×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Load factors for ASME VIII Div.2 5.3.3 Elastic-Plastic Analysis
2

Load factors for ASME VIII Div.2 5.3.3 Elastic-Plastic Analysis

Load factors for ASME VIII Div.2 5.3.3 Elastic-Plastic Analysis

(OP)
Hello Guys,

I am designing a replacement unit for a heat exchanger. The previous unit has failed at the tube to tubesheet joints due to low cycle fatigue. The new design reduced the tubesheet thickness to reduce the stress at the tube to tubesheet joint. The design of the components without failure remained the same. I performed an elastic analysis in accordance with ASME VIII Div.2 Part 5. The design passed the limits on primary stress, fatigue assessment in accordance with 5.5.3 and ratcheting assessment in accordance with 5.5.6. However, the channel barrel to tubesheet joint has failed against the design limits on the primary plus secondary stress and also triaxial stress limit in accordance with 5.3.2. Most of the stress at this location is due to the differential expansion between the channel barrel and tubesheet. No failure has been reported at this joint on the previous unit. The previous design has shown similar stress levels at this location. Therefore, I do not think the overstress is due to the changed tubesheet thickness. The previous unit was designed to a different code using the elastic plastic analysis. Therefore, I am going to perform an elastic plastic analysis to ASME VIII Div.2.
my questions are:
1. I think both primary plus secondary stress limit and Triaxial stress limits from 5.3.2 are for protection against local failure. Therefore, the elastic plastic analysis should be performed in accordance with ASME VIII Div.2 5.3.3 to demonstrate the protection against local failure. Is my understanding correct?
2. In the table 5.5, there is no load case combinations including the thermal loads for local failure assessment. What load factor should I use for this analysis?

Your assistance would be greatly appreciated.


Kind regards,

Bill

RE: Load factors for ASME VIII Div.2 5.3.3 Elastic-Plastic Analysis

2
Regarding the Triaxial stress limit, please refer to the commentary in ASME PTB-1 (2014):

Quote (ASME PTB-1 (2014))

Two issues that are apparent is the use of an elastic stress basis for a local criterion and the
stress category that is used with this criterion. It is not apparent how pseudo elastic stresses, i.e. elastically calculated stresses that exceed the yield strength can be used to evaluate a local fracture strain of a ductile material with strain hardening. In addition, the type of stress used in the criterion (i.e. linearized or average values verse stress at a point) and stress category (i.e. primary, secondary and peak) needs to be resolved. Since local failure is the failure mode being evaluated, the type of stress and stress category used in VIII-3 would appear to more correct. For ductile materials, a local criterion based on elastic analysis may not meaningful and the elastic-plastic method that follows is recommended for all applications.

The elastic primary-plus-secondary stress limit is applicable for Protection Against Failure From Cyclic Loading: Ratcheting.

Focus on each failure mode by themselves - each one has different design margins.

Regarding your second question about Local Failure and Table 5.5 load factors (design margins), and the lack of inclusion of thermal loads - all that I can help you with is that the Code Committee could not determine an appropriate design margin for that load combination. Therefore, it is the responsibility of the User and the Designer to determine appropriate load factors for these circumstances.

RE: Load factors for ASME VIII Div.2 5.3.3 Elastic-Plastic Analysis

(OP)
Hi TGS4,

Thanks very much for your comprehensive response.

Kind Regards,

Bill

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources

White Paper - Effects of AIA Quick Guide to UL 489 or UL 1077
The function of a circuit breaker is to provide overload (thermal) and short-circuit (magnetic) protection to a circuit and its downstream components. A circuit breaker functions like an airbag in a car, protecting circuit components and people by tripping the circuit to interrupt the current flow if it detects a fault condition in the control system. Download Now
White Paper - Guide to Integrate Large-Format Additive
As with any new technology, getting into large-format 3D printing begins with investigation. The first question may be a simple one: what does “large-format” mean? For 3D printers, “large” is a relative term. Many extrusion-based (FFF) 3D printers are referred to as desktop machines, because they fit on table space. Some of these have very respectable build volumes – but when it comes to “large-format,” the machines will need their own dedicated floor space. Large-format 3D printers have significant build volumes and are most often found in professional settings, like manufacturing facilities and R&D centers. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close