If a connection to a web service uses HTTPS (HTTP over Secure Sockets Layer (SSL)) the certificate must be validated before a connection can complete and a request can be completed. If a web service has an invalid certificate an error will be returned.

How PowerShell reacts to different scenarios can be tested. The badssl site can be used to test how PowerShell might react to different SSL scenarios:

https://badssl.com/

For example, when attempting to connect to a site with an expired certificate (using Invoke-WebRequest) the following message will be displayed:

PS> Invoke-WebRequest https://expired.badssl.com/ 
Invoke-WebRequest : The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel. 
At line:1 char:1 
+ Invoke-WebRequest https://expired.badssl.com/ 
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-WebRequest], WebException 
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand  

Invoke-WebRequest cannot bypass or ignore an invalid certificate on its own (using a parameter). Certificate validation behavior may be changed by adjusting the CertificatePolicy on the ServicePointManager:

https://msdn.microsoft.com/en-us/library/system.net.servicepointmanager(v=vs.110).aspx