Incremental Hole-drilling Method Research Articles

Study on Physical Mechanisms of Thickness Effect of Incremental Hole-Drilling Method Based on Energy Analysis

Incremental hole drilling is a commonly employed semi-destructive method for measuring internal residual stresses. It involves calculating internal residual stresses through the measurement of strains. The conversion of strain to stress is achieved through calibration coefficients, the accuracy of which directly influences the precision of residual stress measurements. These calibration coefficients are predominantly determined through finite element simulations, which must consider the sample’s characteristics and realistic experimental conditions. While there has been extensive research on the influence of sample thickness, the impact of thickness under different experimental conditions remains unexplored, and the underlying physical mechanisms driving thickness effects remain ambiguous. This paper addresses this gap by employing finite element simulations to investigate the impact of thickness on calibration coefficients under three commonly utilized experimental conditions. Moreover, this research endeavors to elucidate the physical mechanisms that contribute to variations in these coefficients through energy analysis.

Characterization of residual stresses in fiber metal laminate interfaces – A combined approach applying hole-drilling method and energy-dispersive X-ray diffraction

Fiber metal laminates (FMLs) can be manufactured by direct bonding of fiber reinforced plastics (FRP) and metals using advanced manufacturing approaches, e.g., the prepreg-press-technology. Such materials gain high interest in lightweight applications as they benefit from the superior characteristics of both materials. However, residual stresses are induced in the manufacturing process mainly due to the difference in thermal expansion coefficients between FRP and metal. These residual stresses can lead to the delamination at the transition zone/ interface between FRP and metal and, thus, detrimentally affect the structural integrity and the durability of the FML structure.The objective of the present work is to introduce reliable experimental approaches to accurately measure the distribution of residual stresses across the thickness of the FML, with a special focus on the interface between metal and FRP. Symmetric FMLs with a specific stacking sequence of FRP/steel/FRP are designed and fabricated for residual stress analysis using different approaches, i.e., relaxation and diffraction methods, as well as for results comparison and validation purposes. The incremental hole drilling method (IHD) is used to determine the residual stresses across the thickness of the FMLs. By choosing FRPs with a relatively small thickness, a reliable measurement using IHD can even be achieved across the transition zone between FRP and metal. The energy-dispersive X-ray diffraction method (ED-XRD), as an advanced non-destructive approach, is employed for in-depth residual stress measurement in the metallic part of the FMLs. Each measurement method is characterized by its own advantages, intrinsic limitations as well as uncertainty sources. The obtained results give confidence that both methods are able to assess the qualitative distribution as well as the absolute magnitudes of residual stresses in the FMLs. Based on the approach detailed the reliability of the two measurement methods is cross-validated.

Manufacture of Defined Residual Stress Distributions in the Friction-Spinning Process: Driven Tool and Subsequent Flow-Forming

Friction-spinning as an innovative incremental forming process enables large degrees of deformation in the field of tube and sheet metal forming due to a self-induced heat generation in the forming zone. This paper presents a new tool and process design with a driven tool for the targeted adjustment of residual stress distributions in the friction-spinning process. Locally adapted residual stress depth distributions are intended to improve the functionality of the friction-spinning workpieces, e.g. by delaying failure or triggering it in a defined way. The new process designs with the driven tool and a subsequent flow-forming operation are investigated regarding the influence on the residual stress depth distributions compared to those of standard friction-spinning process. Residual stress depth distributions are measured with the incremental hole-drilling method. The workpieces (tubular part with a flange) are manufactured using heat-treatable 3.3206 (EN-AW 6060 T6) tubular profiles. It is shown that the residual stress depth distributions change significantly due to the new process designs, which offers new potentials for the targeted adjustment of residual stresses that serve to improve the workpiece properties.

Evaluation of residual stress in thick metallic coatings using the combination of hole drilling and micro-indentation methods

Plasma spraying is commonly used to deposit thick metallic coatings. The high deposition temperature and complex interaction of the process parameters result in tensile residual stresses in thick metallic layers. Tensile residual stresses are widely known to affect the integrity of metallic coatings. The present study uses a combination of hole drilling and micro-indentation techniques to evaluate the residual stress developed in Ni-based metallic coatings deposited on stainless steel substrate using a direct current plasma spray torch. The metallic coating samples are first characterized by microscopy, surface roughness measurement, micro-indentation, and scratch tests before through-thickness residual strain measurement via the incremental hole-drilling method. The residual stress in the metallic coating layers is evaluated from the incremental strain measurements and micro-indentation curves. The studies show that the residual stress can be reliably predicted using the combination of hole drilling and micro-indentation measurements. It is found that tensile residual stresses are developed across the depth of both NiCrAl and Ni–20Al coatings. The variation of the tensile residual stresses across the depth is nonlinear and almost equibiaxial. The residual stress strongly influences the adhesion strength of the thick metallic coating layers.

Effect of laser peen forming process parameters on bending and surface quality of Ti-6Al-4V sheets

Laser peen forming (LPF) is a metal forming process that utilizes laser-induced mechanical shock waves to form desired shapes or modify bent structures. The present work focuses on the applicability of LPF to Ti-6Al-4V sheets, to identify an optimal LPF process parameter window and achieve desired bending without compromising the surface quality within the peened region. The effect of LPF process parameters, i.e. laser power density, overlap, type of sacrificial overlay, and the number of peening sequences was investigated for specimens with different thicknesses. The laser power density and number of peening sequences were the most influential parameters that affect the bending of the specimens. Using sacrificial overlay has a significant effect on the bending and surface quality of the specimens. Surface quality after LPF was assessed by measuring the roughness in the peened region. In experiments without a sacrificial overlay, a black titanium oxide residue on the peened region was observed and additionally, small micro-cracks were found in the near surface region. Further characterization of the peened region revealed that the average crack length increased with increase in laser power density. Two possible LPF process parameter combinations were identified to obtain bending in the peened region, where LPF with sacrificial overlay resulted in no surface damage. Furthermore, residual stresses were determined at various LPF process parameters by incremental hole-drilling method in the peened region.

Experimental Analysis of Residual Stresses in CFRPs through Hole-Drilling Method: The Role of Stacking Sequence, Thickness, and Defects

Carbon fiber reinforced plastics (CFRPs) gained high interest in industrial applications because of their excellent strength and low specific weight. The stacking sequence of the unidirectional plies forming a CFRP laminate, and their thicknesses, primarily determine the mechanical performance. However, during manufacturing, defects, e.g., pores and residual stresses, are induced, both affecting the mechanical properties. The objective of the present work is to accurately measure residual stresses in CFRPs as well as to investigate the effects of stacking sequence, overall laminate thickness, and the presence of pores on the residual stress state. Residual stresses were measured through the incremental hole-drilling method (HDM). Adequate procedures have been applied to evaluate the residual stresses for orthotropic materials, including calculating the calibration coefficients through finite element analysis (FEA) based on stacking sequence, laminate thickness and mechanical properties. Using optical microscopy (OM) and computed tomography (CT), profound insights into the cross-sectional and three-dimensional microstructure, e.g., location and shape of process-induced pores, were obtained. This microstructural information allowed for a comprehensive understanding of the experimentally determined strain and stress results, particularly at the transition zone between the individual plies. The effect of pores on residual stresses was investigated by considering pores to calculate the calibration coefficients at a depth of 0.06 mm to 0.12 mm in the model and utilizing these results for residual stress evaluation. A maximum difference of 46% in stress between defect-free and porous material sample conditions was observed at a hole depth of 0.65 mm. The significance of employing correctly calculated coefficients for the residual stress evaluation is highlighted by mechanical validation tests.

Optimization of process-induced residual stresses in automated manufacturing of thermoset composites

The present study investigates the effect of automated fiber placement process on the induced residual stresses in polymer-matrix composites. The robotic fiber placement is utilized to fabricate a total of 150 flat specimens. The specimens are then cured inside an autoclave. The incremental hole-drilling method is applied to estimate the maximum stresses induced in the laminates. Further, an optimization process is conducted based on the response surface method to minimize the generated stresses, while maintaining the highest production rate. A mathematical modeling of the relationship between the process parameters and the residual stresses is generated. The statistical analysis (ANOVA test) and sensitivity analysis are conducted to evaluate the generated model. The results show considerable effect on the stresses when changing the manufacturing process parameters due to the existence of in-situ curing and the change in material porosity. The optimization strategy could significantly decrease the stresses with maximizing the fabrication speed.

Study of the Automated Process-Induced Residual Stresses in Multi-Axis Laminates

The integrity of the composite structures is affected by the manufacturing conditions. Controlling the ply orientation during the manufacturing process plays an important role in defining the induced residual stresses inside the laminate. This work evaluates the generated residual stresses inside multi-axis laminates of carbon fiber reinforced polymers. The samples are manufactured using the robotic fiber placement technique. The residual stresses are measured through the incremental hole-drilling method. Finite element modeling is used to provide the calibration process of the strain measurements. Additionally, microstructure analyses have been performed to show the configuration of the manufactured samples. The effect of the laminate configurations and plies orientations are investigated and compared. The layers’ orientations have shown a considerable effect on the generated stresses.

On the Sensing and Calibration of Residual Stresses Measurements in the Incremental Hole-Drilling Method.

The current study presents three calibration approaches for the hole-drilling method (HDM). A total of 72 finite element models and 144 simulations were established to calibrate the measurements of the strain sensors. The first approach assumed the stresses acted on the boundaries of the drilled hole and thus analyzed the surrounding displacements field. The second analysis considered the loads on the outer surfaces of the specimen while measuring the strains’ differences between the model with and without the drilled hole. The third approach was more comprehensive as it considered the mechanical and thermal effects of the drilling operations. The proposed approaches were applied to two different materials (AISI 1045 and CFRP). The steel specimens were machined using a CNC lathe while the composite laminates were manufactured using the robotic fiber placement (RFP) process. Subsequently, the residual stresses (RSs) were measured using the HDM. The obtained data were compared with X-ray diffraction measurements for validation. The results showed better estimation of the RSs when utilizing the third approach and clear underestimation of the stresses using the second approach. A divergence in RSs values between the three approaches was also detected when measuring the stresses in the internal layers of the composite laminates.

Numerical and Experimental Studies of Fabrication-Induced Thermal Residual Stresses in Microelectronic Packages

Microelectronic packages are typically composed of different layers of materials with un-similar thermo-mechanical properties. Fabrication-induced residual stresses, resulting from the mismatch of layers' thermo-mechanical properties, may lead to adverse effects such as warpage and delamination at the interface of layers. In this study, the fabrication process of the ball grid array (BGA) package was simulated to predict the thermal residual stresses through the thickness of the package. A finite-element model incorporating realistic material behavior of the BGA package layers was developed. To this end, the epoxy molding compound (EMC) was modeled as a viscoelastic material, while the silicon chip, the die-attach, and the composite substrate were simulated using an elastic model during cooling. Since Young's modulus of EMC is time and temperature-dependent, three different process temperatures were simulated to investigate the impact of viscoelastic properties of EMC, defined using Prony coefficients, on numerical residual stresses. To verify the simulation results, the incremental hole-drilling method was used to experimentally measure the residual stress components. It includes drilling a small hole at the center of a rosette strain gauge bonded on the BGA package surface and measuring the released strains on the surface. The relaxed strains were then converted to the residual stresses using a calibration matrix whose coefficients are determined from a finite-element model. The reasonable agreement of numerical and experimental components of the residual stress throughout the thickness of the BGA package confirms the reliability of the proposed simulation approach in estimating residual stresses in microelectronics packages.

Experimental validation of residual stress thermomechanical simulation in as-quenched superalloy discs by using diffraction and incremental hole-drilling methods

Mechanical properties of commercial Ni-based superalloys at elevated temperature rely strongly on the cooling rate during solution heat treatment. Quenching-induced residual stress can be crucial to machining accuracy and fatigue strength of industrial superalloy components.In this study, an existing finite-element-based methodology is improved by inverse heat transfer calculation and phase transformation latent heat input, with a maximum temperature deviation of ∼44℃. The simulated thermal stress and strain rate evolution demonstrate the origin of the quenching stress field. Robust non-destructive methods, i.e., neutron diffraction and X-ray diffraction, are employed to validate the simulated surface and internal residual stresses in both air-cooled and water-quenched discs. The simulated in-depth normal residual stress profiles are proven to be consistent with the results of non-destructive measurement, with a maximum average deviation of ∼34 MPa. After a rapid quenching process, the near-surface stress gradient measured by incremental hole-drilling method at a depth of 0.45 mm is ∼150 % lower than simulation result.

Improvement and validation of residual stress measurement in composite laminates using the incremental hole-drilling method

In this paper, the incremental hole-drilling (IHD) method has been improved by attaching strain gauge rosettes on both top and bottom surfaces, and changing the calculation algorithm for solving the ill-conditioned problem. In order to assess the feasibility and accuracy of the improved IHD method, an experiment of measuring the in-plane stress distribution in a composite laminate under bending was proposed. The composite laminate, which is bended by an external pure bending moment and fixed in a stable state, contains an in-plane stress distribution with obvious gradients in thickness direction, which is essentially similar to residual stresses. A plain woven T800H/803 specimen was tested in the proposed experiment with the improved IHD method. The results calculated using both top-surface and bottom-surface measured strains, and by iteration algorithm, showed good agreement with the finite element method (FEM) simulation and high calculation efficiency.

Effect of Heat Treatments on Residual Stress and Properties of AISI 316L Steel Processed by Directed Energy Deposition

AISI 316L stainless steel samples were produced by means of laser powder-directed energy deposition using optimized building conditions with two different deposition strategies. The samples, in the as-built state, were at first characterized in terms of microstructure, hardness and residual stresses by the incremental hole-drilling strain-gauge method. The results highlighted that the deposition strategy mainly affects the residual stress values, while the material hardness value is not strongly varied. The distribution of the residual stresses along the sample height was also evaluated by measuring the stress at different distances from the building platform. Furthermore, some samples underwent a homogenizing heat treatment for 2 h at 600 and 800 °C and were characterized and compared with the as-built ones. The results showed that the suggested heat treatments allow not only a reduction in residual stresses but also the homogenization of the microstructure confirmed by comparing the variations of the Vickers hardness values along the building direction. Tensile tests were also performed on the as-built and heat-treated samples in order to investigate the effect of the heat treatment on the tensile properties of the material.

Experimental investigation of cooling conditions, MWCNTs and mandrel diameter effects on the thermal residual stresses of multi-layered filament-wound composite pipes

This study employs numerical and experimental approaches to analyze effects of various filament-winding parameters, including mandrel diameter and cooling conditions, on the process-induced thermal residual stresses of composite and nanocomposite pipes. Simultaneous effects of the addition of multi-walled carbon nanotubes (MWCNTs) and the cooling conditions were also analyzed in detail. To accomplish this aim, a few composite and nanocomposite cylinders with various diameters in different cooling conditions were produced using filament-winding process and experimented. The incremental hole-drilling method using an integral inverse solution was employed for measuring the curing-induced residual stresses. The experimental findings confirm that using MWCNTs for reinforcing the matrix, choosing a slow cooling condition, and increasing the diameter generate less residual stresses during the process. Moreover, this study clearly showed that the addition of MWCNTs decreases the sensitivity of the structure to the effects of cooling conditions during the curing process. Therefore, finding optimum amounts of MWCNTs and cooling rate leads to the minimum cost of fabrication.

Residual weld stresses in stiffener-to-deck plate weld of an orthotropic steel deck

A three-dimensional finite element welding simulation procedure is developed with the software Siemens NX and solver type SAMCEF in order to determine the residual stresses of a welded component of an orthotropic bridge deck. The welding process of a deck plate which is welded to a closed trapezoidal stiffener is simulated. A decoupled thermal-mechanical analysis is performed. During the thermal analysis, the temperatures introduced by the passage of the welding torch are calculated for different time steps. This temperature field is used during the thermal analysis to determine the residual welding stresses for the same time steps. The decoupled thermal-mechanical analysis gives the distribution of the residual stresses. For the transverse direction, there are tensile yield residual stresses on the deck plate near the weld region. In between the welded webs of the stiffener, there are compressive residual stresses. For the longitudinal stiffener, there are again tensile yield residual stresses near the weld which decrease at a greater distance and turn into compressive residual yield stresses. For the longitudinal direction, there are tensile yield stresses present near the weld and a compressive residual stress in the middle of the deck plate. The stiffener only shows a small tensile stress peak near the weld root and along the stiffener there are small tensile residual stresses.The results from the finite element analysis are validated by experimental measurements with the incremental hole-drilling method. A residual stress distribution for the top and bottom side of the deck plate and the longitudinal stiffener is determined for both the longitudinal and transverse direction.

Residual stress measurement of single point incremental formed Al/Cu bimetal using incremental hole-drilling method

Residual stress (RS) is an important parameter which affect performance and dimensional stability of components. In the manufacturing process, RS mostly was produced by mechanical and thermal causes. In the present work, concept of residual stress is introduced in the area of single point incremental forming (SPIF) of Al/Cu bimetal. The chosen profile for single point incremental forming is parabolic shape where provide angular and linear displacement. In this regard, influence of process parameters such as tool diameter, step down, and rotational speed are investigated on the residual stress using incremental hole-drilling method. Since there was not any standard for calculation of calibration coefficients in the multilayers, so finite element approach was considered to find the calibration coefficients. Results showed that the tool diameter of 16 mm and step down of 0.25 mm produces less RS and also rotational speed variation does not affect much on RS.

Determination of Residual Welding Stresses in a Steel Bridge Component by Finite Element Modeling of the Incremental Hole-Drilling Method

For welded bridge components, the knowledge of residual stresses induced by welding is essential to determine their effect on the fatigue life behavior resulting in optimal fatigue design and a better knowledge about the fatigue strength of these welded connections. The residual stresses of a welded component in an orthotropic steel bridge deck are determined with the incremental hole-drilling method. This method is specified by the American Society for Testing and Materials ASTM E837-13a and it can be used only when the material behavior is linear-elastic. However in the region of the bored hole, there are relaxed plastic strains present that can lead to significant error of the measured stresses. The hole-drilling procedure is simulated with three-dimensional finite element modeling including a simplistic model of plasticity. The effect of plasticity on uniform in-depth residual stresses is determined and it is concluded that residual stresses obtained under the assumption of linear-elastic material behavior are an overestimation. Including plasticity for non-uniform in-depth residual stress fields results in larger tensile and smaller compressive residual stresses. Larger tensile residual stresses cause premature fatigue failure. Therefore, it is important to take these larger tensile residual stresses into account for the fatigue design of a welded component.

Characterization of thermally sprayed copper and numerically supported residual stress determination by the incremental hole-drilling method

This contribution deals with the determination of the mechanical and thermal properties of thermally sprayed copper. To this end, a comprehensive characterization is carried out by temperature-dependent tensile, three-point bending and caloric tests within a temperature range from 293 K up to 1173 K. The obtained data can be implemented in models for commercial FE software for the simulation of cooling and deformation processes during thermal spraying and the resulting residual stresses. For this purpose, thick coatings were manufactured by wire arc spraying. The resulting volume was further processed by electrical discharge machining into suitable test specimens. Reference specimens from the corresponding solid material were also examined for evaluating the morphological features of the coating. Finally, the residual stress of a thermally sprayed coating system (copper on steel substrate) was determined by the numerically supported hole-drilling strain gage method, using the obtained material parameters. As a result, stress distributions in the coated substrate were computable.

Agglomeration effects of carbon nanotube on residual stresses in polymer nano composite using experimental and analytical method

Since, mechanical properties of carbon nanotubes (CNTs) reinforced polymers and their composites are affected by poor dispersion of CNTs, the present study has been done to investigate the effect of CNT agglomerates on the residual stresses in a fiber reinforced Nano composite using experimental measurements and analytical method. CNTs are incorporated into an epoxy matrix with two weight fractions of 0.1% and 0.5% and are reinforced by unidirectional carbon fibers (CFs) to fabricate laminated composites with [02/902]s layup. The hole-drilling method is employed to measure the residual stresses in cross-ply CNT/CF/epoxy composite laminates. The CNT/epoxy sample is produced where the degree of agglomeration is changed from perfectly dispersed state up to fully agglomerated one. Based on the classical laminate theory, analytical solution is used to calculate the residual stress caused by thermal treatment. The residual stresses measured by incremental hole-drilling method are in good agreement with the theoretical residual stresses calculated by each layer of the laminates. The results show that dispersed samples perfectly are produced lower residual stresses compared to poor dispersed samples and this phenomenon is more significant in case of Nano composite with higher weight fractions of CNTs.

An eigen-strain approach on the estimation of non-uniform residual stress distribution using incremental hole-drilling and slitting techniques

Despite the advantages of incremental hole-drilling and slitting methods for measurement of residual stresses, there are also some limitations in their utilization. For example, decreasing of the measurement accuracy of strain gauges by increasing the hole depth in the incremental hole-drilling method, and necessity of having an assumption for distribution of the residual stress in form of a polynomial in the slitting method are the severe limitations of these two methods. Also, the calculation of the calibration coefficients of the slitting method is a complicated and time-consuming task. In the current study, by using the concept of the eigen-strain as the source of the residual stress, a new experimental-analytical method is developed for estimation of the residual stress using incremental the hole-drilling and slitting methods. To evaluate the accuracy of the proposed method, an aluminum sample with a specific distribution of a pre-induced residual stress has been studied. A comparison of the residual stress distribution obtained by the present method and the pre-induced residual stress in the specimen and theoretical prediction shows the accuracy of the new proposed method.