Sheri Sheppard

Stress intensity factors in spot welds

Abstract This paper looks at stress intensity factors of cracks in resistance spot welded joints. Stress intensity factors have been used in the past to predict fatigue crack propagation life of resistance spot welds. However, the stress intensity factors from all previous work was based on assumed initial notch cracks at the nugget, parallel to the sheets. Physical evidence shows, however, that fatigue cracks in spot welds propagate through the thickness of the sheets rather than through the nugget. In this work, stress intensity factors of assumed notch cracks and through thickness cracks in tensile shear (TS) and modified coach peel (MCP) specimens were determined by the finite element method. The finite element results from the assumed notch cracks were compared with the results in the literature and were found to be in agreement with the results from Zhang’s equations [Int. J. Fract. 88 (1997) 167]. The stress intensity factors of assumed notch cracks were found to be different from those of through thickness cracks. To date, no analytic equations for stress intensity factors of through thickness cracks in spot welds have been published. In the current work, simple equations are proposed to estimate the KI and KII values of through thickness cracks in TS and MCP specimens.

Spot welds fatigue life prediction with cyclic strain range

Even though spot welds have been used widely in automotive manufacturing for years, fatigue life prediction methods for estimating life have been based on welds connecting sheets of the same thickness. A validated fatigue life prediction method for spot welds connecting sheets of different thicknesses has yet to be published. In this study, the stresses and strains at the notch of a spot weld nugget were studied in detail. It was found that significant yielding occurs in spot welds even under relatively low loads. A fatigue life prediction method based on strain is proposed. The work here shows that a strain-based approach does a very good job of describing the fatigue life of mixed-thickness spot welds.

Resistance Spot Weld Failure Loads and Modes in Overload Conditions

The failure modes of two single-weld specimens, the coach-peel and the tensile-shear specimens, were studied in detail. Weld overload experiments, along with optical and scanning electron microscopy, revealed that the coach-peel specimen failed by microvoid coalescence (ductile fracture) near the weld nugget/heat affected zone (HAZ) boundary and that the tensile-shear specimen failed predominately by localized necking (shear localization) near the HAZ/base metal boundary. Empirical data extracted from measurements performed on metallurgical cross sections of interrupted coach-peel and tensile-shear specimens established the deformed characteristic material distance (coach-peel) and the existence of a critical thickness strain for localized necking (tensile-shear). These quantities were used to predict weld failure via finite element analysis as described in Ref I. The work presented here is the first step in a larger project that is focused on developing a methodology for predicting spot weld overload failure in detailed finite element simulations of spot-welded joints. This methodology is based upon the failure phenomena (as reported here) and detailed characterization of the HAZ (as reported in Ref I). The main requirement of this predictive methodology is that it be adaptable to any combination of joint configuration and loading direction. This predictive methodology will serve as the basis for the final step of developing a model of resistance spot weld failure based upon a simpler representation of the spot weld that can be used in car crash simulation models.

Short Subsurface Cracks Under Conditions of Slip and Stick Caused by a Moving Compressive Load

The mechanism of spoiling failure in rolling contact is modeled by an elastic halfplane with a subsurface crack parallel to the surface, loaded by a compressive normal force which moves over the surface. Coulomb friction at the crack faces reduces the Mode II Stress Intensity Factors and results in a number of historydependent slip-stick configurations. The formulation used to study these involves a singular integral equation in two variables which must be solved numerically, and because of the history dependence, requires in an incremental solution. Only crack lengths and coefficients of friction that result in a maximum of two slip or stick zones for any load location are considered in this paper. It is found that the maximum range of stress intensity factors occurs at the trailing crack tip.

Further Refinement of a Methodology for Fatigue Life Estimation in Resistance Spot Weld Connections

The overall objective of this research is to develop a general model of fatigue crack propagation in resistance spot-welded joints. An important feature of this development is that the model and accompanying methodology should be accessible to designers evaluating fatigue response of structures containing multiple welds. This objective is achieved by examining the stress state around a resistance spot weld. A general expression for the structural stress around the weld is formulated that is dependent only on the loading immediately surrounding the weld; as such, it is specimen independent. In previous work [1] it was found that structural stress could be successfully used to estimate life for crack initiation and growth to a length of 0.01 in. (0.254 mm) in resistance spot welds, and that this period represents less than 30% of the total life in as-welded joints. It is important to note that estimation of this period is highly dependent on fatigue-related material properties. In Ref 2 it was shown that structural stress can be related to crack propagation life through linear elastic fracture mechanics. Using the resulting relationship between structural stress and life, life estimations were made for a variety of HSLA steel specimens (e.g., tensile-shear, coach peel) in a number of conditions (e.g., as-welded, prestressed) and were compared with experimentally measured lives. Life estimations were within a factor of 3 of measured lives. In the current work, the methodology developed in Ref 2 for estimating propagation life is extended by explicitly considering the effects of axial loading and of load ratio on life estimates. In addition, issues related to creating an appropriate finite element model are addressed, and a number of modeling guidelines are established. Total life estimations are made and compare favorably with experimentally measured lives of welds in mild steels in a variety of specimen configurations.

Determining the constitutive properties of the heat-affected zone in a resistance spot weld

A methodology is described for determining the mechanical properties of the heat-affected zone (HAZ) in a resistance spot weld. To perform detailed finite-element analyses of spot weld overload failures, it is necessary to have the mechanical properties of the various HAZ subzones. To this end, simulated HAZ material samples were fabricated in a process that utilized a resistance spot welding machine fitted with specially modified electrodes. Tensile tests at several loading rates were performed on miniature notched-bar specimens machined from the simulated HAZ material. These tests provided the empirical data from which the strain hardening capacities and strain rate sensitivities of the weld HAZ were determined through detailed finite-element analyses of the notched-bar tensile tests.

Students connecting engineering fundamentals and hardware design: observations and implications for the design of curriculum and assessment methods

The paper explores how engineering students use fundamental concepts studied in analysis classes as they undertake experiences in hardware design and dissection. Examples are drawn from videotape studies and in situ observations of students. We observed that students learn by reflecting on their experiences and by linking and contextualizing theoretical and practical knowledge. Curriculum design and assessment methods that help foster these skills are discussed.

Stress intensity factors in shafts subjected to torsion and axial loading

Abstract The finite element method has been used to determine stress intensity factors (SIFs) of surface cracks in plain and fillet notched shafts subjected to torsion and axial loadings. The SIFs were obtained as part of a simulation of surface crack growth under fatigue conditions. The shape of the crack front was also predicted and compared to the crack front shapes observed on several test specimens.

The Subsurface Crack Under Conditions of Slip and Stick Caused by a Surface Normal Force

On donne la solution pour le champ de contraintes elastiques dans un demi-plan contenant une fissure plane parallele a la surface et soumise a une pression normale uniforme et a une charge normale concentree. Le glissement de Coulomb peut exister entre les faces de la fissure

Targeting undergraduate students for surveys: Lessons from the Academic Pathways Of People Learning Engineering Survey (APPLES)

The academic pathways of people learning engineering survey (APPLES) was deployed in April 2007 targeting cross-sectional populations from four American universities. The goal of APPLES is to validate earlier findings from the academic pathways study on factors that correlate with persistence in engineering. There is minimal literature detailing the practical process and methodology for engineering education researchers to undertake thorough, statistically-sound survey research, particularly as it relates to reaching specific student groups within the undergraduate engineering population. We outline the APPLES development methodology, and specifically the lessons learned in deploying a multi-site, medium-scale survey. This paper details our process for constructing the sampling plan and the resulting design for the APPLE survey, and addresses issues relating to working with campus liaisons, subject recruitment and deployment. Finally, we share preliminary response rates and feedback from respondents to inform a model for conducting survey design and research for engineering education researchers.