Residual stress and dimensional control management (RSDM) - A165

 

Residual stress and dimensional control management (RSDM)
Systems knowledge article
Document Type	Article
Document Identifier	165
Themes	Residual stress and dimensional control management
Tags	System theme
Prerequisites	Systems Knowledge

Introduction

Residual stress and dimensional control management (RSDM) is a system-level problem concerned with knowing, understanding, and managing how residual stresses build up throughout the manufacturing process so one can consistently produce composite structures with controlled tolerances.

Unlike thermal, material deposition and flow compaction management which are comprised of imperative process steps, RSDM can be perceived as dealing with unwanted stresses and deformations that should ideally be minimized or eliminated in a composite manufacturing system. If not controlled, residual stresses can lead to matrix failure and unexpected dimensional changes which can significantly increase manufacturing time and cost.

While there are ways to mitigate the amount of residual stress developed in a part during manufacture, in practical applications it is impossible to eliminate all residual stresses. Therefore the pragmatic approach is to understand the amount of deformation that will occur and compensate the dimensions of the tool so that the as-produced part — after distortion due to residual stress — has the correct dimensions. Residual stress and deformation simulation is often used for this purpose. The simulation can be used for evaluating dimensional conformance (how well the cured component dimensions will agree with engineering specifications), dimensional stability (the ability of a manufacturing process to consistently produce parts with repeatable dimensions) and dimensional control (how well this agreement can be controlled). However, at the time of the writing, the ability to consistently produce composite structures with controlled tolerance remains a challenge.

Significance

Example of residual stress from the manufacturing process of a composite c-channel (spar) resulting in "spring-in" that makes the interface of the c-channel incompatible with the laminates that it is supposed to mate with.

Link to the outcome matrix

Several manufacturing outcomes are directly related to RSDM. This includes reduced performance, process induced damage and dimensional control problems. Residual stress can lead to matrix failure in the form of matrix cracking and delamination/disbonding, as well as dimensional discrepancies between the true geometry and designed geometry. The dimensional discrepancies often require either custom shimming processes or forced assembly, which can be costly and/or reduce structural performance.

Scope

This page describes RSDM from a systems level perspective. Following the key processing steps in composite manufacturing, the material (M), shape (S), tooling and consumables (T), and equipment (E) and their interactions for each given step are defined in the context of RSDM.

Processing parameters which can affect the RSDM outcomes are categorized using the MSTE approach. These processing parameters can be perceived as knobs one can turn to control the manufacturing process and the consequent outcomes. Following this mindset, the effects of each MSTE parameter class on the RSDM outcomes are analyzed and illustrated in the following subpages:

Effect of material in a residual stress and dimensional control management system

Effect of shape in a residual stress and dimensional control management system

Effect of tooling in a residual stress and dimensional control management system

Effect of equipment in a residual stress and dimensional control management system

Systems level approach

Overview

On a system level, processing parameters (which can be categorized by MSTE) control/define the manufacturing processes, which results in the manufacturing outcomes. The deformation type(s) (manufacturing outcomes), being warpage, thickness change, curvature/radius change, spring-in etc., should be identified when dealing with any dimensional control problems. The inspection and data reduction process should be developed in accordance to the deformation type(s) such that engineers and practitioners are able to simulate/analyze the deformations with the appropriate residual stress sources and deformation mechanisms.

The manufacturing outcomes, i.e. the deformations, are typically small (e.g. angle change < 5 degrees, assembly gaps of a few thousandths of an inch). Due to the large number of processing parameters, residual stress and dimensional control management in practice becomes the management of the variabilities and uncertainties in the composite component manufacturing, inspection and the data reduction processes. See Spring-in of L-shapes for example.

Key processing steps

Thermal transformation

Majority of the residual stress within a composite structure develop during thermal transformation. The residual stress build up as the matrix transforms from a fluid to a solid occurs at many length scales. The residual stress sources and deformation mechanism can be categorized into three scales: micro, macro and component. At the microscale, phase-level residual stresses accumulate due to the mismatch between thermal expansion of resin and fibre, as well as the resin cure/crystallization shrinkage. At the macroscale, laminate-level residual stresses accumulate due to thermal and mechanical property mismatch between layers that are oriented in different directions. In addition, part geometry (shape) plays an important role in the deformation mechanisms and the final residual stress level. Finally, at the component scale, contributors to residual stress include geometric features and constraints, thermal and cure gradients, volume fraction variations due to resin flow, interaction between components in an assembly, tool part interaction, and machining.

• Material (M): material (resin and fibre) properties at microscale (eg. thermal expansion coefficient, resin cure/crystallization shrinkage)
• Material + Shape (M+S)= Part(P): Laminate level (macroscale) properties mismatch (eg. mismatch of CTE and cure shrinkage in the in-plane and through thickness direction) interact with geometric parameters (laminate thickness, radius, length etc.)
• Material + Shape + Tooling and consumables (M+S+T): tool part interaction, Vf gradient caused by consumables
• Material + Shape + Tooling and consumables + Equipment (M+S+T+E): thermal boundary conditions and initial conditions set by equipment (autoclave, oven, hot press etc.). Machining and post processing

	Residual stress sources	Deformation mechanisms
Microscale	Thermal strain(CTE) Cure shrinkage	Mismatch between fibre and matrix
Macorscale	Thermal strain(CTE) Cure shrinkage Vf gradient Elastic and viscoelastic properties	Mismatch in through thickness and in-plane directions Through thickness property gradient Inter-lamina shear Poisson's effect
Component scale	Tooling Geometry Thermal gradient Thermal strain Friction Machining Post processing	Geometrical locking Cure gradient across tool Tool part interaction Stress release

Variabilities and uncertainties prior and during the thermal transformation step include:

• Fibre misalignment from received material
• Fibre misalignment during material deposition
• Cure advancement during shipping, storage and material deposition
• Discrepancies in designed vs actual thermal history due to:
  • Varying part thickness
  • Usage of core material
  • Equipment temperature control
  • Part position in equipment
  • Tooling geometry/substructure effects
  • etc. (See Thermal management for more influencing factors)
• Discrepancies in designed vs actual pressure history due to:
  • Tooling/part geometry
  • Equipment pressure control
• Physical aging

Demoulding

Demoulding has no effect on the final deformation or the residual stress level within a part. However, residual stress induced deformation can be first observed during demoulding. For example, with the curing of a flat strip: a) tool expands more than the laminate in in-plane direction upon heating, inducing tensile stress in the laminate close to interface. b) Inter-ply slippage releases some tensile stress and through thickness stress gradient is locked in. c) Part warps upon demoulding as shown in the figure.

The existence of residual stress and part deformation can either release the part itself from the tool (e.g. warped plate on a flat tool) or create frictional locks, making the part adhere to the tool (e.g. cylindrical part shrinks and grabbing onto the solid mandrel or a C-shape geometry springs-in and clamps onto the tool). This type of frictional locking caused by residual stresses can sometimes make demoulding very difficult. Hence, the effects of residual stresses and deformation on demoulding should be taken into consideration when designing the tool.

• Material + Shape (M+S) = Part (P): cured composite part
• Tooling and consumables (t) = Tool that the part is manufactured on
• Equipment (E) = equipment assisting demoulding e.g. hydraulic press etc.

Trimming and machining

During trimming, machining or drilling, material is being removed from a cured composite part and residual stresses within the part are being released. Many experimental methods for determining/measuring residual stress are based on this. As residual stresses are being released, deformation might occur and interfere with the trimming or machining process. If the trimming process is automated, the onset of deformation during trimming can produce inaccurate trimming results. Depending on the specific part, sometimes the interference can be significant and/or detrimental. For example, when cutting open a cylindrical part along the axial direction, the cylinder can constrict and trap the blade in the part. Hence, the effects of residual stresses and deformation should be carefully considered before executing trimming and machining.

• Material + Shape (M+S) = Part (P): cured composite part
• Tooling and consumables (T) = Tool or fixtures used for trimming and machining
• Equipment (E) = equipment used for trimming, machining or drilling e.g. sander, milling machine, lathe, drills, saws, CNC machines and robotic arms

Variabilities and uncertainties:

• Stress being released during material removal is difficult to quantify
• Defects such as delamination introduced during machining can affect residual stress state within a part
• Heat generation from machining may cause the local thermal history (hence, residual stress) to be different from rest of the part

Inspection (and data reduction)

Inspection, measuring and data reduction play indispensable roles in RSDM. The actual tool (as received and during service), cured part before and after trimming ideally should be measured using the same method and to the same accuracy. A seemingly trivial but commonly made mistake in literature is comparing actual deformed part surface (e.g. from a CMM) to the ideal tool surface (in CAD) or the model outputs (path 5, 6). These comparisons can be inaccurate because the actual tooling imperfections are not considered. A good practice is to compare the actual deformed part surface to the actual tool surface (path 1). The result can then be used as a validation for the parallel digital twin (path 2) to better understand the deformation sources and mechanisms.

• Material + Shape (M+S) = Part (P): cured composite part
• Tooling (T): Tools used to produced the composite part (as received and during service)
• Equipment (E): Measurement equipment (e.g. coordinate measuring machines (CMM), 3D scanner etc.)

Variabilities and uncertainties during the inspection (and data reduction) step can be significant. These Variabilities and uncertainties include:

• Measurement uncertainties (Tool and part should be considered separately)
• Data reduction uncertainties

Assembly

Part deformation has direct and significant influences on the assembly process. In the case of secondary bonding (using adhesive on two or more pre-cured parts, where the only chemical reaction/transformation is only associated with the adhesive), deformed parts may cause variation in the bond-line thickness, which can affect the bond line strength. In worse cases, carefully manufactured shims are required which can be expensive and time consuming. Further heating maybe required to cure the adhesive. The further heating can potentially alter the residual stress states within the pre-cured parts, causing further deformation. In the case of co-curing or co-bonding, the super-position of residual stresses should be considered when evaluating the final deformation.

• Material (M): Adhesive, fasteners
• Shape (S): geometry of the parts to be assembled, mating surfaces
• Tooling and consumables (T): Fixtures, jigs used to align and clamp the parts, dead weights, vacuum bags

• Equipment (E):
  • Aligning equipment: 3D scanners, coordinate measuring machines (CMM), lasers
  • Heating equipment: oven, press, autoclave, heating blanket
  • Adhesive dispenser

Maturity

Level II

Explore this area further

• Residual stress and dimensional control management (RSDM) - A165
  • Effect of material in a RSDM system - A275
  • Effect of shape in a RSDM system - A276
  • Effect of tooling in a RSDM system - A277
  • Effect of equipment in a RSDM system - A278

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles	Machining and assembly management (MAM) - A287 Materials deposition and consolidation management (MDCM) - A157 Quality/inspection management (QIM) - A288 Residual stress and dimensional control management (RSDM) - A165 Thermal and cure/crystallization management (TM) - A107
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents
Case Studies
Perspectives Articles

References

About	Help

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

Outcomes represent the range of response/sensitivity to factory system attributes. Those that fail to satisfy manufacturing requirements are known as defects. Examples of manufacturing outcomes include process parameter outcomes, material structure outcomes, and material performance outcomes.

The failure mode characterized by a partial or full separation of adjacent laminae. Delamination is unique to laminated structures, and can be induced by impact load, fatigue load, and quasistatic load. (Also known as interlaminar separation)

A central processing theme in the manufacturing cycle. This theme relates to management of internal stresses that occur as the material undergoes differential thermal and physical phase change volume changes and viscoelastic property development.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

The act of combining reinforcement fibre, resin to shape on a mould (tool).

There are three main methods to combine resin and fibre to deposit onto a tool (i.e. material deposition):

• Wet Processes: Mixing liquid resin and dry fibre directly in the mould, e.g. Wet Compression Moulding process for parts in BMW 7 series
• Liquid Composite Moulding (LCM) processes: Infusing dry fabric with resin using a pressure gradient, e.g. High Pressure Resin Transfer Moulding process (HP RTM) for parts in BMW i3
• Prepreg Processes: Laying up using prepreg where resin and fibre are already combined, e.g. Automated Tape Laying (ATL) process for some of the parts in Boeing 787.

With regards to modelling & simulation, validation demonstrates the accuracy of a computational model — in the context of its intended use — to check that it represents the physics of the problem and the ‘reality of interest’ (needs are satisfied).