8. Processing of Polymers, Composites, Ceramics and Difficult Materials

Introduction and Context

Advanced manufacturing increasingly depends on materials whose performance cannot be obtained from conventional steels and aluminium alone. Polymers provide low density, corrosion resistance and design freedom; fibre-reinforced composites provide exceptional specific strength and stiffness; ceramics offer hardness, thermal stability and wear resistance; and superalloys retain strength in severe thermal and corrosive environments. Smart materials add the ability to sense, actuate or respond to changes in temperature, stress, electric field or magnetic field.

These advantages are accompanied by difficult processing behaviour. Polymers are sensitive to temperature, shear rate, moisture and cooling history. Composite properties are directional and depend on fibre architecture, resin flow, void content and curing. Ceramics are hard and brittle, so shaping is often performed before sintering and machining is restricted to abrasive or non-traditional methods. Superalloys impose high cutting forces, rapid tool wear and severe thermal loads. Smart materials may be sensitive to contamination, residual stress and process-induced changes in functional response.

Consequently, process selection is not merely a question of choosing a machine. It requires coordination of material state, geometry, tooling, thermal history, stress development, quality assurance and service requirements. This chapter develops an integrated framework for designing processing routes for these material classes.

Learning Objectives

After studying this chapter, the reader should be able to:

1. distinguish the processing characteristics of polymers, composites, ceramics, superalloys and smart materials;

2. select suitable forming, consolidation, curing, sintering and machining routes;

3. explain the effects of anisotropy, porosity, residual stress, thermal history and damage;

4. design practical tooling and process-control strategies;

5. identify common defects and select appropriate inspection methods;

6. evaluate industrial, environmental, safety and regulatory implications; and

7. apply the concepts to an integrated manufacturing case.

Material Behaviour as the Basis of Process Selection

A useful processing route begins with the material's response to heat, pressure, deformation and chemical transformation. Thermoplastics soften reversibly when heated and can therefore be remelted and reshaped. Thermosetting polymers, in contrast, undergo an irreversible cross-linking reaction during curing. Their viscosity initially falls or remains low enough for flow, then rises rapidly as the network forms. The process window must therefore provide adequate impregnation before gelation while avoiding excessive temperature, void formation or incomplete cure.

Ceramics and many powder-based materials are shaped in a relatively weak green state and then densified by sintering. Sintering causes shrinkage, often anisotropically if powder packing or reinforcement is non-uniform. The final dimensions must therefore be predicted from green density and shrinkage behaviour. Superalloys are generally processed in the fully dense state by forging, casting, additive manufacturing and machining. Their low thermal conductivity and strong work hardening make heat management and tool engagement especially important.

Anisotropy is central to composite processing. In a unidirectional laminate, stiffness and strength along the fibres may be many times greater than those transverse to them. The process must preserve fibre alignment and avoid waviness, resin-rich regions and local damage. Even in injection-moulded short-fibre polymers, flow aligns fibres and produces spatially varying properties. A design based on isotropic assumptions can therefore produce unsafe or unnecessarily heavy components.

Processing should also be considered as a chain. A part may pass through material preparation, preforming, mould filling, consolidation, curing or sintering, trimming, machining, joining and inspection. A defect introduced in an early stage may remain hidden until final testing. Process planning must consequently address the complete route rather than optimizing one operation in isolation.

Polymers: From Melt Processing to Precision Finishing

Thermoplastic processing

Injection moulding is the dominant route for high-volume production of thermoplastic components. Pellets are dried when necessary, conveyed into a heated barrel, plasticized by a rotating screw and injected into a closed mould. The cycle comprises filling, packing or holding, cooling and ejection. Important variables include melt temperature, mould temperature, injection speed, pressure, screw speed, back pressure and cooling time.

The filling stage must be fast enough to prevent premature solidification but controlled enough to limit jetting, air entrapment and excessive shear heating. Packing compensates for volumetric contraction as the part cools. Uneven cooling produces differential shrinkage and warpage, particularly in thick sections or parts with non-uniform fibre orientation. Good mould design uses balanced runners, adequate vents, uniform cooling channels and appropriate draft angles.

Extrusion produces continuous products such as pipes, films, sheets, profiles and filaments. The screw must generate stable melting and mixing without degrading the polymer. Die swell results from elastic recovery as the polymer exits the die and must be included in die design. Draw-down, cooling rate and calibration determine final dimensions and surface quality.

Compression moulding is used for thermosets, rubber compounds, sheet moulding compounds and some high-performance thermoplastics. A measured charge is placed in a heated mould, compressed, allowed to flow and then cured or consolidated. Transfer moulding injects a preheated charge into a closed cavity and is useful for encapsulating inserts and producing complex thermoset components.

Thermoforming heats a sheet until it becomes sufficiently deformable and then shapes it over or into a tool by vacuum, pressure or mechanical action. Thickness distribution is a major concern because stretching reduces wall thickness. Plug assistance, pre-stretching and controlled heating improve uniformity.

Polymer additive manufacturing and secondary operations

Material-extrusion additive manufacturing deposits a thermoplastic bead layer by layer. Bond strength depends on interlayer temperature, contact area, deposition speed and cooling. Anisotropy is often pronounced in the build direction. Powder-bed fusion uses thermal energy to fuse polymer powder and can produce complex geometries, but powder refresh, thermal gradients and powder ageing must be controlled.

Machining polymers requires sharp tools, low heat generation and effective chip removal. Excessive heat can soften the workpiece, smear the surface or induce residual stress. Drilling fibre-filled polymers requires attention to delamination, breakout and tool wear. Support fixtures should prevent distortion without causing local crushing.

Fibre-Reinforced Composites: Architecture, Flow and Consolidation

A composite consists of a matrix and a reinforcing phase whose combined properties exceed those of the constituents in a selected application. The matrix transfers load, protects fibres and determines environmental resistance; fibres carry most of the axial load and control stiffness. Processing must achieve adequate wetting, fibre placement, consolidation and interface quality.

Route selection

Hand lay-up is flexible and economical for low-volume large structures, but labour variation can produce inconsistent fibre volume fraction and void content. Prepreg lay-up uses fibres pre-impregnated with partially cured resin. Automated tape laying and automated fibre placement improve repeatability and permit controlled orientation, although they require expensive equipment and careful management of tack, steering and gaps.

Resin transfer moulding places a dry reinforcement preform in a closed mould and injects resin under pressure or vacuum. The resin must fill the preform before gelation. Permeability, fibre compaction, gate location, venting and injection pressure determine flow. Vacuum-assisted resin infusion draws resin through a dry lay-up under atmospheric pressure acting against a vacuum bag. It is suitable for large structures, but thickness variation and dry spots can arise if the flow front is poorly controlled.

Pultrusion continuously pulls fibres through a resin bath or injection chamber and a heated die. It is highly productive for constant cross-sections such as rods, beams and channels. Filament winding places resin-impregnated fibres on a rotating mandrel according to programmed helical or hoop paths. It is especially effective for pressure vessels and cylindrical structures.

Prepreg and resin systems are consolidated by autoclave, out-of-autoclave, compression moulding or hot pressing. Autoclave processing applies controlled temperature, vacuum and external gas pressure, reducing voids and improving laminate quality. Its disadvantages are high capital cost, long cycles and limited component size. Out-of-autoclave systems rely more strongly on resin formulation, vacuum integrity and laminate architecture.

Curing and process-induced defects

Cure is a coupled chemical and thermal process. Temperature raises reaction rate, while the exothermic reaction itself can further increase temperature. Thick laminates may therefore experience an internal temperature peak that differs substantially from the oven air temperature. A cure cycle should be developed using resin kinetics, thermocouple measurements and, where appropriate, dielectric or ultrasonic monitoring.

Common composite defects include voids, resin-rich areas, dry fibres, fibre waviness, delamination, wrinkles, inclusions, incomplete cure and poor bond-line thickness. Their effects depend on location and loading mode. A small void may be tolerable in a non-critical panel but unacceptable near a fatigue-critical joint. Non-destructive testing methods include ultrasonic C-scan, radiography, thermography, shearography and acoustic emission. Visual inspection and dimensional checks remain essential but cannot reveal all internal defects.

Joining composites requires special care. Mechanical fastening introduces holes and local stress concentrations, while adhesive bonding depends on surface preparation, cleanliness, adhesive thickness and cure. Hybrid joints must account for galvanic corrosion when carbon fibres contact aluminium. Edge sealing, insulating layers and compatible fasteners can reduce this risk.

Ceramics and Powder-Based Processing

Ceramics possess high hardness, compressive strength, chemical stability and temperature capability, but their low fracture toughness makes them susceptible to catastrophic brittle failure. Since machining fully sintered ceramics is expensive and can introduce cracks, near-net-shape forming before sintering is preferred whenever possible.

Powder preparation and shaping

The process begins with powder synthesis, classification and conditioning. Particle size distribution affects packing, flow and sintering. Binders and lubricants improve pressing behaviour, while dispersants stabilize ceramic suspensions. Contamination control is critical because small amounts of foreign material can affect colour, electrical properties, grain growth or strength.

Uniaxial pressing is suitable for simple shapes and high production volumes. Pressure is applied through rigid dies, but friction can create density gradients. Cold isostatic pressing applies hydrostatic pressure through a flexible mould and produces more uniform green density, although it is less readily automated for some geometries. Slip casting forms parts by drawing liquid from a ceramic suspension into a porous mould. It can produce complex shapes but requires control of suspension stability and drying.

Injection moulding of ceramics uses a powder-binder feedstock. After shaping, the binder is removed in a carefully controlled debinding stage. Rapid debinding may create cracks or internal pressure. Gel casting and extrusion provide additional routes for complex or continuous forms.

Drying, debinding and sintering

Drying should minimize moisture gradients and capillary stresses. Debinding removes organic constituents through evaporation, diffusion and thermal decomposition. The schedule must allow gases to escape without damaging the compact. Sintering promotes neck growth, pore elimination and densification through diffusion mechanisms. Excessive temperature or holding time may cause grain coarsening, which can reduce strength or toughness.

Hot pressing and hot isostatic pressing combine heat and pressure to achieve high density. Hot isostatic pressing is valuable for eliminating internal pores in castings and additive-manufactured components. However, tooling cost, cycle time and dimensional control must be considered.

Diamond grinding, lapping, polishing and electrical discharge machining of electrically conductive ceramics are used for final shaping. Grinding damage can include microcracks, subsurface damage, pull-out and residual tensile stress. Low-force cutting, appropriate wheel selection, abundant coolant and controlled dressing are important. Inspection may include dimensional measurement, microscopy, ultrasonic testing, X-ray computed tomography and strength testing.

Superalloys and Other Difficult-to-Machine Materials

Nickel-, cobalt- and iron-based superalloys retain useful strength at elevated temperatures and resist oxidation or hot corrosion. These properties make them valuable in turbine disks, blades, combustor components, nuclear systems and chemical equipment. The same properties make them difficult to machine. High strength persists at cutting temperature, thermal conductivity is often low, work hardening is severe and chemical affinity may accelerate tool wear.

Forging refines microstructure and can provide excellent fatigue performance, but requires high loads and close temperature control. Investment casting is used for intricate hot-section components; single-crystal and directionally solidified structures can improve high-temperature performance. Additive manufacturing enables internal cooling channels and complex geometries, but residual stress, lack-of-fusion defects, surface roughness and anisotropic microstructure require post-processing and qualification.

In machining, carbide, ceramic, cermet, polycrystalline cubic boron nitride and diamond tools may be selected according to alloy and operation. Tool geometry should reduce rubbing and control cutting forces. Excessive cutting speed raises temperature and flank wear; excessively low speed may promote built-up edge or unstable cutting. High-pressure coolant, minimum-quantity lubrication, cryogenic cooling and through-tool delivery are possible strategies, but coolant compatibility and operator safety must be evaluated.

When turning thin sections or milling blisks, workholding is as important as tool selection. Components should be supported against deformation, and tool paths should avoid sudden engagement, excessive radial immersion and repeated rubbing. Trochoidal or adaptive milling can maintain a more uniform chip load. Electrical discharge machining, laser machining, abrasive water-jet cutting and electrochemical machining are useful when conventional forces or tool access are limiting. Each process introduces its own concerns, such as heat-affected zones, recast layers, taper, kerf and environmental waste.

Smart Materials and Their Manufacturing Requirements

Smart materials include shape-memory alloys, piezoelectric ceramics, magnetostrictive materials, electroactive polymers and functional coatings. Their manufacture must preserve both structural integrity and functional response.

Shape-memory alloys are commonly formed by machining, laser processing, drawing, rolling or additive manufacturing, followed by heat treatment that establishes transformation behaviour. Surface defects and residual stresses can alter fatigue life and actuation reliability. Piezoelectric ceramics are formed by powder processing and sintering, then electroded and poled in a strong electric field. Porosity, electrode adhesion and poling temperature influence performance. Electroactive polymers may be cast, printed or laminated; thickness uniformity and electrode patterning are important.

Functional quality assurance cannot rely solely on dimensional inspection. It may require electrical impedance measurement, hysteresis characterization, actuation displacement, transformation-temperature measurement, magnetic response or life-cycle testing. The process plan must define acceptance criteria for both geometry and function.

Tooling, Digital Control and Quality Assurance

Tooling for advanced materials must withstand pressure, temperature, chemical exposure and repeated thermal cycling. Composite moulds may be made from steel, aluminium, nickel, invar, carbon fibre or hybrid constructions. Low thermal expansion materials are useful when dimensional stability is critical. Release agents, vacuum seals, heating systems and sensors are part of the tooling system rather than accessories.

A robust quality plan links each critical process variable to a measurable output. Examples include melt temperature and mould filling, vacuum level and composite void content, green density and ceramic shrinkage, or cutting temperature and tool wear. Statistical process control can identify drift before nonconforming parts accumulate. Design of experiments is useful for discovering interactions among temperature, pressure, speed, time and material condition.

Digital manufacturing supports simulation of polymer filling, resin flow, cure kinetics, ceramic shrinkage, metal additive thermal fields and machining forces. Digital twins can combine process data, sensor signals and inspection results to predict quality. Machine learning may assist defect classification or tool-life prediction, but models require representative, traceable data and should not replace engineering validation. A black-box prediction without physical interpretation is risky for safety-critical components.

A typical quality-assurance sequence includes incoming-material verification, equipment calibration, first-article inspection, in-process monitoring, final dimensional inspection, non-destructive testing and documentation of traceability. For critical parts, qualification must demonstrate repeatability under expected environmental and loading conditions.

Illustrative Case Study: Processing a Composite–Superalloy Turbine Module

Background

A manufacturer is developing a small unmanned aircraft engine module. The outer nacelle requires a lightweight carbon-fibre composite shell, while an internal hot-section mounting ring must be made from a nickel-based superalloy. The assembly must tolerate vibration, temperature variation, moisture and repeated maintenance removal.

Situation and problem

The initial design used a thin composite shell bonded directly to a machined aluminium ring. Testing revealed local delamination near fasteners, dimensional distortion after curing and galvanic corrosion at carbon–aluminium interfaces. The ring also suffered rapid tool wear and dimensional drift during machining.

Analysis

The failure investigation identified three interacting causes. First, the composite lay-up had fibres aligned primarily for membrane loading but insufficient reinforcement around attachment holes. Second, the cure cycle produced a temperature gradient through the thick flange, causing residual stress and spring-in. Third, direct contact between carbon fibres and aluminium in a humid environment created an electrochemical couple. On the ring, low thermal conductivity and work hardening caused high cutting temperature and flank wear.

Approach and solution

The shell route was redesigned around an autoclave-cured prepreg with quasi-isotropic plies in the attachment region and local doublers around holes. A low-expansion tooling surface and embedded thermocouples were introduced. The cure cycle was adjusted using measured laminate temperature rather than relying only on oven-air temperature. Ultrasonic inspection was specified after cure and after drilling.

The aluminium ring was replaced by a nickel-based superalloy ring produced near-net shape by investment casting, followed by finish machining. Machining trials established a tool material, conservative engagement strategy and high-pressure coolant condition. Tool wear limits were defined before dimensional drift became unacceptable.

A glass-fibre isolation layer and qualified high-temperature adhesive separated the carbon composite from the superalloy. Fastener holes were sealed, and the joint was designed for inspection and replacement. The assembly was subjected to thermal cycling, vibration, moisture exposure and functional checks.

Outcome

The revised route reduced local damage and improved dimensional repeatability. More importantly, the design no longer treated material selection, processing and joining as independent decisions. Cure monitoring, tool-life control and interface isolation became formal process controls rather than corrective actions.

Learning points

The case demonstrates that advanced-material manufacturing is a system problem. Fibre orientation must reflect load paths; tooling must control thermal distortion; machining parameters must account for alloy behaviour; and dissimilar-material interfaces require environmental assessment. Inspection should be placed at stages where defects can still be corrected economically.

Challenges, Professional Responsibilities and Best Practices

Common mistakes include selecting a process solely from production volume, ignoring material conditioning, assuming isotropic properties, using nominal rather than measured cure temperature, machining ceramics after full densification without considering cost, and accepting dimensional inspection as proof of functional quality. Another frequent error is transferring metal-processing assumptions to polymers or composites without accounting for viscoelasticity, spring-back or fibre damage.

Professional practice requires safe handling of solvents, resins, powders, fibres, coolants and metallic dust. Exposure controls, ventilation, personal protective equipment and waste segregation should be designed into the process. Powder handling may create inhalation or combustible-dust hazards. Electrical poling, high-temperature furnaces, lasers and high-pressure systems require documented operating procedures and interlocks.

Environmental decisions should include energy use, scrap, solvent emissions, coolant disposal and end-of-life recovery. Thermoplastic composites are generally more amenable to remelting than thermoset composites, although recycling may reduce fibre length or property consistency. Design for repair, disassembly and material identification can improve life-cycle performance.

Best practice is to establish a material-process-property relationship before production release. This involves representative trials, calibrated equipment, documented material storage, validated simulation, measurable acceptance criteria and feedback from inspection to process control. For safety-critical products, changes in supplier, tooling, cure schedule, powder batch or cutting conditions should be managed through formal configuration control.

Current Developments and Emerging Directions

Important developments include recyclable thermoplastic composites, automated fibre placement with in-process monitoring, out-of-autoclave consolidation, hybrid additive-subtractive manufacturing, ceramic additive manufacturing, cold-spray repair of difficult alloys and data-driven predictive maintenance. Real-time fibre-optic sensing and embedded dielectric sensors are being investigated for cure and structural-health monitoring.

Sustainable processing is encouraging lower-temperature routes, bio-derived polymer constituents, reduced-solvent systems, closed-loop water and coolant management, and improved recovery of composite materials. Artificial intelligence is increasingly used to optimize process windows and detect defects, but its industrial adoption depends on explainability, cybersecurity, data ownership and robust validation. The central engineering requirement remains unchanged: emerging technologies must demonstrate repeatable performance, not merely produce visually acceptable parts.

Chapter Summary

Processing advanced materials requires integration of material behaviour, geometry, equipment, tooling, thermal history and quality assurance. Thermoplastics are shaped through melt flow and cooling, whereas thermosets require controlled impregnation and irreversible cure. Composite processing must preserve fibre architecture, achieve consolidation and control voids, residual stress and joining damage. Ceramics are usually formed in a green state and densified by sintering, with shrinkage and brittle machining damage as major concerns. Superalloys demand careful thermal and force management during forming and machining. Smart materials require validation of functional behaviour in addition to dimensional accuracy.

Process simulation, sensors, statistical control and non-destructive inspection strengthen reliability, but they must be supported by material knowledge and traceability. The most effective process routes are those designed around the complete product life cycle, including safety, environmental impact, repair, recycling and regulatory responsibility.

Key Terms

Anisotropy: Direction-dependent material properties caused by structure, fibres, texture or processing.

Autoclave curing: Composite consolidation and curing using controlled heat, vacuum and external pressure.

Consolidation: Reduction of voids and improvement of contact or density through pressure, heat or both.

Cure: Chemical cross-linking or hardening of a thermosetting resin.

Debinding: Controlled removal of organic binders from a shaped powder compact.

Delamination: Separation between adjacent layers of a laminated composite.

Die swell: Elastic recovery of an extruded polymer after leaving the die.

Green body: Unfired ceramic compact that has been shaped but not yet sintered.

Hot isostatic pressing: Densification using high temperature and isostatic gas pressure.

Lay-up: Placement of composite plies or reinforcements in a prescribed orientation.

Near-net shape: A product formed close to final geometry, requiring limited finishing.

Permeability: Ability of a porous reinforcement to allow resin or gas flow.

Pultrusion: Continuous production of constant-section composite profiles through a heated die.

Resin transfer moulding: Injection of resin into a dry fibre preform enclosed in a mould.

Sintering: Thermal densification of a powder compact without complete melting.

Spring-in: Geometric distortion of a composite part caused by cure shrinkage and thermal contraction.

Superalloy: High-performance alloy retaining strength and environmental resistance at elevated temperature.

Thermoforming: Shaping a heated polymer sheet over or into a mould.

Void: Unwanted gas-filled or unfilled region within a material.

Review Questions

Short Answer Questions

1. Why is cooling control important in injection moulding?

2. What is the role of the matrix in a fibre-reinforced composite?

3. Why are ceramics commonly shaped before sintering?

4. Identify two reasons why superalloys are difficult to machine.

5. Why must smart materials be tested for function as well as geometry?

Long Answer Questions

1. Explain the stages of injection moulding and discuss the origin of warpage.

2. Compare autoclave curing, resin transfer moulding and vacuum-assisted resin infusion.

3. Describe powder preparation, shaping, debinding and sintering of a ceramic component.

4. Discuss tool wear mechanisms and process-control strategies in machining superalloys.

5. Explain how process-induced defects in composites can be prevented and detected.

Analytical / Application Questions

1. Select a manufacturing route for a large, curved carbon-fibre panel produced in moderate volume. Justify the reinforcement, moulding, curing and inspection decisions.

2. A ceramic component cracks during debinding. Analyze the likely causes and propose changes to the material and thermal schedule.

3. Design a quality-assurance plan for a composite-to-superalloy joint exposed to heat, vibration and moisture. Include process controls, inspection and environmental safeguards.

22. Multiple Choice Questions with Answers

Question 1

In injection moulding, the primary purpose of the packing or holding stage is to:

A. Remove all moisture from the polymer pellets

B. Compensate for volumetric shrinkage during cooling

C. Increase fibre length in the polymer

D. Complete ceramic sintering

Correct Answer: B. Compensate for volumetric shrinkage during cooling

Question 2

Which factor is most directly associated with anisotropy in a unidirectional composite laminate?

A. Random distribution of matrix molecules

B. Uniform cooling of the mould

C. Directional load carrying by aligned fibres

D. Use of a metallic release agent

Correct Answer: C. Directional load carrying by aligned fibres

Question 3

In resin transfer moulding, incomplete filling is most likely when:

A. Resin gels before the flow front reaches all regions

B. The mould is properly vented

C. The preform permeability is correctly characterized

D. The injection pressure is matched to the process window

Correct Answer: A. Resin gels before the flow front reaches all regions

Question 4

Why is debinding of ceramic green bodies performed gradually?

A. To increase polymer melting temperature

B. To prevent gas pressure and cracking within the compact

C. To create fibre waviness

D. To harden the cutting tool

Correct Answer: B. To prevent gas pressure and cracking within the compact

Question 5

Which operation is generally preferred for producing a constant-section composite beam continuously?

A. Pultrusion

B. Vacuum brazing

C. Investment casting

D. Electrochemical machining

Correct Answer: A. Pultrusion

Question 6

A major reason for rapid tool wear when machining nickel-based superalloys is their:

A. Very low strength at cutting temperature

B. High thermal conductivity and easy chip removal

C. High-temperature strength combined with low thermal conductivity

D. Complete absence of work hardening

Correct Answer: C. High-temperature strength combined with low thermal conductivity

Question 7

Which inspection method is particularly suitable for mapping internal delamination in a composite laminate?

A. Ultrasonic C-scan

B. Simple colour measurement

C. Screw-gauge measurement only

D. Melt-flow-index testing

Correct Answer: A. Ultrasonic C-scan

Question 8

A cure cycle for a thick thermoset composite should be based primarily on:

A. Oven-air temperature alone

B. Measured laminate temperature and cure kinetics

C. The colour of the mould surface

D. The cutting speed used after curing

Correct Answer: B. Measured laminate temperature and cure kinetics

Question 9

Direct contact between carbon-fibre composite and aluminium in a humid environment can cause:

A. Sintering shrinkage

B. Galvanic corrosion

C. Die swell

D. Polymer gelation

Correct Answer: B. Galvanic corrosion

Question 10

For a smart-material component, dimensional inspection alone is inadequate because:

A. Smart materials have no measurable dimensions

B. Functional response such as actuation or electrical behaviour must also be verified

C. All smart materials are liquid during manufacture

D. Geometry is irrelevant to service performance

Correct Answer: B. Functional response such as actuation or electrical behaviour must also be verified