 Defects in powder-bed fusion

In powder-bed fusion, a heat source selectively melts or sinters regions of a powder layer. Defects depend on laser or electron-beam power, scan speed, beam diameter, hatch spacing, layer thickness, powder characteristics, shielding atmosphere, and thermal history.

Lack of fusion occurs when adjacent tracks or layers do not bond sufficiently. It is favored by low energy input, excessive scan speed, large hatch spacing, thick layers, poor powder spreading, or contamination. Lack-of-fusion voids are often irregular and elongated, and their planar orientation can make them particularly damaging under tensile or fatigue loading.

Keyhole porosity forms when energy density is high enough to create a deep vapor depression. Unstable keyhole walls can trap gas bubbles as the melt pool solidifies. These pores are often more rounded than lack-of-fusion defects, but their effect depends on size, location, and proximity to the surface.

A useful process-monitoring quantity is volumetric energy density:

where E_v is nominal volumetric energy density in J/mm³, P is beam power in W, v is scan speed in mm/s, h is hatch spacing in mm, and t is layer thickness in mm. The units are consistent because W = J/s and the denominator has units mm³/s. This is an approximate comparative indicator, not a universal predictor of quality. Different beam profiles, scan strategies, absorptivity, preheating conditions, and materials can produce different outcomes at the same nominal value.

Balling is the breakup of a melt track into discontinuous beads. It may result from poor wetting, excessive scan speed, unstable melt-pool behavior, or unsuitable thermal conditions. Balling creates rough surfaces and incomplete bonding.

Recoater interference occurs when a raised or warped region collides with the recoater. It may damage the part, disturb powder distribution, or terminate the build. Causes include thermal distortion, inadequate support, excessive residual stress, or a prior defect.

1.3 Defects in directed energy deposition

Directed energy deposition (DED) feeds powder or wire into a melt pool created by a focused heat source. Typical defects include lack of fusion between deposited beads, excessive dilution into the substrate, porosity from entrapped gas, cracking, underfill, overbuild, and geometrical waviness. Feed rate, travel speed, powder catch efficiency, shielding, and interpass temperature are important variables.

DED repair operations require special attention to the interface between original material and deposited material. Inadequate cleaning, oxide layers, or inappropriate thermal control can create a weak repair boundary.

1.4 Defects in material extrusion

Material extrusion deposits a polymer filament or pellet through a heated nozzle. Common defects include under-extrusion, over-extrusion, stringing, warpage, voids between roads, poor layer adhesion, nozzle blockage, and thermal degradation. Interlayer strength is often lower than in-road strength because bonding depends on polymer diffusion and the thermal history of adjacent roads.

Warping results from differential contraction as deposited material cools. Large flat parts, sharp corners, and excessive thermal gradients increase risk. A brim, raft, heated build platform, enclosure, suitable build orientation, and controlled cooling may reduce it.

1.5 Defects in vat photopolymerization

Vat photopolymerization can produce trapped resin, incomplete curing, support scars, delamination, distortion, and dimensional changes caused by polymerization shrinkage. Excessive exposure may close small holes or enlarge positive features, whereas insufficient exposure can cause weak layers and failed supports. Washing and post-curing are part of the process, not merely cosmetic finishing steps; they affect mass, dimensions, conversion, and mechanical properties.

1.6 Defect formation and failure mechanisms

The effect of a defect depends on its size, shape, orientation, location, and surrounding microstructure. A sharp crack-like discontinuity produces a stronger stress concentration than a smooth spherical pore. A surface-connected pore is generally more harmful than an isolated pore because it can initiate cracking and permit corrosion or leakage.

Residual stress is generated when different regions undergo nonuniform heating and cooling. If the elastic strain exceeds the local ability of the material to accommodate it plastically or through other mechanisms, distortion or cracking may occur. Residual stress can also cause distortion after support removal or heat treatment.

For a uniaxial tensile test, engineering stress and strain are defined as:

where σ is engineering stress in Pa or MPa, F is applied force in N, A₀ is original cross-sectional area in m², ε is engineering strain, L₀ is original gauge length, and L is the current gauge length. These definitions are appropriate for standard tensile reporting before the effects of large deformation make true stress and true strain more informative.

Defects reduce the effective load-bearing area and initiate local plasticity. Under cyclic loading, a defect can initiate a fatigue crack even when the nominal stress is below the static yield strength. Consequently, static tensile strength alone is not sufficient to qualify a fatigue-critical AM component.

2. Metrology and Measurement Strategy

2.1 Measurement principles

Metrology is the science of measurement. An AM inspection plan should begin with the functional requirements of the part, not with a preferred instrument. The plan should define:

1. Measurands: dimensions, form, position, roughness, density, pore size, hardness, or strength.
2. Reference datums and coordinate system.
3. Instrument capability and calibration status.
4. Sampling locations and number of parts.
5. Environmental conditions.
6. Acceptance criteria and decision rules.
7. Measurement uncertainty and traceability.

A result should be reported with units, method, instrument, sampling information, and uncertainty where relevant. Calibration confirms the relationship between an instrument indication and a reference; it does not guarantee that the measurement process is suitable for the application.

2.2 Dimensional inspection

Common tools include calipers, micrometers, height gauges, coordinate measuring machines (CMMs), structured-light scanners, laser scanners, optical microscopes, and industrial CT. Calipers are efficient for accessible dimensions but may not adequately control contact force or complex form. A CMM provides accurate point measurements relative to datums, while optical scanning captures dense surface data and is useful for comparison with CAD.

A simple dimensional deviation is:

and percentage deviation is:

where dimensions must use the same unit. Percentage deviation is meaningful only when the nominal dimension is nonzero and sufficiently large relative to measurement uncertainty.

For freeform parts, a color map of the scanned surface may reveal systematic shrinkage, local warpage, support-related distortion, or staircase effects. Alignment is critical: a poor registration between scan and CAD can create apparent errors that are actually coordinate-system errors. Datum-based alignment is preferable when functional datums exist.

2.3 Surface metrology

Surface texture includes roughness, waviness, and form. Roughness is the short-wavelength component, waviness is a larger-scale undulation, and form is the overall geometric departure. AM surfaces are influenced by layer thickness, orientation, contour strategy, powder size, partially fused particles, stair stepping, and post-processing.

The arithmetic mean roughness, commonly denoted R_a, is:

where z(x) is the filtered profile height relative to the mean line and L is the evaluation length. In discrete measurements:

The filtering method, cutoff length, measurement direction, stylus radius or optical resolution, and surface cleanliness must be reported. R_a alone cannot describe isolated deep valleys, directional texture, or functional bearing behavior. Parameters such as R_z, skewness, kurtosis, and areal parameters may be needed.

2.4 Surface and dimensional error are different

A part may have correct overall dimensions but a rough surface that prevents sealing. Conversely, polishing may improve roughness while removing material and changing dimensions. Inspection should therefore measure both geometry and texture, especially for fits, fluid channels, fatigue surfaces, and biomedical interfaces.

3. Microscopy and Microstructure

3.1 Optical microscopy

Metallographic examination usually involves sectioning, mounting, grinding, polishing, and etching. Optical microscopy reveals melt-pool boundaries, layer structure, porosity, cracks, inclusions, and grain-scale features within its resolution range. The section plane must be documented because a two-dimensional section cannot fully represent a three-dimensional pore network.

3.2 Scanning electron microscopy

Scanning electron microscopy (SEM) provides higher-resolution imaging and may be combined with energy-dispersive X-ray spectroscopy (EDS). SEM can distinguish fracture modes, examine unmelted particles, reveal lack-of-fusion morphology, and identify elemental contrasts. EDS is semi-quantitative and has interaction-volume limitations; it should not be treated as an exact bulk chemical analysis without appropriate standards and preparation.

3.3 Microstructure and anisotropy

Rapid, directional solidification in AM often produces columnar grains, crystallographic texture, and epitaxial growth across layers. Thermal gradients and scan rotation influence grain morphology. Consequently, properties measured parallel to the build direction may differ from properties measured transverse to it.

Anisotropy may occur in yield strength, ductility, fracture toughness, fatigue life, elastic response, thermal conductivity, and dimensional change. It is controlled by process parameters, build orientation, scan strategy, support conditions, powder chemistry, and heat treatment. Mechanical qualification should test orientations representative of service loading rather than assuming isotropic behavior.

3.4 Fractography

Fractography examines the fracture surface to identify failure initiation and propagation. Ductile failure may show dimples, while brittle or cleavage-dominated failure shows relatively flat facets. Fatigue commonly produces a crack-origin region and progressive markings. In AM parts, pores, lack-of-fusion regions, surface notches, inclusions, and unmelted particles are frequent crack origins.

4. Density Measurement and Computed Tomography

4.1 Density methods

The bulk density of a specimen is:

where ρ is density in kg/m³ or g/cm³, m is mass, and V is external volume. In a simple comparison with a theoretical fully dense material:

This assumes that the theoretical density is appropriate for the material composition and phase condition. Relative density is not identical to pore volume fraction when composition, closed pores, surface roughness, or measurement errors are significant.

Archimedes methods are useful for suitable specimens that can be fully wetted and do not absorb the immersion liquid. Gas pycnometry measures skeletal volume and can respond differently to open and closed porosity. The selected method must match the question being asked.

Worked example. A metal coupon has mass 24.60 g and externally measured volume 3.00 cm³. Its bulk density is:

ρ = 24.60 / 3.00 = 8.20 g/cm³.

If the theoretical density is 8.30 g/cm³, relative density is 100 × 8.20 / 8.30 = 98.8% to three significant figures. The missing 1.2% should not automatically be interpreted as exactly 1.2% porosity because measurement uncertainty and composition may contribute.

4.2 Computed tomography

Industrial CT reconstructs internal structure from many X-ray projections. It can reveal enclosed pores, lack-of-fusion defects, internal channels, inclusions, and dimensional deviations without destroying the part. Voxel size, magnification, X-ray energy, detector performance, artifacts, beam hardening, scattered radiation, segmentation threshold, and part size affect results.

CT results should be validated when decisions are critical. Small defects near the resolution limit may be missed or inaccurately sized. High-density materials and thick sections can cause inadequate penetration. CT is powerful but not automatically ground truth.

CT data support several analyses:

• defect volume and equivalent diameter;
• defect position and orientation;
• minimum distance to the external surface;
• channel blockage and wall thickness;
• comparison between as-built geometry and CAD;
• spatial correlation between defects and fracture origins.

A spherical-equivalent diameter is sometimes used to report irregular defect size:

where V is defect volume. This is a reporting convention; it does not imply that the defect is spherical or that its mechanical effect is equivalent to a sphere.

5. Mechanical Characterization

5.1 Tensile testing

A tensile test can provide elastic modulus, yield strength, ultimate tensile strength, elongation, and reduction of area. Specimen geometry, build orientation, surface condition, heat treatment, machining, and test standard must be controlled. AM test bars should be traceable to build location and orientation.

The elastic modulus in the approximately linear region is:

where E is Young’s modulus, ∆σ is the stress change, and ∆ε is the corresponding strain change. The relationship applies only within a selected elastic range and assumes valid strain measurement and appropriate alignment.

Yield strength is often determined using a proof strain, such as 0.2% for metals, when a clear yield point is absent. The selected convention must be stated.

5.2 Hardness and impact behavior

Hardness testing is useful for mapping local variation, heat-treatment effects, and build-to-build consistency. It is not a direct substitute for tensile or fatigue testing. Indent size, surface preparation, specimen thickness, spacing, and proximity to pores influence results.

Impact tests assess energy absorbed during a rapid, notched fracture event. They can reveal sensitivity to notch effects and temperature, but impact energy should not be treated as identical to fracture toughness.

5.3 Fatigue

Fatigue is particularly important for AM because rough surfaces and internal defects act as crack initiators. A stress-life relationship is often represented by a Basquin-type form:

where σ_a is stress amplitude, σ′_f is a fitted fatigue-strength coefficient, N_f is cycles to failure, and b is a fitted exponent. The equation is empirical, applies over the tested regime, and requires consistent definitions of stress ratio, specimen condition, and failure criterion. It should not be extrapolated carelessly beyond the data range.

Fatigue life is affected by surface roughness, pore size, defect morphology, residual stress, mean stress, environment, build orientation, and post-processing. Machining and hot isostatic pressing may improve performance, but their effects must be verified experimentally because they can alter dimensions and microstructure.

5.4 Fracture and leak testing

Fracture toughness testing evaluates resistance to crack growth under defined specimen geometry and validity conditions. It is more directly relevant to defect tolerance than ultimate tensile strength, but valid results require appropriate crack preparation, thickness, loading, and specimen dimensions.

Leak testing is essential for fluid-containing parts. Pressure decay, helium leak testing, bubble testing, or flow testing may be selected according to required sensitivity and product function. A visually dense part is not necessarily leak-tight.

6. Integrated Inspection Workflow

A practical qualification workflow is staged to reduce cost while preserving evidence:

1. Define requirements: Identify critical dimensions, surfaces, loads, pressure boundaries, and allowable defects.
2. Review process records: Examine powder or feedstock certification, machine parameters, build atmosphere, thermal history, and support strategy.
3. Perform visual inspection: Record powder adhesion, cracks, warpage, support damage, discoloration, and incomplete features.
4. Measure external geometry: Use CMM, scanning, or optical methods with controlled datums.
5. Measure surface texture: Select profiles or areal measurements based on function.
6. Assess internal integrity: Use density methods, CT, ultrasonic testing, dye penetrant, or radiography where appropriate.
7. Characterize material: Use microscopy, hardness, tensile, fatigue, or fracture testing.
8. Correlate evidence: Relate defects and properties to process conditions and failure modes.
9. Make a documented disposition: Accept, rework, reject, or investigate further according to predefined criteria.

Non-destructive testing should be complemented by destructive witness specimens when required. Witness specimens cannot perfectly represent every location in a complex part, so their limitations must be considered.

7. Illustrative Case Study: Qualification of a Laser-Powder-Bed-Fused Bracket

Background

A stainless-steel load-bearing bracket is produced by laser powder-bed fusion. The bracket contains curved internal passages and must withstand repeated tensile loading. Its functional requirements include dimensional fit, adequate static strength, and resistance to fatigue cracking.

Problem

Initial parts pass visual inspection but show inconsistent fatigue life. Some parts also require excessive finishing to fit an assembly.

Analysis

Dimensional scanning identifies positive deviation near unsupported overhangs and a hole diameter smaller than nominal. Surface measurements show that upward-facing surfaces are substantially smoother than downward-facing surfaces. CT reveals irregular, layer-oriented discontinuities near an overhang and several rounded pores near a high-energy scan region. Metallography confirms lack-of-fusion features and a directional grain structure.

Tensile specimens built vertically and horizontally show different elongation values. Fractography of failed fatigue specimens identifies crack initiation at a surface-connected lack-of-fusion defect. The evidence indicates two interacting causes: insufficient support and thermal/process conditions near the overhang, plus an unfavorable surface condition at the fatigue-critical region.

Approach and solution

The engineering team:

• revised support geometry and orientation;
• adjusted scan parameters using a validated process window rather than relying only on nominal energy density;
• added CT sampling for critical internal regions;
• machined the fatigue-critical surface;
• introduced orientation-specific tensile and fatigue witness specimens;
• established dimensional and roughness acceptance limits;
• required review of build logs and powder reuse history.

Outcome

The revised process reduced internal discontinuities and improved hole-size consistency. Machining reduced surface crack-initiation sites, while CT confirmed that the internal passage remained open. Qualification testing demonstrated improved and more consistent fatigue life, although the team retained periodic destructive testing because CT resolution could not reliably detect every sub-voxel discontinuity.

Learning points

1. Visual acceptance does not establish internal integrity.
2. Dimensional accuracy and surface quality must be evaluated separately.
3. Defect orientation can be as important as defect volume.
4. Build orientation influences both geometry and mechanical anisotropy.
5. A process window, inspection evidence, and mechanical validation must support one another.

8. Limitations, Governance and Best Practices

Measurement methods have detection limits. A method may produce false negatives when a defect is too small, poorly oriented, shielded by geometry, or obscured by artifacts. False positives can result from segmentation errors, contamination, poor alignment, or instrument drift. Inspection personnel should understand probability of detection rather than treating a pass/fail indication as absolute truth.

Records should preserve material lot, machine identity, parameter set, build orientation, operator, environmental conditions, post-processing, inspection method, calibration status, and disposition. Digital traceability is particularly important because an AM part is inseparable from its process history.

Safety and governance issues include powder inhalation and explosion risks, laser or electron-beam hazards, resin exposure, chemical handling, radiation safety for CT, and protection of proprietary build files. Automated or AI-assisted defect detection can classify CT or surface images rapidly, but training-data bias, false alarms, explainability, and human verification remain important. AI should support—not replace—validated measurement procedures and engineering judgment.

Best practices include:

• define acceptance criteria before inspecting production parts;
• use calibrated equipment and documented procedures;
• report uncertainty and detection limits;
• inspect in functionally relevant orientations;
• combine nondestructive and destructive methods;
• correlate defects with process parameters and failure surfaces;
• avoid using a single metric such as relative density or R_a as a complete quality indicator;
• validate post-processing effects on both properties and dimensions;
• retain raw data where later investigation may be required;
• apply applicable national, industrial, and customer standards rather than informal acceptance rules.

Chapter Summary

AM defects arise from thermal gradients, incomplete bonding, unstable melt pools, poor feedstock delivery, polymer shrinkage, curing behavior, support failure, and process contamination. Their significance depends on morphology, size, location, orientation, and service loading. Geometric inspection, surface metrology, microscopy, density measurement, CT, and mechanical testing provide complementary evidence.

Dimensional deviation compares measured and nominal geometry, while surface parameters quantify texture under specified filtering and sampling conditions. Density measurements provide bulk information but do not necessarily reveal defect location. CT supplies three-dimensional internal evidence but is limited by resolution and artifacts. Microscopy explains microstructure and failure mechanisms, while tensile, hardness, fracture, and fatigue tests establish performance. AM parts frequently exhibit anisotropy because directional thermal histories create textured microstructures.

A robust qualification system begins with functional requirements, uses traceable measurement, accounts for uncertainty, correlates process history with defect evidence, and selects tests appropriate to the expected failure modes. Inspection is most effective when integrated with process control, post-processing, mechanical validation, and documented engineering disposition.

Key Terms

Defect: A condition that fails a requirement or compromises intended performance.

Lack of fusion: Incomplete bonding between adjacent deposited regions or layers.

Keyhole porosity: Gas-trapping porosity associated with an unstable deep vapor depression.

Balling: Breakup of a melt track into discontinuous beads.

Warping: Geometric distortion caused commonly by nonuniform thermal contraction.

Residual stress: Stress remaining in a component after external loads are removed.

Metrology: The science of measurement and its application.

Datum: A reference feature or coordinate basis used for inspection.

Surface roughness: Short-wavelength deviation of a surface profile from its mean line.

Ra: Arithmetic mean absolute profile deviation over an evaluation length.

Microstructure: Arrangement of grains, phases, pores, and other material constituents.

Anisotropy: Direction-dependent material behavior.

Relative density: Measured density expressed as a percentage of theoretical density.

Computed tomography: X-ray-based reconstruction of internal three-dimensional structure.

Segmentation: Separation of features, such as pores, from CT or image data.

Fractography: Examination and interpretation of fracture surfaces.

Fatigue: Progressive damage caused by repeated or fluctuating loading.

Probability of detection: Likelihood that an inspection method detects a specified discontinuity.

Traceability: Ability to relate a result to standards, equipment, material, and process records.

Review Questions

Short Answer Questions

1. What is the difference between a discontinuity and a defect?
2. State two mechanisms that can produce lack-of-fusion defects.
3. Why can dimensional accuracy and surface quality not be treated as the same property?
4. What information does industrial CT provide that external scanning cannot?
5. Why is build orientation important in AM mechanical testing?

Long Answer Questions

1. Explain the formation, morphology, and consequences of lack-of-fusion and keyhole porosity in powder-bed fusion.
2. Compare CMM, optical scanning, surface profilometry, microscopy, and CT in terms of capability and limitation.
3. Discuss how residual stress develops in AM and how it can lead to distortion or cracking.
4. Explain why fatigue performance is often more sensitive than tensile strength to AM surface and internal defects.
5. Design an integrated inspection plan for a pressure-containing AM component.

Analytical / Application Questions

1. A specimen has a mass of 18.75 g and an external volume of 2.50 cm³. Calculate its bulk density. If the theoretical density is 7.50 g/cm³, calculate relative density and explain why the result is not necessarily the exact porosity.
2. A nominal 20.00 mm feature is measured as 20.14 mm. Calculate dimensional deviation and percentage deviation, then discuss whether acceptance can be decided without considering measurement uncertainty and tolerance.
3. A fatigue fracture originates at an irregular, layer-oriented defect near an unsupported overhang. Propose a root-cause investigation that combines process-record review, CT, microscopy, surface metrology, and mechanical testing.

Multiple Choice Questions with Answers

Question 1

Which defect is most directly associated with insufficient overlap or bonding between adjacent melt tracks or layers?

A. Lack of fusion

B. Keyhole porosity

C. Stringing

D. Curing inhibition

Correct Answer: A. Lack of fusion

Question 2

In the nominal energy-density expression E_v = P / (vht), which variable represents hatch spacing?

A. P

B. v

C. h

D. t

Correct Answer: C. h

Question 3

A part has a measured density of 7.84 g/cm³ and a theoretical density of 8.00 g/cm³. What is its relative density?

A. 96.0%

B. 98.0%

C. 99.5%

D. 102.0%

Correct Answer: B. 98.0%

Question 4

Which method is generally most suitable for revealing the three-dimensional location of enclosed pores without sectioning the part?

A. Caliper measurement

B. Industrial computed tomography

C. Surface stylus measurement

D. Visual inspection

Correct Answer: B. Industrial computed tomography

Question 5

What does R_a primarily quantify?

A. Bulk density

B. Average absolute surface-profile deviation

C. Ultimate tensile strength

D. Pore volume fraction directly

Correct Answer: B. Average absolute surface-profile deviation

Question 6

Why may AM tensile properties differ between build directions?

A. The nominal CAD file changes by direction

B. Directional thermal history can produce textured microstructure and interlayer differences

C. Density is always zero between layers

D. CT alignment changes the alloy composition

Correct Answer: B. Directional thermal history can produce textured microstructure and interlayer differences

Question 7

A 10.00 mm nominal feature is measured as 9.90 mm. What is its dimensional deviation?

A. +0.10 mm

B. −0.10 mm

C. +1.00 mm

D. −1.00 mm

Correct Answer: B. −0.10 mm

Question 8

Which defect is commonly associated with excessive energy input and an unstable vapor depression?

A. Keyhole porosity

B. Under-extrusion

C. Support scar

D. Stair stepping

Correct Answer: A. Keyhole porosity

Question 9

Why is a single relative-density value insufficient for complete part qualification?

A. It gives no information about defect location, shape, or orientation

B. It cannot be calculated for metals

C. It always measures surface roughness instead

D. It directly measures fatigue life

Correct Answer: A. It gives no information about defect location, shape, or orientation

Question 10

Which feature is most likely to initiate fatigue cracking?

A. A smooth, isolated spherical pore far from the surface in a lightly loaded region

B. A surface-connected, sharp lack-of-fusion defect in a highly stressed region

C. A calibrated datum plane

D. A correctly sized internal channel

Correct Answer: B. A surface-connected, sharp lack-of-fusion defect in a highly stressed region