Process-induced Distortions Research Articles

Forced-interaction and spring-in – Relevant initiators of process-induced distortions in composite manufacturing

Process-induced distortions are still an issue for today’s CFRP applications. Nonetheless, cost-reduction is considerably pushed by OEMs. Low cost tools, made from aluminum or steel, offer significant cost-saving potential compared to the state-of-the-art Invar (Ni36) tools. However, their high thermal expansion affects the final part shape detrimentally.The present paper reports on an experimental pilot study on forced-interaction and spring-in. Distortions of C- and L-profile specimens with unidirectional, cross-ply and quasi-isotropic laminates and thicknesses up to 8.2mm were manufactured on Invar and aluminum tools. Local and global part distortions were evaluated after manufacturing and compared. The study gives new insight into both acting distortion mechanisms. It quantifies the effect of a high tool CTE on the final part shape, which is of great value for tool and part designers who strive for high-precision composites.

Prediction of Process-Induce Distortions and Residual Stresses of Ancomposite Suspension Blade

This paper deals with a universal simulation strategy for the calculation of process-induceddistortions and residual stresses of a composite part. The mechanical material behavior is describedby a viscoelastic material model depending on temperature and degree of cure . The required materialparameters are derived by dynamic mechanical analyses. For the description of the reaction kinetic aphenomenological based model considering chemical and diffusion-controlled reactions is introduced.The reaction model parameters are fitted to isothermal and dynamic DSC measurements via globaland local optimization. The thermal expansion and chemical shrinkage are characterized by thermalmechanical analysis and using the contact angle measurement method. The simulation strategy isdemonstrated for a GFRP suspension blade for the automobile industry. Based on a sequential coupledtemperature-displacement analysis thermal hot spots, temperature and degree of cure distributions aswell as the final corresponding process-induced distortions and residual stresses are calculated andanalyzed. The development of the stiffness and the correlated stress during the curing process arediscussed in more detail. Furthermore, the effect of a degree of cure dependent stiffness on the stressesis investigated.

Spring-in of curved CFRP/foam-core sandwich structures

This paper reports on a comprehensive study on process-induced distortions (PID) of curved CFRP/foam-core sandwich panels. The study’s aim is to validate the prediction quality of two existing analytical models and a novel straightforward simulation technique, which utilizes measured spring-in distortions of representative monolithic samples as input. Sandwich specimens with a radius of approximately 350mm are manufactured with three different core thicknesses and two different core run-out geometries.The manufactured parts show a significant radius reduction which declines for increasing core thickness which is satisfactorily predicted by both, the analytical models and the simulation. The effect of the run-out shape turned out to be of inferior relevance for the investigated specimen configurations. However, it is found to be essential to use plane-stress assumptions and to capture the stiffness ratio between the skins and the core. Therefore, an extension of the Fernlund model is proposed whose predictions match excellently with the measured distortion of the specimens. Given the simplicity of the simulation approach in combination with the limited number of inputs makes it a tool of great value, particularly early on in the design cycle.

Predicting process-induced distortions in composite manufacturing – A pheno-numerical simulation strategy

Process-induced distortions (PID) of high-performance CFRP structures lead to problems with dimensional control if the distortions are not accounted for by compensating the process tool accordingly. There is a need to predict PID via simulation in industry rather than relying on less reliable information such as past experience or measurements on prototype structures. PID are caused by many phenomena, which can make detailed simulation complex and time consuming. This paper presents three simplified techniques for prediction of PID with sufficient accuracy but that are much faster and cheaper to run than traditional complex process models. The three techniques use solid or shell finite elements and use inputs based on measured spring-in of simple L-angles made of the same material and process conditions as the larger scale structures to be simulated. Application of the three techniques to example structures show that the predictions agree well with each other and that they give good predictions compared to experimentally measured distortions of a CFRP box. Given their speed and simplicity they have the potential to be useful for predicting PID for simpler composite structures, particularly early on in the design cycle where quick approximate calculations are of great value to evaluate different design concepts.

Effects of fabric-based unbalances on process-induced distortions of composite materials

In the present study, process-induced residual stresses of composite materials have been analysed on the mesolevel of a fabric unit cell. The given fabric was the satin-weave G0926, which is used for example in high-lift application of aerospace structures. In the study, a developed viscoelastic cure-dependent material model is applied to analyse the process-induced deformations of the fabric. In fact of the weave structure, this type of fabric tends to be out-of-plane displacement based on chemical and thermal shrinkage. This effect is studied experimentally and numerically, and the results are compared. The knowledge of the sensitivity of this fabric for out-of-plane displacement is an important factor to create real symmetric laminates. Disrespecting can be the source for large scattering warpage on the component level, and therefore, this awareness of the effect of textile structures on process-induced deformations is one key factor to develop robust processes.

Manufacturing distortions of a CFRP box-structure – A semi-numerical prediction approach

Counteracting undesired manufacturing distortions of carbon-composite structures is still a challenge for manufacturers. This paper presents a semi-numerical methodology to predict manufacturing-induced distortions of complex composite parts. The novel approach uses a combination of experimental, analytical and numerical procedures whereas standard shell elements are used for the FE calculations. The main innovation is the significant parameter reduction. Only one simulation parameter is necessary to predict the manufactured shape of a complex CFRP structure. For validation purposes, two CFRP box structures with dimensions of 798mm×206mm×55mm (l×w×h) were manufactured and process-induced distortions are evaluated in detail using a high-precision 3D full-field measurement system. Measurements show that manufacturing distortions are mainly driven by the spring-in effect. However, a global curvature of the part is obtained which is likely due to forced-interaction induced by the aluminum male tool. The predicted process-induced deformations match precisely with the measurement results. Evaluation of the predicted distortions allow a clear distinction between spring-in induced and forced-interaction induced deformations. In consequence, obtained results show that effects due to forced-interaction need to be considered for suchlike structures fabricated on metallic tools when CTEtool is larger than CTECFRP.

Numerical Simulation of a Stretch Bending Process

There are several difficulties associated with the numerical simulation of the stretch bending process of extruded components; the main ones are non-linear material behavior, geometrical non-linearities, modeling of boundary conditions, contact between die and specimen, springback during the unloading phase. Another very complex aspect is the calibration of the numerical model, as rather few experimental results are generally available. This paper deals with a numerical simulation of a complete stretch bending process with the aim to select the most suitable numerical procedure oriented to the evaluation of the process-induced distortions and residual stresses; to validate the proposed procedure, numerical–experimental comparisons have been performed. The activities reported have been developed in collaboration with ALENIA Finmeccanica s.p.a. within the VI FP’s research project called COMPACT.

A holographic phase-shifting interferometer technique to measure in-plane distortion

We describe the use of holographic phase-shifting interferometry (HPSI) to measure in-plane distortion of a substrate in which a shallow grating, produced by interferometric lithography, has been etched. The diffractive metrology inherent in HPSI should enable one to study process-induced distortions down to the level of a few nanometers. We used HPSI to investigate distortion introduced by the anodic bonding of silicon nitride membranes to Pyrex frames in x-ray lithography masks. This was part of an effort to develop an inverted x-ray mask configuration. The HPSI technique gave quantitative measurements of the linear and nonlinear components of distortions in one-dimension. The high levels of distortion found were presumably introduced by the high temperatures used in anodic bonding, combined with the asymmetric manner in which the membranes were brought into contact with the Pyrex frame. The comprehensive, quantitative measurement of distortion provided by HPSI should enable us to modify the mask making process so as to avoid steps that introduce distortion.

Sub-100 nm metrology using interferometrically produced fiducials

Pattern-placement metrology plays a critical role in nanofabrication. Not far in the future, metrology standards approaching 0.2 nm in accuracy will be required to facilitate the production of 25 nm semiconductor devices. They will also be needed to support the manufacturing of high-density wavelength-division-multiplexed integrated optoelectronic devices. We are developing a new approach to metrology in the sub-100 nm domain that is based on using phase-coherent fiducial gratings and grids patterned by interference lithography. This approach is complementary to the traditional mark-detection, or “market plot” pattern-placement metrology. In this article we explore the limitations of laser-interferometer-based mark-detection metrology, and contrast this with ways that fiducial grids could be used to solve a variety of metrology problems. These include measuring process-induced distortions in substrates; measuring patterning distortions in pattern-mastering systems, such as laser and e-beam writers; and measuring field distortions and alignment errors in steppers and scanners. We describe a proposed standard for pattern-placement metrology, which includes both a fiducial grid and market-type marks. Finally, a number of methods through which phase-coherent periodic structures can be patterned are shown, including “traditional” interference lithography, achromatic interference lithography, near-field interference lithography, and scanning-beam interference lithography.

Pattern placement metrology precision evaluation

Meeting the pattern placement requirement for 0.25-μm groundrule x-ray masks is a key development challenge. The e-beam mask lithography system and process-induced distortions influence pattern placement. To understand and control these tools and processes, and to disposition finished x-ray masks, more precise pattern placement metrology is required. This article presents the results of an evaluation of the Leica LMS 2000 Laser Metrology System for pattern placement metrology on x-ray masks.

Influence of X-ray mask repair on pattern placement accuracy

Compared to photomasks, X-ray masks are much more sensitive to mechanical damage, especially to process-induced distortions due to the use of thin silicon membranes (typically 2 μm). One of several possible causes for such distortions is the repair of opaque defects using focused ion beams (FIB). The extent to which distortions are induced by the repair process has been investigated. Extensive experiments have shown that the repair process of opaque defects using focused ion beams does not significantly influence the pattern placement accuracy on X-ray masks.

Electron-beam lithography three-mark silicon automatic registration and its capabilities for process distortion compensation

Submicron e-beam lithography requires that successive levels be accurately registered to each previously processed pattern on a wafer. Using a Perkin–Elmer manufacturing electron-beam exposure system (MEBES)R, we estimate the ability of an automated three-mark registration system to correct for distortion created by typical silicon processing. The results show that this registration system is capable of tracking process-induced distortions typically to within 0.15 μm everywhere on 64-mm-diam silicon wafers.